Semi-automated vectorial analysis of anorectal motion by magnetic resonance defecography in healthy subjects and fecal incontinence

Abstract

Background: Inter-observer variability limits the reproducibility of pelvic floor motion measured by magnetic resonance imaging (MRI). Our aim was to develop a semi-automated program measuring pelvic floor motion in a reproducible and refined manner.

Methods: Pelvic floor anatomy and motion during voluntary contraction (squeeze) and rectal evacuation were assessed by MRI in 64 women with fecal incontinence (FI) and 64 age-matched controls. A radiologist measured anorectal angles and anorectal junction motion. A semi-automated program did the same and also dissected anorectal motion into perpendicular vectors representing the puborectalis and other pelvic floor muscles, assessed the pubococcygeal angle, and evaluated pelvic rotation.

Key results: Manual and semi-automated measurements of anorectal junction motion (r = 0.70; P < 0.0001) during squeeze and evacuation were correlated, as were anorectal angles at rest, squeeze, and evacuation; angle change during squeeze or evacuation was less so. Semi-automated measurements of anorectal and pelvic bony motion were also reproducible within subjects. During squeeze, puborectalis injury was associated (P ≤ 0.01) with smaller puborectalis but not pelvic floor motion vectors, reflecting impaired puborectalis function. The pubococcygeal angle, reflecting posterior pelvic floor motion, was smaller during squeeze and larger during evacuation. However, pubococcygeal angles and pelvic rotation during squeeze and evacuation did not differ significantly between FI and controls.

Conclusion & inferences: This semi-automated program provides a reproducible, efficient, and refined analysis of pelvic floor motion by MRI. Puborectalis injury is independently associated with impaired motion of puborectalis, not other pelvic floor muscles in controls and women with FI.

Figure 1. User interface for semi-automated measurement program

After a series of DICOM images are loaded, any image from that series can be selected for display in the window. In this example, the landmarks have been demarcated by an observer (circles). Any measurement can be displayed by selecting tabs on the left side of screen. Here, the anorectal angle at rest is shown.

Figure 2. MRI images at rest, during squeeze, and evacuation in a healthy subject

Anorectal motion was assessed manually (i.e., by measuring anorectal angles, panels A-C) and by the semi-automated program (panels D-F). Panel E depicts the anorectal motion vector, which connects the anorectal junction at rest and squeeze (white arrow). This vector is decomposed into the puborectalis (white dotted line) and pelvic floor (white solid line) components. Panel F depicts the anorectal motion vector from rest to evacuation (white arrow) and its vector components. Also observe that the pubococcygeal angle decreased from 11° at rest to 8° during squeeze (panels D-E).

Figure 3. MRI images at rest and during squeeze in a patient with fecal incontinence and puborectalis injury

The anorectal motion vector from rest to squeeze (white arrow) is decomposed into puborectalis motion vectors (white dotted line), which is very short, and the longer “other pelvic floor muscle” vector. In this example, the “other pelvic floor muscle” vector constitutes the main component of the anorectal motion vector. In contrast to the asymptomatic subject, the pubococcygeal angle increased from 33° at rest to 40° during squeeze. The anococcygeal ligament is indicated by arrows.

Figure 4. Relationship between anorectal location at rest and motion during evacuation (left panel) and squeeze (right panel)

When the anorectal junction was higher at rest (i.e., closer to pubococcygeal line), the pelvic floor motion vector was longer during evacuation. In contrast, during squeeze, the puborectalis motion vector was longer when the anorectal junction was lower (i.e., further from the pubococcygeal line) at rest.

Figure 5. Assessment of pelvic rotation by MRI

Left panel shows anteroposterior conjugate (1), obstetric conjugate (2), anteroposterior outlet (3), and pubococcygeal line (4) at rest. During squeeze (right panel), observe pelvic rotation as measured by the angle between anteroposterior conjugate and anteroposterior outlet diameters at rest (dashed line) and (solid line) in a control subject.