An instantaneous ECV with no blood sampling: using native blood T1 for hematocrit is as good as standard ECV

Background The extracellular volume fraction (ECV) by T1 mapping measures the size of the myocardial interstitium. T1 changes in blood and myocardium are used to measure the contrast partition coefficient (l), and substituting in the blood volume of distribution (directly measured on a peripheral blood sample as one minus the hematocrit [Hct]) provides the ECV. This methodology is however cumbersome, has significant variability, introduces a delay and is a barrier to wider use of ECV quantification in clinical practice. We have previously observed a strong relationship between ShMOLLI T1blood and Hct [Piechnik, JCMR 2013, 15:13] and hypothesise that this could be used to infer the Hct at the time of scan and permit immediate ECV calculation without blood sampling (ECVNo Hct).


Background
The extracellular volume fraction (ECV) by T1 mapping measures the size of the myocardial interstitium. T1 changes in blood and myocardium are used to measure the contrast partition coefficient (λ), and substituting in the blood volume of distribution (directly measured on a peripheral blood sample as one minus the hematocrit [Hct]) provides the ECV. This methodology is however cumbersome, has significant variability, introduces a delay and is a barrier to wider use of ECV quantification in clinical practice. We have previously observed a strong relationship between ShMOLLI T1 blood and Hct [Piechnik, JCMR 2013, 15:13] and hypothesise that this could be used to infer the Hct at the time of scan and permit immediate ECV calculation without blood sampling (ECV No Hct ).

1
The Heart Hospital Imaging Centre, University College London, London, UK Full list of author information is available at the end of the article Derived ECV No Hct exhibited excellent correlation with conventional ECV Hct (R 2 =0.99; p<0.001) with small~2% bias and~3% SD of differences on Bland-Altman analysis (95% confidence interval -0.7 to +3.9% excluding Amyloid, and -2.6 to +8.0% for Amyloid) close to previously reported 1.4% [Schelbert EB JCMR 2011, 13:16].
ECV No Hct correlated equally well with clinical markers of disease severity (LV mass index, LVEF, stroke volume index, left atrial area index and NT-pro-BNP) as ECV Hct and partition coefficient, and better than postcontrast T1 myocardium (Table 1).

Conclusions
Native T1 blood correlates well with the laboratory-measured values of hematocrit. Our data demonstrates that straight-forward derivation of hematocrit from T1 blood can be used as an immediate measure of ECV that may pave its application for nearly instantaneous clinical diagnosis. It remains to be confirmed if the high correlation of ECV No Hct with the conventional calculations may cause blood sampling to become an obsolete complication in clinical practice.

Funding
TAT and MF are supported by doctoral research fellowships by the National Institute of Health Research (NIHR) and British Heart Foundation, respectively. SKP and MDR are supported by the NIHR Oxford Biomedical Research Centre based at the Oxford University Hospitals Trust at the University of Oxford.