EP1538978A2 - High-resolution magnetoencephalography system and method - Google Patents
High-resolution magnetoencephalography system and methodInfo
- Publication number
- EP1538978A2 EP1538978A2 EP03792953A EP03792953A EP1538978A2 EP 1538978 A2 EP1538978 A2 EP 1538978A2 EP 03792953 A EP03792953 A EP 03792953A EP 03792953 A EP03792953 A EP 03792953A EP 1538978 A2 EP1538978 A2 EP 1538978A2
- Authority
- EP
- European Patent Office
- Prior art keywords
- millimeters
- headrest
- sensors
- head
- dewar
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Withdrawn
Links
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/02—Measuring direction or magnitude of magnetic fields or magnetic flux
- G01R33/035—Measuring direction or magnitude of magnetic fields or magnetic flux using superconductive devices
- G01R33/0354—SQUIDS
-
- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B5/00—Measuring for diagnostic purposes; Identification of persons
- A61B5/24—Detecting, measuring or recording bioelectric or biomagnetic signals of the body or parts thereof
- A61B5/242—Detecting biomagnetic fields, e.g. magnetic fields produced by bioelectric currents
- A61B5/245—Detecting biomagnetic fields, e.g. magnetic fields produced by bioelectric currents specially adapted for magnetoencephalographic [MEG] signals
Definitions
- the invention relates to medical diagnostic systems and methods.
- the invention relates to medical diagnostic systems and methods.
- the invention relates to a system and method for obtaining high-resolution
- Dreyfus-Brisac, C The electroencephalogram of the premature infant and full- term newborn: normal and abnormal development of waking and sleeping patterns.
- P. Kellaway and I. Petersen (eds.) Neurological and electroencephalo ⁇ raphic correlative
- activities from a deep subcortical structure can be detected magnetically outside the brain
- hypoxemic-ischemic encephalopathy in term or near-term infants is between about 3/1000 and 8/1000. Handicapped survivors may be as high as about 42% in such
- the incidence of infants with neonatal seizures is between about 2/1000 and
- the neonatal period is between about 1/1000 and 3/1000.
- EEG electroencephalography
- the staging is useful in detecting a delay or an arrest in brain
- the waveforms and spatial topography such as hemispheric asymmetry of spontaneous EEG are also useful for detecting the presence of a tumor or a necrotic
- magnatoencephalography MEG
- electrophysiological monitoring technique complementing EEG, it may be desirable for
- MEG instrument which may be different from the conventional whole-head MEG instruments.
- a useful MEG instrument must be functional in any ordinary clinical
- the spacing between the patient's head and the sensor should be precisely
- spacing between the head and the sensors may be difficult to maintain in the presence
- FIG. 1 A is a diagrammatic illustration of a high-resolution magneto-
- MEG encephalography
- Fig. 1B is a diagrammatic cross-sectional side view of the system illustrated in
- Fig. 2A is a cross-sectional diagrammatic view of one embodiment of the headrest
- Fig. 2B is a diagram illustrating the wall thickness of a portion of the headrest of
- Fig. 3 is a diagrammatic view of the rear surface of the headrest of Fig. 2B and
- Fig. 4 is a diagram illustrating a single 4-channel sensor module
- Figs. 5A, 5B and 5C illustrate one embodiment of a 4-channel module according to the present invention
- Figs. 6A, 6B and 6C illustrate a coil form for a pickup coil of the module illustrated
- Fig. 6D and 6E illustrates a coil form for the cancellation coil of the module
- Figs. 7 and 8 are charts illustrating noise spectra for a 4-channel module with
- Figs. 9A and 9B are charts showing representative samples of the somatic
- SEFs evoked magnet fields
- Fig. 10 is a chart showing SEFs that are produced by a vibratory stimulation as a
- Fig. 11 illustrates charts showing SEFs measured on a plane above the skull of
- Fig. 12 illustrates charts showing an outline of the somatic evoked potential
- Fig. 13A illustrates charts showing the distortions due to conductivity differences
- Fig. 13B illustrates charts illustrating the Laplacian estimates of currents emerging
- Fig. 14 are charts showing the somatic evoked potentials (SEPs) measured over
- Figs. 15 illustrates charts showing the outputs from a 600 Hz signal for a piglet
- Figs. 16 illustrates charts showing recordings of SEF outside the brain and intracortical SEP
- Fig. 17 are charts showing that the Kyna-insensitive component was localized
- Fig. 18 is a cross-sectional side view of a headrest according to one embodiment
- Fig. 19 is a chart illustrating the cancellation of the ambient earth magnetic field
- Fig. 20 is a chart showing the noise cancellation for the line frequency noise using
- Fig. 21 is a pictorial view of another embodiment of an MEG system, which is
- Fig. 22 is a side elevational view of the system of Fig. 21 , illustrating it with portions thereof partially broken away for illustration purposes;
- Fig. 23 is an enlarged-scale sectional view of the system of Fig. 21 , illustrating the
- Fig. 24 is an enlarged side elevational sectional view of the headrest assembly of
- Fig. 25 is an enlarged sectional view of the headrest assembly of the system of
- FIG. 21 similar to Fig. 24 except taken at a different sectional plane;
- Fig. 26 is an enlarged-scale pictorial view of the array of sensors of the headrest assembly of the system of Fig. 21, illustrating the sensors in their relative positions;
- Fig. 27 is an enlarged face view of the headrest assembly of the system of Fig. 21;
- Fig. 28 is an enlarged face view of the rear surface of the headrest assembly of
- Fig. 29 is an enlarged sectional side elevational view of the headrest assembly
- magnetoencephalography system and method employing a portable cart having a
- SQUID dewar mounted in an inverted manner thereon, and having a headrest assembly
- the headrest assembly includes an array of magnetic sensors of the SQUID
- dewar for responding to electrical activity of the brain of the head.
- the dewar is inverted
- a sensor array plate positions the sensors relative to the headset.
- a plurality of second rods are fixed at one of their ends relative to the dewar and are
- first and second rods is composed of material expandable and contractible with changes
- a plurality of second rods are fixed at one of their ends relative to the dewar and are connected at their opposite Each one of the rods is composed of
- the sensor array plate is supported from below by the first and second rods.
- a movable is positioned below the sensor array plate and the second rods extend
- the headrest is rigidly attached to the dewar.
- disclosed embodiments of the present invention relates to a high-resolution magnetoencephalography (MEG) systemlO for evaluating neurological impairments of MEG
- the system 10 is a portable, non-invasive MEG system that can be used next to a
- crib 12 of any neonatal care unit without a cumbersome magnetically shielded room (not
- the system 10 includes a cart 14 which may be the size and shape of an
- Desirable spatial resolution and sensitivity may be provided by a closely- spaced evenly-distributed array (e.g., 19 x 4-channel modules) of superconducting MEG
- the sensors may be housed between about 1 mm and about
- the entire system is light enough to be portable, and the cart 14 includes
- wheels such as wheels 23 and 25 for rollably supporting the cart on the ground.
- a dewar 27 of a superconducting quantum interference device is
- a patient bed or cushion 29 is mounted on the cart 14 adjacent to the headrest assembly 16 for supporting the body of the
- the SQUID dewar 27 includes a liquid helium reservoir 32 and a fill port 34.
- SQUID phase lock loop electronic circuits and routers unit 36 control the SQUID
- the unit 36 is mounted on cart 14. A
- computer and power supply unit 38 is mounted at the rear end of the cart 14.
- a monitor and keyboard unit 41 are mounted above the unit 38 and communicate electrically
- the system 10 may
- MEG sensors e.g. Ahonen et al., 1992; Buchanan et al., 1994. Its sensitivity may be sufficiently high to measure not only spontaneous neuronal activity, but also evoked
- spatial resolution system 10 will be greater by a factor of about four in comparison to the
- the system 10 takes advantage of the fact that the infant's scalp and skull are
- headrest 21 can have a thickness of about 1.0 mm or only slightly greater and is safe to
- the sensitivity of the system 10 should be high enough to clearly detect evoked cortical
- the MEG sensors are housed in a
- the sensor array 18 of one embodiment of the system 10, includes of 19
- each module having 4 channels of sensors. It will be understood hereinafter described in greater detail, the sensors can be arranged in clusters in accordance with
- Such modules can be fabricated with a noise level
- the sensitivity of the system 10 is sufficiently high to clearly detect evoked cortical
- the system 10 is able to clearly detect such SEFs without signal averaging, since the system 10 requires between about 25 and about 36 times less averaging than the prior known similar MEG systems to obtain signals with comparable signal-to-noise ratios.
- the system 10 is the size of an ordinary examination table in one embodiment of the invention.
- the system 10 itself is provided the cart 14 with the mattress or bed 29
- EEG EEG.
- the measurements of MEG signals can start immediately after placing the baby on the bed 29 and the head in the headrest assembly 16.
- EEG EEG.
- the MEG system 10 is also able to function in clinical rooms without magnetic
- the system 10 includes magnetic field, line frequency and radio frequency noises, for example.
- SQUID control electronics may be equipped with SQUID control electronics with, for example, an effective dynamic range of about 32 bits that resolve magnetic fields between about 10 ⁇ T and ⁇ about 1
- fT may also have a fast slew rate that can follow magnetic field changes as fast as about 10 ⁇ T/ms which is fast enough to follow changes in the line frequency noise
- gradiometers and the reference channels enable the low frequency and 60 Hz noise to
- the system 10 may operate in unshielded environments.
- the above features make the system 10 useful in an ordinary clinical setting.
- the MEG system 10 may have a sufficiently high level of
- system 10 may satisfy this essential requirement by taking advantage of several unique
- the skull and scalp of a newborn are quite thin.
- the human skull at the age of 6 months after birth has an average thickness of about
- the skull should be between about 1.7 and about 2.0 mm thick at birth.
- scalp is also about 1 mm thick in the first several months of age.
- coils can be placed as close as between about 5 and about 6 mm from the cortical
- the system 10 may be thus configured
- mm also increases spatial resolution.
- the sensing coils are tightly packed in the system
- the spatial resolution of the system 10 is about
- abnormal cortical region such as the bilateral strip of cortical tissue along the anterior-
- MEG signals are transparent to the scalp and skull, unlike EEG, even in the presence of
- EEG signals may be significantly distorted by skull defects that are
- the skull is not present within the fontanels; instead the brain is
- the anterior fontanel may be
- midpositions is 3-4 mm for infants between 0 and about 60 days after birth (Eramie and
- sutures are 6 and 1 years, respectively. Thus, EEG signals may be profoundly affected
- fontanels improves the sensitivity of the EEG.
- skull defects may obscure
- the conductivity of the skull is also expected to change with age and across individuals.
- the skull thickness increases rapidly within the first three years of age from
- the 10th-90th percentiles are 2.4-4.6 mm
- the 10th-90th percentile range is 40-50% of the means for
- EEG signals may be distorted not only by skull defects, but also by the brain-skull
- a 6° cortical source may be as much as about 50 times greater for a 6° cortical source compared to a focal source at the center of the brain and as much as about 100 times greater for such a
- focal cortical sources may be small relative to deeper sources and thus some of the cortical components may be difficult to be identified or distinguished, being
- the MEG system 10 assesses neonatal brain functions and serves as a useful non-invasive clinical tool for monitoring physiological functions of the pre- term and full-term neonates born with possible neurological disorders.
- Figs. 2 and 3 show one embodiment of the headrest assembly 16 of a system 10
- honey-comb rear wall design enables the placement of the MEG detection sensor coils at a distance of about 1.5 mm or 2 mm (Fig. 2B) from the outer surface of the headrest
- housed may be in vacuum to provide thermal insulation.
- the sensor array of sensors 18 in one embodiment of the system 10 consists of
- Such modules may be built with a noise level of less than 10 TV I Hz.
- the preferred embodiment of the headrest uses a honey-comb rear-wall design with
- each module (or cluster) including 4 channels of sensing coils.
- the sensors 18 are positioned within the recesses in the rear wall where the headrest wall is the thinnest.
- the headrest assembly 16 was tested to determine whether the individual
- the deflection was less than about 100 ⁇ m for a 1.0 mm-thick window with a
- the deflection was less than about 100 ⁇ m for a 1.0-mm thick window.
- the vacuum was held by the window even for a 0.4-mm thick window.
- honey comb rear wall design with a wall thickness of between about
- chrome-alloy ball bearing was dropped from varying heights onto the center of selected
- the window can withstand a significant impact even for a wall thickness of about 1 mm.
- baby's head was modeled as an ellipsoidal volume with a radius of curvature of about 6
- the headrest may be made from this positive
- Each one of the sensors 18 is a first-order asymmetric, axial superconducting
- the sensors 18 are arranged in clusters of 4 separated by the honeycomb
- the expected noise level for the gradiometers with a pickup coil diameter of about 6 mm may be about 10 TV/ Hz or better.
- Such a module has been fabricated and determined its noise level has been determined.
- Fig. 5 shows the pickup coils, which are about 6 mm in diameter in one
- Figs. 7 and 8 show the noise spectra for the two channels.
