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

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.

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.

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.

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.