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_syn) and synthetic ECV (ECV_syn), to assess accuracy of ECV_syn 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_native,midLV) were correlated with Hct levels from blood sampling within 24 h (Hct_24h) and a formula for calculation of Hct_syn was derived by linear regression.

Results: In the derivation cohort (n = 167), Hct_24h showed a good association with T1_native,midLV (r = -0.711, p < 0.001). The resulting regression equation was Hctsyn = 1/T1_native,midLV * 1355.52-0.310. In the validation cohort (n = 83), Hct_syn and Hct_24h showed good correlation (r = 0.726, p < 0.001), while ECV_syn, and ECV_24h demonstrated excellent correlation (r = 0.940, p < 0.001). ECV_syn 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_syn from native blood T1 on a 3T scanner. The measurement error of ECV_syn is low and comparable to the error due to retest variability of conventional Hct. Scanner- and sequence-specific formulas should be used.