Colon: Non Neoplastic Disease: Spectrum of Disease

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.

CT is particularly valuable for detection and characterization of various inflammatory conditions of the colon including appendicitis, typhlitis, infectious colitis, pseudomembranous colitis, diverticulitis and inflammatory bowel disease. Colonic intussusception, volvulus and other causes of colonic obstruction can be readily demonstrated. In addition, CT can also detect miscellaneous colonic pathology such as fistulae, pneumatosis, varices, traumatic perforations as well as extraluminal conditions which affect the colon such pelvic lipomatosis, pancreatitis, or endometriosis.

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 single or multidetector CT scanner, the abdomen should be routinely imaged from the level of the diaphragm through the symphysis pubis. Consecutive slices with 3-5 mm collimation are obtained at 5 mm intervals. In select cases closer interscan patient can be done as needed.

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.

-Typhlitis

Typhlitis, also known as neutropenic enterocolitis, is a condition which occurs in neutropenic patients undergoing treatment for a malignancy, most frequently in patients with acute leukemia on chemotherapy. Typhlitis is characterized by edema and inflammation of the cecum, ascending colon and sometimes terminal ileum. The inflammation can be so severe that transmural necrosis, perforation and death can result. The etiology of the condition is not known but probably represents a combination of ischemia, infection, mucosal hemorrhage and perhaps neoplastic infiltration. Treatment consists of antibiotic and aggressive fluid and electrolyte replacements.

CT is the study of choice for the diagnosis of typhlitis, due to the risk of perforation with colonoscopy or contrast enema. CT demonstrates cecal distention and circumferential thickening of the wall, which may have low attenuation secondary to the edema. Inflammatory stranding of the adjacent mesenteric fat is a common finding. Complications such as pneumatosis, pneumoperitoneum, or pericolonic fluid collections can also be present. Due to its involvement of the cecum, differentiation of typhlitis from appendicitis can be difficult, although the clinical presentation and history are frequently distinct.

-Radiation Colitis

Radiation therapy can result in injury to colon. More than half of patients undergoing radiation therapy to the pelvis will experience a self-limited acute proctitis. Some patients may experience a severe radiation induced proctitis which may lead to imaging.

Radiation colitis tends to be left sided, as radiation therapy is often given for pelvic pathology. CT findings include nonspecific wall thickening, typically in the rectum, increased pelvic fat accumulation and thickening of the perirectal fibrous tissue are also seen. Stricturing and fistulae are possible complications.

-Inflammatory Bowel Disease

Although contrast studies remain the principle tools for the diagnosis and evaluation of suspected inflammatory bowel disease, CT can sometimes aid in differentiating Crohn’s disease and UC when barium studies are equivocal. In addition, CT plays an important role in detection of complications of inflammatory bowel disease.

There may considerable overlap of the CT findings in Crohn’s disease and ulcerative colitis. However, there are often certain features present which may help distinguish the two. Extensive involvement of right colon and small bowel more common in Crohn’s, although involvement of the left colon and rectosigmoid does occur. In contrast, UC is typically left sided or diffuse, and only rarely involves the right colon exclusively. On CT scan , the most frequent finding in both Crohn’s disease and ulcerative colitis is wall thickening. The mean wall thickness in Crohn’s colitis(11-13 mm) is usually greater than in UC 7.8 mm . Wall thickening in UC may be diffuse and symmetric, while wall thickening in Crohn’s may be eccentric and segmental with skip regions.

The halo sign, a low attenuation ring in the bowel wall due to deposition of submucosal fat, seen more commonly in UC than Crohn’s colitis. In Crohn’s disease, the bowel wall tends to enhance homogeneously, although edema within the wall may result in low attenuation.

Proliferation of mesenteric fat is seen almost exclusively in Crohn’s, while the proliferation of perirectal fat is nonspecific and can be present in Crohn’s, UC, Pseudomembranous colitis, or radiation colitis. The presence of mesenteric lymphadenopathy suggests Crohn’s rather than UC, although is certainly not specific for inflammatory bowel disease..

Complications of IBD can be imaged with CT , and, in the case of Crohn’s disease has been shown to effect patient management in 28% of cases. Abscess formation is detected almost exclusively in Crohn’s, not UC. Abscesses can be confined to the bowel wall and pericolonic fat, or can involve adjacent structures such as the bladder, psoas muscle, or pelvic sidewall. Fistulae can also be reliably detected. Enterovesical, enterocutaneous, perianal, rectovaginal have all been detected with CT.

