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Ankylosing Spondylitis

Imaging

Muhammad Asim Kahn, MD, FRCP, MACP; Nurullah Akkoç, MD 
Reviewed on July 15, 2024
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Introduction

Imaging is almost a necessary element in the diagnostic process of ankylosing spondylitis/ axial spondyloarthritis (AS/axSpA) because many of its clinical features lack specificity, and radiographic evidence of sacroiliitis is a key component of the classification criteria. The sacroiliitis is usually bilateral and symmetrical in patients with primary AS (unassociated with psoriasis or inflammatory bowel disease (IBD)) and typically first involves the lower ventral (synovial) part of the sacroiliac joints (SIJs). Radiographically, it is graded on a scale ranging from 0 (no abnormalities) to 4 (complete fusion or ankylosis of the SIJ) (Table 10-1). But the SIJ has several unique anatomic features with variations in the general population that make it difficult to profile it well on the conventional pelvic anteroposterior (AP) radiograph, which is the first step to look for sacroiliitis (Figures 10-1 to 10-3).

It is not surprising that performance of radiologists and…

Introduction

Imaging is almost a necessary element in the diagnostic process of ankylosing spondylitis/ axial spondyloarthritis (AS/axSpA) because many of its clinical features lack specificity, and radiographic evidence of sacroiliitis is a key component of the classification criteria. The sacroiliitis is usually bilateral and symmetrical in patients with primary AS (unassociated with psoriasis or inflammatory bowel disease (IBD)) and typically first involves the lower ventral (synovial) part of the sacroiliac joints (SIJs). Radiographically, it is graded on a scale ranging from 0 (no abnormalities) to 4 (complete fusion or ankylosis of the SIJ) (Table 10-1). But the SIJ has several unique anatomic features with variations in the general population that make it difficult to profile it well on the conventional pelvic anteroposterior (AP) radiograph, which is the first step to look for sacroiliitis (Figures 10-1 to 10-3).

It is not surprising that performance of radiologists and rheumatologists in detecting sacroiliitis on pelvic X-ray has shown only moderate sensitivity (84.3%/79.8%) and specificity (70.6%/74.7%) and it could not be improved by workshops and by individual training. Oblique views of the SIJs are not advised due to excessive gonadal radiation, whereas posteroanterior (PA) and properly performed Ferguson (angled) views (Figure 10-2) can be useful, although usually not needed.

There are several disorders can cause sacroiliitis-like changes of the SIJs, such as diffuse idiopathic skeletal hyperostosis (DISH) (Table 10-2), osteoarthritis, hyperparathyroidism and many others detailed on the website https://cases.asas-group.org/. For example, the SIJs can become painful during pregnancy or postpartum as they widen and develop increased mechanical stress during pregnancy and childbirth. Therefore, pelvic radiographs of such women may show iliac sclerosis adjacent to the joint that is termed “osteitis condensans ilii,” and it should not be confused with sacroiliitis (Figure 10-4).

Conventional radiography only visualizes the late structural consequences of the inflammatory process, while the early inflammatory changes can be detected by magnetic resonance imaging (MRI), often several years before the appearance of sacroiliitis on radiography. Inclusion of MRI in the Assessment of Spondyloarthritis International Society (ASAS) classification criteria for axSpA has helped in early recognition of the disease. In patients with AS (not non-radiographic axial spondyloarthritis (nr-axSpA)), an initial conventional radiography of the spine (lumbo-thoracic and cervical) may be needed to detect syndesmophytes (which are predictive of development of further syndesmophytes) for long-term monitoring of structural damage and response to treatment over time. Erosions of the vertebral corners and squaring of vertebral bodies can also be seen, along with sclerosis of vertebral corners that led to the appearance of “shiny corners.” Follow-up radiographs, if needed, should only be performed after more than 2 years due to the associated radiation.

Syndesmophytes develop from the vertebral corners along superficial layers of annulus fibrosis, extend vertically toward the adjacent vertebral corner, and can ultimately bridge the adjacent vertebral corners, leading to ankylosis (“bamboo spine”) that can ultimately include the cervical spine (Figure 10-5). Complete fusion of the SIJs and the whole vertebral column in a patient with severe, long-standing and very advanced AS are shown in Figure 10-6.

