Open access peer-reviewed chapter
Abstract
Ankylosing spondylitis (AS) is a chronic inflammatory disease affecting the spine and the sacroiliac joints. AS occurs with the inflammation of the entheses and formation of syndesmophytes and finally sacral and spinal ankylosis. Imaging demonstrates both inflammatory and chronic lesions. Sacroiliitis is the hallmark of the disease. Spinal changes usually take place in advanced stages of the disease. 1984 The Modified New York criteria evaluated for the diagnosis of AS with definite radiological sacroiliitis (bilaterally grade 2 or unilateral grade 3/4 sacroiliitis) on imaging. The Modified New York criteria are well performed in diagnosing the established disease but its sensitivity is too low in early disease identification and leads to a diagnostic delay. So, in 2009 The Assessment in Spondyloarthritis International Society (ASAS) recommended classification criteria for axial spondyloarthritis (axSpA). Patients have sacroiliitis on imaging and ≥1 SpA features (imaging arm) or positive HLA B27 and ≥2 SpA features (clinical arm) are classified as axial SpA. On the imaging arm, either radiographic sacroiliitis according to Modified New York criteria or active inflammation on MRI is required. Imaging is also used for determining extent of disease, monitoring activity and progression of the disease, assessment of the treatment effect, and prognosis in AS patients.
Keywords
- ankylosing spondylitis
- imaging
- conventional radiography
- magnetic resonance imaging
- computed tomography
1. Introduction
Ankylosing spondylitis (AS) is a chronic inflammatory disease affecting the spine and the sacroiliac joints. Patients with AS show both active inflammatory and structural changes on imaging.
Sacroiliitis, spondylitis, spondylodiscitis and facet joint arthritis are the typical inflammatory manifestations. These inflammatory lesions lead to chronic structural lesions, such as syndesmophytes and ankylosis, in the later stages of the disease [1].
Imaging of the sacroiliac joint plays an important role in AS, since almost all patients with AS do have involvement of the sacroiliac joints. Inflammation of sacroiliac joint on imaging is the hallmark of AS in both diagnosis and classification.
1.1 Anatomy of the sacroiliac joint
The sacroiliac joint (SIJ) lies between the sacrum and the ilium, formed within sacral segments S1, S2 and S3, about 1–2 mm in width and a joint on either side of the sacrum is held together by a fibrous capsule. The bony anatomy is highly variable in size, shape and contour among individuals.
The surface of the SIJ can be divided into three parts, corresponding to the three sacral elements (S1, S2, S3) that participate in the (sacral) auricular surface, terms like ventral, middle and dorsal part. The lower portion of the cranial limb and the caudal limb are synovial in construction, whereas the upper part of the cranial limb is more fibrous [2, 3].
Six types of anatomical variants were defined as: accessory joints, iliosacral complex, bipartite iliac bony plate, crescent-like iliac bony plate, semicircular defects at the sacral or iliac side and ossification centers. Accessory joint is the most common anatomic variant in sacroiliac joint [3].
2. Conventional radiography
2.1 Technical aspects
Conventional radiography (CR) of the sacroiliac joints (SIJs) is the first recommended modality for the diagnosis of AS and is the gold standard for the assessment of structural changes in the spine and SIJs. A frontal projection of the SIJs is preferred. An anterior–posterior view of the SIJs is usually performed with the patient in the supine position [4].
İdeal pelvis AP view should include; entirety of the bony pelvis imaged from superior of the iliac crest to the proximal shaft of the femur, obturator foramina appear symmetrical, iliac wings have an equal concavity and greater trochanters of the proximal femur [5].
2.2 Sacroiliac joint
Typical radiographic findings in the SIJs are erosions, pseudo-widening, sclerosis, bony bridging, and ankylosis. Erosions in the iliac side of the SIJs are the earliest radiographic changes visualized in AS. Definition of diagnostic criteria of radiographic changes of the SIJs has been used according to the 1984 modified New York criteria [6] in AS patients and classification of axSpA according to the 2009 ASAS classification criteria [7].
Grade definition of radiographic changes.
0 Normal.
1 Suspicious changes.
2 Minimal abnormalities: small localized areas with erosion and sclerosis, without alteration in the joint width.
3 Unequivocal abnormality: moderate or advanced sacroiliitis with 1 or more signs of erosions, sclerosis, widening, joint space narrowing, or partial ankylosis.
