Deep reflection-mode photoacoustic imaging of biological tissue

A reflection-mode photoacoustic (PA) imaging system was designed and built to image deep structures in biological tissues. We chose near-infrared laser pulses of $804-nm$ wavelength for PA excitation to achieve deep penetration. To minimize unwanted surface signals, we adopted dark-field ring-shaped illumination. This imaging system employing a $5-MHz$ spherically focused ultrasonic transducer provides penetration up to $38mm$ in chicken breast tissue. At the $19-mm$ depth, the axial resolution is $144μm$ and the transverse resolution is $560μm$. Internal organs of small animals were imaged clearly.