- the noise spectra were quite clean with an elbow frequency (where the 1/f noise starts) of about 1 Hz. Based on these results, it appears that a 4-channel module can be constructed with a
- Figure 9 is a graphical representation of the results of the study and show representative samples of the SEFs produced by piezoelectric vibratory stimulations
- the latency delay of the initial component is approximately 35
- the SEFs for airpuffs are averages of 36 epochs
- the system 10 enables a study to be carried out by simply placing the baby's
- the system 10 provides a sense of safety to the parents who would be present at such a
- the 76-channel system 10 tends to speed up the study, since the measurement time is
- the system 10 may well be very useful for neonatal brain assessment.
- EEG is strongly affected by a hole in the skull mimicking the anterior fontanel in the
- system 10 may well be capable of providing new information about the
- skull of the piglet is virtually the same in waveform and spatial distribution with and
- Figure 12 shows an outline (dashed square) of the SEP (somatic evoked potential)
- conductivity of the sucrose-agar should be close to the conductivity of, in this case about
- the scalp is about 1.5 mm in
- Figure 14 shows the SEPs measured over a square
- dashed curves are the scalp SEPs.
- the results of this study indicate that it may be possible to measure SEF over the scalp as if the scalp and skull were absent, whereas the SEP on the dura may be distorted on the scalp.
- electrocorticographic sensors were positioned on the cortex.
- the SEF was first scanned over the SI and
- Fig. 15B shows the wideband (1 Hz - 3,000 Hz) signal at locations SQ2 and SQ4 along with the narrowband (416-2,083Hz) SEF, the difference wave between the SEFs measured at SQ2 and SQ4 and the power of the difference
- microSQUID and thus there is needed about 25 times less average or about 100 epochs
- generator of this 600 Hz signal may be the thalamocortical axonal terminals (Gobbele et al., 1998), inhibitory interneurons in the cortex (Hashimoto et al, 1996a; Mackert et al.,
- Fig. 17, left shows that the Kyna-insensitive component was localized within layer
- the laminar profile at its peak (time point 4) is shown in Fig. 17. This is the first post ⁇
- the MEG sensors are housed in a cryogenic container called a dewar that stores
- the dewar consists of two cylindrical containers.
- the inner container is
- the dewar may be made from a special laminated G-10 fiberglass that is constructed to prevent leakage of helium gas into the vacuum space.
- the dewar may be made from a special laminated G-10 fiberglass that is constructed to prevent leakage of helium gas into the vacuum space.
- the dewar weight may
- the total bed system may be about 300 lb, so that it should be portable on wheels.
- the body of the inner container may be shielded against heat radiation leak using layers of low conductivity material such as aluminum.
- the shielding may be installed in removable packs to enable ease of construction and rework.
- the outside may be about 2 mm via independent mounts.
- the top section of the dewar may be made separately from the bottom cylindrical
- the plate may be a G-10 plate with a cylinder epoxied onto the plate.
- the plate may be a G-10 plate with a cylinder epoxied onto the plate.
- the exhaust hole may be sealed with a removal thermal shield to
- the cylinder may be machined so that
- the headrest can be epoxied onto the cut surface.
- the headrest may have a curvature
- the headrest may be ellipsoidal or semi-ovoid with the radii of curvature of about 6 and about 8 cm. This dimension should be sufficient to accommodate infants of up to about two years of age
- the sensor pick up coils may be mounted at the top and the
- an MEG system 44 may be mounted at the bottom of the inner container of the dewar 27. This sensor assembly may be maintained at the superconducting temperature, by a cold heat sink attached to the bottom of the inner container.
- an MEG system 44 have a
- headrest assembly 46 enables the sensor array 48 to be moved adjustably relative to
- An actuating mechanism 53 may be installed that may enable an operator
- the boil off rate for the system 44 is about 4 liters/day when it is not in use and 8
- a sufficient size dewar such as a 25-liter dewar may
- helium transfers may need to be made twice a week for some applications.
- the actuating mechanism 53 may be made as illustrated in Fig. 18.
- the shafts such as a shaft 50 supporting the sensor array may be spring loaded and moved up
- the array consists of 19 modules of 4 channels each for the disclosed embodiment. Each module may be attached to an
- ellipsoidal support as shown in Fig. 3.
- the distance between the center of a module to the center of an adjacent module is approximately 24 mm for this embodiment.
- Each module consists of 4 channels of first-order, asymmetric, axial gradiometers
- the most suitable pick up coil configuration is a 12 turn, 6 mm-diameter pick up coil and a 6 turn (spaced by 1 mm), 8.49 mm-diameter cancellation coil. This is
- the pickup coils within and across the clusters of 4 are densely packed to
- the coil-to-coil diagonal distance within each module is
- MEG systems made by the CTF Systems and 4D-Neuroimaging.
- the leads from the pick up may be RF shielded.
- coils may be shielded using superconducting lead tubes.
- Each module may be RF shielded by
- An electronic rack may be placed in the body of the portable cart 14 below the
- This rack houses 84 channels of SQUID control units
- the dc-SQUIDs may be connected via RF-
- the SQUID electronics may be powered completely by a suitable dc power
- the dc power reduces the line frequency noise that would be sensed by the MEG
- the system 10 must operate in electromagnetically unshielded environments
- the system 10 is portable and is
- the second stage may be
- Fig. 19 illustrates the
- Figure 20 shows the noise cancellation for the line frequency noise using the 8
- the eight channels of reference consisted of five gradiometer channels and three magnetometer channels comprising a complete field and field gradient measurement.
- the test noise data was acquired at a 2 kHz sample rate for one
- This noise cancellation scheme may be refined by finding the weights that may be time dependent. This refinement may provide a line
- the data acquisition may be carried out by a PCI-based system that may be attached to the cart 14 as shown in Fig. 1.
- control units may be fed to a set of data routers via fiber optic cables. This reduces the rf noise feeding back into the SQUID control unit and into the SQUIDs.
- the control units are
- the fiber optic outputs from four units (12 channels) may be
- level 1 routers fed at a rate of 480 kByte/sec to a level 1 router.
- Four of the level 1 routers are fed to the next router at a rate of 1.92 MB/sec. Since there are 84 channels, 7 level 1 routers
- the level 2 routers may be fed into a PCI card with a transfer rate of 8 MB/s. This data
- the acquisition system continuously acquires 4-byte data at 10 kHz from 84 channels.
- a 1 GHz PCI-based PC may be is sufficient for some applications.
- the STL system may
- the data may be sampled at a rate of at least 8 kHz.
- the data acquisition may be controlled by software. . It provides a complete control over all electronic features.
- An optical technique may be used to determine the head shape of
- This method uses one digital camera to take the pictures of a grid pattern
- the distortions of the grid pattern seen by the camera are used to reconstruct the 3D shape of the head and face. This method may be selected since it is completely remote in operation, non-invasive, fast and economical.
- the image can be obtained in less than 1 second and the reconstruction can be done offline, so that there is no need to worry about movement of the baby's head.
- the conventional optical positioning system 28 from Eyetronics may be any optical positioning system 28 from Eyetronics.
- the input parameters are the relative positions of the camera and the miniature slide projector that may project a single grid pattern onto the face and head,
- overlapping pictures may be taken from different angles.
- a software provided by Eyetronics, may be used to integrate the separate images. This process can be
- the duration of sleep is typically between about 10 min and about 40 min. Monitoring the head position continuously during this period enables
- the useful measurement period may also be extended due to the monitoring , to the period of wakefulness. Oftentimes
- the baby keeps his/her head stationary for more than about 10 sec at a time during the
- head position tracking there are optical, electromagnetic and laser tracking systems. It may be desirable for some applications to continuously monitor the head with an absolute positional accuracy of about 1 mm during measurements.
- Optical tracking systems include two or more cameras of known locations. They track
- the marks can be either passive (for example, reflective
- Optical systems have 0.3 - 0.4 mm accuracy in a tracking volume of about 1 m 3 .
- the update rate is in the 20 - 70 Hz range.
- location can be synchronized with magnetic measurements.
- Laser tracking Laser tracking system has the same advantage as optical systems in that it does not tend to interfere with magnetic measurements, and the object position can be synchronized with the MEG data.
- These systems include a cost effective LaserBird
- the accuracy is about 1 mm at about 1 m distance.
- the tracking is done by scanning the work area with a laser beam.
- Electromagnetic tracking systems are widely used with SQUID biomagnetic measurements (for example, EM tracking systems from Polhemus).
- An electromagnetic transmitter is rigidly attached relative to the pick up coils, and three receivers are
- MEG magnetoencephalography
- the system 55 is generally similar to the system
- system 55 employs clustered sensors instead of modular sensors
- the system 55 is adapted to serve as a non-invasive neurodiagnostic tool in
- System 55 includes a cart 59 having a headrest assembly 62 disposed
- the headrest in an upwardly facing direction for receiving the head of the patient 57.
- assembly 62 is generally similar to the headrest assembly 16, and includes a sensor
- the cart 59 is designed to be portable and roll
- a SQUID dewar 75 is mounted on the cart 57, and includes an optical positioning
- the headrest 62 her head supported from below by the headrest 62.
- a liquid helium reservoir 79 of the SQUID dewar 75 is a
- the electronic components are connected to the SQUID via a group of
- a portable trailer 93 is mounted rollably above the ground by means of a series of
- a group of shielded cables generally indicated at 95
- the trailer including a
- DC power supply 97 and a transformer 99 can be moved along with the cart 57.
- a pair of DC power supply 97 and a transformer 99 can be moved along with the cart 57.
- the headrest assembly 62 has two curvatures for coronal and sagittal (axial)
- the headrest assembly 62 is concave and is
- a more preferred range of the sagittal radius is between about 90 mm and about 110
- a more preferred range of the coronal radius is between about 70 mm and
- coronal radius is about 75 mm.
- the headrest 66 is composed of non-metallic head-insulating structurally strong
- Each one of the sensors 64 is a superconducting gradiometer having a pick-up coil diameter of between about 4 mm and about 8 mm. More preferably, the pick-up coil diameter is between about 5 mm and about 7 mm. Currently, the most preferred pick-up diameter is about 6 mm.
- the gradiometer sensors are uniformly distributed relatively to the head engageable surface of the headrest 66.
- Each one of the sensor pick-up coils being
- spacing distance is more preferably between about 8 mm and about 12 mm.
- the sensors are arranged in groups of four, wherein the spacing
- the distance between adjacent sensors of a group is about 8.5 mm and the spacing distance between diagonally disposed sensors of a group is about 12 mm.
- the sensors 64 are arranged in groups, and
- the headrest 66 has a corresponding series of windows or recesses such as a recess 108 in the rear surface of the headrest 66 positioned opposite to one of the groups of the
- each one of the recesses is generally rectangular in shape and
- the rear surface of the headrest is arranged in a honeycomb configuration to provide a thin wall
- sensors 64 are arranged in clusters of four, it is to be understood that
- each separate sensor may have its own recess or window and not be
- the sensors are arranged in groups of four, and each one of the four sensors of a group provides a
- the channel is usable individually, or in combination
- the channels can be summed in various ways.
- channels can be subtracted, or used individually.
- the dewar 75 is radio frequency interference shielded by multiple techniques.
- the external portions (those at room temperature) of the dewar is shielded by the external portions (those at room temperature) of the dewar.
- coating conductivity and thicknesses is such to provide an eddy current shield with a roll-
- the coating may be applied through various combinations
- the interior portion of the dewar 75 is also shielded by the use of one
- ultra-thin assemblies of conductive material such as aluminum.
- the assemblies are such that they act as an eddy current shield with a
- signals of interest typically less than 10 kHz.
- magnetometer detection sensor coils in the dewar are magnetometer detection sensor coils in the dewar.
- headrest 66 is maintained constant through various temperature changes of the dewar 75.
- the system 10 either cools down or warms up, the differences in
- the coefficient of thermal expansion of the various components may cause the small gap
- a mounting ring 109 is fixed to the headrest assembly 62 and mounts it fixedly to
- a coil array plate 110 supports the sensors 64 in close
- a set of rods such as the rods
- the longer rods such as the rod 111 is composed of
- suitable material such as quartz so that when the system 10 cools down, the quartz rods
- the shorter rods such as the rod 117 are also composed of
- suitable material such as quartz so that when the system cools down, for example, the
- quartz rods such as the rod 117 shrinks in length to pull the coil toward the headrest 66.
- the coil array plate 110 is pre-adjusted to correct lift-off when the system is warm.
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Abstract
According to certain embodiments of the present invention, there is provided a magnetoencephalography and method employing a portable cart having a SQUID dewar mounted in an inverted manner thereon, and having a headrest assembly mounted on the cart for supporting the head of a patient and forming a portion of the dewar. The headrest assembly includes an array of magnetic sensors of the SQUID dewar for responding to electrical activity of the brain of the head.
Description
HIGH-RESOLUTION MAGNETOENCEPHALOGRAPHY SYSTEM AND METHOD
Related Applications The present application is related to U.S. non-provisional patent application entitled "HIGH-
RESOLUTION MAGNETOENCEPHALOGRAPHY SYSTEM, COMPONENTS, AND
METHOD," filed June 26, 2003, Serial No. unknown/unassigned, which is incorporated
herein by reference, and to U.S. non-provisional patent application entitled "HIGH-
RESOLUTION MAGNETOENCEPHALOGRAPY SYSTEM AND METHOD," filed June 26,
2003, Serial No. unknown/unassigned, which is incorporated herein by reference.
