Backward-Mode Photoacoustic Imaging Using Illumination Through a CMUT With Improved Transparency.

In this paper, we describe a capacitive micromachined ultrasonic transducer (CMUT) with improved transparency for photoacoustic imaging (PAI) with backside illumination. The CMUT was fabricated on a glass substrate with indium-tin oxide bottom electrodes. The plate was a 1.5- silicon layer formed over the glass cavities by anodic bonding, with a 1- silicon nitride passivation layer on top. The fabricated device shows approximately 30%-40% transmission in the wavelength range from 700 to 800 nm and approximately 40%-60% transmission in the wavelength range from 800 to 900 nm, which correspond to the wavelength range commonly used for in vivo PAI. The center frequency of the CMUT was 3.62 MHz in air and 1.4 MHz in immersion. Two preliminary PAI experiments were performed to demonstrate the imaging capability of the fabricated device. The first imaging target was a 0.7-mm diameter pencil lead in vegetable oil as a line target with a subwavelength cross section. A 2-mm-diameter single CMUT element with an optical fiber bundle attached to its backside was linearly scanned to reconstruct a 2-D cross-sectional PA image of the pencil lead. We investigated the spurious signals caused by the light absorption in the 1.5- silicon plate. For pencil lead as a strong absorber and also a strong reflector, the received echo signal due to the acoustic excitation generated by the absorption in silicon is approximately 30 dB lower than the received PA signal generated by the absorption in pencil lead at the wavelength of 830 nm. The second imaging target was a "loop-shape" polyethylene tube filled with indocyanine green solution ( ) suspended using fishing lines in a tissue-mimicking material. We formed a 3-D volumetric image of the phantom by scanning the transducer in the - and -directions. The two experimental imaging results demonstrated that CMUTs with the proposed structure are promising for PAI with backside illumination.