Geometric distortion in diffusion-weighted MR imaging of the prostate-contributing factors and strategies for improvement

Rationale and objectives: Image distortion on diffusion-weighted imaging (DWI) of the prostate in 3T endorectal magnetic resonance imaging (MRI) examinations is common. The aim of this study was to determine the degree of distortion on DWI using a state-of-the-art clinical protocol and to explore the main contributors to geometric distortion.

Materials and methods: Forty consecutive patients underwent 3T MRI of the prostate with an endorectal coil filled with air (n = 20) or barium sulfate (n = 20). Distortion was measured as the maximum displacement of the outer boundary of the prostate on DWI relative to T2-weighted imaging. The effects of phase-encoding direction, receiver bandwidth, and parallel imaging were then assessed in a prostate phantom on two MRI scanners from different manufacturers.

Results: There was no statistical difference in the mean displacement of the prostate on DWI between the air cohort (1.8 ± 1.2 mm, range 0-4.2 mm) and barium cohort (1.8 ± 2.2 mm, range 0-9 mm). Displacement of the prostate was observed in the phase-encoding direction. Phantom experiments demonstrated a horizontal displacement of 6.0 mm in the phase-encoding direction, which decreased with the use of parallel imaging and higher bandwidth. Geometric distortion was similar for all b values and across manufacturers.

Conclusions: Geometric distortion on DWI of the prostate is common in the phase-encoding direction and does not improve with inflating the coil with barium sulfate. Strategies to reduce this artifact include the use of higher bandwidth and accelerated imaging. Correction of this phenomenon should improve localization of prostate cancer, particularly important for targeted prostate biopsies or focal therapies.