Validation of T₂-based oxygen extraction fraction measurement with ¹⁵O positron emission tomography

Purpose: To evaluate the accuracy of T₂-based whole-brain oxygen extraction fraction (OEF) estimation by comparing it with gold standard ¹⁵O-PET measurements. Methods: Sixteen healthy adult subjects underwent MRI and ¹⁵O-PET OEF measurements on the same day. On MRI, whole-brain OEF was quantified by T₂-relaxation-under-spin-tagging (TRUST) MRI, based on subject-specific hematocrit. The TRUST OEF was compared to the whole-brain averaged OEF produced by ¹⁵O-PET. Agreement between TRUST and ¹⁵O-PET whole-brain OEF measurements was examined in terms of intraclass correlation coefficient (ICC) and in absolute OEF values. In a subset of 10 subjects, test-retest reproducibility of whole-brain OEF was also evaluated and compared between the two modalities. Results: Across the 16 subjects, the mean whole-brain OEF of TRUST and ¹⁵O-PET were 36.44 ± 4.07% and 36.45 ± 3.65%, respectively, showing no difference between the two modalities (P =.99). TRUST whole-brain OEF strongly correlated with that of ¹⁵O-PET (N = 16, ICC = 0.90, P = 4 × 10⁻⁷). The coefficient-of-variation of TRUST and ¹⁵O-PET whole-brain OEF measurements were 1.79 ± 0.67% and 2.06 ± 1.55%, respectively, showing no difference between the two modalities (N = 10, P =.64). Further analyses on the effect of hematocrit revealed that correlation between PET OEF and TRUST OEF with assumed hematocrit remained significant (ICC = 0.8, P < 2 × 10⁻⁵). Conclusion: Whole-brain OEF measured by TRUST was in excellent agreement with gold standard ¹⁵O-PET, with highly comparable accuracy and reproducibility. These findings suggest that TRUST MRI can provide accurate quantification of whole-brain OEF noninvasively.