The axial plane allows for visualization of the internal and external anal sphincters. The anal mucosa is visualized at the 6 o’clock position, with the internal anal sphincter creating a hypoechoic ring around the anus. The external anal sphincter can be seen as the more hyperechoic structure surrounding the internal anal sphincter (Fig. 8.3). In 360° imaging with a BK 2052 or 8838 probe (BK Ultrasound, Analogic, Peabody, MA, USA), this plane also allows for the evaluation of the levator ani subdivisions (Figs. 8.4, 8.5, and 8.6), which is discussed further in Chap. 5. In posterior 3D imaging, important information about the levator plate anatomy can be obtained. The levator plate is a structure with contribution from the puborectalis and the pubovisceralis (pubococcygeus/iliococcygeus). The integrity of these muscles determines the direction of movement for the levator plate (Fig. 8.7).

The midsagittal plane gives a longitudinal view of the perineal body, external anal sphincter (main and subcutaneous sections), internal anal sphincter, and RVS (Figs. 8.8, 8.9, and 8.10). The levator plate can also be measured in this plane, allowing for dynamic imaging of this structure both at rest and with Valsalva or squeeze (Figs. 8.11, 8.12, and 8.13). The ARA can also be measured (Fig. 8.14), which may be important in the evaluation of pelvic floor dyssynergy [5, 12].

The coronal plane allows the visualization and measurement of both the internal and external anal sphincter thickness (Fig. 8.15). This view also allows visualization of the intricate LAM anatomy that creates a collection funnel to allow for passage of bowel contents.

Ultrasound imaging of the posterior compartment always starts with transperineal 2D imaging (Fig. 8.16). This is performed with the patient in the dorsal lithotomy position, with the hips flexed and abducted. A convex transducer is positioned on the perineum between the mons pubis and anal margin (Fig. 8.17). We use a BK Curved Array 3D convex frequency 4.3–6 MHz probe with a focal range of 6–114 mm (Fig. 8.18). However, it is possible to use any curved-array transducer with frequencies of 3.5–8 MHz [13, 14]. These probes are readily available in any urologist’s or gynecologist’s office and offer a good introduction to dynamic pelvic floor imaging. Depending on the setting of the machine, the image may be upside down on the machine, and most machines have the capability to display the image as if the patient is standing with the left sagittal plane in view (see Fig. 8.18). The transducer is covered with ultrasound gel and covered with a glove, a plastic wrap, or a cover. The pubic symphysis will be on the bottom right of the screen, and the levator plate should be visualized on the lower left. During the examination, dynamic imaging is obtained by asking the patient to perform a Valsalva maneuver or Kegel (Fig. 8.19) [15]. It is important when performing TPUS to avoid excessive pressure on the perineum or inappropriate angle of the transducer (and thus the ultrasound beam) to the anal canal.