Estimating synthetic hematocrit and extracellular volume from native blood pool T1 times at 3 Tesla CMR: Derivation of a conversion equation, accuracy and comparison with published formulas.

PURPOSE

Calculation of extracellular volume fraction (ECV), a marker of myocardial fibrosis in cardiac magnetic resonance imaging (CMR), requires hematocrit (Hct). We aimed to correlate Hct levels with native blood T1 times, to derive a formula for estimating synthetic Hct (Hct) and synthetic ECV (ECV), to assess accuracy of ECV and to compare our model with published formulas.

METHOD

In this retrospective study, a cohort of 250 CMR scans with T1 mapping (3T, MOLLI 5(3)3, endsystolic aquisition), was divided into a derivation and validation cohort. Native T1 times of the left ventricular blood pool (T1) were correlated with Hct levels from blood sampling within 24 h (Hct) and a formula for calculation of Hct was derived by linear regression.

RESULTS

In the derivation cohort (n = 167), Hct showed a good association with T1 (r = -0.711, p < 0.001). The resulting regression equation was Hctsyn = 1/T1 * 1355.52-0.310. In the validation cohort (n = 83), Hct and Hct showed good correlation (r = 0.726, p < 0.001), while ECV, and ECV demonstrated excellent correlation (r = 0.940, p < 0.001). ECV had a minimal bias of 0.28 % and the misclassification rate (8.8 %) was comparable to the variability introduced by repeated Hct measurements (misclassification in 7.5 %). Applying published formulas in our cohort resulted in incorrect classification in up to 60 %.

CONCLUSION

We provide a formula for estimating Hct from native blood T1 on a 3T scanner. The measurement error of ECV is low and comparable to the error due to retest variability of conventional Hct. Scanner- and sequence-specific formulas should be used.