Reporting Sacroiliac Joint Imaging Performed for Known or Suspected Axial Spondyloarthritis: Assessment of SpondyloArthritis International Society Recommendations.

Whereas previous projects attempted to standardize imaging in patients with axial spondyloarthritis (axSpA), few studies have been published about the need for specific details regarding the image acquisition and lesions that may be less familiar to general radiologists. This work reports consensus recommendations developed by the Assessment of SpondyloArthritis International Society (ASAS) that aim to standardize the imaging reports in patients suspected of having or with known axSpA. A task force consisting of radiologists and rheumatologists from ASAS and one patient representative formulated two surveys that were completed by ASAS members. The results of these surveys led to the development of 10 recommendations that were endorsed by 73% (43 of 59) of ASAS members. The recommendations are targeted to the radiologist and include best practices for the inclusion of clinical information, technical details, image quality, and imaging findings in radiology reports. These recommendations also emphasize that imaging findings that indicate differential diagnoses and referral suggestions should be included in the concluding section of the radiology report. With these recommendations, ASAS aims to improve the diagnostic process and care for patients suspected of having or with known axSpA.

Development of international consensus on a standardised image acquisition protocol for diagnostic evaluation of the sacroiliac joints by MRI: an ASAS-SPARTAN collaboration.

BACKGROUND: A range of sacroiliac joint (SIJ) MRI protocols are used in clinical practice but not all were specifically designed for diagnostic ascertainment. This can be confusing and no standard diagnostic SIJ MRI protocol is currently accepted worldwide. OBJECTIVE: To develop a standardised MRI image acquisition protocol (IAP) for diagnostic ascertainment of sacroiliitis. METHODS: 13 radiologist members of Assessment of SpondyloArthritis International Society (ASAS) and the SpondyloArthritis Research and Treatment Network (SPARTAN) plus two rheumatologists participated in a consensus exercise. A draft IAP was circulated with background information and online examples. Feedback on all issues was tabulated and recirculated. The remaining points of contention were resolved and the revised IAP was presented to the entire ASAS membership. RESULTS: A minimum four-sequence IAP is recommended for diagnostic ascertainment of sacroiliitis and its differential diagnoses meeting the following requirements. Three semicoronal sequences, parallel to the dorsal cortex of the S2 vertebral body, should include sequences sensitive for detection of (1) changes in fat signal and structural damage with T1-weighting; (2) active inflammation, being T2-weighted with fat suppression; (3) bone erosion optimally depicting the bone-cartilage interface of the articular surface and (4) a semiaxial sequence sensitive for detection of inflammation. The IAP was approved at the 2022 ASAS annual meeting with 91% of the membership in favour. CONCLUSION: A standardised IAP for SIJ MRI for diagnostic ascertainment of sacroiliitis is recommended and should be composed of at least four sequences that include imaging in two planes and optimally visualise inflammation, structural damage and the bone-cartilage interface.

Clinical information on imaging referrals for suspected or known axial spondyloarthritis: recommendations from the Assessment of Spondyloarthritis International Society (ASAS).

OBJECTIVES: This study aims to establish expert consensus recommendations for clinical information on imaging requests in suspected/known axial spondyloarthritis (axSpA), focusing on enhancing diagnostic clarity and patient care through guidelines. MATERIALS AND METHODS: A specialised task force was formed, comprising 7 radiologists, 11 rheumatologists from the Assessment of Spondyloarthritis International Society (ASAS) and a patient representative. Using the Delphi method, two rounds of surveys were conducted among ASAS members. These surveys aimed to identify critical elements for imaging referrals and to refine these elements for practical application. The task force deliberated on the survey outcomes and proposed a set of recommendations, which were then presented to the ASAS community for a decisive vote. RESULTS: The collaborative effort resulted in a set of six detailed recommendations for clinicians involved in requesting imaging for patients with suspected or known axSpA. These recommendations cover crucial areas, including clinical features indicative of axSpA, clinical features, mechanical factors, past imaging data, potential contraindications for specific imaging modalities or contrast media and detailed reasons for the examination, including differential diagnoses. Garnering support from 73% of voting ASAS members, these recommendations represent a consensus on optimising imaging request protocols in axSpA. CONCLUSION: The ASAS recommendations offer comprehensive guidance for rheumatologists in requesting imaging for axSpA, aiming to standardise requesting practices. By improving the precision and relevance of imaging requests, these guidelines should enhance the clinical impact of radiology reports, facilitate accurate diagnosis and consequently improve the management of patients with axSpA.

