WO2007022499A2 - Reduction d'artefacts de susceptibilite dans une irm au moyen de materiaux composites - Google Patents

Reduction d'artefacts de susceptibilite dans une irm au moyen de materiaux composites Download PDF

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Publication number
WO2007022499A2
WO2007022499A2 PCT/US2006/032566 US2006032566W WO2007022499A2 WO 2007022499 A2 WO2007022499 A2 WO 2007022499A2 US 2006032566 W US2006032566 W US 2006032566W WO 2007022499 A2 WO2007022499 A2 WO 2007022499A2
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susceptibility
composite
diamagnetic
filler
composite material
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PCT/US2006/032566
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WO2007022499A3 (fr
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Steven M. Conolly
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Conolly Steven M
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Priority to EP06801986.8A priority Critical patent/EP1968440A4/fr
Publication of WO2007022499A2 publication Critical patent/WO2007022499A2/fr
Publication of WO2007022499A3 publication Critical patent/WO2007022499A3/fr

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    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/05—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves 
    • A61B5/055—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves  involving electronic [EMR] or nuclear [NMR] magnetic resonance, e.g. magnetic resonance imaging
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
    • G01R33/48—NMR imaging systems
    • G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
    • G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
    • G01R33/565—Correction of image distortions, e.g. due to magnetic field inhomogeneities
    • G01R33/56536—Correction of image distortions, e.g. due to magnetic field inhomogeneities due to magnetic susceptibility variations

Definitions

  • the present invention relates generally to magnetic resonance imaging. More specifically, it relates to devices and techniques for reducing imaging artifacts due to susceptibility differences at tissue-air interfaces.
  • Magnetic resonance imaging is a non-invasive technique developed to produce images of the interior of the human body for medical diagnosis and research.
  • MRI involves exposing a body to a strong magnetic field (called the BO field) which aligns the spins of nuclei in the body into parallel and anti-parallel states, modulating the field at a characteristic radio frequency to induce in specific atomic nuclei resonant transitions between parallel and anti-parallel states, detecting resulting changes in the magnetic field caused by such transitions, and reconstructing an image from the detected changes.
  • the static BO field be both strong and homogenous.
  • MRI devices using a stronger BO field have a greater signal to noise ratio (SNR).
  • SNR signal to noise ratio
  • a homogeneous BO field provides a uniform imaging response.
  • a conventional MRI device is thus designed to produce a strong (typically 1.5 T), almost perfectly homogeneous BO field.
  • the introduction of a body into the MRI device during use creates new inhomogeneities in the BO field due to magnetic susceptibility differences between materials such as air, tissue, and bone. These susceptibility differences distort the BO field and result in "susceptibility artifacts" in the MRI images.
  • One of the largest of these susceptibility differences is found near air-tissue interfaces.
  • Air has a magnetic susceptibility about 10 parts- per-million (ppm) higher than human tissue, which is a sufficiently large difference to produce significant image degradation in regions near air-tissue interfaces. Consequently, susceptibility artifacts are especially problematic when attempting to image tissue near the skin, lungs, mouth, or sinuses.
  • shimming One well known technique used to help produce BO field homogeneity is shimming.
  • active shimming MRI devices incorporate both shim coils which are used to actively adjust the magnetic field over the imaging region.
  • the shim coils are designed to create magnetic field patterns (typically Legendre polynomials) that are independent and orthogonal over a spherical phantom. It is common to have shim te ⁇ ns such as a main field shim, the three linear gradients, and up to a dozen nonlinear terms. By using coils that generate orthogonal field patterns, one can, in principle, optimize the current sources for each shim coil independently.
  • Jezzard (WO 2003-062847) describes a passive shimming technique used in MRI to reduce susceptibility artifacts at air-tissue interfaces.
  • the technique uses passive shims made of a highly diamagnetic material to locally alter the magnetic field to restore homogeneity.
  • the highly diamagnetic material pyrolytic graphite (PG)
  • PGS pyrolytic graphite sheet
  • the material is very anisotropic.
