Implications of tumor oxygenation and blood flow for cancer treatment monitoring using photoacoustic imaging and power Doppler

Photoacoustic (PA) imaging has been proposed for cancer treatment monitoring. Tumor oxygen saturation (sO2) should in principle be related to vascular parameters such as blood flow. In this work, in-vivo PA estimates of sO2 were compared to power Doppler (pD) measures of vascularity hours after the administration of microbubbles (MB), radiation therapy (XRT), individually or combined (MB-XRT). Forty SCID mice were inoculated with subcutaneous, PC3 tumors in the left hind leg. The treatment groups consisted of: MB-XRT (N=12, 3% w/v, 550 kHz insonication, 570 kPa peak negative pressure, 8 Gy radiation), XRT only (N=10) and MB only (N=8) and untreated control (N=10). The VevoLAZR system (FujiFilm-VisualSonics) was used to acquire 3D PA (750/850 nm, 21 MHz ultrasonic frequency) and pD (16 MHz frequency, 2 kHz PRF) datasets at three timepoints: pre-treatment, 2 hours and 24 hours post-treatment. Distributions of sO2 within the tumor were quantified at multiple planes using histograms. The Vascularization Index (VI) was estimated from the pD data for the same tumor planes as PA imaging at each timepoint.