Endoluminal ultrasound is usually performed with the patient in the left lateral decubitus position. Before the probe is inserted into the anus, a digital rectal examination should be performed. If there is an anal stenosis, the finger can check to determine whether it will allow easy passage of the probe. A gel-containing condom is placed over the probe, and a thin layer of water-soluble lubricant is placed on the exterior of the condom. Any air interface will cause a major interference pattern. The probe is then ready for insertion through the anal canal. The patient should be instructed before the examination that no pain should be experienced. Under no circumstances should force be used to advance the probe.
When the spigot for introducing water into the plastic cone is pointing toward the ceiling, by convention, the anterior aspect of the rectum will be superior (12 o'clock) on the screen, right lateral will be left (9 o'clock) on the screen, left lateral will be right (3 o'clock) on the screen and posterior will be inferior (6 o'clock) on the screen (just as in the image on axial CT scan). Some adjustments may have to be made in the gain of the ultrasound unit to provide optimal imaging. It is always possible to perfectly depict all layers of the anal canal circumferentially. This is very important when assessing the canal at different levels. At the origin of the anal canal, the "U"- shaped sling of the puborectalis is the main landmark and should be used for final adjustment . Once the entire canal down to the anal margin has been evaluated, the probe is removed. It is not uncommon to require several passes along the full length of the anus to gain all the information necessary. In some instances, two to six passes may be required to properly evaluate a complex perianal sepsis.
Fig. 8. Normal ultrasonographic six-layer structure of the mid anal canal in a male. Axial image (a), schematic representation (b)
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