  • PGS has a large through-plane diamagnetic susceptibility of -595 ppm. Shimming on a per patient basis is difficult using this technique, however, since PGS is hard and cannot be reshaped to compensate for bodily variations between different patients.
  • the pads contain a gel material (essentially water) whose diamagnetic susceptibility is closely matched with that of tissue. These pads are positioned against specific areas of the body surface to reduce susceptibility artifacts there. There are two significant drawbacks of the gel pads, however. First, the density of the gel is similar to that of water, which makes them fairly heavy and cumbersome. Second, since the pads are placed between the body and the RF coil, they act as a shield between the RF coil and the area to be imaged, reducing signal strength. Because the pads need to remain sufficiently thick to remove artifacts, the shielding is to some extent unavoidable. It would not be safe to place the RF coils inside the gels, as this would make the coils overly heavy, and the fluid would cause problems with high voltages on the capacitors and inductors of the RF coil during transmit phase.
  • a gel material essentially water
  • the present invention provides a susceptibility-matching composite material for reducing susceptibility artifacts in MRI.
  • the material is lightweight, inexpensive, nonconductive, matches the magnetic susceptibility of the human tissue, and may be shaped to an external body part (such as the foot, chin, shoulder, head, knee, spine).
  • the composite material is three to sixty times less dense than the gel pads, making it much easier to handle. Because the composite material is dry and largely nonconductive, RF coils may be embedded within the material to achieve excellent filling factors while minimizing susceptibility artifacts in specific body parts.
  • the composite material enjoys the advantage that its overall shape can be altered to shim specific bodily regions.
  • susceptibility artifacts in magnetic resonance imaging of a portion of a human body can be reduced by positioning the portion of the human body and a diamagnetic material in a magnetic field generated by a magnetic resonance imaging device.
  • the diamagnetic material is characterized in that it is a composite material made of a filler material and a collection of diamagnetic particles uniformly distributed through the filler material.
  • the composite material preferably has a bulk magnetic susceptibility within 10% or less of the bulk magnetic susceptibility of human tissue, and more preferably within 1% or less.
  • the MRJ scan is perfo ⁇ ned. Because of the susceptibility-matching properties of the material, susceptibility artifacts are reduced or eliminated in the resulting image. Field variations persist but they largely exist in the composite material, essentially moving the field interface region to an area outside the human. Hence the imaging area no longer suffers from susceptibility artifacts.
  • the diamagnetic particles are graphite particles e.g., pyrolytic graphite powder, crushed PG, or cut pyrolytic graphite sheet (PGS).
  • the filler material is a deformable, non-conductive, low-density material such as foam (e.g., stryofoam or expanded polystyrene foam, or an EVA foam).
  • foam e.g., stryofoam or expanded polystyrene foam, or an EVA foam.
  • the composite material may be packaged in flexible material container suitable for placement against specific body parts.
  • the composite material may be used as a liner within a container similar to a removable cast so that a limb (such as a foot, leg, hand, or arm) is surrounded by the composite material when the limb is placed in the container.
  • the composite may also be used as a liner in a helmet used when imaging the head.
  • the composite may also be formed into the shape of foam ear plugs that may be placed within the ear.
  • the composite may also be used within the mouth.
  • the composite has the advantage of being deformable if needed, it may also be useful in stiffer forms, such as a gantry table pad upon which the body rests during an MRI scan.
  • the composite may have RF coils embedded in it.
  • the composite may be packaged in a manner similar to bean bags, which could readily conform to the body part.
  • the graphite may be embedded in "balls" of filler material or on the surface of the filler material. The principle of effective operation is the same for each — the bulk volume susceptibility of the composite matches that of human tissue to within 10%, or preferably, to within 1%.
  • FIGS. IA and IB illustrate how a susceptibility-matched composite of highly diamagnetic material and a light filler material leads to a reduction of susceptibility artifacts in an MRI image through a comparison of such a composite with other materials.
