How to Perform and Interpret a Virtual Colonoscopic Examination

Michael Macari and Abraham H. Dachman

In this chapter, we discuss computed tomography (CT) of the cleansed colon performed in a manner to detect polyps and masses. The use of CT colonography (CTC) in the partially prepared or unprepared colon is discussed in chapter 5.

Technical considerations critical to the successful performance and interpretation of CTC are reviewed. This chapter presents an overview of how to perform and interpret an examination and will touch on some of the controversies.

Patient Preparation and Data Acquisition

Accepted principles regarding acceptable CTC technique include adequate colonic cleansing, maximal colonic distension, and data acquisition in the supine and prone positions (Chen et al. 1999; Yee et al. 1999; Fenlon and Ferrucci 1997). While CTC is a relatively noninvasive imaging procedure, there are two aspects of the exam that may produce patient anxiety and potential discomfort. These include the need for bowel preparation and colonic insufflation. We stress that the colon needs to be thoroughly cleaned and properly distended to obtain an adequate examination.

As CTC technique evolves, there is a move toward standardizing techniques for performing this study. Some factors are less critical, although the best and most cost-effective alternatives are not clear. The first set of issues relate to the patient and include: the use of room air vs carbon dioxide (Dachman et al. 1998), the use of manual vs mechanical or even self-insufflation (Macari et al. 2000), the use of a routine hypotonic agent such as glucagon (Johnson and Dachman 2000), and the use of a plain catheter vs a balloon cuff catheter (Fletcher et al. 2000).

Bowel Cleansing

A more comprehensive discussion of bowel cleansing can be found in chapter 5. The minimum requirements are summarized below.

Bowel preparation is currently essential for the confident detection of lesions because residual fecal material may be indistinguishable from polyps or neo plasms, and fecal residue may obscure a polyp (Fletcher et al. 2000; Macari et al. 2001a). The radiologist should take an active role in ensuring that patients understand the importance of the preparation and what is expected of them.

There are two main bowel preparations available: cathartics such as magnesium citrate and oral phospho soda, and lavage solutions such as polyethylene glycol. In our experience, both magnesium citrate and phospho soda provide an acceptable bowel preparation. Radiologists should emphasize the need for bowel preparation and be familiar with the instructions that are provided with these commercial kits to better answer patient's questions. Magnesium citrate should not be used in patients with renal failure and phospho soda should not be used in patients with renal, cardiac, or hepatic insufficiency. We have found that the polyethylene glycol prepa-ration frequently leaves a large amount of residual fluid (Macari et al. 2001). While this preparation is adequate for colonoscopy, large amounts of residual fluid could obscure masses during CTC (whereas at conventional colonoscopy residual fluid can be aspirated out of the colon). Unlike a barium enema examination, in which different projections can be used to redistribute the fluid, in CTC the examination is usually limited to two projections, supine and prone (unless an extra view, such as a decubitus view, is obtained). In this setting, the preparation that provides the least amount of residual fluid will theoretically provide the greatest opportunity to detect polyps by enabling evaluation of the entire mucosal surface of the colon.

Getting Started

At New York University (NYU), the examination is performed entirely by a technologist or nurse. A radiologist is not on-site. Obviously, an experienced technologist or nurse is required, but after adequate training these individuals can perform the examination, minimizing the radiologist's time commitment. Conversely, at the University of Chicago all exams are performed by a radiologist. The patient is asked to evacuate the rectum immediately prior to the examination. Easy access to a nearby bathroom is essential. Some form of informed consent is used, either as required by an institutional review board or as good practice to document that this new procedure was properly explained to the patient. The exam in general takes 10 to 15 minutes of CT room time. The patient is placed on the CT table and at the radiologist's option a rectal exam may be performed. If the CT is part of a screening program offered by the radiology department without need for a referring clinician, we recommend that a digital rectal exam always be included because CTC cannot detect lesions in the anal canal.

Hypotonia

There is no objective evidence that hypotonia improves the quality of the exam (Yee et al. 1999a). After years of experience with the use of glucagon for bar ium enema, some radiologists believe that the added comfort is worth the expense, whereas others limit the use of glucagon to patients who experience severe cramping. When used, a 1.0-mg dose, injected intravenously over 30 seconds, is recommended. In the case of CTC, the use of glucagon has the added disadvantage of decreasing the competency of the ileocecal valve, allowing reflux of gas into the small bowel. As a result, particular attention must be paid to maximally insufflating the bowel for both the supine and prone views by adding more gas immediately prior to scanning.

Rectal Tube

Patients often have sensitive skin at the anus due to the colonic cleansing regime. Jelly, therefore, should be used to perform the rectal exam and insert the rectal tube. Too much jelly, however, may make the catheter tip too slippery. A red rubber catheter (which is smaller and may be more comfortable than a barium enema tip), a Foley catheter, or a plain barium enema tip can be used. If using a barium enema tip, barium enema tubing can be cut into 9-in strips and one end attached to the catheter tip and the other to a hand-held bulb ("blue puffer") for manual insufflation. Some investigators use a tip with a balloon cuff. The tip should be taped in place (butterfly style) to the buttock to minimize the likelihood of the tip dislodging later when the patient turns from the supine to the prone position.

Insufflation

For colonic insufflation, either room air or CO2 can be used. We utilize air because it is easy and inexpensive. Proponents of CO2 argue that it is readily absorbed from the colon and causes less cramping after the procedure in comparison to room air. While mild cramping may be a problem for some patients, most patients find the examination to be quick and not uncomfortable (Svens-son 2002).

