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Everything you need to know about Computed Tomography (CT) & CT Scanning


Colon: Neoplastic Disease: CT of the Large Bowel Including Virtual Colonoscopy

Karen M. Horton, MD

Introduction

Computed tomography (CT) plays an important role in the evaluation of patients with abdominal pain. Recently, the utility of CT in patients with suspected colonic pathology has been documented. A unique imaging feature of CT is its ability to accurately demonstrate the bowel wall as well as adjacent structures. Therefore, abdominal CT provides a highly sensitive method for the detection of intramural pathology as well as extraluminal extension of colonic diseases.

Technique

Routine abdominal CT is usually performed after the administration of intravenous and oral contrast. At our institution, the patient routinely drinks approximately 1200-1250 cc of a 3% oral Hypaque solution 60-90 minutes prior to the scan. If specific colonic pathology is suspected, it is important to adequately opacify the entire colon. Therefore, oral contrast can be administered the night before the study as well as just prior to the scan. This insures that the contrast has reached the colon and is essential for optimal visualization. Alternatively in urgent cases, or in patients in whom limited rectosigmoid disease is suspected, contrast can be gently administered through a rectal tube. The use of air and or water to distend the colon has also been reported to be helpful. In situations where adequate opacification of the colon is not possible, air and feces often provide natural contrast and sometimes allow detection of pathology.

Although the administration of intravenous contrast is not absolutely essential for the diagnosis of colonic pathology, it is often helpful, especially if extracolonic extension of disease is also suspected. At our institution we routinely administer 100 -120 cc of Omnipaque 350, at a rate of 2-3 cc/sec.

Using a standard CT scanner, the abdomen should be routinely imaged from the level of the diaphragm through the symphysis pubis. Consecutive slices with 8 mm collimation are obtained at 10 mm intervals. Using spiral CT 5-8 mm collimation can be performed with a table speed of 8 mm/sec, with reconstruction every 5-6 mm.


Normal Colon

The colon can usually be distinguished from small bowel by its location, size and the presence of haustra. The colon normally frames the abdomen. The cecum lies in the right abdomen and is mostly in the retroperitoneum. The transverse colon is in the peritoneal cavity and crosses the abdomen anteriorly. The descending colon lies in the left abdomen, in the retroperitoneum, while the sigmoid is intraperitoneal and in the pelvis. The colon is surrounded by homogeneous fat.

Variations in colon position are not uncommon, and are usually of little clinical significance. For example. there is significant variation in cecal position depending on the length of its mesentery and extent of retroperitonealization. In addition, the colon can be redundant, resulting the drooping of the transverse colon into the lower abdomen or extension of the sigmoid out of the pelvis into the lower abdomen. Chilaiditi’s syndrome is an anatomic variant where there is interposition of the hepatic flexure and transverse colon between the liver and right hemidiaphragm. This is usually an incidental finding, is often transient and rarely causes symptoms. Another common variant is seen when the colon occupies and empty renal fossa secondary to nephrectomy or renal agenesis.

The transverse diameter of the colon varies. The cecum, the largest portion of the colon, should measure less than 9 cm in diameter. The transverse colon usually measures less than 6 cm in diameter, and the descending/sigmoid colon is usually slightly smaller in caliber. The normal appendix can often be identified on routine CT scan of the abdomen. It appears as a small thin walled tubular structure arising from the posteromedial aspect of the cecum between the ileocecal valve and the cecal tip. The appendix can be variable in length, measuring up to 20 cm. Usually the appendix lies anterior to the cecum although a retrocecal appendix occurs in up to 25% of patients

The wall of the colon is very thin and should measure less than 3 mm. In fact, it should be barely perceptible if the colon is well distended with contrast. Gas, feces, and minimal fluid are normally present within the colon.


Pathologic Conditions

Inflammatory Conditions

-Appendicitis

Acute appendicitis is a common cause of right lower quadrant pain. It occurs when the appendiceal lumen becomes occluded, resulting in an accumulation of fluid, appendiceal dilation, inflammation, ischemia and eventually perforation with possible abscess formation. CT has a 96% positive predictive value for appendicitis.

