WO2000019243A1

WO2000019243A1 - Ultrasound probes implementing waveguide shielding and active compensation of noise

Info

Publication number: WO2000019243A1
Application number: PCT/US1999/022544
Authority: WO
Inventors: Han Wen; David G. Wiesler; Robert S. Balaban
Original assignee: The Government Of The United States Of America As Represented By The Secretary, Department Of Health And Human Services
Priority date: 1998-09-30
Filing date: 1999-09-30
Publication date: 2000-04-06
Also published as: AU6404299A; AU6280299A

Abstract

A shielded ultrasound transducer (50) which employs an electromagnetic waveguide (42) and also preferably active compensation (62) to shield an ultrasound probe/transducer element (50) from noise. The shielded ultrasound probe (50) includes an ultrasound transducer element (32) and an electromagnetic waveguide (42) coupled relative to the ultrasound transducer element (32) such that acoustic energy detected by the ultrasound transducer element (32) first traverses a region substantially bounded by the electromagnetic waveguide (42). In another embodiment, the shielded ultrasound probe (50) includes a conductive element (62) that can be current driven to generate an electromagnetic field that compensates electromagnetic fields in the vicinity of the ultrasound transducer (32). The conductive element (62) that provides active compensation may be located substantially in a region bounded by the electromagnetic waveguide element (42). The shielded ultrasound probe (50) is well suited for use with Hall Effect Imaging (HEI).

Description

ULTRASOUND PROBES IMPLEMENTING WAVEGUIDE SHIELDING AND ACTIVE COMPENSATION OF NOISE

CROSS-REFERENCE TO RELATED APPLICATIONS

This International application is related to concurrently filed

International Application No. , entitled "Ultrasound Array and Electrode Array for Hall Effect Imaging", which claims priority to U.S. Provisional Patent Application Serial No. 60/102,478, filed on September 30, 1998, concurrently with U.S. Provisional Patent Application Serial No. 60/102,479, which is the priority application of the present application.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates generally to ultrasound transducers or probes, and more particularly to an ultrasound probe which is shielded from electrical interference and from ultrasonic interference or noise induced by electrical interference (e.g., pulses) or by the interaction of electrical pulses with a magnetic field.

2. Background Art

Hall effect imaging (HEI) is an ultrasound-based method which provides information about the dielectric and conductivity distributions of the imaged sample, and HEI is described in International Application No.

PCT/US97/11272, filed July 2, 1997, which claims priority to U.S. Provisional

Application No. 60/021,204, filed July 3, 1997. HEI relies on the interaction in the sample medium between a strong static magnetic field and charge displacement induced by either an externally applied radio-frequency current or an externally applied ultrasound signal, as also further described in "Hall Effect Imaging", IEEE

Trans. Biomed. Eng., vol. 45, No. 1, pp. 1 19-124 (1998), which is also incorporated herein by reference. HEI's unique characteristics include its conductivity-based image contrast and its suitability for 2D or 3D imaging with wide-angle signal reception, which characteristics are well suited for biological applications. More specifically, HEI evaluates an object's electrical and mechanical properties based on the Hall effect, which refers to positive and negative charges in a conductive object tending to separate when the object moves in a magnetic field as a result of the opposing Lorentz forces on the charges. This charge separation leads to an externally detectable voltage within the sample, the Hall voltage. In one implementation of HEI, a conductivity-based image of an object can be formed by inducing the motion with ultrasonic pulses. At any moment, the Hall

10 voltage generated by such an ultrasound pulse is a signature of the current position of the pulse. As the pulse sweeps through an object, the associated Hall voltage records its progression in time. In this fashion, the scanning ultrasonic pulse converts spatial information along its path to the time record of the Hall voltage.

^ This relation between space and time is similar to conventional ultrasound imaging.

In another implementation of HEI, a static magnetic field is applied to an object or subject, an electrical pulse is propagated into the object, and an ultrasound signal is detected which is related to the interaction of the electrical pulse

20 generated in the conductive object and the magnetic field. The acquired ultrasound signal, which is dependent on local conductivity as well as local acoustic properties, is then processed to provide an image of the object. Based on the Lorentz force underlying the Hall Effect, the direction of the propagation of the ultrasound signal is in a plane perpendicular to the orientation of the magnetic field, and particularly

25 the ultrasound signal direction is mutually perpendicular to the electric pulse field direction and the magnetic field direction which are preferably orthogonally oriented (i.e., the ultrasound propagation direction is perpendicular to the plane formed by the electric pulse field direction and magnetic field direction).

30 Piezoelectric ultrasound probes are desirable for HEI because they have high sensitivity and are commercially available in many designs, such as focused transducers and arrays. Although conventional ultrasound probes may be used to detect the ultrasound signal in HEI, significant noise, including significant

-. - coherent noise, is encountered when using such probes in HEI. For instance, it may be appreciated that the electrical excitation pulse used in HEI may not only cause direct electrical interference in the ultrasound sensor, but also the electrical voltage picked up by the piezoelectric material in the ultrasound probe may cause excessive noise and instability in the preamplifier electronics and also cause the piezoelectric material to vibrate and send out unwanted (e.g., uncontrolled, unintended, unknown power spectrum) ultrasonic pulses, the echoes of which are later received by the ultrasound transducer as noise. Evidently such interference and noise is not only undesirable but also difficult to compensate in an efficient and robust or universal manner (e.g., independent of the experimental setup or subject being imaged, independent of the local region of the subject being imaged, etc.).

10

It is well known that effective shielding of a device from electromagnetic fields (e.g., static electric fields, low frequency, radiofrequency, etc.) may be provided by enclosing, surrounding, or otherwise encasing the device in a continuous or contiguous conductive material (e.g., metal shield) of sufficient

*5 thickness (which depends on the electromagnetic frequency to be shielded and on the conductivity of the shield). Implementing such shielding for a piezoelectric ultrasonic transducer in the presence of high magnitude electromagnetic fields (i.e., shielding the piezoelectric transducer) is complicated by the need to satisfy o diametric conditions on the thickness of the shield: the layer needs to be thick enough to shield out the electromagnetic fields, and the portion in front of the acoustic aperture of the probe needs to be thin enough to allow ultrasonic signal to pass without noticeable attenuation and reflection.

Further difficulties in shielding noise and interference arise when an 5 ultrasound probe is used in the presence of a magnetic field in addition to an electromagnetic pulse/radiation, such as in HEI. In such instances, the Lorentz interaction between the electric pulse and the magnetic field may result in (unwanted) ultrasonic vibrations being generated in conductive elements

30 acoustically coupled to the ultrasound transducer but which are not the subject or object being imaged or tested (e.g., conductive elements of the ultrasound transducer itself), and these unwanted ultrasonic vibrations may thus couple into the ultrasound transducer. More specifically, when an ultrasound probe is used in HEI there is a very strong magnetic field (on the order of 1 Tesla or more) present, and 5 when the high voltage impulse occurs, the induced currents in any conductive shield layer would experience Lorentz forces, which would cause the layer to vibrate. If the vibrations occur in the acoustic aperture area, or propagate to this area, they will enter the probe as coherent ultrasonic noise, and will also propagate into the sample as unwanted ultrasonic pulses which give rise to unwanted echoes (i.e., noise or interference).