Air should be inflated slowly and the patient encouraged to retain the air. We ask patients to let the technologist know when they are beginning to feel discomfort from bowel distension. In general, this signals that the colon is well distended. In general, approximately 40 puffs is sufficient to distend the colon. However, we do not use a set number of puffs because the length of the human colon is variable. Also, reflex of air via an incompetent ileocecal valve will result in the need for more insufflation. It is important to be aware of the stoic patient who will wiggle their toes in silence as you puff away!

Some researchers use a mechanical pump such as a lap-aroscopic insufflator. This pump can be connected to CO2 or compressed air. A commercial pump dedicated to CTC is also available. A set pressure setting is in general used.

Colonography Air Adequate Topogram

Figure 4.1. Adequate bowel distension. After insufflation of the colon, a scout topogram should be obtained. If the colon is distended as demonstrated here, then proceeding with data acquisition may proceed. If not, additional air insufflation is necessary. The arrow points to a thin catheter tip with no balloon permitting visualization of mucosa to the internal anal sphincter.

Figure 4.1. Adequate bowel distension. After insufflation of the colon, a scout topogram should be obtained. If the colon is distended as demonstrated here, then proceeding with data acquisition may proceed. If not, additional air insufflation is necessary. The arrow points to a thin catheter tip with no balloon permitting visualization of mucosa to the internal anal sphincter.

Performing the Scan

After insufflation, the catheter is left in the rectum and a single or biplane supine scout CT image is obtained to verify adequate bowel distension. If adequate bowel distension is present, the CT examination is performed (Fig 4.1). If adequate bowel distension is not achieved, additional air is insufflated into the rectum. Fol lowing air insufflation, CTC is performed first in the supine position in a cephalo-caudad direction encompassing the entire colon and rectum. The display field of view (DFOV) should be adjusted so as not to exclude any part of the abdomen or pelvis. That is why some technologists like to use both anteroposterior and lateral scouts. The scan range should extend several finger breaths above the top of the most cephald colon so as not to accidentally omit some colon due to a variable in the inspiration. Caudally, the scan should extend below the anal verge.

The patient is asked to hyperventilate to maximize the length of the breath hold. Some investigators use nasal oxygen, in particular in the elderly or when the technique calls for a scan longer than 30 seconds. It is best not to break up the scan into multiple breath holds. At the University of Chicago, we have found the following patient instructions to be effective in minimizing or eliminating respiratory motion: "Explain to the patient that movement of the belly will ruin the scan; take several deep breaths, as though you were going to hold your head under water. Try to hold your breath for the entire scan. If you can't, then breathe out as slowly as possibly so there will not be any rapid movement of your belly. If you must, then breathe in as slowly as possible."

As soon as the supine scan is complete, the patient is then placed in the prone position. A second scout localizing image is obtained, repeating the process over the same z-axis range. The image is reviewed to determine if the colon is adequately distended. If not, it may be necessary to insufflate scout additional gas into the colon, depending on how much the patient can tolerate. The patient is reminded that adequate distention of the bowel is critical for this study, but no additional air is insufflated into the colon against the patient's wishes.

As soon as the prone scan is complete, the catheter is positioned vertically, the puffer removed, and the rectal tip left in place. Towel or tissues should be available to cap the tubing if there is a liquid return. With the dry prep, however, there is in general little or no liquid. By finishing in the prone position and leaving the rectal tube in place open to room air for 30 to 60 seconds, postprocedure cramping should be minimized. After the tip is removed, the patient is sent to the restroom.

Supine and prone imaging doubles the radiation dose but is essential to allow optimal bowel distension, redistribution of residual fluid, and differentiation of fecal material from polyps because visualization of mobility of a filling defect implies residual fecal material. If a wet prep is used and a large amount of retained fluid is seen on the supine scan, one can optionally add a decubitus scan, optimized or tailored to move the fluid out of the loop of interest (a technique first suggested by Ken Hopper, MD, at Hershey Medical Center).

Other Technical Scan Parameters

Thin sections on a multislice scanner are strongly preferred. At NYU, we utilize a 4- X 1-mm slice detector configuration, 120 kV, 0.5-second gantry rotation, and effective 50 mAs. Pitch (table feed per gantry rotation/nominal slice thickness) should be varied between 6 and 7 such that the entire abdomen and pelvis may be covered during a 30-second breath hold. The pitch is varied to account for differences in patient's body length so that the acquisition can be completed in 30 seconds. This results in 12 and 14 mm of coverage per second. CT images are reconstructed as 1.25-mm-thick sections with a 1-mm reconstruction interval. The examination is networked to a workstation for interpretation.

At the University of Chicago, we use a 1.25-mm collimation, 7.5-mm/sec table speed, HS (high-speed) mode, with overlapping reconstructions to 1.0 mm, kV = 120, mA = 100, and soft algorithm (GE LightSpeed, GE Medical Systems). Nasal oxygen is used for scans longer than 30 seconds.

Regardless of the scanner type, it must be stressed that interpretation of multiplanar reformations (MPR) and 3D endoluminal data is facilitated by thin section (<3 mm) image acquisition (Fig 4.2.). If one does not own a multislice scanner, in our opinion, the thickest acceptable sections are 2.5 mm with a pitch = 1.5, and overlapping reconstruction to 1 to 1.5 mm.

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