Toxic megacolon is a severe, life threatening fulminant transmural colitis most commonly associated with UC. Toxic megacolon, however has also been reported to occur infrequently with Crohn’s, amebiasis, salmonella, pseudomembranous colitis and ischemic colitis. The patient typically presents with profuse bloody diarrhea, abdominal pain , fever and leukocytosis.. Histologically there is mucosal sloughing as well as thinning and necrosis of the muscular layers of the bowel wall. On CT, there is distention of the colon, most commonly involving the transverse colon, containing large amounts of fluid and air. The haustra appear edematous and distorted, or may be absent. The presence of pneumatosis signifies ischemia and necrosis. The major complication of toxic megacolon is perforation with resulting in sever sepsis, shock and possibly death.

-Ischemic colitis

Ischemic colitis is a common cause of abdominal pain in the elderly and results when blood flow to the colon is compromised, usually as a result of hypoperfusion in the inferior mesenteric artery distribution. Watershed areas of the colon,(splenic flexure and rectosigmoid) are most commonly affected. Most patients are over 50 years of age and many have a history of heart disease. The extent and severity of the affected colon varies with etiology (hypoperfusion vs. thrombus vs. trauma). Most cases of transient ischemia result in full resolution. Strictures are common complications of more serious episodes. If severe, ischemic bowel may become infarcted, which is a life threatening condition requiring immediate surgical resection.

CT demonstrates circumferential segmental wall thickening with fold enlargement. Inflammatory changes in the pericolonic fat may also be present. Ischemic colitis can be diffuse or left sided, rarely will it be localized to the right colon. Pneumatosis with or without air in the mesenteric vessels or portal vein are ominous findings suggesting necrosis. Although often not pathognomonic, CT findings can suggest the diagnosis of ischemic colitis in the appropriate clinical setting.

-Inflammation of Appendage Epiploicae

Appendices epiploicae are fat containing peritoneal outpouchings which are attached to the serosal surface of the colon. They are most abundant in the transverse colon and sigmoid.

These structures can become torsed resulting in ischemia and infraction. The clinical presentation and imaging findings are often nonspecific, and the diagnosis is usually made at surgery. On CT, the diagnosis of inflammation of the appendices epiploicae should be suspected when there is focal localized inflammatory changes in the pericolonic fat. However, this finding is very nonspecific and the distinction of this entity from diverticulitis is usually not possible with CT.

Infectious Disease

-Infectious Colitis

There are many causes of infectious colitis. Bacterial infections include Shigella, Salmonella, Yersinia, Campylobacter, Staphylococcus., and Chlamydia trachomatis. Fungal infection such as histoplasmosis, mucormycosis, and actinomycosis can involve the colon. Viral causes of colitis include herpes, CMV, and Rotavirus. Amebiasis, a protozoan causes a colitis which can resemble inflammatory bowel disease. TB is another cause of colitis. Overall, the infectious colidities are typically diagnosed clinically and do not rely on CT for detection or differential diagnosis. They may, however, be identified on CT incidentally or in cases where the patients clinical symptoms are not straight forward.

On CT, patients with infectious colitis, of any etiology, demonstrate wall thickening, which usually displays homogenous enhancement. Sometimes low attenuation is detected within the wall, representing edema. Ascites or inflammation of the pericolic fat can may be present. Increased fluid within the colon results in multiple air fluid levels.

Typically, most infectious colitis is limited to involvement of the right colon, although diffuse involvement also occurs. In contrast, Gonorrhea, herpes and Chlamydia Trachomatis (lymphogranuloma venereum) typically involve the rectosigmoid colon. In addition, schistosomiasis, involvement is usually confined to the descending and sigmoid colon, as the adult worm have a tendency to enter the inferior mesenteric vein.

In summary, there is considerable overlap of the appearances of infectious colitis on CT, and usually a specific diagnosis cannot be suggested without laboratory studies.

-Pseudomembranous Colitis

Pseudomembranous colitis results from an overgrowth of the organism Clostridium dificile. Although first described as a complication of antibiotic therapy, pseudomembranous colitis has also been described with hypotensive episodes, chemotherapeutic agents, following abdominal surgery, and proximal to a large bowel obstruction.

