Venous Blood Oxygenation Measurements Using TRUST and T2-TRIR MRI During Hypoxic and Hypercapnic Gas Challenges

Background: Oxygen extraction fraction (OEF) and cerebral metabolic rate of oxygen (CMRO2) may serve as biomarkers in several diseases. OEF and CMRO2 can be estimated from venous blood oxygenation (Y-v) levels, which in turn can be calculated from venous blood T2 values (T2(b)). T2(b) can be measured using different MRI sequences, including T2-relaxation-under-spin-tagging (TRUST) and T2-prepared-blood-relaxation-imaging-with-inversion-recovery (T2-TRIR). The latter measures both T2(b) and T1 (T1(b)) but was found previously to overestimate T2(b) compared to TRUST. It remained unclear, however, if this bias is constant across higher and lower oxygen saturations. Purpose: To compare TRUST and T2-TRIR across a range of O-2 saturations using hypoxic and hypercapnic gas challenges. Study Type: Prospective. Population: Twelve healthy volunteers (four female, age 3610years). Field Strength/Sequence: A 3T; turbo-field echo-planar-imaging (TFEPI), echo-planar-imaging (EPI), and fast-field-echo (FFE). AssessmentTRUST- and T2-TRIR-derived T2(b), Y-v, OEF, and CMRO2 were compared across different respiratory challenges. T1(b) from T2-TRIR was used to estimate Hct (Hct(TRIR)) and compared with venipuncture (Hct(VP)). Statistical Tests: Shapiro-Wilk, one-sample and paired-sample t-test, repeated measures ANOVA, Friedman test, Bland-Altman, and correlation analysis. Bonferroni multiple-comparison correction was performed. Significance level was 0.05. Results: A significant bias was observed between TRUST- and T2-TRIR-derived T2(b), Y-v, and OEF values (-1311msec, -5.3%3.5% and 5.9 +/- 4.1%, respectively). For Y-v and OEF, this bias was constant across the range of measured values. T1(b) was significantly lower during severe hypoxia and hypercapnia compared to baseline (1712 +/- 86msec and 1634 +/- 79msec compared to 1757 +/- 90msec). While no significant bias was found between Hct(VP) and Hct(TRIR) (0.02% +/- 0.06%, P = 0.20), the correlation between these Hct values was significant but weak (r=0.19). Data Conclusion: Given the constant bias, TRUST- and T2-TRIR-derived venous T2(b) values can be used interchangeably to estimate Y-v, OEF, and CMRO2 across a broad range of oxygen saturations. Hct from T2-TRIR-derived T1-values only weakly correlated with Hct from venipuncture.