Dependence of blood T₂ on oxygenation at 7 T: In vitro calibration and in vivo application

Purpose: The calibratable relationship between blood oxygenation (Y) and T₂ allows quantification of cerebral venous oxygenation. We aim to establish a calibration plot between blood T₂, Y, and hematocrit at 7 T, and using T₂-relaxation-under-spin-tagging MRI, determine human venous blood oxygenation in vivo. Methods: In vitro experiments were performed at 7 T on bovine blood samples using a Carr-Purcell-Meiboom-Gill-T₂ sequence, from which we characterized the relationship among T₂, Y, and hematocrit. T₂-relaxation-under-spin-tagging MRI was implemented at 7 T to measure venous blood T₂ in vivo, from which oxygenation was estimated using the in vitro calibration plot. Hyperoxia was performed to test the sensitivity of the method to oxygenation changes, and the 7 T results were compared with those at 3 T. Results: In vitro data showed that arterial and venous T₂ at 7 T are 68 and 20 ms, respectively, at a typical hematocrit of 0.42. In vivo measurement showed a cerebral venous oxygenation of 64.7 ± 5.0% and a test-retest coefficient-of-variation of 3.6 ± 2.4%. Hyperoxia increased Y_v by 9.0 ± 1.4% (P = 0.001) and the 3 and 7 T results showed a strong correlation (R = 0.95) across individuals. Conclusion: We provided an in vitro calibration plot for conversion of blood T₂ to oxygenation at 7 T and demonstrated its utility in vivo.