Optimizing light delivery through ball-shaped multimode fiber tips in co-registered photoacoustic and ultrasound endo-cavity imaging: Simulation and experimental validation

Adenocarcinoma is a type of cancers that forms in mucus-secreting glands throughout the body. For example, adenocarcinomas make up around 96% of colorectal cancers and some cervical cancers (10%). Depth of invasion for early adenocarcinoma is generally around 1-7 mm. In this report, we propose an optimal design, implementation, and evaluation of an optical fiber-based transvaginal photoacoustic/ultrasound prototype imaging probe for endo-cavity imaging of adenocarcinoma with ball-shaped fiber tips (refractive index = 1.517, radius 0.75 mm), to improve the light illumination homogeneity and increase the light delivery fluence on the central imaging area of the tissue surface. The light delivery system consists of light coupling optics, a custom-made transducer sheath, four 1-mm-core multimode optical fibers with the ball-shaped fiber tip arranged around the transducer. The probe design was optimized by simulating the light fluence distribution of the fibers with and without ball-shaped tips using a 3D model in Zemax for different design parameters such as fiber effective numerical aperture, fiber displacement from the probe base, sheath taper angle, ball-shaped fiber tips' diameter and refractive index, etc. The laser fluence profiles were experimentally recorded through calibrated intralipid solution by camera at various imaging depths. The output power on the central imaged areas were experimentally measured by calculating the PAT signal strengths from the black threads buried inside chicken breast tissue at various depths and in-vivo photoacoustic imaging of one palmar vein proximal to the human wrist.