WO2008033528A2 - Therapeutic and diagnostic electrostrictor ultrasonic arrays - Google Patents

Abstract

Electrostrictor material is used as transducer elements in an ultrasonic array comprising bias and signal dimensions. The bias dimension is used for Fresnel focusing and the signal dimension is used for beam forming. Positive and negative polarity may used along the bias dimension. In some cases, a zero bias is also used within the Fresnel aperture. Dynamic switching of the bias and signal dimensions may also be employed.

Description

ACOUS.006VPC PATENT

THERAPEUTIC AND DIAGNOSTIC ELECTROSTRICTOR ULTRASONIC ARRAYS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001} The present application claims priority to U.S. Provisional Applications 60/844,554 filed on September 13, 2006 and 60/891,455 filed on February 23, 2007, both of which are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

[00021 The present invention relates to ultrasound systems and their use in therapeutic and diagnostic applications.

Description of the Related Art

[0003] An acoustic transducer is an electronic device used to emit and receive sound waves. Ultrasonic transducers are acoustic transducers that operate at frequencies above 20KHz, and more typically, in the 1-20 MHz range. Ultrasonic transducers are used in medical imaging, non-destructive evaluation and other applications. Because ultrasound waves are able to penetrate the human body, ultrasound has been widely used to image internal human organs for diagnostic purposes. To avoid thermally damaging tissue, the power level in diagnostic ultrasound imaging is kept very low. Typical ultrasound intensity (power per unit area) used in imaging is less than 0.1 watt per square centimeter. Conversely, high intensity ultrasound (HIFU) may have intensities exceeding 1000 watts per centimeter and can raise the tissue temperature at the region of the spatial focus to above 60 degrees Celsius in a few seconds and can cause tissue necrosis almost immediately. HIFU applications include tumor ablation, hemostasϊs, etc.

[0004] The most common forms of ultrasonic transducers are piezoelectric transducers. Conventional therapeutic arrays are typically annular arrays or mechanically focused pistons. The annular arrays allow for only beam focusing along the z-axis where x = 0 and y = 0 (note: z is the axis perpendicular to the face of the therapeutic array or depth). This maximizes the peak intensity from the annular array on-axis through delay or phase control of individual rings of the annular array. The mechanically focused array is typically created through a machining technique that creates a fixed focus from the therapeutic device. This device is typically described as a piston, since it has only one channel for electrical excitation. Unfortunately, the fixed focus of the mechanically focused piston is a severe limitation that is usually circumvented through the use of water standoffs during treatment. Both array geometries can create peak intensities in tissue that are sufficient for lesions. Although the concepts lesion, they suffer from lack of beam control along the x-y axes. Neither the annular array nor the piston can move the therapy beam off of the z-axis unless moved through manual or mechanical means. During tissue heating, this increases the possibility of cavitation and limits potential lesion size in the x-y plane at the intended focus in the z plane.

[0005] Ideally, a 2-D matrix of elements with individual delays would allow for beam control in three dimensions. This approach offers the best control over minimizing the possibility of cavitation and increasing the lesion size through electronic beam steering and focusing. However, this severely increases both system and array complexity. For example, a 50 mm by 50 mm 2-D array with a pitch of 1 mm at a frequency of 1.5 MHz would require over 2,500 separate connections at the array. The added complexity for such an improvement would add significant cost. Thus, there is a need for improved ultrasonic arrays for therapeutic and imaging applications.

Summary of the Invention

[0006] In one embodiment, the invention includes an ultrasound transducer array, comprising an array of transducer elements comprising electrostrictor material, a bias control circuit coupled to each row of the array and configured to apply a positive or negative bias voltage selectively to a plurality of rows in the array, and a signal control circuit coupled to each column of the array and configured to either apply a periodic signal to a plurality of columns in the array or detect a periodic signal from a plurality of columns in the array.

[0007] In another embodiment, the invention includes an ultrasound transducer array, comprising an array of transducer elements comprising electrostrictor material, a switching circuit configured to selectively apply bias voltages to a plurality of rows in the array and apply a periodic signal to a plurality of columns in the array, or apply bias voltages to a plurality of columns in the array and either apply a periodic signal to a plurality of rows in the array or detect a periodic signal from a plurality of rows in the array.

[0008] In another embodiment, the invention includes a method of driving an ultrasound transducer array, the method comprising selectively applying a positive or negative bias voltage to a plurality of rows of transducer elements in an array of electrostrictor transducer elements and applying a periodic signal to a plurality of columns in the array.

[0009] In another embodiment, the invention includes a method of driving an ultrasound transducer array, the method comprising selectively applying a positive or negative bias voltage to a plurality of rows of transducer elements in an array of electrostrictor elements during a first period of time applying a periodic signal to a plurality of columns of transducer elements in the array during the first period of time selectively applying a positive or negative bias voltage to the plurality of columns of transducer elements in the array during a second period of time and applying a periodic signal to the plurality of rows of transducer elements in the array during the second period of time.

[0010] In another embodiment, the invention includes a method of imaging with an ultrasound transducer array, the method comprising selectively applying a positive or negative bias voltage to a plurality of rows of transducer elements in an array of electrostrictor elements during a transmit period of time applying a periodic signal to a plurality of columns of transducer elements in the array during the transmit period of time selectively applying a positive or negative bias voltage to the plurality of columns of transducer elements in the array during a detection period of time and detecting a periodic signal from the plurality of rows of transducer elements in the array during the detection period of time.

