WO1999024967A1

WO1999024967A1 - Shielded ultrasound probe

Info

Publication number: WO1999024967A1
Application number: PCT/US1998/024152
Authority: WO
Inventors: Han Wen; Eric B. Bennett
Original assignee: The Government Of The United States Of America As Represented By The Secretary, Department Of Health And Human Services
Priority date: 1997-11-12
Filing date: 1998-11-12
Publication date: 1999-05-20
Also published as: EP1031135A1; AU1403299A

Abstract

A shielded ultrasound transducer (10) which includes an ultrasound probe (16) having an acoustic aperture and enclosed by a conductive member (18). The transducer includes an ultrasound reflector (32) which redirects acoustic signals from a first direction into a second direction incident on the acoustical aperture of the ultrasound probe. The portion (18a) of conductive member that encloses the acoustic aperture is substantially planar, and is oriented substantially perpendicular to a magnetic field direction present during use of the ultrasound transducer (10). The conductive member may include a first conductive shield layer (18) and a second conductive shield layer (20) which surrounds the first conductive shield region (18) except in a portion of the first conductive shield region (18) that encloses the acoustic aperture of the ultrasound probe. The shielded ultrasound probe (10) is well suited for use with Hall Effect Imagining (HEI), and for implementation for modifying or adapting a conventional commercial ultrasound scanner or array probe.

Description

SHIELDED ULTRASOUND PROBE

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.

BACKGROUND OF THE INVENTION

Recently, a new imaging method referred to as Hall Effect Imaging (HEI) was invented, and HEI is described in International Application No. PCT/US97/11272 filed July 2, 1997, and in U.S. Provisional Application No. 60/021,204 filed July 3, 1997, each in the name of one of the present inventors, and each of which is incorporated herein by reference. In one 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 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. More specifically, 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 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).

Although conventional ultrasound transducers (probes) may be used to detect the ultrasound signal in HEI, 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.). 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 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 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 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 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 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).

It is appreciated, therefore, that further advancements in ultrasound transducers are needed, particularly to provide an ultrasound probe which eliminates, mitigates, or is otherwise not susceptible to electrical interference or to electromagnetically induced ultrasonic noise or interference, and which is 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.

SUMMARY OF THE INVENTION

The present invention overcomes the above mentioned problems and other limitations, by providing a shielded ultrasound transducer which includes an ultrasound probe having an acoustic aperture and enclosed by a conductive member. The transducer includes an ultrasound reflector which redirects acoustic signals from a first direction into a second direction incident on the acoustic aperture of the ultrasound probe. In accordance with an aspect of the present invention, the portion of the conductive member that encloses the acoustic aperture is substantially planar, and is oriented substantially perpendicular to a magnetic field direction present during use of the ultrasound transducer. In accordance with another aspect of the present invention, the conductive member includes a first conductive shield layer and a second conductive shield layer which surrounds the first conductive shield region except in a portion of the first conductive shield region that encloses the acoustic aperture of the ultrasound probe.

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 drawings, wherein:

FIG. 1 shows a schematic cross-sectional side view of an embodiment of a shielded ultrasound transducer 10 in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

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 KB A Alpha 1.0

MHZ/0.75 diameter transducer, and the hereinbelow described embodiment of the present invention is representative of such a modification of these 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.

Referring now to FIG. 1, there is shown a schematic cross-sectional side view of an embodiment of a shielded ultrasound transducer 10 in accordance with the present invention. Ultrasound transducer 10 includes preamplifier 12, coaxial coupling 14, ultrasound probe 16, first conductive shield layer 18 having continuous surface portions 18a, 18b, and 18c, second conductive shield layer 20, ultrasound prism 22 having reflective surface 32, probe encasement 26, and acoustic coupling medium 28.

In operation, an ultrasound (acoustic) signal 30 (generated by some means, e.g., HEI) propagates from the sample into the ultrasound prism 22, which reflects ultrasound signal 30 into the acoustic aperture of ultrasound probe 16. Ultrasound probe 16 transduces the acoustic signal to provide an electrical signal to preamplifier 12 via coaxial coupling 14. Preamplifier 12 amplifies the electrical signal, providing an amplified signal via a coaxial signal line (not shown) to additional electronics (not shown) for further acquiring and processing the amplified signal representative of the ultrasound (acoustic) signal.