  • FIG. 2 illustrates a composite material in the form of small composite beads contained within an envelope whose inner surface is deformable so that it conforms to the shape of a body part and whose outer surface is designed to retain the shape of a cylinder.
  • FIG. 3 illustrates a composite material formed to serve as ear plugs.
  • FIG. 4 illustrates a composite material in the form of a wrappable pad.
  • FIG. 5 illustrates a composite material in the form of a gantry pad.
  • a composite susceptibility-matching material for reducing susceptibility artifacts in MRI imaging.
  • the composite has a low- density filler material such as foam with particles of a highly diamagnetic material embedded or mixed homogeneously within or around the filler.
  • the filler material is preferably low-density, substantially non-conducting, and inexpensive.
  • a low density material in this context is defined as a material preferably having less than 0.5 specific density.
  • the filler material preferably possess magnetic susceptibility very similar to air (e.g., within 10% or less, and preferably within 1% or less).
  • the diamagnetic particles are primarily a graphite powder, pyrolytic graphite, another highly diamagnetic material, or some mixture of such materials.
  • the diamagnetic particles used in the composite may be a mixture of different diamagnetic materials.
  • PPS Pyrolytic graphite sheet
  • Xavcragc / XPGS / (1 + CX ⁇ PG s), where/is the volume fraction of the PGS particles, and ⁇ is the demagnetizing factor determined by the shape of the PGS particles (e.g., spherical particles have a « 1/3). Because the magnitude of X PGS is very small, however, we can approximate
  • the shape factor is not significant in this case, so any shape of PGS particles should yield the same bulk susceptibility.
  • a homogeneous volume reduction of PGS particles within the filler material by a factor of 66 can be used to create a composite with bulk diamagnetic properties very similar to human tissue. Because PGS has an in-plane thermal and electrical conductivity similar to copper, it is desirable to ensure that the particles of the powder are sufficiently small to break up conducting paths in the PGS, which may otherwise create noise in the receiver coil.
  • Narrow (1 mm) strips of PGS material 10 were glued to low density balsa wood filler layers 12 to form a vertically stacked PGS-wood composite block 18 encased in a plastic protective cube whose edges have length 2.5 cm.
  • the strip separation distance in each layer and the layer thicknesses are selected to create a net volume reduction by a factor of 66 so that the PGS composite block accurately mimics the susceptibility of human tissue.
  • the orientation of the strips is alternated between adjacent layers of balsa wood to homogenize the blurring effect in 3D rather than 2D.
  • a solid piece of wood 14 was encased in a plastic cube to form a wood block 20, and an air-filled block 22 was formed by filling a similar plastic block with air 16.
  • the figure shows an MRI image of the three blocks within a cylindrical container of water doped with MnC ⁇ .
  • the composite block 20 is oriented so that the strip planes are perpendicular to the BO field.
  • Bright regions 24 adjacent to the water-cube interfaces are characteristic dipole field patterns that represent image artifacts due to susceptibility differences.
  • the field pattern around the PGS composite cube is significantly smaller than the patterns surrounding the air cube 22 and wood cube 20.
  • this simple PGS composite is very effective at removing the characteristic susceptibility artifact at the interface between the cube and the surrounding water.
  • PGS is pure crystal since the material is synthesized by chemical vapor deposition.
  • PGS is currently quite expensive, so in many embodiments it is preferable to use a pyrolytic graphite powder formed by crushing mined graphite crystal, which is relatively inexpensive.
  • PG powders are commercially available, some have been found to have magnetic contaminants.
  • FIG. IB is an image of B0-field intensity comparing field patterns for water 28, air 30, and a composite 32 formed from PG powder in a polyurethane foam matrix. The figure clearly shows that the susceptibility artifact near the surface of the composite 32 is greatly reduced compared to air 30, and is close to that of water 28. In the image, all three phantoms are immersed in water.
  • the concentration should be increased by a factor of 2 compared to PGS, resulting in a net volume dilution factor of approximately 30-32.