Patients typically present with a profuse watery diarrhea with abdominal pain and fever. Histologically the condition is characterized by pseudomembranes which represent exudate of necrotic cells from the denuded mucosa. The diagnosis is typically made with stool assay for the C. difficile toxin or by stool culture. Treatment with metronidazole and vancomycin is usually effective.

CT findings include marked low attenuation wall thickening, which can be circumferential or eccentric. In one series the average wall thickness was 14.7 mm, significantly greater than wall thickening seen in other inflammatory conditions. Haustral folds are thickened and can appear as broad transverse bands, referred to as "accordion pattern. The colon wall may enhance secondary to the hyperemia. Classically PMC is a pancolitis although there are reports of PMC sparing the rectum . Ascites can occasionally be present.

-Colonic Malignancies

-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.

-Lymphoma

Primary lymphoma of the colon is rare and comprises less than 1% of all colonic neoplasms. These lesions are predominantly non-Hodgkin lymphomas and most often involve the cecum (52%) or rectum (21%).

Three distinct appearances of primary colonic lymphoma have been described on CT: a focally infiltrative lesion,, a diffuse mural infiltration or a discrete mass . Classically, the appearance which should prompt consideration of lymphoma is the presence of a focal or diffuse infiltrative colonic process with abdominal or pelvic adenopathy.

Without adenopathy, however, it is often difficult to distinguish primary colonic lymphoma from adenocarcinoma on CT, especially if a solitary mass is present. However certain features can be used to help distinguish the two neoplasms. First, colonic lymphomas tend to be right sided, colorectal cancers tend to left sided, . Second, lymphomas may present with circumferential wall thickening, colorectal cancers usually present as a focal mass. Third, lymphomas typically do not result in a significant desmoplastic response, unlike colorectal cancers which tend to causes a desmoplastic reaction, often leading to obstruction and also involvement adjacent organs.

-Benign Neoplasms

The most common benign tumor of the colon is an adenoma. Adenomas are typically solitary, and can the have significant malignant potential depending on size and histology. Certain syndromes such as familial polyposis, Gardner’s, and Turcots, are characterized by multiple colonic polyps.

The double contrast barium enema and colonoscopy are the primary imaging tools for the diagnosis of colonic polyps. However, polyps can be identified on CT. On CT polyps appear as soft tissue density filling defects in the colon.

Lipoma is the most common benign submucosal tumor in the colon. A colonic lipoma more commonly occurs in the cecum and ascending colon and is usually an incidental finding in asymptomatic patient. On CT. a colonic lipoma is identified as a solitary submucosal mass of uniform fat density. There is no risk of malignant degeneration. rare complications include intussusception. Lipomatous infiltration of the ileocecal valve can also occur but is not thought to be neoplastic in nature.

Leiomyomas can occur in the colon, although they are more common in the small bowel and esophagus. On CT the appearance of leiomyomas is nonspecific. A leiomyoma appears as a submucosal soft tissue density mass, without any distinct characteristics, except for round shape and well defined margins.

The most common appendiceal tumor is carcinoid. The vast majority of appendiceal carcinoid tumors are benign. They rarely metastasize or result in carcinoid syndrome. Appendiceal carcinoid tumors are usually asymptomatic and are found incidentally at surgery for unrelated conditions. Occasionally. and appendiceal carcinoid tumors can obstruct the lumen of the appendix and result in periappendiceal inflammation, indistinguishable for typically appendicitis. On, a stellate radiating pattern may be seen in the mesentery secondary to the carcinoids desmoplastic reaction.

Non appendiceal large bowel carcinoids, usually involve the rectum. They are usually discovered incidentally on endoscopy or barium enema. Rectal carcinoids are low grade malignancies with metastases occurring in only 10% of cases.

- Obstruction/Ileus

Mechanical obstruction of the colonic can be caused by a variety or pathologic processes including malignancies, inflammatory structures, volvulus, intussusception, hernias, and fecal impaction. CT is not only able to diagnosis the presence of a colonic obstruction, but is also often able to determine the etiology.

The key to detection of a colonic obstruction on CT is the presence of dilated colon proximal to a transition to normal caliber or collapsed bowel distally. This distinguishes obstruction from nonobstructive ileus.

Colonic ileus occurs when there is selective distention of the colon without a mechanical obstruction. The etiology of colonic ileus is not known but is thought to relate to a problem with sympathetic and parasympathetic innervation of the colon. Colonic ileus usually occurs in hospitalized patients after abdominal surgery.