International Application No. PCT/US98/24152 (publication No. WO 99/24967), which claims priority to U.S. Provisional patent application No. 60/065,111 (filed 1 1/12/97), entitled "Shielded Ultrasound Probe", discloses a shielded ultrasound probe which eliminates and/or significantly mitigates the above- described noise sources, including the sources of coherent noise. The shielded ultrasound probe includes an ultrasound reflector which redirects acoustic signals incident from a first direction into a second direction incident on the acoustic aperture of the ultrasound probe, the second direction typically being substantially perpendicular to the first direction. The ultrasound probe is substantially enclosed by a conductive shield and, in accordance with the arrangement of the ultrasound reflector, the probe is oriented such that the conductive shield is substantially parallel to the magnetic field. The conductive shield and its orientation results in eliminating and/or mitigating noise sources, including noise related to vibrations generated by the Lorentz interaction between the electric pulse and the magnetic field. Although this shielded ultrasound probe is well suited for HEI and other applications having significant electromagnetic and/or magnetic fields present, it may be appreciated that further advancements in shielded ultrasound transducers are needed.

That is, it may be generally appreciated that further advancements in ultrasound transducers are needed, particularly for providing additional ultrasound probes which eliminate, mitigate, or are otherwise not susceptible to, electrical interference and/or electromagnetically induced ultrasonic noise or interference, and which are thus well suited for HEI or other industrial or medical applications where an ultrasonic transducer is used in an environment having significant electromagnetic and/or magnetic fields present. For instance, additional and/or alternative shielded ultrasound probe configurations and/or ultrasound probe shielding mechanisms may be needed and/or be advantageous, for example, for o implementing various imaging system mechanical/structural designs as well as for providing efficacious noise-shielding of the probe in a given application and/or system configuration.

SUMMARY OF THE INVENTION

5

The present invention overcomes the above mentioned problems and other limitations, by providing a shielded ultrasound probe which employs an electromagnetic waveguide and also preferably active compensation to shield an ultrasound probe element from noise, including cross-talk noise and Lorentz

10 vibration noise. In accordance with an aspect of the present invention, an apparatus is provided which includes an ultrasound transducer element and an electromagnetic waveguide coupled relative to the ultrasound transducer element such that acoustic energy detected by the ultrasound transducer element traverses a region

, - substantially bounded by the electromagnetic waveguide. The waveguide element may be provided by, for example, a conductive cylindrical waveguide member that defines a boundary of the electromagnetic waveguide.

In accordance with another aspect of the present invention, an apparatus is provided which includes an ultrasound transducer element and a

20 conductive element that can be current driven to generate an electromagnetic field that compensates electromagnetic fields in the vicinity of the ultrasound transducer. The conductive element may include a pair of conductive loops respectively oriented such that their respective magnetic fields are constructively additive. In

25 accordance with yet a further aspect of the present invention, the conductive element is located substantially in a region bounded by the electromagnetic waveguide element. Preferably, the ultrasound transducer is shielded by both the electromagnetic waveguide and the conductive element that provides active compensation.

30

BRIEF DESCRIPTION OF THE DRAWINGS Additional aspects, features, and advantages of the invention will be understood and will become more readily apparent when the invention is considered in the light of the following description made in conjunction with the accompanying

35 drawings, wherein: FIG. 1 is a schematic cross-sectional lengthwise view of a shielded ultrasound probe, including a waveguide shielded piezoelectric probe, in accordance with an embodiment of the present invention;

FIG. 2 is a schematic cross-sectional view of a shielded ultrasound probe, including a waveguide shielded piezoelectric probe, in accordance with another embodiment of the present invention;

FIG. 3 is a schematic cross-sectional lengthwise view of a shielded ultrasound probe, including a piezoelectric probe shielded by a waveguide and by active compensation, in accordance with another embodiment of the present

10 invention;

FIG. 4 illustrates a plan view of a compensation coil element implemented in the embodiment of the invention illustrated in FIG. 3;

FIG. 5 shows an illustrative setup for Hall effect imaging (HEI)

15 experiment;

FIG. 6A shows a configuration that was used to measure EM crosstalk between a probe and an excitation pulse outside the presence of an applied static magnetic field; 20 FIG. 6B schematically depicts features of the ultrasound transducer of FIG. 6 A;

FIG. 7 shows the measured signal voltage plotted against time, with zero time set at the instant of the excitation pulse, for the experimental measurement of the coherent noise due to EM cross-talk between the probe and the excitation

25 pulse;

FIG.8 shows the results from an experimental EM cross-talk measurement, performed outside the presence of an applied static magnetic field, for a commercial probe modified to include a waveguide shield in accordance with an 30 embodiment of the present invention;

FIG. 9 depicts, in cylindrical coordinates, modeling of the waveguide shield according to the present invention as a semi-infinite cylinder extending from z = 0 along the positive z axis, wherein the material of the wall is - _ assumed to be a perfect conductor; FIG. 10 is an illustration of the electric field and the magnetic field produced in the vicinity of a probe by the excitation pulse across the electrodes;

FIG. 11 shows an HEIS image of a polycarbonate block immersed in a saline tank, collected with a waveguide shielded probe in accordance with an embodiment of the present invention;

FIG. 12 schematically depicts an ultrasound prism fitted to a probe, which allows the probe to align with the magnetic field for Lorentz vibration noise reduction;

FIG. 13 is an HEI image of a polycarbonate block immersed in a saline tank collected with a probe having an ultrasound prism as depicted in FIG. 12;

FIG. 14 schematically depicts an HEI experimental setup having a pair of coil windings placed across a saline tank to create a compensting field for active cancellation of the rf magnetic field generated by an excitation pulse, in accordance with an embodiment of the present invention; and

FIG. 15 is an HEI image of a polycarbonate block in a saline tank collected with the active shielding arrangement schematically depicted in FIG. 14, in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a shielded ultrasound transducer or probe, and is particularly well suited for adapting or modifying a conventional, commercial ultrasound transducer such as, for example, the Panametrics V314

Unfocussed Immersion Transducer or the Krautkramer Branson KBA Alpha 1.0

MHZ/0.75 diameter transducer, and various aspects of one or more of the hereinbelow described embodiments of the present invention are representative of such modification of commercial probes. It will be appreciated, however, that the present invention is not limited to embodiments which represent modifications of commercial probes, and that one skilled in the art may design and/or manufacture a custom shielded ultrasound transducer (not constrained by practicalities of adapting a commercial probe) in accordance with the present invention. As mentioned, the IEEE paper, supra, elaborates basic principles underlying HEI, which, in practice, can be carried out in two modes: the forward mode, which involves applying an ultrasonic pulse and receiving a resulting electrical signals; and the reverse mode, which involves applying an electrical excitation pulse and receiving a resulting ultrasonic signal. The reverse mode of HEI is advantageous and/or implemented more easily in many situations because this mode allows the use of receive-only array ultrasonic probes for fast image formation. Although the ensuing description of embodiments of the invention and of HEI experiments refer more specifically to this reverse mode, it may be

10 appreciated that the present invention is not limited to the reverse mode, but may also be practiced in accordance with the forward mode.

