MRI Steatosis Grading: Development and Initial Validation of a Color Mapping System

OBJECTIVE. The purpose of this article is to develop and validate a chemical-shift imaging-derived color mapping system for evaluation of liver steatosis. MATERIALS AND METHODS. Opposed phase MRI was evaluated for 85 subjects (51 with presumed nonalcoholic fatty liver disease and 34 healthy volunteers). Liver signal intensity loss was compared with histologic analysis for 52 subjects, assuming grade 0 steatosis for healthy volunteers, to determine signal-intensity-loss threshold points differentiating steatosis grades and subsequent Spearman correlation. Color scale grading was then applied for 78 subjects. Interpretation of color maps for steatosis severity and heterogeneity was performed by three readers. Analyses of agreement among readers and of color map steatosis grade with biopsy were performed using weighted kappa values. RESULTS. The numbers of subjects with steatosis grades 0, 1, 2, and 3 were 41, 12, 13, and 19, respectively. A correlation of 0.90 was obtained using selected threshold values of 5.9% or less, 6-26.1%, 26.2-36.8%, and greater than 36.8% for steatosis grades 0, 1, 2, and 3, respectively. Interobserver agreement for color map grading of steatosis was excellent (kappa = 0.93-0.94). Color map interpretation for all readers also showed excellent agreement with histologic findings for whole liver (kappa = 0.82-0.86) and estimated biopsy site location (kappa = 0.810.86; anterior region of right lobe). Heterogeneous steatosis on color maps was identified in 56-60% of subjects with nonalcoholic fatty liver disease and in 7% of healthy volunteers and was associated with greater disagreement between color map and histology grading (61-74%) compared with the whole group (37-40%). CONCLUSION. MRI-derived color map estimation of liver steatosis grade appears to be reproducible and accurate.