This systematic and controlled evaluation of erosion and related features on SIJ MRI in patients with AS had two findings of clinical relevance. First, the reliability of detecting erosion on SIJ MRI was substantial and comparable to BME, provided that readers are trained to recognize abnormalities on T1SE MRI. Familiarity with the variable features of erosions on MRI may also improve reliability. These features include EE and BF. Second, erosions occurred statistically significantly more frequently in the ilium compared with the sacrum. This finding has to be taken into account when comparing various lesions detected on SIJ MRI in AS patients, because BME and FI showed a similar distribution across both joint surfaces.
A debriefing analysis of the reading exercise in the original study population [6
] retrospectively identified EE and BF as major sources of inter-observer disagreement regarding erosion. It is possible that these appearances represent stages in the evolution of an erosive lesion because 15 (78.9%) of 19 AS patients showing BF also showed EE. We, therefore, consider it appropriate to incorporate these features into a comprehensive definition of SIJ erosion. We interpret the signal alteration of BF on T1SE sequences as metaplastic tissue which refills the excavated subchondral bone. For obvious ethical reasons, histopathologic evidence in support of this assumption is not available. Serial MRI assessments of the SIJ over several years may help determine if BF represents an intermediate stage between SIJ erosion and ankylosis. In contrast to rheumatoid arthritis, where regression of erosion has been reported only rarely [19
], erosion in many SpA patients may be followed by refilling of the excavated bone and eventually ankylosis.
Two previous reports focused on agreement data for structural MRI lesions of the SIJ [9
]. In a cohort of 68 patients with inflammatory back pain according to Calin criteria, the concordance rate for structural lesions defined as a composite index of ankylosis, sclerosis and erosions was 81% and 88%, and kappa values were 0.37 and 0.66, respectively [9
]. However, differences in study design preclude a direct comparison with our reliability data. Unlike our study cohort, the study population consisted of patients with inflammatory back pain and only 14 patients (20.6%) met the radiographic modified New York criteria, structural MRI lesions were less frequent (16%), assessment was based on different definitions of MRI lesions, lesions were recorded for the entire right and left SIJ rather than according to joint surface, and there was no control group. Another study which evaluated SIJ MRI in 41 inflammatory back patients meeting the European Spondylarthropathy Study Group (ESSG) criteria [21
], reported an interreader agreement (percentage agreement/kappa value) between two senior radiologists of 77%/0.54 for erosion [10
]. The lesion analysis according to eight SIJ quadrants noted a predominance of erosions in the iliac joint portion, but no formal comparison of lesion frequency between the two joint surfaces was performed. Again, there were major differences in study design compared to our work regarding study population, MRI lesion definitions and imaging technique, and lack of a control group.
Our study was conducted with T1SE and STIR sequences representing the routine protocol used for MRI evaluation of SpA patients. Additional so-called 'cartilage MRI sequences', such as T1-weighted fat saturated (T1FS) or T2-weighted gradient echo (T2GE) sequences, may offer advantages to recognize erosion. However, they require further evaluation as to their reliability for detection of erosion compared to T1SE sequence alone and their implementation into routine MRI scanning protocols with regard to examination time and costs [22
]. A study using both T1SE and T1FS sequences to detect SIJ erosions in 37 SpA patients meeting the ESSG criteria reported a good inter-observer agreement by kappa statistics between two trained radiologists (0.76 for erosion at joint level and 0.80 at patient level) [10
]. Erosion was defined as loss of marrow signal on T1SE and T1FS images together with a defect in the overlaying cortical bone; erosions were scored regarding their extent on the joint surface and the presence of ankylosis was added to the erosion score. However, this study did not directly compare T1SE and T1FS sequences with regard to reliability for detection of SIJ erosions, a separate analysis without the contribution of ankylosis to the erosion score was not performed, and there was no control group. A recent report compared T1FS sequences with two variants of T2GE sequences, three-dimensional -fast low angle shot (3D-FLASH) and three-dimensional-double excitation in the steady state (3D-DESS) sequences, in a retrospective analysis of scans of 30 patients with clinically suspected sacroiliitis and nine healthy controls [23
]. There was no difference in the number of erosions detected by all three sequences, but erosion scores based on the extent of joint involvement were significantly higher in both T2GE sequences compared with T1FS sequences. The reliability for detection of erosion by these three 'cartilage MRI sequences' could not be assessed because only one reader evaluated the scans and there was no comparison with the T1SE sequence often used in daily routine.
There are no controlled data on whether CT may have higher sensitivity to detect SIJ erosions compared with MRI. However, two recent studies consistently reported an increased risk of malignancy associated with CT of the pelvis [3
]. The adjusted lifetime attributable risk of cancer was two cancers per 1,000 exposed women for 20-year-old women who undergo pelvic CT examination [4
]. This concern about radiation dose may limit the use of CT to assess SIJ erosions in daily routine and particularly in studies involving healthy controls.
We found a marked difference for erosion kappa values in the iliac versus the sacral joint portion. However, kappa values depend on the prevalence of the finding under observation [24
]. Kappa values for erosion and BME were virtually identical in the ilium, which may be partly due to a higher occurrence of erosion in this area, comparable to the occurrence of BME. In contrast, erosion kappa values for the sacrum were lower than for BME which occurred more frequently than erosion in the sacrum. Another reason for the difference in detecting erosion between the ilium and sacrum may be different MRI appearances of erosion in the two articular surfaces. Such differences in erosion phenotype may relate to cartilage thickness (which is usually thinner on the iliac compartment), size and depth of erosions, or MRI artifacts, such as chemical shift, which may impair the assessment of subchondral bone [27