Appearance of the levator ani muscle subdivisions in magnetic resonance images

Abstract

Objective: Identify and describe the separate appearance of 5 levator ani muscle subdivisions seen in axial, coronal, and sagittal magnetic resonance imaging (MRI) scan planes.

Methods: Magnetic resonance scans of 80 nulliparous women with normal pelvic support were evaluated. Characteristic features of each Terminologia Anatomica-listed levator ani component were determined for each scan plane. Muscle component visibility was based on pre-established criteria in axial, coronal, and sagittal scan planes: 1) clear and consistently visible separation or 2) different origin or insertion. Visibility of each of the levator ani subdivisions in each scan plane was assessed in 25 nulliparous women.

Results: In the axial plane, the puborectal muscle can be seen lateral to the pubovisceral muscle and decussating dorsal to the rectum. The course of the puboperineal muscle near the perineal body is visualized in the axial plane. The coronal view is perpendicular to the fiber direction of the puborectal and pubovisceral muscles and shows them as "clusters" of muscle on either side of the vagina. The sagittal plane consistently demonstrates the puborectal muscle passing dorsal to the rectum to form a sling that can consistently be seen as a "bump." This plane is also parallel to the pubovisceral muscle fiber direction and shows the puboperineal muscle.

Conclusion: The subdivisions of the levator ani muscle are visible in MRI scans, each with distinct morphology and characteristic features.

Fig. 1

A. Three-dimensional model of levator ani subdivisions including the pubic bone and pelvic viscera. This model was created by using the magnetic resonance images shown in Figures 2, 3, and 4. The pubovaginal, puboperineal, and puboanal muscles are all combined into a single structure, the pubovisceral muscle. Inferior, left 3-quarter view. B. The same model without the pubic bone. PB, pubic bone; V, vagina; U, uterus; Ur, urethra; B, bladder; IC, iliococcygeus muscle; PR, puborectal muscle; PVi, pubovisceral muscle; EAS, external anal sphincter. © DeLancey 2006.

Fig. 2

Axial scan of 25-year-old nullipara showing subdivisions of the levator. Level of scan in centimeters relative to the arcuate pubic ligament (A) is indicated in lower left corner with positive numbers cranial to the ligament and negative numbers caudal. Additional abbreviations: PP, puboperineal muscle; PVa, pubovaginal attachment; PA, puboanal muscle; OI, obturator internus muscle; STP, superficial transverse perineal muscle; R, rectum. White arrows indicate puborectal muscle progression. © DeLancey 2006.

Fig. 3

Coronal scans of the same subject depicted in Figure 2. The level of the scan in centimeters relative to the arcuate pubic ligament is indicated in the lower left corner with negative numbers dorsal to the ligament. Note that scans −1.0 and −3.5 have been omitted so that other scans could be included. Additional abbreviations: VB, vestibular bulb; PM, perineal membrane. © DeLancey 2006.

Fig. 4

Sagittal images of the same subject from the right side of the pelvis. The level of the scan in centimeters to the right of the midsagittal plane is indicated in the lower left corner. Additional abbreviations: SQ-EAS, subcutaneous external anal sphincter. © DeLancey 2006.