Graves’ ophthalmopathy and idiopathic orbital inflammation are characterized by increased orbital fat volume, lymphocytic infiltration and edema. Orbital fat volumetry and fat fraction measurement using IDEAL-FSE imaging are potential methods for quantitatively evaluating orbital fat inflammation^1-3. The aim of this study was to evaluate the feasibility of volumetry and fat fraction measurement of orbital fat using IDEAL-FSE imaging and assess the age correlation and gender difference of orbital fat volume and the fat fraction ratio in healthy adults. Fifty-two orbits of 26 healthy volunteers (14 men, 12 women) were scanned twice to evaluate interobserver agreement and interscan repeatability in the measurements of orbital fat volume and fat fraction. Axial T2-weighted IDEAL-FSE images (TR/TE, 6000/100 ms; flip angle, 90°; image matrix, 288 × 160; field of view, 160 × 160 mm; slice thickness/gap, 2/0 mm; number of slices, 32) were obtained on a 3-T MR unit. Two observers measured the orbital fat volume of both eyes from fat images. After separating fat tissue from other structures by using an adequate threshold value and manually removing the fatty marrow of the orbital bone and the outer fat, the orbital fat volume was automatically determined on a workstation (Fig. 1). The orbital fat fraction was measured from the signal intensities of intraorbital fat on fat images (SIf) and water images (SIw): fat fraction ratio = SIf / (SIf + SIw) (Fig. 2). We determined measurement repeatability and interobserver agreement with the Pearson correlation coefficient, which was also used for evaluating the right–left correlation and the age correlation of the orbital fat volume and fat fraction. The independent samples t-test was used to assess gender differences. The average interobserver difference was 1.0%, and the interobserver correlations were excellent (r = 0.99) for the orbital fat volumetry. The average volume difference on the first and second scans was 2.2%; measurement repeatability was excellent (r = 0.98) (Fig. 3a). The average volume of the right and left orbital fat was 12.3 and 12.1 cm3, respectively. The right–left correlation was excellent (r = 0.99) (Fig. 3b). There were no gender differences and no age correlation for orbital fat volume (Fig. 3c). The average difference of orbital fat fraction ratios on the first and second scans was 3.6%; measurement repeatability was good to excellent (r = 0.81) (Fig. 4a). Each of the average fat fraction ratios of right and left orbits was 0.79. The right–left correlation of the orbital fat fraction ratio was moderate-to-good (r = 0.70) (Fig. 4b). There was a significant gender difference (p = 0.02) and no age correlation for orbital fat fraction ratios (Fig. 4c).Orbital fat volumetry and fat fraction measurement using IDEAL-FSE imaging is feasible and could be useful for evaluating orbital fat abnormalities.