4 Severe changes: total ankylosis.
According to the modified New York criteria, the radiographic definition of sacroiliitis has a high specificity for axial SpA, but a low sensitivity (30–50%) especially in early disease [8]. Therefore, using only these criteria in the diagnosis of AS may delay the diagnosis of the disease (Figure 1).
Figure 1.
Grading sacroiliitis according to the New York criteria. a)Grade 1: subtle findings that does not indicate defnite AS. b)Grade 2: minimal changes with bilateral small sclerotic areas and erosions on right SIJ. c)Grade 3:bilateral joint narrowing and erosions on right SIJ. d)Grade 4: bilateral total ankylosis.
2.3 Spine
Radiograhic findings of AS patients in the spine are vertebral corner erosions, enthesophytes, vertebral squaring (precursor “shiny corners” seen on x-ray represent circumscribed areas of postinflammatory fatty bone marrow degeneration), sclerosis and erosions of the vertebral endplate, disk calcifications, spondylophytes, syndesmophytes, bony bridging, and/or intervertebral ankylosis and then bamboo spine.
New bone formation, syndesmophytes, and ankylosis of the vertebral column are almost pathognomonic for AS. Syndesmophytes are the best predictors of radiographic progression [9]. Most of the data regarding radiological progression of AS pertains to CR (Figure 2) [10].
Figure 2.
Radiographs of spine in AS. a)AP view of thoracolumbar vertebrae:extensive syndesmophytes leading bamboo spine b)lateral view demonstrating syndesmophytes c) lateral view of cervical spine: vertical syndesmophytes bridging the anterior vertebral corners.
2.4 Scoring
The most common findings of ankylosing spondylitis in the vertebral column are syndesmophytes and ankylosis. These findings indicate new bone formation, erosions are less common in the vertebral column. Erosion and sclerosis are predictive factors for syndesmophyte formation. Syndesmophyte and ankylosis are best evaluated on conventional radiographs [11, 12]. Syndesmophytes grow and merge and appear as bamboo spine in advanced disease [13]. Radiograhic grading and scoring help us to evaluate the disease activity and progression.
To date, there are two accepted scoring systems using conventional radiography in AS patients; SASSS/mSASSS [14] and BASRI scores [15].
2.4.1 Modified stoke ankylosing spondylitis spinal score (mSASSS)
mSASSS derived from sum of scores for lumbar spine and cervical spine (range 0–72)
nominal scoring system used for each site
0 = no abnormality
1 = erosion, sclerosis, or squaring
2 = syndesmophyte
3 = total bony bridging
cervical sites were lower border of second cervical vertebra up to and including upper border of first thoracic vertebra; third cervical vertebra scored for erosions and sclerosis, but not squaring.
2.4.2 BASRI
For the lumbar spine, examine both the anteroposterior and lateral radiographs together. The score for the lumbar spine is a composite of the two views. For the cervical spine lateral view is scored.
| Score | Grade | Lumbar and cervical spine (grade each as 0–4) |
|---|---|---|
| 0 | Normal | No change |
| 1 | Suspicious | No definite change |
| 2 | Mild | Any number of erosions, squaring or sclerosis with/without syndesmophytes on ≤2 vertebrae |
| 3 | Moderate | Syndesmophytes on ≥3 vertebrae with/without fusion involving 2 vertebrae |
| 4 | Severe | Fusion involving ≥3 vertebrae |
A modification has been accepted and called BASRI-s for the spine only, BASRI-h for the hips only, and BASRI-t for the summation of both [16].
A study comparing three of these methods (i.e. BASRI, SASSS and mSASSS) concluded that mSASSS is the most appropriate method by which to score radiographic progression in AS [17]. mSASSS is the preferred scoring method for radiographic progression in AS.
2.5 Radiographic progression
According to the definition of mSASSS, radiographic damage is defined as more than 2 points change from baseline [18]. This means that at least 1 new syndesmophyte is formed. This change can be evaluated as improvement or worsening depending on the emergence or disappearance of the new lesion. Definite radiographic damage at baseline is a prognostic factor associated with continuing radiographic progression over time, possibly independent of treatment.
3. Magnetic resonance imaging
Magnetic resonance imaging (MRI) can detect both active inflammatory and structural lesions and capable of detecting both bone marrow edema (BME) or osteitis and erosions before CR [19]. Therefore, MRI is particularly useful for the early diagnosis of AS.