Preamplifier 12, ultrasound probe 16, and coaxial coupling 14 may be conventional elements present in conventional, commercial ultrasound transducers such as those described above. The outer housing of preamplifier 12 is preferably conductive to shield the enclosed preamplifier circuitry, and is also preferably grounded. As described, preamplifier 12 is electrically coupled via coaxial (shielded) coupling 14 to receive electrical signals generated (transduced from acoustic vibrations) by ultrasound probe 16.

Ultrasound probe 16 generally includes a piezoelectric material and two opposing substantially planar conductive electrodes, one of which provides the acoustic aperture (i.e., the surface area of ultrasound probe 16 onto which acoustic signals impinge). Typically, and in the present embodiment, the electrode surface of ultrasound probe 16 which provides the acoustic aperture surface is electrically grounded. It is also noted that commercial ultrasound probes may have a slightly curved acoustic aperture surface, with possibly an array of piezoelectric elements either sharing a common ground electrode or having individual ground electrodes, and either of these ultrasound probes may be used for practicing the present invention. Either of these probes can be used to focus or orient the direction of acoustic responsivity, and can be used in this manner to practice the present invention. Alternatively, conventional single-element ultrasound probes may be used.

The typical commercial single-element probe is cylindrical in shape, and may be packaged or otherwise encased in a plastic-like or metallic housing, with the conductive electrodes available for electrical connection (e.g., to coaxial coupling 14). In accordance with the shielding implementation of the present embodiment of the present invention, probe encasement 26 is used in the present embodiment in order to facilitate shielding of a cylindrically shaped ultrasound probe 16, based on the shielding techique (copper foil) used in an experimental probe constructed by the inventors by adapting a conventional commercial ultrasound probe. Probe encasement 26 is generally rectilinear (e.g., cubic) in shape, and includes a cylindrical bore completely through probe encasement 26, the cylindrical axis of the bore normally intersecting opposite faces of probe encasement 26. Ultrasound probe 16 tightly fits into the bore (and may include a surrounding O-ring coaxial with the bore and ultrasound probe cylindrical axes, and which is not shown), and a recessed set screw (not shown) through probe encasement 26, transverse to the cylindrical axis, engages and affixes ultrasound probe 16, such that the acoustic aperture of ultrasound probe 16 is substantially parallel to and preferably substantially flush with the adjacent surface of probe encasement 26. It may be appreciated, however, that probe encasement 26 is not necessary for practicing the present invention, and may not be desirable in alternative embodiments.

First conductive shield layer 18, having continuous surface portions 18a, 18b, and 18c, encloses all surfaces (in all dimensions) of probe encasement 26 (and thus similarly encloses ultrasound probe 16) except for the back surface which is oriented adjacent preamplifier 12. First conductive shield layer 18 extends beyond the back surface of probe encasement 26 and is electrically coupled to the grounded, conductive package of preamplifier 12 along the entire edge (in all dimensions) of the back surface of probe encasement 26. Accordingly, ultrasound probe 16 is substantially enclosed or encompassed by a contiguous layer of conductive material (i.e, conductive shield layer 18, and conductive package of preamplifier 12), which preferably is of high conductivity. As described, the conductive shield layer 18 needs to be thick enough to shield out the electromagnetic fields, and the portion in front of the acoustic aperture of ultrasound probe 16 needs to be thin enough to allow ultrasonic signal to pass without noticeable attenuation and reflection (which is generally satisfied if the thickness is much less than an acoustic wavelength). In practice, 0.001 inch (i.e., 1 mil) thick copper foils were found to be sufficient. These foils included an adhesive backing which facilitated mounting on probe encasement 26. In addition, in order to facilitate acoustic coupling, a thin layer of silicone rubber type compound (e.g., RTV) may be inserted between the front surface portion 18a of conductive shield layer 18 and ultrasound transducer 16 in the region of the acoustic aperture. It will be appreciated, however, that various other conductive materials, thicknesses, and mechanisms for providing the conductive material (e.g., electroplating, evaporation, sputtering, etc.) may be employed. As is also understood, it is not strictly necessary for conductive shield 18 to lack any apertures; for instance, conductive shield layer 18 may include small apertures provided they still render conductive shield layer 18 "opaque" to the electromagnetic radiation present and conductive shield 18 remains electrically contiguous in surrounding ultrasound probe 16.