Referring now to FIG. 1, there is shown a schematic cross-sectional length-wise view of an illustrative embodiment of a shielded ultrasound probe 10 in

15 accordance with the present invention. More specifically, for this illustrative embodiment, shielded probe 10 is generally cylindrical in shape, and the cross- sectional view of FIG. 1 represents a section along the cylindrical axis (not shown) which extends length-wise with respect to probe 10. In more detail, shielded probe

20 1 includes a conductive (e.g., copper) outer casing 22, isolation layer 30, acoustic couplant 26, acoustic window 24, piezoelectric (PZ) element 32, acoustic backing 34, acoustic matching layer 38, and waveguide element 42. Isolation layer 30 may be made of foam or cork, for example. Acoustic couplant 26 is formed of a low dielectric constant material, such as Fluorinert, available from 3M Corp., or may be

25 implemented with mineral oil. Acoustic window 24, which may be a cellophane membrane, facilitates separation and/or containment of acoustic couplant 26 from the external environment. The back electrode 32a of PZ element 32 is a thin layer of conductive material (e.g., 50 nm of sputter-deposited nickel) and is connected to

30 the inner conductor 36b of mini-coaxial cable 36. Acoustic backing 34 is preferably a non-conductive (e.g., non-metallic) material (as will be further understood hereinbelow); however, it is noted that in commercial ultrasound probes (which may be modified according to the present invention) acoustic backing 34 is often metallic

-.. (e.g., tungsten). The outer shield conductor 36a (which is typically grounded) of the mini-coaxial cable is connected (e.g., soldered) to outer casing 22. Mini-coaxial cable 36 connects the shielded probe 10 to a preamp (not shown).

In additional detail, the PZ element is bonded to the backing material which is then bonded to the casing via an acoustic dampening layer (RTV for instance). The acoustic matching layer is bonded to the PZ element tightly and is made of specially formulated epoxy which has the desired acoustic impedance. The joining between the acoustic matching layer and the casing is water tight, normally with RTV or other glue. The electrodes on the PZ element are typically deposited by sputtering and the front electrode connects to the casing via silver paint/epoxy

10 around the circular edge of the front face.

Waveguide element 42 is a thin conductive (e.g., copper) cylinder coaxial with PZ element 32 and the overall structure of probe 10, with one end (i.e., the proximal end) connected (e.g., soldered) to the conductive outer casing 22 of

15 probe 10 and the other end (i.e., the acoustic aperture or distal end) capped with acoustic window 24 (e.g., cellophane membrane) which contains acoustic couplant 26. The inner surface of waveguide element 42 is lined with isolation layer 30 (e.g., foam layer) for insulation against acoustic noise (as will be explained

20 in more detail below). In the "Waveguide Shielding Model" section below, it is shown that the electromagnetic field decreases roughly exponentially into the interior of the waveguide; thus with sufficient distance between the probe and the front end, the EM cross-talk noise should be greatly reduced, which was experimentally verified as also described hereinbelow.

25

FIG. 2 illustrates another embodiment of a shielded ultrasound probe in accordance with the present invention. Using the same reference numerals for similar components with respect to FIG. 1, shielded probe 14 includes a conductive (e.g., copper) encasement 52, isolation layer 30, lining 28, acoustic couplant 26,

30 acoustic window 24, piezoelectric (PZ) element 32, conductive layer 40, and non- metallic acoustic backing 34. In this embodiment, conductive encasement 52 is an integral (i.e., undivided) conductive member which corresponds to both outer casing 22 and waveguide element 42 of the embodiment shown in FIG. 1. Lining

_ _ 28 dampens acoustic coupling between encasement 52 and PZ element 32 and may be made of a silicone rubber compound (e.g., RTV). Conductive layer 40 may be provided by depositing (e.g., sputtering) a thin layer (e.g., 50 nm) of a conductive material (e.g., nickel) onto both the front surface of PZ element 32 and the RTV lining 28, such that they form a continuous layer when PZ element 32 and its backing 34 are inserted into the lining bounded region. Alternatively, conductive layer 40 may be provided by depositing a thin, continuous layer of conductive material on the front, side, and bottom (except for a central circular region) surfaces of PZ element 32 . The back electrode 32a of PZ element 32 is also a thin layer of conductive material (e.g., 50 nm of nickel) and is connected to the inner conductor 36b of mini-coaxial cable 36. The outer shield conductor 36a (which is typically grounded) of the mini-coaxial cable is connected (e.g., soldered) to the outer casing 22, which is electrically connected to conductive layer 40. The smooth, continuous outer surface structure of conductive encasement 52, provided by the overall design of shielded probe 20 renders it well suited for implementation (e.g., mounting in a housing) as a packaged ultrasound array (e.g., linear array, two-dimensional matrix array, circular/cylindrical array with probes oriented radially to receive ultrasound signals from a two-dimensional slice or slab, etc.). By way of example, an illustrative circular/cylindrical ultrasound sensor array implementation is disclosed in concurrently filed International Application No. , entitled "Ultrasound Array and Electrode Array for Hall Effect Imaging", which claims priority to U.S. Provisional Patent Application Serial No. 60/102,478, filed on September 30, 1998. As may be appreciated, and as will be further explained hereinbelow, a shielded probe design according to the present invention, such as each embodiment illustrated in FIGS. 1 and 2, is well suited for eliminating or mitigating noise sources which may arise during HEI measurements. For example, isolation layer 30 and lining 28 provide acoustic isolation and damping, acoustically isolating the PZ element 32 from the Lorentz force induced vibration of the outer casing 22 (as further explained hereinbelow). Additionally, waveguide element 42 (or, for

FIG. 2, the portion of outer casing 22 extending from the PZ element) and acoustic couplant 26 form a waveguide which reduces direct cross-talk noise. Also, a non- metallic (e.g., alumina) backing 34 does not generate noise. Further, the small thickness of the deposited conductive layer(s) 40 ensures that their Lorentz vibration noise is at negligible levels. Additional aspects and features of noise sources and their elimination or reduction in accordance with the present invention and with the illustrative embodiments of FIGS. 1 and 2 may be appreciated in view of the description which follows the ensuing description of a further embodiment of the present invention.