Also, patients with signs or symptoms indicative of SpA but not have structural lesions on conventional radiography, MRI can detect active- chronic lesions for sacroiliitis or spondylitis. These patients can be classified as having ‘axial non-radiographic spondyloarthritis (nr-axSpA)’ according to criteria developed by the Assessment of Spondyloarthritis International Society (ASAS) [7]. So, MRI is useful for classifying patients as SpA.
3.1 Protocol
An MRI of the sacroiliac joints is conducted with the patient in the supine position. Semicoronal T1w sequence and either a STIR or fat-saturated T2-weighted (T2FS) sequence, should be included in the routine evaluation of the SIJs by MRI. T1w images are mandatory for evaluation of structural (chronic) changes, such as bone erosion, new bone formation and fat infiltrations. Active inflammatory changes are visualized best by fat saturated T2-weighted turbo spin-echo sequence or a short tau inversion recovery (STIR) sequence, which can detect bone marrow edema [20].
Bone marrow abnormalities in both sacroiliac joints and spine are detected almost equally well with the STIR and contrast-enhanced T1w FS sequences in patients with SpA, so contrast injection is generally not needed [21].
The whole sacral bone image should be included both its anterior to its posterior border, which usually requires at least 10–12 slices. Transverse slices are useful for assessment of the posterior parts of the spine. However, for routine imaging of the spine transverse sequences are time consuming and therefore less feasible.
Characteristic lesions in the sacroiliac joints and the spine of patients with AS [8].
| Inflammatory (active) changes | Structural (chronic) changes |
|---|---|
| Sacroiliitis/bone marrow edema/osteitis | Subchondral sclerosis |
| Synovitis | Erosions |
| Capsulitis | Fat metaplasia |
| Enthesitis | Ankylosis |
ASAS group defines sacroiliitis by MRI as ‘active inflammatory lesions of sacroiliac joints with definite bone marrow edema/osteitis’ as suggestive of spondyloarthritis [22].
3.2 Inflammatory (active) changes
3.2.1 Bone marrow edema (BME)
Bone marrow edema is the term given to abnormal fluid signal seen within the bone marrow on MRI and reflects osteitis. BME is hypointense in T1 sequences and hyperintense in T2-FS and STIR sequences. To define it as sacroiliitis, BME should be associated with SIJ (periarticularly located), approximately 1 cm in width (especially in the inferior part of the joint), and should be observed in at least 2 consecutive sequences or more than 1 lesion in a single image. The stronger the signal intensity, the stronger the association with the disease. BME generally associated with structural damage such as erosions or sclerosis (Figures 3 and 4).
Figure 3.
STIR sequences of sacroiliac joint demonstrates bilateral bone marrow edema on three consecutive slices.
Figure 4.
(a) T1-weighted image shows bilateral large erosions. (b) STIR images demonstrate bilateral bone marrow edema on SIJs. (c) Prominent synovitis is shown with T1-weighted contrast enhanced images.
Bone edema and osteitis are highly suggestive of active sacroiliitis. However, bone edema can be found in other conditions and between 2.6 and 20% in healthy patients [23].
3.2.2 Synovitis, capsulitis and enthesitis
Synovitis, capsulitis and enthesitis can be demonstrated on contrast-enhanced T1w images. STIR sequences do not differentiate synovitis from joint fluid (Figure 5).
Figure 5.
(a) Right iliac and sacral erosions is seen on T1-weighted image. (b) large subchondral sclerosis (T1-w). (c) subchondral fat metaplasia (T1-w). (d) bilaterally synovitis on T1-w contrast enhanced image.
3.3 Structural (chronic) changes
3.3.1 Subchondral sclerosis
Sclerosis is seen as low signal intensity areas in all sequences (T1, STIR, T2 FS) and shows no signal enhancement after contrast medium administration. Sclerosis should extend at least 5 mm from the SI joint space.
3.3.2 Erosions
Erosions are bony defects at the joint margin. Erosions may occur throughout the cartilaginous compartment of the joint. Erosions initially appear as single lesions. Confluence of erosions may be seen as pseudodilation of the sacroiliac joints. Erosions are seen as low signal intensity on T1-weighted images. T2 gradient-echo or T1 fat saturated sequences maybe more useful in detecting erosions.
3.3.3 Periarticular fat deposition
Periarticular fat deposition is characterized on MRI by an increased signal intensity on T1-weighted images and low signal intensity on T2w FS and STIR sequences. Accumulation occurs mostly periarticular bone marrow areas and is not a specific finding for AS.
3.3.4 Ankylosis
Ankylosis is bony bridges across the joint with low signal intensity on all MRI sequences.
