An Ellipsoidal Focused Ultrasound Transducer for Extend-Focus Photoacoustic Microscopy

Abstract

Limited by spherical focused ultrasound transducer (SUT) with a high acoustic numerical aperture, photoacoustic microscopy (PAM) suffers a rapidly degrading sensitivity and lateral resolution as increased depth. In this study, an ellipsoidal focused ultrasound transducer (EUT) was developed to address the above restriction via providing high sensitivity and lateral resolution over a large depth of field (DOF). To fabricate the EUT, the piezoelectric element was laminated onto a curved steel surface for self-focusing (the ellipsoidal continuous-focus geometry was employed instead of the spherical single-focus one). Additionally, phantoms and in vivo animal experiments were performed by an extend-focus PAM equipped with EUT to characterize its performance. The EUT involved over 30 MHz center frequency and -6 dB bandwidth of 124% with a resolution-invariant focal depth of 1.39 mm, more than 3 times the DOF of the SUT. The in vivo imaging results demonstrated that the EUT was capable of extending the focal depth to get rid of the restriction of the visual field, while the DOF of the SUT was limited by the nature of spherical geometry. The EUT markedly enhances the image quality at different imaging depths, which has great potential for promoting the biomedical development of in vivo rapid-noninvasive PAM.