Comparison of changes in rectal area and volume during MR evacuation proctography in healthy and constipated adults

Abstract

Background: During proctography, rectal emptying is visually estimated by the reduction in rectal area. The correlation between changes in rectal area, which is a surrogate measure of volume, is unclear. Our aims were to compare the change in rectal area and volume during magnetic resonance (MR) proctography and to compare these parameters with rectal balloon expulsion time (BET).

Methods: In 49 healthy and 46 constipated participants, we measured BET and rectal area and volume with a software program before and after participants expelled rectal gel during proctography.

Key results: All participants completed both tests; six healthy and 17 constipated patients had a prolonged (>60 seconds) BET. During evacuation, the reduction in rectal area and volume was lower in participants with an abnormal than a normal BET (P < 0.01). The reduction in rectal area and volume were strongly correlated (r = 0.93, P < 0.001) and equivalent for identifying participants with abnormal BET. Among participants with less evacuation, the reduction in rectal area underestimated the reduction in rectal volume. A rectocele larger than 2 cm was observed in eight of 18 (44%) participants in whom the difference between change in volume and area was ˃10% but only 14 of 77 (18%) participants in whom the difference was ≤10% (P = 0.03).

Conclusions: Measured with MR proctography, the rectal area is reasonably accurate for quantifying rectal emptying and equivalent to rectal volume for distinguishing between normal and abnormal BET. When evacuation is reduced, the change in rectal area may underestimate the change in rectal volume.

Keywords: anorectum; constipation; defecatory disorders; defecography; pelvic floor dysfunction.

Figure 1.. Assessment of rectal area and volume with MR proctography.

Panels A and B, midsagittal sections. Panels C and D, 3D volumetric rendered images of the rectum before and after evacuation in the same participant. Panel E shows that rectal area and rectal volume were equivalent for distinguishing between participants with normal and abnormal BET. Panel F depicts the strong correlation between the change in rectal area and volume during evacuation. Panel G shows the Bland Altman plot comparing the change in rectal area and volume. Solid red lines represent 95% limits of agreement for participants with normal BET, dotted red lines represent 95% limits of agreement for participants with abnormal BET. While most data points are within 2 SD of the mean difference between the (change in) rectal area and volume, the difference between measurements is inversely correlated with the mean change.