ULTRASOUND SYSTEM WITH ASYMMETRIC TRANSMIT SIGNALS

Abstract

An ultrasound system drives the elements of an ultrasound probe with asymmetric transmit signals which reinforce poling of the probe transducer. The use of asymmetric transmit signals enables a transducer element to withstand a significantly higher RF transmit voltage without degradation, which in turn enables higher acoustic output and improved reliability. This is particularly beneficial with single crystal transducer material when used to generate high energy pressure waves of long duration such as shear wave push pulses.

Claims

1. An ultrasound system for transmitting ultrasonic energy into a body, the system comprising: an ultrasound probe having an array of ultrasonic transducer elements; and a transmit beamformer having transmit channels coupled to the ultrasonic transducer elements and configured to apply asymmetric transmit signals to the elements during their respective transmit intervals, wherein each transmit channel comprises a hardware addressable memory device or shift register configured to store digital data of transmit signals.

2. The ultrasound system of claim 1, wherein the transducer elements further comprise piezoelectric ceramic transducer elements.

3. The ultrasound system of claim 2, wherein the piezoelectric ceramic transducer elements further comprise single crystal transducer elements.

4. The ultrasound system of claim 3, wherein the single crystal transducer elements further comprise single crystal PMN-PT, PZN-PT, or PIN-PMN-PT.

5. The ultrasound system of claim 1, further comprising a T/R switch coupling each transmit channel to a transducer element.

6. The ultrasound system of claim 1, further comprising a probe cable coupling the T/R switches to the transducer elements.

7. The ultrasound system of claim 1, wherein the ultrasonic transducer elements further comprise negatively poled transducer elements; and wherein the asymmetric transmit signals exhibit a mean amplitude value which is negative with respect to a zero reference voltage.

8. The ultrasound system of claim 1, wherein the ultrasonic transducer elements further comprise positively poled transducer elements; and wherein the asymmetric transmit signals exhibit a mean amplitude value which is positive with respect to a zero reference voltage.

9. (canceled)

10. The ultrasound system of claim 1, wherein each transmit channel further comprises a digital-to- analog converter coupled to receive digital transmit signal data to convert the data to an analog transmit signal.

11. The ultrasound system of claim 10, wherein each transmit channel further comprises a high voltage transmit amplifier coupled to receive an analog transmit signal and apply a high voltage transmit signal to a transducer element.

12. The ultrasound system of claim 1, wherein the asymmetric transmit signals further comprises push pulse transmit signals.

13. The ultrasound system of claim 12, wherein the push pulse transmit signals further produce push pulses of 50 to 1000 microseconds in duration.

14. The ultrasound system of claim 1, wherein the transmit beamformer is further configured to apply asymmetric RF signals.

15. The ultrasound system of claim 1, wherein the system comprises a DC bias circuit and the asymmetric signals comprise a symmetric RF signal on a DC bias.

Description

[0011] In the drawings:

[0012] FIG. 1 illustrates in block diagram form an ultrasonic diagnostic imaging system which drives the elements of the probe transducer array with asymmetric transmit signals.

[0013] FIG. 2 is a schematic illustration of two channels of a transmit beamformer coupled to two elements of a transducer array.

[0014] FIG. 3 is a plot of measured transmit waveforms illustrating an asymmetric transmit signal of the present invention.

[0015] FIG. 4 illustrates major components of an ultrasound probe, in accordance with an embodiment of the present invention.

[0016] In certain embodiments of the present invention, an ultrasound system is described which drives the transducer elements of the probe with asymmetric transmit signals. The amplitude asymmetry of the transmit signal creates an electric field with larger magnitudes in the direction that reinforces the polarization of the piezoelectric material, and smaller magnitudes in the direction that opposes and degrades the polarization of the piezoelectric material. The asymmetric transmit signals reinforce polarization because net energy delivered to the transducer element over the duration of the transmit pulse creates net mechanical and electric forces that work to keep domains in alignment and polarization maintained.

