Common incidental findings on sacroiliac joint MRI: Added value of MRI-based synthetic CT.

PURPOSE: To determine the prevalence of incidental findings on sacroiliac joint MRI and to determine the added value of MRI-based synthetic CT in the detection and evaluation of these incidental findings. METHOD: In this retrospective study 210 patients clinically suspected of spondyloarthritis who underwent MRI of the sacroiliac joint with synthetic CT sequence were included. The images were reviewed by two radiologists in consensus for the prevalence of sacroiliitis, incidental findings, and the ability of synthetic CT and the conventional MRI to detect and diagnose these findings. RESULTS: In 44.7% of patients sacroiliitis was present. In 89.0% of patients MRI showed at least one incidental finding other than sacroiliitis. Degeneration of the sacroiliac joint was the most prevalent finding (140 patients, 66.6%). The most frequent incidental findings outside the sacroiliac joint were facet joint degeneration (29.0%), disc degeneration (25.2%), enostosis (19.5%) and lumbosacral transitional vertebrae (14.3%). A total of 788 lesions was recorded and synthetic CT was found to be problem solving or necessary for diagnosis in 543 (68.9%) of these lesions. 42.1% of lesions were not visible on conventional MRI (T1 TSE and STIR), most often degenerative osteophytes in the sacroiliac joint or lower lumbar spine. CONCLUSION: Incidental findings are seen more frequently on sacroiliac joint MRI than sacroiliitis, which is relevant as some will have clinical significance or require treatment. Nearly half of these incidental lesions were only visible on synthetic CT, which additionally has been shown to be problem solving for diagnosis in many other cases.

Blurring and Irregularity of the Subchondral Cortex in Pediatric Sacroiliac Joints on T1 Images: Incidence of Normal Findings That Can Mimic Erosions.

OBJECTIVE: To determine prevalence of variations of subchondral bone appearance that may mimic erosions on T1-weighted magnetic resonance imaging (MRI) of pediatric sacroiliac (SI) joints according to age and sex. METHODS: With ethics committee approval and informed consent, SI joint MRIs of 251 children (132 girls), mean age 12.4 years (range 6.1-18.0 years), were obtained in 2 cohorts: 127 children imaged for nonrheumatic reasons, and 124 children with low back pain but no features of sacroiliitis at initial clinical MRI review. MRIs were reviewed by 3 experienced radiologists, blinded from each other, for 3 features of the cortical black line representing the subchondral bone plate on T1-weighted MRI: visibility, blurring, and irregularity. RESULTS: Based on agreement from 2 or more readers, the cortical black line was partially absent in 88.4% of the children, blurred in 34.7%, and irregular in 41.4%. All these features were most common on the iliac side of SI joints and at the first sacral vertebra level. Clearly visualized, sharply delineated SI joints with none of these features were seen in only 8.0% of children, or in 35.1% if we conservatively required agreement of all 3 readers to consider a feature present. There was no significant difference between sexes or cohorts; findings were similar across pediatric age groups. CONCLUSION: Understanding the normal MRI appearance of the developing SI joint is necessary to distinguish physiologic findings from disease. At least two-thirds (65%) of normal pediatric SI joints showed at least 1 feature that is a component of the adult definition of SI joint erosions, risking overdiagnosis of sacroiliitis.

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.

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.

MRI-based synthetic CT of the lumbar spine: Geometric measurements for surgery planning in comparison with CT.

PURPOSE: MRI is the imaging modality of choice for soft tissue-related spine disease. However, CT is superior to MRI in providing clear visualization of bony morphology. The purpose of this study is to test equivalency of MRI-based synthetic CT to conventional CT in quantitatively assessing bony morphology of the lumbar spine. METHOD: A prospective study with an equivalency design was performed. Adult patients who had undergone MRI and CT of the lumbar spine were included. Synthetic CT images were generated from MRI using a deep learning-based image synthesis method. Two readers independently measured pedicle width, spinal canal width, neuroforamen length, anterior and posterior vertebral body height, superior and inferior vertebral body length, superior and inferior vertebral body width, maximal disc height, lumbar curvature and spinous process length on synthetic CT and CT. The agreement among CT and synthetic CT was evaluated using equivalency statistical testing. RESULTS: Thirty participants were included (14 men and 16 women, range 20-60 years). The measurements performed on synthetic CT of pedicle width, spinal canal width, vertebral body height, vertebral body width, vertebral body length and spinous process length were statistically equivalent to CT measurements at the considered margins. Excellent inter- and intra-reader reliability was found for both synthetic CT and CT. CONCLUSIONS: Equivalency of MRI-based synthetic CT to CT was demonstrated on geometrical measurements in the lumbar spine. In combination with the soft tissue information of the conventional MRI, this provides new possibilities in diagnosis and surgical planning without ionizing radiation.

