MRI compared with low-dose CT scanning in the diagnosis of axial spondyloarthritis

Objectives To compare the performance of conventional radiography, ldCT, and MRI in the diagnosis of sacroiliitis in suspected axial spondyloarthritis (axSpA). Methods Patients presenting with > 3 months chronic back pain were assessed by axSpA-experienced rheumatologists and diagnosed as axSpA or not; axSpA patients were then considered nr-axSpA or AS using plain radiography. Non-axSpA patients were recruited as controls, and divided into non-inflammatory and inflammatory groups on the basis of inflammatory back pain and/or CRP/ESR elevation. Clinical variables, pelvic radiography, sacroiliac joint (SIJ) ldCT, and SIJ MRI were obtained. Results A total of 121 patients were included and had SIJ radiography and ldCT, of whom 71 additionally had an SIJ MRI. These included 23 non-inflammatory controls, 21 inflammatory controls, 32 nr-axSpA cases, and 45 AS cases. Fourteen of 32 (44%) nr-axSpA patients had positive ldCT scans, 21/24 (88%) had MRI-BMO, and 11/24 (46%) had MRI-structural lesions. ldCT had high specificity with only 1/23 (4%) non-inflammatory controls being positive. MRI-BMO had the highest sensitivity for nr-axSpA, but compared with ldCT lower specificity, with 5/15 (33%) of non-inflammatory controls being positive, and similar sensitivity for AS (20/22 (91%) vs 44/44 for ldCT). Conclusions ldCT identifies evidence of radiographic change in a significant proportion of nr-axSpA cases and is highly specific for axSpA. MRI-BMO lesions are more sensitive than either conventional radiography or MRI-structural assessment for axSpA. The relative position of these imaging modalities in screening for axSpA needs to be reconsidered, also taking into account the costs involved. Key Points • ldCT is more sensitive for erosions or sclerosis in axSpA than plain radiography, with 44% of patients with nr-axSpA having evidence of AS-related sacroiliac joint changes on ldCT. • MRI-structural lesions are no more sensitive but are less specific for AS than ldCT. • MRI-BMO is the most sensitive test for nr-axSpA of the modalities tested but is less specific for axSpA than for ldCT.