Detection of synovial inflammation in the sacroiliac joint space through intravoxel incoherent motion imaging: an alternative to contrast agents.

Purpose: We investigated the diagnostic accuracy of simplified intravoxel incoherent motion (IVIM) imaging for detecting synovial inflammation in the sacroiliac joint (SIJ) in a population with active sacroiliitis.

Methods: In accordance with the Assessment of Spondyloarthritis International Society criteria, 86 SIJs of 46 patients with active sacroiliitis were included in this retrospective study conducted between November 2020 and January 2022. Based on T1-weighted post-gadolinium images, the SIJs were divided into two groups: synovial inflammation positive (SIP) (n = 28) and synovial inflammation negative (SIN) (n = 58). Synovial areas in the SIJ space were independently and blindly reviewed for the presence of inflammation by two radiologists with differing levels of expertise in radiology. Using four b values, apparent diffusion coefficient (ADC)= ADC (0, 800) and the simplified 3T IVIM method parameters true diffusion coefficient (D₁)= ADC (50, 800), D= ADC (400, 800), f₁= f (0, 50, 800), f₂= f (0, 400, 800), pseudodiffusion coefficient (D*)= D* (0, 50, 400, 800), ADC_low = ADC (0, 50), and ADC_diff= ADC_low - D were generated voxel by voxel for each patient. The IVIM and ADC parameters at the SIN and SIP joints were compared.

Results: The D parameter was significantly increased in SIP areas (1.23 ± 0.34 × 10^-3 mm²/s) compared with SIN areas (1.02 ± 0.16 × 10^-3 mm²/s) (P = 0.004). Conversely, the D* parameter was significantly decreased in SIP areas (21.78 ± 3.77 × 10^-3 mm²/s) compared with SIN areas (16.19 ± 4.58 × 10^-3 mm²/s) (P < 0.001). When the optimal cut-off value of 1.11 × 10^-3 mm²/s was selected, the sensitivity for the D value was 71% and the specificity was 72% [area under the curve (AUC): 0.716)]. When the optimal cut-off value of 21.06 × 10^-3 mm²/s was selected, the sensitivity for the D* value was 78.6%, and the specificity was 79.3% (AUC: 0.829). The interclass correlation coefficient was excellent for f₁, f₂ D*, D, and ADC_diff, good for ADC_low and D₁, but reasonable for ADC.

Conclusion: The presence of synovial inflammation in the SIJ can be evaluated with high sensitivity and specificity using only four b values through the simplified IVIM method without the need for a contrast agent.

Clinical significance: IVIM imaging is a technique that allows us to gain insights into tissue perfusion without the administration of contrast agents, utilizing diffusion-weighted images. In this study, for the first time, we demonstrated the potential of detecting synovial inflammation in the SIJ using IVIM, specifically through the pseudodiffusion (D*) parameter, without the need for contrast agents.