WO2008135896A1

WO2008135896A1 - Methods and apparatuses of aperture control and multiplexing with adjustable fluid lenses

Info

Publication number: WO2008135896A1
Application number: PCT/IB2008/051626
Authority: WO
Inventors: Anna T. Fernandez; Christopher S. Hall; Jan F. Suijver
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-05-03
Filing date: 2008-04-28
Publication date: 2008-11-13
Also published as: CN101675470A; JP2010525861A

Abstract

An acoustic imaging system (1300) includes: an acoustic transducer (40); a plurality of variably-refracting acoustic lens elements (10) coupled to the acoustic transducer, a controller (1326) adapted to generate a plurality of control signals for electrodes (250, 260) of the variably-refracting acoustic lens elements (10); a multiplexer (1322) adapted to multiplex the control signals for the electrodes of the variably-refracting acoustic lens elements (10) to produce a multiplexed control signal; a decoder (1312) adapted to demultiplex the multiplexed control signal to produce the plurality of control signals for the electrodes (250, 260) of the variably-refracting acoustic lens elements (10); and a cable (1330) adapted to provide the multiplexed control signal from the multiplexer (1322) to the decoder (1312). The variably-refracting acoustic lens elements (10) are adapted, in response to control signals applied thereto, to adjust an effective aperture of the acoustic transducer (40) to have a selected aperture size.

Description

Methods and apparatuses of aperture control and multiplexing with adjustable fluid lenses

FIELD OF THE INVENTION

This invention pertains to acoustic imaging methods, acoustic imaging apparatuses, and more particularly to methods and apparatuses for focus control of acoustic waves for optimizing imaging resolution (e.g., by changing elevation focus, lateral focus, and depth-of- field) employing an adjustable fluid lens.

BACKGROUND OF THE INVENTION

Acoustic waves (including, specifically, ultrasound) are useful in many scientific or technical fields, such as medical diagnosis, non-destructive control of mechanical parts and underwater imaging, etc. Acoustic waves allow diagnoses and controls which are complementary to optical observations, because acoustic waves can travel in media that are not transparent to electromagnetic waves.

Acoustic waves are generally launched from an acoustic transducer. Figure 1 shows an example piston transducer 100 for illustrating the effect of the size of an aperture of an acoustic transducer on the focus of an acoustic wave. The solid lines 110 illustrate how the acoustic energy is focused in a relatively narrow region at distance z for the large, solid piston aperture 112. In contrast, the lines 120 illustrate how the smaller "dashed-line" aperture 122 produces a wider focus region at distance z.

In acoustic imaging, in the so-called "far- field" the aperture size determines the resolution (R) at a distance z as:

(1) R = ^ = X(F /#)

where λ is the acoustic wavelength and D is the aperture length (i.e. the diameter of a piston), and the F-number (F/#) equals (z/D).

The aperture size may need to be changed for different applications. For example, in ultrasonic therapy treatment, a smaller aperture could be used to spread high- intensity focused energy over a wider region initially (with less overall energy density) before changing the aperture to a tighter focus for better targeting and sending more energy at certain focal positions. In acoustic imaging and therapy, the aperture might need to be changed because there are near- field obstacles (i.e. ribs or important vessels) that make the larger aperture not useful, and even though the smaller aperture will cause a loss in resolution, the imaging/therapy could still be useful at the reduced resolution.

Dynamically changing the aperture size to make these aperture tradeoffs has not been practical with single element transducers, or a transducer having only a few elements, because there was no way to control the energy.

Accordingly, acoustic transducer arrays have been provided which use a large number of elements to allow the "effective aperture" size to be adjusted and controlled, allowing for focus control.

In an acoustic transducer array made up of a large number of transducer elements (e.g., 50 or more transducer elements) the system can control the effective aperture size by applying an appropriate signal to each transducer element. For example, the system may control the effective aperture size by applying an apodization function to the transmitted waveforms of the transducer elements that will essentially "zero-out" the peripheral transducer elements - i.e. these transducer elements will fire a transmit waveform that will be multiplied by a relatively small (e.g. zero) weighting factor that effectively nulls the element's contribution to the focusing wavefront. Alternatively, some transducer elements can simply not be used in a particular transmit configuration. On transmit, this allows for the effective aperture to change when focusing at different depths up to the maximum available aperture. On receive, the electronics can expand the effective aperture as acoustic signals are received at different depths - this is to ensure a constant F/# in the system that leads to uniform resolution. Toward this end, acoustic imaging equipment has been developed with a relatively large number of transducer elements, including traditional one-dimensional ("ID") acoustic transducer arrays, and fully sampled two-dimensional ("2D") acoustic transducer arrays employing microbeamforming technology.

However, when the number of transducer elements is increased, there is a corresponding increase in the amount of electronics and cabling that are required.

SUMMARY OF THE INVENTION

Accordingly, it would be desirable to provide an acoustic imaging apparatus that provides the functionality of a transducer array having a large number of transducer elements, but that requires less electronics, that can have simplified cabling requirements, and that potentially can be much cheaper to deploy. It would further be desirable to provide such an acoustic imaging apparatus that will provide aperture control with reduced computational requirements. In one aspect of the invention, an acoustic probe comprises: an acoustic transducer; and a plurality of variably-refracting acoustic lens elements coupled to the acoustic transducer and being adapted, in response to control signals applied thereto, to adjust an effective aperture of the acoustic transducer to have a selected aperture size for at least one of transmitting an acoustic wave and receiving an acoustic signal. In another aspect of the invention, a method of controlling a focus of an acoustic probe comprises: providing a plurality of variably-refracting acoustic lens elements coupled to an acoustic transducer; and applying control signals to the plurality of variably- refracting acoustic lens elements to adjust an effective aperture of the acoustic transducer to have a selected aperture size for at least one of transmitting an acoustic wave and receiving an acoustic signal.

