O 2 ‐sensitive MRI distinguishes brain tumor versus radiation necrosis in murine models

Purpose The goal of this study was to quantify the relationship between the 1 H 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 (ΔR 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 ΔR 1 were calculated for each cohort. Results O 2 r 1 in the brain was 9 × 10 −4  ± 3 × 10 −4 mm Hg −1 · s −1 at 4.7T. R 1 and ΔR 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 ΔR 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. Magn Reson Med 75:1–2, 2016. © 2015 Wiley Periodicals, Inc.