  The penetration depth of photoacoustic imaging in biological tissues has been
fundamentally limited by the strong optical attenuation when light is delivered
externally through the tissue surface. To address this issue, we previously
reported internal-illumination photoacoustic imaging using a customized
radial-emission optical fiber diffuser, which, however, has complex
fabrication, high cost, and non-uniform light emission. To overcome these
shortcomings, we have developed a new type of low-cost fiber diffusers based on
a graded-scattering method in which the optical scattering of the fiber
diffuser is gradually increased as the light travels. The graded scattering can
compensate for the optical attenuation and provide relatively uniform light
emission along the diffuser. We performed Monte Carlo numerical simulations to
optimize several key design parameters, including the number of scattering
segments, scattering anisotropy factor, divergence angle of the optical fiber,
and reflective index of the surrounding medium. These optimized parameters
collectively result in uniform light emission along the fiber diffuser and can
be flexibly adjusted to accommodate different applications. We fabricated and
characterized the prototype fiber diffuser made of agarose gel and intralipid.
Equipped with the new fiber diffuser, we performed thorough proof-of-concept
studies on ex vivo tissue phantoms and an in vivo swine model to demonstrate
the deep-imaging capability (~10 cm achieved ex vivo) of photoacoustic
tomography. We believe that the internal light delivery via the optimized fiber
diffuser is an effective strategy to image deep targets (e.g., kidney) in large
animals or humans.
