Multiangle Long-Axis Lateral Illumination Photoacoustic Imaging to Evaluate Tumor Oxygenation

Photoacoustic imaging (PAI) is an emergent technique with a wide range of biomedical applications. An interesting application of PAI is the non-invasive estimation of tissue oxygenation, which is an important parameter for early prediction of tumor response to treatment. PAI is directly affected by light fluence distribution within the target; therefore, efficient light delivery is essential to generate high quality images. Recently, we proposed a novel multiangle long-axis lateral illumination (MALLI) scheme for hand-held PAI. This new illumination strategy showed remarkable improvement in light delivery to targets smaller than the ultrasound transducer width. Subcutaneous xenograft mouse model is a typical example where the target is smaller than the transducer width. Therefore, in this study multispectral PA images were acquired from subcutaneous tumors implanted in mice with the objective to evaluate the efficiency of MALLI to map tumor vascularization and oxygen saturation (SO
2
). To this end, this approach was first validated acquiring multispectral PA images of tissue-mimicking phantoms. After this validation step, multispectral PA images were acquired from prostate carcinoma (PC-3) and melanoma (HT-144). The PA images clearly showed the blood vessels irrigating the tumors. For those blood vessels surrounding the tumors, a mean SO
2
value of 95% was estimated, and hypoxic regions were identified within the tumor with a mean SO
2
value of 86%. These results demonstrated the efficiency of MALLI-PAI to map SO2