SPA

Imaging of the sacroiliac joint has a primary role in the diagnosis of SpA.
In routine clinical practice radiographic imaging has become an integral part of the investigation of patients with suspected sacroiliitis (SII). While plain x-rays remain the first and most widespread imaging modality employed in the investigation of SII, MRI has a significantly improved sensitivity and specificity for the diagnosis of SII.
Unfortunately, the determination of the exact sensitivities and specificities of these imaging modalities is severely hindered by the absence of a ‘gold standard’ investigation. MRI is the defacto imaging gold standard.
Other imaging techniques used to diagnose SII include CT and scintigraphy. Conventional tomography was employed prior to the use of cross-sectional imaging.

Screening examination of choice in the investigation of SpA -widely available, low cost
However lack of sensitivity in detecting early SpA, high false positive rate, the use of ionizing radiation, and the high inter- and intraobserver variation are limiting factors.
In addition radiographs can only assess chronic changes and are unable to assess presence and extent of acute inflammatory changes
If radiographs are diagnostic then no further imaging may be required.
If non-diagnostic then further evaluation with MRI is advised

MRI is the imaging gold standard
Assessment of acute and chronic changes of the whole spine and sacroiliac joints
Allows for detection of pre-radiographic changes of SpA.
Can serve as a biomarker for active disease.
Excellent at evaluating alternate pathology including degenerative disc disease, infection, insufficiency fractures, osteitis condensa ilia
However expensive, may not be widely available.

Not routinely used in the diagnosis SpA
High radiation dose and can not assess acute inflammatory changes
Excellent in demonstrating the osseous anatomy of the SIJs and allows high-resolution axial acquisition with coronal and sagittal reconstruction
Comparative studies between plain radiographs and CT have demonstrated marked advantage of CT in the delineation of chronic changes including erosions, subchondral sclerosis, ankylosis and fat-replacement of bone marrow.
May be helpful if the patient is not MRI compatible and radiographs are non-diagnostic, in the assessment of alternate pathologies e.g. fractures.
Note prior CT imaging should be reviewed which may include non-dedicated imaging of the SIJs, e.g.CT pelvis

Scintigraphy had previously been a commonly used modality in the investigation of SII in clinical practice.
It is very sensitive in the detection of early articular inflammatory change but is non-specific. Radionuclide normally accumulates at the SIJs and the differentiation of normal uptake and early SII can be difficult.
Quantitative analysis of the SIJs has proven more sensitive however limited by the the wide range of variation in quantitative evaluation in the normal population. Other factors including age and prominent first sacral spine may all cause difficulties.
Scintigraphy has poor sensitivity when compared with MRI in symptomatic spondyloarthropathy patients.
Due to the above limitations, and given the significant associated radiation dose, scintigraphy is not part of our diagnostic imaging algorithm.

Ultrasound is not currently utilized in our clinical practice in the assessment of SII.
Ultrasound can evaluate the posterior portions of the sacroiliac joints with Doppler and has shown in some studies increased vascularization with decreased resistive indices in this portion of sacroiliac joint in patients with active SII.
Further research is warranted to assess what role ultrasound will play in clinical assessment SII.

Conventional tomography is associated with a higher radiation burden than plain radiographs and are no longer used.
The newer technique of tomosynthesis holds promise but our initial evaluation demonstrated a higher radiation burden than low dose CT and is not indicated until further research is performed to evaluate its position in the investigation of sacroiliitis.

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Imaging Algorithm