Unmet Needs in Spondyloarthritis: Imaging in Axial Spondyloarthritis.

Imaging biomarkers in axial spondyloarthritis (axSpA) are currently the most specific biomarkers for the diagnosis of this condition. Despite advances in imaging, from plain radiographs-which detect only damage-to magnetic resonance imaging (MRI)-which identifies disease activity and structural change-there are still many challenges that remain. Imaging in sacroiliitis is characterized by active and structural changes. Current classification criteria stress the importance of bone marrow edema (BME); however, BME can occur in various diseases, mechanical conditions, and healthy individuals. Thus, the identification of structural lesions such as erosion, subchondral fat, backfill, and ankylosis is important to distinguish from mimics on differential diagnosis. Various imaging modalities are available to examine structural lesions, but computed tomography (CT) is considered the current reference standard. Nonetheless, recent advances in MRI allow for direct bone imaging and the reconstruction of CT-like images that can provide similar information. Therefore, the ability of MRI to detect and measure structural lesions is strengthened. Here, we present an overview of the spectrum of current and cutting-edge techniques for SpA imaging in clinical practice; namely, we discuss the advantages, disadvantages, and usefulness of imaging in SpA through radiography, low-dose and dual-energy CT, and MRI. Cutting-edge MRI sequences including volumetric interpolated breath-hold examination, ultrashort echo time, zero echo time, and deep learning-based synthetic CT that creates CT-like images without ionizing radiation, are discussed. Imaging techniques allow for quantification of inflammatory and structural lesions, which is important in the assessment of treatment response and disease progression. Radiographic damage is poorly sensitive to change. Artificial intelligence has already revolutionized radiology practice, including protocolization, image quality, and image interpretation.

ESR Essentials: Imaging of sacroiliitis-practice recommendations by ESSR.

Sacroiliitis is commonly seen in patients with axial spondyloarthritis, in whom timely diagnosis and treatment are crucial to prevent irreversible structural damage. Imaging has a prominent place in the diagnostic process and several new imaging techniques have been examined for this purpose. We present a summary of updated evidence-based practice recommendations for imaging of sacroiliitis. MRI remains the imaging modality of choice for patients with suspected sacroiliitis, using at least four sequences: coronal oblique T1-weighted and fluid-sensitive sequences, a perpendicular axial oblique sequence, and a sequence for optimal evaluation of the bone-cartilage interface. Both active inflammatory and structural lesions should be described in the report, indicating location and extent. Radiography and CT, especially low-dose CT, are reasonable alternatives when MRI is unavailable, as patients are often young. This is particularly true to evaluate structural lesions, at which CT excels. Dual-energy CT with virtual non-calcium images can be used to depict bone marrow edema. Knowledge of normal imaging features in children (e.g., flaring, blurring, or irregular appearance of the articular surface) is essential for interpreting sacroiliac joint MRI in children because these normal processes can simulate disease. CLINICAL RELEVANCE STATEMENT: Sacroiliitis is a potentially debilitating disease if not diagnosed and treated promptly, before structural damage to the sacroiliac joints occurs. Imaging has a prominent place in the diagnostic process. We present a summary of practice recommendations for imaging of sacroiliitis, including several new imaging techniques. KEY POINTS: • MRI is the modality of choice for suspected inflammatory sacroiliitis, including a joint-line-specific sequence for optimal evaluation of the bone-cartilage interface to improve detection of erosions. • Radiography and CT (especially low-dose CT) are reasonable alternatives when MRI is unavailable. • Knowledge of normal imaging features in children is mandatory for interpretation of MRI of pediatric sacroiliac joints.

Standardized reporting of spine and sacroiliac joints in axial spondyloarthritis MRI: from the ESSR-Arthritis Subcommittee.

