The Effect of Oral Laxatives on Rectal Distension and Image Quality in Magnetic Resonance Imaging of the Prostate

Abstract

Introduction Increasing rectal size is associated with increased artefacts on magnetic resonance imaging (MRI) of the prostate and has the potential to degrade image quality. The objective of this study was to analyse the effect of oral laxative medication on rectal distension and image quality in prostate MRI. Methods Eighty patients prospectively received either 15 mg of oral senna (laxative group) or no medication (control group). Patients underwent prostate MRI according to standard local protocol and seven rectal dimensions on axial and sagittal images were measured. A subjective assessment of rectal distension was also made using a five-point Likert scale. Finally, artefacts on diffusion-weighted sequences were assessed using a four-point Likert scale. Results There was a small reduction in rectal diameter on sagittal images in the laxative group compared to the control group, with mean diameters of 27.1 mm and 30.0 mm respectively, p=0.02. There was no significant difference in rectal measurements of anteroposterior diameter, transverse diameter, or rectal circumference on axial imaging. Subjective scoring also demonstrated no significant difference in diffusion-weighted imaging quality between the laxative group and control group, p=0.82. Conclusion Bowel preparation with the oral laxative, senna, provided only a marginal decrease in rectal distension on one measure and no reduction in artefacts on diffusion-weighted sequences. The findings of this study do not support the routine use of this medication in patients undergoing prostate MRI.

Keywords: artefact; bowel preparation; laxatives; magnetic resonance imaging; prostate neoplasms.

Figure 1. Axial T2 image (A) demonstrating the measurement of the anteroposterior diameter, transverse diameter, and area of the rectum on the slice with maximal prostatic dimensions. Sagittal T2 image (B) demonstrates the measurement of maximal sagittal rectal diameter perpendicular to the rectal wall.

Figure 2. Axial DWI images demonstrate none (A), mild (B), and moderate (C) artefact secondary to rectal gas.

DWI: diffusion-weighted imaging

Figure 3. The rectal measurements in the laxative group (dark grey) and control group (light grey) are illustrated. The anteroposterior and transverse diameters are shown for the two axial slices analysed. The significant reduction in maximal diameter measured on the selected sagittal slice is also illustrated.