MRI hypoxia measurements

Tumor hypoxia is a key feature of solid tumors. It influences tumor growth, metastatic risk, and resistance to a number of important treatment strategies including radiation, chemotherapy, and thermal ablations. Accordingly, much effort has been spent on the development of noninvasive image-based measurements of tissue oxygenation. MR oxygenation measurements, in particular, are subject of extensive research, due to the favorable soft-tissue contrast, extensive availability, and lack of ionizing radiation, which is particularly beneficial in longitudinal or treatment follow-up studies. Information on tissue oxygenation can be derived from the MR relaxation parameters R₂*, i.e., the reversible transverse relaxation rate, and R₁, i.e., the longitudinal relaxation rate. Historically, the blood-oxygen-level-dependent (BOLD) imaging technique is the most prominent and frequently used. Oxygen-enhanced MRI R₁- and/or R₂*-weighted imaging or quantification is becoming increasingly popular, since it allows the separation of the effect of oxygenation on the relaxation properties from other influences and offers a novel contrast mechanism distinct from BOLD. Quantitative measurements of the oxygen partial pressure can be obtained from ¹⁹F imaging techniques and approaches that combine electron spin resonance (ESR) and Overhauser effects. Other methods are sensitive to hypoxia itself, but do not provide direct pO₂estimates. This chapter elaborates on the methodology of different MR-based oxygenation measurements and presents exemplary results from their evaluation in oncologic applications. It concludes with a comparison of techniques and an outlook on their future role in oncology.