OBJECTIVES: Apply a modified Delphi-based approach and produce a practical, radiology-specific set of definitions for interpretation and standardization of the multiple MRI findings in axial spondyloarthritis (ax-SpA), specifically to aid the general radiologist with a musculoskeletal interest, working with gold standard basic MRI protocols. MATERIALS AND METHODS: We report the results of a modified Delphi-based consensus of 35 experts from 13 countries in the Arthritis Subcommittee of the European Society of Musculoskeletal Radiology (ESSR). Seventeen definitions were created (i.e., nine for the spine and eight for the sacroiliac joint) and two Delphi rounds were conducted on an electronic database, collated and revised by the project leader with agreement. Group leads were appointed for each definition following the first round. Final definitions included only those that reached a consensus > 80%; if > 50% agreed on exclusion consensus, definitions were excluded. Final results have been shared during the Arthritis meeting at the Annual ESSR Congress. RESULTS: Fourteen definitions, eight for the spine and six for the sacroiliac joint were agreed for standardized reporting. Andersson's, anterior corner sclerotic and costovertebral joint inflammatory lesions of the spine, with active and non-active erosions, and fat metaplasia of the sacroiliac joint reaching the highest consensus (≥ 95%). More than 50% of the experts agreed to exclude joint space inflammation in the sacroiliac joint and tissue backfill. Syndesmophytes reached 76% agreement. CONCLUSIONS: Agreed definitions by expert radiologists using a modified Delphi process, should allow standardized actionable radiology reports and clarity in reporting terminology of ax-SpA. CLINICAL RELEVANCE STATEMENT: The proposed definitions will support reporting from musculoskeletal and general radiologists working with gold-standard basic MRI, improve confidence in lesion assessment, and standardize terminology to provide actionable reports on MRI in patients with ax-SpA. KEY POINTS: Experts applied a modified Delphi method to optimize the definitions of MRI findings of ax-SpA. After two Delphi rounds and one in-person meeting, fourteen definitions reached the agreement threshold. These consensus-based definitions will aid in actionable reporting specifically for the general radiologist with a musculoskeletal interest.

Association of anatomical variants of the sacroiliac joint with bone marrow edema in patients with axial spondyloarthritis.

OBJECTIVE: To determine the prevalence of sacroiliac joint variants in patients with axial spondyloarthritis (axSpA) using MRI-based synthetic CT images and to evaluate their relationships with the presence of bone marrow edema, as this may potentially complicate diagnosing active sacroiliitis on MRI in patients with suspected axSpA. METHODS: 172 patients were retrospectively included. All patients underwent MRI because of clinical suspicion of sacroiliitis. The diagnosis of axSpA was made by a tertiary hospital rheumatologist. Two readers independently determined the presence of bone marrow edema and the presence of one or more of the nine known sacroiliac joint (SIJ) variants. RESULTS: SIJ variants were common in axSpA patients (82.9%) and the non-SpA group (85.4%); there were no significant differences in prevalence. Bone marrow edema was frequently found in axSpA (86.8%) and non-SpA patients (34%). AxSpA patients with SIJ variants (except for accessory joint) demonstrated 4 to 10 times higher odds for bone marrow edema, however not statistically significant. The more variants were present in this group, the higher the chance of bone marrow edema. However, some multicollinearity cannot be excluded, since bone marrow edema is very frequent in the axSpA group by definition. CONCLUSION: SIJ variants are common in axSpA and non-SpA patients. SIJ variants were associated with higher prevalence of bone marrow edema in axSpA patients, potentially due to altered biomechanics, except for accessory joint which may act as a stabilizer.

Performance of an MRI scoring system for inflammation of joints and entheses in peripheral SpA: post-hoc analysis of the CRESPA trial.