[0011] In another embodiment, the invention includes a method of imaging with an ultrasound transducer array, the method comprising acquiring a first image by selectively applying a positive or negative bias voltage to a plurality of rows of transducer elements in an array of electrostrictor elements during a first transmit period of time applying a periodic signal to a plurality of columns of transducer elements in the array during the first transmit period of time selectively applying a positive or negative bias voltage to the plurality of rows of transducer elements in the array during a first detection period of time and detecting a periodic signal from the plurality of columns of transducer elements in the array during the first detection period of time. Brief Description of the Figures

[0012] Exemplary embodiments of the invention are explained in greater detail in the following description and are illustrated in the drawings, in which:

{0013) Figure 1 is a schematic of an electrostrictive transducer array architecture.

[0014J Figure 2 is a perspective view of a crossbar interconnect in a 2D array of transducer elements.

[0015) Figure 3 is a diagram of a conventional lens compared to a Fresnel lens in optics.

[0016] Figure 4 is a diagram of a conventional lens compared to a Fresnel lens in ultrasound.

[0017] Figure 5 depicts two graphs comparing the phase for a conventional lens to a Fresnel lens.

[0018] Figure 6 depicts two graphs comparing a Fresnel lens (left) to a discrete lens modeled in electrostrictive material (right).

[0019] Figure 7 is a diagram of path length differences between two elements.

[0020] Figure 8 is a graph depicting the achievable rms pressure with two PMN elements.

[0021] Figure 9 is a surface plot of point-spread-function (PSF) in x-y plane with natural focus and a Fresnel focus.

[0022] Figure 10 depicts two graphs comparing the differences in signal dimension between natural focus and Fresnel focus un-normalized and normalized.

[0023] Figure 11 depicts two graphs comparing the PSF Differences in bias dimension between natural focus and Fresnel focus un-normalized and normalized.

[0024] Figure 12 is a surface plot of PSF in x-z plane a) natural focus b) Fresnel focus.

[0025] Figure 13a shows the pressure in X-Y plane at a depths 20 mm.

[0026] Figure 13b shows the pressure in X-Y plane at a depth of 25 mm.

[0027] Figure 13b shows the pressure in X-Y plane at a depth of 30 mm.

[0028] Figure 13d shows the pressure in X-Y plane at a depth of 35mm.

[0029] Figure 13e shows the pressure in X-Y plane at a depth of 40mm. [0030] Figure 14 is a flowchart of the algorithm used to compute the Fresnel aperture.

[0031] Figure 15 is a picture of independent transducer electrostrictor arrays.

[0032] Figure 16 is a picture of part of a deep bleeder acoustic coagulation cuff.

[0033] Figure 17 depicts two graphs comparing apodization patterns with and without zeros for two different foci.

[0034] Figure 18 depicts three graphs comparing beam patterns formed by steered and unsteered Fresnel patterns with and without zeros.

[0035] Figure 19 shows an example of a switched Fresnel array aperture.

[0036] Figure 20 shows an example of a fixed Fresnel array aperture.

[0037] Figure 21 shows the one-way response from a Fresnel aperture with the bias and signal lines configures as shown in either the either the transmit/receive configuration of the Fixed Fresnel Array Aperture of Figure 20 or the transmit configuration of Switched Fresnel Array Aperture of Figure 19.

[0038] Figure 22 shows the one-way response from a Fresnel aperture with the bias and signal lines configured as shown in the receive configuration of the switched Fresnel array aperture of Figure 21.

[0039] Figure 23 shows the two-way response from a Fresnel aperture with switchable bias and signal lines.

Detailed Description

Therapeutic and Diagnostic Arrays with Fresnel Focus for 3D Beam Steering and Optimal Focusing

[0040] One type of an ultrasonic transducer is an electrostictive transducer. Unlike normal piezoelectric materials such as PZT, electrostictive materials,^' also termed "relaxors", require a DC bias voltage to exhibit piezoelectric properties. When the DC bias voltage is removed, the electric field-induced polarization disappears and the material ceases to have piezoelectric properties. This means that entire groups of transducers can be turned on or off by application or removal of the bias field. As described below, this enables the number of driver channels to be greatly reduced, simplifying the interconnection and control issues significantly, as well as the manufacturing cost and complexity of conventional piezoelectric 2-D arrays. [0041] One of several families of relaxor type electrostrictive materials may be used in the electrostrictor arrays described herein.. In one embodiment, lead-magnesium-niobate modified with lead titanate (PMN-PT) relaxors are used. Non-limiting advantageous properties of PMN-PT materials for ultrasonic applications include large field-induced piezoelectric coefficients comparable to PZTs, variable transmit/receive sensitivity by adjusting the DC bias (e.g. apodization), high dielectric constant, which improves electrical impedance matching, spectral response similar to PZT-type transducers, sensitivity and bandwidth comparable to PZT with slightly higher sensitivity being observed in PMN-PT, relaxor properties conducive to use for both detection and high power therapy, and relatively stable transducer performance over the operating temperature range despite the fact that the dielectric constant and coupling constant is a function of temperature. Three different electrostictive PMN-PT materials have been developed having operating temperature ranges of 0-30C, 10-50 C and 75-96C, respectively. A variety of other electrostrictive materials may also be used and/or designed.

