Fabrication and performance of a miniaturized 64-element high-frequency endoscopic phased array

We have developed a 42 MHz, 64-element phased array transducer packaged in a 2.5 mm by 3.1 mm endoscopic form factor. The array is a forward looking semi-kerfed design based on a 0.68Pb(Mg_1/3Nb_2/3)O ₃-0.32PbTiO₃ (PMN-32%PT) single-crystal wafer with an element-to-element pitch of 38 microns. In order to achieve a miniaturized form factor, a novel technique of wire bonding the array elements to a polyimide flexible circuit board oriented parallel to the forward looking ultrasound beam and perpendicular to the array was developed. A technique of dicing partially into the back of the array was also implemented in order to improve the directivity of the array elements. The array was fabricated with a single layer P(VDF-TrFE)-copolymer matching layer and a polymethylpentene (TPX) lens for passive elevation focusing to a depth of 7 mm. The two-way -6 dB pulse bandwidth was measured to be 55% and the average electromechanical coupling (k _T) for the individual elements was measured to be 0.62. The one-way -3 dB directivity from several array elements was measured and found to be ± 20 degrees, which was shown to be an improvement over an identical kerfless array. The -3 dB one-way elevation focus resulting from the TPX lens was measured to be 152 microns at the focal depth, and the focused lateral resolution was measured to be 80 microns at a steering angle of 0°. To generate beam profiles and images the probe was connected to a Visualsonics Vevo 2100 imaging platform which was reprogrammed to allow for phased array transmit beamforming and receive data collection. The collected RF data was then processed offline using a Matlab script to generate sector images. Focused one-way transmit radiation patterns were collected with a needle hydrophone. Two-way images were generated with a dynamic range of 60 dB for wire phantoms in water and a tissue-equivalent medium. Finally, ex-vivo tissue images were generated of porcine brain tissue.