OBJECTIVES: The aim of this study was to investigate the reliability, validity, and sensitivity to change of a novel MRI scoring system in early peripheral SpA (pSpA). METHODS: MRI of the pelvis and lower extremities was performed before initiation of the TNF inhibitor golimumab in 56 patients and repeated in 46 patients who achieved sustained clinical remission after 24, 36 or 48 weeks. Three readers applied a semi-quantitative MRI scoring system for lower-extremity joint and entheseal inflammation. Four lesion types were assessed: entheseal osteitis, entheseal soft-tissue inflammation, joint osteitis, and joint synovitis/effusion. MRI response was defined as a decrease in MRI lower-extremity inflammation index (sum of scores from 75 sites, each scored 0-3) above the smallest detectable change (SDC). RESULTS: At follow-up, the MRI index decreased in 34 of 46 patients (74%), and 15 (33%) patients achieved MRI response, i.e. a decrease above SDC of 2.8. When restricting the analysis to patients with clinical involvement of lower-extremity sites that were assessed by MRI, 13 of 28 (46%) achieved MRI response. Interreader reliability was very good, with an average-measure intraclass correlation coefficient of 0.92 (95% CI: 0.85-0.95) for status scores and 0.89 (0.80-0.94) for change in scores. The MRI index correlated with other measures of disease activity, including CRP [Spearman's rho 0.41 (0.23-0.56)], swollen joint count of 6 joints [0.47 (0.27-0.63)], tender enthesis count of 14 entheses [0.32 (0.12-0.50)] and pain score [0.28 (0.08-0.46)], all P < 0.05. CONCLUSION: The proposed MRI lower-extremity inflammation index demonstrated reliability, validity, and sensitivity to change in patients with early pSpA. TRIAL REGISTRATION: Clinicaltrials.gov, http://clinicaltrials.gov, NCT01426815.

Neural network algorithm for detection of erosions and ankylosis on CT of the sacroiliac joints: multicentre development and validation of diagnostic accuracy.

OBJECTIVES: To evaluate the feasibility and diagnostic accuracy of a deep learning network for detection of structural lesions of sacroiliitis on multicentre pelvic CT scans. METHODS: Pelvic CT scans of 145 patients (81 female, 121 Ghent University/24 Alberta University, 18-87 years old, mean 40 ± 13 years, 2005-2021) with a clinical suspicion of sacroiliitis were retrospectively included. After manual sacroiliac joint (SIJ) segmentation and structural lesion annotation, a U-Net for SIJ segmentation and two separate convolutional neural networks (CNN) for erosion and ankylosis detection were trained. In-training validation and tenfold validation testing (U-Net-n = 10 × 58; CNN-n = 10 × 29) on a test dataset were performed to assess performance on a slice-by-slice and patient level (dice coefficient/accuracy/sensitivity/specificity/positive and negative predictive value/ROC AUC). Patient-level optimisation was applied to increase the performance regarding predefined statistical metrics. Gradient-weighted class activation mapping (Grad-CAM++) heatmap explainability analysis highlighted image parts with statistically important regions for algorithmic decisions. RESULTS: Regarding SIJ segmentation, a dice coefficient of 0.75 was obtained in the test dataset. For slice-by-slice structural lesion detection, a sensitivity/specificity/ROC AUC of 95%/89%/0.92 and 93%/91%/0.91 were obtained in the test dataset for erosion and ankylosis detection, respectively. For patient-level lesion detection after pipeline optimisation for predefined statistical metrics, a sensitivity/specificity of 95%/85% and 82%/97% were obtained for erosion and ankylosis detection, respectively. Grad-CAM++ explainability analysis highlighted cortical edges as focus for pipeline decisions. CONCLUSIONS: An optimised deep learning pipeline, including an explainability analysis, detects structural lesions of sacroiliitis on pelvic CT scans with excellent statistical performance on a slice-by-slice and patient level. CLINICAL RELEVANCE STATEMENT: An optimised deep learning pipeline, including a robust explainability analysis, detects structural lesions of sacroiliitis on pelvic CT scans with excellent statistical metrics on a slice-by-slice and patient level. KEY POINTS: • Structural lesions of sacroiliitis can be detected automatically in pelvic CT scans. • Both automatic segmentation and disease detection yield excellent statistical outcome metrics. • The algorithm takes decisions based on cortical edges, rendering an explainable solution.

Update on Pediatric Spine Imaging.