[0042] Bias controllable electrostrictor ultrasound elements enable a straightforward crossbar approach to activation of a single element in a Two-D matrix. In one example, a given element in the array is only activated if a DC bias is applied by the electrically conductive strip above (column) and simultaneously with this, an AC drive signal from below (row). Elements which have an AC drive signal applied with no DC bias applied lack any piezoelectric properties, hence have no acoustic output. By appropriate activation of multiple DC bias columns and AC drive rows, a rectangular area within the array may be selected for active operation. The number of rows and/or columns selected depends upon the acoustic wavefront that is desired to achieve the acoustic focusing objectives to a remote target point. In addition, the combination of the DC bias selected for each column and the AC drive signal selected for each row (e.g., the phase or time delay selected for each row) determine the characteristics of the acoustic wavefront generated by the array.

[0043] In one embodiment, array and system complexity is reduced by utilizing a Fresnel Focus along one dimension (e.g. y-axis) with electronic beamforming in the other dimension (e.g. x-axis). The device may also be mechanically shaped in the y-dimension (e.g. analogous to a lens) such that a Fresnel pattern is used to move the focus around this intended mechanical focus. Electrostrictive material allows a Fresnel lens to be discretely modeled by switching the poling dimension through the application of a bias voltage. In this case, the aperture is subdivided into discrete elements along the bias dimension to model the Fresnel lens. The bias for each element is varied depending on the frequency and focus position. For a 50x50 array, only 50 electronic beamformer channels are required along with 50 bias lines (100 connections at the array), rather than 2,500 connections for a fully sampled array.

[0044] Another embodiment creates a 2-D matrix of elements that improves the electro-mechanical coupling coefficient when compared with the prior art. Another embodiment offers a significantly larger sweet spot when compared to 1.25D.

[0045] Figure 1 is a schematic illustrating the design and bias control of an ultrasonic transducer array based on electrostrictive transducers. This architecture may be used to provide ultrasonic therapy and/or ultrasonic detection/localization. Operation control of the architecture for detection, localization and therapy is discussed more fully below. One of skilled in the art will appreciate that the electrostrictive array techniques described herein may be utilized in any ultrasound system and are not limited to the applications described.

[0046] In one embodiment, the architecture shown in Figure 1 may be used in an 80 x 40 cm array where the relaxor transducer elements 600 cover the entire array area. At an operating frequency of 1 MHz, this area would result in an array of 320,000 (800 x 400) elements. Using the biasing control method to piezoelectrically activate individual rows, only 800 channels are needed to control this array for both Doppler detection/localization and therapeutic ultrasound. Along each column, one side (positive) of the elements is electrically connected together to a system control channel 602. Along each row, the backsides of the elements 600 are connected to a multiplexer 604. Individual rows of the array are made piezoelectrically active by application of a bias voltage. Control of individual elements 600 along the activated row is via the 800 system channels 602. Furthermore, the polarization direction is varied by using a positive or negative bias voltage. This configuration is also illustrated in a perspective view in Figure 2, where bias controllable piezoelectric materials (such as electrostrictor) enable a straightforward crossbar approach to activation of a single element in a 2D array of elements 600.

[0047] Bias controllable electrostrictor ultrasound elements enable a straightforward crossbar approach to activation of a single element in a Two-D array as illustrated in Figure 2. In this example, a given element in the array 200 is only activated if a DC bias is applied by the electrically conductive strip below 204 (row) and simultaneously with this, an AC drive signal (for transmission) or an AC receiver signal (for detection) from above 202 (column). Elements which have an AC drive signal applied with no DC bias applied lack any piezoelectric properties, hence have no acoustic output. By appropriate activation of multiple DC bias columns and AC drive rows, a rectangular area within the array is selected for active operation and a desired wavefront, and hence focal location and apodization, is achieved.

[0048] In therapeutic ultrasound configuration, when a row of elements 600 is turned "on" in Figure 1, the focusing position, steering, focal size and power intensity is controlled through the 800 system channels 602. Any elements 600 that have the acoustic path to the therapeutic site obstructed by bone or other obstructions may be turned off through a system channel 602. The selection of the number of rows to be turned on may be determined by the depth and size of the desired treatment area. A Fresnel lens design concept, as discussed below, can be used to select the voltage applied to each row. This approach provides the best beam shape for detection, localization and therapy in the array architecture. The beam shape and intensity can also be controlled through the magnitude of the DC bias, which can be used to shade selective parts of the aperture.

[0049] The concept of a Fresnel lens was initially used in lens designs for lighthouses where the goal is to send out collimated light over a large distance. In lighthouse applications, it is critical to design a lens that has a short focal length (small radius of curvature) and has a large light gathering capability. If conventional optics were applied, the lighthouse's lens would be extremely thick due to the short focal length and light gathering requirements. This increases the light attenuation through the lens and reduces the view distance. Fresnel realized that collimated light is still achieved by discretely modeling the lens curvature through separate rings. This reduces lens weight, decreases the light attenuation, and minimizes manufacturing complexity. Figure 3 shows a conventional lens 300 compared to a Fresnel lens 302, which is significantly thinner. Fresnel lenses have found applications in other areas such as aircraft landing systems, projection televisions, and automobile and traffic lights.