  • a volume fraction of 1/31 gives a susceptibility almost exactly matching that of water.
  • Most human tissues are extremely close to the susceptibility of water, and the volume fraction can be further tuned to match human tissue even more accurately.
  • another advantage of the PG powder is that it is not directionally dependent once the crystal particle orientations are randomized in the filler (i.e., the composite has isotropic magnetic susceptibility even though the crystals are anisotropic).
  • the PG powder may also be distributed between tiny styrofoam beads. Care should be taken to ensure that the graphite powder has no magnetic contaminants.
  • the size and separation of the particles determines how close local field distortions exist within the subject. This is due to the blurring effect of magnetic field sources.
  • the field a distance 1 cm away from a complex magnetic field source is convolved with a function of width 1 cm; at 10 cm it is convolved with a function of width 10 cm.
  • microvariations within the composite are largely blurred out a distance approximately equal to the separation between the particles.
  • the graphite particles and their separation be small relative to a pixel size. For example, a 100 micron particle size or smaller would be preferred.
  • graphite powder is used in addition to, or instead of PGS.
  • Graphite powder has the advantage that it is extremely inexpensive and isotropic once randomized. The average susceptibility of graphite powder, however, is about 19 to 33 times that of tissue), so composites using graphite will require a larger volume fraction of diamagnetic particles in the foam (specifically 19 to 33 times volume dilution) than composites using PGS alone (which requires a volume dilution of about 60-66 fold).
  • a composite is made of a high purity graphite powder suspended in a low density filler such as styrofoam or expanded polystyrene (EPS) foam or EVA foam.
  • EPS expanded polystyrene
  • raw polystyrene beads may be mixed with graphite powder then the mixture is heated (e.g., with steam or pentane) to create EPS-graphite foam which can then be formed as desired, e.g., into small (i.e., 1 mm diameter) beads.
  • EPS-graphite foam which can then be formed as desired, e.g., into small (i.e., 1 mm diameter) beads.
  • These beads of styrofoam-graphite may then be enclosed in a containment envelope to form a susceptibility-matching composite package.
  • the air gaps between beads increase the volume, reducing the net susceptibility. This, however, is easily corrected for since the susceptibility of the air and the filler material are virtually identical at about 0.36 ppm.
  • the tolerance for the sampling pattern of the composite material's homogeneity is best understood from a Green's function (or convolution) analysis.
  • the magnetic fields fall off as the convolution of the magnetic field source with a kernel whose width is proportional to the distance r.
  • the variations in the composite due to discrete graphite flakes will be blurred out nearly perfectly at a distance equal to the composite array spacing.
  • this effect can be made nearly negligible.
  • susceptibility-matching composites can be made to have susceptibilities within 1% of human tissue, reducing shifts of ⁇ 300 Hz at 1.5 T to ⁇ 3 Hz shift at the skin, producing negligible image artifacts.
  • the high level of homogeneity attained using these composites can be helpful for demanding applications like RF ablation monitoring in an interventional setting, since a 15 C temperature increase induces only about 3 Hz frequency shift at 0.5 T, which is quite small relative to 100 Hz variations due to air. It also may be helpful for routine applications such as robust water-fat separation, or robust fat saturation with selective RF pulses, or SSFP at 3 T. At 1.5 T, fat and water are separated in resonance frequency by about 245 Hz.
  • Air-tissue interfaces experience +/- 320 Hz field shifts which are enough to cause poor water-fat separation using most clinically accepted methods (spectral saturation, three-point Dixon, Chemsat, etc).
  • this ⁇ 320 Hz field shift can be reduced to just ⁇ 32 Hz with 10% tolerance (or preferably to ⁇ 3.2 Hz with 1% tolerance).
  • These classic methods of fat suppression and/or water-fat imaging will be far more robust with deviations of 32 Hz or 3.2 Hz. Essentially these methods will never fail with such a perfect field variation. It is common for them to fail in current clinical practice, and this can be a significant clinical confoundant.