In still another aspect of the invention, an acoustic imaging system comprises: an acoustic transducer; a plurality of variably-refracting acoustic lens elements coupled to the acoustic transducer, each variably-refracting acoustic lens element having one or more electrodes adapted to adjust at least one characteristic of the variably-refracting acoustic lens element in response to a selected voltage applied thereto; a controller adapted to generate a plurality of control signals for the electrodes of the variably-refracting acoustic lens elements; a multiplexer adapted to multiplex the control signals for the electrodes of the variably- refracting acoustic lens elements to produce a multiplexed control signal; a decoder adapted to demultiplex the multiplexed control signal to produce the plurality of control signals for the electrodes of the variably-refracting acoustic lens elements; and a cable adapted to provide the multiplexed control signal from the multiplexer to the decoder.

In yet another aspect of the invention, a method is provided for controlling an acoustic probe of an acoustic imaging apparatus. The method comprises: generating a plurality of control signals for controlling variably-refracting acoustic lens elements of the acoustic probe; multiplexing the control signals to produce a multiplexed control signal; transporting the multiplexed control signal to the acoustic probe; demultiplexing the multiplexed control signal to produce the plurality of control signals for controlling variably- refracting acoustic lens elements of the acoustic probe; and providing the plurality of demultiplexed control signals to electrodes of the variably-refracting acoustic lens elements of the acoustic probe.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows an example piston transducer for illustrating the effect of the size of an aperture of an acoustic transducer on the focus of an acoustic wave.

FIGs. 2A-B illustrate one embodiment of a variably-refracting acoustic lens element that can be employed in an acoustic probe.

FIG. 3 shows one embodiment of an acoustic probe including a space-filling variably-refracting acoustic lens coupled to an acoustic transducer.

FIGs. 4A-B illustrate one embodiment of an arrangement that uses variably- refracting acoustic lens elements to control the effective aperture of an acoustic transducer.

FIGs. 5A-B illustrate another embodiment of an arrangement that uses variably-refracting acoustic lens elements to control the effective aperture of an acoustic transducer.

FIGs. 6A-B illustrate yet another embodiment of an arrangement that uses variably-refracting acoustic lens elements to control the effective aperture of an acoustic transducer.

FIGs. 7A-B plot depth-of- field (DOF) and resolution (R) as a function of frequency for an arrangement such as that shown in FIGs. 4A-B.

FIGs. 8A-B show an example of an acoustic probe with three variably- refracting acoustic lens elements arranged in close proximity along the elevation dimension for elevation effective aperture control.

FIG. 9 shows one example of a grid of variably-refracting acoustic lens elements that can be used to form a smaller aperture.

FIG. 10 shows another example of a grid of variably-refracting acoustic lens elements that can be used to form a smaller aperture.

FIG. 11 is a block diagram of an embodiment of an acoustic imaging apparatus using an acoustic probe including a variably-refracting acoustic lens coupled to an acoustic transducer to provide real-time axial focus control while maintaining constant lateral resolution.

FIG. 12 shows a flowchart of one embodiment of a method of controlling an acoustic imaging apparatus.

FIG. 13 illustrates one embodiment of an acoustic imaging apparatus. FIG. 14 is a flowchart illustrating one embodiment of a method for adjustment of the acoustic focus in an acoustic probe.

DETAILED DESCRIPTION OF EMBODIMENTS The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided as teaching examples of the invention. Variable-focus fluid lens technology is a solution originally developed for the express purpose of allowing light to be focused through alterations in the physical boundaries of a fluid filled cavity with specific refractive indices (see Patent Cooperation Treat (PCT) Publication WO2003/069380, the entirety of which is incorporated herein by reference as if fully set forth herein). A process known as electro-wetting, wherein the fluid within the cavity is moved by the application of a voltage across conductive electrodes, accomplishes the movement of the surface of the fluid. This change in surface topology allows light to be refracted in such a way as to alter the travel path, thereby focusing the light.

Meanwhile, acoustic waves, and particularly ultrasonic waves, propagate in a fluid medium. In fact the human body is often referred to as a fluid incapable of supporting high frequency acoustic waves other than compressional waves. In this sense, the waves are sensitive to distortion by differences in acoustic speed of propagation in bulk tissue, but also by abrupt changes in speed of sound at interfaces. This property is exploited in embodiments of an acoustic probe and an acoustic imaging apparatus as disclosed in the "METHODS AND APPARATUSES OF MICROBEAMFORMING WITH ADJUSTABLE FLUID LENSES," U.S. Provisional Patent Application 60/915,703 filed on 3 May 2007, the entirety of which is hereby incorporated herein for all purposes as if fully set forth herein.

In the discussion to follow, description is made of an acoustic imaging apparatus and an acoustic probe including a variably-refracting acoustic lens. In the context of the term "variably-refracting acoustic lens" as used in this application, the word "lens" is defined broadly to mean a device for directing or focusing radiation other than light (possibly in addition to light), particularly acoustic radiation, for example ultrasound radiation. While a variably-refracting acoustic lens may focus an acoustic wave, no such focusing is implied by the use of the word "lens" in this context. In general, a variably-refracting acoustic lens as used herein is adapted to refract an acoustic wave, which may deflect and/or focus the acoustic wave.

FIGs. 2A-B illustrate one embodiment of a variably-refracting acoustic lens element 10 that can be employed in an acoustic probe. Each variably-refracting acoustic lens element 10 includes a housing 210, a coupling element 220, first and second fluid media 241 and 242, first electrode 250, and at least one second electrode 260a. Housing 210 may be of cylindrical shape, for example. Beneficially, the top end and bottom end of housing 210 are substantially acoustically transparent, while the acoustic waves do not penetrate through the side wall(s) of housing 210. For example, an acoustically-absorbing material may be provided along the side wall(s) of housing 210. Not shown in FIGs. 2A-B is an acoustic transducer that would be provided at the "top" of variably-refracting acoustic lens element 10, above electrode 250, when variably-refracting acoustic lens element 10 is employed in an acoustic probe. In that case, an acoustic wave would be transmitted or received from "below" variably-refracting acoustic lens element 10 via coupling element 220. Each variably-refracting acoustic lens element 10 is adapted to adjust at least one acoustic signal processing characteristic thereof in response to at least one selected voltage applied thereto. For example, beneficially variably-refracting acoustic lens element 10 includes the ability to vary the focus of an acoustic wave along the axis of propagation ("focus"), and/or perpendicular to this axis ("deflection"). Beneficially, coupling element 220 is provided at one end of housing 210.