More specifically, in accordance with another embodiment of the present invention, active shielding is implemented to compensate electromagnetic fields generated near the probe aperture, such as those which are induced by the electrical excitation pulse in HEI, thus mitigating or eliminating eddy currents which may result in noise from Lorentz induced vibrations. As will be explained in more detail in the hereinbelow section "Measurement and Shielding of Lorentz Vibration Noise", the Lorentz vibration noise is the part of the coherent noise which only occurs in the static magnetic field, and may be understood as follows. The excitation pulse applied to the sample also produces rf electric and magnetic fields in the vicinity of the PE probe. These rf fields induce eddy currents in the metallic components of the probe. Referring generally to FIGS. 1 and 2 , these components include the electrode platings of the PE element, the acoustic backing if it is metal, and also the metal walls of the waveguide if it is used for shielding. In the presence of the static magnetic field, the Lorentz forces on the eddy currents cause vibrations in these components. These vibrations either directly enter the piezoelectric element or propagate into the tank and create spurious echoes. Both result in coherent noise, which is called the Lorentz vibration noise.

Referring now to FIG. 3, there is shown a schematic cross-sectional view of another embodiment of a shielded ultrasound probe 50 which includes a coil pair to generate a magnetic field which compensates or nulls the magnetic field induced in the vicinity of the shielded ultrasound probe. Using the same reference numerals for similar components with respect to FIGS. 1 and 2, shielded ultrasound probe 50 of FIG. 3 includes waveguide element 42, acoustic window 24, acoustic couplant 26, and cylindrical membrane 61 (e.g., a plastic cylindrical member which contains acoustic couplant 26). It is noted that for purposes of clarity of exposition, probe device 47 is generically depicted in a block form and may embody various elements (e.g., PZ element, backing, etc.) of an array or single element ultrasound probe. Similarly, an outer casing is not shown and waveguide element 42 is not explicity shown as being connected to such a casing or to some other grounded (or fixed potential) conductor or shield; however, it may be understood that such a casing or shield surrounding probe device 47 may be implemented in various ways, and that waveguide element 42 may be connected in various ways to such a casing, shield or other grounded element.

As shown in FIG. 3, a cylindrical compensation coil element 62, for example made of a single-sided flexible printed circuit (pc) board, is positioned between waveguide element 42 and probe device 47, and is sandwiched between cylindrical acoustic insulation foam layers 60a. and 60b Coils are patterned on the pc board of cylindrical compensation coil element 62, and are connected via a coaxial cable 67 penetrating waveguide element 42 (e.g., using a via/feedthrough) to a current driver (not shown) which provides the precise amplitude and shape of a pulsed current to cancel the rf magnetic field in the vicinity of the piezoelectric element(s) of the probe element 47 which is induced by an excitation pulse (e.g., an

HEI excitation pulse).

FIG. 4 illustrates a plan view of compensation coil element 62 when it is not flexed into a cylindrical shape but is relaxed and substantially flat. It is seen that compensation coil element 62 includes printed circuit board 64 (e.g., single cladded pc board) and patterned (e.g., etched) conductor 66 comprising current loop 66a. (square coil) and current loop 66b (square coil). By way of example, pc board 64 may be implemented as fiber-glass reinforced teflon of about 0.02 inches (i.e., about 20 mils) thickness to provide the desired flexibility. For clarity, reference lines 68a and 68b are shown as the center lines of the coil squares; these center lines are diametrically opposed on the cylinder when pc board 64 is flexed in the assembled probe. Conductor 66 terminates in two driving ports or terminals 66c and 66d which are connected to the current driver (not shown) via coaxial cable 67. As may be seen, loops 66a and 66b are serially coupled such that when they are current driven (i.e., made to conduct a current, regardless of how the current is generated) their respective magnetic fields add at the center of the cylinder.

It is appreciated that such compensating coil pairs may be implemented in various ways depending on various factors such as the system design and application in which one or more such probes are implemented, which relates to factors such as the magnitude of the fields present, the spatial uniformity and orientation of the electric-pulse-induced magnetic fields relative to the ultrasound probe(s), and the presence of other shielding techniques. For instance, in the circular array described in concurrently filed International Application No. , entitled "Ultrasound Array and Electrode Array for Hall Effect Imaging", which claims priority to U.S. Provisional Patent Application Serial No. 60/102,478, filed on September 30, 1998, depending on the overall diameter of the circular array, it may be sufficient to implement pairs of compensating coils equal in number (for example) to the number of (opposing) electrode pairs (instead of an individual pair of compensating coils for each ultrasound probe in the array), each (opposing) electrode pair in effect being associated with an effectively transverse pair of compensating coils. These compensating coils could, for example, be provided as thin-film conductive loops patterned on an inner portion of the dielectric substrate that houses the ultrasound sensor array. In configurations, however, where several ultrasound probes concurrently receive acoustic signals but are subject to substantially dissimilar electric-pulse-induced magnetic fields (e.g., dissimilar in magnitude and/or direction), it may be preferable for each probe to have a dedicated compensating coil.

The ensuing description elaborates additional aspects, features, and principles related to HEI, to noise sources present in HEI, and to reduction and/or elimination of such noise sources in accordance with the present invention and with the hereinabove described embodiments. This ensuing description also includes several examples which are also presented to illustrate features and characteristics of the present invention, which is not to be construed as being limited thereto.

3. Hall Effect Imaging Signal Strength An arrangement of a typical HEI experiment is shown in FIG. 5.

The sample to be imaged (shown here as a polycarbonate block 76) is immersed in a tank 72 of saline 74. The saline concentration may be, for example, 0.5% to mimic biological conductivities. The tank is placed in a static magnetic field Bo, shown as being applied in the "y"direction, out of the page. A pulsed electric field E(r, t) is applied to the saline along the "x" direction via two electrodes 70 which are coupled to signal/pulse generator 78. In the IEEE article, supra, it is shown that ultrasonic pulses are produced at conductivity discontinuities, such as the saline-block interfaces. The ultrasonic signal is detected along the "z" direction with a piezoelectric probe 80.

To obtain a general estimate of the HEI signal level, note that the quasi-static approximation is valid at medical ultrasonic frequencies (below 10 MHz), so that the electric field in the saline follows the same time course everywhere. E(r, f) can then be expressed as Eo(t)g(r), where g(r) represents the spatial distribution. Following the derivation in the IEEE article, supra, assuming that the block 76 is acoustically matched to saline 74 so that there is no acoustic reflection at the interface, the HEI ultrasound signal in the vicinity of the saline- block interface is expressed in terms of the acoustic pressure:

p(t) = ^ Bo P_s c [A(-)] g_x Q(t) , (1)

2 p where p_s is the mass density of saline, c is the speed of sound in saline, g_x is the x component of g(r), Δ(σ/p) is the difference in the ratio of conductivity to mass density, σ/p, between saline and the polycarbonate block, and Q(t) is the integral

This equation allows an estimate of the signal level. For example, in a 2.0 tesla magnetic field, an electrical pulse of 0.5 μs duration and 1600 V/m magnitude produces an ultrasonic pulse of 1.1 Pascal peak pressure.