If the diagnosis of AS/axSpA cannot be established based on clinical features and conventional radiography, but it is still clinically suspected, MRI of the SIJs is recommended. T1-weighted spin echo (T1SE) and T2-weighted short tau inversion–recovery (STIR) MRI sequence of the SIJs are showing bilateral sacroiliitis in Figure 10-7. Please note that cortical bone has dark appearance on both T1 and STIR images. The T1-images are better for evaluating chronic structural changes, while the STIR sequences suppress fat signals and bring out signals from free water present in bone marrow edema (BME) in active axSpA. In daily practice, the combination of STIR and T1-weighted sequences is usually sufficient, and they are complementary; therefore, concurrent viewing by scrolling simultaneously through synchronized images of both of these sequences is needed. One should consider both active inflammatory lesions (primarily BME) and structural lesions (such as bone erosion, new bone formation, joint fusion, bone sclerosis and fat infiltration) for evaluating MRI findings (Table 10-3). A normal MRI result does not completely exclude axSpA. In certain cases, such as very young patients with suspected axSpA and those with short symptom duration or those with juvenile SpA, MRI of the SIJs is an alternative imaging method rather than conventional pelvic radiograph.

The MRI findings that indicate active ongoing inflammation in the SIJs (sacroiliitis) include BME, enthesitis, capsulitis and synovitis. Enthesitis at the interosseous ligament insertions in the retro-articular space of the SIJs may also occur. The presence of structural damage is indicated by erosions (full-thickness loss of dark appearance of the cortical bone and change in normal bright appearance of adjacent bone marrow on T1-weighted images), sclerosis, entheseal/periarticular fatty tissue deposition (focal increased signal in bone marrow on T1-weighted images) and fusion (bright signal on T1-weighted images extending across the SIJs). ASAS has recently updated the definition of active SpA-related sacroiliitis on MRI as a part of the classification and not the diagnostic approach.

According to this definition, MRI of SIJ is positive if there is BME clearly present in a typical anatomical area (subchondral bone) and the appearance of BME is highly suggestive of SpA. Other active inflammatory changes such as capsulitis, enthesitis, joint space enhancement and joint space fluid may also be manifestations of SpA-related SIJ inflammation. Structural lesions observed on MRI images of the SIJs in axSpA were also defined by the ASAS-MRI working group. A recent ASAS initiative proposes data-driven lesion cut-offs on SIJ MRI considered highly suggestive of axSpA if the BME is present in ≥4 SIJ quadrants or ≥3 consecutive SIJ slices, erosion on ≥3 SIJ quadrants, or fat lesions in ≥5 SIJ quadrants. The sensitivity and specificity of T1-weighted MRI as compared to computed tomography (CT) as gold standard in detecting erosions in the SIJ is 63.4% and 88%, respectively. Using the MRI volumetric interpolated breath-hold examination technique to produce thinner image slices helps increase the sensitivity (71%), with no loss in specificity (87%).

The typical MRI findings in the spine that indicate active disease are enthesitis at the corners of vertebral bodies, spondylodiscitis and arthritis/synovitis of the facet, costovertebral and costotransverse joints (Figure 10-8). Chronic spinal changes include bone erosions, focal fat infiltration following resolution of active vertebral corner inflammation, syndesmophyte formation (bright signal on T1-weighted images extending from a vertebral angle towards the adjacent vertebral angle) and ankylosis (bright signal on T1-weighted images extending across a vertebral endplate towards the adjacent one). Inflammation and fatty lesions have predictive value for subsequent development of syndesmophytes at that level.

Combination of MRI of the spine and SIJs adds little incremental value compared with SIJ MRI alone for recognizing patients with early axSpA. Therefore, MRI of the spine is not generally recommended to diagnose axSpA, but in rare instances MRI may detect spinal inflammation in some patients with nr-axSpA without SIJ inflammation. The increasing use of MRI in SpA has, however, revealed several limitations that have been very well reviewed by Weber and colleagues.

