Three-dimensional biomechanical properties of the human rectum evaluated with magnetic resonance imaging

A method to evaluate the three-dimensional (3-D) geometry of the human gastrointestinal wall may be valuable for understanding tissue biomechanics, mechano-sensation and function. In this paper we present a magnetic resonance imaging (MRI) based method to determine rectal geometry and validation of data obtained in three volunteers. A specially designed rectal bag was filled in a stepwise manner while MRI and bag pressure were recorded. 3-D models of curvatures, radii of curvature, tension and stress were generated and the circumferential and longitudinal strains were calculated. The computed bag volumes corresponded to the infused volumes. A pronounced bag elongation and decrease in wall thickness was observed during the bag filling. The spatial distributions of the biomechanical parameters were distinctly different between individuals and non-homogeneous throughout the rectal wall due to its complex geometry. The average tension and stress increased as a function of infused volume and circumferential strain. The present study provides a method for characterizing the complex in vivo 3-D geometry of the human rectum. The non-homogenous spatial curvature distribution suggests that simple estimates of tension based on pressure and volume do not reflect the true 3-D biomechanical properties of the rectum.     

Geometric and mechanosensory properties of the sigmoid colon evaluated with magnetic resonance imaging

The aim of this study was to use magnetic resonance imaging (MRI) to evaluate the three-dimensional geometry and mechanosensory properties of the sigmoid colon. The sigmoid colon was stepwise distended by a water-filled bag in eight subjects. Simultaneous MRI, bag pressure recording and sensory assessment were performed before and after smooth muscle relaxation with butylscopolamine. The surface distributions of principal curvature radii, wall thickness, tension, stress and circumferential strain were calculated. The geometry of the distended sigmoid colon was complex and the spatial distributions of the biomechanical parameters were non-homogeneous. The circumferential length, strain, pressure and wall stress increased as a function of bag volume (all P < 0.001). In response to butylscopolamine, the pressure and wall stress were reduced (P < 0.05) and the stress-strain curves were shifted to the right. The sensory response was a linear function of the biomechanical parameters (all P < 0.001) and decreased in response to butylscopolamine as a function of volume (P = 0.02). The stimulus-response data indicate that the mechanosensitive afferents are affected by smooth muscle tone. The present study provides a method for characterizing the complex geometry and mechanical properties of the sigmoid colon, including the role of smooth muscle tone. This may be valuable in understanding of the biomechanical and mechanosensory functions in colonic diseases.