Traditional high numerical aperture spherical focused ultrasound transducers (SUT) are widely used in photoacoustic microscopy (PAM) to provide high resolution, but it has a rapidly degrading sensitivity and lateral resolution as depth increasing. And there is the same situation in ultrasound imaging. This study aims to extend the acoustic focal depth by developing an ellipsoidal focused ultrasound transducer (EUT). The proposed transducer has continuous focus instead of the spherical single-focus one, which provides a 3 times larger focal depth range and better lateral resolution over a large region.
In order to obtain greater bandwidth and the appropriate acoustic impedance matching the human body, we select the piezoelectric element of Polyvinylidene Fluoride Trifluoroethylene P(VDF-TrFE) to fabricate the EUT, which was laminated onto a curved steel surface for self-focusing. A 2 mm hole is set in the center of the transducer to allow the laser to pass through the transducer. After pulse-echo test by a pulser/receiver and analysis spectrum result, EUT has the center frequency of over 30 MHz and -6 dB bandwidth of 124%. Additionally, a Fiber-optic hydrophone system was used to verify the acoustic characteristic and acoustic field distribution. And a PAM system was built to evaluate fine temporal and spatial features. The stability of the PAM system was proved by the phantom experiment. In vivo mouse ear and brain experiments were performed to characterize the performance of EUT. By comparing the results of in vivo imaging, it can be seen that EUT has good resolution and sensitivity at different depths.