[0050] The Fresnel "lens" concept can also be applied to ultrasound, where the velocity and path length determines the wave direction. Figure 4 shows an acoustic Fresnel "lens" 402, as opposed to a conventional acoustic "lens" 400, applied to ultrasound with benefits similar to optics. An analogous acoustic Fresnel aperture may be created in an electrostrictor array where each discrete component of the Fresnel aperture is a DC bias-controlled row of electrostrictor elements. The Fresnel "lens" concept in ultrasound allows for shallow or deep foci, gathers ultrasound from a large aperture and minimizes loss through the "lens."

[0051] In ultrasound, the lens velocity and path length (delay) determine the direction of the wavefront. The Fresnel lens for ultrasound achieves this by eliminating the majority of lens material in which multiple wavelengths propagate through such that the direction of the wavefront remains the same. Figure 5 diagrams the effect of a Fresnel lens for ultrasound. On the left is the phase required by a lens for different elevation (bias) dimensions. This assumes a 60 mm aperture with a focus of 70 mm (depth) at 1 MHz. On the right is the phase required by a Fresnel lens where multiple wavelengths have been removed (e.g. a wavelength is equivalent to 360 degree phase shift, specifically the x-axis of the left graph in Figure 5 illustrates the total phase offset, where as the x-axis on the right graph in Figure 5 represents the remainder of the total phase offset when divided by 360 degrees). In this case, the phase is bounded from 0° to 360° or one wavelength.

[0052] Electrostrictive material allows a Fresnel aperture to be discretely modeled by switching the poling dimension through the application of a bias voltage. In this case, the aperture is subdivided into discrete elements along the bias dimension to model a Fresnel lens. The bias for each element is varied depending on the frequency and focus position. In this case, the position of the elevation focus may vary unlike a conventional lens. Figure 6 shows the resulting Fresnel lens approximation in an electrostrictive material (right figure) for the same aperture discussed in Figure 5. In this case, the pitch of the elements in the elevation dimension is 1 mm.

[0053] Figure 7 diagrams the problem with two point sources 700 and 702 separated by a distance p. The distance from element 1 to the focus is d and the distance to the focus from element 2 is d + δ. In order to calculate the configuration for the two elements, we assume that the intensity must be maximized at the focus. Therefore, the generalized rms pressure detected at the focus for a single frequency is:

P_rms = 1 + cosfø) where φ is the phase delay in radians between the two elements due to the distance and defined by:

where λ is the wavelength in the propagation medium. Since electrostrictors (e.g., PMN) allows the poling direction to be switched, the actual possible pressure detected at the focus is: P_rms = maxQ ± cos(φ))

[0054] Figure 8 shows the achievable rms pressure with two PMN elements. If just one element existed, then the measured rms pressure at the focus is simply 0.707 (assuming that δ « d). If regular PZT material is used, then only the dark curve is achievable. This assumes both elements are poled in the same direction. As Figure 8 shows, it is possible for the two elements to destructively interfere at the focus when φ is 180 degrees. However, the phase can be switched by 180 degrees with PMN. In this case, the red curve is possible. If the goal is to maximize rms pressure, then the possible magnitude ranges from 1 to 1.4 for two PMN elements. This is an increase of 3 dB to 6 dB over PZT. Figure 8 suggests that the poling direction should be switched when the phase applied by the signal line is between 90 degrees and 270 degrees from the main waveform. This technique can be generalized to N elements in an array. In this case, the elements with ideal phase will be assumed to be the middle of the array.

Focusing Straight Ahead

[0055] Suppose a therapy array is 40 mm by 40 mm with a pitch of 1 mm in both dimensions (40 bias lines and 40 channels). The operation frequency is 0.75 MHz and the focus is at 75 mm. The calculated Fresnel configuration (e.g., the combination of signal phase applied to each column and the bias poling applied to each row) is given in Table 1. Only half of the elements are listed, with the remaining elements being symmetric. Table 1

[0056] Table 1 shows that Fresnel focusing offers an advantage if the aperture is greater than 18 mm tall since smaller apertures are diffraction limited.

[0057] Figure 9 shows the differences in the x-y plane at 75 mm between the natural focus and the Fresnel focus. Only 18 elements are used in the natural focus case where as all 40 elements are used for the Fresnel focus. The beam is significantly narrower for the Fresnel case in the x dimension (bias dimension).

[0058] Figures 10a and 10b show the rms pressure, both un-normalized and normalized in the signal dimension. The peak intensity for the Fresnel focus is 9.5 dB higher than the natural focus (note: this is two-way). Furthermore, when both PSF's are normalized, the beam widths and off-axis energy are identical.

[0059] Figures 11a and lib show the rms pressure un-normalized and normalized in the bias dimension. Again, the peak intensity is 9.5 dB higher than the natural focus as previously shown in Figure 10a. When the PSF's are normalized, the beam width differences are significant. The -6 dB beam width of the natural focus and Fresnel focus is 8.3 mm and 3.8 mm respectively. This significant improvement, which is inversely proportional to the aperture size used, suggests improved reliability in imaging applications (e.g., locating a small diameter bleed inside a human body).

