Version July 2025
INTRODUCTION —
Computed tomographic (CT) colonography (also referred to as virtual colonoscopy) provides a computer-simulated endoluminal perspective of a prepared and gas-distended colon. The technique uses conventional 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 the colon with a variety of post-processing techniques.
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 tomographic (CT) colonography' and "Tests for screening for colorectal cancer", section on 'Computed tomography colonography'.)
PATIENT SELECTION
Indications — Indications for CT colonography include:
●Screening for colorectal cancer (CRC) – CT colonography is an option for CRC screening in asymptomatic individuals with average or moderate risk [1-3]. While there is consensus that CT colonography should not be used for screening in patients at higher risk (eg, those with inflammatory bowel disease, hereditary polyposis, or nonpolyposis cancer syndromes), most guidelines include CT colonography as a colorectal screening option 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 CRC in whom a complete optical colonoscopy cannot be performed due to the inability to pass the colonoscope beyond an obstructing tumor, CT colonography can evaluate for 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 CRC as it permits biopsy of the lesion, CT colonography may be performed in patients with an incomplete colonoscopy or in whom a colonoscopy is contraindicated. (See "Clinical presentation, diagnosis, and staging of colorectal cancer", section on 'Computed tomographic (CT) colonography' and "Overview of colonoscopy in adults", section on 'Contraindications'.)
Contraindications — The following 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 endoscopic intervention such as endoscopic mucosal resection (EMR) or endoscopic submucosal dissection (ESD) (see "Overview of endoscopic resection of gastrointestinal lesions")
●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 CT colonography, we avoid CT colonography for at least two weeks after EMR or ESD and for six weeks after 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 colonic 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 a low-residue diet followed by clear liquids for 24 hours prior to the examination and ingesting a bowel preparation. Adequate bowel preparation is critical for CT colonography as stool can obscure underlying polyps or mass lesions, and in some cases, can simulate polyps (image 1). Several regimens (eg, polyethylene glycol [PEG], magnesium citrate, sodium picosulfate) have been used. While lower volume hyperosmolar preps such as magnesium citrate or sodium picosulfate are preferred over higher volume PEG-based preparations because they tend to leave less residual colonic fluid, it is important to consider the patient's comorbid illnesses when choosing a preparation. As an example, iso-osmolar preparations (eg, PEG solution) are safer for patients with kidney, liver, and cardiac disease. The efficacy and safety of bowel preparations are discussed in more detail separately. (See "Bowel preparation before colonoscopy in adults".)
Despite the use of a bowel preparation, retained fluid or stool in the lumen may obscure or mimic small polyps. Thus, residual fecal and fluid material is tagged by administering oral, water-soluble iodinated contrast alone or in combination with low-volume barium contrast [4-6]. The contrast-tagged residual material can then be differentiated from the surrounding colonic mucosa. Typical iodinated contrast agents include hypertonic options such as 30 to 60 mL of diatrizoate dimeglumine and diatrizoate sodium or isotonic agents such as iohexol or iopamidol. If barium is selected, we use a 2 percent CT-appropriate barium formulation. Of note, contrast allergy to orally administered iodinated contrast is exceedingly rare. However, the safety of orally administered iodinated contrast in patients with documented anaphylaxis to intravenous contrast is uncertain. Thus, some centers do not use iodinated contrast tagging, or they administer barium alone in such patients.
The protocol for bowel preparation and contrast tagging is center-specific and informed by the available published evidence and clinical experience.
We typically use the following protocol:
●One day prior to CT colonography:
•All day: Clear liquids only
•5 PM: 300 mL magnesium citrate and 20 mg bisacodyl
•8 PM: 300 mL magnesium citrate and 250 mL 2 percent barium
•Before bed: 30 mL oral iohexol
●Morning of examination: 30 mL oral iohexol
In the setting of a same-day CT colonography following an incomplete colonoscopy, we condense the schedule for stool and fluid tagging to improve polyp detection, particularly if significant residual fluid remains in the lumen following bowel preparation. We typically administer at least 30 mL of diatrizoate meglumine and diatrizoate sodium as soon as the patient recovers from sedation and wait at least two hours before performing CT colonography. Studies have suggested that the iodinated contrast reaches the distal colon in approximately 75 percent of patients when using this approach [4,7,8].
While bowel preparation is typically given, CT colonography without a bowel preparation has been evaluated [9-17]. Studies suggested that avoiding or limiting bowel preparation improved patient compliance but reduced the accuracy of the test [18]. When patients were informed of the limitations of CT colonography without bowel preparation, most patients opted to complete a bowel preparation [19].
PROCEDURE
Technique and data acquisition — Following placement of a thin and flexible rectal catheter, the colon is distended with carbon dioxide or ambient air. Carbon dioxide has improved patient tolerance compared with air due to its more rapid post-procedure absorption. We do not routinely use smooth muscle relaxants such as glucagon. We evaluate the amount of colon distension prior to image acquisition by reviewing the planar CT scout image. This helps to ensure that imaging will be technically adequate.
