Measuring the levator hiatus with axial MRI sequences: Adjusting the angle of acquisition

Abstract

We aimed to compare MRI measurements of the female levator hiatus and the most caudal "levator ani" muscles between image slices in a plane axial to the body (AxB) and an axial plane parallel to the direction of the puborectalis muscle (AxPRM). We performed high-resolution, T2-weighted axial magnetic resonance imaging (in two different planes) on 20 sequentially recruited asymptomatic nulliparous women. Levator hiatus measurements were compared between the two planes. The mean tilt angle of the AxPRM slices relative to the AxB slices was 18.9° (SD 8.5) and the area of the levator hiatus was 10% greater (P=0.04) in the AxPRM plane. By rotating the acquisition plane to be parallel to the puborectalis muscle (sometimes called the plane of minimal hiatal dimensions), some of the measurements of the distal and medial pelvic floor muscles (and the hiatus defined by them) are altered. This issue is important because both MRI and 3D ultrasound are increasingly being used to evaluate the pelvic floor hiatus, and its relationship to childbirth injury and pelvic floor disorders. To make meaningful comparisons, it is important that both modalities are measuring the same thing.