The spine is often difficult to evaluate clinically in children, increasing the importance of diagnostic imaging to detect a wide variety of spinal disorders ranging from congenital abnormalities to severe infections. Clinical history and physical examination can help determine whether imaging is needed and which imaging technique would be best. The most common cause for back pain, even in children, is muscular strain/spasm that does not require any imaging. However, red flags such as pain at age < 5 years, constant pain, night pain, radicular pain, pain lasting > 4 weeks, or an abnormal neurologic examination may require further investigation. Imaging can be of great value for diagnosis but must be interpreted along with the clinical history, physical examination, and laboratory findings to achieve an accurate diagnosis. We discuss imaging for the most common and/or important spine pathologies in children: congenital and developmental pathologies, trauma, infectious processes, inflammatory causes, and tumors.

Current Role of Conventional Radiography of Sacroiliac Joints in Adults and Juveniles with Suspected Axial Spondyloarthritis: Opinion from the ESSR Arthritis and Pediatric Subcommittees.

This opinion article by the European Society of Musculoskeletal Radiology Arthritis and Pediatric Subcommittees discusses the current use of conventional radiography (CR) of the sacroiliac joints in adults and juveniles with suspected axial spondyloarthritis (axSpA). The strengths and limitations of CR compared with magnetic resonance imaging (MRI) and computed tomography (CT) are presented.Based on the current literature and expert opinions, the subcommittees recognize the superior sensitivity of MRI to detect early sacroiliitis. In adults, supplementary pelvic radiography, low-dose CT, or synthetic CT may be needed to evaluate differential diagnoses. CR remains the method of choice to detect structural changes in patients with suspected late-stage axSpA or established disease and in patients with suspected concomitant hip or pubic symphysis involvement. In children, MRI is the imaging modality of choice because it can detect active as well as structural changes and is radiation free.

Intermediate-weighted MRI with fat suppression (IW-FS): diagnostic performance for bone marrow edema and erosion detection in axial spondyloarthritis.

BACKGROUND: Bone marrow edema (BME) and erosion of the sacroiliac joint are both key lesions for diagnosing axial spondyloarthritis (axSpA) on magnetic resonance imaging (MRI). PURPOSE: To qualitatively and quantitatively compare intermediate-weighted MRI with fat suppression (IW-FS) with T2-weighted short tau inversion recovery (T2-STIR) in assessment of sacroiliac BME and erosion in axSpA. MATERIAL AND METHODS: Patients aged 18-60 years with axSpA were prospectively enrolled. All patients underwent a 3.0-T MRI examination of the sacroiliac joints. Para-coronal IW-FS, T2-STIR, and T1-weighted (T1W) images were acquired. BME and erosion were scored by two readers in consensus on IW-FS and STIR using a modified Spondyloarthritis Research Consortium of Canada (SPARCC) scoring system. Consensus scores on T1WI were used as the reference for erosion. Signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were measured for BME. RESULTS: In total, 49 patients (mean age=33.4 ± 7.6 years) were included. More patients were scored as having BME on T2-STIR (36 vs. 29, P = 0.016). SPARCC-BME score on IW-FS was lower than that acquired on T2-STIR (mean, 11.5 vs. 14.7, P = 0.002). SNR and CNR of BME were both lower on IW-FS than on T2-STIR (mean SNR, 118 vs. 218, P < 0.001; mean CNR, 44 vs. 137, P < 0.001). The sensitivity of erosion detection was higher on IW-FS (83%) than on T2-STIR (54%, P = 0.006). CONCLUSION: IW-FS is not sufficient for BME detection using T2-STIR as the reference standard in patients with axSpA. IW-FS has a much higher sensitivity than T2-STIR for erosion detection in the sacroiliac joint.

MRI in pediatric sacroiliitis, what radiologists should know.