Coupling element 220 is designed for developing a contact area when pressed against a body, such as a human body. Beneficially, coupling element 220 comprises a flexible sealed pocket filled with a coupling solid substance such as a Mylar film (i.e., an acoustic window) or plastic membrane with substantially equal acoustic impedance to the body. Housing 210 encloses a sealed cavity 212 having a volume Fin which are provided first and second fluid media 241 and 242. In one embodiment, for example the volume V of the cavity within housing 210 is about 0.8 cm in diameter, and about 1 cm in height, i.e. along the axis of housing 210.

Advantageously, the speeds of sound in first and second fluid media 241 and 242 are different from each other (i.e., acoustic waves propagate at a different velocity in fluid medium 241 than they do in fluid medium 242). Also, first and second fluid medium 241 and 242 are not miscible with each another. Thus they always remain as separate fluid phases in the cavity. The separation between the first and second fluid media 241 and 242 is a contact surface or meniscus which defines a boundary between first and second fluid media 241 and 242, without any solid part. Also advantageously, one of the two fluid media 241,

242 is electrically conducting, and the other fluid medium is substantially non-electrically conducting, or electrically insulating.

In one embodiment, first fluid medium 242 consists primarily of water. For example, it may be a salt solution, with ionic contents high enough to have an electrically polar behavior, or to be electrically conductive. In that case, first fluid medium 241 may contain potassium and chloride ions, both with concentrations of 1 moll^"1, for example. Alternatively, it may be a mixture of water and ethyl alcohol with a substantial conductance due to the presence of ions such as sodium or potassium (for example with concentrations of 0.1 moll^"1). Second fluid medium 241, for example, may comprise silicone oil that is insensitive to electric fields. Beneficially, the speed of sound in first fluid medium 242 may be 1480 m/s, while the speed of sound in second fluid medium 241 may be 1050 m/s.

Beneficially, first electrode 250 is provided in housing 210 so as to be in contact with the one of the two fluid mediums 241, 242 that is electrically conducting, In the example of FIGs. 2A-B, it is assumed the fluid medium 241 is the electrically conducting fluid medium, and fluid medium 242 is the substantially non-electrically conducting fluid medium. However it should be understood that fluid medium 241 could be the substantially non-electrically conducting fluid medium, and fluid medium 242 could be the electrically conducting fluid medium. In that case, first electrode 250 would be arranged to be in contact with fluid medium 242. Also in that case, the concavity of the contact meniscus as shown in FIGs. 2A-B would be reversed.

Meanwhile, second electrode 260a is provided along a lateral (side) wall of housing 210. Optionally, two or more second electrodes 260a, 260b, etc., are provided along a lateral (side) wall (or walls) of housing 210. Electrodes 250 and 260a are connected to two outputs of a variable voltage supply (not shown in FIGs. 2 A-B).

Operationally, variably-refracting acoustic lens elements 10 operate in conjunction with one or more acoustic transducer elements as follows. In the exemplary embodiment of FIG. 2A, when the voltage applied between electrodes 250 and 260 by the variable voltage supply is zero, then the contact surface between first and second fluid media 241 and 242 is a meniscus Ml. In a known manner, the shape of the meniscus is determined by the surface properties of the inner side of the lateral wall of the housing 210. Its shape is then approximately a portion of a sphere, especially for the case of substantially equal densities of both first and second fluid media 241 and 242. Because the acoustic wave W has different propagation velocities in first and second fluid media 241 and 242, the volume V filled with first and second fluid media 241 and 242 acts as a convergent lens on the acoustic wave W. Thus, the divergence of the acoustic wave W entering acoustic lens element 10 is reduced upon crossing the contact surface between first and second fluid media 241 and 242. The focal length of variably-refracting acoustic lens element 10 is the distance to a source point of the acoustic wave, such that the acoustic wave is made planar and with the highest convergence of acoustic intensity by the variably-refracting acoustic lens element 10.

When the voltage applied between electrodes 250 and 260 by the variable voltage supply is set to a positive or negative value, the shape of the meniscus is altered, due to the electrical field between electrodes 250 and 260. In particular, a force is applied on the part of first fluid medium 241 adjacent the contact surface between first and second fluid media 241 and 242. Because of the polar behavior of first fluid medium 241, it tends to move closer to or further away to electrode 260, depending on the sign of the applied voltage, as well as on the actual fluids that are used. Accordingly, the contact surface between the first and second fluid media 241 and 242 changes as illustrated in the exemplary embodiment of FIG. 2B. In FIG. 2B, M2 denotes the shape of the contact surface when the voltage is set to a non-zero value. Such electrically-controlled change in the form of the contact surface is called electro wetting. In case first fluid medium 241 is electrically conductive, the change in the shape of the contact surface between first and second fluid media 241 and 242 when voltage is applied is the same as previously described. Because of the change in the form of the contact surface, the focal length of variably-refracting acoustic lens element 10 is changed when the voltage is non-zero.

Beneficially, in the example of FIGs. 2A-B, in a case where fluid medium 241 consists primarily of water, then at least the "bottom" wall of housing 210 (shown in FIGs. 2A-B towards the top of the page) is coated with a hydrophilic coating 270. Of course in a different example where fluid medium 242 consists primarily of water, then instead the top wall of housing 210 may be coated with a hydrophilic coating 270 instead.