Following this, an uninterrupted volume of data is acquired through the abdomen in several seconds during a single breath-hold. Because of the presence of stool, fluid, or bowel spasm, we acquire data using two patient positions (eg, supine, prone) to redistribute fluid and colon gas, thereby facilitating polyp detection and optimizing visualization of the luminal circumference. Scanning parameters are designed to cover a large volume of data with thin slices (eg, submillimeter slices) to optimize subsequent image reformation.
For selected patients (eg, those with known colorectal cancer who require tumor staging), we may use intravenous iodinated contrast during the examination to evaluate other abdominal organs. (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 [20]. The data are then used to render multiplanar reformatted images (MPR) in coronal, sagittal, and axial planes to create mucosal relief profiles and to produce hybrid surface-shaded or volume-rendered endoluminal perspectives (image 2 and image 3 and image 4).
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. Additional options include using a field of view similar to endoscopic visualization, evaluating bowel wall transparency, measuring lesions and bowel wall thickness, and recording images.
Approaches to image interpretation include primary 2D evaluation and primary 3D evaluation. Primary 2D evaluation typically involves initial evaluation of the source planar images (axial and other MPR views) to detect polyps and masses with secondary cross-referencing of findings with the 3D endoluminal view. Cross-referencing with the 3D view serves to confirm lesion morphology and obtain more accurate size measurements. Primary 3D evaluation involves the initial evaluation of 3D rendered images (using one of the rendering techniques described below) with secondary cross-referencing of findings with the 2D source images to confirm CT attenuation. This process facilitates differentiating true polyps and masses from residual stool or other pseudopolypoid findings. Regardless of which primary evaluation technique is employed, the use of a combination of 2D and 3D approaches is necessary for optimal interpretation.
3D rendering techniques in routine clinical practice include:
●3D endoluminal fly-through – 3D rendering of an interior view of the colonic lumen simulates endoscopic visualization during optical colonoscopy (hence the term “virtual colonoscopy”). Surface rendering relies on selecting a specific range of CT attenuation values, whereby only pixels within a preselected range are included in the 3D 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 accurately represents the true anatomic structures being imaged.
●Colon map – 3D rendering of the interface between the colon wall and endoluminal gas in a frontal projection simulates the appearance of a double-contrast barium enema examination. We typically use these views as a “road map” to localize the site of the endoluminal “camera” or “virtual colonoscope” during 3D endoluminal fly-through (image 5).
●Dissection or filet views – This 3D display method takes the 3D model of the colon, stretches it out longitudinally, virtually sections it open along the long axis, and flattens the colon wall into a rectangular field of view that simulates a gross pathology specimen (ie, “virtual pathology” view) (image 6). The mucosal surface of the colon is displayed as a series of segmented strips, which can generally be viewed more quickly than other primary 3D and 2D evaluation methods; however, this technique inadvertently involves distortion in some areas of the colon, especially at the splenic and hepatic flexures, and requires additional experience and training. Suspected lesions require cross-referencing with 2D images to confirm attenuation as well as undistorted 3D endoluminal images for more accurate size measurement.
●Unfolded cube or panoramic projections – On 3D endoluminal projections, unfolded cube or panoramic projections are variations that may extend the capabilities of a unidirectional virtual fly-through technique. These virtual image projections show a 360-degree endoluminal field of view, either as an unfolded six-sided cube or as a wide-angle endoluminal image (with associated distortion along the end of the field of view). With these approaches, both sides of haustral folds can be visualized on a single fly-through, rather than performing an endoluminal fly-through in antegrade and retrograde projections for each patient position.
The development of software designed to enhance polyp detection (computer-aided detection) is ongoing, with several US Food and Drug Administration (FDA)-approved products available [20-22].
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 tomographic (CT) colonography'.)
ADVERSE EVENTS
Adverse events related to bowel preparation — Bowel preparations can cause adverse effects, such as fluid and electrolyte abnormalities, nausea, vomiting, abdominal bloating, and abdominal discomfort. Adverse events related to bowel preparation are discussed separately. (See "Bowel preparation before colonoscopy in adults".)
Colonic perforation — CT colonography has a very low risk of colonic perforation. In one review of three studies that included 50,860 individuals who underwent a CT colonography, there were 18 perforations (0.035 percent) [23-26]. 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, endoscopic mucosal intervention, inguinal hernia, and obstructing colorectal cancer (CRC). (See "Overview of colonoscopy in adults", section on 'Perforation'.)