[0017] Referring now to FIG. 1, an ultrasound system which produces asymmetric transmit signals for an ultrasound probe in accordance with the principles of the present invention is shown in block diagram form. In the illustrated implementation the ultrasound system is set up to transmit push pulses for the measurement of shear waves in the body. The ultrasound probe 10 has a transducer array 12 of transducer elements which operate to transmit and receive ultrasound signals. The elements of the transducer array 12 are made of a piezoelectric ceramic material such as PZT, PMN-PT, PZN-PT or PIN-PMN-PT (lead indium niobate-lead magnesium niobate-lead titanate). The array can be fabricated as a one dimensional (1D) or a two dimensional (2D) array of transducer elements. Either type of array can scan a 2D plane and the two dimensional array can be used to scan a volumetric region in front of the array. A probe cable 40 connects the probe to the ultrasound system mainframe. FIG. 4 is a side view of a typical ultrasound probe 10, with the probe cable 40 attached at the proximal end of the probe handle. The transducer array 12 is in the distal end 11 of the probe, positioned to transmit ultrasound and receive echo signals over a scanning area 80 in front of the probe when the distal end is in acoustic contact with the body of a patient.

[0018] The transducer array elements of the probe 10 are coupled to a transmit beamformer 18 and a multiline receive beamformer 20 in the ultrasound system by a transmit/receive (T/R) switch 14. Transmit beamformers are well known in the art and are described in US Pat. pub. no. 2013/0131511 (Peterson et al.), .U.S Pat. No. 6,937,176 (Freeman et al.), U.S. Pat. No. 7,715,204 (Miller), and U.S. Pat. No. 5,581,517 (Gee et al.) for instance, each of which is incorporated by reference in its entirety. As described in these publications, a transmit beamformer for a transducer array has multiple channels, each of which can transmit a drive signal or pulse or waveform for a transducer element at an independently programmed time in relation to the other channels. It is the selected relative timing of the application of the drive signals to the individual transducer elements which provides transmit beam focusing and steering. Coordination of transmission and reception by the beamformers is controlled by a beamformer controller 16, which is controlled by user operation of a user control panel 38. The user can operate the control panel to command the ultrasound system to transmit a single push pulse or multiple simultaneous push pulses during shear wave imaging, for instance. The multiline receive beamformer produces multiple, spatially distinct receive lines (A-lines) of echo signals during a single transmit-receive interval. Multiline beamformers are known in the art as described in U.S. Pat. No. 6,482,157 (Robinson), U.S. Pat. No. 6,695,783 (Henderson et al.), and U.S. Pat. No. 8,137,272 (Cooley et al.), for instance, each of which is incorporated by reference in its entirety. The echo signals are processed by filtering, noise reduction, and the like by a signal processor 22, then stored in an A-line memory 24, a digital memory which stores the echo signal data received along the A-lines. Temporally distinct A-line samples relating to the same spatial vector location are associated with each other in an ensemble of echoes relating to a common point in the image field. The RF echo signals of successive A-line sampling of the same spatial vector are cross-correlated by an A-line RF cross-correlator 26, a processor programmed to perform cross-correlation of signal data, to produce a sequence of samples of tissue displacement for each sampling point on the vector. Alternatively, the A-lines of a spatial vector can be Doppler-processed to detect shear wave motion along the vector, or another phase-sensitive techniques can be employed. A wavefront peak detector 28 is responsive to detection of the shear wave displacement along the A-line vector to detect the peak of the shear wave displacement at each sampling point on the A-line. In a preferred embodiment this is done by a processor performing curve-fitting, although cross-correlation and other interpolative techniques can also be employed if desired. The time at which the peak of the shear wave displacement occurs is noted in relation to the times of the same event at other A-line locations, all to a common time reference, and this information is coupled to a wavefront velocity detector 30, a processor which differentially calculates the shear wave velocity from the peak displacement times on adjacent A-lines. This velocity information is coupled into a velocity display map 32 stored in a buffer, which indicates the velocity of the shear wave at spatially different points in a 2D or 3D image field. The velocity display map is coupled to an image processor 34 which processes the velocity map, preferably overlaying an anatomical ultrasound image of the tissue, for display on an image display 36. Further details of the ultrasound system components of FIG. 1 can be found in US Pat. pub. no. 2013/0131511 (Peterson et al.), which is incorporated by reference in its entirety.