Although the typical MRI changes in the SIJs and the spine due to axSpA have been well described, there are many problems associated with these definitions because similar lesions may occur in individuals who do not have SpA. For example, in a Danish cohort study, 21% of 1,020 consecutive young subjects with chronic back pain showed bone marrow edema (BME) of their SIJs that met the ASAS definition of positive MRI for AS/axSpA. In a retrospective controlled cohort study of patients with juvenile SpA, a 20% prevalence of non-specific BME of the SIJs was observed in healthy control subjects with a mean age of 15.1 years. The reasons for this lack of specificity of the isolated BME, particularly when it is subtle in appearance, are poorly understood. The extent and distribution of MRI findings resembling those seen in the SIJ of axSpA patients can be commonly seen in athletes (25%), and women in post-partum period with or without back pain (>62%). These findings suggest that SIJ BME lesions can be induced by mechanical strain, and this can create a diagnostic challenge. Therefore, in women with back pain in the postpartum period a waiting period of at least 6 months has been advised before performing an SIJ MRI.

It is of utmost importance that MRI should be only ordered and interpreted while taking into consideration the clinical evaluation and pretest probability of the diagnosis of AS/axSpA, which remains a process of pattern recognition. Incidentally, there is no need to stop the use of nonsteroidal anti-inflammatory drugs (NSAIDs) prior to getting the MRI. Moreover, gadolinium enhancement is not needed, except when infection is suspected, or in a rare patients with juvenile SpA when a high STIR signal limited to the SIJ is a result of synovitis. When the diagnosis has been established, MRI may be needed to further evaluate disease activity and treatment response. For such purpose, in general, STIR sequences are sufficient to detect inflammation and the use of contrast medium is not needed.

Involvement of all widespread sites of inflammation can be visualized by the promising method of whole body MRI, without additional inconvenience for the patients. It can very nicely identify the presence of enthesitis/osteitis at axial as well as peripheral sites, such as the anterior chest wall, and the manubriosternal, sternoclavicular and acromioclavicular junctions (Figure 10-9). Presence of extensive MRI inflammatory activity (i.e., BME), particularly in the spine, in addition to clinical findings and elevated level of C-reactive protein (CRP) might aid in the decision to initiate anti-TNF therapy as they seem to be a predictor of good therapeutic response.

Novel MRI techniques have been developed for the better assessment of structural lesions in the sacroiliac joints, such as volumetric interpolated breath-hold examination (VIBE), susceptibility-weighted imaging (SWI), and AI-generated synthetic CT-like images which offer comparable accuracy to CT scans.

Imaging modalities other than conventional radiography and MRI are not generally recommended, but CT may provide additional information on structural damage if conventional radiography is negative and MRI is unavailable or cannot be performed. However, its clinical utility is limited due to excessive radiation, lack of ability to detect presence of active inflammation, provide information regarding the soft tissues and its lower sensitivity than MRI for detecting axSpA. Attempts are underway to develop CT of the SIJs that is associated with much less radiation that can be consistently performed with less than 1 mSv effective dose, placing it in the same minimal risk category (0.1–1 mSv) as the SIJ radiography. A significant proportion of nr-axSpA cases show evidence of radiographic change when examined with low dose CT. Moreover, a recent study showed that low dose CT is more sensitive than conventional radiography in detecting bone proliferation in the spine of patients with AS, but the radiation dosing of 4 mSv might still be too high.

When spinal fracture in AS/axSpA is suspected, conventional radiography is the recommended initial imaging method. If conventional radiography is negative, CT should be performed. MRI is an additional imaging method to CT, which can also provide information on soft tissue lesions (Figure 7-3). It is important to prevent instability of the suspected spinal fracture site during all the imaging procedures in such patients.

Dual-energy computed tomography (DECT), which is a new technique using dual-source spiral CT scanners with the same radiation dose as standard CT is emerging as an alternative imaging technique for detecting BME in SpA patients, with a very good sensitivity and specificity. Bone scintigraphy of the SIJs is neither sensitive nor specific.

