Localization and quantitative assessment of inflammation in frozen shoulder: a PET/MRI fusion imaging analysis.

BACKGROUND AND OBJECTIVE: Frozen shoulder is a common clinical condition; however, its pathophysiology and precise anatomical localization of inflammatory activity remain unclear. This study was aimed at identifying the localization and intensity of inflammation in patients with frozen shoulder using PET/MRI fusion imaging. PATIENTS AND METHODS: Seven patients with frozen shoulder who underwent 18F-FDG PET/CT were analyzed (2 men, 5 women; mean age, 59 years), all in the freezing phase. PET/CT and MRI datasets were coregistered, and standardized uptake values (SUVs) were obtained. The SUVmax of the asymptomatic contralateral shoulder served as the reference threshold. On T2-weighted oblique sagittal images, nine slices centered on the labrum were evaluated. Area-weighted SUV was calculated as the product of the area above threshold and mean SUV. On the labral slice, SUVmax was measured radially using a clockface-based method. SUVmax values of the rotator interval, coracohumeral ligament, axillary recess, and glenoid labrum were compared. RESULTS: The mean SUVmax of the asymptomatic shoulder was 1.6 ± 0.3. Area-weighted SUV peaked at the glenoid labrum and decreased with distance (ρ = - 0.709, p < 0.0001). SUVmax was highest in the anterosuperior and anteroinferior labrum. FDG uptake was most consistently observed at the glenoid labrum, which showed the highest SUVmax in all patients. DISCUSSION: Inflammation was localized to the anterosuperior and anteroinferior glenoid labrum, suggesting the labrum as a preferential site of inflammatory activity. These findings provide insight into disease mechanisms and support further imaging studies focusing on the labrum.