Muscle fat-fraction and mapping in Duchenne muscular dystrophy: evaluation of disease distribution and correlation with clinical assessments

To examine the usefulness of dual-echo dual-flip angle spoiled gradient recalled (SPGR) magnetic resonance imaging (MRI) technique in quantifying muscle fat fraction (MFF) of pelvic and thighs muscles as a marker of disease severity in boys with Duchenne muscular dystrophy (DMD), by correlating MFF calculation with clinical assessments. We also tried to identify characteristic patterns of disease distribution. Twenty consecutive boys (mean age, 8.6 years +/- 2.3 [standard deviation, SD]; age range, 5-15 years; median age, 9 years;) with DMD were evaluated using a dual-echo dual-flip angle SPGR MRI technique, calculating muscle fat fraction (MFF) of eight muscles in the pelvic girdle and thigh (gluteus maximus, adductor magnus, rectus femoris, vastus lateralis, vastus medialis, biceps femoris, semitendinosus, and gracilis). Color-coded parametric maps of MFF were also obtained. A neurologist who was blinded to the MRI findings performed the clinical assessments (patient age, Medical Research Council score, timed Gower score, time to run 10 m). The relationships between mean MFF and clinical assessments were investigated using Spearman's rho coefficient. Positive and negative correlations were evaluated and considered significant if the P value was < 0.05. The highest mean MFF was found in the gluteus maximus (mean, 46.3 % +/- 24.5 SD), whereas the lowest was found in the gracilis muscle (mean, 2.7 % +/- 4.7 SD). Mean MFF of the gluteus maximus was significantly higher than that of the other muscles (P < 0.01), except for the adductor magnus and biceps muscles. A significant positive correlation was found between the mean MFF of all muscles and the patients age (20 patients; P < 0.005), Medical Research Council score (19 patients; P < 0.001), timed Gower score (17 patients; P < 0.03), and time to run 10 m (20 patients; P < 0.001). A positive correlation was also found between the mean MFF of the gluteus maximus muscle and the timed Gower score. Color-coded maps provided an efficient visual assessment of muscle fat content and its heterogeneous distribution. Muscle fat fraction calculation and mapping using the dual-echo dual-flip angle SPGR MRI technique are useful markers of disease severity and permit patterns of disease distribution to be identified in patients with DMD.