It can be seen, therefore, that the probes in HEI are required to detect signals on the order of 1 Pascal. For piezoelectric transducers, the corresponding output voltages are on the order of μV's, which is commensurate with the coherent noise from EM coupling between the excitation pulse and the PE probes. In the following the EM cross-talk noise and the Lorentz vibration noise are discussed separately. o

4. EM Cross-Talk Noise

The mechanism of electromagnetic cross-talk between the excitation pulse and the PE probe may be understood as follows. When the excitation pulse is applied to the sample, it also produces an electromagnetic field in the vicinity of the PE probe. The PE element of the probe acts as an antenna and picks up the field, resulting in a voltage across it. This voltage induces strains in the element and results in acoustic emissions into the sample. The echoes of these acoustic emissions from within the sample are received later by the PE probe and appear as coherent noise, but at 0 double the elapsed time of the corresponding HEI signals. The EM cross-talk is independent of the static magnetic field employed in HEI, and therefore can be studied outside the magnet, as described below.

Experiment 1 5 To measure the level of the cross-talk noise, an experimental setup shown in FIG. 6A was used. For reasons as stated above, the experiment was conducted outside the presence of a static magnetic field. Two brass electrodes 70 were placed in 0.5% saline 74 contained in a tank 72 of dimensions 23.5 cm x 13.5 _π cm x 5.8 cm (height). The electrodes 70 were connected to a pulse generator

(Panametrics not shown) which produced an exponentially decaying pulse of 400- volt peak voltage and 300-ns half-height width. The ultrasonic probe 80, shown in more detail in FIG. 6B, was a spherical-focused piezoelectric 2.25 MHz, 1.84 cm- diameter broadband transducer (KrautKramer-Branson) with a focal length of 76 5 mm, and which includes piezoelectric element 82 (including electrode plating), acoustic matching layer 88, tungsten backing 81, and metal jacket 83. A 1.0 cm layer of silicone oil 75 was poured onto the saline 74 to prevent direct electrical contact between the metal jacket of the probe and the saline. 0 Averaging over multiple data acquisitions was used to isolate coherent noise from random electronic and acoustic noises. FIG. 7 is the signal from the probe after averaging over 1000 acquisitions. Since no magnetic field was present, the peaks in FIG.7 are not Hall effect signals. By their times of flight, they can be assigned to echoes of a pulse emitted by the PE probe 80 due to EM cross-talk between the 5 excitation pulse and the probe. More specifically, the peaks are echoes from the interfaces in the saline tank, wherein: peak (A) is the echo from the silicon oil-saline interface; peak (B) is the echo from the bottom of the saline tank; and peak (C) is the multiple reflection echo from the silicone-saline interface and the bottom of the tank. Although the front surface of the PE element 82 in the probe 80 is sputtered with a nickel layer, the thickness of this layer is smaller than the rf penetration depth at 2.25 MHz (i.e., 100 μm). Thus, the PE element 82 can still pick up a small voltage from the excitation pulse. A direct approach to reducing the cross-talk noise measured in FIG. 7 would be to put shielding layers in front of the probe 80 that are much thicker than the rf penetration depth. Generally, however, it is difficult to find a material which does not disrupt the acoustic signal at those thicknesses.

Experiment 2

In accordance with the present invention, a shielded ultrasound probe as depicted in FIG. 1 was constructed and tested to demonstrate the efficacy of an electromagnetic waveguide to reduce cross-talk noise in an ultrasound probe. This shielded ultrasound probe was constructed by modifying a commercial ultrasound probe as used in Experiment 1 to include waveguide element 42, acoustic couplant 24, isolation layer 30, and acoustic window 26, as shown in FIG. 1. Acoustic backing 34 was metallic (i.e., tungsten), as per the manufacture of the commercial probe, and was not modified.

More specifically, referring to FIG. 1, waveguide element 42 was a thin copper cylinder coaxial to the probe, with the proximal end connected to the metal jacket of the probe and the distal end capped with acoustic window 24 implemented with a cellophane membrane. The diameter of the waveguide was 26 mm, and the distance between the piezoelectric element surface and the cellophane membrane was 37 mm. The waveguide was filled with mineral oil as the acoustic couplant 26. Isolation layer 30 was implemented as a foam layer for insulation against acoustic noise (as will be explained in detail later).

The experimental test of this shielding scheme was slightly modified from the setup in FIG. 6A. The silicone oil layer was no longer needed, as the mineral oil in the waveguide acts as the electrical insulator. FIG. 8 shows the result after averaging over 9000 acquisitions. The EM cross-talk noise was suppressed by at least a factor of 200 compared to the results of Experiment 1 where a conventional probe was used without a waveguide structure (note the difference in the vertical scale between FIG. 7 and FIG. 8), and the cross-talk noise cannot be seen above the random noise floor, which was reduced to 0.01 μV by the extensive averaging. The waveguide shielding technique, therefore, clearly and significantly suppressed cross-talk noise. To further appreciate the waveguide shielding mechanism, in the subsequent section hereinbelow, it is shown that the electromagnetic field decreases roughly exponentially into the interior of the waveguide; thus, with sufficient distance between the probe piezoelectric element and the distal end of the waveguide, the EM cross-talk noise should be greatly reduced.

For clarity, it is also noted that with the mentioned reduction of the noise floor, two more coherent peaks of approximately 0.1 μV amplitude were revealed. These peaks are not part of the cross-talk noise, since their times of flight are exactly half those of the expected cross-talk peaks from the waveguide-saline interface and the bottom of the saline tank. This suggests that they were generated at these interfaces at the same instant the excitation pulse was applied, and are most likely due to thermoelastic expansions of the saline body caused by ohmic heating of the excitation pulse. In particular, peak (A) is the thermoelastic signal from the waveguide-saline interface, and peak (B) is the thermoelastic signal from the bottom of the saline tank. The significance of this thermoelastic expansion phenomenon is discussed further hereinbelow, after description of the Lorentz induced noise experimental measurements.

Waveguide Shielding Model

A simplified model of the waveguide is a semi-infinite cylinder 100 of radius as schematically depicted in FIG. 9. A uniform static electric field Eo is applied perpendicular to the axis of the waveguide. Assuming that the waveguide is at ground potential, the potential function Fin the interior of the waveguide satisfies the following equation, V ²V = 0, r < a,

(3) V = 0, r = a.

Because of the azimuthal dependence imposed by the asymptotically uniform external field, the solution to Eq. (3) can be written as a summation of 1st order Bessel functions:

where the k_t's are the roots of the equation

J,(k,a) = 0. (5)

This expression shows that the electric field generally follows a multiple exponential decay into the waveguide. The first term exp(- ιz) with the smallest root k_\ dominates at large z. For 1st order Bessel functions, k_\a = 3.832; thus the electric field at depth z into the waveguide decays as exp(-3.832 z/a). With the dimensions used in Experiment 2, this means the waveguide reduces the electric field by a factor of 3χl0⁴.

