The degenerative and inflammatory disease are the main etiology of sacroiliac joint.
They have a common clinical and radiological signs,
making their distinction difficult.
Conventional radiography is the first-line examination of any symptomatology affecting the sacroiliac joint.
however,
it remains insufficient and only visualizes structural lesions already established at a late stage of the disease,
including erosions,
hyperostosis and ankylosis.
MRI allows the diagnosis of spondyloarthropathy at an early stage,
when only inflammatory lesions are visible namely medullary edema.
In 2009,
the assessement in spondyloarthritis international society (ASAS) integrated MRI of sacroiliac into its classification criteria for spondyloarthritis.
They consern patients aged less than 45 years old with active low back pain for more than three months and it is based on clinical and radiological criteria.
MRI of sacroiliac joint,
when positive,
is sufficient for diagnosis.
Spinal exploration can help to guide the diagnosis,
especially in cases where MRI of sacroiliac joint is negative or in doubtful cases,
investigating inflammatory disco-vertebral involvements,
including Romanus spondylitis,
Andersson spondylodiscitis and enthesopathy,
and to rule out discarthrosis.
It is necessary to acquire at least one T2 sequence with fat saturation to detect bone marrow edema sites,
and a T1-weighted sequence to visualize structural abnormalities,
including fatty infiltration and ankylosis.
These MRI abnormalities may sometimes lack of specificity.
Topographic analysis of lesions may help diagnosis.
The degenerative lesions are electively localized in the anterior and middle parts of the sacroiliac joints corresponding to the zones of maximum mechanical stress.
while inflammatory attacks may affect the whole sacroiliac joint,
but involvement of the posterior and inferior parts is very suggestive of the diagnosis.
when the doubt persists,
the interpretation must be always confronted with the clinical and biological context and sometimes with the evolution of the patient avoiding to over diagnose an inflammatory attack or to omit it in the beginning stages engaging the functional prognosis.
In summary,
MRI is the gold standard examination for inflammatory disease that may require a CT scan to rule out degenerative disease.