[0060] Figures 12a and 12b show the surface plot of the normalized rms pressure in the x-z plane for the natural focus and the Fresnel focus. The surface plot clearly shows the differences in beam width near the focus of 75 mm. The natural focus array distributes significantly more energy away from the focus.

[0061] As has been shown, the generalized Fresnel approach for large apertures appears to maximize intensity and minimize beam width and clutter.

Beam Steering and Focusing

[0062] Beam steering is also possible using the Fresnel concept described above. The practicality of beam steering was demonstrated by calculating the Fresnel configuration necessary to achieve focusing to 10 mm along x, 7.5 mm along y and 30 mm along z in a 30 mm by 20 mm array with a pitch of lmm and operated at 1.3 MHz. Table 2 shows the resulting driving parameters. Table 2

[0063] Beam plots were measured in a test tank using a hydrophone. The 30 mm by 20 mm aperture was placed in the test tank and the hydrophone was moved in the x-y plane at different five different depths. Figures 13a-13e show the measured rms pressure within the x-y plane. The peak appears at a depth of 30 mm with the intended x position of 10 mm and y position of 7.5 mm. The pressure decreases by approximately 3 dB when moving 5 mm away. The pressure has decreased by approximately 6 dB when moving 10 mm away from the intended focus. The psf in the x-y plane at 30 mm is an ellipse since the aperture is rectangular and so it is expected to be slightly larger in the bias dimension.

[0064] Figure 14 is a flowchart illustrating one algorithm for determining Fresnel configurations necessary to achieve a desired focal location. In state 1402, the x, y and z positions of all of the elements in the transducer are known for delay calculations. The array may be planar as well as curved in either the bias or signal dimensions. This curvature may be convex or concave. In addition to the array geometry, anything else that may modify the delay of the ultrasound wave at an element may be included. For example, a lens may be added over the electrostrictive material that already places delays over individual elements and this would affect the Fresnel function across the aperture. Secondly, a decision is made as to whether or not the distances should be calculated using independent equations in state 1404. There are two methods for calculating the Fresnel pattern across an aperture: independent and dependent. In state 1406 the independent case, the delays in the signal dimension are calculated independently from the delays in the bias dimension. This is similar to how a lens is made for a conventional 1 D ultrasound transducer. The lens defines a focus only in the elevation dimension for a specific depth. In state 1408 system delays are calculated based on the azimuth location of each element. for the dependent case. In the dependent case the delay error across the aperture is minimized by using a weighted mean approach in the signal dimension and bias dimension. In other words, the delays used in the signal dimension are dependent on the ideal delays across the entire aperture. In State 1412 after the signal delays are calculated, the signal delay is subtracted from the ideal delay required on each element and the remaining delay is applied using bias poling again using the mean approach. The choice of dependent or independent solution determines how the delays are computed. Next the Fresnel pattern (the poling along the bias dimension) is computed by discretely modeling the phase (note: the delays along the bias dimension have been converted to a phase for the particular operation frequency) as either a 0 degree phase or a 180 degree phase (via PMN material. In State 1414 the Fresnel pattern is varied by varying the phase offset when converting the delays to an actual phase for the operation frequency. In State 1416 by simulating the beam performance for each phase offset in State 1414, the optimal beam may be chosen (based upon intensity, main lobe to side lobe ration, etc).

[0065] It was found that the peak intensity can vary by as much as 40% depending on the phase angle offset. Therefore, the offset may be varied and the optimal Fresnel configuration calculated for any given aperture for any focus. The use of the dependent solution over the independent solution may improve the clutter levels and the pressure at the focus.

[0066] One application of flat electrostrictor arrays using the Fresnel concepts described above is as ultrasound array panels in a deep bleeder acoustic coagulation cuff, such as the cuff described in U.S. Patent Application Publication 2007-0066897, filed July 13, 2006, which is incorporated herein by reference in its entirety. Figure 15 illustrates 4 independent transducer electrostrictor arrays 1502, 1504, 1506, and 1508 that were constructed, each having an area of 38.1 cm² having 1440 elements composed of 72 beam former channels and 20 bias channels. Each array operates at 700 kHz. Illustrated in Figure 16 is part of a deep bleeder acoustic coagulation cuff 1602. Panels 110 and 112 each contain 4 of the transducer arrays 1604 depicted in Figure 15 and are capable of focusing high intensity focused ultrasound using the disclosed Fresnel implementation. Imaging transducer 130 is a commercially available imaging transducer used to detect and localize the area of bleeding and the HIFU therapeutic transducers are used to deposit thermal energy (via HIFU) at the site of the internal bleeding so as to stop the internal bleeding (e.g. causing hemostasis).

Introduction of Zeroes into the Fresnel Pattern

[0067] As discussed above, bias controllable electrostrictors used in an array structure can take advantage of Fresnel concepts due to the fact that the acoustic wave generated by an individual element can be in phase or 180 degrees out of phase with respect to the AC drive signal simply by reversing the polarity of the DC bias applied. A pattern of positive bias and negative bias columns across the chosen active area of the array can be applied to achieve the desired wavefront. This pattern may be referred to as the "Fresnel pattern".

