Orbital conditions and their Hounsfield unit characteristics.

Purpose: Computed tomography (CT) uses Hounsfield units (HU) for density, which is a standardized unit of measure. This study created a HU reference guide and evaluates the use of HU for orbital lesions and normal structures.

Methods: Retrospective chart review at two academic centers identified patients with biopsy proven orbital lesions and CT imaging. Patients with normal orbital structures were included as a normative database. HU was measured.

Results: The study included 219 patients with orbital lesions and 50 control patients with a total of 341 images. Of these, 241 images were orbital lesions, with 137 non-contrast and 104 contrast. There were 50 non-contrast and 50 contrast normal scans. The orbital lesion group represented 29 distinct orbital diagnoses. Major orbital structures were distinguishable by HU, and extraocular muscles were found to be significantly different between muscles. A subset of orbital lesions had significantly different mean HU compared to normal orbital structures.

Conclusions: Analysis of Hounsfield units provides additional depth to orbital imaging that may aid in diagnostic yield and therapeutic intervention for certain lesions. This study summarizes HU data for orbital lesions as well as a normative database of orbital structures.