Quantitative analysis of orbital soft tissues on computed tomography to assess the activity of thyroid-associated orbitopathy

To evaluate the validity of using quantitative volume and density measurements from orbital computed tomography (CT) images to assess the inflammatory activity of patients with thyroid-associated orbitopathy (TAO). Computed tomography (CT) scans were obtained from 80 TAO patients and 40 controls, and 3D image analysis was conducted to measure the volume and density (in HU units) of intraorbital and extraorbital fat, extraocular muscle (EOM), and the lacrimal gland. Volume and density measurements of the orbital tissues were compared among active TAO, inactive TAO, and control subjects by ANCOVA. To determine the predictive value of each parameter for TAO activity, logistic regression was performed. The mean volume of extraorbital and intraorbital fat was significantly higher in patients with TAO than controls (p = 0.0019, p = 0.0004), with no significant difference between active and inactive TAO subjects. The mean total EOM volume and lacrimal gland volume was greater in active TAO patients than other groups (p < 0.0001, p < 0.0001). The mean density of extraorbital fat and the lacrimal gland was significantly different between active TAO, inactive TAO, and control groups (p = 0.0002, and p = 0.0487, respectively). Regression models incorporating total EOM volume, lacrimal gland volume, intraorbital fat volume, and density of extraorbital fat and the lacrimal gland could predict active inflammation in patients with TAO with accuracy of 84.5 %. The measurements of orbital soft tissue volume and density using CT scans can be used as a reliable and feasible technique to establish active inflammation in patients with TAO.