MRI is used for early detection of inflammation of sacroiliac joints as it shows active lesions of sacroiliitis long before radiographs show damage to the sacroiliac joints. Early diagnosis of arthritis allows early treatment of inflammation and can help delay disease progression and prevent irreversible damage. Also, early identification of axial involvement in juvenile spondyloarthropathy is crucial, as treatment options are different than for peripheral juvenile spondyloarthropathy. In general, standard sequences used in adults are also applied to children. However, interpreting MR images of pediatric sacroiliac joints is more challenging than in adults, because of normal physiological changes during skeletal maturation, which can simulate disease on MR images. Furthermore, classical definitions of sacroiliitis used in adults, for both active inflammatory and structural lesions, can be difficult to extrapolate to children. The development of reliable pediatric-specific definitions for sacroiliitis is still in active study. Understanding both normal and pathological signal changes in children is important to distinguish physiologic findings from disease and to make a correct diagnosis. In this review, the main imaging characteristics of sacroiliitis on MRI in children and its frequent pitfalls will be illustrated, while also citing some discussion points regarding the scan protocol.

Novel imaging techniques for sacroiliac joint assessment.

PURPOSE OF REVIEW: Imaging of the sacroiliac joints is one of the cornerstones in the diagnosis and monitoring of axial spondyloarthritis. We aim to present an overview of the emerging imaging techniques for sacroiliac joint assessment and provide an insight into their relevant benefits and pitfalls. RECENT FINDINGS: Evaluation of structural and active inflammatory lesions in sacroiliitis are both important for understanding the disease process. Dual-energy computed tomography (CT) can detect inflammatory bone marrow edema in the sacroiliac joints and provides an alternative for magnetic resonance imaging (MRI). Three-dimensional gradient echo sequences improve the visualization of erosions on MRI. Susceptibility weighted MRI and deep learning-based synthetic CT are innovative MRI techniques that allow for generating 'CT-like' images and better depict osseous structural lesions than routine MRI sequences. SUMMARY: New imaging innovations and developments result in significant improvements in the imaging of spondyloarthritis. Advanced MRI techniques enhance its potential for the accurate detection of structural and active inflammatory lesions of sacroiliitis in one single imaging session.

EULAR points to consider for the use of imaging to guide interventional procedures in patients with rheumatic and musculoskeletal diseases (RMDs).

OBJECTIVES: To develop evidence-based Points to Consider (PtC) for the use of imaging modalities to guide interventional procedures in patients with rheumatic and musculoskeletal diseases (RMDs). METHODS: European Alliance of Associations for Rheumatology (EULAR) standardised operating procedures were followed. A systematic literature review was conducted to retrieve data on the role of imaging modalities including ultrasound (US), fluoroscopy, MRI, CT and fusion imaging to guide interventional procedures. Based on evidence and expert opinion, the task force (25 participants consisting of physicians, healthcare professionals and patients from 11 countries) developed PtC, with consensus obtained through voting. The final level of agreement was provided anonymously. RESULTS: A total of three overarching principles and six specific PtC were formulated. The task force recommends preference of imaging over palpation to guide targeted interventional procedures at peripheral joints, periarticular musculoskeletal structures, nerves and the spine. While US is the favoured imaging technique for peripheral joints and nerves, the choice of the imaging method for the spine and sacroiliac joints has to be individualised according to the target, procedure, expertise, availability and radiation exposure. All imaging guided interventions should be performed by a trained specialist using appropriate operational procedures, settings and assistance by technical personnel. CONCLUSION: These are the first EULAR PtC to provide guidance on the role of imaging to guide interventional procedures in patients with RMDs.

MRI-based synthetic CT of the hip: can it be an alternative to conventional CT in the evaluation of osseous morphology?

OBJECTIVES: MRI is the gold standard for soft tissue evaluation in the hip joint. However, CT is superior to MRI in providing clear visualization of bony morphology. The aim of this study is to test the equivalency of MRI-based synthetic CT to conventional CT in quantitatively assessing bony morphology of the hip. MATERIALS AND METHODS: A prospective study was performed. Adult patients who underwent MRI and CT of the hips were included. Synthetic CT images were generated from MRI using a deep learning-based image synthesis method. Two readers independently performed clinically relevant measurements for hip morphology, including anterior and posterior acetabular sector angle, acetabular version angle, joint space width, lateral center-edge angle, sharp angle, alpha angle, and femoral head-neck offset on synthetic CT and CT. Inter-method, inter-reader, and intra-reader reliability and agreement were assessed using calculations of intraclass correlation coefficient, standard error of measurement, and smallest detectable change. The equivalency among CT and synthetic CT was evaluated using equivalency statistical testing. RESULTS: Fifty-four hips from twenty-seven participants were included. There was no reported hip pathology in the subjects. The observed agreement based on reliability and agreement parameters indicated a strong degree of concordance between CT and synthetic CT. Equivalence statistical testing showed that all synthetic CT measurements are equivalent to the CT measurements at the considered margins. CONCLUSION: In healthy individuals, we demonstrated equivalency of MRI-based synthetic CT to conventional CT for the quantitative evaluation of osseous hip morphology, thus obviating the radiation exposure of a pelvic CT examination. KEY POINTS: •MRI-based synthetic CT images can be generated from MRI using a deep learning-based image synthesis method. •MRI-based synthetic CT is equivalent to CT in the quantitative assessment of bony hip morphology in healthy individuals. •MRI-based synthetic CT is promising for use in preoperative diagnosis and surgery planning.

