Study on Needle Hydrophone for Low Frequency Photoacoustic Imaging

Abstract

The low frequency photoacoustic imaging system usually uses several MHz ultrasound signals. In order to detect the signals, several kinds of ultrasound transducers including needle hydrophones have been developed so far. However, the sensitivity and frequency bandwidth of those transducers are still not enough to detect very small photoacoustic signals from specimen with low thermoelastic effect, such as biological tissues. In this paper, a design and fabrication of a new type of needle hydrophone with high sensitivity and wide bandwidth in the range of 1~10 MHz is described in detail. The piezoelectric material of the hydrophone is lead magnesium niobate-lead titatnate (PMN-PZT) which has very high electromechanical coupling coefficient ( > 0.9 ). The sensing part of the hydrophone is small (0.5x0.5 mm2) and has two matching layers on the tip of the needle. Construction and performance are compared with other types of needle hydrophones. The receiving frequency responses of the hydrophones are simulated by the PiezoCAD program based on the KLM equivalent circuit model. It was characterized to be more sensitive than PVDF needle hydrophones of the similar structure, and it was confirmed by experiment. Furthermore an Acoustic Needle Imaging System for low frequency photoacoustic imaging was established using this fabricated PMN-PZT needle hydrophone to get some images of specimens. The images of the ICG specimen and human hair obtained by the established system have high resolution and high contrast. Conclusively, it is shown that the hydrophone is very useful in photoacoustic imaging.