Quality improvement of acoustic-resolution photoacoustic imaging of skin vasculature based on practical synthetic-aperture focusing and bandpass filtering

We previously developed a compact acoustic-resolution photoacoustic (PA) imaging system with optical fiber-based illumination, by which blood vessels in the rat skin were visualized in vivo. However, there were issues to be improved in the imaging characteristics: blurring in the out-of-focus region of the ultrasound sensor and degraded resolution due to low-frequency components in the PA signals. These are common limitations in acoustic-resolution PA imaging methods. In this study, we attempted to solve these problems by introducing signal processing methods based both on the synthetic-aperture focusing technique, which was applied only to signals in the detection zone of the sensor in the horizontal direction that was determined in advance in a phantom study, and on digital acoustic signal filtering with three specific frequency bands. Ex vivo and in vivo PA imaging of blood vessels in the rat skin and subcutaneous tissue showed the validity of the present signal processing methods.