Imaging of Structural Abnormalities of the Sacrum: The Old Faithful and Newly Emerging Techniques.

The sacrum and sacroiliac joints pose a long-standing challenge for adequate imaging because of their complex anatomical form, oblique orientation, and posterior location in the pelvis, making them subject to superimposition. The sacrum and sacroiliac joints are composed of multiple diverse tissues, further complicating their imaging. Varying imaging techniques are suited to evaluate the sacrum, each with its specific clinical indications, benefits, and drawbacks. New techniques continue to be developed and validated, such as dual-energy computed tomography (CT) and new magnetic resonance imaging (MRI) sequences, for example susceptibility-weighted imaging. Ongoing development of artificial intelligence, such as algorithms allowing reconstruction of MRI-based synthetic CT images, promises even more clinical imaging options.

MRI-based Synthetic CT in the Detection of Structural Lesions in Patients with Suspected Sacroiliitis: Comparison with MRI.

Background Evaluation of structural lesions in the sacroiliac (SI) joints can improve the accuracy for diagnosis of spondyloarthritis. However, structural lesions, such as erosions, are difficult to assess on routine T1-weighted MRI scans. Purpose To determine the diagnostic performance of MRI-based synthetic CT (sCT) in the depiction of erosions, sclerosis, and ankylosis of the SI joints compared with T1-weighted MRI, with CT as the reference standard. Materials and Methods A prospective study (clinical trial registration no. B670201837885) was performed from February 2019 to November 2019. Adults were referred from a tertiary hospital rheumatology outpatient clinic with clinical suspicion of inflammatory sacroiliitis. MRI and CT of the SI joints were performed on the same day. SCT images were generated from MRI scans using a commercially available deep learning-based image synthesis method. Two readers independently recorded if structural lesions (erosions, sclerosis, and ankylosis) were present on T1-weighted MRI, sCT, and CT scans in different reading sessions, with readers blinded to clinical information and other images. Diagnostic performance of sCT and T1-weighted MRI scans were analyzed using generalized estimating equation models, with consensus results of CT as the reference standard. Results Thirty participants were included (16 men, 14 women; mean age, 40 years ± 10 [standard deviation]). Diagnostic accuracy of sCT was higher than that of T1-weighted MRI for erosion (94% vs 86%, P = .003), sclerosis (97% vs 81%, P < .001), and ankylosis (92% vs 84%, P = .04). With sCT, specificity for erosion detection (96% [95% CI: 90, 98] vs 89% [95% CI: 81, 94], P = .01] and sensitivity for detection of sclerosis [94% [95% CI: 87, 97] vs 20% [95% CI: 10, 35], P < .001] and ankylosis (93% [95% CI: 78, 98] vs 70% [95% CI: 47, 87], P = .001) were improved. Conclusion With CT as the reference standard, synthetic CT of the sacroiliac joints has better diagnostic performance in the detection of structural lesions in individuals suspected of having sacroiliitis compared with routine T1-weighted MRI. © RSNA, 2020 Online supplemental material is available for this article. See also the editorial by Fritz in this issue.

Axial involvement in patients with early peripheral spondyloarthritis: a prospective MRI study of sacroiliac joints and spine.