Ultrasonography is not recommended for detection of sacroiliitis, but when peripheral SpA is suspected, ultrasonography or MRI may be used to detect peripheral synovitis/enthesitis. Furthermore, ultrasonography or MRI might be used to detect tenosynovitis, dactylitis and bursitis, and to monitor disease activity (particularly synovitis and enthesitis) in peripheral SpA, providing additional information on top of clinical and biochemical assessments. Ultrasonography with high-frequency color power Doppler is sufficient to detect inflammation, and the use of ultrasonography contrast medium is not needed (Figure 10-10).

In patients with AS/axSpA without syndesmophytes in the lumbar spine on conventional radiography, osteoporosis should be assessed by DEXA scans of hip and spine AP views. When syndesmophytes are present, osteoporosis should be assessed by hip DEXA, supplemented by either spine DEXA in lateral projection or possibly quantitative CT of the spine.

Lastly, readers are strongly recommended to access the ASAS website (https://cases.asas-group.org/) to learn how to read and evaluate imaging findings in the context of clinical features and laboratory test results.

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Enlarge  Figure 10-1A: Right Sacroiliac Joint of a Patient With Axial Spondyloarthritis. Suspicious changes in the right sacroiliac joint of a patient with axial spondyloarthritis; subsequent MRI showed evidence of inflammation in that joint.
Figure 10-1A: Right Sacroiliac Joint of a Patient With Axial Spondyloarthritis. Suspicious changes in the right sacroiliac joint of a patient with axial spondyloarthritis; subsequent MRI showed evidence of inflammation in that joint.
Enlarge  Figure 10-1B: Sacroiliitis. Schematic drawing showing the location of the sacroiliac joint; the red line depicts sacroiliitis. Source: Khan MA. Spondyloarthropathies. In: Hunder GG, ed. Atlas of Rheumatology. 4th ed. Philadelphia, PA: Current Medicine; 2005:151-180.
Figure 10-1B: Sacroiliitis. Schematic drawing showing the location of the sacroiliac joint; the red line depicts sacroiliitis. Source: Khan MA. Spondyloarthropathies. In: Hunder GG, ed. Atlas of Rheumatology. 4th ed. Philadelphia, PA: Current Medicine; 2005:151-180.
Enlarge  Figure 10-2: Anteroposterior Roentgenographic View of Pelvis Showing Bilateral Sacroiliitis in AS. Erosions and blurring of the subchondral bone plate and reactive bone sclerosis are more prominent on the iliac side of the joint (Ferguson view). Source: Khan MA. Spondyloarthropathies. In: Hunder GG, ed. Atlas of Rheumatology. 4th ed. Philadelphia, PA: Current Medicine; 2005:151-180.
Figure 10-2: Anteroposterior Roentgenographic View of Pelvis Showing Bilateral Sacroiliitis in AS. Erosions and blurring of the subchondral bone plate and reactive bone sclerosis are more prominent on the iliac side of the joint (Ferguson view). Source: Khan MA. Spondyloarthropathies. In: Hunder GG, ed. Atlas of Rheumatology. 4th ed. Philadelphia, PA: Current Medicine; 2005:151-180.
Enlarge  Figure 10-3: Partial Ankylosis of the Right Sacroiliac Joint of a Patient With Spondyloarthritis and Recurrent Episodes of Acute Anterior Uveitis. Source: Provided with permission Courtesy of Dr. Mazen Elyan.
Figure 10-3: Partial Ankylosis of the Right Sacroiliac Joint of a Patient With Spondyloarthritis and Recurrent Episodes of Acute Anterior Uveitis. Source: Provided with permission Courtesy of Dr. Mazen Elyan.
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Enlarge  Figure 10-4: Pelvic Radiograph Showing Osteitis Condensans Ilii. Pelvic radiograph shows osteitis condensans ilii (red arrowheads) on 32-year-old, HLA-B27–negative woman with persisting inflammatory back pain starting in the second trimester of her first pregnancy. Source: Weber U, et al. Curr Rheumatol Rep. 2016;18(9):58.