Meanwhile, PCT Publication WO2004051323, which is incorporated herein by reference in its entirety as if fully set forth herein, provides a detailed description of tilting the meniscus of a variably-refracting fluid lens. FIG. 3 shows one embodiment of an acoustic probe 300 including a spacefilling variably-refracting acoustic lens 30 coupled to an acoustic transducer 40. Variably- refracting acoustic lens 30 comprises an array of AT variably-refracting acoustic lens elements 10. Each variably-refracting acoustic lens element 10 may be constructed essentially as described above. Acoustic transducer 40 is coupled to the bottom of the housing 210 of each acoustic lens element 10, beneficially by one or more acoustic matching layers 230. The need for the acoustic matching layer is driven primarily by the choice of acoustic transducer material and may not be necessary in some implementations, as is the case with piezoelectric micromachined ultrasound transducers (PMUTs) or capacitive micromachined ultrasound transducers (CMUTs).

Acoustic transducer 40 can be a single element transducer as illustrated in FIG. 3, or alternatively could comprise a ID transducer array or a 2D transducer array of P transducer elements. In the acoustic probe 300, the number of variably-refracting acoustic lens elements 10, K, is greater than the number of transducer elements, P. Beneficially, for each transducer element there is a corresponding plurality oϊK/P variably-refracting acoustic lens elements 10. In one beneficial arrangement, when transducer 40 comprises a ID transducer array, there are at least three (3) acoustic lens elements 10 for each transducer element (i.e., KJP > 3). In another beneficial arrangement, when transducer 40 comprises a 2D transducer array, there are at least nine (9) acoustic lens elements 10 for each transducer element (i.e., KJP > 9), including at least three acoustic lens elements 10 in each of the two dimensions.

FIG. 3 illustrates the ability to apply a different signal to the electrodes of each variably-refracting acoustic lens element 10 to construct an effectively- larger, smoothly- varying variably-refracting acoustic lens 30. However, the effectively- larger meniscus needs not to be continuous. For example, there could be a vertical displacement from compartment to compartment. This is the same principle that is used for a Fresnel-lens. Ideally the coupling fluid 242 has a similar impedance to the layer in contact with a patient. When the surface reaches the correct topology, then acoustic transducer 40 will be excited, for example, with either a short time imaging pulse for time-resolved echo information in traditional acoustic imaging, or a time-resolved tone burst to allow for detection of motion along a line of site.

In one embodiment, acoustic probe 300 is adapted to operate in both a transmitting mode and a receiving mode. In that case, in the transmitting mode acoustic transducer element 40 converts one or more electrical signals input thereto into one or more acoustic wave which it outputs. In the receiving mode, acoustic transducer element 40 converts acoustic waves which it receives into electrical signals which it outputs.

In an alternative embodiment, acoustic probe 300 may instead be adapted to operate in a receive-only mode. In that case, a transmitting transducer is provided separately. In yet another embodiment, the acoustic probe 300 may instead be utilized in a transmit only mode. Such a mode would be useful for therapeutic applications where ultrasonic energy is intended to interact with tissue or the insonified object to deliver a therapy. Adjustment of variably-refracting acoustic lens element 10 can be controlled by external electronics (e.g., a variable voltage supply) that, for example, can adjust the surface topology within 20 ms when variably-refracting acoustic lens element 10 has a diameter of 3 mm, or as quickly as 100 microseconds when variably-refracting acoustic lens 10 has a diameter of 100-microns. When acoustic probe 300 operates in both a transmit mode and a receive mode, then variably-refracting acoustic lens elements 10 will be adjusted to alter the effective transmit and receive focusing. In a transmitting mode, transducer 40 will be able to send out short time (broad-band) signals operated in M-mode or B-mode, possibly short tone-bursts to allow for pulse wave Doppler or other associated signals for other imaging techniques. A typical application might be to image a plane with a fixed focus adjusted to the region on clinical interest. Another use might be to image a plane with multiple foci, adjusting the focus to maximize energy delivered to regions of axial focus. The acoustic signal can be a time-domain resolved signal such as normal echo, M-mode or PW Doppler or even a non-time domain resolved signal such as CW Doppler.

FIGs. 4A-B illustrate one embodiment of an arrangement that uses acoustic lens elements to control the effective aperture of an acoustic transducer. The example illustrated in FIGs. 4A-B illustrates the use of an acoustic lens assembly 400 comprising three variably-refracting acoustic lens elements (10-1, 10-2 and 10-3) to control the effective aperture of acoustic transducer 40. For simplicity of illustration, only three variably- refracting acoustic lens elements 10 are shown in the example of FIGs. 4A-B, but the number of variably-refracting acoustic lens elements 10 could be more than three. Acoustic lens assembly 400 includes an acoustically absorbing material 410 around a periphery thereof to minimize acoustic reflections/reverberations from off-axis directed energy. Beneficially, a sidewall of each variably-refracting acoustic lens element 10 may include the acoustically absorbing material 410. In both FIG. 4A and FIG. 4B, transducer 40 generates and emits acoustic energy. In FIG. 4A, all three variably-refracting acoustic lens elements 10-1, 10-2 and 10-3, are used to focus the acoustic energy using the full aperture of acoustic transducer 40. In contrast, in FIG. 4B, a smaller effective aperture is created by using acoustic lens elements 10-1 and 10-3 to direct some of the acoustic energy away from the center focus (where some of that energy is absorbed by acoustically absorbing material 410). Thus, the effective acoustic on-axis aperture in FIG. 4B comprises the portion of transducer 40 that lies beneath acoustic lens 10-2.

In FIG. 4A, where all three variably-refracting acoustic lens elements 10-1, 10-2 and 10-3 are employed to make a full-aperture, all acoustic energy is transmitted or received on-axis along the center of the array. This larger (full) aperture will provide a higher lateral resolution than the smaller effective aperture of FIG. 4B. In contrast, in FIG. 4B the smaller effective aperture provides a broader beamwidth throughout the transmit/receive path. FIGs. 5A-B illustrate another embodiment of an arrangement that uses variably-refracting acoustic lens elements 10-1 and 10-2 to control the effective aperture of an acoustic transducer 40. In particular, FIGs. 5 A-B illustrate the use of an acoustic lens assembly 500 comprising two variably-refracting acoustic lens elements 10-1 and 10-2 stacked on top of each other to form a smaller "effective aperture" for receiving an acoustic wave. For simplicity of illustration, only two stacked variably-refracting acoustic lens elements 10 are shown in the example of FIGs. 5A-B, but the number of stacked variably- refracting acoustic lens elements 10 could be more than two.

