System and method of operating microfabricated ultrasonic transducers for harmonic imaging
A capacitive microfabricated ultrasonic transducer (cMUT) is operated to improve its performance during harmonic imaging in non-linear media, such as in contrast agents or in human tissue. The cMUT is operated by inverting the transmit waveform to adjacently spaced azimuth elements, and combining at least two additional firings without adjacent inversion, for each transmit vector, thereby canceling the second harmonic generation of the cMUT; and thus, the performance of harmonic imaging using the cMUTs can achieve improvement.
1. A method of ultrasonically imaging an object that reduces undesired transmitted harmonic distortion comprising the steps of:
generating a plurality of transmit waveforms centered around a predetermined fundamental frequency from a plurality of microfabricated ultrasonic transducer transmit elements that are within a microfabricated transducer array, wherein at least one of the plurality of transmit waveforms generated contains the fundamental frequency and transmitted harmonic frequencies, and wherein at least a second of the plurality of transmit waveforms generates information at harmonic frequencies, the information modeling distortion created by the microfabricated transducer array;
detecting return waveforms associated with the transmit waveforms that are received by microfabricated ultrasonic transducer receive elements within the microfabricated transducer array and generating return signals corresponding thereto, at least one of the return signals having the fundamental frequency and the transmitted harmonic frequencies, and wherein at least a second of the return signals having the information at the harmonic frequencies, the information modeling distortion created by the microfabricated transducer array;
combining the return signals such that signal components having the transmitted fundamental frequency and the transmitted harmonic frequencies are minimized, wherein combining the return signals comprises subtracting out the information at the harmonic frequencies, the information modeling distortion created by the microfabricated transducer array; and
imaging by providing data from the combined return signals which represents a harmonic image of the object having reduced transmitted harmonic distortion therein;
wherein the step of generating the plurality of transmit waveforms applies a transmit signal to different transducer transmit elements, the transmit signal applied to the different transducer transmit elements differing in a delay profile; and the information at the harmonic frequencies modeling distortion created by the microfabricated transducer array are generated by alternating the polarity of the transmit signal applied to adjacent transducer transmit elements.
2. The method according to claim 1 , wherein the step of generating the plurality of transmit waveforms includes the steps of:
repeating the steps of generating, detecting, and combining for each of a plurality of vectors, each vector formed from three transmit waveforms and three associated return signals.
3. The method according to claim 1 , wherein the step of generating the plurality of transmit waveforms includes the step of generating arbitrary signals that are applied to different transducer transmit elements; and
the step of detecting detects the return waveform corresponding to the generated arbitrary signals and generates return signals corresponding thereto; and
the step of combining the return signals includes combining the return signals to minimize inherent asymmetries and distortions of the microfabricated transducer array.
4. The method according to claim 1 wherein the steps of generating and detecting are applied to multiple focal zones to improve cancellation of the fundamental frequency.
5. The method according to claim 1 , wherein the steps of generating and receiving are performed independent of bias applied to the transducer transmit elements and the transducer receive elements.
6. The method according to claim 1 , wherein the information at the harmonic frequencies modeling distortion created by the microfabricated transducer array contains substantially only even harmonics generated by the microfabricated transducer array.
7. A method of ultrasonically imaging an object that reduces undesired transmitted harmonic distortion comprising the steps of:
generating a plurality of transmit waveforms centered around a predetermined fundamental frequency from a plurality of microfabricated ultrasonic transducer transmit elements that are within a microfabricated transducer array, wherein at least one of the plurality of transmit waveforms generated contains the fundamental frequency and transmitted harmonic frequencies, and wherein at least a second of the plurality of transmit waveforms generates information at harmonic frequencies, the information modeling distortion created by the microfabricated transducer array;
detecting return waveforms associated with the transmit waveforms that are received by microfabricated ultrasonic transducer receive elements within the microfabricated transducer array and gene rating return signals corresponding thereto, at least one of the return signals having the fundamental frequency and the transmitted harmonic frequencies, and wherein at least a second of the return signals having the information at the harmonic frequencies, the information modeling distortion created by the microfabricated transducer array;
combining the return signals such that signal components having the transmitted fundamental frequency and the transmitted harmonic frequencies are minimized, wherein combining the return signals comprises subtracting out the information at the harmonic frequencies, the information modeling distortion created by the microfabricated transducer array; and
imaging by providing data from the combined return signals which represents a harmonic image of the object having reduced transmitted harmonic distortion therein;
wherein the step of generating the plurality of transmit waveforms includes the step of repeating the steps of generating, detecting, and combining for each of a plurality of vectors, each vector formed from three transmit waveforms and three associated return signals; and
wherein the three transmit waveforms include:
a first transmit waveform having the fundamental frequency and the transmitted harmonic frequencies;
a second transmit waveform having the fundamental frequency and the transmitted harmonic frequencies, the second transmit waveform being inverted from the first transmit waveform; and
a third transmit waveform having the transmitted harmonic frequencies and a greatly reduced fundamental frequency component, the third transmit waveform generated by alternating a polarity of a transmit signal applied to adjacent transducer transmit elements.
8. The method according to claim 7 , wherein the first, second and third transmit waveforms are sequentially generated.
9. The method according to claim 8 , wherein the step of combining implements the algorithm F1+F2−(2*F3), wherein F1 is received voltage v. time data from the first transmit waveform, F2 is received voltage v. time data from the second transmit waveform, and F3 is received voltage v. time data from the third transmit waveform.
10. An ultrasound system for imaging an object that reduces undesired transmitted harmonic distortion comprising:
a microfabricated transducer array comprising:
a plurality of microfabricated ultrasonic transducer transmit elements operable to generate a plurality of transmit waveforms centered around a predetermined fundamental frequency, wherein a first transmit waveform is of the fundamental frequency and transmitted harmonic frequencies, wherein a second transmit waveform is of the fundamental frequency and the transmitted harmonic frequencies, and wherein a third transmit waveform is of substantially only the transmitted harmonic frequencies, the third transmit waveform generated by alternating a polarity of a transmit signal applied to adjacent transducer transmit elements; and
a plurality of microfabricated ultrasonic transducer receive elements operable to detect return waveforms associated with the transmit waveforms and generate return signals corresponding thereto, wherein a first return signal is of the fundamental frequency and the transmitted harmonic frequencies, wherein a second return signal is of the fundamental frequency and the transmitted harmonic frequencies, and wherein a third return signal is of substantially only the transmitted harmonic frequencies;
a mathematical processor operable to create image data representing a harmonic image of the object by:
combining the first and second return signals; and
subtracting out the third return signal; and
a display operable to show the image data.
11. The ultrasound system according to claim 10 , wherein the transducer transmit elements are further operable to invert the second transmit waveform from the first transmit waveform by a difference in delay profile from the first transmit waveform.
12. The ultrasound system according to claim 10 , wherein the transducer transmit elements are further operable to generate the first, second, and third transmit waveforms serially for each of a plurality of image frame vectors.
13. The ultrasound system according to claim 10 , wherein the mathematical processor is operable to subtract out the third return signal by subtracting out the third return signal twice.