Figure 10-4: Pelvic Radiograph Showing Osteitis Condensans Ilii. Pelvic radiograph shows osteitis condensans ilii (red arrowheads) on 32-year-old, HLA-B27–negative woman with persisting inflammatory back pain starting in the second trimester of her first pregnancy. Source: Weber U, et al. Curr Rheumatol Rep. 2016;18(9):58.
Enlarge  Figure 10-5: Actual Specimen of the Pelvis of a Patient With Advanced Long-Standing AS. There is complete ankylosis of both sacroiliac joints and the fourth and fifth lumbar vertebrae. Moreover, there is almost complete fusion of his public symphysis. Source: Permission provided courtesy of Dr. Heinz Baumberger and the “Schweizerische Vereinigung Morbus Bechterew” (SVMB) or the Swiss Ankylosing Spondylitis Society in English.
Figure 10-5: Actual Specimen of the Pelvis of a Patient With Advanced Long-Standing AS. There is complete ankylosis of both sacroiliac joints and the fourth and fifth lumbar vertebrae. Moreover, there is almost complete fusion of his public symphysis. Source: Permission provided courtesy of Dr. Heinz Baumberger and the “Schweizerische Vereinigung Morbus Bechterew” (SVMB) or the Swiss Ankylosing Spondylitis Society in English.
Enlarge  Figure 10-6: Actual Specimen of the Axial Skeleton of the Same Patient Whose Pelvis is Shown in Figure 10-5. There is marked kyphosis with complete fusion (‘bamboo spine’) of the whole spine (including the facet joints and even the interspinous ligaments) but sparing of the intervertebral fossa. Red line in the cervical spine marks the site of the neck fracture that lead to his death. Source: Permission provided courtesy of Dr. Heinz Baumberger and the “Schweizerische Vereinigung Morbus Bechterew” (SVMB) or the Swiss Ankylosing Spondylitis Society in English.
Figure 10-6: Actual Specimen of the Axial Skeleton of the Same Patient Whose Pelvis is Shown in Figure 10-5. There is marked kyphosis with complete fusion (‘bamboo spine’) of the whole spine (including the facet joints and even the interspinous ligaments) but sparing of the intervertebral fossa. Red line in the cervical spine marks the site of the neck fracture that lead to his death. Source: Permission provided courtesy of Dr. Heinz Baumberger and the “Schweizerische Vereinigung Morbus Bechterew” (SVMB) or the Swiss Ankylosing Spondylitis Society in English.
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Enlarge  Figure 10-7: Morphology of the Most Relevant Lesion Types on Sacroiliac Joint Coronal Oblique MRI in Early Axial Spondyloarthritis. 21-year-old, HLA-B27–positive male patient with inflammatory back pain for 3 years. The MRI shows extended BME (green arrows) predominantly on the sacral side of both SI joints on the STIR sequence (bottom). The T1 Spin Echo (T1SE) sequence (top) displays fat metaplasia (red arrows) mainly in the right SI joint within and outside areas with BME. Erosion (yellow arrowheads) in the right SI joint can be demonstrated already at this early disease stage. Source: Weber U, et al. Curr Rheumatol Rep. 2016;18(9):58.
Figure 10-7: Morphology of the Most Relevant Lesion Types on Sacroiliac Joint Coronal Oblique MRI in Early Axial Spondyloarthritis. 21-year-old, HLA-B27–positive male patient with inflammatory back pain for 3 years. The MRI shows extended BME (green arrows) predominantly on the sacral side of both SI joints on the STIR sequence (bottom). The T1 Spin Echo (T1SE) sequence (top) displays fat metaplasia (red arrows) mainly in the right SI joint within and outside areas with BME. Erosion (yellow arrowheads) in the right SI joint can be demonstrated already at this early disease stage. Source: Weber U, et al. Curr Rheumatol Rep. 2016;18(9):58.
