O2-Sensitive MRI Distinguishes Brain Tumor Versus Radiation Necrosis in Murine Models

Purpose: The goal of this study was to quantify the relationship between the H-1 longitudinal relaxation rate constant, R-1, and oxygen (O-2) concentration (relaxivity, r(1)) in tissue and to quantify O-2-driven changes in R-1 (Delta R-1) during a breathing gas challenge in normal brain, radiation-induced lesions, and tumor lesions. Methods: R-1 data were collected in control-state mice (n = 4) during three different breathing gas (and thus tissue O-2) conditions. In parallel experiments, pO(2) was measured in the thalamus of control-state mice (n = 4) under the same breathing gas conditions using an O-2-sensitive microprobe. The relaxivity of tissue O-2 was calculated using the R-1 and pO(2) data. R-1 data were collected in control-state (n = 4) mice, a glioma model (n = 7), and a radiation necrosis model (n = 6) during two breathing gas (thus tissue O-2) conditions. R-1 and Delta R-1 were calculated for each cohort. Results: O-2 r(1) in the brain was 9 x 10(-4) +/- 3 x 10(-4) mm Hg-1.s(-1) at 4.7T. R-1 and Delta R-1 measurements distinguished radiation necrosis from tumor (P<0.03 and P<0.01, respectively). Conclusion: The relaxivity of O-2 in the brain is determined. R-1 and Delta R-1 measurements differentiate tumor lesions from radiation necrosis lesions in the mouse models. These pathologies are difficult to distinguish by traditional imaging techniques; O-2-driven changes in R-1 holds promise in this regard. (C) 2015 Wiley Periodicals, Inc.