Objective
The objective of the study was to describe a framework for visualizing the perineal body’s complex anatomy using thin-slice magnetic resonance (MR) imaging.
Study Design
Two millimeter thick MR images were acquired in 11 women with normal pelvic support and no incontinence/prolapse symptoms. Anatomic structures were analyzed in axial, sagittal, and coronal slices. Three-dimensional (3-D) models were generated from these images.
Results
Three distinct perineal body regions are visible on MR imaging: (1) a superficial region at the level of the vestibular bulb, (2) a midregion at the proximal end of the superficial transverse perineal muscle, and (3) a deep region at the level of the midurethra and puborectalis muscle. Structures are best visualized on axial scans, whereas craniocaudal relationships are appreciated on sagittal scans. The 3-D model further clarifies interrelationships.
Conclusion
Advances in MR technology allow visualization of perineal body anatomy in living women and development of 3-D models that enhance our understanding of its 3 different regions: superficial, mid, and deep.
The perineal body is an anatomical structure encountered daily by obstetricians and gynecologists. Known to some as the anchor of the pelvis, its components have long been debated in the literature in cadaver dissections and histological studies. Like the word shoulder, which describes a distinct region of the body recognizable by most, the words perineal body would cause most to point to the region between the vagina and anus. But the question remains, what structures are involved in that region? Is there a distinct, identifiable structure called the perineal body, or is it made up of its component pieces all traversing or inserting into this region?
Understanding the relationships of its components is crucial for successful obstetrical laceration and fistula repairs in addition to other procedures such as perineocele repair. In this study, we sought to utilize advanced thin-slice magnetic resonance imaging (MRI) of living women to identify structures within the perineal body region, define their 3-dimensional (3-D) location, and provide a framework for visualizing this region’s complex anatomy.
Materials and Methods
Thin-slice MRI scans were performed on 11 women (all controls) in an ongoing University of Michigan Institutional Review Board–approved (#1999-0395) case-control study of pelvic organ prolapse. Women in the control group were asymptomatic based on Pelvic Floor Distress Inventory (PFDI) and Pelvic Floor Impact Questionnaires (PFIQ), had negative full bladder stress tests, had not had previous surgery for pelvic floor disorders, and had Pelvic Organ Prolapse Quantification system (POP-Q) points at least 1 cm above the hymenal ring. Women were recruited by newspaper advertisements and matched for age, race, and parity with study subjects.
Each woman underwent supine MRI at rest using a 3 Telsa Philips Achieva scanner (Philips Medical Systems, Best, The Netherlands) with a 6-channel phased array coil. Turbo spin echo techniques were used to image the sagittal, coronal, and axial planes. Thirty images were obtained in each plane (repetition time range, 2300–3000, echo time 30, 0.2 mm gap for 2 mm slices, and 1 mm gap for 5 mm slices, number of signal averages 2, 256 × 255). To maximize visibility of small perineal body structures, 2 mm thick multiplanar proton density MRIs were acquired in axial and coronal planes.
Utilizing the description of the female perineal body location by Oh and Kark, individual structures were identified by the region bounded anteriorly by the posterior vaginal wall and posteriorly by the anterior anal wall on axial, sagittal, and coronal scans. The longitudinal extent of the perineal body region was defined as extending from the superficial transverse perineal muscle and external anal sphincter to the fusion of the longitudinal muscle of the rectum with the internal anal sphincter, which occurs at the caudal extent of the rectovaginal space.
The structures were consistently identified by 2 different observers with oversight by the senior author. Images were reviewed and a representative subject selected for the clarity of her anatomy from whom to generate a computer model of this region using 3-D Slicer program (version 2.1b1; Brigham and Women’s Hospital, Boston, MA). The original 2 mm axial and coronal Digital Imaging and Communications in Medicine images were aligned, ensuring that structures colocalized in these axes by simultaneous review of scan planes in the viewer.
A 3-D model was made of the pelvic bones, bladder, urethra, vagina, rectum, and perineal body structures by tracing the structure outlines on axial images and creating 3-D models from these outlines ( Figure 1 ). Each structure was validated by overlaying the model with original source images in orthogonal planes. Although only 1 model was made for demonstration purposes, observations were based on all 11 subjects.
Names conforming to Terminologia Anatomica were used except for the levator ani muscles in which we have chosen the term pubovisceral muscle rather than pubococcygeal muscle to more accurately reflect its insertions. As described by Kearney et al, the components of the pubovisceral muscle are the same as those referenced in the Terminologia Anatomica for the pubococcygeal muscle: the pubovaginalis, puboperinealis, and puboanalis.
Results
The mean age of the 11 study participants was 61 ± 10 years (SD), mean body mass index (BMI) was 24.8 ± 4.7 kg/m 2 , median parity was 2, and 91% were white. POP-Q points are shown along with the demographics in Table 1 . No subject reported having a significant obstetrical laceration with their deliveries. No subject had undergone a hysterectomy or pelvic organ prolapse surgery. None had pelvic floor dysfunction symptoms as determined by responses to validated questionnaires (PFDI and PFIQ).
