ABSTRACT Objective To evaluate the effect of US-CT image fusion of the sacroiliac joints (SIJ) of ankylosing spondylitis(AS) patients to locate the active inflammatory activity as visualized by CDUS in these patients to confirm the sources of abnormal vascularization. The aim of this study is to provide an effective basis for confirming that Doppler abnormal blood flow signal is a reliable index to evaluate AS. Methods Based on preliminary methodology study, after successful US-CT image fusion of 84 patients at levels of 3 posterior sacral foramens, CDUS was performed to detect abnormal blood flow signals and to estimate their positions according to our studies of gross and sectional anatomy. The located abnormal signals were grouped into 3 regions according to the 3 levels. Results Among the successfully fused US-CT images of the SIJs in the patients group, CDUS revealed abnormal blood flow signals in 128 SIJs at level 1, 105 at level 2, and 61 at level 3. The interior of the SIJ and the area surrounding the SIJ were used to describe the location of these signals. The differences in abnormal blood flow signals among the 3 levels were significant (χ2 = 9.101, P = 0.011). Combined the fused US-CT images with anatomical study of SIJ, the locations of abnormal blood flow signals are initially confirmed: Almost all the cases at level 1 and level 2 were located at ligament part. However, the majority of signals at level 3 were located at synovial part. Only a few signals from level 2 and level were uncertain. The differences among the 3 levels were significant (χ2=174.607, P < 0.01). Conclusion This study demonstrated that US-CT fusion imaging can be used to locate abnormal blood flow signals in the SIJs visualized by CDUS. The signals at different levels indicate the occurrence of inflammation in different tissues of the posterior SIJ. US-CT imaging provides an effective basis for confirming that abnormal Doppler signal is a reliable index to evaluate AS and may be helpful in the diagnosis of AS patients at the early stage. However, several limitations still need to be resolved by fusion of US-MRI in the further studies.