3D Volumetric Photoacoustic (PA) Imaging of Multimodal Porphysome Nanoparticles

Photoacoustic imaging adds functional information on the conventional B-mode ultrasound images. Combining photoacoustic information with micro-ultrasound has potential to aid localization and delineation of cancerous lesions to be targeted for focal therapies. The photoacoustic signal in the tumor is primarily generated by hemoglobin but exogenous photoacoustic contrast agents such as novel, biocompatible porphysome nanoparticles can be used to enhance photoacoustic signal. This work demonstrates 3D in vivo micro-ultrasound and photoacoustic imaging of porphysome nanoparticles in a subcutaneous mouse tumor model up to 48 hours after the tail-vein injection. Linear spectral unmixing was used to separate the photoacoustic signal contribution from oxygenated, deoxygenated hemoglobin and porphysomes. 3D imaging revealed the spatial distribution of nanoparticles in the tumor over time. Longitudinal imaging with porphysomes showed that the photoacoustic signal from contrast agent increased after the injection and was co-localized with oxygenated hemoglobin. The presence of nanoparticles in the tumor was also confirmed by the fluorescence imaging which showed the signal peak at 24 hours post-injection potentially due to disassociation of the particle in vivo. Fluorescence histology also confirmed the presence of nanoparticles in the tumor.