Colon Imaging: Conventional Imaging and Computed Tomography

Technical Aspects

Before cross-sectional imaging, the double-contrast enema was the foremost radiologic method for detection of colonic mucosal lesions and precancerous polyps. Diagnostic high-quality double-contrast barium enema examination is an art, requiring skillful maneuvering of the patient and barium pool while optimally using fluoroscopy. With the advent of computed tomography (CT), intramural as well as extraluminal extension of colonic diseases can be detected. This chapter discusses the principles and techniques for safe and accurate barium enema and review technical considerations for the performance of colorectal CT.

Barium Enema Examination

Single-contrast examination is preferred in immobile, elderly, or incontinent patients. The single-contrast examination is less sensitive than the double-contrast examination for detection of small polypoid lesions and evaluation of inflammatory bowel disease ( Figure 29-1 ). Double-contrast barium enema should be performed 1 week after a recent polypectomy, mucosal cautery, or large forceps biopsy to avoid the risk for colonic perforation.

Bowel Preparation.

Rigorous colonic cleansing is necessary for an optimal barium enema examination. Several colon cleansing regimens have been described.

A standard bowel preparation is outlined in Box 29-1 . Contraindications for the standard bowel preparation include bedridden patients, postoperative patients, patients with diabetes and hypothyroidism, and patients taking opiates.

Box 29-1

Suggested Outline for Bowel Preparation

On the day before the study:

• 1.

  Twenty-four hours of clear liquids for breakfast, lunch, and dinner

• 2.

  Plenty of liquids or water between meals to maintain hydration

• 3.

  Sixteen ounces of magnesium citrate at 5 pm

• 4.

  Four bisacodyl tablets at 8 pm

On the day of the examination: Nothing by mouth

Procedure.

Before starting the study, compliance with the bowel preparation should be confirmed. A preliminary abdominal radiograph can assess bowel preparation adequacy. The procedure may need to be rescheduled if there is significant colonic residue.

Rectal Catheter Insertion.

The patient lies in a recumbent, left side down position. Lubricant is applied on the external anal surface and the rectal catheter tip, and the catheter is gently inserted into the anal canal. Balloon insufflation is performed only in patients who are leaking barium or air out of the anal canal.

Technique Tips

• Rectal balloon inflation should be performed only after outlining the rectum with barium.

• Contraindications for inflating rectal balloons include suspected colitis, a history of pelvic radiation or colitis, suspected rectovaginal fistula, and Crohn's disease.

Barium Instillation

The patient is placed prone and barium is slowly instilled. Once a full column has reached the descending colon, fully open the tube. The radiologist turns the patient 360 degrees to coat the colon. The patient is turned prone to advance the barium into the mid–transverse colon. The distal rectum is then drained by dropping the barium bag to the floor to avoid bubbles when air is insufflated.

Air Insufflation

Air is used to aid barium across the transverse colon into the ascending colon. The patient is turned right side down to fill the proximal transverse colon and then turned into the supine position to fill the posteriorly located hepatic flexure.

The enema tip can then be removed, providing physical relief to the patient and allowing better evaluation of the distal rectum. The enema tip may need to be left in place in patients expelling gas and who may need additional air to visualize the terminal ileum (e.g., patients with Crohn's disease).

Radiographic Documentation: Spot Films and Overhead Views

Complementary radiologic projections evaluate the colon. Twelve to 14 spot films and 4 to 5 overhead films are obtained. A routine sequence obtained in a double-contrast barium enema study is suggested in Table 29-1 ( Figure 29-2 ).

Some technical challenges during a double-contrast barium enema examination and plausible solutions are summarized in Box 29-2 .

Water-Soluble Contrast Enema

In patients with suspected colonic perforation or in postoperative patients, a water-soluble contrast agent can be used to look for fistulas or perforations ( Figure 29-3 ).

In cystic fibrosis patients, a hyperosmolar water-soluble molar contrast agent can liquefy the viscous stool by drawing fluid into the bowel.

Colostomy Enema

Double-contrast examinations can be performed through a colostomy.

Pros and Cons

The pros and cons of a barium enema study are outlined in Table 29-2 .

Complications

The most serious complication is colonic perforation, which can be both intraperitoneal and extraperitoneal and is usually seen in a diseased rectum secondary to inflation of rectal balloon. Air insufflations can cause abdominal discomfort and can be minimized by the use of carbon dioxide. Allergic reactions may be observed to barium, glucagon, or latex in gloves and rectal catheters. Transient bacteremia and septicemia are very rare complications of barium enema. Barium impaction can cause delayed complications with varying degrees of constipation and/or abdominal pain. Venous intravasation may result from barium breaching the colonic mucosa and is a feared complication because of its high mortality rate.

Controversies

The continued use of the double-contrast barium enema for detecting colorectal polyps is controversial. Although the procedure is reimbursed by Medicare for colon screening, two large prospective clinical trials show the sensitivity of double-contrast barium enema examinations is approximately 50% for polyps measuring at least 10 mm in diameter.

Normal Anatomy

The goal of the double-contrast barium enema is uniform coating of the mucosal surface with barium without formation of barium pools, which can obscure pathologic processes. It is important to be well versed with normal appearances and variations on a double-contrast study.