Magnetic resonance imaging findings in the normal pediatric sacroiliac joint space that can simulate disease.

BACKGROUND: Magnetic resonance imaging (MRI) features of active sacroiliac joint inflammation include joint space fluid and enhancement, but it is unclear to what extent these are present in normal children. OBJECTIVE: To describe normal MRI appearances of pediatric sacroiliac joint spaces in boys and girls of varying ages. MATERIALS AND METHODS: In this ethics-approved prospective study, 251 children (119 boys, 132 girls; mean age: 12.4 years, range: 6.1-18.0 years), had both oblique-coronal T1-weighted and short tau inversion recovery (STIR) sacroiliac joint MRI. Of these, 127 were imaged for other reasons and had asymptomatic sacroiliac joints ("normal cohort") while 124 had low back pain with no features of sacroiliitis on initial clinical MRI review ("low-back-pain cohort"). Post-gadolinium T1-weighted sequences were available in 16/127 normal and 124/124 low-back-pain subjects. Three experienced radiologists scored high signal in the sacroiliac joint space on STIR (score 0=absent; 1=high signal compared to normal bone marrow present anywhere in the joint but not as bright as fluid [compared to vessels, cerebrospinal fluid]; 2=definite fluid signal in part of the joint; 3=definite fluid signal, entire vertical height, majority of slices) and, when available, joint space post-contrast enhancement (0=no high signal/enhancement; 1=thin, symmetrical, mildly increased linear high signal present in the joint space; 2=focal, thick or intense enhancement). Associations between joint signal scores, age, gender and sacral apophyseal closure were analysed. RESULTS: Increased signal on STIR (score 1-3) was present in 74.7% of pediatric sacroiliac joint spaces, as intense as fluid in 18.4%. There was no significant difference in proportion by gender, side or cohort, but girls showed peak signal earlier than boys (10 years old vs. 12 years old, respectively). On post-gadolinium T1-weighted sequences, a thin rim of increased signal was nearly universally seen in sacroiliac joint spaces without focal, intense or thick post-contrast enhancement. CONCLUSION: Sacroiliac joint spaces of most children demonstrate mildly increased signal on STIR, compared to normal bone marrow, and thin rim-like enhancement on post-contrast T1 images, likely related to cartilage. These findings should not be confused with sacroiliitis.

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.

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.

Spectrum of injuries associated with paediatric ACL tears: an MRI pictorial review.

OBJECTIVE: Magnetic resonance imaging (MRI) findings in anterior cruciate ligament (ACL) injury are well known, but most published reviews show obvious examples of associated injuries and give little focus to paediatric patients. Here, we demonstrate the spectrum of MRI appearances at common sites of associated injury in adolescents with ACL tears, emphasising age-specific issues. METHODS: Pictorial review using images from children with surgically confirmed ACL tears after athletic injury. RESULTS: ACL injury usually occurs with axial rotation in the valgus near full extension. The MRI findings can be obvious and important to management (ACL rupture), subtle but clinically important (lateral meniscus posterior attachment avulsion), obvious and unimportant to management (femoral condyle impaction injury), or subtle and possibly important (medial meniscocapsular junction tear). Paediatric-specific issues of note include tibial spine avulsion, normal difficulty visualising a thin ACL and posterolateral corner structures, and differentiation between incompletely closed physis and impaction fracture. CONCLUSION: ACL tear is only the most obvious sign of a complex injury involving multiple structures. Awareness of the spectrum of secondary findings illustrated here and the features distinguishing them from normal variation can aid in accurate assessment of ACL tears and related injuries, enabling effective treatment planning and assessment of prognosis. TEACHING POINTS: • The ACL in children normally appears thin or attenuated, while thickening and oedema suggest tear. • Displaced medial meniscal tears are significantly more common later post-injury than immediately. • The meniscofemoral ligaments merge with the posterior lateral meniscus, complicating tear assessment. • Tibial plateau impaction fractures can be difficult to distinguish from a partially closed physis. • Axial MR sequences are more sensitive/specific than coronal for diagnosis of medial collateral ligament (MCL) injury.

MR imaging findings and MR criteria for instability in osteochondritis dissecans of the elbow in children.

