In this poster: i) we illustrate MRI typical features of axSpA in the SIJs,
with emphasis on the ancillary findings supporting the diagnosis; ii) we review the spectrum of main pathologies and mimickers of SIJ BMO; iii) we analyze the role for MRI within a multidisciplinary approach to axSpA.
i) SIJ MRI typical features of axSpA and ancillary findings
The presence of periarticular/subchondral BMO in the SIJs represents the main feature of active sacroiliitis.
According to ASAS criteria,
sacroiliitis is defined as BMO that is present on at least two consecutive slices if only one lesion is seen,
or at least two lesions on one slice should be present [5,6].
SIJ MRI provides radiologists with many other findings that can help make a more confident diagnosis of sacroiliitis.
Inflammatory changes in the SIJs can be divided into active and chronic lesions (Table 2).
EARLY FEATURES OF ACTIVE INFLAMMATION
- BONE MARROW OEDEMA - BMO presents as subchondral/periarticular areas of high signal intensity on STIR and of low signal intensity on T1-weighted images (Fig. 1).
Fig. 1: Bilateral sacroiliitis in a 17-years-old male with spondyloarthritis and a positive human leukocyte antigen (HLA) B27 test. Semicoronal T1-weighted (a) and STIR (b) MR images show BMO in the SIJ (arrows), more conspicuous in the sacral region on both side.
- ENTHESITIS - Enthesitis is depicted as a high signal intensity area at the junctional sites between bone and tendons,
fascia,
ligaments,
or capsules on STIR images and fat-suppressed contrast-enhanced T1-weighted images (Fig. 2).
Ligaments surrounded by vessels and some coil artifacts may mimic enthesitis.
Fig. 2: Pelvic enthesitis (arrows) in a 24-years-old female with IBD-associated spondyloarthritis. Semicoronal STIR image (a) shows high signal intensity in the context of right gluteus maximum muscle close to its origin on the surface of the sacrum, confirmed by intense enhancement after gadolinium administration on fat-suppressed contrast-enhanced T1-weighted MR image (b).
- SYNOVITIS AND CAPSULITIS - Synovitis (Fig. 3) and capsulitis are depicted as high signal intensity areas in the synovial or anterior and posterior capsules of the sacroiliac joints on fat-suppressed contrast-enhanced T1-weighted images and on STIR images.
Gadolinium administration can differentiate between synovitis and joint fluid.
Fig. 3: Synovitis on the iliac side of the left SIJ (arrows) in a 24-years-old female with IBD-associated spondyloarthritis. Semicoronal STIR image (a) shows high signal intensity in the left SIJ, confirmed by intense contrast enhancement after gadolinium administration on fat-suppressed T1-weighted MR image (b).
CHRONIC INFLAMMATORY CHANGES
- FATTY TRANSFORMATION (Fig. 4) of the bone marrow is depicted as increased signal intensity in periarticular bone marrow on T1-weighted images.
This finding is non-specific; it often indicates areas of previous inflammation in patients with SpA,
but it may also be observed in healthy individuals,
distributed in a heterogeneous manner in the central sacrum.
- SCLEROSIS (Fig. 4) on both iliac and/or sacral articular surfaces present as areas of low signal intensity on STIR and T1-weighted images and are not enhancing on fat-suppressed contrast-enhanced T1-weighted images.
Sclerosis typically extends at least 5 mm from the SIJ surface [7],
and it is localized in subchondral/periarticular areas.
- EROSIONS (Fig. 4) are bony defects at the joint surface that appear as areas of low signal intensity on T1-weighted images and may occur throughout the cartilaginous compartment of the joint.
Erosions appear initially as single lesions and may subsequently become confluent,
causing a false appearance of SIJ enlargement.
- ANKYLOSIS is the fusion of bone surfaces to form BONY BRIDGES across a joint and usually and it shows low signal intensity in all MRI sequences.
Fig. 4: A mild degree of articular surface erosions (arrows), associated with subtle aspects of sclerosis (asterisks), and fatty degeneration of bone marrow (arrowheads) on the sacral side of both SIJ, in a 48-years-old male with psoriasis and low back pain. T1-weighted MR images show bilateral bony defects with low signal (arrows) at the sacral side of the SIJ surface surrounded by areas of fatty degeneration that presents as high signal intensity on T1-weighted images (arrowheads).
ii) The spectrum of main pathologies and mimicker of SIJ BMO
- INFECTIOUS ARTHRITIS - Infectious arthritis is depicted as an area of intense BMO in the SIJs,
associated with excessive fluid and synovitis. Involvement of the surrounding soft tissue,
with oedema and abscess formation in the iliac muscle,
can help differentiate between infectious sacroiliitis and inflammatory sacroiliitis (Fig. 5). Structural damage can be detected after the acute phase of infection [8].
Infectious arthritis is usually unilateral whereas inflammatory sacroiliitis is often bilateral.
Fig. 5: Unilateral infectious sacroiliitis in a 44-years-old female, with a history of i.v. drug addiction. Axial T1-weighted (a), STIR (b) and fat-suppressed gadolinium-enhanced T1-weighted (c) MR images demonstrate BMO (arrowheads), intramuscular abscess, and inflammatory soft tissue changes (arrows) on the right side.
- STRESS FRACTURE - Stress fracture BMO can be distinguished from inflammatory sacroiliitis by looking at its location and shape.
A stress reaction is usually seen in the sacrum without the involvement of the joint surface.
A stress fracture includes a full cortical break which is depicted as a vertically oriented hypointense irregular line,
especially on T1-weighted images,
that is surrounded by BMO (Fig. 6).
