In 2009, the International SpA Assessment Task Force (ASAS) published classification criteria for medial SpA that differ from previous criteria. According to this criterion, the diagnosis of SpA must first meet the age of onset <45 years and the duration of back pain ≥3 months, and those who meet the conditions should further consider whether the following 4 points are met.
① back pain with inflammatory features.
②Human leukocyte antigen (HLA)-B27 positivity
(iii) elevated C-reactive protein levels.
(4) Non-steroidal anti-inflammatory drug (NSAID) treatment is effective. The diagnosis of SpA can be confirmed if the above conditions are met. For those who do not meet all the conditions, imaging of the sacroiliac joint should be performed to determine whether there are inflammatory changes. For the diagnosis of SpA, only sacroiliac joint examination is the most valuable among imaging examinations, while examination of the spine and other parts is not recommended for the time being. Among the diagnostic indicators of SpA, imaging has the highest specificity but low sensitivity, so it is not recommended as a screening method. The correct determination of sacroiliac arthritis based on imaging findings is a method that should be mastered by rheumatologists.
Principles of imaging test selection
Sacroiliac arthritis plays an important role in the diagnosis of both mid-axis and peripheral SpA. The methods currently available for sacroiliac joint examination include X-ray plain film, magnetic resonance imaging (MRI) and CT, with X-ray plain film being preferred. In the absence of X-ray findings, CT and MRI should not be performed. If the diagnosis is not confirmed by X-ray, it should be considered whether the diagnosis of SpA has an important impact on drug selection, otherwise, other tests to confirm the diagnosis of SpA will only increase the financial burden of the patient; if the definite diagnosis of SpA will affect the choice of therapeutic drugs and the diagnosis cannot be confirmed by X-ray plain film, MRI is recommended as the second test in the classification criteria by ASAS in 2009. CT is recommended only when there is a contraindication to MRI examination. Because CT has 2 important drawbacks: radiation increases the risk of cancer; there is no uniform standard of review for CT of the sacroiliac joint, and there are often significant differences between reviewers in the determination of the same CT findings.
Radiological examination of the sacroiliac joint
X-ray examination
X-ray examination of the sacroiliac joint requires only a pelvic radiograph with the hip joint. Anteroposterior radiographs are an important method to determine the extent of sacroiliac joint pathology in SpA. The rheumatologist should evaluate the sacroiliac joints separately on both sides.
Each sacroiliac joint is scored from 0 to 4 according to the following grading
Grade 0: normal (Figure 1).
Grade 1: suspicious lesion, but not completely sure if it is normal (Figure 2, left sacroiliac joint).
Grade 2: definite lesion with slight local erosion or sclerosis and no significant widening of the joint space (Figure 2, right sacroiliac joint).
Grade 3: definite lesions, moderate or severe inflammation of the sacroiliac joint with erosion, sclerosis, widening (or narrowing) in one change, and partial ankylosis (Figure 3).
Grade 4: severe joint changes and complete ankylosis of the sacroiliac joint (Figure 4).
For the determination of SpA sacroiliac arthritis, pelvic radiographs were considered positive if the following conditions were met: ① total bilateral sacroiliac joint score ≥ 4; ② sacroiliac joint score ≥ 3 on either side.
CT examination
There is no CT criteria for the diagnosis of SpA, and there may not be similar criteria in the future, because CT examination for SpA usually does more harm than good. A few scholars have used a scoring method similar to that of radiographs in their studies, and there are no multicenter studies to verify the reliability of the CT scoring system. Sometimes CT examination can reveal significant abnormalities of the sacroiliac joint (Figure 5).
MRI of the sacroiliac joint
Conditions for implementation
Many early SpA have negative radiographs and only MRI can show positive results. Because MRI is expensive, it should only be used when there is a change in drug selection if the diagnosis of SpA is clear. For example, MRI may be performed when biologic therapy is being considered once the MRI is positive.
Clinical investigators often repeat the examination to test the effect of treatment on MRI results. However, in clinical practice, MRI should only be used for diagnosis and not for monitoring treatment response.
Examination methods
For the MRI examination site and the selected sequence, the following aspects can be referred to.
①There is no uniform understanding of MRI examination of the spine, and MRI examination for the diagnosis of SpA is only recommended for the sacroiliac joint area
②Since the sacrum is not in a vertical position, the MRI scan should be performed in an oblique coronal position;
③The MRI scan layer thickness is 4mm;
④The T1 sequence must be shown, but it is not necessary to do the enhancement scan, which only increases the financial burden of the patient without increasing the diagnostic accuracy;
The most important MRI sequences are the T2 sequence with fat suppression (or T2FS) and the short T1 inversion recovery (STIR) sequence, one of which can be chosen.
Sometimes it is difficult for the reviewer to distinguish between T1 and STIR sequences by referring to the L5/S1 intervertebral disc, with the disc showing white as a STIR sequence and black as a T1 sequence (Figure 6). All sacroiliac MRI findings should be reviewed by a rheumatologist in addition to the imaging physician for a final diagnosis.
Diagnostic features of MRI of sacroiliac arthritis
According to the consensus issued by the 2009 ASAS/Rheumatoid Arthritis Clinical Trial Efficacy Assessment Criteria (OMERACT) MRI Working Group, the criteria for diagnosing SpA on MRI include only signs of edema in the bone marrow. The signs of bone marrow edema appear as high signal (white) in T2 sequences or STIR sequences with fat suppression and should be present in subchondral or periarticular areas. If only one area within a level has bone marrow edema, a similar lesion must be found at another level, and if two or more areas have edema signal, one level is required for diagnosis. synovitis, arthritis and tendon telangiectasia shown by MRI are not suggestive for the diagnosis of SpA. Figures 6 to 8 show negative and positive MRI presentations, respectively.
Summary
For the diagnosis of SpA, the primary basis is clinical indicators, especially whether there are inflammatory features in back pain and whether there is finger (toe) inflammation in peripheral joints. Before performing imaging examinations, HLA-B27 and C-reactive protein should be tested, and patients should be observed to see if their condition improves significantly after the application of NSAID.
X-rays are preferred for imaging of the sacroiliac joint. The reading of radiographs must avoid subjective speculation, and the scoring system published by ASAS above should be used. If there is clearly SpA sacroiliac arthritis, no other imaging examinations are necessary. In patients with early negative radiographs, MRI should be performed if the diagnosis of SpA would affect the choice of therapeutic agents, as bone marrow edema of the sacroiliac joint is the only diagnostic feature of SpA in MRI findings.