The subject patent application also claims priority to U.S. provisional patent
application, entitled HIGH-RESOLUTION MAGNETOENCEPHALOGRAPHY SYSTEM,
Serial No. 60/393,045, filed June 28, 2002, which is incorporated herein by reference.
BACKGROUND OF THE INVENTION Field of the Invention
The invention relates to medical diagnostic systems and methods. In particular,
the invention relates to a system and method for obtaining high-resolution
encephalographs.
Related Art The information contained in this section relates to the background of the art of
the present invention without any admission as to whether or not it legally constitutes prior art.
The following is a list of articles relating to various diagnostic techniques
contemplated for assessing brain functions, as follows:
Ahonen, A. I., Hamalainen, M. S., Kajola, M. J., Knuutila, J. E. T., Laine, P.
P., Lounasmaa, 0. V., Parkkonen, L. T., Simola, J. T., and Tesche, C. D. 122 channel SQUID instrument for investigating the magnetic signals from the human brain. Physica
Scripta, 1993, T49: 198-205; Barth, D. S., Sutherling, W., Broffman, J., and Beatty, J. Magnetic localization of a dipolar current source implanted in a sphere and a human cranium. Electroenceph. in. Neurophvsio 1986, 63: 260-273;
Buchanan, D. S., Crum, D. B., Cox, D., & Wikswo, J. P. jr. MicroSQUID: A
close-spaced four channel magnetometer. In S. J. Williamson et al., (eds.), Advances in
Biomagnetism, Plenum Press, New York, 1989, pp. 677-679; Curio, G., Mackert, B.-M., Abraham-Fuchs, K., and Harer, W. (1994) High-
frequency activity (600 Hz) evoked in the human primary somatosensory cortex: a survery
of electric and magnetic recordings. In C. Patev et al., eds. Oscillatory Event-Related Brain
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incorporated herein by reference.
The need for finding useful diagnostic techniques is becoming increasingly urgent
today in assessing brain functions of infants. With advances in medicine, more and
more pre- and full-term newboms survive even with neurological disabilities (Sunshine,
1997; Volpe, 2000). According to Sunshine (1997), the number of newborns with
neurological impairments is quite large. The incidence of perinatal asphyxia is between
about 2/1000 and about 47/1000. Between about 4% and about 26% of those newborns
who survive such an event will have severe neurological deficits. The incidence of
hypoxemic-ischemic encephalopathy in term or near-term infants is between about 3/1000 and 8/1000. Handicapped survivors may be as high as about 42% in such
cases. The incidence of infants with neonatal seizures is between about 2/1000 and
about 9/1000. The incidence of handicaps in the survivors is between about 11% and about 50%. The incidence of moderate-to-severe cerebral palsy in infants who survive
the neonatal period is between about 1/1000 and 3/1000. The prevalence of severe
mental retardation is between about 3/1000 and about 4/1000 school-age children. The
incidence of mild mental retardation is between about 23/1000 and about 30/1000 in the
same population. According to Volpe (2000), the percentage of preterm infants with proven periventricular white matter injury is about 45% for those with birth weight of less
than about 1500g, about 38% for those with gestational age of less than about 33 weeks
and about 24% for those with gestational age of less than about 38 weeks. The
percentage of asphyxiated term infants with some form of central nervous system injury is as high as about 62%, a common form of the injury being the parasagittal cerebral injury. Infants with germinal matrix hemorrhage is between about 23% and about 32%
of all births delivered through the vaginal route when the delivery lasts more than six
hours.
Survival of neurologically impaired neonates raises an important responsibility for the health care community in this country. Currently, electroencephalography (EEG) is used to monitor electrical activity of the brain of newborns (Sunshine, 1997). The use of
EEG for perinatal monitoring was started in the late 1950's (Dreyfus-Brisac et al., 1958; Dreyfus-Brisac, 1962, 1964). Its use is increasing in recent years due to its usefulness
in staging the development of the nervous system, in detecting the presence of hypoxic and intracranial injuries, in providing the prognosis of recovery and in differential
diagnosis of seizures from non-seizures in paroxysmal motor behavior (Tharp, 1997; de
Weerd, 1995). The staging is useful in detecting a delay or an arrest in brain
development. The waveforms and spatial topography such as hemispheric asymmetry of spontaneous EEG are also useful for detecting the presence of a tumor or a necrotic
area in the brain. In order for magnatoencephalography (MEG) to become useful as a clinical
electrophysiological monitoring technique, complementing EEG, it may be desirable for
certain applications to have a MEG instrument which may be different from the conventional whole-head MEG instruments. In this regard, to be competitive with EEG instrumentation, a useful MEG instrument must be functional in any ordinary clinical
rooms without any special cumbersome electromagnetic shielding such, for example, as
a large and expensive, special purpose magnetically shielded room being currently used
when conventional MEG techniques are employed.
Prior known MEG systems (such as those manufactured by Canadian Thin Films
or CTF, Vancouver, Canada, 4-D Neuroimaging, San Diego, CA, and Helsinki, Finland)
are relatively large and heavy, and are used mostly in an expensive magnetically
shielded room.
In order to use an MEG system to detect infant brainwaves with a sufficient high
resolution, it would be desirable to be able to position the MEG sensors in very close proximity to the head of the infant patient. This would necessitate maintaining such
close spacing between the MEG sensors and the patient's head without altering the
spacing to any substantial extent during the use of this system.
The spacing between the patient's head and the sensor should be precisely
maintained within reasonable tolerances for at least some applications. Due to the
different coefficients of thermal expansion of the various components, this gap or
spacing between the head and the sensors may be difficult to maintain in the presence
of temperature changes for certain applications. Such temperature changes occur, for
example, when the SQUID dewar warms from cryogenic temperatures toward room
temperature. Also, when the dewar is cooled to its cryogenic temperatures, the
components tend to change dimensionally for some applications.
BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 A is a diagrammatic illustration of a high-resolution magneto-
encephalography (MEG) system according to one embodiment of the present invention;
Fig. 1B is a diagrammatic cross-sectional side view of the system illustrated in
Fig. 1A;
Fig. 2A is a cross-sectional diagrammatic view of one embodiment of the headrest
assembly of the system of Fig. 1 A;
Fig. 2B is a diagram illustrating the wall thickness of a portion of the headrest of
the headrest assembly of Fig. 2A;
Fig. 3 is a diagrammatic view of the rear surface of the headrest of Fig. 2B and
the headrest and illustrates one embodiment of the arrangement of sensor modules of
the headrest assembly of Fig. 2;
Fig. 4 is a diagram illustrating a single 4-channel sensor module;
Figs. 5A, 5B and 5C illustrate one embodiment of a 4-channel module according to the present invention;
Figs. 6A, 6B and 6C illustrate a coil form for a pickup coil of the module illustrated
in Figs. 5A, 5B and 5C;
Fig. 6D and 6E illustrates a coil form for the cancellation coil of the module
illustrated in Figs. 5A, 5B and 5C;
Figs. 7 and 8 are charts illustrating noise spectra for a 4-channel module with
simulated dewar noise;
Figs. 9A and 9B are charts showing representative samples of the somatic
evoked magnet fields (SEFs) produced by piezoelectric vibratory stimulations and SEFs
produced by air puff stimulations;
Fig. 10 is a chart showing SEFs that are produced by a vibratory stimulation as a
function of number of epochs in the average;
Fig. 11 illustrates charts showing SEFs measured on a plane above the skull of
the piglet;
Fig. 12 illustrates charts showing an outline of the somatic evoked potential
measurement area on the skull with a square hole over the left hemisphere;
Fig. 13A illustrates charts showing the distortions due to conductivity differences;
Fig. 13B illustrates charts illustrating the Laplacian estimates of currents emerging
through a filter paper for various conditions;
Fig. 14 are charts showing the somatic evoked potentials (SEPs) measured over
a square area;
Figs. 15 illustrates charts showing the outputs from a 600 Hz signal for a piglet;
Figs. 16 illustrates charts showing recordings of SEF outside the brain and intracortical SEP;
Fig. 17 are charts showing that the Kyna-insensitive component was localized
within layer IV and that the presynaptic component was generated by the thalamocortical terminals within the cortex;
Fig. 18 is a cross-sectional side view of a headrest according to one embodiment
of the invention;
Fig. 19 is a chart illustrating the cancellation of the ambient earth magnetic field
changes achieved in the frequency range below about 5 Hz; Fig. 20 is a chart showing the noise cancellation for the line frequency noise using
8 reference channels;
Fig. 21 is a pictorial view of another embodiment of an MEG system, which is
constructed in accordance with certain embodiments of the present invention;
Fig. 22 is a side elevational view of the system of Fig. 21 , illustrating it with portions thereof partially broken away for illustration purposes;
Fig. 23 is an enlarged-scale sectional view of the system of Fig. 21 , illustrating the
headrest assembly and dewar;
Fig. 24 is an enlarged side elevational sectional view of the headrest assembly of
the system of Fig. 21; Fig. 25 is an enlarged sectional view of the headrest assembly of the system of
Fig. 21 , similar to Fig. 24 except taken at a different sectional plane;
Fig. 26 is an enlarged-scale pictorial view of the array of sensors of the headrest assembly of the system of Fig. 21, illustrating the sensors in their relative positions;
Fig. 27 is an enlarged face view of the headrest assembly of the system of Fig. 21;
Fig. 28 is an enlarged face view of the rear surface of the headrest assembly of
Fig. 27; and
Fig. 29 is an enlarged sectional side elevational view of the headrest assembly
and its supporting structure.
DESCRIPTION OF CERTAIN EMBODIMENTS OF THE INVENTION
According to certain embodiments of the present invention, there is provided a
magnetoencephalography system and method employing a portable cart having a
SQUID dewar mounted in an inverted manner thereon, and having a headrest assembly
mounted on the cart for supporting the head of a patient and forming a portion of the
dewar. The headrest assembly includes an array of magnetic sensors of the SQUID
dewar for responding to electrical activity of the brain of the head. The dewar is inverted
in that its sensors are disposed above its reservoir.
According to certain embodiments of the present invention, there is provided a
mounting arrangement for a magnetoencephalography (MEG) system headrest
assembly forming a portion of a SQUID dewar and having a fixed headrest and an array
of sensors, wherein a sensor array plate positions the sensors relative to the headset. A
plurality of first rods interconnecting the array plate and the movable mounting member.
A plurality of second rods are fixed at one of their ends relative to the dewar and are
connected at their opposite ends to the movable mounting member. Each one of the
first and second rods is composed of material expandable and contractible with changes
in temperature, whereby the movable mounting member tends to be moved in one
direction toward or away from the sensor array plate, and tends to be moved in an
opposite direction away from or toward the array plate, resulting in temperature changes affecting the first and second rods, so that the sensor array plate and its sensors
maintain a substantially constant spacing between the sensors and the headrest with
changes in temperature. A plurality of second rods are fixed at one of their ends relative to the dewar and are connected at their opposite Each one of the rods is composed of
material expandible and contractible with changes in temperature, whereby the movable mounting member tends to be moved in one direction toward or away from the sensor
array plate, and tends to be moved in an opposite direction away from or toward the
array plate resulting in temperature changes affecting the first and second rods, so that the sensor array plate and its sensors maintain a substantially constant spacing between
the sensors and headrest with changes in temperature.
According to the disclosed embodiment of the invention, the first and second rods
are each composed of quartz material.
According to other features of the invention as disclosed herein, the sensor array plate is supported from below by the first and second rods. Additionally, as disclosed
herein, a movable is positioned below the sensor array plate and the second rods extend
upwardly therefrom. Additionally, according to the disclosed embodiment of the invention, the headrest is rigidly attached to the dewar.
MODULAR SENSOR MEG SYSTEM Referring now to the drawing and more particularly to Figs. 1 A and 1 B thereof, the
disclosed embodiments of the present invention relates to a high-resolution magnetoencephalography (MEG) systemlO for evaluating neurological impairments of
preterm and term babies, for example. The system 10 provides a non-invasive neurodiagnostic tool that may complement electroencephalography (EEG) in assessing
possible neurological dysfunctions and brain development in neonates through its
capability to detect electrophysiological functions in focal areas of the cortex in real time with or preferably without signal averaging.
The system 10 is a portable, non-invasive MEG system that can be used next to a
crib 12 of any neonatal care unit without a cumbersome magnetically shielded room (not
shown). The system 10 includes a cart 14 which may be the size and shape of an
examination table with a headrest assembly 16 for receiving the head of a patient (not shown). Desirable spatial resolution and sensitivity may be provided by a closely- spaced evenly-distributed array (e.g., 19 x 4-channel modules) of superconducting MEG
sensors 18 housed below the outer surface of a headrest 21 (Fig. 1 B) of the headrest
assembly 16. For example, the sensors may be housed between about 1 mm and about
3 mm below the outer surface. The sensors may be protected against radio frequency (rf) noise. The entire system is light enough to be portable, and the cart 14 includes
wheels such as wheels 23 and 25 for rollably supporting the cart on the ground.
A dewar 27 of a superconducting quantum interference device (SQUID) is
mounted in an inverted manner on the cart 14. A patient bed or cushion 29 is mounted on the cart 14 adjacent to the headrest assembly 16 for supporting the body of the
patient with his or her head supported by the headrest.
