Magnetic resonance imaging of the rectum during distension

PURPOSE: A knowledge of the relationships between the rectum and its surrounding structures during distention may improve our understanding of the results of studies assessing rectal sensory-motor responses to distention. This magnetic resonance imaging study was designed to assess the shape of the rectum and the degree of distention at which the surrounding structures are compressed. METHODS: Nine healthy patients underwent magnetic resonance imaging of the rectum under resting conditions and after the inflation of a plastic bag to volumes of 50, 100, 150, 200, and 250 ml. The thickness of the rectovesical space was assessed as a measure of the compression of the perirectal structures, and the perception of sensations were recorded. RESULTS: The shape of the rectum changed from being quasicylindrical at distention volumes of <100 ml to bean-shaped at larger volumes. The thickness of the rectovesical space at a distention volume of 50 ml was the same as when the bag was not inflated, but it progressively decreased until the difference became statistically significant at distention volumes of ≥200 ml, corresponding to a mean ± standard deviation rectal radius of 2.66 ± 0.37 cm. Statistically significant compression of the rectovesical space was recorded when the sensations of gas, desire to defecate, and urgency were perceived. CONCLUSIONS: The shape of the rectum changes during distention; it significantly compresses the extrarectal structures in the tested range of distention that induces nonpainful sensations. Magnetic resonance imaging is a useful means of assessing the morphologic changes in the rectum during distention.