OBJECTIVES: To assess axial involvement on MRI in early peripheral spondyloarthritis (pSpA) and to evaluate whether axial inflammation predicts relapse on treatment withdrawal. METHODS: Fifty-six patients with early, active, newly diagnosed pSpA underwent MRI of the sacroiliac joints (SIJs) and spine prior to golimumab initiation. At sustained clinical remission of pSpA, treatment was withdrawn and a second MRI was performed. Bone marrow oedema (BME) was scored by three readers according to the Spondyloarthritis Research Consortium of Canada (SPARCC) method. Scores were compared with an axial spondyloarthritis cohort (Belgian Arthritis and Spondylitis cohort). Structural lesions were assessed using a similar method. Furthermore, fulfilment of the Assessment of Spondyloarthritis International Society (ASAS) definition of a positive MRI for sacroiliitis was assessed. Spinal images were evaluated for BME and structural lesions using the Canada-Denmark MRI spine scoring system by two readers. RESULTS: Thirty-six per cent showed SIJ BME at baseline, all fulfilling the ASAS definition of sacroiliitis. No association with back pain was found. Twenty-one per cent displayed SIJ structural lesions. Spinal BME was limited: the median inflammation scores were low and no patients had ≥5 inflammatory corner lesions. On clinical remission, a significant decrease in SIJ SPARCC scores was detected. On clinical remission, no significant differences in SIJ SPARCC scores were noted between patients relapsing and those maintaining remission after treatment discontinuation. CONCLUSION: In patients with early pSpA, a surprisingly high prevalence of sacroiliitis on MRI was observed; SPARCC scores decreased significantly on tumour necrosis factor inhibition. Residual inflammation on MRI was not predictive of relapse of peripheral manifestations. No relevant inflammatory spinal involvement was detected. Collectively, our findings suggest a higher inflammatory burden in patients with early pSpA than anticipated.

Normal subchondral high T2 signal on MRI mimicking sacroiliitis in children: frequency, age distribution, and relationship to skeletal maturity.

OBJECTIVES: To determine patterns of variation of subchondral T2 signal changes in pediatric sacroiliac joints (SIJ) by location, age, sex, and sacral apophyseal closure. METHODS: MRI of 502 SIJ in 251 children (132 girls), mean age 12.4 years (range 6.1-18.0), was obtained with parental informed consent. One hundred twenty-seven out of 251 had asymptomatic joints and were imaged for non-rheumatologic reasons, and 124 had low back pain but no sign of sacroiliitis on initial clinical MRI review. After calibration, three subspecialist radiologists independently scored subchondral signal changes on fat-suppressed fluid-sensitive sequences from 0 to 3 in 4 locations, and graded the degree of closure of sacral segmental apophyses. Associations between patient age, sex, signal changes, and apophyseal closure were analyzed. RESULTS: Rim-like subchondral increased T2 signal or "flaring" was much more common at sacral than iliac SIJ margins (72% vs 16%, p < 0.001) and was symmetrical in > 90% of children. Iliac flaring scores were always lower than sacral, except for 1 child. Signal changes decreased as sacral apophyses closed, and were seen in < 20% of subjects with fully closed apophyses. Signal changes were more frequent in boys, and peaked in intensity later than for girls (ages 8-12 vs. 7-10). Subchondral signal in iliac crests was high throughout childhood and did not correlate with other locations. CONCLUSIONS: Subchondral T2 "flaring" is common at SIJ of prepubertal children and is generally sacral-predominant and symmetrical. Flaring that is asymmetrical, greater in ilium than sacrum, or intense in a teenager with closed apophyses, is unusual for normal children and raises concern for pathologic bone marrow edema. KEY POINTS: • A rim of subchondral high T2 signal is commonly observed on MRI at pediatric sacroiliac joints, primarily on the sacral side before segmental apophyseal closure, and should not be confused with pathology. • Unlike subchondral signal changes elsewhere, high T2 signal underlying the iliac crest apophyses is a near-universal normal finding in children that usually persists throughout adolescence. • The following patterns are unusual in normal children and are suspicious for pathology: definite iliac flaring, iliac flaring more intense than sacral flaring, left-right difference in flaring, definite flaring of any pattern in teenagers after sacral apophyseal closure.