Characterization of an improved polyimide-etalon all-optical transducer for high-resolution ultrasound imaging

摘要

All-optical transduction of ultrasound provides high-frequency (>20 MHz) operation in the absence of electrical noise and distortion that hinders small-scale piezoelectric probes. Although fabrication of an all-optical 2-D array suitable for in vivo imaging remains incomplete, a thin-film structure integrating a polyimide film with a Fabry-Perot (etalon) receiver has been shown to be a viable candidate. We present here incremental improvements in the performance of a polyimide-etalon transducer and demonstrate imaging with an array configuration alternative to our previous study. We first show that a gain of more than 30% in output pressure is achieved when increasing the thickness of a bare polyimide film from 3 to 15 ;C;m. This motivated the choice of polyimide as the etalon medium-a configuration made possible by utilizing a dielectric mirror that transmits wavelengths used for generation of ultrasound (ultraviolet) and reflects those for detection (near infrared). The increased reflectivity of the dielectric mirror resulted in a 2-fold decrease in noise-equivalent pressure to 3.3 kPa over a bandwidth of 47.5 MHz (0.48 Pa/ Hz). The transmit/receive center frequency increased from 37 to 49 MHz with a -6-dB bandwidth of 126%, and a maximum pressure of 213 kPa was produced using a 43 ;C;m UV spot. A 2 x 2 mm synthetic array of 957 transmitters centered on a 1 x 1 mm synthetic array of four receivers was used to image two wire targets. Offline reconstruction indicated lateral resolutions of 70 and 114 ;C;m at depths of 2.4 and 5.8 mm, respectively, with an average axial resolution of 35 ;C;m. Finally, we explore the challenges of imaging in this configuration, which provides the best opportunity for real-time performance pending further development.