Quantitative Magnetic Resonance Imaging of the epicardial adipose tissue with free-running cardiac Dixon

There is a growing interest to better understand the pathophysiological role of cardiac fat towards cardiovascular degradation in metabolic diseases. As epicardial adipose tissue (EAT) can become an inflammatory substrate under pathological conditions, emerging therapies are aiming at modulating its metabolic functions. However, there is a lack of non-invasive tools that can probe EAT, which is thin and moves with cardiac and respiratory motion. This study aims at developing quantitative MRI biomarkers that can characterize EAT. As detailed in the Fig. 1, a custom-built free-running Dixon-MRI technique enabled high resolution mapping of proton density fat-fraction (PDFF) and R2*, free-breathing, in the heart. Cardiac Dixon MRI was implemented on a 3 T MRI with a self-navigated gradient echo radial sequence (Fig. 1.1) acquiring N = 13-echo bipolar 3D (TE1/ΔTE = 1.12/1.07ms). The “free-running’ self-navigation (Fig. 1.2) reconstructed 4 respiratory and 8-12 cardiac phases of 100 ms. Measurements were performed on end-expiratory peak-systole volume. K-space trajectories were corrected (Fig. 1.3) with the gradient impulse function (GIRF). The 5D reconstruction (Fig. 1.4) of volumes was performed by Compressed Sensing followed by iterative least-square IDEAL water-fat separation (Fig. 1.5). Eventually, R2*&PDFF 3D maps (overlayed in Fig. 1 over echo TE1 image) are resolved in cardiac and respiratory time. The sequence proved highly accurate and precise with mean biases of PDFF and R2* estimated by simulation under 0.05% and 0.05 s-1 and an accuracy of ±1.2% and 5.0 s-1. As expected, in the healthy population, epicardial fat had a significantly lower fat fraction than subcutaneous fat (PDFF EAT = 81.6 ± 9.6% vs. PDFF ScAT = 92.7 ± 4.2%, P < 0.001) and paracardiac fat (PDFF PAT = 90.6 ± 3.7%). Preliminary results in type-2 diabetic patients also showed a lower PDFF in EAT compared to neighbor paracardiac fat, which remains to be confirmed in a larger cohort. This study demonstrated precise and highly-resolved PDFF and R2* 3D maps to probe cardiac fat, in particular epicardial adipose tissue, thanks to free-running cardiac Dixon-MRI at 3 T. Due to the proximity of the lungs, the B0 inhomogeneities field map had large spatial variations, hampering unbiased R2* quantification. Nevertheless, 3D maps of PDFF in the heart allow characterization of the EAT, whose brown-beige fat composition was confirmed in controls and few type 2 diabetic patients.