The application value of quantitative analysis of orbital soft tissue parameters on plain CT scans in evaluating the activity of thyroid-associated ophthalmopathy

Purpose Using plain orbital computed tomography (CT) scan images, we quantitatively extracted indicators such as diameters, cross-sectional areas, volumes, and densities of intraorbital soft tissues to investigate the feasibility of these metrics for assessing inflammatory activity status in patients with thyroid-associated ophthalmopathy (TAO). Methods This study retrospectively recruited 31 patients with active TAO, 46 with inactive TAO, and 46 normal controls between April 2022 and October 2025 (note: counts of inactive TAO patients and normal controls overlapped due to contralateral eye classification in TAO patients). Clinical and imaging data were collected, and eyes were stratified into three groups: 50 active TAO eyes, 79 inactive TAO eyes, and 87 normal eyes. Orbital CT scans and 3D image analysis (3D Slicer 5.7.0) were performed to measure exophthalmos, axial length, globe width, cross-sectional areas (CSA) of individual/total extraocular muscles (EOMs), lacrimal gland CSA/volume (LGV), optic nerve CSA, intraorbital fat thickness/volume (IOFV), and densities of lacrimal gland, vitreous, and intraorbital fat. Ratios of LGV, IOFV, lacrimal gland CSA, and total EOMs CSA (OM) to total orbital volume (TV)/CSA (TOA) were calculated. The Kruskal–Wallis H test was used for intergroup comparisons, with pairwise Mann–Whitney U tests and Bonferroni correction ( α =0.017) applied for significant parameters. Point-biserial correlation was used to quantify associations with active TAO, and receiver operating characteristic (ROC) curve analysis (maximum Youden index) was used to evaluate diagnostic value. Results This study included 67 TAO patients (men:women = 35:32) with a 92.5% (62/67) bilateral involvement rate. Kruskal–Wallis H test showed significant differences among the active TAO, inactive TAO, and control groups in OM, CSA of the superior/inferior/medial rectus muscles, superior/inferior oblique muscles, TOA, lacrimal gland CSA, Intraorbital fat density, IOFV, vitreous density, lacrimal gland density, and the ratios of IOFV/TV, OM/TOA, and lacrimal gland CSA/TOA (all p <0.05). Mann–Whitney U tests revealed that patients with active TAO had significantly higher superior/medial rectus CSA, OM, Intraorbital fat density, and OM/TOA ratio than those in the inactive TAO and control groups (all p < 0.017). Point-biserial correlation analysis confirmed positive correlations between these five parameters and active TAO status ( r > 0; all p < 0.05). ROC curve analysis demonstrated their diagnostic efficacy for active TAO: Intraorbital fat density ≥ − 78.5 HU (area under the curve [AUC] = 0.830, sensitivity: 76.0%, specificity: 89.2%), OM/TOA ratio ≥ 0.301 (AUC=0.829, sensitivity: 86.0%, specificity: 70.5%), superior rectus CSA ≥ 49.115 mm 2 (AUC=0.812, sensitivity: 64.0%, specificity: 87.3%), OM ≥ 254.12 mm 2 (AUC=0.825, sensitivity: 68.0%, specificity: 82.5%), and internal rectus CSA ≥ 43.525 mm 2 (AUC=0.749, sensitivity: 58.0%, specificity: 84.3%) (all p < 0.001). Conclusions The measurement of the CSA of the superior rectus muscle at 7 mm posterior to the orbital globe, the CSA of the medial rectus muscle, OM, Intraorbital fat density, and the OM/TOA ratio via plain CT scan serves as a reliable and feasible technical approach for evaluating disease activity in patients with TAO in primary ophthalmic healthcare institutions.