Axial Spondyloarthritis: Mimics and Pitfalls of Imaging Assessment

Abstract

Axial spondyloarthritis (axSpA) is a chronic inflammatory disorder that predominantly involves the axial skeleton. Imaging findings of axSpA can be divided into active changes, which include bone marrow edema, synovitis, enthesitis, capsulitis, and intra-articular effusion, and structural changes, which include erosions, sclerosis, bone fatty infiltration, fat deposition in an erosion cavity, and bone bridging or ankylosis. The ability to distinguish between imaging lesions suggestive of axSpA and artifacts or lesions suggestive of other disorders is critical for the accurate diagnosis of axSpA. Diagnosis may be challenging, particularly in early-stage disease and magnetic resonance imaging (MRI) plays a key role in the detection of subtle or inflammatory changes. MRI also allows the detection of structural changes in the subchondral bone marrow that are not visible on conventional radiography and is of prognostic and monitoring value. However, bone structural changes are more accurately depicted using computed tomography. Conventional radiography, on the other hand, has limitations, but it is easily accessible and may provide insight on gross changes as well as rule out other pathological features of the axial skeleton. This review outlines the imaging evaluation of axSpA with a focus on imaging mimics and potential pitfalls when assessing the axial skeleton.

Keywords: axial spondyloarthritis; computed tomography; differential diagnosis; magnetic resonance imaging; mimic; normal variant; pitfall; radiography.

Figure 1

Imaging of the sacroiliac joint—Topographic distribution of main anatomical variants and pathological conditions that mimic axSpA, separated by quadrants of each articular surface (A) and orthogonal planes (B), namely coronal oblique (right upper image) and axial oblique (right lower image).

Figure 2

T1WI (A) and STIR image (B) of a military subject showing a small, peri-articular, area of bone edema (arrow) on the iliac side of the right sacroiliac joint. T1WI (C,E) and STIR (D,F) images of a post-partum female with bilateral foci of bone edema (arrows) adjacent to the sacroiliac joint.

Figure 3

Normal variants and incidental findings of the sacroiliac joint (arrows). CT reconstructions with oblique orientation depict bilateral iliosacral complexes (A), the most common sacroiliac joint variant; right accessory sacroiliac joint (B); bilateral bipartite iliac bony plate (C); left iliac bone pneumatocyst (D). A patient with an incidental finding on the right sacroiliac joint seen on pelvic radiography performed MRI, which revealed an iliac bone cleft filled with fluid (E,F). Note the sclerosis of the symphysis pubis (E), compatible with osteitis pubis.

Figure 4

Bilateral transitional vertebra (sacralization of L5), with neo-articulation of both hypertrophic transverse apophyses with the sacrum (arrows).

Figure 5

CT axial slice showing degenerative changes of the sacroiliac joint (A), with marginal osteophytes and bone sclerosis. Modic endplate changes at the weight bearing surfaces of the distal lumbar spine (arrows), seen on coronal T1 (B) and sagittal fluid-sensitive (C) sequences. Bone marrow signal changes are high on T1WI and fat-saturated T2WI, compatible with Modic type 2.

Figure 6

Fifty seven-year-old female patient with bilateral osteitis condensans ilii evident on pelvis radiography (A) and CT (B). Another patient, with post-partum bilateral bone marrow edema of the sacroiliac joint and sclerotic changes compatible with osteitis condensans ilii (arrows, asterisks), shown on MRI sequences (C–E) and CT (F).

Figure 7

Lateral cervical radiography of a patient with cervical undulating anterior longitudinal ligament ossification (arrows), compatible with DISH (A). Another patient (B) with cervical DISH and ossification of the posterior longitudinal ligament; differential diagnosis with posterior syndesmophytes is not always straightforward, as seen in a CT sagittal reconstruction of a patient with ankylosing spondylitis [(C), arrow].

Figure 8

CT (left), T1WI (middle), and fat-saturated PD (right) MRI of a patient with sacral insufficiency fractures (arrows). T1WI shows diffuse slightly hypointense signal of the left sacral wing and a linear hypointensity compatible with a sacral fracture; corresponding fat-saturated PD image documents marked bone marrow edema.

Figure 9

Fat-saturated PD (A) and T1WI (B) coronal slices, fat-saturated PD (C) axial and CT axial slices (D) of a 12-year-old female patient with proven Streptococcus spp. osteomyelitis of the right sacrum (arrow). A lytic lesion is seen adjacent to the right sacroiliac joint. Lateral lumbar radiography (E), post-contrast fat-saturated T1WI (F) and TIRM (G) sagittal slices of a 26-year-old female patient with confirmed tuberculous spondylodiscitis of L3–L4 segment (arrow).

Figure 10

Fat-saturated T2WI sagittal MRI sequence (A) of the lumbar spine in a 16-year-old female patient with CRMO (arrows), mimicking corner inflammatory lesions. Fat-saturated PD axial slice (B) of the same patient depicting involvement of the right SIJ. Fat-saturated T2WI axial slice (C) of the neck shows involvement of the left mandibular ramus.

Figure 11

CT sagittal reconstruction of the dorsal and lumbar spine (A,B) of a patient with renal osteodystrophy depicts abnormal bone turnover and mineralization, with diffuse osteosclerosis and multiple areas of subperiosteal resorption. Lateral lumbar spine radiography (C) shows the characteristic “rugger jersey” spine, with alternating bands of increased and normal bone density of the vertebral bodies. Note a large brown tumor of the left iliac bone on CT (D).

Figure 12

Lumbosacral radiography (A), CT axial slice (B) and post-contrast fat-saturated T1WI (C) of a 65-year-old male patient with Paget disease of the sacrum and right iliac bone. Typical findings include an expanded bone with coarsened trabecular pattern and sclerotic changes that are more evident on conventional radiography. There is increased uptake after intravenous contrast injection (C).

Figure 13

CT axial slice (A) of an iliac bone enostosis mimicking peri-articular sclerosis; CT sagittal reconstruction (B) of the dorsal and lumbar spine in a patient with diffuse osteoblastic metastasis due to prostate cancer; lateral lumbar radiograph of the same patient (C); T1WI axial slice (D); and post-contrast fat-saturated T1WI coronal slice (E) of a patient with leukemic infiltration of the sacrum and iliac bones, showing diffuse bone marrow T1 hypointensity due to tumoral infiltration and multifocal patchy uptake, respectively; fat-saturated PD (F), post-contrast fat-saturated T1WI (G) MRI and CT axial slice (H) of a 18-year-old male patient with Ewing sarcoma; fat-saturated PD (I) MRI sequence of an aneurysmatic bone cyst of the left iliac bone.

Figure 14

Fat-saturated PD (A) MRI sequence, CT axial slice (B), and sagittal lumbar spine reconstruction (C) of a 31-year-old male patient with Sickle cell disease and extensive bone marrow changes causing widening of the medullary spaces and thinning of cortical bone.