Improved depth-of-field photoacoustic microscopy with a custom high-frequency annular array transducer

Researchers have been using single element transducers for photoacoustic microscopy (PAM), but such systems have limited depth of field due to a single focus. The aim for this project was to develop a high-frequency annular array transducer for improved depth-of-field PAM. We have designed a concave 40 MHz ultrasound transducer which has 8 annular array elements with equal area. The outer ring is 12 mm in diameter, the geometric focus is 12 mm, and the space between each annulus is 100 μm. The array was fabricated by lithographically patterning metalized polyimide film to define back electrodes and signal leads. 9-micron-PVDF film was then press-fit into the array pattern with epoxy as a backing material and a single drop of epoxy as a bonding layer. The array exhibits high sensitivity to high-frequency photoacoustic signals. Dynamic focusing of amplified and digitized signals permits extended depth-of-field imaging compared to the single-element transducer case. Dark-field light-delivery and 3-axis motorized scanning permits 3-D photoacoustic microscopy. Imaging performance in phantoms is discussed.