ObjectiveTo evaluate the effect of ultrasound and CT fusion imaging in locating inflammatory blood flow signals in sacroiliac joint（SIJ）of ankylosing spondylitis patients

and to explore its clinical application value.MethodsUS-CT image fusion of SIJs were performed on 84 patients at levels of 3 posterior sacral foramens

color Doppler ultrasound（CDUS）was performed on the successful image fusion.Two pelvic specimens were dissected with SIJ system and sectional anatomy.According to their anatomical characteristics

the positions of low-impedance blood flow signals detected at 3 levels of AS patients were classified to determine their tissue origin.Results Among the successfully performed image fusion of the AS patients

CDUS revealed abnormal low-impedance blood flow signals were 128 SIJs at level 1

105 at level 2

and 61 at level 3

respectively.The position of inflammatory blood flow signals among the three levels was compared according to the distribution of localization in and around the joints

the differences were statistically significant（χ2=9.101

P=0.011）.Combined with the fusion image and anatomical study of SIJ

the locations of abnormal low-impedance blood flow signals were initially confirmed:almost all the cases at level 1 and level 2 were located at joint ligament part（128/128

98/105）.However

the majority of signals at level 3 were located at synovial part（39/61）.Only a few signals located at the junction of synovium and ligament at level 2（2）and level3（10）were cannot be accurately located in the fusion image.The inflammatory blood flow signals between the 3 levels were compared according to the location of the anatomical type

and the differences were statistically significant（χ2=174.607

P<0.01）.ConclusionUS-CT fusion imaging can be used to locate abnormal low-impedance blood flow signals in the SIJs accurately in the joint ligament or synovial region. The signals at different levels indicate the different types of inflammation. The lowimpedance blood flow signal is a reliable index to evaluate AS.