In order to mitigate or null induced ultrasound vibration effects, ultrasound transducer 10 is shown oriented such that the substantially planar front surface portion 18a of first conductive shield layer 18 which covers, and is preferably parallel to, the acoustic aperture of ultrasound probe 16 is oriented perpendicular to the magnetic field B₀ (e.g., used in HEI). More particularly, as may be appreciated, when the probe is used in the presence of electromagnetic radiation (e.g., pulses) and strong magnetic fields (e.g., if the probe is used in HEI, there is a very strong magnetic field on the order of 1 tesla or more), when the high voltage impulse occurs, the induced currents in the shield layer experience Lorentz forces, which 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 propagate into the sample as unwanted ultrasonic pulses. Avoiding such vibrations in the acoustic area is an important feature of the present invention. It may be understood, therefore, that by making planar front surface portion 18a (of first conductive shield layer 18) which is in front of the acoustic aperture of ultrasound probe 16 perpendicular to the direction of the magnetic field B₀, the induced currents in this planar front surface portion 18a of first conductive shield layer 18 are confined within the layer, and thus the Lorentz forces on them are also parallel (tangential) to the layer (from the right hand rule of

Lorentz force). In this configuration, there is no vibration of planar front surface portion 18a of first conductive shield layer 18 in a direction incident on the acoustic aperture (i.e., in a direction normal to the acoustic aperture plane) of ultrasound probe 16, but rather a sideways sliding motion in a plane parallel to the acoustic aperture plane and to planar front surface portion 18a, which thus does not cause acoustic noise. It is understood that for any specific implementation or use of ultrasound transducer 10, proper orientation may be provided by a variety of electromechanical and/or mechanical mechanisms (e.g., robotic arm, motor driven stages with linear and rotational motion), and that it is possible to also implement a calibration procedure to finely adjust the orientation of the probe to minimize orientation dependent noise (e.g., ultrasonic vibrations coupled into the ultrasound probe from the shield when the shield covering the acoustic aperture is not properly oriented). In the embodiment of the present invention shown in FIG. 1, away from the acoustic aperture, first conductive shield layer 18 bends back to encase the probe (i.e., planar surface portion 18b and 18c), and therefore it loses its perpendicular direction relative to the magnetic field. If these portions of the shield are not precisely parallel to the magnetic field, then they may vibrate such that the vibrations propagate to the acoustic aperture as noise.

In order to eliminate such vibrations, in accordance with an embodiment of the present invention, preferably a second conductive shield layer 20 encompasses the first conductive shield layer 18 except within the acoustic aperture region, with a small air gap in between the two layers to prevent the acoustic vibrations of the second conductive shield layer 20 from reaching the first conductive shield layer 18. Second conductive shield layer 20 is also electrically connected to the conductive package of preamplifier 12. Thus, the only portion of the first conductive shield layer 18 exposed to the electromagnetic field is the acoustic aperture, which, as described above, does not produce acoustic noise. It is appreciated, however, that in the embodiment shown in FIG. 1 with first conductive shield layer 20 encompassing ultrasound transducer 16 and having front surface 18a oriented as described with respect to the magnetic field, second conductive shield layer 20 is not essential for practicing the present invention.

It is also appreciated that myriad and various alternative embodiments are possible for implementing the shielding in accordance with the present invention. For example, since the second conductive shield layer 18 surrounds the first conductive shield layer 16 in all but the acoustic aperture area, it is not necessary that the inner shield include portions which are not parallel to the magnetic field direction. In one such implementation, inner first conductive shield layer 18 would include only a planar portion perpendicular to the magnetic field and covering the acoustic aperture of ultrasound probe 16 (e.g., planar portion 18a of first conductive shield layer 18, exclusive of planar portions 18b and 18c). Such an alternative first conductive shield layer may be grounded by using the grounded electrode surface of ultrasound probe 16 as the acoustic aperture and connecting the shield thereto, or by extending a small portion of (or attaching a small wire from) the inner first conductive shield layer to the second conductive shield layer 20, or by leaving it floating but strongly capacitively coupled to the outer shield by ensuring a very small gap and significant areal overlap therebetween. In another variation, the metallic electrode of the ultrasound probe 16 itself may be used as a first (inner) conductive shield layer, with additional conductive material deposited thereon if necessary to provide sufficient shielding, and possibly including a planar conductive member (in order to enlarge the area) having an aperture, with the periphery of the aperture electrically contacting the outer periphery of the metallic electrode of ultrasound probe 16. In another variation, the plastic housing of the ultrasound probe itself may be covered (e.g., evaporated or sputtered) with a conductive layer to replace and provide all or part of first conductive shield 18. Alternatively, encasement 26 may be made of a conductive material in order to effectively replace portions 18a and 18b of first conductive shield layer 18. These variations may be used in various combinations, and are not provided as an exclusive, exhaustive, and complete list of possible variations and embodiments, but merely to indicate that many variations are possible for implementing the various elements or components in order to provide a shielded probe in accordance with the present invention.

