Diffusion-weighted Imaging Distortion in Prostate MRI: A Cross-sectional Study Comparing Supine and Prone Positioning

Diffusion weighted imaging (DWI) is a key component of multiparametric (mp) prostate MRI. DWI using echo-planar techniques is susceptible to distortion at the recto-prostatic air-tissue interface. This study was to determine whether prone patient positioning reduces adjacent rectal air and DW image distortion when compared with standard-of-care supine positioning.

This prospective study included consecutive patients undergoing mpMRI for suspected PCa between 2023 and 2024. Prostate segmentation was performed on DW and contrast-enhanced images. DWI distortion was measured quantitatively. Qualitative image quality of DWI and T2-weighted imaging (T2WI) was evaluated using PI-QUAL version 2; a separate 5-point clinically based Likert scale was employed to evaluate the volume of rectal air adjacent to the prostate.

Fifty-two patients were enrolled. In total, 58% of patients expressed a preference for supine imaging versus 20% for prone imaging. Qualitative DWI image quality improved significantly in the prone position [median: 4 (3 to 4)] versus supine [3 (1 to 4)]; P < 0.001. In contrast, prone T2WI quality [1 (1 to 1)] was significantly inferior than supine T2WI [3 (3-4)]; P < 0.001. Quantitative measures of rectal air were significantly lower for prone [1.13 cm 3 (0.34-2.43)] compared with supine imaging [1.96 cm 3 (0.47 to 5.81); P = 0.005]. There was no significant difference in distortion between prone [3.21 mm (2.42 to 3.82) and supine [2.95 mm (2.25 to 4.21)] positioning across all patients ( P = 0.80); however, in patients with >4 cm 3 of supine rectal air (n = 19), distortion was significantly reduced by prone imaging [3.49 mm (2.84 to 4.03)] compared with supine [4.60 mm (3.17 to 5.95)]; P = 0.02. The mean additional scanning time for the necessary prone imaging was 8 minutes 18 seconds.

Prone positioning significantly reduces DWI distortion artefact when rectal air is present, but consistently results in degraded T2WI quality.