Fluoroscopy retains central role in defecation studies

Dynamic imaging has become an integral part of pre- and postoperative examinations for pelvic floor dysfunction. Such assessments, which show the precise effect of anatomical trauma or surgery on neighboring structures, can reveal the underlying cause of numerous excretory disorders.

Functional cine MR imaging is emerging as a viable tool for diagnoses of pelvic organ prolapse. But traditional imaging approaches can still play a valuable role in exploring defecation difficulties, according to Prof. Clive Bartram, clinical director and consultant radiologist at St. Mark's Hospital in Harrow, U.K.

During a special focus session at the European Congress of Radiology 2004, Bartram outlined how basic voiding proctography can be used to determine the significance of anal-rectal morphology. Patients drink a dilute barium suspension 30 minutes before imaging to opacify the small bowel, aiding the hunt for enteroceles. The subject's rectum is then filled with approximately 120 mL barium paste to simulate fecal matter. Evacuation into a commode takes place behind a screen, and excretory function is recorded on spot films (one film per second) using a low-dose protocol with added filtration.

Complete assessment should include observation of the before, during, and after stages. Any opening of the anal canal prior to defecation indicates sphincter weakness. If the pelvic floor descends more than 3 cm as evacuation starts, this is also abnormal, he said.

Failure to evacuate rapidly, along with incomplete rectal emptying, is another finding. This condition can be caused by anterior rectoceles, which are especially common following childbirth. The presence of a rectocele becomes problematic if it prolapses through the vaginal wall to such an extent that patients must press the posterior vaginal wall to release trapped feces.

Fluoroscopic diagnosis of intussusception requires good understanding of how the rectum collapses, Bartram said. Lateral views may suggest longitudinal collapse through the tubelike structure, in common with esophageal collapse. An anteroposterior view is often needed to reveal how the rectum actually folds over on itself.

One method of diagnosing intussusception from lateral views alone involves comparing the internal diameter of the distal part of the rectum, as it enters the anal canal, with the canal width. A large increase in the ratio of these measurements, due to the rectum wall entering twice in a double fold, indicates intussusception.

"This is a useful criterion to think about, but it is also helpful sometimes just to turn the patient around to see the anal canal fold more clearly," he said. "When in doubt, I would go for the AP view. When the intra-anal intussusception externalizes, then you have got rectal prolapse."

Opacification of the bladder and vaginal areas permits a more extensive assessment of the pelvic floor area, including detection of cystoceles. Alternate filling and emptying of different areas in the pelvic cavity can then show how individual abnormalities interact. Because space is limited, presence of a rectocele or cystocele, for example, may prevent an enterocele from developing.

This procedure requires that patients be catheterized to receive barium contrast to the bladder, however. Views of the pelvic floor are also indirect, Bartram said. While fluoroscopy remains a cost-effective tool for examining anal-rectal dysfunction, radiologists seeking a wider view of the pelvic area should consider using MRI.

Defecation is a complex physiological event, and it is important not to isolate the pelvic floor when considering functional abnormalities, he said.

"Pelvic floor disorders are more common than previously anticipated," said Dr. Jaap Stoker, senior radiologist in abdominal imaging at the Academic Medical Center, University of Amsterdam. "People weren't aware of these problems because they didn't talk about them with their neighbors or did not think anything would help. But in the last few years, there have been more treatment possibilities."

MRI provides an accurate means of determining the extent of pelvic floor dysfunction and associated pelvic floor prolapse. Functional cine MRI is now well accepted as a method for visualizing the structures responsible for organ descent and prolapse, detecting enteroceles, and following patients after surgery.

Effective imaging with functional cine MRI requires a high-field magnet, fast gradient-echo sequences, midsagittal slice orientation, and a stack of slices or repeated measurements in the same slice positions while the patient is in a supine or sitting position and at rest or straining.

Both endosonography and endo-anal MRI achieve high spatial resolution. MRI is the modality of choice in the assessment of atrophy or tears involving the external sphincter, and endo-anal ultrasound is often preferred in the evaluation of internal sphincter abnormalities. For patients who are candidates for surgical repair of a tear in the sphincter, endo-anal MRI is performed to exclude the presence of atrophy, a negative prognostic factor.

"Radiologists need to become more familiar with the normal pelvic anatomy as it relates to endo-anal imaging of sphincter lesions and pelvic floor prolapse, which gets little attention in medical school and radiology training and is relatively complex," Stoker said.