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Overview of computed tomographic colonography

Overview of computed tomographic colonography
Author:
Stephan Anderson, MD
Section Editors:
Kenneth K Tanabe, MD
Jonathan B Kruskal, MD, PhD
Deputy Editor:
Shilpa Grover, MD, MPH, AGAF
Literature review current through: Dec 2022. | This topic last updated: Mar 16, 2021.

INTRODUCTION — Computed tomographic (CT) colonography (also virtual colonoscopy or CT colography) provides a computer-simulated endoluminal perspective of the air-filled distended colon [1]. The technique uses conventional CT scan images acquired as an uninterrupted volume of data and employs sophisticated post-processing software to generate images that allow the operator to evaluate a cleansed colon in any chosen direction.

This topic will review the preparation, technique, and risks of CT colonography. The effectiveness of CT colonography in screening and the diagnosis of colorectal cancer are discussed in detail, separately. (See "Clinical presentation, diagnosis, and staging of colorectal cancer", section on 'Computed tomography colonography' and "Tests for screening for colorectal cancer", section on 'Computed tomography colonography'.)

PATIENT SELECTION

Indications — Potential indications for CT colonography include the following:

Screening for colorectal cancer (CRC) – CT colonography is an option for CRC screening in asymptomatic average-risk individuals [2,3]. However, the role of CT colonography in screening for CRC is controversial. While there is consensus that CT colonography should not be used for screening in patients at increased risk for CRC (eg, history of adenomas, inflammatory bowel disease, familial CRC syndrome), guidelines differ in their recommendations for CT colonography in average-risk individuals. Guidelines for CRC screening are discussed in detail, separately. (See "Screening for colorectal cancer: Strategies in patients at average risk".)

Evaluation for synchronous CRC – In patients with a CRC in whom a complete colonoscopy cannot be performed due to the inability to pass the colonoscope beyond an obstructing tumor, a CT colography can rule out a proximal synchronous CRC. (See "Screening for colorectal cancer: Strategies in patients at average risk".)

Evaluation of patients with signs or symptoms suggestive of CRC – While colonoscopy is the preferred initial diagnostic test in patients with signs or symptoms of a CRC as it permits biopsy of the lesion, a CT colonography may be performed in patients with an incomplete or failed colonoscopy or in whom a colonoscopy is contraindicated. (See "Clinical presentation, diagnosis, and staging of colorectal cancer", section on 'Computed tomography colonography' and "Overview of colonoscopy in adults", section on 'Contraindications'.)

Contraindications — The following situations are relative contraindications for CT colonography:

Active colonic inflammation (eg, acute diarrhea, active inflammatory bowel disease)

Symptomatic colon-containing abdominal wall hernia

Recent acute diverticulitis

Recent colorectal surgery

Recent deep endoscopic biopsy/polypectomy/mucosectomy

Known or suspected colonic perforation

Symptomatic or high-grade bowel obstruction

While there is little evidence to inform the duration of a safe interval before performing a CT colonography, we avoid CT colonography for at least 15 days after a deep biopsy/polypectomy/mucosectomy and six weeks after an episode of acute diverticulitis. The length of the interval between colorectal surgery and CT colonography should be based on the underlying disease and the location of the colonic anastomosis, but should be at least six weeks. (See "Clinical manifestations and diagnosis of acute diverticulitis in adults", section on 'Exclusion of an underlying malignancy' and "Management of anastomotic complications of colorectal surgery", section on 'Risk factors'.)

PATIENT PREPARATION — Patient preparation consists of dietary restriction with a low-residue diet and clear liquids for 24 hours or more and bowel preparation with a laxative. Patient preparation is critical for computed tomographic (CT) colonography as stool can obscure underlying polyps or mass lesions, and in some cases, can simulate polyps (picture 1). Several regimens (eg, polyethylene glycol, phosphosoda, magnesium citrate) have been used. It is important to consider the patient's comorbid illnesses when choosing an appropriate preparation or combination of preparations. (See "Bowel preparation before colonoscopy in adults".)

Even with the use of cathartic colon preparations, retained fluid in the lumen may obscure or mimic small polyps. Residual material is therefore tagged with oral administration of water-soluble contrast alone (typically with several meals prior to the examination) or in combination with a low-volume barium contrast agent [4,5]. The contrast-enhanced residual material can then be differentiated from the surrounding colonic mucosa. While bowel preparation is required at the present time, CT colonography without a cathartic bowel preparation is also being evaluated [6-14].

PROCEDURE

Technique and data acquisition — Following placement of a thin and flexible rectal catheter, the colon is distended with air or carbon dioxide throughout its length. Carbon dioxide has an improved patient tolerance as compared with air due to more rapid post-procedure absorption. Distension is also facilitated by use of smooth muscle relaxants, such as glucagon or scopolamine (hyoscine), which reduce peristalsis. However, these agents also enhance reflux of gas into the small bowel, which complicates interpretation of the study and automated computerized tracking along the gas-filled lumen. Colonic distension is evaluated on the computed tomographic (CT) table immediately prior to image acquisition by reviewing the planar CT scout image in order to ensure technical adequacy of the resultant acquisition.

