Quantifying fat and lean muscle in the lower legs of women with knee osteoarthritis using two different MRI systems

Decreased muscle mass and increased fat mass are commonly seen in the thighs of individuals with knee osteoarthritis (OA). Despite the role of calf muscles in activities of daily living and knee mechanics, little work has investigated calf changes in knee OA. Unlike the thigh, muscle and fat in the lower leg can be imaged using a peripheral magnetic resonance imaging (MRI) scanner. We aimed to assess agreement between subcutaneous fat, intermuscular fat (IMF), intramuscular fat (intraMF), and lean muscle volumes acquired using a peripheral 1.0T as compared to a reference whole-body 3.0T MRI scanner. A calf MRI scan from each scanner was acquired from twenty women >55 years with knee OA. The different tissues were segmented on each of ten axial slices for every participant using SliceOmatic 5.0 (Tomovision, Magog, QC). Tissue volumes were determined for each outcome. Agreement between tissue volumes from the two scanners was assessed using intraclass correlation (ICC(2,1)) coefficients, standard error, and Bland-Altman plots. Agreement between tissue volumes was strong to very strong, with ICCs ranging from 0.842 to 0.991 for all outcomes. However, wide confidence intervals for IMF and intraMF suggest there is less confidence in agreement with segmentation of images from the 1.0T scanner generally underestimating fat volume relative to the 3.0T scanner. The 3.0T's superior between-tissue contrast likely resulted in more accurate segmentation of IMF and intraMF compared to the 1.0T scanner. Comparisons of tissue volume between studies using different scanners/sequences should be interpreted cautiously.