Enlarge  Figure 10-8: Spinal MRI Can Identify SpA-Associated Inflammation. Three MRI images with STIR sequence applied of a 48-year-old, HLA-B27–negative male patient with AS who presented with interscapular pain on deep breathing and rotation of the trunk. (A) A central MRI slice, showing the central spinal canal, without inflammation. (B) A lateral MRI slice through the pedicles displays BME in the fifth and sixth thoracic vertebra, representing costovertebral joint inflammation (red arrows). In addition, BME is visible in pedicles (yellow arrowheads). (Cw) A far lateral MRI slice shows BME both in the rib (red arrow) and transverse process (yellow arrow), corresponding to costotransversal joint inflammation. See a normal rib (blue arrow) and transverse process (green arrow) for comparison. Inflammatory lesions in the lateral spinal compartment (costovertebral, costotransverse, and facet joint) are highly specific for SpA and can be captured by dedicated spinal MRI protocols for SpA that include lateral slices. Source: van Tubergen A, Weber U. Nat Rev Rheumatol. 2012;8:253-261.
Figure 10-8: Spinal MRI Can Identify SpA-Associated Inflammation. Three MRI images with STIR sequence applied of a 48-year-old, HLA-B27–negative male patient with AS who presented with interscapular pain on deep breathing and rotation of the trunk. (A) A central MRI slice, showing the central spinal canal, without inflammation. (B) A lateral MRI slice through the pedicles displays BME in the fifth and sixth thoracic vertebra, representing costovertebral joint inflammation (red arrows). In addition, BME is visible in pedicles (yellow arrowheads). (Cw) A far lateral MRI slice shows BME both in the rib (red arrow) and transverse process (yellow arrow), corresponding to costotransversal joint inflammation. See a normal rib (blue arrow) and transverse process (green arrow) for comparison. Inflammatory lesions in the lateral spinal compartment (costovertebral, costotransverse, and facet joint) are highly specific for SpA and can be captured by dedicated spinal MRI protocols for SpA that include lateral slices. Source: van Tubergen A, Weber U. Nat Rev Rheumatol. 2012;8:253-261.
Enlarge  Figure 10-9: The Whole-Body MRI (STIR technique) Coronal and Sagittal Sections in a Patient With Ankylosing Spondylitis. Curved red arrows indicate inflammation of the manubriosternal articulation and straight red arrows point to corner lesion BME that is better visualized on the magnified insert. Source: Weber U, et al. Intl J Adv Rheumatol. 2007;5:2-7.
Figure 10-9: The Whole-Body MRI (STIR technique) Coronal and Sagittal Sections in a Patient With Ankylosing Spondylitis. Curved red arrows indicate inflammation of the manubriosternal articulation and straight red arrows point to corner lesion BME that is better visualized on the magnified insert. Source: Weber U, et al. Intl J Adv Rheumatol. 2007;5:2-7.
Enlarge  Figure 7-3: MRI Lateral View Showing Spinal Fracture Between C7 and T1. Source: Khan MA. Clin Rheumatol. 2016;35(6):1637-1641.
Figure 7-3: MRI Lateral View Showing Spinal Fracture Between C7 and T1. Source: Khan MA. Clin Rheumatol. 2016;35(6):1637-1641.
Enlarge  Figure 10-10: Enthesitis in Juvenile SpA. Quadriceps tendon enthesitis in a 12-year-old boy. (A) Long, grayscale ultrasound image demonstrates focal hypoechogenicity within the distal aspect of the quadriceps tendon (arrow) which is confirmed on the transverse image (B) with mild focal tendon enlargement (arrows) Long (C) and transverse (D) Power Doppler images demonstrate abnormal, increased signal within the tendon (dashed arrows) consistent with hyperemia. A small amount of joint fluid (arrow) is seen in D. Source: Gmuca S, Weiss PW. Curr Opin Rheumatol. 2015;27(4):364-372.
Figure 10-10: Enthesitis in Juvenile SpA. Quadriceps tendon enthesitis in a 12-year-old boy. (A) Long, grayscale ultrasound image demonstrates focal hypoechogenicity within the distal aspect of the quadriceps tendon (arrow) which is confirmed on the transverse image (B) with mild focal tendon enlargement (arrows) Long (C) and transverse (D) Power Doppler images demonstrate abnormal, increased signal within the tendon (dashed arrows) consistent with hyperemia. A small amount of joint fluid (arrow) is seen in D. Source: Gmuca S, Weiss PW. Curr Opin Rheumatol. 2015;27(4):364-372.

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