Figure 4.

Utility of deep-tissue photoacoustic imaging to monitor PDT efficacy. The ultrasound image demarcates the location and size of the subcutaneous tumor (glioblastoma U87 cells). Spectroscopic photoacoustic imaging provides blood oxygen saturation map of the tumor at the same cross-section. The oxygen saturation maps are pseudo colored on a black (0%) to red (100%) scale. Immunofluorescence image at the same cross section of the tumor is obtained post-euthanasia. The vasculature is stained in green while red stain shows the hypoxic regions in the tumor. Hypoxic conditions are caused in PDT either due to consumption during the process or via vascular coagulation post-PDT. Here we observe that deeper tumor regions had no hypoxia stain (indicated by yellow arrows) or reduction in oxygen saturation indicating insufficient light dose reaching these deeper tissues. Incorporating therapy monitoring techniques to identify non-responsive or untreated areas is highly important and critical to prevent subsequent regrowth of these regions by designing appropriate therapy. Figure adapted from Mallidi et al. ⁴⁷