Similar reasoning can be applied to the shielding of an rf magnetic field, assuming that the waveguide cylinder is perfectly conducting and thus prevents flux from running through its side wall. It can be shown that the dominant term in the magnet field is exp(-h_\z), where h_\ is the first root of the equation

J\'(h_\a)=0. With the waveguide dimensions used in the experiment, the magnetic field is suppressed by approximately a factor of 2x10².

These analyses do not give the exact solutions to the field distributions in the waveguide, which would involve solving multiple integral equations; however, they provide order-of-magnitude estimates of the shielding effect, elucidate the effect of the waveguide, and clarify design principles for implementing a waveguide shielded ultrasound probe. o

5. Measurement and Shielding of Lorentz Vibration Noise

As mentioned above, the Lorentz vibration noise is the part of the coherent noise which only occurs in the static magnetic field, and may be understood as follows. The excitation pulse applied to the sample also produces rf electric and magnetic fields in the vicinity of the PE probe. These rf fields induce eddy currents in the metallic components of the probe. Referring generally to FIGS. 1-3 and 6B, these components include the electrode platings of the PE element, the acoustic backing if it is metal, and also the metal walls of the waveguide if it is used for shielding. It is noted that in each of the waveguide-shielded probes shown in

FIGS. 1-3, the waveguide cylinder is acoustically insulated from the probe (e.g., isolation layer 30) and the saline (because the acoustic complant which couples to the imaging region (e.g., saline) extends beyond the distial end of the conductive waveguide element which can thus be electrically and acoustically separated from 5 the imaging region (e.g., saline)); thus the Lorentz vibration noise comes from the metallic components inside the probe. In the presence of the static magnetic field, the Lorentz forces on the eddy currents cause vibrations in these components. These vibrations either directly enter the piezoelectric element or propagate into the tank 0 and create spurious echoes. Both result in coherent noise, which is called the

Lorentz vibration noise.

To further elucidate the mechanism by which induced currents arise, as well as their reduction based on active compensation according to the present invention, FIG. 10 shows the electric (indicated by arrows) and magnetic fields 5 (indicated by the conventional "pencil point" notation as directed out of the page) around the probe 10 of FIG. 1 produced by an excitation pulse. A conservative electric field Eo results from the voltage difference between the electrodes, and a solenoidal rf magnetic field B is generated by the current flowing in the saline tank. 0 A small portion of these fields reaches into the waveguide and induces currents in the probe. It is shown in the hereinbelow section "Relative Eddy Current Magnitude" that the current Is induced by the magnetic field B is of the order of 10 times the current I_E induced by the electric field Eo. Thus, it is appreciated that the 5 rf magnetic field is the dominant source of the Lorentz vibration noise. Based on this estimation, active cancellation of the rf magnetic field in accordance with the present invention should greatly reduce the Lorentz vibration noise, as was experimentally observed as described hereinbelow.

More particularly, first an experiment was conducted to measure the Lorentz vibration noise in a waveguide-shielded ultrasound probe which did not incorporate active shielding. Second, the Lorentz vibration noise was experimentally measured for a shielded ultrasound probe made in accordance with the invention disclosed in International Application No. PCT/US98/24152 (publication No. WO 99/24967), which claims priority to U.S. Provisional Patent Application Serial No. 60/065, 1 11. Finally, the Lorentz vibration noise was measured for an ultrasound probe having both waveguide shielding and active compensation shielding in accordance with an embodiment of the present invention. These measurements clearly demonstrated the efficacy of actively compensating the electric-pulse- induced magnetic field in a waveguide-shielded ultrasound probe.

Experiment 3

To measure the Lorentz vibration noise, an HEI experiment was carried out with a waveguide shielded piezoelectric probe (i.e., shielded by the waveguide as described above in connection with FIG. 1 and with Experiment 2, and not including any active shielding). Because the waveguide removes the EM cross-talk noise, any additional noise will be related to the magnetic field. The experimental setup was the standard one shown in FIG. 5. The excitation pulse was an exponentially decaying pulse of 400 volt peak voltage and 300 ns width. A polycarbonate target block of 6.2 cm (width) x 1.2 cm (thickness) was placed in a saline tank of 23.5 cm x 13.5 cm x 6.0 cm (height). A 2D cross-sectional image of the tank was acquired by moving the probe at 0.5 cm increments across the saline surface and collecting a scan line at each position. FIG. 11 is the image from 26 line scans at 1000 averages. Although the boundaries of the block are visible, the amplitude of the coherent noise was approximately 0.7 μV, which is at least 50 times higher than the noise level outside the magnet (FIG. 8) and 40% that of the maximum HEI signal.

To reduce the Lorentz vibration noise from the waveguide shield, a foam layer (isolation layer 30 of FIG. 1) was used to acoustically isolate it from the o ultrasonic probe. The rest of the noise is attributable to the metallic parts inside the probe, especially the ones in direct contact with the PE element, including the nickel electrode platings and the tungsten backing (e.g., refer to FIG. 6B for components of the commercial probe)

Experiment 4

As mentioned, two shielded probe techniques were demonstrated as being effective in reducing the Lorentz vibration noise from inside the probe. The first shielded probe tested is shown in FIG. 12, and corresponds to the shielded probe disclosed in International Application No. PCT/US98/24152 (publication No.

WO 99/24967), which claims priority to U.S. Provisional Patent Application Serial No. 60/065,111. This shielded probe design is based on the principle that if the metallic platings on the PE element of probe 112 are made normal to the magnetic - field, then the Lorentz forces on the platings will be tangential to the PE element and therefore will not emit acoustic noise into the PZ element or the sample. With such a probe 112 orientation an ultrasound prism 102 is used to redirect the HEI signal 114 into the probe 112. The prism 102 may be implemented with silicone oil as the acoustic coupling medium. The acoustic reflection surface 106 may be 0 implemented as a thin cellophane membrane. This prism 102 adds an additional acoustic distance between the probe 112 and the sample. To minimize this, the waveguide shield was replaced with a thin conductive film 104 (e.g., a 15μm aluminum foil). This film 104 proved sufficient in shielding the EM cross-talk, 5 which is already reduced by the added distance between the probe and the sample from the prism. The side of the probe was covered with a conductive shield 108 by using a layer of copper tape which was lined with a foam layer 110 to prevent induced currents from developing in the metal jacket of the probe 112. FIG. 13 shows an image of the saline tank acquired with this shielding method at 1000 averages. The Lorentz vibration noise was reduced by at least a factor of 20, and cannot be detected above the noise floor of 0.03 μV.