On CT, an abnormal appendix will appear dilated (>6mm) with a thickened wall which may homogeneously enhance after the administration of intravenous contrast. The appendix may be filled with fluid or debris. An appendicolith can be detected in up to 25%-40% of cases. The presence of an appendicolith along with pericecal inflammation or mass is considered diagnostic for appendicitis. A hallmark of acute appendicitis is the presence of varying degrees of inflammatory thickening in the fat surrounding the diseased appendix. However, the presence of a dilated thickened appendix, even in the absence of pericolonic inflammation, is suggestive of possible appendicitis. In addition, the presence of pericecal inflammatory changes , without definite identification of an abnormal appendix is suspicious for acute appendicitis, but not diagnostic, as many other conditions such as Crohn’s disease or cecal diverticulitis may have a similar appearance.

Perforation is a potential complication of appendicitis and appears as small pockets of extraluminal air or pneumoperitoneum. Appendiceal abscesses appear as a soft tissue mass containing air or necrotic debris and surrounded by inflammatory changes. Less common complications include hepatic abscess or small bowel obstruction.

-Diverticulosis/Diverticulitis

Diverticulosis is a common condition in the U.S.A. Diverticula can occur anywhere throughout the colon but are most common in the sigmoid. They represent small outpouchings of the colonic mucosa and submucosa through the muscular layers of the wall. Diverticula usually range in size from 2-3 mm up to 2 cm. A giant sigmoid diverticulum is a distinct entity which can range up to 25 cm in size. This may result from a ball-valve mechanism or could represent a walled off cyst secondary to inflammation. The exact etiology is not known.

The most common complications of diverticular disease include bleeding, due to erosion of a feeding nutrient artery, and infection. Acute diverticulitis occurs when the neck of a diverticulum is occluded by stool, inflammation or food particles resulting in a microperforation of the diverticula and surrounding pericolonic inflammation. Left sided diverticula more commonly lead to infection, while right sided diverticula more frequently bleed.

CT is well suited for the evaluation of diverticular disease, as it is able to image the wall of the colon as well as the surrounding pericolonic fat.

On CT, diverticulosis appears as small air filled outpouchings of the wall of the colon, most abundant in the sigmoid colon. The wall of the colon may appear thickened due to muscular hypertrophy. Diverticulitis appears as segmental wall thickening, hyperemia and inflammatory changes in the pericolonic fat. A frank abscess can be seen in up to 30% of cases. A diverticular abscess appears as a soft tissue mass with surrounding inflammatory changes. The center of the collection may contain air or air fluid levels or have low attenuation representing necrotic debris. CT also allows detection of other complications of diverticulitis such as colovesical fistula or perforation. Colovesical fistula is suspected when air is seen in the bladder and there is thickening of the bladder wall adjacent to a diseased segment of bowel (usually sigmoid.) Focal contained perforations appear as small extraluminal pockets of air or extravasation oral contrast material.

CT also provides guidance for percutaneous drainage of diverticular abscesses, which can eliminate the need for emergent surgery, thus permitting a single elective resection after proper patient preparation.

-Colorectal Cancer

Colorectal cancer is the second most frequently diagnosed malignancy in the United States, resulting in significant morbidity and mortality. Initial diagnose is usually made with endoscopy or barium enema. However, computed tomography continues to play a significant role in staging adenocarcinoma of the colon and rectum and in detecting recurrent disease.

On CT, adenocarcinoma of the colon usually appears as a soft tissue density mass with irregular borders. Larger masses may have a low density necrotic center or occasionally may contain gas, resembling an abscess. Rectal cancers may appear as asymmetric wall thickening which narrows the lumen.. CT is able to detect extension of tumor into the pericolonic fat, invasion of adjacent organs, such as bladder or pelvic muscles, and adenopathy. CT is the study of choice for the detection of liver METS, which will appear as multiple hypodense lesions within the liver after injection of intravenous contrast. Compilations of colorectal cancer can also be imaged with CT such as obstruction and perforation.

Recurrent tumor after surgery usually appears as a soft tissue density mass with irregular borders. This can often be distinguished from postoperative fibrosis which usually appears more linear without a discrete mass.