PURPOSE: Osteochondritis dissecans (OCD) of the elbow is an uncommon cause of elbow pain in adolescents and occurs at different locations in the elbow joint. Early diagnosis and treatment may prevent surgery. The aim of the study is to describe the MR imaging features of OCD at initial imaging, and to correlate these findings with surgical findings of stability and instability with arthroscopic findings as the reference standard. METHODS: Patients were identified through a keyword search of the radiology information system from 2000 to 2009. Twenty-five patients (26 elbows) with OCD of the elbow were identified (age 10.4-18 years, mean age 14 years). MR studies were retrospectively reviewed by two radiologists in consensus to define MR imaging findings and to determine the presence of previously described MR imaging criteria for OCD instability (i.e., high T2 signal rim, surrounding cysts, high T2 signal fracture line, fluid-filled osteochondral defect). Sensitivity of the individual and combined criteria was calculated. RESULTS: OCD occurred in the capitellum in 24 patients (92%), in the trochlea in 2 patients (8%) and radial head in 1 patient (4%). Loose bodies were identified in 11 (42%) patients. Eighteen patients demonstrated MRI findings in keeping with unstable lesions. In all 11 patients who had surgery the surgical findings of instability correlated with the MRI findings. When combined, the MR criteria were 100% sensitive for instability of OCD lesions of the elbow. CONCLUSION: The vast majority of OCD of the elbow occurs in the capitellum. When used together, the MR criteria for instability were 100% sensitive for evaluation OCD lesions of the elbow.

MRI differentiates femoral condylar ossification evolution from osteochondritis dissecans. A new sign.

OBJECTIVE: To determine if MRI (magnetic resonance imaging) of the femoral condyles in children can differentiate variations in ossification from osteochondritis dissecans (OCD). METHODS: MRI studies of the knee of 315 patients demonstrated ossification defects of the femoral condyles involving the subchondral bone plate. MRI features categorized the defects as ossification variability (N = 150) or OCD (N = 165). Both groups were compared for age, residual physeal cartilage, site, configuration, 'lesion angle' and associated findings. RESULTS: (a) Ossification variability did not occur in girls >10 year. and boys >13 year., OCD did not occur in children younger than 8 year. (b) Ossification variability was not seen in patients with 10% or less residual physeal cartilage, OCD was rare in patients with 30% or greater residual physeal cartilage. (c) Ossification variability was located in the posterior third of the femoral condyle, OCD occurred most commonly in the middle third. (d) Intracondylar extension was seen in OCD and not in ossification variability. (e) Perilesional oedema was very common with OCD and absent with ossification variability. (f) Lesion angle <105° was a feature of ossification variability. CONCLUSION: MRI may help differentiate variations in ossification of the femoral condyles from OCD.

Evolution of femoral condylar ossification at MR imaging: frequency and patient age distribution.

PURPOSE: To determine how the magnetic resonance (MR) signal intensity seen with variability in distal femoral epiphyseal ossification in children varies with (a) age, (b) sex, (c) distribution to the medial or lateral condyles, and (d) residual physeal cartilage. MATERIALS AND METHODS: Ethics committee approval was obtained, and informed patient consent was waived. Two pediatric radiologists retrospectively reviewed the consecutive knee MR imaging studies of 910 children (457 boys, 453 girls; age range, 0.7-16.9 years) for variability in ossification and categorized the variability as preossification center, early ossification center, puzzle piece, incomplete puzzle piece, spiculation, or accessory ossification center. Patient age and sex, ossification variability site, residual physeal cartilage, and associated findings were analyzed. Basic descriptive statistical analysis, Student t tests for comparison of continuous variables, and κ statistics analysis of interobserver agreement were performed where appropriate. RESULTS: In 202 (22.2%) patients (278 condyles), ossification variability was present. In the 910 patients, early ossification center (n = 172, 18.9%) and spiculated configuration of the secondary ossification center (n = 151, 16.6%) were the most common variants. Preossification center (50 [5.5%] patients), puzzle piece (26 [2.9%] patients), accessory ossification center (nine [1.0%] patients), and incomplete puzzle piece (two [0.2%] patients) were seen less often. Ossification variability was more common in the medial condyles (169 [18.6%] of 910 cases) than in the lateral condyles (109 [12.0%] of 910 cases), nearly always posteriorly located (277 [99.6%] of 278 condyles), and more common in boys (153/457 [33.5%]) than in girls (49/453 [10.8%]). Ossification variability was less common with decreasing residual physeal cartilage. Peak patient age ranges for ossification variability were 2-12 years for boys and 2-10 years for girls. CONCLUSION: Ossification variability in the femoral condyles is common in children and should not be confused with abnormal processes.