Mapping Fatty Acid Composition in the Human Knee: Short‐Term Repeatability at <scp>3T</scp>

BACKGROUND: Alterations in periarticular lipid composition are implicated in musculoskeletal diseases, yet short-term reliability of MRI-based triglyceride composition mapping in the knee is not fully established. PURPOSE: To evaluate 1-week repeatability of proton-density fat fraction (PDFF) and triglyceride fatty-acid composition-saturated (SFA), monounsaturated (MUFA), and polyunsaturated (PUFA)-in periarticular knee tissues. STUDY TYPE: Prospective. POPULATION: ). FIELD STRENGTH/SEQUENCE: 3T; 12-echo 3D spoiled gradient-echo acquisition for chemical shift-encoded fat quantification and a proton density-weighted SPACE sequence for segmentation (0.6 mm isotropic). ASSESSMENT: Participants underwent repeated MRI 1 week apart. Femoral and tibial bone marrow, patella, Hoffa's fat pad, prefemoral fat pad, quadriceps fat pad, posterior fat pad, and subcutaneous adipose tissue were segmented and rigidly aligned. Voxelwise spectral fitting was used to estimate PDFF and fatty acid composition, including SFA, MUFA, and PUFA components. Repeatability metrics included bias, within-subject standard deviation (wSD), within-subject coefficient of variation (wCV%), coefficient of repeatability, and intraclass correlation coefficient (ICC). STATISTICAL TESTS: Paired t-tests assessed systematic differences (α = 0.05); ICCs used a two-way random-effects, absolute-agreement model (ICC(2,1)). RESULTS: PDFF showed lowest variability across all regions (wCV: 1.5%-5.9%; ICC: 0.33-0.96). SFA demonstrated similar stability (wCV: 2.4%-12.6%; ICC: 0.19-0.87). MUFA exhibited anatomy-dependent reliability (wCV: 4.1%-21.1%; ICC: 0.17-0.97), with highest repeatability in subcutaneous adipose tissue (ICC: 0.97) and Hoffa's fat pad (ICC: 0.85). PUFA displayed the greatest variability (wCV: 3.6%-52.8%; ICC: 0.10-0.94), with the greatest instability in periarticular fat pads. No paired comparisons were significant (all p > 0.05; range p = 0.14-0.98). Regional ordering remained consistent across sessions. DATA CONCLUSION: A 12-echo chemical shift-encoded MRI protocol provides repeatable PDFF and SFA measurements over 1 week. MUFA reliability varies by tissue, while PUFA remains least stable. EVIDENCE LEVEL: 2 (Prospective cohort). TECHNICAL EFFICACY STAGE: 2 (Reproducibility/feasibility evaluation).