Israel Medical Association Journal

Background: Ultra-short-echo magnetic resonance imaging (MRI) sequences improve visualization of bone cortex and enable the generation of an MRI-based computed tomography (CT)-like (CT-l) images.

Objectives: To compare the agreement between CT-l images derived from merged fast-field echo (MFFE) sequence with conventional CT (cCT) for detecting sacroiliitis-associated structural lesions.

Methods: Consecutive MRI and conventional CT examinations (maximum one-year interval) of the sacroiliac joints (SIJ) of patients with suspected sacroiliitis, performed between 2022 and 2023 were retrospectively evaluated by a musculoskeletal radiologist and a third-year resident who evaluated half of the study’s images for the presence of erosions, sclerosis, and ankylosis on semicoronal CT-l and cCT-SIJ images. Cohen’s kappa was used to compare the results between the two modalities. The diagnostic performance of the CT-l was analyzed using cCT as the reference standard. Inter-reader reliability was assessed using intraclass correlation coefficients (ICCs) between the two readers.

Results: Sacroiliitis was detected in 11 patients (21%). Erosions, sclerosis, and ankylosis were detected in 44%/31%, 48%/42%, 4%/4% on CT-l/cCT, respectively. Statistical analysis regarding ankylosis was not feasible due to its low prevalence. Cohen's kappa agreement between modalities was substantial for erosions (0.68) and almost perfect for sclerosis (0.92). Sensitivity and specificity of CT-l were 0.90 and 0.69 for erosions and 0.72 and 0.91 for sclerosis, respectively. The ICC on CT-l/cCT for detecting erosions was 0.72/0.68 and 0.84/0.91 for sclerosis.

Conclusions: CT-l MRI sequence is a promising, radiation-free alternative to conventional CT for detecting structural lesions in SIJs, with high diagnostic performance and inter-reader agreement.

Background: Magnetic resonance imaging (MRI) is the most sensitive imaging modality for the detection of sacroiliitis. Diagnosing sacroiliitis on MRI is not always straightforward and can be challenging in some cases.

Objectives: To evaluate the prevalence of alternative diagnoses suggested by MRI and characterize the MR appearance of the most common ones.

Methods: Consecutive MRI examinations of the sacroiliac joints (SIJ) performed between 2005 and 2012 were retrospectively evaluated for the presence of structural and active sacroiliitis findings according to the Assessment of SpondyloArthritis International Society guidelines. Alternative diagnoses, including degenerative changes, diffuse idiopathic skeletal hyperostosis (DISH), Osteitis condensans ilii (OCI), septic sacroiliitis/discitis, stress reaction as well as anatomic variants, were registered

Results: We evaluated 281 MRI examinations, 116 males, 165 females, average age 44 ± 15 years. Sacroiliitis was found in 71 examinations (25%) and alternative diagnoses were suggested in 87 (31%) (OCI 8.9%, anatomic variants 5.3%, septic sacroiliitis 5.3%, degenerative findings 4.3%, diffuse idiopathic skeletal hyperostosis [DISH] 1.5%, stress reaction 0.7%, tumor 0.3%). A normal examination was found in the remaining 123 examinations. Patients with alternative diagnoses were older than those with sacroiliitis (62 vs. 47 years of age, respectively, P > 0.05). Alternative pathologies in the SIJ were significantly more common in females (66) than males (21), P < 0.05.

Conclusions: A substantial proportion of patients with suspected sacroiliitis had normal SIJ while the rest were more commonly diagnosed with other pathologies. A referral by an experienced rheumatologist may improve the sensitivity and specificity of this important examination.