  • small (i.e., 1 mm diameter or less) EPS or EVA foam- graphite composite beads 200 are contained within an envelope 202 that is at least partly composed of a flexible material (e.g., a fabric, rubber, or other membrane) 204, as shown in FIG. 2.
  • the graphite density in the beads is adjusted to account for the sphere packing factor so that the overall volume concentration of the PG graphite mix (including the air voids) is precisely matched to human magnetic susceptibility.
  • the composite beads behave as an ultralight liquid or sand to provide high conformability to the individual geometry of the surface of the body part 206 being imaged.
  • an outside portion of the envelope not contacting the body may be rigid or semi-rigid hull 208 that approximates a cylindrical, spherical, or hemispherical shape, e.g., by using a semi-rigid supporting structure for the envelope, or using a semi-rigid material for the envelope itself.
  • Using a cylindrical or spherical shell as an outside hull of the package helps to reduce image artifacts at the outside surface of the composite package.
  • the envelope may be configured in two or more parts attached with a hinge 212 or other mechanism that allows the device to be opened and closed around a body part.
  • the envelope may also be similar to a removable cast having an outside shell and an inner padding that contains composite beads which conform to the body part in the cast.
  • FIG. 2 illustrates the specific case of a foot
  • the envelope may be easily adapted for use with a hand, and elbow or knee (e.g., by providing an opening at the bottom of the cylindrical hull shown for the limb to exit), a head (e.g., by providing a hemispherical hull so that the device fits on a head like a helmet), a back/spine (e.g., by providing a horizontal gantry pad for a patient to lay upon), and various other body parts.
  • elbow or knee e.g., by providing an opening at the bottom of the cylindrical hull shown for the limb to exit
  • a head e.g., by providing a hemispherical hull so that the device fits on a head like a helmet
  • a back/spine e.g., by providing a horizontal gantry pad for a patient to lay upon
  • various other body parts e.g., by providing a horizontal gantry pad for a patient to lay upon
  • the flexible envelope 204 may be used without semi-rigid hull 208 as a bean-bag like composite that can be wrapped around a body part and held in place by gravity, hook-and-loop fasteners, straps, or various other devices.
  • the composite beads 200 may be replaced by a solid foam composite whose surface is shaped during manufacture to approximate the expected shape of the body 206.
  • the solid foam is defo ⁇ nable, it provides more support than the conforming beads.
  • Such solid foam composites may be preferred to fit softer regions of tissue, such as the breast, abdomen, or thigh.
  • Such solid foam embodiments in most cases would not need a semi-rigid hull 208 since the foam itself provides adequate structural support.
  • Preferred embodiments may include an integrated RF coil or RF coil array 210 in the composite that fits snugly around or against a body part (such as a foot, hand, knee, breast, head, spine, inner ear, chin, mouth, shoulders, etc.).
  • a body part such as a foot, hand, knee, breast, head, spine, inner ear, chin, mouth, shoulders, etc.
  • Such devices provide excellent RF coil filling factor as well as excellent magnetic field homogeneity.
  • RF coil arrays are well known to improve spatial coverage with excellent signal to noise ratio. They are also critical for certain high-speed imaging applications such as SENSE and SMASH.
  • the combination of a coil array with the susceptibility matching composite should provide a very homogeneous region over each small coil.
  • This combination would enable the use of certain fast-imaging pulse sequences that are known to suffer from susceptibility artifacts (e.g., echo planar MRI, spiral MRI, balanced Steady State Free Precession, RARE or Fast Spin Echo, and fast gradient recalled echo imaging).
  • susceptibility artifacts e.g., echo planar MRI, spiral MRI, balanced Steady State Free Precession, RARE or Fast Spin Echo, and fast gradient recalled echo imaging.
  • FIG. 3 Another preferred embodiment is a diamagnetically doped ear plug 300, as shown in FIG. 3.
  • the plug 300 could be formed of a continuous compressible doped foam, or as an envelope containing doped beads.