The absence of excess synovial fluid or structural provides radiologists other important features to distinguish it from inflammatory sacroiliitis.
Stress reactions are more frequently seen in runners and patients with osteoporosis.
Fig. 6: Sacrum insufficiency fracture in a 58-old woman suffering from osteoporosis and low back pain. Pelvic x-ray shows no clear sign either of fracture or sacroiliitis (a). Further investigations provided a diagnosis of left sacrum fracture visible on CT (b) and MRI (c, d). Fracture appears on MRI as a hypointense vertical line on T1-weighted image (c) corresponding to an extensive area of high signal on STIR image (d).
- WOMEN WITH POSTPARTUM BACKPAIN and OSTEITIS CONDENSANS ILII (OCI) - Osteitis condensans ilii (OCI) is a condition related to remodeling of bone following stress across the SIJ.
It usually affects childbearing women,
who show BMO in the perinatal period (Fig. 7).
Patients with OCI show areas of triangular-shaped subchondral sclerosis localized anteriorly in the articular iliac side.
No signs of erosions or joint space widening should be detected [9].
Clinical context is fundamental to distinguish it from inflammatory sacroiliitis.
Fig. 7: Post-partum low back pain in a 26-years-old woman. Semicoronal T1-weighted (a) and STIR (b) images show BMO in the SIJ (arrows), more conspicuous on the left side of the sacrum.
- HEALTHY INDIVIDUALS - A proportion of healthy active individuals without any symptoms of back pain may display BMO on MRI [10,11]. In individuals performing a variety of sports activities,
the presence of BMO on MRI has been described for several joints [12,13]. The meaning of BMO in active individuals is under debate; it does not always have a pathological meaning and may be correlated to “wear and tear”.
The presence of BMO-like lesions in healthy and asymptomatic individuals underlines its low specificity in settings without a proper clinical evaluation.
SIJ MRI does not represent an effective method to screen individuals for inflammatory active sacroiliitis [14].
- OSTEOARTHRITIS - degenerative changes of the SIJ due to age include joint irregularity with minimal subchondral sclerosis,
subchondral cysts (Fig. 8),
interosseous space narrowing and osteophytosis.
Osteophytes usually involve the anterosuperior and/or anteroinferior limit of the articular cavity.
Age and clinical context are crucial for proper differential diagnosis.
Fig. 8: Low back pain in a 45-years-old male. Semicoronal STIR image (a) and T1-weighted image (b) show subchondral cysts and no evidence of BMO (arrows).
- NEOPLASTIC LESIONS - Metastases are the most common malignant tumors of the sacrum. Primary tumors of the sacrum are rare and represent only 5% of all bone neoplasms [15]. From an imaging-oriented point of view,
the presence of blood,
necrosis,
soft-tissue extension and spinal canal involvement may help differentiate between tumor and active sacroiliitis (Fig. 9).
Fig. 9: Low back pain in a 57-years-old man with multiple myeloma. Focal lesions in multiple myeloma may appear as area of low signal on T1-weighted (a) and high signal on STIR (b and c) close to the SIJ. The presence of other multiple lesions with the same signal aspects (arrowheads), spread across the pelvic bones allows prompt differentiation between neoplastic infiltration and inflammatory sacroiliitis.
- DISH - Diffuse Idiopathic Skeletal Hyperostosis (DISH) is a systemic disease in which diffuse calcification involves ligaments,
entheses,
and soft tissues [16]. Anterior bridging osteophytes at the area of the SIJ anterior capsule characterize diffuse idiopathic skeletal hyperostosis.
Bridging osteophytes can also occasionally be seen posterior to the capsule,
potentially leading to confusion with end-stage inflammatory sacroiliitis or degenerative changes in osteoarthritis.
- ANATOMICAL VARIANTS - Various anatomical variants can mimic sacroiliitis.
For example,
hemisacralization of the L5 vertebra to the sacrum can lead to degenerative abnormalities with BMO caused by the pseudarthrosis between the sacrum and the transverse process (Fig. 10).
Fig. 10: Various examples of hemisacralization of L5: complete bone fusion on the left side displayed on CT 3D reconstruction image (arrow in image a). Hemisacralization of L5 associated with scoliosis on X-ray (b). Pseudarthrosis between the sacrum and the right transverse process of L5, due to its hemisacralization (c and d).
- MRI ARTIFACTS - Wraparound artifacts,
coil artifacts,
and motion artifacts can complicate the analysis of the SIJs. Motion artifacts are only seen in the phase-encoding direction,
and changing the phase direction enables adequate image interpretation [8].
Enlarging the FOV and using pre-saturation bands on areas outside the FOV or using surface coils may help with wraparound artifacts.
iii) The role of MRI within a multidisciplinary approach to axSpA
SIJ MRI represents a key imaging biomarker of axSpA,
especially for identification of early signs of inflammation. The presence of BMO in the SIJ is essential to meet the definition of active sacroiliitis. A positive MRI for ax
SpA remains controversial due to limited sensitivity and specificity of lesions suspicious for BMO [17].
Thus,
MRI should only be requested in the appropriate clinical context.
In the setting of low-grade MRI changes,
a diagnosis should not be made based on bone marrow lesions alone,
as they are common in back pain conditions not related to SpA and even in healthy subjects.
Contextual evaluation of ancillary findings,
such as erosions,
may improve radiologist confidence in MRI assessment of the SIJs [18,19,20].
SIJ MRI is helpful to identify early imaging signs of axSpA,
to monitor disease activity and to assess structural changes (Table 3) [19].