Radiation exposure — CT colonography exposes patients to ionizing radiation, and repeated examination theoretically may lead to a cumulative radiation exposure that has been associated with cancer risk. However, rates of cancer induction fall precipitously in patients who are 35 years of age and older. In studies involving pediatric populations with less than 100 mSv cumulative radiation exposure, the risk of adverse health effects has been minimal to none [27]. In a modeling 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 [28]. This study assumed a radiation dose of 8 mSv for females and 7 mSv for males, which is higher than the average dose of 4.4 mSv [29]. Further dose reduction to 1 mSv with iterative reconstruction or deep-learning image reconstruction algorithms and lower tube voltage may further decrease any risk of cancer induction [30-32]. (See "Radiation-related risks of imaging".)
Incidental extracolonic findings — Although CT colonography can identify important extracolonic diseases, in some cases it may lead to unnecessary evaluation of incidental findings [33,34]. The incidence of extracolonic findings increases with age [35]. 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 clinically important. These include abdominal aortic aneurysms, adrenal masses, nonmalignant kidney lesions, and uncommonly, extracolonic cancers. Renal cell cancer, lung cancer, and lymphoma are the most common extracolonic cancers detected incidentally by CT colonography, and they are usually localized at the time of diagnosis [36-39].
Other complications — Cardiovascular complications are rarely reported in patients undergoing CT colonography. However, vasovagal reactions may occur due to pain related to colonic distension. In a retrospective review of 17,067 CT colonography examinations, there were three vasovagal episodes and one episode of angina [24]. No cardiac complications required hospital admission and no deaths were reported.
In general, examination with CT colonography is well-tolerated and does not require sedation or post-procedure activity or diet restrictions. In studies comparing patients undergoing CT colonography with optical colonoscopy, patient tolerance was not significantly different between groups [40-43].
QUALITY INDICATORS —
The goal of applying quality indicators is to obtain high-quality imaging and decrease the number of missed lesions. Quality metrics for CT colonography include quality of bowel preparation, polyp detection rate, and other outcome measures (eg, perforation rate, management of extracolonic findings).
The American College of Radiology National Radiology Data Registry for CT colonography provides regional and national benchmarks for quality assurance and improvement.
SOCIETY GUIDELINE LINKS —
Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Colon polyps".)
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
●Background – Computed tomographic (CT) colonography (also referred to as virtual colonoscopy) provides a computer-simulated endoluminal perspective of the prepared and gas-filled colon. The technique uses conventional helical CT scan images acquired as an uninterrupted volume of data and employs interactive 3D post-processing software to generate images that allow the operator to evaluate the colon. (See 'Introduction' above.)
●Indications – CT colonography is an option for colorectal cancer (CRC) screening in asymptomatic average- or moderate-risk individuals over the age of 45 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.)
●Contraindications – 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 endoscopic mucosal resection (EMR) or endoscopic submucosal dissection (ESD)
•Known or suspected colonic perforation
•Symptomatic or high-grade small bowel obstruction
There is little evidence to inform a safe time interval between an event such as therapeutic colonoscopy or acute diverticulitis and CT colonography. We avoid CT colonography for at least two weeks after EMR or ESD and for six weeks after an episode of acute diverticulitis. The time interval between colorectal surgery and CT colonography is informed by the underlying disease and location of the colonic anastomosis, and it should be at least six weeks. (See 'Contraindications' above.)
●Patient preparation – Patient preparation consists of a low-residue diet followed by clear liquids for 24 hours prior to the examination and ingesting a bowel preparation. To reduce the number of false positives, stool and fluid tagging is usually performed by administering an oral iodinated contrast agent alone or in combination with low-volume barium contrast. (See 'Patient preparation' above.)
●Procedure – Following placement of a thin and flexible rectal catheter, the colon is distended with air or carbon dioxide. Data are then acquired through the abdomen in several seconds during a single breath-hold. The imaging data are used to render multiplanar reformatted images, mucosal relief profiles, and hybrid surface-shaded or volume-rendered endoluminal perspectives (image 2 and image 3 and image 4). (See 'Procedure' above.)
●Adverse events – Adverse events of CT colonography include colon perforation and events related to patient preparation. CT colonography also exposes patients to ionizing radiation, and repeated examinations theoretically may lead to a small cumulative radiation exposure with an associated risk of cancer. However, the risk of adverse health effects has been minimal to none when cumulative radiation exposure does not exceed 100 mSv. The average radiation dose for CT colonography is 4.4 mSv. (See 'Adverse events' above.)
CT colonography may identify important extracolonic diseases but may also lead to unnecessary evaluation of incidental findings.
●Quality indicators – The goal of applying quality indicators is to obtain high-quality imaging and decrease the number of missed lesions. Quality metrics for CT colonography include quality of bowel preparation, polyp detection rate, and other outcome measures (eg, perforation rate, extracolonic findings). The American College of Radiology National Radiology Data Registry for CT colonography provides regional and national benchmarks for quality assurance and improvement. (See 'Quality indicators' above.)