The SQUID dewar 27 includes a liquid helium reservoir 32 and a fill port 34. A
SQUID phase lock loop electronic circuits and routers unit 36 control the SQUID
functions and are used for data acquisition. The unit 36 is mounted on cart 14. A
computer and power supply unit 38 is mounted at the rear end of the cart 14. A monitor and keyboard unit 41 are mounted above the unit 38 and communicate electrically
therewith.
A short sensor distance combined with improved sensor noise provide
unprecedented sensitivity and spatial resolution for infant studies. The system 10 may
be about one order of magnitude better in sensitivity than the conventional whole-head
MEG sensors (e.g. Ahonen et al., 1992; Buchanan et al., 1994). Its sensitivity may be sufficiently high to measure not only spontaneous neuronal activity, but also evoked
activity of the cortex of the newborns in real time preferably without signal averaging. Its
spatial resolution system 10 will be greater by a factor of about four in comparison to the
existing whole-head MEG sensors.
The system 10 takes advantage of the fact that the infant's scalp and skull are
thin. This make it possible to measure MEG signals at a distance of only between about 5 mm and about 6 mm from the brain surface. This short distance results in a very large
amplitude of MEG signals from the newborns, since the magnetic field is inversely
proportional to the square of the distance. The short distance and a high density of
detectors also result in high spatial resolution. It has been determined that a 4-channel module with a noise level less than 10
fTΛ/Hz for detection coils with a diameter of about 6 mm can be built. Further, the
headrest 21 can have a thickness of about 1.0 mm or only slightly greater and is safe to
use for the system 10 (i.e., that holds the vacuum without breakage) can be constructed. The sensitivity of the system 10 should be high enough to clearly detect evoked cortical
activity preferably without signal averaging.
In certain embodiments of the invention, the MEG sensors are housed in a
vacuum section just below the headrest assembly 16. The wall of the headrest is thin
(about 1 mm or only slightly greater) so that the detection coils of the sensors can be
placed as close to the head as possible (~2 mm).
The sensor array 18 of one embodiment of the system 10, includes of 19
modules, each module having 4 channels of sensors. It will be understood hereinafter described in greater detail, the sensors can be arranged in clusters in accordance with
another embodiment of the invention. Such modules can be fabricated with a noise level
of less than 10 fϊV Hz for sensor pickup coils with a diameter of 6 mm. This is possible
even though the cross sectional area of the coils is about 10 times smaller than those of
the conventional whole-head sensors.
The sensitivity of the system 10 is sufficiently high to clearly detect evoked cortical
activity without signal averaging. A test was carried out by measuring the somatic
evoked magnetic fields (SEFs) in 1-7 month old infants with a microSQUID (not shown)
which was similar to the system 10 in that it has a 4-channel module in the vacuum
space just below the tail section of a conventionally oriented dewar. The distance between the pickup coils and the outer dewar wall surface was very short (about 1.2
mm) and comparable to the distance for the system 10. The noise level of the system
10 may be about 5 to about 6 times better than that of the prior known similar MEG
systems. Thus, one can extrapolate from the SEFs measured with the system 10 to infer whether evoked cortical responses can be indeed measured without signal averaging using the system 10. In one embodiment of the invention, clear signals were
measured non-invasively by averaging 16 responses to vibratory stimuli and 36 responses
to air puffs applied to the tip of the index finger. Extrapolating from these results, it may be concluded that the system 10 is able to clearly detect such SEFs without signal averaging, since the system 10 requires between about 25 and about 36 times less averaging than the prior known similar MEG systems to obtain signals with comparable signal-to-noise ratios.
The system 10 is the size of an ordinary examination table in one embodiment of the invention. The system 10 itself is provided the cart 14 with the mattress or bed 29
on top thereof, adjacent to the headrest. The acquisition of brain signal is relatively
rapid, since there is no need to attach electrodes, as in the case, for example, of an
EEG. The measurements of MEG signals can start immediately after placing the baby on the bed 29 and the head in the headrest assembly 16. For conventional EEG,
placing electrodes on the scalp is time consuming and often troublesome since the baby may easily resist. The baby may wake up if not sedated. This practical problem limits
the number of electrodes to be used in any diagnosis, thereby severely limiting the
inference that can be made regarding the nature of brain abnormality.
The MEG system 10 is also able to function in clinical rooms without magnetic
shielding. An ideal system should function without interference from the ambient
magnetic field, line frequency and radio frequency noises, for example. The system 10
may be equipped with SQUID control electronics with, for example, an effective dynamic range of about 32 bits that resolve magnetic fields between about 10 μT and < about 1
fT. It may also have a fast slew rate that can follow magnetic field changes as fast as about 10 μT/ms which is fast enough to follow changes in the line frequency noise
without loosing the lock on the flux-locked feedback loop. This fast slew rate enables
the system 10 to maintain the SQUIDs operating continuously in the midst of low frequency magnetic field changes and line frequency noises. The short-line baseline
gradiometers and the reference channels enable the low frequency and 60 Hz noise to
be cancelled with a suppression rate of between about 30 dB and about 90 dB. Thus,
the system 10 may operate in unshielded environments.
The above features make the system 10 useful in an ordinary clinical setting. In
addition to these features, the MEG system 10 may have a sufficiently high level of
sensitivity and spatial resolution to provide new information for certain applications. The
system 10 may satisfy this essential requirement by taking advantage of several unique
features of the head of infants. First, the skull and scalp of a newborn are quite thin.
The human skull at the age of 6 months after birth has an average thickness of about
2.5 mm (minimum of about .3 mm, maximum of about 3.7 mm) for girls and about 2.7
mm (minimum of about 1.7 mm, maximum of about 3.8 mm) for boys (Hansman, 1966).
Extrapolating from their normative data in connection with certain embodiments of the
invention, the skull should be between about 1.7 and about 2.0 mm thick at birth. The
scalp is also about 1 mm thick in the first several months of age. Thus, MEG sensing
coils can be placed as close as between about 5 and about 6 mm from the cortical
surface when the MEG system 10 is built with about a 2 mm distance between the
outer surface of the pillowcase and the sensing coils. The system 10 may be thus
capable of measuring MEG signals at a distance of about 5 mm as opposed to between
about 25 to about 30 mm for the conventional whole-head MEG instruments. Thus, the
measurement distance for a cortical source 3 mm below the surface of the brain would
be about 8 mm and about 30 mm for the system 10 and a conventional whole-head
MEG system. This implies that the signal may be more than 10 times stronger when
measured with the system 10. Therefore, it may be possible to monitor cortical activity
in real time without signal averaging. Monitoring cortical activity at a distance of about 5
mm also increases spatial resolution. The sensing coils are tightly packed in the system
10 to maximize the spatial resolution. The spatial resolution of the system 10 is about
four times higher than that of the conventional whole-head MEG systems. For an
example of a whole-head MEG system, reference may be made to U.S. patent
6,023,633, which is incorporated herein by reference. All sensor coils are assembled
below the headrest, so that there is no need to position each sensor precisely as it would
be required by EEG if one were to carry out a quantitative EEG analysis. The high-
density packing of the sensors in the system 10 make it possible to delineate the
abnormal cortical region such as the bilateral strip of cortical tissue along the anterior-
posterior direction expected in the case of parasagittal cerebral injury typical of term
infants who have suffered a temporary ischemia or anoxia.
Certain embodiments of the system may be capable of measuring cortical activity
as if its sensing coils are placed a few millimeters above the cortical surface without the
intervening scalp and skull, that is as if the scalp and skull are removed and the sensors
lie very close to the exposed cortical surface. The magnetic field above the head is
essentially the same as the field present at the cortex except for an attenuation of signal
amplitude and spread of the spatial distribution of the magnetic field merely due to the
slightly larger distance of measurement. This means that an MEG signal above the
scalp should be quite similar in information content to the signal just above the cortex.
MEG signals are transparent to the scalp and skull, unlike EEG, even in the presence of
skull defects created by the fontanels and sutures. It has been argued that the skull is
"transparent" to MEG signals (Kaufman et al., 1981) on the basis of theoretical results
obtained by Geselowitz and Grynszpan (Geselowitz, 1970; Grynszpan and Geselowitz,
1973). This conclusion is supported by simulations studies (Hamalainen and Sarvas,
1989), by measurements of MEG signals outside the head of a cadaver with a hole in
the skull (Barth et al., 1986) and by a careful comparison of the topography of the
somatic evoked magnetic fields before and after a hole is introduced in the skull in an in
vivo piglet study (Okada et al., 1999b, see Phase I Final Report).
In comparison, EEG signals may be significantly distorted by skull defects that are
unique to the human neonates. The fontanels are present at the midline junctions of the
bregma and lambda. The skull is not present within the fontanels; instead the brain is
protected by a thick dura filling the windows. They are small during the delivery, but
become larger in the first several months, up to between about 3 cm and about 4 cm
along the coronal suture, and then eventually they close. The anterior fontanel may be
large enough to admit an adult's thumb. The unclosed sutures can be quite wide near
the fontanels. The mean width of the coronal and lambdoidal sutures at their
midpositions is 3-4 mm for infants between 0 and about 60 days after birth (Eramie and
Ringertz, 1976). In abnormal cases such as hydrocephalous, the sutures may change
its width with the development of the disease and become as wide as 10 mm or more
(Eramie and Ringertz, 1976). Moreover, the sutures do not close for many years
(Hansman, 1966). The earliest age for complete closing of the sagittal and coronal
sutures are 6 and 1 years, respectively. Thus, EEG signals may be profoundly affected
by the fontanels and sutures since they create paths of low conductivity for the volume
currents in the brain and funnels the currents through these openings in the skull.
It might be argued that the skull defects are not a disadvantage for EEG, but on
the contrary, are an advantage. In diagnosing seizures the breech provided by the
fontanels improves the sensitivity of the EEG. However, the skull defects may obscure
the asymmetry of the signals, especially when the generator is deep, making it difficult to
determine the epileptiform tissue when it can not be easily visualized by CT or MRl.
The conductivity of the skull is also expected to change with age and across individuals. The skull thickness increases rapidly within the first three years of age from
about 2 mm at birth for term newborns to about 5 mm at 3 years of age for boys, then the growth slows down (Hansman, 1966). The skull thickness reaches a mean of about
8.3 mm and about 9.5 mm for 25 year-old women and men, respectively. These
changes in skull thickness are associated with thickening of the dense, poorly-
conducting inner and outer bony tables of the skull relative to the spongy middle layer containing blood and with a decrease in effective conductivity of the skull with age. Also,
important is the variability in skull thickness. The 10th-90th percentiles are 2.4-4.6 mm
and 3.0-4.9 mm for 1-1/2 year old girls and boys. That is, the range is 50-60% of the
means. At the age of 25, the 10th-90th percentile range is 40-50% of the means for
women and men.
EEG signals may be distorted not only by skull defects, but also by the brain-skull
and scalp-air boundaries of EEG waveforms in certain circumstances. Goff et al. (1978) have shown that the attenuation of scalp potential is highest for focal cortical sources and lower for extended cortical and subcortical sources. The attenuation of potential
may be as much as about 50 times greater for a 6° cortical source compared to a focal source at the center of the brain and as much as about 100 times greater for such a
shallow focal source compared to extended sources subtending a solid angle between
about 72° and about 180° regardless of depth. This large variation, depending on source
depth and extent, implies that the EEG signals on the scalp would be most likely different or deformed in comparison to the signals on the cortex. The components due
to focal cortical sources may be small relative to deeper sources and thus some of the
cortical components may be difficult to be identified or distinguished, being
overshadowed by signals from extended cortical or deeper subcortical sources.
In summary, the MEG system 10 assesses neonatal brain functions and serves as a useful non-invasive clinical tool for monitoring physiological functions of the pre- term and full-term neonates born with possible neurological disorders.
Figs. 2 and 3 show one embodiment of the headrest assembly 16 of a system 10
configured as a contoured semi-ovoid having a honey-comb shaped rear wall. The
honey-comb rear wall design enables the placement of the MEG detection sensor coils at a distance of about 1.5 mm or 2 mm (Fig. 2B) from the outer surface of the headrest
21 and, at the same time, provides a sufficient strength to protect the headrest 21
against the vacuum. The space where the sensor detection coils and SQUID dewar are
housed may be in vacuum to provide thermal insulation.
The sensor array of sensors 18 in one embodiment of the system 10 consists of
19 clusters of 4 sensors, each arranged as shown in Fig. 3, such that there is a reduced amount or minimum of space between individual sensors 18, given the honeycomb
structure. Such modules may be built with a noise level of less than 10 TV I Hz. The preferred embodiment of the headrest uses a honey-comb rear-wall design with
individual circular or elliptical windows or recesses (Fig. 3) for the 19 modules, each module (or cluster) including 4 channels of sensing coils. The primary advantage of
using individual windows or wells is the ability to achieve close spacing to the head while
providing strength to withstand against vacuum. In this regard, the sensors 18 are positioned within the recesses in the rear wall where the headrest wall is the thinnest.