The presence of synovitis, capsulitis, or enthesitis without concomitant subchondral bone marrow edema/osteitis is not sufficient for making a diagnosis of active sacroiliitis. Structural lesions such as fat deposition, sclerosis, erosions or bony ankylosis represents previous inflammation.
3.4 ASAS positive MRI
ASAS defines sacroiliitis as BME/osteitis in the subchondral bone. There should be BME at least 2 consecutive sequences or more than 1 lesion in a single image [24].
3.5 SPINE
Active lesions of the spine in AS patients are spondylitis, spondylodiscitis and arthritis of the facet, costovertebral and costotransverse joints. Enthesitis may affect the interspinal and supraspinal ligaments and the interosseous ligaments of the sacroiliac joints.
3.5.1 Spondylitis
Spinal MRI shows us active spinal lesions, disease activity and response to therapy. Inflammation of vertebral body can be seen as shiny corners, Romanus lesions and vertebral osteitis, most frequently seen in the thoracolumbar region (T10-T12). The typical appearance of spondylitis is not limited to vertebral edges but also spread to the entire vertebral body. Syndesmophytes occur as a result of inflammation and repair reaction (Figure 6) [25, 26].
Figure 6.
Sagittal MRI view of lumbar spine in AS patient. (a) T1-w image and (b) T2-w FS images demonstrate vertebral endplate osteitis (Romanus lesions).
3.5.2 Spondylodiscitis
Spinal inflammation also seen in the intervertebral space (diskitis) and diskitis with vertebral body inflammation (spondylodiskitis-Andersson lesions). Andersson lesion is inflammation involving the intervertebral disc and adjacent vertebrae. Asymptomatic spondylodiskitis may occur in early disease [27].
3.6 Scoring the MRI
Several systems for assessment of disease activity in the sacroiliac joints and in the spine have been proposed. The SPARCC method had the highest sensitivity to change [28].
3.6.1 SPARCC
This method is based on the evaluation on STIR sequence six consecutive semicoronal slices focusing on the synovial part of the SIJ. Six coronal slices are assessed and only STIR sequences are scored. The sacroiliac joint is divided into four quadrants (upper iliac, lower iliac, upper sacral and lower sacral) and each quadrant is examined separately. For each quadrant, it is recorded whether it has an hyperintense lesion in the STIR sequence. Each quadrant is also scored due to its signal intensity. Total maximum score is 72 [29]. Assessment of the chronic inflammatory changes is important for disease monitoring.
MRI also help us to determine peripheral lesions of AS, like peripheral arthritis and enthesitis. Achilles tendon and plantar fascia are the most affected sites of entesitis (Figure 7).
Figure 7.
AS patients also have peripheral clinical findings as arthritis and enthesitis. (a) sagittal image shows tibiotalar-talonavicular arthritis with plantar fasciitis (white arrow). (b)T1-w axial image also demonstrates plantar fasciitis (black arrow).
4. Computed tomography
CT permits imaging the structural changes without superimposition of overlying structures. CT shows chronic changes more clearly than conventional radiography.
Semicoronal CT is used for the diagnosis of sacroiliitis. Semicoronal technique permits an overview of the cartilaginous and ligamentous portions of the SIJ with less radiation dose—6–8 contiguous 5-mm slices [30].
CT can detect osteoporosis or osteosclerosis quite well but these changes are very nonspecific. The primary value of CT in AS is its ability to detect and clearly define erosion of bone at any joint or enthesis, and for documenting fractures.
Typical changes for sacroiliitis at CT are joint erosions, subchondral sclerosis and ankylosis [31]. Joint space narrowing and pseudo-widening can be viewed clearly (Figure 8). However, CT findings may be misleading in elderly patients because subchondral sclerosis in the iliac part of the SIJs can be seen due to aging.
Figure 8.
CT images of sacroiliac joints (a) axial CT image demonstrates bilateral subchondral sclerosis (b) coronal image shows right sided large erosions.
CT is superior to MRI, especially in detecting sclerosis, bone production, and chronic bone changes in the ligamentous portion of the joint. But CT cannot reveal bone marrow edema, which makes the diagnosis of active sacroiliitis. A new promising technique; low-dose CT of the SIJs may replace CR as a method of structural damage and new bone formation monitoring [32, 33].
In the spine, CT can demonstrate complications of the disease such as spondylodiscitis or spinal fracture.
