MRI-index: an automatic tool for early quantitative evaluation of fat infiltration at muscle MRI in neuromuscular diseases

Introduction. Quantitative skeletal muscle MRI (mMRI) has been widely studied as a reliable outcome measure in patients with Neuromuscular Disorders (NMD). However the standard 3-point Dixon technique scoring system is not widely used in all centers for technical limitations. Conversely, we have developed a MRI-index, a measure extrapolated with an algorithm of image analysis able to derive automatically the fat percentage in mMRI images from NMD patients. Normative values, stratified for ages in healthy subjects, were acquired in our earlier studies. Methods. Using a standard mMRI protocol in the past two years, we acquired myoimaging results of 46 patients (age range 7-70) with variable disease severity. Of these cases, 39 were ge- netically characterized and included 27 BMD boys, 4 subjects presenting mutations in RYR1, 2 in MHY7, 2 in TPM3, 1 DMD carrier, and 3 patients harboring mutations in other genes. Im- ages of the medial third of the thighs of each subject were taken into account for the analysis of MRI-index. Genotype and phe- notype correlations were subsequently made. Results. Analyzing patients with the same genotype (BMD patients) we observed that disease severity correlates with the degree of fat percentage in muscle (high MRI-index). Considering patients with milder phenotype and no fat infiltration at qualitative imaging, MRI-index showed higher values when mutations occur in genes encoding structural muscular proteins. In asymptomatic hyperCKemia and mutations in RYR1, MRI- index was in the normal range. Conclusions. The MRI-index can be a valid tool to detect muscle involvement and early genotype-phenotype correlation in NMD.