The headrest assembly 16 was tested to determine whether the individual
windows or recesses in the honey comb rear wall design can withstand against the
vacuum (one atmosphere pressure) of the dewar 27. Extending the theoretical results,
the expected deflection was calculated at the center of a G-10 fiberglass window or
recess against the differential vacuum pressure of one atmosphere as a function of
window or recess diameter and thickness. The deflection was calculated using a
Mathcad routine based on Roark's formula for deflection (Roark and Young, 1975). A
specific case of flat circular plate with fixed edges and uniformly distributed pressure was
exploited. Young's modulus (E) of 2 x 106 was used for G-10. Table 1 shows the
results:
Table 1. Theoretical deflection at center of G-10
The deflection was less than about 100 μm for a 1.0 mm-thick window with a
about 30 mm were chosen to be slightly larger than the diameter needed to
accommodate a 20-mm diameter module. These theoretical results were verified
empirically. We constructed a 28-mm diameter G-10 window of varying thicknesses,
fixed the window on top of a fiberglass cylinder using epoxy seal and then evacuated the
inside of the cylinder. The deflection at the center of the window was measured with a
micrometer. Table 2 shows the results:
Table 2. Measured deflection at center of G- 10 window
The empirical results were quite close to the predicted values shown in Table 1.
Interpolating from Table 2, the deflection was less than about 100 μm for a 1.0-mm thick window. The vacuum was held by the window even for a 0.4-mm thick window. Based
on these results, the honey comb rear wall design with a wall thickness of between about
1.0 mm and about 1.5 mm appears to be safe to use for the system 10.
The safety of the window was evaluated with yet another test. A 25-mιm diameter
chrome-alloy ball bearing was dropped from varying heights onto the center of selected
28-mm diameter windows. The windows were epoxied onto the vacuum test fixture and a vacuum gauge measured the integrity of the window during impact. Table 3
summarizes the results:
The results of this drop test appears to demonstrate that a G-10 fiberglass
window can withstand a significant impact even for a wall thickness of about 1 mm. The
1-mm-thick window did not show any visible damage nor leak even when the large steel
ball was dropped from the height of about 60 cm. This second test, thus, reinforces the
conclusion that a honey comb design with a wall thickness of between about 1.0 and
about 1.5 mm appears to be safe to use for the system 10.
In addition to testing individual windows, the safety of an entire headrest 21 was
tested. For the purpose of this safety evaluation, a headrest with a uniform wall
thickness was constructed and determined the deflection of the center of the headrest
and its ability to withstand against one atmospheric pressure was determined. The
baby's head was modeled as an ellipsoidal volume with a radius of curvature of about 6
cm along the coronal section and 8 cm along the sagittal section, using a standard
reference for the head sizes (Lusted and Keats, 1978). The negative mold for the
headrest was then carefully made with gelucel wood, sealed with polyurethane varnish
and then heavy carnuba wax was applied as a mold release prior to applying the
fiberglass. Polyester resin and glass cloth were laminated directly into the negative
mold. In manufacturing the system 10, a positive mold of the representative baby's head
may be made from the negative mold and the headrest may be made from this positive
mold to ease removal of the cast form.
Each one of the sensors 18 is a first-order asymmetric, axial superconducting
gradiometer. The sensors 18 are arranged in clusters of 4 separated by the honeycomb
rear wall of the window or recess (Fig. 3), although the result is substantially equal
spacing between sensors. The expected noise level for the gradiometers with a pickup
coil diameter of about 6 mm may be about 10 TV/ Hz or better. Such a module has been fabricated and determined its noise level has been determined.
Fig. 5 shows the pickup coils, which are about 6 mm in diameter in one
embodiment, since the distance between the pickup coils and the cortical surface will be
about 6 mm. This diameter is optimal for this measurement distance in considering
spatial resolution and sensitivity.
To design a 4-channel module, it may be necessary to calculate the crosstalk
between coils for some applications. This was done as a function of coil separation.
The calculations demonstrated that the coils in the 4-channel module had to be spaced
diagonally by about 12 mm in order to maintain less than about 2 % crosstalk. Based on
the noise and the crosstalk considerations a 4-channel module was designed, as shown
in Figs. 5 and 6.
Figs. 7 and 8 show the noise spectra for the two channels. The noise spectra were quite clean with an elbow frequency (where the 1/f noise starts) of about 1 Hz. Based on these results, it appears that a 4-channel module can be constructed with a
noise level of 10 TV/ Hz.
A study of SEFs in human infants using the microSQUID was carried out. The
purpose was to show by extrapolation that it should be possible to measure cortical evoked activity without signal averaging using the system 10.
Figure 9 is a graphical representation of the results of the study and show representative samples of the SEFs produced by piezoelectric vibratory stimulations
(left) and SEFs produced by air puff stimulations (right). The data with vibratory
stimulations were obtained from 9 sessions (9 subjects) and those with airpuffs were obtained from 8 successful sessions (4 subjects). For both stimuli, the SEF reversed its
polarity near C3, indicating that its generator was located in the somatosensory cortex
below C3. The SEF was stronger for the vibratory stimuli than for the airpuffs, but the waveforms were similar. The initial component was directed into the head over the
upper region of the measurement area, whereas it was directed out of the head over the lower part, indicating that the underlying current was directed from the deep layer to the
superficial layers of area 3b. The latency delay of this initial component for the air puff
stimulation relative to the vibratory stimulation is due to the delay in the arrival of the
compressed air to the finger via the tubing. The timing of the air puff stimulation,
measured with a pressure transducer at the end of the tube, shows a delay of about 35 ms in the start of the pressure and a further delay of about 85 ms before the pressure
reached the maximum. The latency delay of the initial component is approximately 35
ms indicating that the initial component was triggered the very early phase of the
pressure change.
The SEFs obtained as a result of the study strongly support the prediction that similar evoked cortical activity with a comparable signal-to-noise ration (SNR) can be
seen in real time using the system 10. The SEFs for airpuffs are averages of 36 epochs,
whereas those for vibratory stimulation are averages of 16 epochs. Therefore, it
appears that it is possible to obtain SEFs of comparable quality in single trials with the
system 10, even for airpuffs, since its noise level is expected to be about 6 times less than that of a conventional microSQUID. In this regard, the SEFs shown in Fig. 10 are
produced by a vibratory stimulation as a function of number of epochs in the average. It is clear that reliable SEFs can be seen even after averaging less than 25 epochs, after
as little as 4 epochs. The bandwidth of the recordings was 50 Hz, passing between 1 Hz
and 50 Hz.
The system 10 enables a study to be carried out by simply placing the baby's
head on the headrest assembly 16, instead of placing the detector above the head as
was the case for the conventional microSQUID. This inverted design of the dewar 27 of
the system 10 provides a sense of safety to the parents who would be present at such a
study. Parents would tend to express concern after seeing the baby's head being
sandwiched between a conventional detector and the bed. The coverage expected with
the 76-channel system 10 tends to speed up the study, since the measurement time is
very limited when no sedative is used. The set up time for the measurements is reduced
or minimal with the system 10 since no electrodes are required to be placed on the scalp
unlike EEG. This also greatly increases the usefulness of the system 10 for routine
measurements.
The system 10 may well be very useful for neonatal brain assessment. The
results of testing indicate that: (a) an MEG system has little or no affect by the skull (b)
EEG is strongly affected by a hole in the skull mimicking the anterior fontanel in the
infants, (c) EEG signals are strongly distorted as a function of depth of the active tissue,
and (d) the system 10 may well be capable of providing new information about the
physiology of the thalamocortical fibers and cortical neurons in certain circumstances.
Okada et al. (1999b) have shown that the SEF measured on a plane above the
skull of the piglet is virtually the same in waveform and spatial distribution with and
without the skull (Fig. 11). The SEFs measured in the skull-on (wave forms A), skull-off
(wave forms B), and skull-on again (wave forms C) conditions could be superimposed
with no clearly visible differences (wave forms D). Little, if any distortion was seen when
the SEF was measured with the skull intact and after the dorsal portion of the skull was
removed (Okada et al., 1999a). A quantitative evaluation of the similarity has, however,
shown that the skull does influence the external MEG signal as the generator becomes
deeper, due to the fact that the skull is not spherical. This result confirmed a theoretical
result by Hamalainen and Sarvas (1989) showing that the MEG signal outside the head
calculated with a boundary element model is the same regardless of whether the scalp and skull are present. It also confirms an experimental finding by Barth et al. (1986)
showing that a large hole in the skull of a human cadaver does not distort MEG signals
produced by an artificial source embedded in the cranium.
There does not appear to be any study which explicitly demonstrated a distortion
in the waveform and spatial pattern of EEG signals caused by a hole such as one mimicking the fontanel in human infants. The large size of cranium in the piglet preparation used in Dr. Okada's laboratory enables one to test this question empirically.
Figure 12 shows an outline (dashed square) of the SEP (somatic evoked potential)
measurement area on the skull with a square hole (12 mm x 12 mm) over the left
hemisphere. The hole was over the cortical area generating the early responses in the
primary somatosensory cortex produced by snout stimulation. The SEP was mapped
with a 16-channel electrode array placed on a 0.9% NaCI saline-soaked filter paper
covering the entire measurement area and mimicking the scalp that was removed. The
SEP pattern was measured in three conditions differing widely in the electrical conductivity of the hole, so that the distortion can be measured as a function of
conductivity. The hole was filled with air r= 0), isotonic 3% agar containing 330
milliequivalent of sucrose (fr~ 0.01 S/m close to the conductivity of the skull), with
isotonic 3% agar containing 0.9% NaCI (<r~1.28 S/m). Under these conditions, the SEPs
(bandwidth: 1- 200 Hz. 60 epochs/ave), especially their early components, were large in sucrose- and saline-agar compared to the air conditions.
The distortion due to conductivity differences in the hole is clearly revealed by the difference map shown in Fig. 13A (each trace bandwidth: 5-200 Hz, 60 epochs/ave). The differences were large between the sucrose and air conditions where the
conductivity of the sucrose-agar should be close to the conductivity of, in this case about
a 2.5 mm thick, skull. The differences were also quite reproducible as seen by the
similarity in the sucrose1-air and sucrose2-air different maps.
The Laplacian transformation is becoming more commonly used in deblurring the scalp EEG pattern (Gevins et al., 1994, 1999; Le et al., 1994; Nunez et al., 1994). This
method is used to estimate the current emerging or entering the scalp along the direction perpendicular to the surface as a result of neuronal activity in the brain. This current is
conventionally estimated by solving the Poisson equation. In practice, the current Im is
estimated from a discrete version of this equation: Im 4V(xO,yO)-V(dx,yO)-V(-dx,yO)- V(x0,dy)-V(x0,-dy), when the potential is measured at five positions centered at position
x=x0 and y=y0. The Laplacian estimate of currents emerging through the filter paper
(mimicking the scalp) is shown for the three conditions in Fig. 13B. The calibration is
based on an electrode separation (dx=dy) of 4 mm and a conductivity of 0.016
Siemens/m for the sucrose-agar and 1.27 Siemens/m for the saline-agar. Note that the current magnitudes are much larger over the saline agar. This result clearly
demonstrates that currents leak through a hole and distort the EEG pattern on the scalp when such a hole is above an active area of the cortex. In sum, these results empirically demonstrate that EEG signals are distorted by a hole in the skull, suggesting that the
skull defects such as the fontanels and sutures may profoundly distort EEG signals in infants.
The following study compared the SEPs measured on the dura and on the scalp
and, similarly, the SEFs measured on the dura and scalp in order to determine whether
these signals seen outside the scalp are similar to those over the cortex. Goff et al.
(1978) showed that the potentials due to focal cortical generators are more attenuated than those due to extended cortical or deeper sources when the potential is measured
on the scalp. This implies that the waveform of the SEP on the cortex or dura should be
distorted when it is measured on the scalp. The "transparency" of MEG, on the other
hand, implies that the SEF on the scalp should be similar in waveform to that on the
cortex or dura.
These experiments were carried out on piglets of less than 1 week of age (1-6 days) as a neonatal model of human newborns. The skull of these piglets are 0.5 -1.5
mm in thickness over the dorsal portion of the cranium. The scalp is about 1.5 mm in
thickness. They do not possess fontanels, but the sutures are still present and the
cranial bones are loosely attached. Figure 14 shows the SEPs measured over a square
area (indicated by the solid line) and the SEFs measured over the dashed area, both roughly centered over the active area in SI responsible for early cortical responses. The
snout was electrically stimulated and the SEPs were measured first on the scalp, then
with the scalp and skull removed and finally on the scalp again in order to control for possible changes in the physiological condition of the animal. The procedure was
repeated for SEF measurements. The scalp and skull were cut before the experiment, then the skull was replaced and the scalp was attached to the intact skin with a suture.
The suture was removed for dura measurements and then the scalp was sutured again.
The combined thickness of the skull and scalp was 3 mm. It can be seen that the SEP
and SEF were very similar in the scalp 1 and scalp 2 conditions, indicating the animal
was in stable physiological condition and other extraneous factors did not contaminate
the results (signal bandwidth = 1-200 Hz, 30 and 60 epochs/ave for SEF and SEP,
respectively).
The inset in Fig. 14 labeled "scalp and dura EEG" compares the SEP at its
potential extremum for the early cortical component (indicated by an open circle) in the
scalpl , dura and scalp2 conditions. The solid curve in (a) is the dura SEP and the two
dashed curves are the scalp SEPs. The peak amplitude of the first component of the
scalp SEP was magnified to match that of the SEP on the dura as shown in (b). Clearly,
the later components of the magnified scalp SEP are larger than the corresponding
components of the SEP on the dura, with a good reproducibility for the two scalp
conditions. Thus, the attenuation ratios are different for different components of the SEP
and thus the scalp SEP is distorted in waveform in comparison with the dura SEP.