5. Ultrasonography
Ultrasonography (US) is an evolving imaging technique increasingly used by the rheumatologist in daily clinical practice. The role of US in assessment of sacroiliac and spine involvement in AS and other types of axial SpA is minimal [34]. Only the superficial part of the SIJs is accessible to visualization by US including the surrounding soft tissue structures and the posterior stabilizing ligaments, while the cartilaginous portion is inaccessible by this imaging modality.
However, US can be used to guide SIJ corticosteroid injections, particularly where these appear to be the primary affected joints [35, 36, 37, 38].
Ultrasonography is more useful in the diagnosis of peripheral involvement of the disease. US has a high sensitivity and specificity in the diagnosis of Achilles and plantar enthesitis, especially using the power Doppler. It is also a guide for interventional treatment for arthritis and enthesitis (Figure 9).
Figure 9.
(a) Normal view of sacroiliac joint (b) normal US view of Achilles tendon.
While US assessment is safe, noninvasive, comparably cheap and conveys no radiation, it is highly operator-dependent and influenced by the quality of the US equipment.
6. Radionuclide methods
6.1 Bone scintigraphy with technetium-99 labeled methylene diphosphate
Bone scintigraphy can be used to detect inflammation by demonstrating increased uptake in the sacroiliac joints [39]. It also gives the chance to evaluate the inflammation quantitatively by comparing the radionuclide signal intensity [39, 40]. But it has a limited value in detecting AS in clinical practice. Scintigraphy of sacroiliac joints has low sensitivity for diagnosis of suspected or established ankylosing spondylitis.
A review by Song et al. on the performance of bone scintigraphy showed an overall sensitivity of about 50% and specificity not higher than about 80% for the diagnosis of sacroiliitis [41]. Also, the radiation exposure of bone scintigraphy limits its daily use in patients with suspected AS.
6.2 Single-photon emission computed tomography (SPECT) and combined SPECT-CT
Quality and sensitivity of the bone scintigraphy can be increased with using SPECT. SPECT provides a better anatomical evaluation of the joint. SPECT has been shown to be superior in quantifying the SIJ to sacrum ratio [42]. Kim et al. showed that SPECT with low dose CT is superior to bone scintigraphy in demonstrating early sacroiliitis, with sensitivity of 80% and specificity of 84% [43].
7. Novel/future modalities
7.1 Whole-body MRI (wbMRI)
In recent years, new magnetic resonance imaging modalities has been developed for diagnosing AS patients. Multichannel systems and multiple coils are used to scan larger areas, like wbMRI. This MRI modality allows a clearer assessment of peripheral involvement [44, 45]. wbMRI includes T1w and STIR sequences of the entire spine, shoulder girdle, arms, anterior chest wall, pelvis including the SIJs and the lower extremities [46]. wbMRI reduces imaging times and spatial resolution is similar to standard MRI [46].
This modality is mostly helpful for the assessment of enthesitis [47, 48]. wbMRI may contribute for the early diagnosis of AS and was shown to detect active inflammation and structural changes in active nr-axSpA and AS [45, 49].
7.2 Diffusion-weighted MRI (DWI)
a new MRI modality, the image contrast is yielded by the random motion of the water molecules in different biological tissue environments, within the cellular and extracellular tissue compartments, providing both quantitative [apparent diffusion coefficient (ADC)] and qualitative functional information [50]. Inflammation leads to higher ADC values through increased water in extracellular spaces. DWI was shown to identify active sacroiliitis based on conventional MRI on qualitative analysis, and to differentiate active from inactive sacroiliitis by quantitative ADC measurements [51]. Preliminary data with high-resolution MRI show an increased detection rate of erosions on the SIJs when compared to conventional MRI and low-dose CT [52].
8. Differential diagnosis
BME of the SIJ can be observed in several conditions other than AS. Most common conditions are infectious sacroiliitis, osteitis condensans ilii, diffuse idiopathic skeletal hyperostosis (DISH), and pelvic fractures.
BME/osteitis in infectious sacroiliitis extends to the surrounding soft tissue [53]. MRI detects early signs of infection, while CR is usually normal in the first few weeks.
8.1 Osteitis condensans ilii
Osteitis condensans ilii is depicted as a triangular-shaped area of sclerosis of the iliac side of the SIJ, on MRI as on CR or CT [54]. It is frequently seen in middle-aged women after pregnancy, although it can rarely occur in men (Figure 10).
Figure 10.
Osteitis condensans ilii. Both conventional radiography and MRI (STIR) demonstrates bilateral inferior triangular shaped sclerosis.