In the example illustrated in FIG. 5 A, some of the incoming received acoustic wavefronts are redirected by variably-refracting acoustic lens element 10-2 away from variably-refracting acoustic lens element 10-1 and into the absorbing material 410. The signals then pass through variably-refracting acoustic lens elements 10-1 with no additional focusing, and the resulting acoustic energy at transducer 40 is received at the small "effective aperture" 550A. In the example illustrated in FIG. 5B, variably-refracting acoustic lens element 10-1 focuses the received acoustic energy in the manner of a conventional lens. In that case, an even smaller "effective aperture" 550B is produced. In the embodiment of FIGs. 6A-B, transducer 40 comprises three transducer elements 41, 42 and 43.

FIGs. 6A-B illustrates another embodiment of an arrangement that uses acoustic lens elements to control the effective aperture of an acoustic transducer 40. In particular, FIGs. 6A-B illustrate the use of an acoustic lens assembly 600 comprising two variably-refracting acoustic lens elements 10-1 and 10-2 stacked on top of each other to form a smaller "effective aperture" for transmitting an acoustic wave. In the embodiment of FIGs. 6A-B, transducer 40 comprises three transducer elements 41, 42 and 43.

In the example illustrated in FIG.6A, some of the outgoing transmitted energy is redirected by variably-refracting acoustic lens element 10-1 into the absorbing material 510 410 (shown as a white arrow into absorbing material 410). The acoustic waves then pass through variably-refracting acoustic lens element 10-2 with no additional focusing and the resulting acoustic energy from transducer 40 is transmitted as if it came from a smaller "effective aperture" 650A than the physical aperture of transducer 40. In the example illustrated in FIG.6B, variably-refracting acoustic lens element 10-2 focuses the transmitted energy that passed through variably-refracting acoustic lens element 10-1 producing a focused, narrower transmit beam than in FIG. 6A, but from a lens-focused effective aperture 650B similar to the effective aperture 650A of FIG. 6A.

FIGs. 7A-B plot depth-of- field (DOF) and lateral resolution (R) as a function of frequency for an arrangement such as that shown in FIGs. 4A-B. In the example whose characteristics are plotted in FIGS. 7A-B, the transducer 40 is a 6mm diameter square, single- element piston, and the acoustic energy is focused at a distance of 2 cm. Variably-refracting acoustic lens element 10-2 covers a portion of the transducer 40 that is 4mm x 6mm (lateral x elevation dimension), and variably-refracting acoustic lens element 10-1 and variably- refracting acoustic lens element 10-3 each cover a portion of the transducer 40 that is lmm x 6mm, on either side of variably-refracting acoustic lens element 10-2, respectively.

For a 5 MHz transducer, the lateral resolution worsens by 50% when going from the full aperture (1.0mm resolution at 2cm focus) to a narrower 4mm x 6mm aperture using the portion of transducer 40 covered by variably-refracting acoustic lens element 10-2 (1.5mm lateral resolution), but there is a gain in DOF from 2.7cm to 6.2cm when going from full aperture to the portion of transducer 40 covered by variably-refracting acoustic lens element 10-2. This may be good for gaining uniformity in imaging or energy deposition over a longer range or for deeper therapeutic range scenarios.

Imaging ultrasound for most clinical scenarios operates in the 2.5-10 MHz imaging range. For ocular and other short-depth applications, higher frequencies (10-30 MHz) are used. For therapeutic applications, lower frequencies are used and may be most affected by changing apertures (by increasing DOF).

Arrays with acoustic transducer elements in both azimuth and elevation are costlier to build than conventional 1-D (linear) arrays. The aperture even on single-element or multiple-element arrays can be modified with the multi-lens system without having to incorporate more transducer elements.

FIGs. 8A-B show an example of an acoustic probe 800 with three acoustic lens elements 10 along the elevation dimension. In particular, FIGs. 8A-B illustrate an example of a square single element array (or an array with a few elements) along the azimuth dimension. The three variably-refracting acoustic lens elements 10 can be used to control the aperture in elevation.

For high- intensity ultrasound or therapeutic heating ultrasound, the control of the aperture will be useful to deliver slightly different intensities over a broader region initially using a narrow aperture, then deliver higher intensities over a more focused region using a larger aperture. This may be needed to elevate areas to a "base temperature" and then refocus energy (increasing temperature) over a more focalized region. For example, using an arrangement as shown in FIGs. 4A-B, the full aperture of 6mm diameter will give a resolution of λ*z/D, and the narrower effective aperture of 3mm diameter, corresponding to variably-refracting acoustic lens element 10-2, will provide a resolution twice as broad. In that case, only the variably-refracting acoustic lens element 10-2 could be used during the initial stage of therapy, and then the full aperture could be used to increase energy deposition over a narrower area.

When using a large single element transducer, or an m-element transducer array, an (n x 1) or (n x ή) array of variably-refracting acoustic lens elements 10 can be used to control the size of the aperture. This would be useful when nxl > m or nxn > m and can provide increased aperture control that cannot be obtained just by turning on transmit/receive functionality for some of the m transducer elements. For example, with an « x κ grid of variably-refracting acoustic lens elements 10, the outer variably-refracting acoustic lens elements 10 (positioned along the periphery of the grid) can be used to reduce the aperture by re-directing acoustic waves away from the center. The tilt angle of the variably-refracting acoustic lens elements 10 would have to be greater than or equal to the critical angle of the lens to insure no energy transfer to the axial direction. The critical angle is defined as sin^"1 (c2/cl), where cl is the speed of sound in the medium where the waves are initially traveling (incident medium), and c2 is the speed of sound in the refractive medium.