The same comparison was made for the SEFs. The SEF distribution on the dura
was more compact than the distribution over the scalp due to the shorter measurement
distance, just as was the case for the SEPs. Taking this into account, the waveforms
were compared at the field extremum of the earliest component of the SEF in the three
conditions. The inset in Fig. 14 labeled "scalp and dura MEG" compares the waveforms
at the location indicated by a circle in the three conditions. In (a), the scalp SEFs (two
dashed curves) were weaker than the dura SEF (solid curve), but they could be scaled
by a constant (see b), again by enlarging the scalp SEF to match the peak of its earliest
component with that of the dura SEF. Thus, the SEF waveforms were comparable on
the dura and scalp. In sum, the results of this study indicate that it may be possible to
measure SEF over the scalp as if the scalp and skull were absent, whereas the SEP on the dura may be distorted on the scalp.
The above result implies that it should be possible to non-invasively measure
cortical activity with MEG as if the skull and scalp were removed and the sensors are
practically placed on the cortex if the skull and scalp are thin as it is the case for
neonates and furthermore if the sensors can be placed very close to the scalp. This suggests that it may be possible to non-invasively measure cortical activity as if we had
electrocorticographic sensors were positioned on the cortex.
It has been suggested that the high-frequency signal, the so-called 600 Hz signal,
may reveal spike activity (Curio et al., 1994a, b; Hashimoto et al., 1996a; Haueisen et al., 2000a, b). This study has shown that it is possible to detect the 600 Hz signal and determine the origin of this signal in the piglet model using the microSQUID. Figures
15A-D show that the 600 Hz signal from the SI of the piglets has properties very similar
to those found in the human SEP and SEF. The SEF was first scanned over the SI and
then the 4-channel detector was placed directly above the generator of the first cortical
response (marked by x in Fig. 15A). In some experiments, the SEP within the cortex
was measured simultaneously or sequentially with the SEF in the same animals using a glass micropipette or a 16-channel microelectrode array. In others, SEPs were
measured separately. Fig. 15B shows the wideband (1 Hz - 3,000 Hz) signal at locations SQ2 and SQ4 along with the narrowband (416-2,083Hz) SEF, the difference wave between the SEFs measured at SQ2 and SQ4 and the power of the difference
wave (square of the difference wave). The amplitude spectrum of the wideband signal (Fig. 15C) was very similar to that found in the humans with a clear peak around 600 Hz
(Curio et al., 1994b; Hashimoto et al., 1996a). The SNR of this signal compared to the
noise level during the prestimulus period was 10:1. Also in agreement with human
results (Emerson et al., 1988; Yamada et al., 1988; Hashimoto, 1996a), the amplitude of
the first SEF component corresponding to the human N20m increased while the
amplitude of the 600 Hz decreased with an increase in depth of sleep induced by an
increase in the level of the anesthetics (ketamine/xylazine) (Fig. 15D).
Signals with the quality shown in Figs. 15A-D were obtained from the humans
with an average over about 1,500-5,000 epochs (Curio et al., 1994b, Hashimoto et al.,
1996a). The signals are based on average of 3,000 epochs, even though the
measurement distance was about 8 mm in the present study and probably 40 mm in the
human studies. If the source strength was the same in the humans and piglets, the
signal should be about 25 times ((40/8)2) stronger. The signals in humans are typically
10-30 fT peak-to-peak. In the present study the signal was as much as 400 fT as
compared to the expected value of 250-450 fT. Thus, the magnitudes do scale. The test
results indicate that the system 10 is 5-6 times more quiet than a conventional
microSQUID and thus there is needed about 25 times less average or about 100 epochs
to obtain signals of comparable quality in the piglets. This implies that it should be
possible to measure this signal from infants by averaging about several hundred signals
since the scalp and skull add to the distance of measurement.
Taking advantage of the animal model, the origin of this 600 Hz signal was also
determined. Simultaneous recordings of SEF outside the brain and intracortical SEP
revealed that the narrowband signals measured with MEG were very similar to the
extracellular intracortical activity (Fig. 16A). This indicated that the 600 Hz signal was
generated in the cortex. Based on human studies, it has been proposed that the
generator of this 600 Hz signal may be the thalamocortical axonal terminals (Gobbele et
al., 1998), inhibitory interneurons in the cortex (Hashimoto et al, 1996a; Mackert et al.,
2000) or excitatory cortical neurons. Kynurenic acid (Kyna, 20 mM), a non-specific
antagonist of excitatory amino acid neurotransmitters, was used to separate the
presynaptic and postsynaptic components of the 600 Hz signal. The SEF and
intracortical SEP were measured simultaneously with the microSQUID and a 16-channel
electrode array fixed in place in the cortex during the control condition without Kyna (Fig.
16A, control). This showed that the SEF power consisted of two components, one
around 8 ms and the other between 10-15 ms post-stimulus. Application of Kyna with a
pair of glass micropipettes in the projection area of the snout greatly reduced the second
component, whereas the first component was intact (Fig. 16B). This second component
showed partial recovery during the washout phase (Fig. 16C). Fig. 16D shows the
selective effects of Kyna on the first component of the simultaneously measured SEF
and SEP. Thus, there are Kyna-insensitive and sensitive components which indicate
that there are pre- and post-synaptic contributions to the high-frequency signal.
Fig. 17, left, shows that the Kyna-insensitive component was localized within layer
IV which is the receiving area for specific thalamocortical afferents. A detailed analysis
of the presynaptic component, Fig. 17, right, shows that this component was generated
by the thalamocortical terminals within the cortex. The laminar profiles within the control
and Kyna conditions showed an initial wave, starting about 6 ms post-stimulus, of
extracellular negativity that were strongest near the white matter. This wave did not
reverse polarity within the cortex. Then, two components (at time points 2 and 3)
showing polarity reversal appeared at 7.2-7.9 ms. The extracellular potential was
negative in layer IV (~1 ,000-1 ,200μm) and positive in the superficial and deeper layers,
indicating that it was generated by the thalamocortical terminals in the receiving layer. A
trigeminal brainstem study showed that the snout stimulation produces activity in the
trigeminal nuclei with a shortest latency of about 4.5 ms and that the impulses were
carried by the fast, myelinated Aβ fibers with a conduction velocity of 32-42 m/sec. This
implies that the thalamocortical volley may arrive in the cortex as early as 7.3 ms, taking
into account one synaptic delay in the thalamus. Thus, the Kyna-insensitive component
at around 8 ms appears to be of thalamocortical origin on this ground as well. About 1.1
ms later, there was a large Kyna-sensitive component in layer III in the control condition.
The laminar profile at its peak (time point 4) is shown in Fig. 17. This is the first post¬
synaptic component produced by the cortex. Fig. 17, left, shows that this post-synaptic
component was produced in layers II-VI with a delay between the activity in layer lll/IV
and layer V/VI. Also, importantly, Fig. 17, left, shows that this post-synaptic component
became larger 3-4 hrs after the injection of Kyna, indicating that this component showed
hyper-excitability. This suggests that it was generated by excitatory cortical neurons due
to partial blocking of inhibitory neurons by Kyna. It has been thought for a long time that
the detection of synchronize population spikes would be extremely improbable based on
the careful work of Humphrey (1968a, b) who has shown that the antidromically
generated action potentials contribute little to the cortical surface record. However, it
appears possible from outside the brain to detect axonal spikes from the thalamocortical
fibers and synchronized populations spikes produced by cortical neurons using a high-
resolution MEG sensor.
The MEG sensors are housed in a cryogenic container called a dewar that stores
liquid helium. The dewar consists of two cylindrical containers. The inner container
stores a sufficient quantity of liters of liquid helium which is used to maintain the
superconducting sensors operating at the critical temperature such as about 4.2°K. The
space between the two containers is in vacuum to provide thermal insulation. The inner
and outer containers may be made from a special laminated G-10 fiberglass that is constructed to prevent leakage of helium gas into the vacuum space. The dewar may
be placed on the floor of the portable cart. In one embodiment, the dewar weight may
be about 100 lb and the total bed system may be about 300 lb, so that it should be portable on wheels.
The body of the inner container may be shielded against heat radiation leak using layers of low conductivity material such as aluminum. The shielding may be installed in removable packs to enable ease of construction and rework. The gap from the inside to
the outside may be about 2 mm via independent mounts.
The top section of the dewar may be made separately from the bottom cylindrical
containers. It may be a G-10 plate with a cylinder epoxied onto the plate. The plate may
also accommodate an exhaust hole from the inner cylinder that lets helium vapor escape into the atmosphere. The exhaust hole may be sealed with a removal thermal shield to
prevent heat from leaking into the inner container. The cylinder may be machined so that
the headrest can be epoxied onto the cut surface. The headrest may have a curvature
that may accommodate the head of infants. As mentioned above the headrest may be ellipsoidal or semi-ovoid with the radii of curvature of about 6 and about 8 cm. This dimension should be sufficient to accommodate infants of up to about two years of age
(Lusted and Keats, 1978) for some applications.
As shown in Fig. 1 , the sensor pick up coils may be mounted at the top and the
associated dc-SQUIDs may be mounted at the bottom of the inner container of the dewar 27. This sensor assembly may be maintained at the superconducting temperature, by a cold heat sink attached to the bottom of the inner container. In
accordance with another embodiment as shown in Fig. 18, an MEG system 44 have a
headrest assembly 46 enables the sensor array 48 to be moved adjustably relative to
headrest 51. An actuating mechanism 53 may be installed that may enable an operator
to adjust the height of the sensor as array 48 relative to the headrest 51 so that the pickup coils may be as close to the inner surface of the headrest as possible. This may
be about 2.0 mm spacing between the outer surface of the headrest and the pickup coils
when the system 10 is in use, but about 1 cm below the closest position when it is not in
use in order to conserve liquid helium.
The boil off rate for the system 44 is about 4 liters/day when it is not in use and 8
liters/day when in use according to one embodiment of the invention. Assuming the system 44 to be used 12 hours/day, a sufficient size dewar such as a 25-liter dewar may
be able to hold liquid helium for 4 days. Thus, helium transfers may need to be made twice a week for some applications.
The actuating mechanism 53 may be made as illustrated in Fig. 18. The shafts such as a shaft 50 supporting the sensor array may be spring loaded and moved up
and down by a wedge 54 that may be moved horizontally by a thermally shielded rod 52
that may protrude from the side of the dewar.
Reverting again to the system 10 of Fig. 1 , the array consists of 19 modules of 4 channels each for the disclosed embodiment. Each module may be attached to an
ellipsoidal support as shown in Fig. 3. The distance between the center of a module to the center of an adjacent module is approximately 24 mm for this embodiment.
Each module consists of 4 channels of first-order, asymmetric, axial gradiometers
in the preferred embodiment. However, other numbers of gradiometers may be
employed. The most suitable pick up coil configuration is a 12 turn, 6 mm-diameter pick
up coil and a 6 turn (spaced by 1 mm), 8.49 mm-diameter cancellation coil. This is
desirable for the 3 μΦ0Λ/Hz SQUIDS by Quantum Design, San Diego, CA, or the
SQUIDs made by Jena SQUIDs, Jena, Germany. This has a noise level of 2 μΦ0Λ/Hz
rather than 3 μΦ0Λ/Hz with a inductance of 0.6 μH rather than 1.8 μH. Thus, the ideal
noise (without the noise from the superinsulation and fiberglass wall) should be reduced
from 7 TV I Hz to 4.7 fT/ Hz. Furthermore, the number of turns in the gradiometer
assembly can be reduced for some applications, thereby making the fabrication of the
gradiometers coils relatively easier.
The pickup coils within and across the clusters of 4 are densely packed to
increase the spatial resolution. The coil-to-coil diagonal distance within each module is
about 12 mm for some applications. The coil-to-coil distance across the modules
between the closed coils is about 10 mm. This separation is about three times less than
the separation between channels of the latest high-density 200-300 channel whole-head
MEG systems made by the CTF Systems and 4D-Neuroimaging.
As the system 10 is designed to operate in an unshielded environment, special
precautions may be taken to provide adequate RF shielding. All cryocables (from room
temperature to the SQUID sensors) may be RF shielded. The leads from the pick up
coils may be shielded using superconducting lead tubes. The dewar superinsulation
provides partial RF shielding for the pick up coils. Each module may be RF shielded by
wrapping it with a superinsulation material.
An electronic rack may be placed in the body of the portable cart 14 below the
mattress or bed 29 (see Fig. 1). This rack houses 84 channels of SQUID control units,
76 channels for the detection coils plus three magnetometers and five first-order
gradiometers to be used as reference channels for noise cancellation. All control units
may fit into one 19-inch rack-mounted card cage, sufficiently small for the entire rack
to fit comfortably within the body of the cart. The dc-SQUIDs may be connected via RF-
shielded coaxial cables to the control units.
The SQUID electronics may be powered completely by a suitable dc power
supply that may be located close to the AC power outlet of the hospital room. The use of
the dc power reduces the line frequency noise that would be sensed by the MEG
detection coils.