8.2 DISH
DISH is characterized by wide, bulky osteophytes with concomitant ossification of the anterior longitudinal ligament. The ossification extends three to four consecutive vertebrae and osteophytes can be seen at the shoulder, elbow, knee, or calcaneus [55]. The absence of ankylosis at the facet-joint interface and absence of sacroiliac joint erosion, sclerosis, or fusion are used in the distinction of DISH and AS (Figure 11) [56].
Figure 11.
AP view thoracal spine shows right sided large osteophytes on the anterior longitudinal ligament in patient with DISH.
8.3 Insufficiency fractures
Insufficiency fractures of the sacrum may present with low back pain and as the fracture line is not always visible, may lead to a misdiagnosis. MR image may be confused with bone marrow edema (Figure 12).
Figure 12.
STIR image of sacroiliac joint in AS patient also shows insufficiency fracture in left superior sacral bone.
8.4 CPPD
Calcium pyrophospahtel deposition in the spine can cause spine stiffness with bony ankylosis and can be misdiagnosed with AS or DISH. In addition, crystal deposition may lead to spinal cord compression syndromes. CPPD should be in the differential diagnosis of AS especially in elderly and patients with familial chondrocalcinosis.
8.5 Postpartum transient sacroilitis
Inflammatory low back pain can be seen in young women in the postpartum period and bone marrow edema can be seen in MR imaging. Transient sacroiliitis should not be confused with AS so MRI is not recommended in the first year of delivery to screen AS (Figure 13).
Figure 13.
(a) 26 year old male patient having inflammatory back pain after using isotretinoin and T2 FS image shows bone marrow edema on inferior right SIJ. (b) 28 year old female with inflammatory back pain 4 months after delivery. STIR sequence demonstrates bone marrow edema on the right superior sacral side of the joint, favoring postpartum transient sacroiliitis.
The most important differential diagnoses in the spine are degenerative/mechanical lesions, blood vessels and hemangioma, fractures, and septic spondylitis/spondylodiscitis.
9. Recommendations for imaging
The diagnosis of AS is delayed approximately 7 years in many patients [57, 58]. The main reason for the delay in diagnosis is that radiographic sacroiliitis must be present in the x-ray for diagnosis among the modified New York criteria. These criteria are quite specific in patients with established disease, but they are insensitive for diagnosing early stages of disease. MRI is mostly preferred to prevent delay in diagnosis and to detect early signs of inflammation. Despite all the developments and different techniques in the field of imaging in recent years, conventional radiography is still the gold standard imaging method, especially for the assessment of structural damage.
Compared to other imaging modalities (conventional radiography, CT, scintigraphy), MRI is significantly superior in both detecting disease and showing active lesions [59, 60].
9.1 EULAR recommendations for diagnosis of axial SpA
In 2015, European League Against Rheumatism (EULAR) taskforce developed recommendations on the use of musculoskeletal imaging in the clinical management of axial Spondyloarthritis [61]. Conventional radiography is the first recommended imaging method. In young patients with short symptom duration, MRI can be preferred in the first line imaging. MRI of the spine is not generally recommended to diagnose axial SpA. Imaging modalities, other than conventional radiography and MRI are generally not recommended in the diagnosis of axial SpA. Monitoring disease activity MRI with STIR sequences are sufficient to detect inflammation. Conventional radiography of the SI joints and/or spine are used to monitor structural changes.
Abbreviations
| ADC | apparent diffusion coefficient |
| AP | anteroposterior |
| AS | ankylosing spondylitis |
| ASAS | assessment of spondyloArthritis international society |
| axSpA | axial spondyloarthritis |
| BASRI | bath ankylosing spondylitis radiology index |
| BME | bone marrow edema |
| CR | conventional radiography |
| CT | computed tomography |
| DISH | diffuse idiopathic skeletal hyperostosis |
| DWI | diffusion-weighted MRI |
| EULAR | European league against rheumatism |
| MRI | magnetic resonance imaging |
| mSASSS | modified stoke ankylosing spondylitis spinal score |
| nr-axSpA | nonradiographic axial spondyloarthritis |
| SIJ | sacroiliac joint |
| SPARCC | spondyloarthritis research consortium of Canada |
| SPECT | single-photon emission computerized tomography |
| STIR | short tau inversion recovery |
| T1-W | T1 weighted |
| T2 FS | T2 fat saturated |
| US | ultrasonography |
| wbMRI | whole-body MRI |
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