Deep reflection-mode photoacoustic imaging and resolution scalability with depth

A deep reflection-mode photoacoustic (PA) imaging system was designed and implemented to visualize deep structures in biological tissues. To achieve good penetration depth, we chose near IR laser pulses at 804 nm wavelength for the generation of photoacoustic waves. To avoid overshadowing the deep PA signals by the surface PA signals, we employed dark-field illumination. To achieve good lateral resolution, we chose spherically focused high-numerical-aperture ultrasonic transducers with 5 MHz or 10 MHz center frequencies. By using these transducers, we achieved 153 μm and 130 μm axial resolutions, respectively, at 19.5 mm depth in 10% porcine gelatin containing 1% intralipid. The system was applied to imaging internal organs of small animals. Compared with our previous high-frequency (50-MHz) photoacoustic microscope, we scaled up the imaging depth while maintaining the ratio of the imaging depth to axial resolution more than 100. In addition, we studied the scalability of the imaging depth and the resolution with ultrasound frequency.