On CT, colonic ileus appears as dramatic dilation of all or part of the colon. The walls of the colon are smooth and not usually thickened or edematous, which may help distinguish ileus from obstruction. However, it may be difficult to distinguish colonic ileus from obstruction on CT and a contrast enema may be necessary to definitely exclude obstruction. Despite the fact that colonic ileus is a nonobstructive dissension of the colon, perforation can occur. Impending perforation has been reported with cecal diameters of 9-12cm.

Specific causes of colonic obstruction

-Intussusception

An intussusception occurs when a segment of bowel prolapses into the lumen of an adjacent segment. Intussusception is more common in children and is usually idiopathic. In adults however, an anatomic lead point can often be identified. Certain conditions such as celiac disease, scleroderma, Whipple’s disease predispose to intussusception. Recently AIDS associated intussusception in adults has been described.

The CT appearance of intussusception is very distinctive. Intussusceptions can appear as a target mass with alternating rings of soft tissue and fat, representing the wall of the intussusceptum, mesenteric fat, and the wall of the intussuscipiens. As edema of the bowel wall increases, these distinct layers may be obscured. The presence of pneumatosis indicates significant ischemia. Dilatation and or air fluid level in proximal loops suggests obstruction.

-Volvulus

Cecal volvulus occurs in patients with an unusually long mesentery to the right colon. This results in a mobile cecum which can twist upon its luminal axis and cause obstruction. The twisted distended cecum flips superiorly and to the left so that it is now located in the left upper quadrant. this has a distinctive appearance on plain film and contrast enema. The key to the CT diagnosis is distention of the cecum which is displaced into the left upper quadrant. A "whirl" pattern of dilated bowel around twisted mesenteric vessels was first described in midgut volvulus , but has also described in volvulus of the colon.

Sigmoid volvulus occurs when a long mobile segment of sigmoid twists upon its self. This results in a closed loop obstruction which tends to fill the lower abdomen. On plain film this appears as a bean shaped collection of air representing the displaced and distended closed loop of sigmoid. CT diagnosis of volvulus is suggested when the sigmoid appears distended and displaced superiorly. A whirl sign as well as pericolonic inflammation may also be present.

-Hernia

An external hernia results when a peritoneal sac herniates through a weakness in the muscular layers of the abdominal wall. Common external hernias that may contain colon include inguinal, femoral, spigelian, and incisional hernias. Obturator, sciatic notch, and umbilical hernia are much less common.

Inguinal hernias can be direct or indirect depending on their position relative to the inferior epigastric artery and vein. Indirect inguinal hernias are most common. Right sided inguinal hernias often contain small bowel while left sided inguinal hernias often contain sigmoid colon.

In a patient with a indirect inguinal hernia, bowel is identified within the inguinal ring and may extend inferiorly into the scrotum or labia.

Direct inguinal hernias are within the aponeurosis of the external oblique muscle and rarely involve the scrotum. Femoral hernias are less common than inguinal hernias and represent herniations through the femoral ring. Femoral hernia sacs lies below and lateral to the pubic tubercle (inguinal hernias are above and medial) Femoral hernias have higher incidence of incarceration. Spigelian hernias, herniation of bowel at the lateral border of the rectus sheath, along the linea semilunaris, can also contain colon.

The key to the diagnosis of an external hernia on CT is the identification of

bowel in an abnormal location. The bowel loops contained within the hernia must be examined carefully. Dilation of the herniated loops or proximal bowel loops indicates obstruction. Inflammatory changes, wall thickening or pneumatosis signify incarceration and ischemia, which is a surgical emergency.

Diaphragmatic hernias can be congenital or acquired.. A foramen of Morgagni hernia typically present in adults, and are located on the right side through the anteromedial foramen of morgagni. The hernia may be comprised totally of omental fat and vessels, or may contain bowel, especially transverse colon.

Foramen of Bochdalek Hernia are typically on the left and are located posterolaterally. These hernias may contain small or large bowel, stomach, omentum, liver or spleen. Most congenital hernias occur through the foramen of Bochdalek.

Miscellaneous

-Pneumatosis

Pneumatosis Coli is an uncommon but important condition characterized by air within the wall of the colon. Classically the condition is separated into cystic (pneumatosis cystoides intestinalis) vs. linear depending on the morphology of the thin walled noncommunicating air collections. Traditionally, cystic air collections were thought to be due to "benign" conditions such as chronic obstructive pulmonary disease or high dose corticosteroids, while linear collections were thought to be more indicative of serious conditions such as bowel ischemia or necrosis. However, it is not always possible to classify the condition based on the morphology of the air collections, as there is considerable overlap in appearance. The condition can be indolent or fatal depending on the severity and etiology which can be difficult to predict from imaging. Therefore, correlation with the patients clinical condition, possible predisposing factors, and extent of bowel involvement is necessary.

