Is a timely assessment of the hematocrit necessary for cardiovascular magnetic resonance–derived extracellular volume measurements?

BACKGROUND: Cardiovascular magnetic resonance (CMR)–derived extracellular volume (ECV) requires a hematocrit (Hct) to correct contrast volume distributions in blood. However, the timely assessment of Hct can be challenging and has limited the routine clinical application of ECV. The goal of the present study was to evaluate whether ECV measurements lead to significant error if a venous Hct was unavailable on the day of CMR. METHODS: 109 patients with CMR T1 mapping and two venous Hcts (Hct(0): a Hct from the day of CMR, and Hct(1): a Hct from a different day) were retrospectively identified. A synthetic Hct (Hct(syn)) derived from native blood T1 was also assessed. The study used two different ECV methods, (1) a conventional method in which ECV was estimated from native and postcontrast T1 maps using a region-based method, and (2) an inline method in which ECV was directly measured from inline ECV mapping. ECVs measured with Hct(0), Hct(1), and Hct(syn) were compared for each method, and the reference ECV (ECV(0)) was defined using the Hct(0). The error between synthetic (ECV(syn)) and ECV(0)was analyzed for the two ECV methods. RESULTS: ECV measured using Hct(1) and Hct(syn) were significantly correlated with ECV(0) for each method. No significant differences were observed between ECV(0) and ECV measured with Hct(1) (ECV(1); 28.4 ± 6.6% vs. 28.3 ± 6.1%, p = 0.789) and between ECV(0) and ECV calculated with Hct(syn) (ECV(syn); 28.4 ± 6.6% vs. 28.2 ± 6.2%, p = 0.45) using the conventional method. Similarly, ECV(0) was not significantly different from ECV(1) (28.5 ± 6.7% vs. 28.5 ± 6.2, p = 0.801) and ECV(syn) (28.5 ± 6.7% vs. 28.4 ± 6.0, p = 0.974) using inline method. ECV(syn) values revealed relatively large discrepancies in patients with lower Hcts compared with those with higher Hcts. CONCLUSIONS: Venous Hcts measured on a different day from that of the CMR examination can still be used to measure ECV. ECV(syn) can provide an alternative method to quantify ECV without needing a blood sample, but significant ECV errors occur in patients with severe anemia.