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

Abstract

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 (Hct₀: a Hct from the day of CMR, and Hct₁: 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 Hct₀, Hct₁, and Hct_syn were compared for each method, and the reference ECV (ECV₀) was defined using the Hct₀. The error between synthetic (ECV_syn) and ECV₀was analyzed for the two ECV methods.

Results: ECV measured using Hct₁ and Hct_syn were significantly correlated with ECV₀ for each method. No significant differences were observed between ECV₀ and ECV measured with Hct₁ (ECV₁; 28.4 ± 6.6% vs. 28.3 ± 6.1%, p = 0.789) and between ECV₀ 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, ECV₀ was not significantly different from ECV₁ (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.

Keywords: Cardiovascular magnetic resonance; Extracellular volume fraction; Hematocrit; T1 mapping.

Fig. 1

Schematic illustration of the conventional (a) and inline extracellular volume (ECV) methods (b). a The conventional ECV method derives a partition coefficient (λ_conv) using averaged T1 values from regions-of-interests (ROIs) drawn in the myocardium and blood in native and postcontrast T1 maps. The ECV values are calculated from three different hematocrits (Hct). ECV₀ is derived from an Hct₀ measured on the same day as cardiovascular magnetic resonance (CMR) exam; ECV₁ is derived using Hct₁, which is measured on a different day; ECV_syn is derived using Hct_syn[conv] which is calculated from the native blood T1. b Inline partition coefficient (λ) mapping is automatically generated on a pixelwise basis after image registration of the native and postcontrast T1 maps. The blood mask is automatically generated from native T1 mapping and used to estimate Hct_syn[inline]. Three inline ECV maps were reconstructed with Hct₀, Hct₁, and Hct_syn[inline]. Three ECV values, ECV₀, ECV₁, and ECV_syn, are directly measured from three different inline ECV maps using the same myocardial ROI for T1 mapping

Fig. 2

A flow chart showing the selection process in this study. 812 consecutive patients were available for inclusion in this cohort. A total of 770 subjects were included in this study; 37 patients that did not have hematocrit (Hct) data were excluded. 149 patients had both Hct data and CMR performed on the same day. Among these patients, 109 patients had a second Hct (HCT₁) measure and were included for further analysis

Fig. 3

a Linear regression between native blood 1/T1 (R1) and Hct measured on the same day as that of CMR (Hct₀). b Correlation graph comparing synthetic ECV error (synECV_error) obtained from the conventional ECV method (red line) and the inline ECV method (black line). Both errors are positively associated with the Hct levels

Fig. 4

Correlation and comparisons among three extracellular volumes (ECVs) derived from three different hematocrits (Hcts) using the conventional ECV and the inline ECV methods. ECV₁ and ECV_syn are both strongly correlated with ECV₀ in the conventional method (a) and the inline ECV method (b). In the conventional method, Bland–Altman plots indicate minimal bias between ECV₁ and ECV₀ (c) and between ECV_syn and ECV₀ (e). Similarly, Bland–Altman plots also indicate minimal bias between ECV₁ and ECV₀ (d) and between ECV_syn and ECV₀ (f) in inline ECV method. No significant differences among 3 ECVs in the conventional method (g) and the inline ECV method (h) were observed. ECV₀ is derived using an Hct obtained on the day of CMR (CMR; Hct₀), ECV₁ is derived with Hct₁ is measured from a different day than CMR, and ECV_syn is derived with Hct_syn, which is calculated from native blood T1 mapping

Fig. 5

Comparisons among three different hematocrits (Hcts) and between two native blood T1. Bland–Altman plots indicate the minimal bias between Hct₁ and Hct₀ (a), between Hct_{syn [conv]} and Hct₀ (c), and between Hct_{syn [inline]} and Hct₀ (e). There is no statistical difference between Hct₁ and Hct₀ (b), between Hct_{syn [conv]} and Hct₀ (d), and between Hct_{syn [inline]} and Hct₀ (f). Bland–Altman plot indicates a 1.5 ms bias and confidence limit (7.2–6.9 ms) using the inline method compared with using the conventional method (g), which resulted in no statistical difference between the conventional and inline ECV methods for native blood T1 measurements (h). Hct₀ is Hct obtained on the same day as theCMR. Hct₁ is Hct obtained on a different day from the CMR. Hct_{syn [conv]} and Hct_{syn [inline]} are Hcts obtained synthetically from native blood T1mapping using the conventional and inline ECV methods, respectively

Fig. 6

Interpretation of equivalence of tested hematocrits (Hct₁, Hct_syn) to the standard hematocrit (Hct₀) for ECV measurements. The dashed vertical lines indicate the predefined equivalence margins (− 2 to 2%) and the horizontal lines represent the 95% confidence interval (CI) of the difference in ECV between compared Hcts. Equivalence is concluded if the entire CI is within the equivalence margins. Hct_{1 [conv]} and Hct_{1 [inline]} are hematocrits obtained on a different day from that of CMR f using the conventional and inline ECV methods, respectively. Hct_{syn [conv]} and Hct_{syn [inline]} are Hcts obtained synthetically from native blood T1 mapping using the conventional and inline ECV methods, respectively