Tumor response to hypoxic gas breathing challenge was explored using oxygen-sensitive MRI in rat 13762NF breast tumors and compared with the more conventional hyperoxic gas breathing challenge. In viable tumor regions, blood oxygenation decreased with 16% O2 breathing (demonstrated by an increase in R2*). A unique “on-and-off” pattern in T1-weighted signal intensity, which was closely related to tumor hypoxia level, was triggered by 16% O2 breathing but not 100% O2 breathing. When combined with hypoxic gas breathing, oxygen-sensitive MRI may reveal tumor hypoxia threshold, which is important for assessing the efficacy of hypoxia-activated prodrugs.
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