[0068] If only a positive bias is applied in the active area, the voltage potential between one DC bias line and the adjacent bias line is either 0 V or the DC bias voltage amount (e.g., at the edges of the active area). With the concept of a Fresnel pattern utilizing positive and negative bias applied to an array, the voltage potential between one DC bias line and the adjacent bias line can be twice the DC bias voltage. This effectively doubles the voltage isolation requirement between elements in order to prevent electrical breakdown. Surface treatments can be used to improve isolation between elements, but they can be expensive and time consuming to apply. In addition, they can adversely affect the acoustic performance of the transducer.

[0069] Accordingly, in one embodiment, an alternative to the increased isolation requirement is achieved by intelligent design of a Fresnel Pattern that contains not only positive and negative bias, but also a zero bias state, which then introduces elements with zero bias voltage between elements of opposite polarity. The result is that the voltage isolation requirement between elements has been restored to levels that would have only been required in a single polarity bias voltage array design, while still being able to obtain the benefits of Fresnel focusing.

[0070] In addition to the voltage isolation benefits, it was found that the utilization of the improved Fresnel pattern with zero bias states reduces energy away from the focus when used for therapy compared to standard Fresnel patterns, increases contrast ratio for detection and localization compared to standard Fresnel patterns and increases acoustic efficiency. These results were demonstrated in simulation data in that near equal power at the focal target was achieved, with significantly less aperture being excited.

[0071 J Fresnel patterns are constructed by thresholding elements into two categories based on their phase. These thresholds are moved around the unit circle to find the Fresnel pattern that has the highest intensity at the focus. The elements that have phases closest to these thresholds (-1 and 1 boundaries) contribute the least to the focal intensity. Zeros are added to Fresnel patterns by zeroing out the element in each bias switch that is closest to the boundary and thus contributes the least to the beam at the focal intensity. Figure 17 shows two examples of standard Fresnel patterns and the associated patterns with zeros.

[0072) Any negative consequences of introducing zeros into Fresnel patterns are very small, even if a large number of zeros are introduced. A 60x60 element array with 1 mm pitch operating at a frequency of 0.75MHz was simulated focusing at depths between 30 and 180mm. The greatest decreases in focal intensities were seen when focusing to shallow depths. However, the greatest difference in focal intensities was only 1.IdB and this difference was found when over a third of the array's elements were turned off (as shown in Table 3). This difference decreases as focal depth increases. Table 3

[0073] Table 4 compares the number of zeros and focal intensity changes for a 60 x 60 array with different amounts of bias steering. When the beam is steered in the bias dimension, adding zeros to the Fresnel pattern has a greater effect on focal intensity. This is possibly due to the increased in phase transitions as the beam is steered more. Table 4

Depth (mm) Bias steer (mm) Number of zeros Intensitv change (dB)

70 0 12 -0.34

[0074] Introducing zeros into the Fresnel pattern appears to slightly increase contrast. This is likely because elements that contributed only slightly to the focus are turned off. Simulations on the same 60x60 element array described above show that when the beam is focused on axis at 70 mm, adding zeros to the Fresnel pattern increases contrast by 3.7%.

[0075] Figure 18 shows simulation results on the x, y, and z axes for beams focused at 70mm, with and without steering 30mm in the bias dimension, and with and without zeros. The differences between the two beams without steering are very slight. The differences between the two steered beams are more apparent.

Switchable Electrostrictor Fresnel Arrays for Optimal Therapy and Diagnostic Performance

[0076J Although the above mention Fresnel method of implementing diagnostic and therapeutic focusing of ultrasound allows for many benefits including minimizing channel count, the number of interconnections and the ability to steer and focus in 3 dimensions, the resulting aperture with a Fresnel focus is not ideal. Typically, a significant amount of energy remains away from the intended focus, since the Fresnel lens is quantized using a phase shift of 0 to 180 degree, large steering angles create significant energy away from the focus especially in a pulsed mode (wide bandwidth).

[0077] Accordingly, in one embodiment the aperture is switched (e.g., rotated) in order to achieve a symmetric beam in the x and y direction yielding improved contrast and detail in imaging applications as well as improved targeting of high intensity focused ultrasound with regards the therapeutic applications. This aperture switching is achieved by switching the signal and Fresnel (bias) dimension electronically. For imaging applications, during the receive period, the aperture would still be steered via the original focus, but the signal and Fresnel dimension would be switched. In imaging this minimizes the negative effect of the poor focusing along the Fresnel dimension since the effective aperture is the multiplication of the transmit aperture beam profile and the received aperture beam profile. [0078] The switching of the aperture is also advantageous to therapy. Since therapy is a one-way beam profile, the advantage to having switching apertures is the ability to minimize off focal energy and increased symmetry of the lesion. By switching the array, it also allows for improved steering of the beam in three dimensions, since the signal dimension can be rotated 90 degrees.

[0079] Switchable Fresnel arrays may minimize clutter or energy away from the intended focus when compared to the standard Fresnel array and may allow for improved imaging/diagnostics and therapy. They may improve the ability of the aperture to steer in azimuth and elevation directions since the side lobe energy is essentially eliminated in the two- way response (e.g. switching the aperture between transmit and receive). Additionally, switchable Fresnel arrays may yield a symmetric beam in x and y which is advantageous in imaging and therapy. They may minimize the amount of heating occurring away from the intended therapeutic focus and they may create images with improved contrast and detail resolution when compared to a static standard Fresnel aperture.

