Author + information
- Andrei Codreanu, MD∗ (, )
- Pauline Ferry, MBS,
- Marine Beaumont, PhD and
- Pierre-Yves Marie, MD, PhD
- ↵∗Service de Cardiologie, Centre Hospitalier de Luxembourg, 4 rue Barblé, L-1210, Luxembourg
We read with much interest the recent publication by White et al. (1), assessing the accuracy of the contrast bolus T1 mapping cardiac magnetic resonance technique for measuring myocardial extracellular volume fraction (ECV). The study provides the first validation of the bolus technique against collagen volume fraction from myocardial biopsy. The bolus technique was also compared with the gold standard infusion technique in 5 representative conditions. The 2 techniques provide equivalent results, except for pathological states with an ECV >0.4, where the bolus approach consistently and increasingly overestimates the ECV value.
To date, T1 mapping has been used mainly for differentiation between healthy and disease states in clinical settings associated with an increased ECV. The technique should provide even more clinical benefit through its ability to differentiate between different degrees of pathological states associated with scar or edema in such settings as post-infarction remodeling, myocarditis, and transplant rejection follow-up. In this regard, the lower precision of the bolus technique in the ECV range of myocardial scar or edema is a matter of concern.
As suggested by the authors, a possible reason for such ECV overestimation is that renal clearance might be faster than the exchange rate between the intravascular and interstitial compartments, leading to lower ΔR1 in blood compared with ΔR1 of myocardium with time. This is in line with previous observations in subjects with normal or modestly increased ECV, showing small but significant changes in ECV with time using the bolus approach (2,3). This should cause a slight overestimation in the high ECV range with the bolus approach, as a limitation of the 2-compartment model, but independent of noticeable differences in blood T1 related to the underlying clinical state.
The data in the present study show some intriguing differences in blood T1 between groups. Post-contrast blood T1 is higher for bolus than for infusion in all subjects except for the healthy and the HCM-remote groups. This includes therefore all cases of high ECV (i.e., the Amyloid, HCM LGE Zone, and Infarct Zone groups in Table 1 in White et al. (1)). As such, the relative difference in blood T1 between bolus and infusion (with pre-contrast T1 as the reference) is up to 4.3 times higher in the high ECV subjects compared with the healthy or HCM-remote group. The lower ΔR1 for blood with the bolus approach in the high ECV subjects is therefore likely to contribute to the higher calculated ECV with this method. This is most striking for the HCM-LGE subgroup compared with the HCM-remote group and raises the question of whether the observed differences in blood T1 between groups do not reflect different equilibrium states between blood and myocardium according to the study groups.
Multiple factors may contribute to the higher blood T1 with the bolus approach in subjects with disease. Heart rate or flow-dependent variations in blood inversion could lower the accuracy of T1 measurement (4). Altered blood clearance through renal impairment and synovial third-space penetration of contrast may also act as confounders. More complex examinations in disease may produce lower image quality, altering intrastudy ECV reproducibility as a factor of time (5).
We are aware of the complex nature of myocardial T1 measure, of multiple factors interfering with ECV calculation and we much appreciate the transparency and completeness of data provided. We would like to know the authors' interpretation of the blood T1 data. We believe this is an issue of practical interest in a field expected to provide a key biomarker in cardiac disease in the future.
- American College of Cardiology Foundation