A comparative study of synthetic and venous hematocrit for calculating cardiovascular magnetic resonance-derived extracellular volume

The extracellular volume (ECV) fraction derived from cardiac magnetic resonance (CMR) can reflect various pathologies. The application of ECVs was limited by the strict requirement that hematocrit (Hct₀) should be obtained within 24 hours of CMR scan. The aim of this study was to obtain accurate and convenient ECV calculated from the venous Hct and synthetic Hct in CMR. A total of 839 subjects were retrospectively enrolled. The subjects were divided into derivation cohort for local sex-specific models and validation cohort for assessing the accuracy of different ECVs. In the validation cohort, venous Hcts from 7 days before the scan (Hct_1 − 7), outside 7 days (Hct_> 7), the closest day (Hct_closest), and Hct_syn were compared with Hct₀. The agreement and correlation of the conventional ECV (ECV₀) with the corresponding ECVs were analyzed. The factors affecting the accuracy of ECV_syn were assessed. ECV_1–7 and ECV_closest had the best correlation and smallest bias with ECV₀ (R = 0.959 and 0.951, bias = 0.02% and − 0.03%). When using an absolute 2% error as the standard, the performance of ECV_1–7 was the best, with an accuracy of 81.0%, followed by ECV_closest (78.8%), ECV_> 7 (77.2%) and ECV_syn (70.7%). Abnormally low and high Hcts and decreased left ventricular ejection fractions were associated with miscalculation of ECV_syn, especially patients with dilated cardiomyopathy. We recommend extending the time interval between a Hct and a CMR scan to 7 days for ECV calculation. The synthetic ECV should be used cautiously, especially for patients with extremely low or high Hcts, decreased cardiac function, and dilated cardiomyopathy.