Following this, an uninterrupted volume of data is then acquired through the abdomen in several seconds during a single breath-hold. Because of the presence of stool, fluid, or bowel spasm, data are often acquired in both the supine and prone positions in order to redistribute fluid and colonic gas, thereby facilitating polyp detection. Scanning parameters are designed to cover a large volume of data with thin slices (in many cases, submillimeter) in order to optimize subsequent image reformation.

Intravenous injection of iodinated contrast medium is reserved for patients with known colorectal cancer in order to improve staging, in patients with symptoms of colorectal cancer, or in whom the extracolonic organs need to be further evaluated. (See "Clinical presentation, diagnosis, and staging of colorectal cancer", section on 'Clinical staging evaluation'.)

Image processing and reconstruction — Once image data are acquired, post-processing is performed on a computer using a variety of commercially available software packages [15]. The data are then used to render multiplanar reformatted images (in coronal, sagittal, and axial planes), mucosal relief profiles, or hybrid surface-shaded or volume-rendered endoluminal perspectives (picture 2).

Each technique provides different information, and studies should be tailored to answer the specific clinical question. For the virtual endoluminal study, interactive viewing of the study is performed at near real-time speeds, with the operator capable of simultaneously viewing other anatomic projections at the same site while being able to control the speed and direction of flight. Shutter widths, bowel wall transparency, lesion and bowel wall measurements, and image recording can all be performed.

Three rendering techniques are in clinical use:

Surface rendering relies on selection of a specific range of CT attenuation values, whereby only pixels within a preselected range are included in the three-dimensional reconstruction. A simulated light source provides a sense of depth.

With volume rendering, voxels are assigned different degrees of opacity and the entire CT data set is used, producing a final image that more faithfully represents the true anatomic structures being imaged.

Perspective rendering, an adjunct to volume rendering, provides a sense of motion, depth, and the ability to navigate or fly-through the bowel lumen.

The development of software designed to enhance polyp detection automatically (computer-aided detection) is ongoing, with several US Food and Drug Administration-approved products available [15-17].

TEST PERFORMANCE — The acceptability and effectiveness of CT colonography in the detection of colorectal neoplasia in asymptomatic patients undergoing colorectal cancer screening and those with symptoms of colorectal cancer are discussed in detail, separately. (See "Clinical presentation, diagnosis, and staging of colorectal cancer", section on 'Computed tomography colonography'.)

COMPLICATIONS

Complications related to preparation — All bowel preparations can cause adverse effects, which include fluid and electrolyte disturbances, nausea, vomiting, abdominal bloating, abdominal discomfort, aspiration, and esophageal tears from vomiting. Complications related to the various colonoscopy preparations are discussed in detail elsewhere. (See "Bowel preparation before colonoscopy in adults".)

Colonic perforation — Computed tomographic (CT) colonography has a low risk of colonic perforation as compared with colonoscopy. In one review of three studies that included 50,860 subjects who underwent a CT colonography for evaluation of symptoms or for colorectal cancer (CRC) screening, there were a total of 18 perforations (0.035 percent) [18-21]. Potential factors associated with colonic perforation include the use of a rectal balloon for inflation, rigid rectal catheters, manual air insufflation, and patient factors including active inflammatory bowel disease, prior colon surgery, recent deep colonic biopsies, inguinal hernia, and obstructing CRC. (See "Overview of colonoscopy in adults", section on 'Perforation'.)

Radiation exposure — CT colonography exposes patients to ionizing radiation, and repeated examination leads to a small but significant cumulative radiation exposure with associated risk of cancer. In one study, CT colonography performed every five years for patients between the ages of 50 and 80 years would result in 150 radiation-related cancers/100,000 individuals screened, but would prevent 3580 to 5190 CRCs/100,000 individuals screened [22]. The benefit-risk ratio for cancers prevented to the number of cancers induced varied from 24:1 to 35:1. This study assumed a radiation dose of 8 mSv for women and 7 mSv for men, which is higher than the present average dose of 4.4 mSv [23]. Further dose reduction to 1 mSv with iterative reconstruction algorithms and lower tube voltage may further decrease the risk of cancer [24-26]. (See "Radiation-related risks of imaging".)

