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 ¹H longitudinal relaxation rate constant, R₁, and oxygen (O₂) concentration (relaxivity, r₁) in tissue and to quantify O₂-driven changes in R₁ (ΔR₁) during a breathing gas challenge in normal brain, radiation-induced lesions, and tumor lesions. Methods R₁ data were collected in control-state mice (n = 4) during three different breathing gas (and thus tissue O₂) conditions. In parallel experiments, pO₂ was measured in the thalamus of control-state mice (n = 4) under the same breathing gas conditions using an O₂-sensitive microprobe. The relaxivity of tissue O₂ was calculated using the R₁ and pO₂ data. R₁ 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 O₂) conditions. R₁ and ΔR₁ were calculated for each cohort. Results O₂ r₁ in the brain was 9 × 10^-4 ± 3 × 10^-4 mm Hg^-1· s^-1 at 4.7T. R₁ and ΔR₁ measurements distinguished radiation necrosis from tumor (P< 0.03 and P< 0.01, respectively). Conclusion The relaxivity of O₂ in the brain is determined. R₁ and ΔR₁ measurements differentiate tumor lesions from radiation necrosis lesions in the mouse models. These pathologies are difficult to distinguish by traditional imaging techniques; O₂-driven changes in R₁ holds promise in this regard.