The system 10 must operate in electromagnetically unshielded environments
such as almost any room in a hospital. Shielded rooms are expensive, costing in some
cases between about $500K and $1M, depending on the size and level of magnetic
shielding desired. It also makes the MEG systems non-portable. This lack of portability
has inhibited a wide-spread use of MEG systems. The system 10 is portable and is
capable of operating in many or most clinical settings, in almost any room in a clinic
without electromagnetic shielding. In unshielded environments, there may be three
types of noise: (1) low frequency magnetic field changes due to the variation in the
earth's magnetic field, due to movements of cars, trains, etc., (2) line frequency noise
and (3) RF noise. Considering noise problems plus an additional problem of sampling
requirement, a new SQUID control unit manufactured by the Systemtechnik Ludwig
(STL), GmbH, Konstanz, Germany is currently preferred. The initial stage of noise
cancellation is accomplished by using first-order gradiometers which can reject uniform
magnetic fields with a rejection ratio of at least about 100:1. The second stage may be
accomplished by software cancellation, after recording the data. Fig. 19 illustrates the
cancellation of the ambient earth magnetic field changes that have been achieved in the
frequency range below about 5 Hz. The data in square symbols ("before subtraction") illustrate the noise level sensed by a second-order gradiometer of liver iron
biosusceptometer installed in Torino, Italy. The reference channel in this case was a fluxgate magnetometer. The curve in triangles ("after subtraction") shows the same
curve with the same scale after the cancellation. The curve in diamonds ("after subtraction, x32") shows the noise-cancelled curve after magnifying it by a factor of 32
which corresponds to the ratio of the standard deviations of the signals before and after
the noise cancellation. The cancellation by a factor of about 30 achieved in our
preliminary work is comparable to the shielding factor of the commercially available two- layer magnetically shielded rooms in the frequency range below 1 Hz.
Figure 20 shows the noise cancellation for the line frequency noise using the 8
reference channels. Noise cancellation was performed on data from a 29 channel
gradiometer sensor array installed at the Vanderbilt University with an additional eight
channels of reference. The eight channels of reference consisted of five gradiometer channels and three magnetometer channels comprising a complete field and field gradient measurement. The test noise data was acquired at a 2 kHz sample rate for one
minute with no signal present. The noise was dominated by 60Hz line noise and its harmonics. A multivariate, linear, least-squares fit was applied to the first 30 sec of the
data to determine a linear functional dependence of the signal array channels on the
reference channels. The 29 by 8 matrix of fit coefficients was then applied to each time step in the second thirty seconds of the test data. This generated a prediction of the
noise in the sensor array channels that was then subtracted from the actual noise
measured. Over the 29 channels, it appears that a noise cancellation may be achieved under certain circumstances of up to 90. This noise cancellation scheme may be refined
by finding the weights that may be time dependent. This refinement may provide a line
frequency cancellation by a factor of greater than about 100.
The data acquisition may be carried out by a PCI-based system that may be attached to the cart 14 as shown in Fig. 1. The outputs from the 84 SQUID control
units may be fed to a set of data routers via fiber optic cables. This reduces the rf noise feeding back into the SQUID control unit and into the SQUIDs. The control units are
housed in units of three. The fiber optic outputs from four units (12 channels) may be
fed at a rate of 480 kByte/sec to a level 1 router. Four of the level 1 routers are fed to the next router at a rate of 1.92 MB/sec. Since there are 84 channels, 7 level 1 routers
may be required for some applications feeding to two level 2 routers. The two outputs of
the level 2 routers may be fed into a PCI card with a transfer rate of 8 MB/s. This data
acquisition system continuously acquires 4-byte data at 10 kHz from 84 channels. A 1 GHz PCI-based PC may be is sufficient for some applications. The STL system may
work with the Windows NT. In some applications, such as in the studies aimed at measuring the 600 Hz
activity from the cortex, it is necessary to use a signal bandwidth of 3 kHz for some applications. Thus, the data may be sampled at a rate of at least 8 kHz. The data acquisition may be controlled by software. . It provides a complete control over all electronic features. An optical technique may be used to determine the head shape of
the infant. This method uses one digital camera to take the pictures of a grid pattern
projected onto the baby's head. The distortions of the grid pattern seen by the camera are used to reconstruct the 3D shape of the head and face. This method may be selected since it is completely remote in operation, non-invasive, fast and economical.
The image can be obtained in less than 1 second and the reconstruction can be done offline, so that there is no need to worry about movement of the baby's head.
The conventional optical positioning system 28 from Eyetronics may be
employed. The accuracy better than 1 mm can be achieved. A conventional lithographic
technique is used to prepare the grid pattern. Only a single picture, taken from a slightly
different angle than the projection angle, may be sufficient to extract 3D shape
information. The input parameters are the relative positions of the camera and the miniature slide projector that may project a single grid pattern onto the face and head,
and the deformed grid on the object. To obtain a full 3D model of the head, several
overlapping pictures may be taken from different angles. A software, provided by Eyetronics, may be used to integrate the separate images. This process can be
completed before the EEG/MEG measurements start.
It may be important for some applications to be able to continuously monitor the position and orientation of the baby's head in order to maximize or at least increase the
time available for useful measurements. In studies involving normal, healthy babies, the presently preferred procedure involves no sedative. This means that the study may be
done while the baby is asleep. The duration of sleep is typically between about 10 min and about 40 min. Monitoring the head position continuously during this period enables
the data to be analyzed during the motion-free periods. The useful measurement period may also be extended due to the monitoring , to the period of wakefulness. Oftentimes
the baby keeps his/her head stationary for more than about 10 sec at a time during the
transition to the sleep state. Thus, it would be possible to measure the brain activity
during the awake stage if the baby's head can be continuously monitored.
There are also other techniques available for both 3D head shape mapping and
head position tracking. For the head position tracking there are optical, electromagnetic and laser tracking systems. It may be desirable for some applications to continuously monitor the head with an absolute positional accuracy of about 1 mm during measurements.
Optical tracking
Optical tracking systems include two or more cameras of known locations. They track
three or more marks in 3D. The marks can be either passive (for example, reflective
tape) or active (for example, infrared LED's). Optical systems have 0.3 - 0.4 mm accuracy in a tracking volume of about 1 m3. The update rate is in the 20 - 70 Hz range.
Optical tracking does not tend to interfere with MEG measurements, so that the object
location can be synchronized with magnetic measurements.
Laser tracking Laser tracking system has the same advantage as optical systems in that it does not tend to interfere with magnetic measurements, and the object position can be synchronized with the MEG data. These systems include a cost effective LaserBird
system from Ascension Technology. The accuracy is about 1 mm at about 1 m distance.
The tracking is done by scanning the work area with a laser beam. The sensors attached
to a baby's head pick up the laser beam. From these data signals the information on the
object position is derived.
Electromagnetic tracking
Electromagnetic tracking systems are widely used with SQUID biomagnetic measurements (for example, EM tracking systems from Polhemus). An electromagnetic
transmitter is rigidly attached relative to the pick up coils, and three receivers are
attached to the head. The accuracy of this system is on the order of about 1 mm.
CLUSTER SENSOR SHIELDED MEG SYSTEM
Referring now to Figs. 21 through 29, there is shown a high-resolution
magnetoencephalography (MEG) system 55 for evaluating neurological impairments of
preterm and term babies, for example. The system 55 is generally similar to the system
10, except that the system 55 employs clustered sensors instead of modular sensors,
and has other features such as being shielded in a particular manner.
The system 55 is adapted to serve as a non-invasive neurodiagnostic tool in
assessing possible neurological dysfunctions and brain development in neonates such
as a patient 57. System 55 includes a cart 59 having a headrest assembly 62 disposed
in an upwardly facing direction for receiving the head of the patient 57. The headrest
assembly 62 is generally similar to the headrest assembly 16, and includes a sensor
array generally indicated at 64 mounted below a concave headrest 66 similar to the
headrest 21, and as shown in Fig. 55. The cart 59 is designed to be portable and roll
along the ground on a set of wheels such as the wheels 68, 71 and 73.
A SQUID dewar 75 is mounted on the cart 57, and includes an optical positioning
system 76, which is similar to the optical positioning system 28 of Fig. 1 , for the purpose
of determining the position of the head of the patient 57. A bed or cushion 77 on the
upper surface of the cart 57 permits the patient 57 to lie in a reclined position with his or
her head supported from below by the headrest 62.
As best seen in Fig. 22, a liquid helium reservoir 79 of the SQUID dewar 75 is a
fill port 82 extending above the upper surface of the cart 57. SQUID data acquisition
electronic equipment 84 and SQUID phase lock loop electronic equipment 86 are
mounted on the cart 57, together with a power hub 88. These electronic components
are mounted within a shielded container 91 to protect them against radio frequency
interference. The electronic components are connected to the SQUID via a group of
cables generally indicated at 92.
A portable trailer 93 is mounted rollably above the ground by means of a series of
wheels such as the wheel 94. A group of shielded cables generally indicated at 95
provide power and data channels for the electronic equipment mounted on the cart 57.
In this manner, when the cart 57 is moved to the desired location, the trailer including a
DC power supply 97 and a transformer 99 can be moved along with the cart 57. A pair
of personal computers 102 and 104 are mounted [on] the trailer 93 and communicate
with a notebook computer 106 mounted on top of the trailer 93 to be utilized by an
attendant.
The headrest assembly 62 has two curvatures for coronal and sagittal (axial)
coverage. As indicated in Figs. 24 and 25, the headrest assembly 62 is concave and is
configured in an ellipsoid shape having a sagittal radius of curvature of between about
80 mm and about 120 mm for the sagittal axis of the head and having a coronal radius
of curvature of between about 60 mm and about 90 mm at the coronal axis of the head.
A more preferred range of the sagittal radius is between about 90 mm and about 110
mm, and a more preferred range of the coronal radius is between about 70 mm and
about 80 mm. Currently, the most preferred sagittal radius is about 100 mm, and
currently the most preferred coronal radius is about 75 mm.
The headrest 66 is composed of non-metallic head-insulating structurally strong
material. This material is currently preferred to be fiberglass G 10.
Each one of the sensors 64 is a superconducting gradiometer having a pick-up coil diameter of between about 4 mm and about 8 mm. More preferably, the pick-up coil diameter is between about 5 mm and about 7 mm. Currently, the most preferred pick-up diameter is about 6 mm.
The gradiometer sensors are uniformly distributed relatively to the head engageable surface of the headrest 66. Each one of the sensor pick-up coils being
spaced apart by a spacing distance of between about 6 mm and about 14 mm. This
spacing distance is more preferably between about 8 mm and about 12 mm. As
currently contemplated, the sensors are arranged in groups of four, wherein the spacing
distance between adjacent sensors of a group is about 8.5 mm and the spacing distance between diagonally disposed sensors of a group is about 12 mm.
Referring now to Figs. 26, 27 and 28, the sensors 64 are arranged in groups, and
the headrest 66 has a corresponding series of windows or recesses such as a recess 108 in the rear surface of the headrest 66 positioned opposite to one of the groups of the
sensors. In this regard, each one of the recesses is generally rectangular in shape and
is dimensioned to the approximate size of its group of sensors. In this regard, the rear surface of the headrest is arranged in a honeycomb configuration to provide a thin wall
construction for the sensors so that the sensors may be positioned in a close relationship to the head of the patient 57. The honeycomb configuration of the rear wall
of the headrest 66 is best seen in Fig. 28.
While the sensors 64 are arranged in clusters of four, it is to be understood that
there could be other numbers of such sensors clustered together. Also, it is contemplated that each separate sensor may have its own recess or window and not be
clustered with other sensors. In the currently preferred example, the sensors are
arranged in groups of four, and each one of the four sensors of a group provides a
separate communication channel. The channel is usable individually, or in combination
or subtracted from one another. Thus, the channels can be summed in various
combinations for signal averaging purposes, albeit with a decrease in spatial resolution.
Additionally, the differences between channels can be taken, such as by electronic
subtraction of their respective voltages to effectively measure planar gradients.
Furthermore, the channels can be subtracted, or used individually.
The dewar 75 is radio frequency interference shielded by multiple techniques.
The external portions (those at room temperature) of the dewar is shielded by the
application of a thin coating or coatings of conductive material. The product of the
coating conductivity and thicknesses is such to provide an eddy current shield with a roll-
off frequency of less than 500 kHz. The coating may be applied through various
methods including (but not limited to), flame spraying or the use of a metallic enclosure,
as well as others. The interior portion of the dewar 75 is also shielded by the use of one
or more ultra-thin assemblies of conductive material, such as aluminum. The thickness
or thicknesses of the assemblies are such that they act as an eddy current shield with a
typical roll-off frequency of less than 500 kHz, but do not significantly attenuate magnetic
signals of interest (typically less than 10 kHz).
Additionally, the power cables DC power supply carts or trailer are filtered to
prevent radio frequency interference from entering either the electronics or the
magnetometer detection sensor coils in the dewar.
Headrest Constant Sensor Gap
Referring now to Fig. 29, the relative close spacing of the sensors 64 and the
headrest 66, is maintained constant through various temperature changes of the dewar
75. In this regard, when the system 10 either cools down or warms up, the differences in
the coefficient of thermal expansion of the various components may cause the small gap
between the sensors 64 and the headrest 66 to change in an undesirable manner.
