Feasibility of US-CT image fusion to identify the sources of abnormal vascularization in posterior sacroiliac joints of ankylosing spondylitis patients

Ultrasound (US) can be used to evaluate the inflammatory activity of the sacroiliac joints (SIJs) in ankylosing spondylitis (AS) patients, but to precisely locate the abnormal vascularization observed on color Doppler US (CDUS) was difficult. To address this issue, we performed US and computed tomography (CT) fusion imaging of SIJs with 84 inpatients and 30 controls, and then assessed the sources of abnormal vascularization in the posterior SIJs of AS patients based on the fused images. Several possible factors impacting the fusion process were considered including the lesion classes of SIJ, the skinfold thickness of the sacral region and the cross-sectional levels of the first, second and third posterior sacral foramina. Our data showed high image fusion success rates at the 3 levels in the AS group (97.0%, 87.5% and 79.8%, respectively) and the control group (96.7%, 86.7%, and 86.7%, respectively).The skinfold thickness was identified as the main factor affecting the success rates. The successfully fused images revealed significant differences in the distribution of abnormal vascularization between 3 levels, as detected via CDUS (P = 0.011), which suggested that inflammation occurred in distinct tissues at different levels of the SIJ (intraligamentous inflammation in Regions 1 and 2; intracapsular inflammation in Region 3).