Objective To explore the value of water -based material decomposition images for detection of bone marrow edema (BME)in sacroiliitis with energy spectral CT.Methods The sacroiliac (SI)joints of nine spondyloarthropathy patients with sacroiliitis (the research group)and eight healthy volunteers (the control group) were underwent MRI and energy spectral CT.The mixed energy image of energy spectral CT was reconstructed to be mono energy image.Then,the mono image was divided into water -based image and calcium image by material divid-ed and analyzed software.The SI para -articular marrow region water -calcium relative concentration of research group was compared with that of control group.The BME diagnosis efficiency and optimal water -based concentration of energy spectral CT was analyzed by receiver operating characteristic (ROC)curve.Then,the sensitivity,specificity, positive likelihood ratio and negative likelihood ration were calculated.Results The water -based concentration of BME in research group (1 067.43 ±10.84)g/L was higher than that of control group (1 039.43 ±8.01)g/L(t =-3.14,P =0.003).Meanwhile,the calcium -based concentration of research group (68.98 ±20.53)g/L was not different from control group (78.03 ±26.39)g/L(t =1.88,P =0.066).ROC curve showed that the diagnosis efficiency was medium as the area of under curve was 0.75.When the optimal concentration of water -based was 1 052.00g/L,the diagnosis efficiency was the best.The sensitivity and specificity was 84.00%,62.50% respectively. Conclusion There are reference value and potential clinical value with energy spectral CT water -based concentra-tion detection for diagnosis BME in patients with sacroiliitis.