FIGs. 9-10 show two examples of an n x n grid of variably-refracting acoustic lens elements 10 that can be used to form different smaller apertures. An acoustically absorbing material on the outside of the multi-acoustic-element lens absorbs the energy; thus transmitting/receiving energy from a reduced aperture. FIG. 9 illustrates an example of tilting the variably-refracting acoustic lens elements 10 around the border of the n x n grid (the tilted lenses are denoted by the shaded boxes) to form a smaller square aperture. FIG. 10 illustrates the use additional tilts in variably-refracting acoustic lens elements 10 to form a smaller aperture that resembles a "round" shape aperture. The transducer underneath the FFUS lens grid can be either a one-element piston, an m-element 1-D acoustic element array, or an "m x m" 2-D acoustic element array. Beneficially, m < n.

Various modifications and additional features are possible. For example, variations of each of the lens apertures along the direction of acoustic propagation (axial axis) will increase or decrease the depth-of -field and the focus. Also, the n lenses can be made with different materials which could vary their rate of stability change.

FIG. 11 is a block diagram of an embodiment of an acoustic imaging apparatus 1100 using an acoustic probe including a variably-refracting acoustic lens coupled to an acoustic transducer to provide real-time axial focus control with control over lateral and elevation resolution by changing the size of the aperture. Acoustic imaging apparatus 1100 includes processor/controller 1110, transmit signal source 1120, transmit/receive switch 1130, acoustic probe 1140, filter 1150, gain/attenuator stage 1160, acoustic signal processing stage 1170, focus controller 1180, and variable voltage supply 1190. Meanwhile, acoustic probe 1140 includes a plurality of variably-refracting acoustic lens elements 1142 coupled to an acoustic transducer 1144 comprising one or more transducer elements.

Acoustic probe 1140 may be realized, for example, as acoustic probe 300 as described above with respect to FIG. 3, or as illustrated in FIGs. 4A-B, 5A-B and 6A-B. In that case, beneficially the two fluids 241, 242 of each variably-refracting acoustic lens element 1142 have matching impedances, but differing speed of sounds. This would allow for maximum forward propagation of the acoustic wave, while allowing for control over the direction of the beam. Beneficially, fluids 241, 242 have a speed of sound chosen to maximize flexibility in the focusing and refraction of the acoustic wave.

Variable voltage supply 1190 supplies controlling voltages to electrodes of each variably-refracting acoustic lens element 1142. In one embodiment, voltages are supplied to variably-refracting acoustic lens elements 1142 to control an effective size of an aperture presented by acoustic transducer 1144.

Acoustic transducer 1144 may comprise a ID array of acoustic transducer elements.

Operationally, acoustic imaging apparatus 1100 operates as follows. Focus controller 1180 controls voltages applied to electrodes of variably- refracting acoustic lens elements 1142 by variable voltage supply 1190. As explained above, this in turn controls a refraction of each variably-refracting acoustic lens element 1142 as desired. When the surface of the meniscus defined by the two fluids in variably- refracting acoustic lens elements 1142 reach the correct topology, then processor/controller 1110 controls transmit signal source 1120 to generate one or more desired electrical signals to be applied to acoustic transducer 1144 to generate a desired acoustic wave. In one case, transmit signal source 1120 may be controlled to generate short time (broad-band) signals operating in M-mode or B-mode, possibly short tone-bursts to allow for pulse wave Doppler or other associated signals for other imaging techniques. A typical use might be to image a plane with a fixed elevation focus adjusted to the region of clinical interest. Another use might be to image a plane with multiple foci, adjusting the elevation focus to maximize energy delivered to regions of axial focus. The acoustic signal can be a time-domain resolved signal such as normal echo, M-mode or PW Doppler or even a non-time domain resolved signal such as CW Doppler.

In one embodiment, acoustic probe 1140 is adapted to operate in both a transmitting mode and a receiving mode. As explained above, in an alternative embodiment acoustic probe 1140 may instead be adapted to operate in a receive-only mode. In that case, a transmitting transducer is provided separately, and transmit/receive switch 1130 may be omitted.

FIG. 12 shows a flowchart of one embodiment of a method 1200 of controlling the elevation focus of an acoustic imaging apparatus. In a first step 1205, the acoustic probe 1140 is coupled to a patient.

Then, in a step 1210, focus controller 1180 controls a voltage applied to electrodes of variably-refracting acoustic lens elements 1142 by variable voltage supply 1190 to focus at a target elevation. As explained above, this in turn controls a refraction of each variably-refracting acoustic lens element 1142 as desired. In one embodiment, voltages are supplied to variably-refracting acoustic lens elements 1142 to control an effective size of an aperture presented by acoustic transducer 1144.

Next, in a step 1215, processor/controller 1110 controls transmit signal source 1120 and transmit/receive switch 1130 to apply one or more desired electrical signals to acoustic transducer 1144. Variably-refracting acoustic lens elements 1142 operate in conjunction with acoustic transducer 1144 to generate an acoustic wave and focus the acoustic wave in a target area of the patient, including the target elevation.

Subsequently, in a step 1220, variably-refracting acoustic lens elements 1142 operate in conjunction with acoustic transducer 1144 to receive an acoustic wave back from the target area of the patient. At this time, processor/controller 1110 controls transmit/receive switch 1130 to connect acoustic transducer 1144 to filter 1150 to output an electrical signal(s) from acoustic transducer 1144 to filter 1150.

Next, in a step 1230, filter 1150, gain/attenuator stage 1160, and acoustic signal processing stage 1170 operate together to condition the electrical signal from acoustic transducer 1144, and to produce therefrom received acoustic data.

Then, in a step 1240, the received acoustic data is stored in memory (not shown) of acoustic signal processing stage 1170 of acoustic imaging apparatus 1100.

Next, in a step 1245, processor/controller 1110 determines whether or not it to focus at another axial distance or with different lateral and/or elevation resolution. If so, then in step 1250, the new focus is selected, and process repeats at step 1210. If not, then in step 1255 acoustic signal processing stage 1170 processes the received acoustic data (perhaps in conjunction with processor/controller 1110) to produce and output an image.