  • the ear plug can double as an acoustic noise dampener while reducing field artifacts near the ear canal.
  • the composite takes the form of a flexible diamagnetically doped foam pad 400, as shown in the cross-sectional illustration of FIG. 4.
  • the pad may be wrapped around a body part multiple times to provide the desired net thickness.
  • the pad is about lcm to 3cm in thickness. It could have a ramped thickness, allowing tight conformity close to the body part, and providing speedier attainment of desired outer radius. Once wrapped, it may be secured using any of various types of fasteners.
  • a similar but larger and thicker pad 500 may be used in an unrolled configuration as shown in FIG. 5.
  • a patient can lay on a flexible composite foam pad 500 positioned on a MRI scanner gantry, for example, to reduce artifacts at the surfaces of the underside of the patient in contact with the pad.
  • the pad 500 may be flat, or it may be shaped to conform to an approximate expected shape of a body lying on the pad.
  • the pad may include extensions that conform to specific parts of the body, including shoulder pads, chin pads, a bowl shaped region to support the bottom of the head, or positioning pads under the knee.
  • the PG-foam pad may also double as the attachment device for securing the RF coil to the patient, perhaps in conjunction with a hook-and-loop fastener for securing. This will ensure that the MRI technologist uses the PG foam strap and prevents poor imaging when the MRI technologist is too rushed to remember to use the susceptibility matching material.
  • a composite material is uniformly doped with a highly diamagnetic material (such as pyrolytic graphite or graphite powder) at a dilution factor designed to match the bulk magnetic susceptibility of the test substance (e.g., the liquid in a test tube).
  • a highly diamagnetic material such as pyrolytic graphite or graphite powder
  • Uniform doping is defined in the present description to be doping that is sufficiently uniform that inhomogeneities in the diamagnetic particle distribution are not resolvable by the MRI imaging device.
  • the diamagnetic agent is conductive (such as graphite) then it preferably takes the form of small pieces or a powder to prevent eddy current paths from introducing RF losses in the receive coil.
  • An envelope similar to that shown in FIG. 2 may be filled with a bead composite that is susceptibility-matched to a test substance. The test substance (contained in a test tube or other container) is then surrounded by the susceptibility-matched composite to reduce image artifacts in MRI.

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Abstract

La présente invention concerne un matériau composite (200) correspondant à la susceptibilité composé d'un matériau hautement diamagnétique et d'un matériau de charge qui réduit les artefacts de susceptibilité dans une I.R.M. Ce composite, par exemple, peut être formé par l'intégration de particules hautement diamagnétiques (par exemple de la poudre de graphite) dans un matériau de charge léger (par exemple un styrofoam ou une mousse de polystyrène expansé ou une mousse EVA) de sorte que la susceptibilité nette de ce composite corresponde au tissu humain (206) ou à une autre substance de test à imager. La charge est de préférence un matériau non conducteur, de faible densité qui est soit déformable soit formé sous la forme d'une collection de petites billes emballées dans un récipient de sorte que ce composite (200) prenne la forme de la surface d'un corps (206) pressé contre lui.
PCT/US2006/032566 2005-08-18 2006-08-18 Reduction d'artefacts de susceptibilite dans une irm au moyen de materiaux composites WO2007022499A2 (fr)

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WO2017080562A1 (fr) * 2015-11-12 2017-05-18 Rigshospitalet Dispositif pour réduire un artéfact de susceptibilité magnétique
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Cited By (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US8320647B2 (en) 2007-11-20 2012-11-27 Olea Medical Method and system for processing multiple series of biological images obtained from a patient
US9123100B2 (en) 2007-11-20 2015-09-01 Olea Medical Method and system for processing multiple series of biological images obtained from a patient
WO2011017705A3 (fr) * 2009-08-07 2011-06-23 University Of Florida Research Foundation, Inc. Appareil et procédé compatibles avec la résonance magnétique et à susceptibilité accordée, et procédé d'imagerie et de spectroscopie par rm
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