[0080] Figure 19 illustrates the Switchable Electrostritor Fresnel Array concept compared to a standard Fresnel aperture illustrated in Figure 20. In Figure 19 the Switchable Electrostrictor Fresnel Array concept is shown in the imaging mode in the first row. The Fresnel array 1902 shows the Switched Fresnel Array in transmit mode having the signal lines parallel to the y-axis and the bias lines parallel to the x-axis, resulting in the one-way response (transmit) illustrated in Figure 21. The Fresnel array 1904 shows the same Fresnel array in receive mode with the signal and bias lines having been electronically switched during the receive mode resulting in the one-way response (receive) illustrated in Figure 22. In Fresnel array 1904, the signal lines are parallel to the x-axis and the bias lines are parallel to the y-axis. The 2 lower Fresnel arrays show the same switched Fresnel array, however, being used for therapy. In Fresnel array 1906, the therapy is delivered at time t0 with the signal being parallel to the y-axis and the bias lines being parallel to the x-axis. At time tl the Fresnel array aperture has been electronically switched and the therapy is delivered with the signal lines being parallel to the x- axis and the bias lines being parallel to the y-axis. Figure 20 illustrates a conventional Fresnel aperture which has dedicated signal and bias lines. As shown in the figure, the bias lanes remain parallel to the x-axis and the signal lines remain parallel to the y-axis in Fresnel arrays 2102 and 2104. Fresnel array 2102 shows the array during the transmit mode while imaging or during time tO while applying therapy. Fresnel array 2104 shows the array during receive mode while imaging or during time tl while applying therapy. For imaging, the transmit and receive apertures are identical as illustrated by the modeled transmit profile shown in Figure 21. Therefore, the side lobe energy that exists away from the main lobe on transmit also exists on receive. The resultant image or two way beam profile is the product of the transmitted (Figure 21) and received (Figure 21) beam profile results in reduced contrast resolution in B-mode images and reduce the signal-to-noise ratio in Doppler. If this static Fresnel aperture configuration was used in therapy, there is a chance, depending on the amount of off-axis energy, that heating away from the intended target may occur.

[0081] These side lobe issues are circumvented by applying a Fresnel aperture that can switch the location of the bias and signal lines without physically rotating the aperture. For example, each connection of the array along x and along y could be switched from a bias line to a signal line or vice versa. The focus between the transmit and receive apertures would stay the same when this technique is applied to imaging. Since the two-way beam profile is the multiplication of the transmit (Figure 21) and Fresnel Switched receive beam (Figure 22) profiles, this method results in a symmetric beam two-way response beam ( Figure 23) which minimizes the effects of any energy that may occur away from the main lobe. This technique has advantages in B-mode as well as Doppler. In B-mode, minimizing energy away from the main lobe increases contrast resolution. The simulations also show that the width of the main beam is symmetric and improved along the y axis when compared to the conventional static Fresnel aperture implementation. This switchable Fresnel aperture allows for improved resolution and symmetric speckle size if a 3D scan is made.

[0082] In therapy, the ability to switch the apertures minimizes the possible risk of heating tissue away from the intended focus. However, the effective aperture is not the multiplication of the switched apertures as in the detection and localization (D&L) case. In this case, the effective aperture is the weighted average of the two beam profiles. The switching of the aperture also allows for a more symmetric heating pattern to be created in tissue as well as better control if painting the beam around a lesion site.

[0083] Figure 21 shows the one-way beam profile from a Fresnel aperture (i.e. the top left array configuration of Figure 19 shows the Switched Fresnel Array in transmit mode having the signal lines distributed on the x-axis and the bias lines distributed along the y-axis, resulting in the one-way response). This beam profile was created from an aperture 60 mm by 60 mm at a depth of 70 mm (focus). The center frequency of the aperture was 1 MHz with a 60% bandwidth. The excitation frequency was a 1 MHz 2-cycle sine wave. The bottom graph of Figure 21 shows how widespread the energy is in the dimension with the bias lines.

[0084] Figure 22 shows the one-way beam profile from a Fresnel aperture with the bias and signal lines switched when compared to Figure 21. The energy away from the main beam has rotated 90 degrees because of the switch.

[0085] Although the invention has been described with reference to embodiments and examples, it should be understood that numerous and various modifications can be made without departing from the spirit of the invention. Accordingly, the invention is limited only by the following claims.

Claims

WHAT IS CLAIMED IS:

1. An ultrasound transducer array, comprising: an array of transducer elements comprising electrostrictor material, a bias control circuit coupled to each row of the array and configured to apply a positive or negative bias voltage selectively to a plurality of rows in the array, and a signal control circuit coupled to each column of the array and configured to either apply a periodic signal to a plurality of columns in the array or detect a periodic signal from a plurality of columns in the array.

2. The array of claim 1, wherein the bias control circuit is configured to apply a positive, negative, or zero bias voltage selectively to each row.

3. The array of claim 1, wherein the bias control circuit is configured to apply multiple levels of positive or negative bias voltages selectively to each row so as to perform apodization

4. The array of claim 1, comprising a processor configured to determine bias voltages and periodic signals sufficient to achieve a desired focus.

5. The array of claim 1, wherein the electrostrictor material comprises lead- magnesium-niobate modified with lead titanate.