Finally, in a step 1260, acoustic imaging apparatus 1100 outputs the image. In general, the method 1200 can be adapted to make measurements where the acoustic wave is a time-domain resolved signal such as normal echo, M-mode or PW Doppler, or even a non-time domain resolved signal such as CW Doppler.

An apparatus similar to that described above with respect to FIG. 11, and a method similar to that shown in FIG. 12, can also be employed with these same principles to change the lateral focusing with an acoustic probe having only one or a few transducer elements.

In some embodiments of the devices discussed above a large number of acoustic lens element may be necessary. In particular, a space-filling large array of acoustic lens elements (additional details of which may be found in U.S. Provisional Patent Application 60/915,703 filed on 3 May 2007) requires that a substantial portion of space be filled with active lenses while minimizing the excess surface area taken by electrodes or non- lens material. The electrodes for each of the acoustic lens elements in the acoustic probe need to be connected to different control voltage signals to control their operation to provide a acoustic transmit and/or receive characteristic for the acoustic probe. This could lead to a large number of cables. This added complexity can sometimes obviate one of the attractive feature of the acoustic lens element technology for acoustic imaging and/or ultrasound therapy - in particular the fact that using acoustic lens elements can reduce the number of transducer elements and associated cables, amplifiers, control systems, etc. needed to steer a beam. Accordingly, in one embodiment multiplexing and demultiplexing circuitry is employed to transport control signals from an imaging system to an array of acoustic lens elements for an acoustic probe.

FIG. 13 illustrates one embodiment of an acoustic imaging apparatus 1300. Acoustic imaging apparatus 1300 includes an acoustic probe 1310 connected to an imaging system 1320 by cable 1330. Acoustic probe 1310 includes acoustic transducer 40 and associated acoustic lens array of acoustic lens elements 10, connected by signal lines 1314 to a decoder (or demultiplexer) 1312. Imaging system 1320 includes an imaging controller 1326 connected to a multiplexer 1322 by signal lines 1324. Beneficially, acoustic probe 1310 is enclosed within a first housing, and imaging system 1320 is enclosed in a second housing, and cable 1330 extends between the first and second housings.

Operationally, imaging controller 1326 outputs control signals on lines 1324 for controlling acoustic lens elements 10 alter the meniscus geometry on the acoustic lens array to allow for optimal focusing. Multiplexer 1322 multiplexes the control signals on signal lines 1324 and outputs a multiplexed control signal on cable 1330. The operation of multiplexer 1322 will be described in greater detail below. Decoder 1312 receives the multiplexed control signal from cable 1330 and outputs the control signals on signal lines 1314 for controlling acoustic lens elements 10 alter the meniscus geometry on the acoustic lens array. In one exemplary embodiment, the aperture of acoustic transducer 40 is a square 3x3 cm². In this embodiment, each acoustic lens element 10 is constructed in such a way to be 5 mm in size on each side. Accordingly, there are 36 acoustic lens elements 10 in the array. In one embodiment, the control signals control two electrodes per acoustic lens element 10 in order to allow non- spherical deformation of the lens. In this case, multiplexer 1322 multiplexes together 72 control signals into a single multiplexed control signal carried by cable 1330. Multiplexer 1322 needs to be capable of handling 0.72 MHz signals (36 lens x 2 electrodes/lens x 10 KHz (ac voltage frequency)) at voltages of ~10 volts, which is easily achievable.

A second embodiment pertains to an acoustic transducer 40 used for thermal ablation. In this class of transducers used for extra-corporeal ablation, a large aperture size is used to allow for tight spatial focusing with minimal bio-effects outside the treatment zone. These apertures can typically be on the order of 10 cm or more in diameter. In this geometry, an acoustic transducer 40 having a 10 cm diameter implies an area of approximately 7853 mm². In that case, the acoustic lens array may comprise approximately 277 acoustic lens elements 10 each being 6mm in size on a side. Accordingly, multiplexer 1322 needs to be capable of multiplexing signals to a frequency greater than 5 MHz (277 lens x 2 electrodes/lens x 10 KHz (ac voltage frequency)) at voltages of ~10 volts. Again, this is easily achievable. FIG. 14 is a flowchart illustrating one embodiment of a method 1400 for adjustment of the acoustic focus in an acoustic probe.

In a first step 1410, an imaging system decides upon a desired focus position for an acoustic probe.

In a next step 1420, the imaging system calculates a geometry for the acoustic lens array to achieve the desired focus position.

In a step 1430, the imaging system produces control signals for the electrode(s) of each lens element in the acoustic lens array.

In a step 1440, a multiplexer combines the individual control signals to produce a multiplexed control signal. In a step 1450, the multiplexer provides the multiplexed control signal via a cable to an acoustic probe.

In a step 1460, a decoder at the acoustic probe demultiplexes the multiplexed signal to produce individual control signals.

In a step 1470, the decoder sends the individual control signals to corresponding electrodes for acoustic lens elements of the acoustic lens array.

In a step 1480, the acoustic lens elements respond to the control signals to cause the acoustic lens array to produce a desired acoustic focus. In the case of a transmitting operation, then an acoustic signal is transmitted from the acoustic transducer via the acoustic lens array. In the case of a receive operation, then an acoustic signal is received by the acoustic transducer via the acoustic lens array.

While preferred embodiments are disclosed herein, many variations are possible which remain within the concept and scope of the invention. Such variations would become clear to one of ordinary skill in the art after inspection of the specification, drawings and claims herein. The invention therefore is not to be restricted except within the spirit and scope of the appended claims.

Claims

CLAIMS:

1. An acoustic probe (300, 400, 500, 600, 1140, 1310), comprising: an acoustic transducer (40); and a plurality of variably-refracting acoustic lens elements (10) coupled to the acoustic transducer (40) and being adapted, in response to control signals applied thereto, to adjust an effective aperture of the acoustic transducer (40) to have a selected aperture size for at least one of transmitting an acoustic wave and receiving an acoustic signal.