[0019] FIG. 2 is a schematic drawing of components of two channels of transmit and receive beamformers coupled by T/R switches 14 and signal lines of a probe cable 40 to elements e.sub.M and e.sub.M+1 of an N element probe array transducer. The T/R switches 14 are shown as set in the transmit position.

[0020] When set in the receive position, a switch couples a transducer element to an amplifier 42, 52 at the input to a receive beamformer channel. T/R switches are commercially available, such as the TX810 from Texas Instruments of Dallas, Tex., USA. When set as shown for transmit, a channel of the transmit beamformer 18 is coupled to a transducer element. Each transmit channel 49, 50 in the illustrated implementation has a hardware addressable memory device or shift register 48, 58, into which is loaded digital data of a transmit signal as depicted by the sampled waveform illustrations. When it is time for the element coupled to a channel to transmit an ultrasound signal during a transmit interval, the stored digital signal data is addressed or clocked out of the memory device or shift register to a digital-to-analog converter 46, 56. The converter converts the digital data to an analog transmit signal which is amplified by a high voltage transmit amplifier 44, 54. The peak drive voltages on the transducer elements are typically in the range of 5 to 100 volts, depending on the imaging mode. The high voltage transmit signal is applied to a transducer element e.sub.M, e.sub.M+1 by a T/R switch 14 and a signal line of the probe cable 40.

[0021] In accordance with the principles of the present invention, the transmit signals applied to the transducer elements are asymmetric in relation to a zero volt reference potential as illustrated in FIG. 3. This waveform drawing depicts samples of a conventional symmetrical transmit signal 60 and an asymmetric transmit signal 70. The symmetrical transmit signal 60 ranges from peaks of +1 to ?1 and has a mean amplitude value of about zero. The asymmetric transmit signal 70 ranges from a positive peak of 0.5 to a negative peak of ?1 and has a negative mean value of about ?0.25. When applied to a negatively poled transducer element, the asymmetric waveform 70 reinforces the negative poling. When a positively poled transducer is used, the asymmetry of the transmit waveform will have a greater positive peak and/or duration than its negative phase and will have a positive mean value to reinforce the positive poling.

[0022] Another embodiment of transmit signals that reinforce transducer polarization is the combination of a symmetric RF signal and a DC bias voltage. This embodiment includes extra circuitry in the transducer or the ultrasound system to create the DC bias. In a practical implementation, the DC bias circuit can be placed between an AC coupling capacitor and the transducer elements of the transducer array. The DC bias can be generated within the transducer assembly or routed to it from the ultrasound system. An advantage of this alternate embodiment is that it permits the use of simpler transmit signal circuitry within the ultrasound system.

[0023] An asymmetric transmit signal (e.g., the asymmetric RF signal or the combination of a symmetric RF signal and DC bias voltage) in accordance with the present invention is particularly beneficial in the case of high voltage signals of long duration such as those used to produce a push pulse for shear wave diagnosis. For a push pulse, pulses of high MI (e.g., 1.5 to 1.9) and long durations are used so that sufficient energy is transmitted to displace the tissue downward along the beam direction and cause the development of a shear wave. In a typical implementation a push pulse is a long pulse of 50 to 1000 microseconds in duration. A typical duration is 500 microseconds, for instance. With conventional symmetrical transmit signals there is a significant risk of depoling the transducer elements, whereas the asymmetric transmit signals can actually reinforce the poling. This is particularly the case when the transducer elements are of a single crystal material, which will depole at lower electric fields than conventional PZT material. The use of asymmetric transmit signals enables a transducer element to withstand a significantly higher transmit voltage without degradation, which in turn enables higher acoustic output and improved reliability.