Diagnostic Imaging for Sacroiliitis

Initial Evaluation

The American College of Radiology recommends conventional radiography of the sacroiliac joints as the first imaging modality for the evaluation of suspected sacroiliitis, followed by MRI if radiographs are negative or equivocal but clinical suspicion remains high 1, 2, 3
Radiography of the sacroiliac joints demonstrates chronic erosions, sclerotic changes, and ankylosis as sequelae of inflammatory sacroiliitis, with limitations including low sensitivity (19%-72%) for early disease detection and fair to moderate interobserver agreement 1
Complementary radiography of the spine should be performed alongside sacroiliac joint radiographs, including at minimum cervical and lumbar spine radiographs 1, 3, 5

MRI of the sacroiliac joints is the highest rated second-line investigation, with a protocol that includes T1-weighted sequences, fat-suppressed fluid-sensitive sequences, and evaluation of both active inflammatory lesions and structural lesions 2, 3, 4, 7, 8
MRI without contrast is generally sufficient, with a sensitivity of 79% and specificity of 89% for diagnosis of axial spondyloarthritis 3, 4, 8
MRI with contrast may be considered in select cases, but has not been shown to significantly increase diagnostic accuracy 3, 4, 7

MRI of the spine is not generally recommended for initial diagnosis of axial spondyloarthritis, but may be considered if sacroiliac joints are normal but clinical suspicion remains high 1, 2, 7
Bone scintigraphy/SPECT is not routinely recommended, with low to moderate sensitivity and variable specificity 1, 3, 4, 7
Ultrasound is not recommended for diagnosis of sacroiliitis 1, 2
PET/CT is not routinely obtained as initial imaging modality, with uncertain utility in diagnosis of axial spondyloarthritis 1, 8

Radiographs may miss more than half of patients with structural changes of axial spondyloarthritis when compared to CT 1
Bone marrow edema on MRI can be seen in non-inflammatory conditions and in up to 30% of healthy controls, but deep bone marrow edema lesions extending at least 1 cm deep to the articular surface are more specific for axial spondyloarthritis 8
Interobserver agreement for radiographic findings of sacroiliitis is only fair to moderate 1
Early sacroiliitis may be radiographically occult for 3-7 years after symptom onset 1, 8