Experiment 5 5 The above shielded probe technique of Experiment 4 is simple to implement, but it is limited to probes of planar piezoelectric elements and requires careful alignment of the probe with the magnetic field. These problems are avoided in the second method tested, which employed active compensation in accordance with the present invention. In this scheme, compensating electromagnetic fields were used to decrease the induced currents in the metal parts of the probe. As described, this active shielding can be realized with a pair of coil windings appropriately configured to produce a compensating magnetic field near the probe. This field can be adjusted to cancel the field from the current in the saline. Referring to FIG. 14, in this experiment, the coils 122 were supplied by the excitation pulse generator 78 via an adjustable current divider circuit 120, which was manually adjusted to minimize the residual magnetic field. The rest of the experimental setup was essentially identical to that described in FIG. 5 (but using a waveguide-shielded probe, which is not shown for clarity). The resulting acquired image is shown in FIG. 15. The peak due to Lorentz vibration noise (i.e., corresponding to the light trace labeled "LV echo") was just above the random noise floor of 0.03 μV, roughly 1/10 the noise level measured without active shielding (i.e, FIG. 11) and about 5% of the maximum HEI signal. This peak is the echo from the upper surface of the polycarbonate block, where there is a 46% acoustic impedance mismatch. In biological samples the mismatch of soft tissue interfaces is below 5%, and the Lorentz vibration noise is expected to be lower still.

As may be appreciated, compared to the ultrasound prism method of Experiment 4, active shielding does not limit the orientation of the probe, and permits real-time compensation with feedback circuits. Additionally, precise orientation of the compensating field is not critical because the compensation current is actively set to null the undesirable peaks and because the current needed to null these peaks is a broad function of orientation angle. More sophisticated current control schemes may allow the compensation field to be both amplitude and phase-matched to the excitation field for precise field nulling at all relevant frequencies. For these reasons active shielding may be more suitable for in vivo applications. Relative Eddy Current Magnitude Estimation

To more fully appreciate aspects and features of an actively shielded ultrasound probe according to the present invention, the following presents a rough estimate of the ratio between the eddy currents induced by the rf magnetic field and electric field, Iv/k- The electric field E₀ and the magnetic field B affect the probe differently and will be considered separately. As shown hereinabove in the "Waveguide Shielding Model" section, a residual of the E₀ field reaches into the waveguide and gives an electric field E' around the front nickel plating of the PΕ element of the probe. Electric currents arise in the plating such that the resulting charge distribution nulls the electric field in the plating. Denote α as the amount of electric field leakage into the waveguide:

E' ~ a Eo , (6) and denote the radius of the waveguide as a. The current I in the plating provides the charge accumulation q to counter the field E'. Thus

Iι; ~ —r ~ ω q ~ ω a² ε_εo E' - a co a ε _εoEo ■ (7) dt where ε is the relative dielectric constant of the liquid in the waveguide.

Similarly, the rf magnetic field leakage into the waveguide B' can be described as

B'~ β B , (8) and the induced currents 7_B in the nickel platings and the metallic backing of the PE element counter this magnetic

_B'- l± _{. (9)} a

Thus

I_B ~ B'a/μ₀ ~ βaB/μ_t o - (10)

The magnetic field B is related to the current in the saline tank, and therefore the electric field E₀. If the current in the tank is 7₀, the size of the tank is d, and the conductivity of the saline is σ, then

Substituting Eq. (11) into Eq. (10) gives

~ β ∑₀ σ a d . (12)

Comparing Eq. (12) and Eq. (7). the ratio IQ I_B can be estimated:

Using the length dimensions of the experimental setup, and assuming that electric and magnetic field leakage into the waveguide are comparable (α and β in Eq. (13) are similar), the ratio is approximately 8χl0³ at 1MHz. According to the "Waveguide Shielding Model" section, the electric field leakage is smaller than the magnetic field leakage by two orders of magnitude; thus the actual ratio ofls/ can be even larger. This estimate, therefore, shows that the rf magnetic field is the dominant source of induced currents in the probe.

In view of the foregoing description, including description of the embodiments, experiments, noise sources, and models of the noise sources and/or shielding techniques, it may be appreciated that, in accordance with the present invention, active shielding is preferably implemented in combination with waveguide (passive) shielding of the EM cross-talk noise. Additionally, it may be appreciated that since the rf magnetic field is typically the main source of the Lorentz vibration noise, there are various features of the piezoelectric probe design which are preferably designed to minimize the eddy currents. For instance, non- metallic materials should be used for acoustic backings (e.g., acoustic backing 34). Additionally, eddy currents in the PE element electrodes may be reduced by minimizing the electrode plating thickness. Further, other metallic components, including the outer casing and the connector should be acoustically insulated from the PE element. These measures combined with the waveguide shield may reduce the Lorentz vibration noise to a level where the shielding methods described above will not be needed in certain implementations or applications. . It should be noted that because the forward and reverse mode of HEI are strictly reciprocal, all of the shielding methods described above (i.e., including waveguide, active compensaton, and ultrasound prism) are also applicable to the forward mode.

It may also be appreciated that in incorporating the hereinabove described shielding principles into clinical imagers or piezoelectric arrays, additional factors must be addressed. For example, the waveguide, necessary for eliminating the EM cross-talk, will also modify the diffraction pattern of the piezoelectric transducer. For the designs tested hereinabove the distortion was not significant, but it may become a problem for wide acceptance angle array transducers. Modification of the waveguide shape from cylindrical to conical may reduce the distortion, although the overall waveguide length would need to be increased for the same degree of cross-talk noise isolation. An additional illustrative factor is that the waveguides act as acoustic standoffs and may cause reverberations, as seen in FIG. 7. To reduce these reverbations, care should be exercised to maintain good impedance matching and acoustic contact (e.g., no trapped air bubbles) between the sample and the end of the waveguide.

As mentioned above, FIG. 8 shows that after EM cross-talk noise was removed with the waveguide shield, there were residual peaks which were likely from the thermoelastic expansions of the saline tank, as suggested by their times of flight. This thermoelastic signal is due to ohmic heating of the sample medium by the excitation pulse, and therefore it is expected to scale with the square of the current density in the sample. With the 400-volt 300-ns excitation pulse, the amplitudes of these peaks were about 30dB below the HEI signal at 2.5 tesla. In measurements with a larger pulser capable of producing a sine wave of 1 μs period and 3 kV amplitude, the thermoelastic signal reached 1/4 of the HEI signal. These two signals cannot be distinguished by timing, since they both occur simultaneously with the excitation pulse, and they both occur at conductivity discontinuities. They can, however, be separated by reversing the polarity of the electrodes relative to the static magnetic field. The HEI signal reverses sign under this condition, while the thermoelastic signal does not.