Virtual Colonocopsy

CT colonography (CTC) is considered by many to be a potential breakthrough in the screening for colorectal polyps and cancer. This technique combines the volume acquisition of CT data, using single or multidetector spiral CT, with a variety of two-dimensional and three-dimensional visualization techniques. The viewing technique which has gained widespread attention is the simulated endoluminal view of the colon (virtual colonoscopy). This method allows the colon to be viewed from the inside, similar to the view through an endoscope as used by the gastroenterologists. Although this method of display can be useful, it is time consuming and computer intensive. In addition, most radiologists are not familiar with this endoluminal perspective, and will thus require considerable training. We have found that in our practice, it is often necessary to utilize a variety of imaging display techniques to best visualize the anatomy and pathology. This lecture reviews the display techniques we utilize when performing CTC.

Technique

Patient Preparation CTC requires bowel cleansing for optimal results. Most centers use 1 gallon of GoLytely the evening before the study. Other preparations such as Fleets prep can be utilized. Immediately before the exam, an enema tip is inserted into the rectum and either room air or carbon dioxide is instilled to maximum patient tolerance (1-2L). Both prone and supine imaging is necessary. Bowel relaxants such as Glucagon can be administered if necessary. However, a recent study of 60 patients by Yee et al showed than IV Glucagon did not significantly improved colonic distention. Further investigation is necessary.

Scanning Parameters

There are many different protocols being investigated for CT colonography. Most are dependent on the type of scanner and its capabilities. The optimal technique has not yet been determined. With helical scanners, investigators have utilized 3-5mm collimation with a pitch of 1.3-2.0. This technique usually requires a 40-50 second breath hold. We currently have the Siemens Volume Zoom Multidetector Scanner. With this scanner we can obtain 1.25 mm slices by utilizing the 4 X 1 mm collimator configuration. Prone and supine imaging should be obtained, to allow visualization of the entire mucosa. In one study, by Chen et al, 59% of the scans have adequate distention for polyp detection using either the prone or supine acquisition, compared with 87% having adequate distention using a combination of the supine and prone acquisition together.

Data rendering

Data rendering can be performed using shaded surface or volume rendering techniques. Shaded surface technique require less computer power, but are limited by threshold settings. Volume rendering techniques allow interactive manipulation of image thresholds and are considered superior to shaded surface. For endoluminal imaging, a central axis can be created which allows simply navigation through the colon. Volume rendering has been found to be superior to shaded surface for the detection and characterization of polyps.

Our Technique

Scanner:

Siemens Somatom Plus 4 Volume Zoom scanner (Siemens Medical Systems, Iselin , N.J.) with 8 detectors.

kVp    120

mA    180

collimation    4 X 1 mm

slice thickness    1.25mm

reconstruction interval    1-2mm

breath hold    approximately 25 seconds


Results

Many studies have been carried out using different scanning techniques, different software and computer display and real vs. simulated polyps. In a study of 30 endoscopically proven polyps by Hara et al, a combination of 2D and 3D viewing detected 100% of polyps greeter than 1 cm, 71% of polyps between 0.5 and 0.9cm and 28% polyps less than 0.5cm. In a study of 20 proven masses by Royster et al, all were identified on 2D CT colonography and only 19/20 were visualized on the 3D virtual colonoscopy. However, there were 2 false positive using the 2D, and no false positives using the 3D. In the same study, 2D CT colonography detected 14/15 polyps, with 3 false positive. 3D virtual colonoscopy detected 13/15 polyps with no false positives. In a blinded prospective study of 70 patients by Johnson at al, the sensitivity of detection of polyps of 1 cm or more was 75% and the specificity is 90%. In a recent study published by Fenlon et al, comparison was made between virtual and conventionalcolonoscopy in a high risk group of 100 patients. Virtual colonoscopy identified all 3 cancers and 20/22 polyps that were 10mm or more in diameter (91%). There were 19 false postivies. Results were less accurate for smaller polyps 33/40 for polyps 6-9mm, and 29/53 for polyps 5mm or smaller.

The main limitations to using these techniques include, retained colonic fluid and feces, underdistended segments, and long interpretation times.

Most investigators use a combination of 2D and 3D displays and find them complementary. However several recent studies suggest that the 2D images alone, may be adequate for lesion detection and would therefore decreased the time necessary for evlaution.


Future Directions

Despite marked technical advancement in the last several years, standardization and validation of CT colonography techniques in necessary before widespread implementation. Groups are currently working on "intelligent soft ware" and artifical intelligence to potentially triage the data-set and identifiy only suspicious areas for review. This may increase accuracy and reduce exam time. Groups are also working on perfecting the bowel prep, including stoll tagging regimes.

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