Fat quantification of the rotator cuff muscles using CT histogram analysis in comparison with Dixon MRI sequence

Objectives: To quantify the proportion of fat within the rotator cuff muscles using CT histogram analysis (HA) and to compare fat fraction (FF) values obtained from CT and T2*-corrected 6-echo three-dimensional gradient-echo Dixon sequences. Methods: Twenty-six patients who underwent both shoulder CT and MRI were enrolled. The FF of rotator cuff muscles was measured on FF maps of the Dixon technique. Using CT, the FF values were obtained in two ways: 1) CT mean Hounsfield Unit (HU) measurement-based fat quantification using subcutaneous fat and the teres major muscle as internal standards; and 2) CT HA-based fat quantification in which pixels from −190 to −30 HU were arbitrarily classified as fat, and those from −30 to 150 HU as muscle. Afterward, the relationship between FF values from MRI (FF-MR), CT mean HU-based analysis (FF-HU), and CT HA (FF-HA) was assessed. Results: There were strong positive correlations (p < 0.001) between FF-MR and FF-HA, which were higher when compared with FF-HU (r = 0.90 vs. 0.77 for total). For a total of 104 rotator cuff muscles, the intraclass correlation between FF-MR and FF-HA was excellent (ICC, 0.90), which was higher than that between FF-MR and FF-HU (ICC, 0.76). The ICCs showed excellent inter-observer agreement between FF-MR, FF-HU, and FF-HA (ICCs, 0.93–0.96). Conclusions: Fat quantification within the rotator cuff muscles using CT HA is both feasible and reliable. Advances in knowledge: CT HA provides reliable quanti-fication of intramuscular fat and has strengths regarding its retrospective applicability to conventional CT studies without additional radiation dose to subjecting patients.