High frequency optoacoustic transducers for ultrasonic and photoacoustic imaging

The front end of all ultrasound and photoacoustic imaging systems includes an ultrasonic transducer to convert pressure waves into electric signals (and vice versa in the case of ultrasound imaging). Immediately after World War II, piezoelectric crystals (e.g., quartz) were the transducer materials in the first biomedical applications of ultrasound. They were quickly replaced with more efficient transducer materials developed during the war, the piezoelectric ceramics (e.g., lead zirconate titanate, or PZT) [1-3]. Ultrasound applications exploded in the 1970s when high efficiency PZT array transducers were developed for electronic, real-time scanning [4]. Although piezoelectric ceramic arrays have been constructed for a wide range of applications, and sophisticated manufacturing 224methods have been developed in the last two decades to produce very high yields, these transducers are still primarily manufactured using “dice and fill” technology [5], limiting their complexity. In particular, high-frequency operation is limited by the small required kerf widths, (several micrometers) electrical connections, and cross talk (both acoustical and electrical) between elements. While there has been significant progress [6-12], it is still extremely difficult to produce 1D piezoelectric arrays operating at 50 MHz or higher. High-frequency 2D arrays pose even greater challenges.