Real-Time Assessment of Tissue Hypoxia In Vivo with Combined Photoacoustics and High-Frequency Ultrasound

In preclinical cancer studies, non-invasive functional imaging has become an important tool to assess tumor development and therapeutic effects. Tumor hypoxia is closely associated with tumor aggressiveness and is therefore a key parameter to be monitored. Recently, photo- acoustic (PA) imaging with inherently co-registered high-frequency ultrasound (US) has reached preclinical applicability, allowing parallel collection of anatomical and functional information. Du- al-wavelength PA imaging can be used to quantify tissue oxygen saturation based on the absorb- ance spectrum differences between hemoglobin and deoxyhemoglobin. Experimental Design: A new bi-modal PA/US system for small animal imaging was employed to test feasibility and reliability of dual-wavelength PA for measuring relative tissue oxygenation. Murine models of pancreatic and colon cancer were imaged, and differences in tissue oxygenation were compared to immunohistochemistry for hypoxia in the corresponding tissue regions. Results: Functional studies proved feasibility and reliability of oxygenation detection in murine tissue in vivo. Tumor models exhibited different levels of hypoxia in localized regions, which pos- itively correlated with immunohistochemical staining for hypoxia. Contrast-enhanced imaging yielded complementary information on tissue perfusion using the same system. Conclusion: Bimodal PA/US imaging can be utilized to reliably detect hypoxic tumor regions in murine tumor models, thus providing the possibility to collect anatomical and functional infor- mation on tumor growth and treatment response live in longitudinal preclinical studies.