Background Imaging is an important tool in diagnosing axial spondylo-arthritis (SpA). In contrast to magnetic resonance imaging, conventional bone scintigraphy has not been so helpful for early detection of axial SpA. However, SPECT (single positron emission computed tomography)/CT has the advantage of better delineating the changes of the sacroiliac (SI) joints as well as the underlying structure.

Objectives To investigate the usefulness of bone SPECT-CT with volume of interest (VOI) analysis in early axial SpA.

Methods Fifteen patients presenting with inflammatory back pain meeting the Amor criteria of SpA were recruited from a single center in South Korea. Bone scintigraphy was performed 180 min after intravenous injection of 1110 MBq of Tc-99m-HDP, followed by bone SPECT-CT. The ratio between the entire SI joint and sacrum (SIS ratio) was measured by both bone SPECT-CT and bone scintigraphy. Data from 13 controls were also evaluated. The ROC curve was plotted for further analysis, and the correlation between the SIS ratio and SI joint grade by plain radiography was assessed.

Results Figure A, B are bone SPECT-CT images of a representative early axial SpA patient, whereas Figure C, D are those of a control subject. The SIS ratio of early axial SpA patients vs. control subjects was significantly increased (p = 0.000) in bone SPECT-CT. However, no significant difference was detected in bone scintigraphy. ROC curve analysis showed a significant difference in the AUC of bone SPECT-CT vs. bone scintigraphy (0.873 vs. 0.560, respectively; p = 0.000). With a cutoff SIS ratio of 1.5, ROC curve analysis showed a sensitivity of 83.3% and specificity of 76.9% in bone SPECT-CT. The SIS ratio measured in SPECT-CT, but not that measured in bone scintigraphy, was significantly increased with a higher grade of SI joint changes in plain radiography (p = 0.000).

Conclusions Bone SPECT-CT is more useful than conventional bone scintigraphy in identifying sacroiliitis in axial SpA patients, even with mild SI joint changes in plain radiography.

Disclosure of Interest None Declared