Since the substantially planar front surface portion 18a of first conductive shield layer 18 which covers, and is preferably parallel to, the acoustic aperture of ultrasound probe 16 is oriented perpendicular to the magnetic field B₀, ultrasonic signal 30 is redirected into the aperture by ultrasound prism 22, which is mounted in a fixed (and preferably finely adjustable) orientation with respect to o acoustic aperture of ultrasound transducer 16. Further, in accordance with an embodiment of the present invention, acoustic coupling is facilitated by including acoustic coupling medium 28, such as a silicone rubber compound (e.g., RTV) between ultrasound prism 22 and planar front surface portion 18a of first conductive 5 shield layer 18. Ultrasound prism 22 employs a reflection surface 32 of large acoustic impedance mismatch to reflect the acoustic beam into the desired direction. The reflection surface can be flat, or can be curved to realize specific focusing into the sample or a desired ultrasonic field of view, much like the techniques used in curved- , _π surface mirrors. In an experimental probe, ultrasound prism 22 was made by machining a hollow plastic frame structure which included apertures for receiving and transmitting the ultrasound signal. The frame was covered with a cellophane-like material and filled with a silicone oil (e.g., dimethyl silicone fluid). The silicon oil was selected because it is electrically inert and has an acoustic impedance close to

15 water, which was the medium used in the sample region to couple the signal from the sample to the transducer). It is appreciated that various media may be employed to optimize the acoustic coupling and reflectivity of ultrasound prism 22.

Although the above description provides many specificities, these 0 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 5 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.

0

5

Claims

We claim:

1. An ultrasound transducer, comprising: an ultrasound probe having an acoustic aperture; a conductive member that encloses said ultrasound probe; an ultrasound reflector that redirects acoustic signals from a first direction into a second direction incident on said acoustic aperture.

2. The ultrasound transducer according to claim 1, wherein said conductive member includes a first conductive shield layer which includes a portion that encloses said acoustic aperture.

3. The ultrasound transducer according to claim 2, wherein said portion that encloses said acoustic aperture is substantially planar, and is oriented substantially perpendicular to a magnetic field direction.

4. The ultrasound transducer according to claim 2, further comprising a second conductive shield layer which surrounds said first conductive shield region except in said portion of said first conductive shield region that encloses said acoustic aperture.

5. The ultrasound transducer according to claim 1, wherein said conductive member includes a plurality of electrically coupled conductive elements.

6. The ultrasound transducer according to claim 5, wherein said plurality of electrically coupled conductive elements includes a first conductive shield layer which includes a portion that encloses said acoustic aperture, and a second conductive material which is part of an electronic component or housing of the ultrasound transducer.

7. The ultrasound transducer according to claim 5, wherein said plurality of electrically coupled conductive elements includes a first conductive shield layer and a second conductive shield layer.

8. An ultrasound transducer for use in an environment having a magnetic field, said ultrasound transducer comprising: an ultrasound probe having an acoustic aperture; a conductive shield layer having a substantially planar portion located adjacent said acoustic aperture, said substantially planar portion oriented substantially orthogonal to the direction of said magnetic field; an ultrasound reflector that redirects acoustic signals from a first direction into a second direction incident on said acoustic aperture.

9. The ultrasound transducer according to claim 8, further comprising a second conductive shield layer which surrounds said conductive shield region except in said portion of said conductive shield region that encloses said acoustic aperture.

10. An ultrasound transducer, comprising: means for transducing an acoustic signal into an electrical signal; means for shielding said transducing means from electromagnetic radiation or interference; means for redirecting acoustic signals from a first direction into a second direction incident on said transducing means.