The traditional external light illumination in PACT usually limits the penetration depth to a few centimeters from the tissue surface, mainly due to strong optical attenuation along the light propagation path from the outside in. By contrast, internal light illumination, with external ultrasound detection, can potentially detect much deeper targets. We have been developing a variety of different internal-illumination PACT systems using customized optical fibers with a diffuser attached to the fiber tip, which can homogeneously illuminate the surrounding space and substantially enlarge the field of view. Our results have collectively demonstrated that the internal light illumination combined with external ultrasound detection might be a useful strategy to improve the penetration depth of PACT in imaging deep organs of large animals and humans.
