In general,
for the diagnosis of axSpA,
radiography is the first-line imaging modality.
Anteroposterior and lateral views of the entire spine,
as well as an erect anteroposterior view of the pelvis,
should be obtained to evaluate both the sacroiliac joints and the hips (oblique sacroiliac joint views are not necessary).
Although the sacroiliac joints are the most important joints to image because the disease nearly always starts in this location of the axial skeleton,
radiographs of the spine in the same setting can be useful to document the presence and progression of structural changes.
Published MR imaging protocols for the sacroiliac joints include the oblique coronal plane (tangent to the posterior surface of the S2 vertebral body) and the oblique axial plane (perpendicular to the oblique coronal plane) used with T1-weighted and fluid-sensitive sequences,
such as the STIR or T2-weighted fat-suppressed sequences.
MR imaging with the STIR sequence provides more-homogeneous fat suppression,
compared with chemical-shift techniques.
Inclusion of a fat-saturated T1-weighted or T2-weighted gradient-echo sequence is useful to improve sensitivity for the detection of erosions.
In general,
administration of intravenous gadolinium-based contrast material is not necessary for evaluation of the sacroiliac joints but can be helpful in certain scenarios,
such as when the diagnosis is in doubt.
The entire spine should be imaged because any region can be affected,
but the thoracic levels appear to be the most commonly affected areas in nr-AxSpA and the lumbar spine in Ax-SpA.
Sagittal T1-weighted and STIR MR images should be obtained with coverage extending laterally to include the paravertebral synovial joints,
including the facet,
costovertebral,
and costotransverse joints.
As outlined by ASAS,
both active inflammatory lesions (bone marrow edema,
capsulitis,
synovitis,
and enthesitis) and chronic structural lesions (sclerosis,
erosions,
fat deposition,
and ankylosis) may be seen at MR imaging of the sacroiliac joints [1].
On MR (Figure 7) the patient had low-signal-intensity osteophyte bridges around the ventral joint bonders,
which is a typical change associated with osteoarthrosis of the sacroiliac joints.
Detection of subchondral sclerosis was based on the finding of low signal intensity to nulled signal in subchondnal zones on images obtained with precontrast Ti- and T2*ıweighted sequences and lack of enhancement after injection of gadolinium.
Erosions were depicted as contrast-enhanced areas continuously connected to the inner joint spaces,
with consecutive pseudodistention of the joint cavities.
While large ( > 2 mm diameter) erosions were detected in precontrast images as hypointense with Ti-weighted sequences and hyperintense with T2*I weighted sequences,
small ( < 1 mm diameter) erosions were distinctly visible on postcontnast images [4].
It is crucial to recognize that some degree of periarticular BME signal can appear in patients with non-specific low back pain and even in the normal population ,
emphasizing the importance of a proficient and expert reading of the MRI images for an accurate diagnosis of inflammatory sacroiliitis [5].