Mono-exponential and bi-exponential model-based diffusion-weighted MR imaging and IDEAL-IQ sequence for quantitative evaluation of sacroiliitis in patients with ankylosing spondylitis

To evaluate the utility of mono-exponential and bi-exponential model-based diffusion-weighted MR imaging and IDEAL-IQ sequence for differentiating the activity of sacroiliitis in ankylosing spondylitis (AS). AS patients were divided into active group (n = 30) and inactive group (n = 28) according to Ankylosing Spondylitis Disease Activity Score (ASDAS) with C-reactive protein (CRP). In addition, 30 healthy volunteers were chosen as healthy group. Subjects were scanned by conventional MRI, diffusion-weighted imaging, and IDEAL-IQ sequence. Apparent diffusion coefficient (ADC), true diffusion coefficient (D), pseudodiffusion coefficient (D*), perfusion fraction (f), and fat fraction (FF) values were measured, and their relative values (rADC, rD, rD*, rf) were calculated by the formula ADC (D,D*,f)_lesion/ADC (D,D*,f)_reference, respectively. The ADC, D, rADC, and rD of active group were the highest among the three groups, followed by inactive and healthy group. However, D* and rD* showed no significant difference among the three groups. FF was significantly higher in inactive group than in healthy and active group. ADC and D had significantly higher AUCs than f for differentiating active group from healthy group, while FF had the highest AUC for distinguishing inactive sacroiliitis from healthy group. DWI and IDEAL-IQ imaging are helpful in quantitatively assessing the activity of sacroiliitis in AS patients.