Incidental extracolonic findings — Although CT colonography can identify important extracolonic diseases, in a large number of cases it leads to unnecessary evaluation of incidental findings [27,28]. The incidence of extracolonic findings increases significantly with age [29]. Additional evaluation for potentially important extracolonic findings occurs in approximately 10 percent of cases. Approximately 2 to 5 percent of findings on CT colonography are ultimately of importance. These include abdominal aortic aneurysms, adrenal masses, nonmalignant renal masses, and in 0.5 percent of cases, extracolonic cancers. Of the extracolonic cancers detected on CT colonography, renal cell cancer, lung cancer, and lymphoma are most prevalent and are usually localized at the time of diagnosis [30-33]. However, it is unknown if detection of these malignancies improves outcomes.

Other complications — Given the absence of sedation, cardiovascular complications are rare in patients undergoing CT colonography. However, vasovagal reactions may occur due to the pain induced by colonic distension. In one retrospective review of 17,067 CT colonography examinations, there were three vasovagal episodes and one episode of angina [19]. No cardiac complications required admission and no deaths were reported.

QUALITY INDICATORS — Monitoring of several quality metrics for CT colonography is recommended for continuous quality improvement. These include process measures (eg, optimal bowel cleansing and distension) to ensure that the CT colonography is adequate for at least the detection of ≥10 mm lesions and preferably ≥6 mm lesions in all colonic segments and outcome measures (eg, rate of colonic perforation, true positive rate for polyps ≥10 mm, extracolonic findings that require additional imaging).

INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5th to 6th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10th to 12th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)

Basics topics (See "Patient education: Colon and rectal cancer screening (The Basics)" and "Patient education: Colonoscopy (The Basics)".)

Beyond the Basics topics (see "Patient education: Screening for colorectal cancer (Beyond the Basics)")

SUMMARY AND RECOMMENDATIONS

Computed tomographic (CT) colonography (also virtual colonoscopy or CT colography) provides a computer-simulated endoluminal perspective of the air-filled distended colon. The technique uses conventional spiral or helical CT scan images acquired as an uninterrupted volume of data and employs sophisticated post-processing software to generate images that allow the operator to evaluate a cleansed colon in any chosen direction. (See 'Introduction' above.)

CT colonography is an option for colorectal cancer (CRC) screening in asymptomatic average-risk individuals over the age of 50 years. Other indications for CT colonography include the evaluation of the proximal colon in patients with an obstructing CRC or evaluation of signs or symptoms of CRC in whom a colonoscopy cannot be performed due to intolerance, technical difficulty, or in whom a colonoscopy is contraindicated. (See 'Indications' above.)

Relative contraindications to CT colonography include the following:

Active colonic inflammation (eg, acute diarrhea, active inflammatory bowel disease)

Symptomatic colon-containing abdominal wall hernia

Recent acute diverticulitis

Recent colorectal surgery

Recent deep endoscopic biopsy/polypectomy/mucosectomy

Known or suspected colonic perforation

Symptomatic or high-grade small bowel obstruction

There is little evidence to inform a safe interval between colonoscopy with deep biopsy/polypectomy/mucosectomy or acute diverticulitis and CT colonography. In order to minimize the risk of perforation, we avoid CT colonography for at least 15 days after a deep biopsy and six weeks after an episode of acute diverticulitis. The length of the interval between colorectal surgery and CT colonography should be based on the underlying disease and the location of the colonic anastomosis, and should be at least six weeks. (See 'Contraindications' above.)

Patient preparation consists of dietary restriction with a low-residue diet and clear liquids for 24 hours or more and bowel preparation with a laxative. To reduce the number of false positives due to residual fluid or stool, fecal tagging is performed with administration of an oral contrast agent (typically with several meals prior to the examination) alone or in combination with a low-volume barium contrast agent. (See 'Patient preparation' above.)

Following placement of a thin and flexible rectal catheter, the colon is distended with air or carbon dioxide throughout its length. Distension is also facilitated by use of smooth muscle relaxants, such as glucagon or scopolamine (hyoscine), which reduce peristalsis. An uninterrupted volume of data is then acquired through the abdomen in several seconds during a single breath-hold. Once image data are acquired, they are then used to render multiplanar reformatted images, mucosal relief profiles, or hybrid surface-shaded or volume-rendered endoluminal perspectives (picture 2). (See 'Procedure' above.)

Complications of CT colonography include those related to patient preparation and colonic perforation. CT colonography also exposes patients to ionizing radiation, and repeated examination leads to a small, but clinically significant, cumulative radiation exposure with associated risk of cancer. Although CT colonography can identify important extracolonic diseases, in a large number of cases it leads to unnecessary evaluation of incidental findings. (See 'Complications' above.)

Monitoring of several quality metrics for CT colonography is recommended for continuous quality improvement. These include process measures (eg, optimal bowel cleansing and distension) to ensure that the CT colonography is adequate for at least the detection of ≥10 mm lesions and preferably ≥6 mm lesions in all colonic segments and outcome measures (eg, rate of colonic perforation, true positive rate for polyps ≥10 mm, extracolonic findings that require additional imaging). (See 'Quality indicators' above.)

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