Therefore, according to the disclosed embodiment of the present invention, the gap
between the sensors and the headrest remains substantially constant as a result of the
mounting structure for the headrest assembly 62.
A mounting ring 109 is fixed to the headrest assembly 62 and mounts it fixedly to
the dewar 75 in a rigid manner. A coil array plate 110 supports the sensors 64 in close
proximity to the headrest 66 as hereinbefore described. A set of rods such as the rods
111 , 112 and 113 support the plate 110 from below and have their lower ends
connected to an intermediate mounting ring 115. A set of shorter rods such as the rods
117, 118, 119 support the intermediate mounting ring 115 from above and are
connected to a fixed mounting ring 122. In this regard, the mounting ring 122 is fixed
with respect to the dewar 75. The longer rods such as the rod 111 is composed of
suitable material such as quartz so that when the system 10 cools down, the quartz rods
shrink in length to pull the coil array plate 110 away from the headrest 66 to thereby
increase lift-off. However, the shorter rods such as the rod 117 are also composed of
suitable material such as quartz so that when the system cools down, for example, the
quartz rods such as the rod 117 shrinks in length to pull the coil toward the headrest 66.
In this regard, the longer and shorter rods compensate one another for temperature
changes to help maintain a constant gap between the sensors and the headrest.
The rods are composed of quartz material since quartz has a low characteristic of
thermal contraction between room temperature and cryogenic temperatures.
Additionally, the coil array plate 110 is pre-adjusted to correct lift-off when the system is warm.
While particular embodiments of the present invention have been disclosed, it is
to be understood that various different modifications and combinations are possible and
are contemplated within the true spirit and scope of the invention. There is no intention,
therefore, of limitations to the exact disclosure herein presented.
Claims
1. A magnetoencephalography system, comprising:
a portable cart for moving along the ground;
a SQUID dewar mounted in an inverted manner on the cart;
a headrest assembly mounted on the cart and having a headrest for
supporting a head of a patient and forming a portion of the dewar;
the headrest assembly includes an array of magnetic sensors of the
SQUID dewar for responding to electrical activity of the brain of the head; and
a patient bed mounted on the cart adjacent to the headrest for supporting
the body of the patient with his or her head supported by the headrest.
2. A magnetoencephalography system according to claim 1 , wherein said
headrest is concave and is configured in an ellipsoid shape having a sagittal radius of
curvature of between about 80 millimeters and about 120 millimeters for the sagittal axis
of the head and having a coronal radius of curvature of between about 60 millimeters
and about 90 millimeters at the coronal axis of the head.
3. A magnetoencephalography system according to claim 2, wherein the
sagittal radius is between about 90 millimeters and about 110 millimeters, and wherein
the coronal radius is between about 70 millimeters and about 80 millimeters.
4. A magnetoencephalography system according to claim 3, wherein said
sagittal radius is about 100 millimeters, and wherein the coronal radius is about 75
millimeters.
5. A magnetoencephalography system according to claim 1 , wherein the
headrest is composed of non-metallic head-insulating structurally strong material.
6. A magnetoencephalography system according to claim 5, wherein said material is G-10 fiberglass.
7. A magnetoencephalography system according to claim 1 , wherein each
one of said sensors is disposed at a spacing distance from the outer head engaging surface of between about one millimeter and about three millimeters.
8. A magnetoencephalography system according to claim 7, wherein said
spacing distance is between about one millimeter and about two millimeters.
9. A magnetoencephalography system according to claim 1 , wherein each
one of said sensors is disposed at a spacing distance of greater than about one
millimeter from the outer head engaging surface of the headrest assembly.
10. A magnetoencephalography system according to claim 1 , wherein each
one of said sensors is a superconducting gradiometer having a pick-up coil diameter of
between about four millimeters and about eight millimeters.
11. A magnetoencephalography system according to claim 10, wherein said
pick-up coil diameter of between about five millimeters and about seven millimeters.
12. A magnetoencephalography system according to claim 11 , wherein said
pick-up diameter is about six millimeters.
13. A magnetoencephalography system according to claim 1 , wherein each
one of said sensors is a superconducting gradiometer having a pick-up coil, said sensors being substantially uniformly distributed relative to the head engageable surface of the
headrest, each one of the sensor pick-up coils being spaced apart by a spacing distance
of between about 6 millimeters and about 14 millimeters.
14. A magnetoencephalography system according to claim 13, wherein said spacing distance is between about 8 millimeters and about 12 millimeters.
15. A magnetoencephalography system according to claim 13, wherein said
sensors are arranged in groups of four, wherein the spacing distance between adjacent
sensors of a group is about 8.5 millimeters and the spacing distance between diagonally
disposed sensors of a group is about 12 millimeters.
16. A magnetoencephalography system according to claim 1 , wherein said
sensors are arranged in groups thereof, and wherein said headrest has a corresponding
series of recesses in the rear surface thereof positioned opposite to said groups of said
sensors.
17. A magnetoencephalography system according to claim 16, wherein each
one of said recesses is dimensioned to the approximate size of its group of sensors.
18. A magnetoencephalography system according to claim 17, wherein said
recesses are arranged in a honeycomb configuration.
19. A magnetoencephalography system according to claim 18, wherein each
one of said group comprises four sensors.
20. A magnetoencephalography system according to claim 1 , wherein said
sensors are arranged in groups of four, each one of said four sensors of a group
provides a separate communication channel, the channels being useable individually, or
combined or subtracted.
21. A magnetoencephalography system according to claim 1 , wherein said cart
includes electronic equipment for data acquisition from said sensors, and further
including a container composed of conductive material for confining said electronic
equipment to shield it from radio frequency interference.
22. A magnetoencephalography system according to claim 21 , wherein said
dewar has an external coating of conductive material for radio frequency interference
shielding.
23. A magnetoencephalography system according to claim 21 , further
including a direct current power supply for supplying electrical power to said electronic
equipment.
24. A magnetoencephalography system according to claim 23, further
including a trailer having said power supply mounted thereon and being connected
mechanically to said cart.
25. A headrest assembly for a magnetoencephalography system, comprising:
a concave headrest assembly having a headrest for supporting a head of a
patient;
an array of ultra high spatial resolution cryocooled superconducting
sensors disposed adjacent to the headrest for responding to electrical activity of the
brain of the head; and
said headrest being configured in an elipsoid shape having a sagittal
radius of curvature of between about 80 millimeters and about 100 millimeters for the
sagittal axis of the head and having a coronal radius of curvature of between about 60
millimeters and about 90 millimeters at the coronal axis of the head.
26. A headrest assembly according to claim 25, wherein said headrest is
concave and is configured in an ellipsoid shape having a sagittal radius of curvature of
between about 80 millimeters and about 120 millimeters for the sagittal axis of the head
and having a coronal radius of curvature of between about 60 millimeters and about 90
millimeters at the coronal axis of the head.
27. A headrest assembly according to claim 26, wherein the sagittal radius is
between about 90 millimeters and about 110 millimeters, and wherein the coronal radius
is between about 70 millimeters and about 80 millimeters.
28. A headrest assembly according to claim 27, wherein said sagittal radius is
about 100 millimeters, and wherein the coronal radius is about 75 millimeters.
29. A headrest assembly according to claim 25, wherein the headrest is
composed of non-metallic head-insulating structurally strong material.
30. A headrest assembly according to claim 29, wherein said material is G-10
fiberglass.
31. A headrest assembly according to claim 25, wherein each one of said
sensors is disposed at a spacing distance from the outer head engaging surface of
between about one millimeter and about three millimeters.
32. A headrest assembly according to claim 31 , wherein said spacing distance
is between about one millimeter and about two millimeters.
33. A headrest assembly according to claim 25, wherein each one of said
sensors is disposed at a spacing distance of greater than about one millimeter.
34. A headrest assembly according to claim 25, wherein each one of said
sensors is a superconducting gradiometer having a pick-up coil diameter of between
about four millimeters and about eight millimeters.
35. A headrest assembly according to claim 34, wherein said pick-up coil
diameter of between about five millimeters and about seven millimeters.
36. A headrest assembly according to claim 35, wherein said pick-up diameter
is about six millimeters.
37. A method of magnatoencephalography for the brain of a head of a patient,
comprising: supporting the head on a headrest assembly including a headrest mounted
on a cart having an array of magnetic sensors forming a part of an inverted SQUID
dewar; supporting the body of the patient on a bed mounted on the cart; and detecting
electrical activity of the brain of the patient.
38. A method according to claim 37, further including moving the headrest and
the sensors relative to one another to positionally adjusting them.
39. A mounting arrangement for a magnetoencephalography system headrest
assembly forming a portion of a SQUID dewar and having a fixed headrest and an array
of sensors, comprising:
a sensor array plate for positioning the sensors relative to the headrest;
an intermediate moveable mounting member;
a plurality of first rods interconnecting said array plate and said moveable
mounting member, each one of said rods being composed of material expandable and
contractible with changes in temperature; and
a plurality of second rods fixed at one of their ends relative to the dewar
and being connected at their opposite ends to said moveable mounting member, said
second rods being composed of said material;
whereby said moveable mounting member tends to be moved in one
direction toward or away from said sensor array plate, and tends to be moved in an
opposite direction away from or toward said array plate resulting in temperature changes
affecting the first and second rods, so that said sensor array plate and its sensors
maintain a substantially constant spacing between said sensors and said headrest with
changes in temperature.
40. A mounting arrangement according to claim 39, wherein said first and
second rods are each composed of quartz material.
41. A mounting arrangement according to claim 39, wherein said sensor array plate is supported from below by said first and second rods.
42. A mounting arrangement according to claim 39, wherein said moveable
mounting member is positioned below said sensor array plate and said second rods
extend upwardly therefrom.
43. A mounting arrangement according to claim 42, wherein said moveable
mounting member is a ring.
44. A mounting arrangement according to claim 39, further including a fixed
mounting ring rigidly attached to the dewar.
45. A mounting arrangement according to claim 39, wherein said headrest
assembly being fixedly attached to the dewar.
46. A method of mounting a headrest and an array of sensors to a SQUID
dewar of a magnetoencephalography system, comprising:
positioning the sensors relative to the headrest with a sensor array plate; interconnecting the sensor array plate and an intermediate moveable
mounting member with a plurality of first rods, each one of said rods being composed of material expandable and contractible with changes in temperature; and interconnecting the moveable mounting member and the dewar with a
plurality of second rods, each one of said second rods being composed of said material;
whereby said moveable mounting member tends to be moved in one direction toward or away from the sensor array plate, and tends to be moved in an
opposite direction away from or toward said array plate resulting in temperature changes affecting the first and second rods, so that said sensor array plate and its sensors
maintain a substantially constant spacing between said sensors and said headrest with
changes in temperature.
47. A method according to claim 46, wherein said sensor array plate is
supported from below by said first and second rods.
48. A method according to claim46, wherein said moveable mounting member
is positioned below said sensor array plate and said second rods extend upwardly
therefrom.
49. A method according to claim 48, further including rigidly attaching the
headrest to the dewar.
Applications Claiming Priority (7)
Application Number | Priority Date | Filing Date | Title |
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US609259 | 1984-05-11 | ||
US10/228,694 US6784663B2 (en) | 2001-08-27 | 2002-08-26 | Self-adjusting assembly and method for close tolerance spacing |
US228694 | 2002-08-26 | ||
US10/608,725 US7130675B2 (en) | 2002-06-28 | 2003-06-26 | High-resolution magnetoencephalography system and method |
US608725 | 2003-06-26 | ||
US10/609,259 US7197352B2 (en) | 2002-08-26 | 2003-06-26 | High-resolution magnetoencephalography system, components and method |
PCT/US2003/020523 WO2004017811A2 (en) | 2002-08-26 | 2003-06-27 | High-resolution magnetoencephalography system and method |
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EP1538978A4 EP1538978A4 (en) | 2005-11-23 |
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DE4227878A1 (en) * | 1992-08-22 | 1994-02-24 | Philips Patentverwaltung | Shielding cover for SQUID magnetometer against electromagnetic interference fields |
JP3194695B2 (en) | 1995-12-14 | 2001-07-30 | 学校法人金沢工業大学 | Magnetic measuring device, method of assembling and repairing the same, and diagnostic device for magnetic measurement |
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- 2003-06-27 WO PCT/US2003/020523 patent/WO2004017811A2/en not_active Application Discontinuation
- 2003-06-27 AU AU2003263757A patent/AU2003263757A1/en not_active Abandoned
- 2003-06-27 EP EP03792953A patent/EP1538978A4/en not_active Withdrawn
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BARTHELMESS H -J ET AL: "Low-noise biomagnetic measurements with a multichannel dc-SQUID system at 77 K" IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY IEEE USA, vol. 11, no. 1, March 2001 (2001-03), pages 657-660, XP002346913 ISSN: 1051-8223 * |
See also references of WO2004017811A2 * |
Also Published As
Publication number | Publication date |
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AU2003263757A8 (en) | 2004-03-11 |
WO2004017811A2 (en) | 2004-03-04 |
WO2004017811A3 (en) | 2005-04-14 |
AU2003263757A1 (en) | 2004-03-11 |
EP1538978A4 (en) | 2005-11-23 |
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