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Background  The extracellular volume (ECV), derived from cardiovascular magnetic resonance (CMR) T1 mapping, is a biomarker of the extracellular space in the myocardium. The hematocrit (HCT), measured from venipuncture, is required for ECV measurement. We test the clinic values of synthetic ECV, which is derived from the longitudinal relaxation of blood–based (T1 blood ) synthetic hematocrit in 3 T CMR.    Methods  A total of 226 subjects with CMR T1 mapping and HCT measurement taken on the same day as the CMR were retrospectively enrolled and randomly split into derivation ( n  = 121) and validation ( n  = 105) groups, comprising healthy subjects ( n  = 45), type 2 diabetes mellitus (T2DM) patients ( n  = 60), hypertrophic cardiomyopathy (HCM) patients ( n  = 93), and 28 other patients. Correlation of T1 blood  with the measured HCT (HCTm) was established in the derivation group and used in both the derivation and the validation groups. The relationships between the ECV values derived from both the synthetic HCT (HCTsyn) and HCTm were explored. In addition, the differences in the ECV values among the HC, T2DMs, and HCMs were compared.    Results  Regression between the HCTm and 1/T1 blood  was linear (R 2  = 0.19,  p  < 0.001), and the regression equation was: HCTsyn = [561.6*(1/T1blood)] + 0.098 in the derivation group. The measured ECV (ECVm) was strongly correlated with the synthetic ECV (ECVsyn) (R 2  = 0.87,  p  < 0.001) and mildly correlated with the difference between the ECVsyn and ECVm (R 2  = 0.10,  p  < 0.001) in the derivation group. Also in this group, the ECVm was larger in T2DMs than that in healthy cohort (29.1 ± 3.1% vs. 26.4 ± 2.4%,  p  = 0.002), whereas, the ECVsyn did not differ between T2DMs and healthy cohort (28.3 ± 2.9% vs. 26.9 ± 2.2%,  p  = 0.064). Compared with the healthy cohort, the HCMs were associated with higher ECVsyn and ECVm of the mid-ventricle in both the derivation and the validation groups. Using our center’s normal cut-off of 31.8%, the use of ECVsyn would lead to a 6–25% incorrect categorization of patients in the derivation and validation groups.    Conclusions  ECVsyn derived from HCTsyn may lead to clinical errors in 3 T CMR, especially for patients who have only a subtle elevation in ECV.    Electronic supplementary material  The online version of this article (10.1186/s12968-018-0475-6) contains supplementary material, which is available to authorized users.

journal of cardiovascular magnetic resonance

Extracellular volume fraction measurements derived from the longitudinal relaxation of blood-based synthetic hematocrit may lead to clinical errors in 3 T cardiovascular magnetic resonance