With high voltage pulsers, the thermoelastic effect may itself be a suitable mechanism for imaging. To have sufficient sensitivity, the peak excitation current density should be approximately 2 amperes/cm³ or higher, depending on the required spatial resolution. The mechanism of the thermoelastic effect involves not only the conductivity of the medium but also its heat capacity and thermal expansion coefficient, and possibly other parameters. Thus, the thermoelastic signal contains information that is different from both echo-based ultrasound and HEI. It is noted that another source of coherent signal that also occurs concurrently with the HEI signal is the electro-acoustic effect, or "electro-sound", in electrolytes. Although the details of this phenomenon are not completely understood, the general mechanism is that under an external electric field, the translational motion of charged molecules in an electrolytic sample pulls the surrounding medium with them and creates acoustic pressure waves. The electro- acoustic signal is generally emitted along the electric field, as opposed to the HEI signal, which generally propagates perpendicular to the electric field. However there is mixing of the two to various degrees. In certain media such as cooked egg white, where the charged molecules are large or extensively cross-linked, the electro- acoustic signal was observed to be as large as the HEI signal at 2.5T. In more dilute media such as agarose, it decreased to 10 to 20 dB below the HEI signal. The electro-acoustic effect inherently occurs in electrolytic media, and it can be separated from the HEI signal by changing the direction of the static magnetic field relative to the sample.

It is further noted that an alternative to piezoelectric sensors which completely avoids the electromagnetic interference problem is optical ultrasonic sensors, such as laser beam based techniques and optical-fiber based sensors. Currently these technologies have not reached the sensitivity and robustness of piezoelectric transducers, but they hold potential for ultrasonic imaging applications where EM interference is expected.

In summary, in view of the foregoing it may be appreciated that shielded ultarsound probes in accordance with the present invention provide many features, advantages, and attendant advantages which are particularly well suited for use in HEI which typically emplosys large electric excitation pulses which induce coherent electromagnetic interference noise in piezoelectric probes used for signal reception. As described, two main noise mechanisms present in HEI are direct cross-talk between the probe and the excitation pulse, and the Lorentz vibration noise. In accordance with the present invention, a waveguide shield has been shown to remove the cross-talk noise. The Lorentz vibration noise, which in typical configurations is mostly induced by the rf magnetic field produced by the excitation pulse, has been shown to be effectively eliminated by employing an active compensating magnetic field in accordance with the present invention. Additionally, it is appreciated that the active compensation technique according to the present invention has certain advantages relative to using an ultrasound prism type probe which, however, was also shown to be effective in effectively eliminating the Lorentz vibration noise.

It may further be appreciated that HEI technologies can benefit greatly from the existing tools of echo-based sonography, including sensor arrays, data acquisition and data processing; however, one critical issue is the adaptation of the piezoelectric array transducers. With a better understanding of the electromagnetic interference (EMI) issues and the shielding methods described hereinabove, it may soon be possible to test HEI with modified conventional echo scanners. Further, as the same EMI-related noise sources are also present in combined ultrasound-MRI applications, these shielding methods may also be useful and applicable.

Although the above description provides many specificities, these enabling details should not be construed as limiting the scope of the invention, and it will be readily understood by those persons skilled in the art that the present invention is susceptible to many modifications, adaptations, and equivalent implementations without departing from this scope and without diminishing its attendant advantages. It is therefore intended that the present invention is not limited to the disclosed embodiments but should be defined in accordance with the claims which follow.

Claims

We claim:

1. An apparatus, comprising:

an ultrasound transducer element; and an electromagnetic waveguide coupled relative to said ultrasound transducer element such that acoustic energy detected by said ultrasound transducer first traverses a region substantially bounded by the electromagnetic waveguide.

2. The apparatus according to claim 1, wherein said electromagnetic waveguide is a cylindrical waveguide.

3. The apparatus according to claim 1, wherein said electromagnetic waveguide includes an outer conductive member that defines a boundary of the electromagnetic waveguide.

4. The apparatus according to claim 3, wherein said electromagnetic waveguide includes an acoustic couplant medium within said outer conductive member.

5. The apparatus according to claim 4. wherein said acoustic couplant medium includes a portion that extends beyond a distal end of said outer conductive member.

6. The apparatus according to claim 4, wherein said acoustic couplant medium and said outer conductive member are acoustically isolated.

7. The apparatus according to claim 1. wherein conductive portions of said electromagnetic waveguide element are acoustically isolated from said ultrasound transducer element.

8. The apparatus according to claim 1, further comprising a conductive shield layer substantially enclosing said ultrasound transducer element.

9. The apparatus according to claim 8. wherein said conductive shield layer is electrically connected to said electromagnetic waveguide.

10. The apparatus according to claim 1, further comprising a conductive element that can be current driven to generate an electromagnetic field that compensates electromagnetic fields in the vicinity of the ultrasound transducer.

11. The apparatus according to claim 10, wherein said conductive

5 element is substantially located in a region bounded by the electromagnetic waveguide.

12. The apparatus according to claim 10, wherein said conductive i_Q element includes a pair of conductive loops respectively oriented such that their respective magnetic fields are constructively additive.

13. The apparatus according to claim 12, further comprising a flexible printed circuit board upon which said pair of conductive loops are

15 patterned.

14. The apparatus according to claim 10, wherein said conductive element and said acoustic couplant are acoustically isolated.

^ 15. The apparatus according to claim 10, wherein said conductive element and said ultrasound transducer element are acoustically isolated.

16. An apparatus comprising:

an ultrasound transducer element; and

25 a conductive element that can be current driven to generate an electromagnetic field that compensates electromagnetic fields in the vicinity of the ultrasound transducer element.

17. The apparatus according to claim 16, wherein said conductive 30 element is substantially located in a region bounded by an electromagnetic waveguide that is coupled relative to said ultrasound transducer element such that acoustic energy detected by said ultrasound transducer first traverses a region substantially bounded by the electromagnetic waveguide.

35 18. The apparatus according to claim 16, wherein said conductive element includes a pair of conductive loops respectively oriented such that their respective magnetic fields are constructively additive.

19. The apparatus according to claim 18, further comprising a flexible printed circuit board upon which said pair of conductive loops are patterned.

20. An apparatus, comprising:

means for transducing an acoustic signal into an electrical signal; and means for selectively guiding electromagnetic radiation according to frequency to shield said transducing means from electromagnetic fields.

21. The apparatus according to claim 20, wherein said guiding means is oriented such that acoustic energy detected by said transducing means first traverses a region substantially bounded by the guiding means.

22. The apparatus according to claim 20, further comprising means for actively shielding said transducing means from electromagnetic fields.

23. An ultrasound transducer, comprising:

means for transducing an acoustic signal into an electrical signal; and means for actively shielding said transducing means from electromagnetic fields.

24. The apparatus according to claim 23, wherein said means for actively shielding is substantially located in a region bounded by a means for selectively guiding electromagnetic radiation according to frequency to shield said transducing means from electromagnetic fields, said guiding means oriented such that acoustic energy detected by said transducing means first traverses a region substantially bounded by the guiding means.

25. The apparatus according to claim 24, wherein said means for actively shielding includes a conductive element that can be current driven to generate an electromagnetic field that compensates electromagnetic fields in the vicinity of the transducing means.

26. The apparatus according to claim 25, wherein said conductive element includes a pair of conductive loops respectively oriented such that their respective magnetic fields are constructively additive.