2. The acoustic probe (300, 400, 1140, 1310) of claim 1, wherein the plurality of variably-refracting acoustic lens elements (10) includes at least three variably-refracting acoustic lens elements (10-1, 10-2, 10-3) disposed laterally with respect to each other across a surface of the acoustic transducer (40).

3. The acoustic probe (300, 400, 500, 600, 1140, 1310) of claim 1, wherein the acoustic transducer (40) comprises P acoustic transducer elements, and wherein the plurality of variably-refracting acoustic lens elements (10) comprises AT variably-refracting acoustic lens elements disposed in close proximity with respect to each other across a surface of the acoustic transducer (40), where K > P.

4. The acoustic probe (500, 600, 1140, 1310) of claim 1, wherein the plurality of variably-refracting acoustic lens elements (10) includes at least a first variably-refracting acoustic lens element (10-1) disposed on the acoustic transducer (40), and a second variably- refracting acoustic lens element (10-2) disposed on the first variably-refracting acoustic lens element (10-1), wherein a refraction of the first variably-refracting acoustic lens element (10- 1) may be controlled independently of a refraction of the second variably-refracting acoustic lens element (10-2).

5. The acoustic probe (300, 400, 500, 600, 1140, 1310) of claim 1, further comprising an acoustically-absorbing material (410) provided around at least part of a periphery of the variably-refracting acoustic lens elements (10).

6. A method of controlling a focus of an acoustic probe (300, 400, 500, 600, 1140, 1310), comprising: providing a plurality of variably-refracting acoustic lens elements (10) coupled to an acoustic transducer (40); and applying control signals to the plurality of variably-refracting acoustic lens elements (10) to adjust an effective aperture of the acoustic transducer (40) to have a selected aperture size for at least one of transmitting an acoustic wave and receiving an acoustic signal.

7. The method of claim 6, wherein the plurality of variably-refracting acoustic lens elements (10) are disposed in an array across a surface of the acoustic transducer (40), and wherein the control signals are applied to tilt a focus of at least one of the variably- refracting acoustic lens elements (10) away from an acoustic axis of the acoustic probe to adjust the effective aperture of the acoustic transducer (40).

8. The method of claim 6, wherein the plurality of variably-refracting acoustic lens elements (10) includes at least a first acoustic lens element (10-1) disposed on the acoustic transducer (40), and a second variably-refracting acoustic lens element (10-2) disposed on the first acoustic lens element (10-1), and wherein the second variably-refracting acoustic lens element (10-2) is controlled such that a portion of an acoustic wave received by the second variably-refracting acoustic lens element (10-2) is directed by the second variably- refracting acoustic element (10-2) away from the first variably-refracting acoustic lens element (10-1).

9. The method of claim 6, wherein the plurality of variably-refracting acoustic lens elements (10) includes at least a first variably-refracting acoustic lens element (10-1) disposed on the acoustic transducer (40), and a second variably-refracting acoustic lens element (10-2) disposed on the first variably-refracting acoustic lens element (10-1), and wherein the first variably-refracting acoustic lens element (10-1) is controlled such that a portion of an acoustic wave received by the first variably-refracting acoustic lens element (10-1) is directed by the first variably-refracting acoustic element (10-1) away from the second variably-refracting acoustic lens element (10-2).

10. A system (1300), comprising : a plurality of variably-refracting acoustic lens elements (10), each variably- refracting acoustic lens element (10) having one or more electrodes (250, 260) adapted to adjust at least one characteristic of the variably-refracting acoustic lens element (10) in response to a selected voltage applied thereto; a controller (1326) adapted to generate a plurality of control signals for the electrodes (250, 260) of the variably-refracting acoustic lens elements (10); a multiplexer (1322) adapted to multiplex the control signals for the electrodes (250, 260) of the variably-refracting acoustic lens elements (10) to produce a multiplexed control signal; a decoder (1312) adapted to demultiplex the multiplexed control signal to produce the plurality of control signals for the electrodes (250, 260) of the variably-refracting acoustic lens elements (10); and a cable (1330) adapted to provide the multiplexed control signal from the multiplexer to the decoder (1312).

11. The system (1300) of claim 10, further comprising an acoustic transducer (40) coupled to the plurality of variably-refracting acoustic lens elements (10).

12. The system (1300) of claim 10, wherein the acoustic lens elements (10) are controlled to operate as a single variably-refracting acoustic lens (30) having an effective size greater than each variably-refracting acoustic lens element (10).

13. The system (1300) of claim 10, wherein each variably-refracting acoustic lens element (10) comprises: a cavity (212); first and second fluid media (241, 242) disposed within the cavity (212); and the pair of electrodes (250, 260), wherein a speed of sound of an acoustic wave in the first fluid medium (241) is different than a corresponding speed of sound of the acoustic wave in the second fluid medium (242), wherein the first and second fluid media (241, 242) are immiscible with respect to each other, and wherein the first fluid medium (241) has a substantially different electrical conductivity than the second fluid medium (242).

14. A method (1400) of controlling an acoustic probe (300, 400, 500, 600, 1140, 1310) of an acoustic imaging apparatus ( 1300), the method comprising : generating (1430) a plurality of control signals for controlling variably- refracting acoustic lens elements (10) of the acoustic probe (300, 400, 500, 600, 1140, 1310); multiplexing (1440) the control signals to produce a multiplexed control signal; transporting (1450) the multiplexed control signal to the acoustic probe (300,

400, 500, 600, 1140, 1310); demultiplexing (1460) the multiplexed control signal to produce the plurality of control signals for controlling variably-refracting acoustic lens elements (10) of the acoustic probe (300, 400, 500, 600, 1140, 1310); and providing (1470) the plurality of demultiplexed control signals to electrodes of the variably-refracting acoustic lens elements (10) of the acoustic probe (300, 400, 500, 600, 1140, 1310).