POS0518 CLINICAL FEASIBILITY STUDY OF IDENTIFYING STRUCTURAL LESIONS OF SACROILIITIS BY VERY LOW DOSE CT SI-SCAN
Authors: M. A. Korteweg, M. I. Wessels, B. F. Bartels, I. C. J. Koetser, S. C. Heslinga, M. Gerritsen, C. J. Van der Laken
Journal: Annals of the Rheumatic Diseases (2024)

<h3>Background:</h3> To diagnose sacroiliitis, according to ASAS guidelines, X-rays of the sacroiliac (SI) joints are performed, which are known to have low sensitivity and specificity. [1] If inconclusive, MRI is used as the next modality in the diagnostic workup. It would be beneficial if a single imaging modality provided a more accurate diagnosis. CT is useful for detecting structural lesions, indicating damage due to sacroiliitis, and has improved sensitivity and specificity compared to X-rays. [2,3] New CT developments facilitate scanning SI-joints with an effective dose lower than or similar to X-rays, which have an effective dose considered &lt;1mSv. [4]. This study investigates the clinical feasibility of very low-dose (ld) CT compared to conventional X-rays. <h3>Objectives:</h3> To describe preliminary clinical experiences of very ld CT SI-scans for detection of structural lesions due to sacroiliitis correlated with clinical findings in comparison to X-rays. <h3>Methods:</h3> In an outpatient rheumatology center, all adult patients suspected of sacroiliitis who gave permission for scientific use of their data were included between 15/6/23-15/12/23. Each received spine/pelvis X-rays, including SI-joints and a very ld CT SI-scan. 2 musculoskeletal radiologists blindly and anonymously scored X-rays according to modified New York (mNY) criteria and CT images for the probability of sacroiliitis on a 0-4 scale (very unlikely - unequivocal) based on structural lesions associated with sacroiliitis; number/location of erosions, joint space narrowing/widening, extent of ankyloses/sclerosis. [2] Imaging was defined as positive if the scan was overall scored 3 or 4, based on the extent and combination of structural lesions. [2] CRP and HLA-B27 were noted. CRP &gt;10mg/L was used as cut-off. Final clinical diagnosis made by a rheumatologist was applied as outcome variable and gold standard, and retrieved from clinical files. Sensitivity (sens.), specificity (spec.), negative predictive value (NPV), positive predictive value (PPV), false positive rate (FPR) and false negative rate (FNR) were calculated. <h3>Results:</h3> 59 (38 female, mean age 43y (range 24-68y)) gave informed consent and were included. In n=19 patients (32%) a clinical diagnosis of sacroiliitis was determined. X-rays vs CT scans respectively result in sens., 42% vs 95%; spec., 80% vs 83%; NPV, 74% vs 97%; PPV, 50% vs 72%; FPR, 20% vs 18%; FNR 58% vs 5% (Figure 1). Interobserver agreement was good. Mean mNY score; 2,1(±Sd0,8) and for CT; 2,0(±Sd1,5). Mean effective dose of very ld CT SI-scans; 0,44mSv (±Sd0,07). The majority of sacroiliitis patients are male and HLA-B27 positive. Elevated CRP was equally present in the positive and negative clinical sacroillitis group. The only imaging characteristic exclusively associated with clinical sacroiliitis is complete ankyloses on CT (Figure 2). Of the 19 patients with sacroiliitis; 18 were correctly identified by CT. 1 CT SI-scan was false negative; it showed only pseudowidening. 7 CT SI-scans were false positive; diagnosed with Paget or osteoarthritis. Of these patients all had lesions interpreted as erosions; 2 had ≥6 erosions, 5 showed periarticular sclerosis of whom one ≥1cm and 2 showed joint space widening. <h3>Conclusion:</h3> Very ld CT SI-scans have superior diagnostic values for diagnosing sacroiliitis compared to X-rays. Especially the potential of CT for excluding sacroiliitis is an improvement, as X-rays miss more than half of sacroiliitis patients and CT only approximately 1 in 20. Additionally, very ld CT SI-scans cause less harm by having on average a lower effective dose than X-rays. Concluding, very ld CT SI-scans help prevent the development of structural damage as they enable more accurate diagnosis and thus earlier treatment. We therefore advocate the use of very ld CT SI-scans for diagnostic workup of sacroillitis. <h3>REFERENCES:</h3> [1] Sudoł-Szopińska I, et al. Diagnostics of Sacroiliitis... Pol J Radiol 2015. [2] Hermann KGA, et al. … structural damage defines sacroiliitis: a CT study. RMD Open 2022. [3] Diekhoff T, et al. Choose wisely... Ann Rheum Dis. 2022. [4] Mellor FE, et al. The radiation dose received from lumbar spine… Radiography 2014. <h3>Acknowledgements:</h3> We would like to thank all patients who give permission for using their medical data anonymously for scientific and teaching purposes, as this is essential to spreading and increasing knowledge in the medical field. <h3>Disclosure of Interests:</h3> Mies A. Korteweg: None declared, Mascha I. Wessels: None declared, Bart F. Bartels: None declared, Inge C.J. Koetser: None declared, Sjoerd C. Heslinga: None declared, Martijn Gerritsen Horizon Therapeutics, Conny J. van der Laken GlaxoSmithKline GSK, UCB Pharma, Galapagos, Lilly, GlaxoSmithKline UCB Pharma, Novartis, Abbvie, Pfizer.