Clinical application of MOLLI T1* for extracellular volume calculation in healthy volunteers and aortic stenosis

Background

The calculation of the extracellular volume fraction (ECV) requires accurate quantification of myocardial and blood pool T1. Some Modified look locker inversion recovery (MOLLI) sequences provide a T1 and T1* output. T1* does not use a look locker correction, and so it is theoretically a more accurate estimation of true T1 blood T1 because fresh spins are flowing into the imaging plane. It is therefore recommended to use T1* for the quantification of the pre- and post-contrast blood pool. The aim of this study was to investigate the effect on ECV of using T1* (ECV_T1*) rather than T1 (ECV_T1) and assess accuracy, precision and bias.

Methods

57 patients with aortic stenosis (AS) (mean age= 71±10 years, 33 female) and 25 healthy volunteers (HV) (mean age= 40±11 years, 19 female) were recruited. 4 chamber and mid ventricular short axis (SA) T1 maps were acquired pre-contrast and 15 minute post-contrast using 5s(3s)3s and 4s(1s)3s(1s)2s sequences respectively. Regions of interest (ROI) were drawn carefully to avoid blood-myocardium border and copied across series with correction only for patient movement. ECV was calculated as (Δ[1/T1_myo] / Δ[1/T1_blood]) * (1-haematocrit).

Results

ECV_T1* was significantly lower than ECV_T1 (mean 27.1±3.4% vs 28.1±3.2%, p<0.0001). ECV_T1* showed excellent correlation with ECV_T1 (R= 0.88) (Figure 1). Bland-Altman analysis revealed no bias or variability (Figure 2). There was no statistical difference in variance between groups (F test, p= 0.66). In this group of subjects there was no difference in ECV between AS and HV groups using either ECV_T1 (28.1±3.2% vs 28.2±3.4%) or ECV_T1* (27.3±3.6% vs 26.5±3.0%).

Figure 1

Excellent correlation between ECV BloodT1 and ECV BloodT1*

Figure 2

Little bias and variability between ECV BloodT1 and BloodT1* using Bland-Altman analysis

Conclusions

ECV quantification using T1* can measure ECV across disease and normal populations, but its own normal values need to be referenced. It has similar variability, and no bias when compared to ECV using T1_blood. ECV_T1* is therefore practically feasible and encourages further work to explore its theoretical accuracy by histological correlation.

Funding

N/A.