6. The array of claim 1, wherein the array is substantially flat.

7. The array of claim 1, wherein the array is curved.

8. The array of claim 1, wherein the bias control circuit comprises a multiplexer coupled to a positive voltage source and a negative voltage source.

9. The array of claim 1, wherein the signal control circuit comprises a plurality of control channels.

10. An ultrasound transducer array, comprising: an array of transducer elements comprising electrostrictor material, a switching circuit configured to selectively: apply bias voltages to a plurality of rows in the array and apply a periodic signal to a plurality of columns in the array, or apply bias voltages to a plurality of columns in the array and either apply a periodic signal to a plurality of rows in the array or detect a periodic signal from a plurality of rows in the array.

11. The array of claim 10, comprising: a bias control circuit coupled to the switching circuit and configured to apply a positive, negative, or zero bias voltage selectively to each row or column through the switching circuit; and a signal control circuit coupled to the switching circuit and configured to either apply a periodic signal to a plurality of rows or columns through the switching circuit or to detect a periodic signal from a plurality of rows or columns through the switching circuit.

12. The array of claim 10, comprising a processor configured to determine bias voltages and periodic signals sufficient to achieve a desired focus.

13. The array of claim 10, wherein the electrostrictor material comprises lead- magnesium-niobate modified with lead titanate.

14. A method of driving an ultrasound transducer array, the method comprising: selectively applying a positive or negative bias voltage to a plurality of rows of transducer elements in an array of electrostrictor transducer elements; and applying a periodic signal to a plurality of columns in the array.

15. The method of claim 14, comprising selectively applying a positive, negative, or zero bias voltage to a plurality of rows of transducer elements in the array.

16. The method of claim 15, wherein a zero bias is applied to at least one row that is in between a positively biased row and a negatively biased row.

17. The method of claim 14, comprising: selecting a desired focal location; and determining bias voltages and periodic signals sufficient to achieve a desired location of focused ultrasound energy.

18. The method of claim 17, wherein the desired location is not centered relative to the array.

19. The method of claim 17, wherein determining the bias voltages comprises modeling the plurality of rows of transducer elements as a Fresnel aperture.

20. The method of claim 17, wherein the energy density of the focused ultrasound at the desired location is at least 4 Watts/cm³.

21. The method of claim 17, wherein the focused ultrasound energy is applied to a site of bleeding in a human body and is sufficient to cause hemostasis.

22. The method of claim 14, wherein the periodic signal applied to at least one column has a different phase than the periodic signal applied to at least one other column.

23. The method of claim 14, wherein the periodic signal is pulsed and wherein a time delay is present between a pulse applied to at least one column and a pulse applied to at least one other column.

24. A method of driving an ultrasound transducer array, the method comprising: selectively applying a positive or negative bias voltage to a plurality of rows of transducer elements in an array of electrostrictor elements during a first period of time; applying a periodic signal to a plurality of columns of transducer elements in the array during the first period of time; selectively applying a positive or negative bias voltage to the plurality of columns of transducer elements in the array during a second period of time; and applying a periodic signal to the plurality of rows of transducer elements in the array during the second period of time.

25. The method of claim 24, comprising: selectively applying a zero bias voltage to a plurality of rows of transducer elements in an array of electrostrictor elements during a first period of time; and selectively applying a zero bias voltage to a plurality of columns of transducer elements in the array of electrostrictor elements during a second period of time.

26. A method of imaging with an ultrasound transducer array, the method comprising: selectively applying a positive or negative bias voltage to a plurality of rows of transducer elements in an array of electrostrictor elements during a transmit period of time; applying a periodic signal to a plurality of columns of transducer elements in the array during the transmit period of time; selectively applying a positive or negative bias voltage to the plurality of columns of transducer elements in the array during a detection period of time; and detecting a periodic signal from the plurality of rows of transducer elements in the array during the detection period of time.

27. A method of imaging with an ultrasound transducer array, the method comprising: acquiring a first image by selectively applying a positive or negative bias voltage to a plurality of rows of transducer elements in an array of electrostrictor elements during a first transmit period of time; applying a periodic signal to a plurality of columns of transducer elements in the array during the first transmit period of time; selectively applying a positive or negative bias voltage to the plurality of rows of transducer elements in the array during a first detection period of time; and detecting a periodic signal from the plurality of columns of transducer elements in the array during the first detection period of time.

28. The method of claim 27, comprising constructing an image based on the detected periodic signal from the plurality of columns of transducer elements in the array during the first detection period of time.

29. The method of claim 27, comprising: acquiring a second image by selectively applying a positive or negative bias voltage to a plurality of columns of transducer elements in an array of electrostrictor elements during a second transmit period of time; applying a periodic signal to a plurality of columns of transducer elements in the array during the second transmit period of time; selectively applying a positive or negative bias voltage to the plurality of columns of transducer elements in the array during a second detection period of time; and detecting a periodic signal from the plurality of rows of transducer elements in the array during the second detection period of time.

30. The method of claim 29, comprising: constructing a first image based on the detected periodic signal from the plurality of columns of transducer elements in the array during the first detection period of time; constructing a second image based on the detected periodic signal from the plurality of rows of transducer elements in the array during the second detection period of time; and combining the first and second images to construct a compounded image.