To investigate imaging features and clinical diagnostic value of pyogenic sacroiliitis (PS) in children.

A total of 7 children with PS admitted to Children′s Hospital, Captial Institute of Pediatrics from January 2016 to June 2021 were selected into this study. The clinical data, X-ray images result of sacroiliac joint, CT and MRI findings of these 7 PS children were analyzed retrospectively. The study was conducted in accordance with World Medical Association Declaration of Helsinki revised in 2013. Informed consent was obtained from each participate′ guardian.

① These 7 PS children aged from 6 to 14 years old, including 4 boys and 3 girls, were involved in unilateral PS. ②Only one case (children No.1) had an abnormality suggested by X-ray orthopantomogram of the coarse articular surface of iliac crest. ② CT images showed sacroiliac joint space narrowing in 2 cases (children No.1, 7) and articular surface destruction in 6 cases (children No.1-3, 5-7), 5 (children No. 1-3, 5, 6) of which were located in ilium surface. Euphotic zone under articular surface of iliac was seen in 3 cases (children No.1, 2, 5). In 3 cases (children No. 1, 2, 7), there was soft tissue swelling around sacroiliac joint. One (child No.2) of the 2 cases (children No.1, 2 ) performed enhanced CT showed abscess formation. ④The results of MRI showed that no definite sacroiliac joint stenosis was found in 7 children, and high signal shadow of fat-suppression protein density weighted imaging(FS PDWI) in sacroiliac joint space were seen in 6 children (children No.2-7), and 5 cases (children No.1, 2, 5-7) showed articular surface of sacroiliac joint destruction. Among 6 children (children No.1, 2, 4-7) with bone marrow involvement in bilateral articular surfaces of sacroiliac joint, only one child (child No.7) was located on sacrum side, and the rest were located on ilium side. Soft tissue of sacroiliac joint abscess occurred in 4 cases (children No.2, 5-7). FS PDWI of MRI showed that there was a round-like high signal focus in the diffuse high signal area, with clear edge and partial wall structure. After enhancement, annular enhancement of MRI could be seen in T1WI images, and 4 cases (children No.1, 2, 5, 6) showed signs of lymph node enlargement around sacroiliac joint.

The clinical signs of PS in children lack of specificity and can be easily misdiagnosed. MRI has high sensitivity and specificity to auxiliary diagnosis of PS, it can fully evaluate sacroiliac joint inflammation, bone marrow edema under the sacroiliac joint and soft tissue swelling of sacroiliac. It has high detection rate of abscess and it′s the first choice for early diagnosis of this disease, which is better than X-ray and CT. CT can show the bone changes in progressive stage of lesions clearly (including articular surface bone destruction and joint space stenosis), which is helpful to judge prognosis of children, and is an important complementary imaging method for MRI examination.