Dual-frequency super harmonic imaging piezoelectric transducers for transrectal ultrasound

In this paper, a 2/14 MHz dual-frequency single-element transducer and a 2/22 MHz sub-array (16/48-elements linear array) transducer were developed for contrast enhanced super-harmonic ultrasound imaging of prostate cancer with the low frequency ultrasound transducer as a transmitter for contrast agent (microbubble) excitation and the high frequency transducer as a receiver for detection of nonlinear responses from microbubbles. The 1-3 piezoelectric composite was used as active materials of the single-element transducers due to its low acoustic impedance and high coupling factor. A high dielectric constant PZT ceramic was used for the sub-array transducer due to its high dielectric property induced relatively low electrical impedance. The poßible resonance modes of the active elements were estimated using finite element analysis (FEA). The pulse-echo response, peak-negative preßure and bubble response were tested, followed by in vitro contrast imaging tests using a graphite-gelatin tißue-mimicking phantom. The single-element dual frequency transducer (8 × 4 × 2 mm³) showed a-6 dB fractional bandwidth of 56.5 % for the transmitter, and 41.8 % for the receiver. A 2 MHz-transmitter (730 μm pitch and 6.5 mm elevation aperture) and a 22 MHz-receiver (240 μm pitch and 1.5 mm aperture) of the sub-array transducer exhibited-6 dB fractional bandwidth of 51.0 % and 40.2 %, respectively. The peak negative preßure at the far field was about-1.3 MPa with 200 V_pp, 1-cycle 2 MHz burst, which is high enough to excite microbubbles for nonlinear responses. The 7th harmonic responses from micro bubbles were succeßfully detected in the phantom imaging test showing a contrast-to-tißue ratio (CTR) of 16 dB.