CT is more sensitive than plain film for the detection of pneumatosis and can sometimes identify the etiology. The small air collections can readily be detected within the wall of the colon, especially when lung windows are obtained. Intramural air should be demonstrated in a dependent wall, as confusion with intraluminal air can thus be avoided. Additional CT findings such as distention, wall edema, pericolonic inflammation, or portal venous air suggest bowel ischemia/infarction.

Although pneumoperitoneum can occur in benign, incidental cases or in life threatening conditions, the clinical status of the patients will usually aid in the differential diagnosis. Portal venous air, however, is almost always a sign of serious bowel necrosis and carries a grave prognosis.

Pneumatosis coli is has recently been described as a late manifestation in patients with AIDS, typically involves the right colon, and is thought to be innocuous, not requiring emergent surgery.

-Varices

Colonic varices may develop in patients with portal hypertension as collaterals between the mesenteric and systemic venous systems enlarge. Cecal varices in the distribution of the superior mesenteric vein have been described but are very rare. Rectal varices, however are not infrequently seen. Rectal varices are dilated venous collaterals between the superior rectal vein, a branch of the inferior mesenteric vein, and the middle/inferior rectal veins, branches of the internal iliac vein. On CT after rapid IV administration of contrast, rectal varices can be visualized and serpiginous enhancing vessels surrounding the rectum.

-Traumatic perforation

The value of CT in the diagnosis of injury to solid abdominal organs after trauma has been well established. However the role of CT for the diagnosis of bowel rupture after blunt abdominal trauma continues to be debated . With improvements in CT imaging techniques, more recent studies have been encouraging.

CT findings thought to be diagnostic of bowel rupture include pneumoperitoneum, pneumoretroperitoneum, or free air within the mesentery. Demonstration of oral contrast extravasation or of discontinuity in the bowel wall is also diagnostic. CT findings suspicious, but not diagnostic for colon perforation include segmental wall thickening, pericolonic fluid or blood.

The findings are often subtle.

Extrinsic Pathology Effecting Colon

-Pelvic Lipomatosis

Pelvic lipomatosis is usually an incidental finding of unknown etiology and is characterized by a proliferation of fat within the pelvis which can result in compression of pelvic organs including the colon, typically sigmoid colon. CT demonstrates increased fat within the pelvis with minimal fibrosis or inflammatory changes. The fatty accumulation can narrow and stretch the rectosigmoid colon resulting in a characteristic appearance. The fat can also elevate and deform the bladder. Ureteral obstruction is an important complication of pelvic lipomatosis, resulting in hydronephrosis and can lead to renal failure if not recognized and corrected.

-Pancreatitis

CT is the imaging modality of choice for evaluation of acute pancreatitis and its complications. In cases of acute pancreatitis, inflammation and edema may spread to involve the transverse mesocolon and transverse colon. On CT , the transverse colon may be dilated with thickened folds and surrounding inflammation within the pericolonic fat.

-Endometriosis

Endometriosis is characterized by the presence of extrauterine foci of endometrial tissue. Endometriosis can involve the bowel, most commonly the sigmoid and rectosigmoid junction, although the cecum may also be involved. The ectopic endometrial tissue adheres to and invades the subserosal layer of the bowel and under hormonal influence may bleed. This may result in smooth muscle hyperplasia and fibrosis which may narrow the bowel lumen. Symptomatic patents present with cyclical abdominal cramps and diarrhea.

On CT endometriosis may appear as an eccentric focal region of bowel wall thickening. The lumen of the bowel may appear narrowed. A discrete endometrioma may appear as an intramural mass whose attenuation may vary depending on the blood products present.

Conclusions

Computed tomography (CT) plays an important role in the evaluation of patients with a variety of colonic diseases due to its unique ability to accurately demonstrate the bowel wall as well as adjacent structures. If colonic disease is suspected, careful attention to technique is required for optimal evaluation. With the advent of Multidetector CT, CT will continue to play a significant role in the imaging of colonic pathology. The impact of techniques including virtual colonoscopy will continue to drive the role of radiology in colonic imaging.