 improved outcomes in colon and rectal surgery Figure 11.3 Small Bowel Obstruction. Plain radiograph demonstrates dilated loops of small bowel with absence of colonic gas indicating a complete/high grade small bowel obstruction. There is air in the right inguinal canal. This patient had a surgery proven incarcerated right inguinal hernia. Figure 11.4 Large Bowel Obstruction. Supine radiograph demonstrates dilated colon as well as dilated small bowel in the right lower quadrant, indicating a large bowel obstruction with incompetent ileocecal valve. levels, and absence of colonic gas indicate a complete/high grade small bowel obstruction. SBO can be simple (blood supply not impaired) or strangulating (blood supply impaired). Most stran- gulating obstructions are closed-loop obstructions (blocked at both ends), and this occurs typically with incarcerated hernias (Figure 11.3) and volvulus.(4) Plain films are not able to reliably differentiate simple from strangulating obstruction. However, extensive mucosal thickening or edema, portal venous gas, or a closed loop obstruction indicates high risk for strangulating obstruction.(5) Large bowel obstructions (LBO) occur commonly in the sigmoid colon, where stool is more formed and the colon is narrower. Air fluid levels distal to the hepatic flexure are strong evidence of obstruction unless the patient has had an enema. When the ileocecal valve is com- petent, the small bowel usually contains little gas. When the ileocecal valve is incompetent (Figure 11.4), gaseous distention of the small bowel is often present as the colon decompresses into the ileum.(6) Toxic Megacolon Toxic megacolon is a manifestation of sever colitis with absent peristalsis, and extreme dilation of all or a portions of the colon. A markedly dilated (>6 cm) colon with “thumbprinting” (thick- ened mucosal folds projecting into the lumen caused by bowel wall edema) is concerning for toxic megacolon (Figure 11.5). Figure 11.5 Toxic Megacolon. Plain radiograph demonstrates a markedly dilated transverse colon with “thumbprinting” (thickened folds projecting into the lumen caused by bowel wall edema – see arrows) in a patient with ulcerative colitis and toxic megacolon. When the cecum exceeds 10 cm in diameter, it is at risk for perforation. Ulcerative colitis is the most common cause of toxic megacolon and other causes include: Crohn’s colitis and infec- tious colitis. Barium enema is contraindicated because of the per- foration risk.(4) Sigmoid and Cecal Volvulus Sigmoid volvulus is a closed loop obstruction that occurs most common in the elderly. On plain films, the sigmoid colon appears as a large gas-filled loop without haustral makings, arising from the pelvis and extending high into the abdomen. The three white  limitations of colorectal imaging studies Figure 11.6 Sigmoid Volvulus. Radiograph of the abdomen demonstrates the characteristic massive dilation of the sigmoid colon arising from the pelvis and extending to the right diaphragm. Three lines representing the twisted walls of the sigmoid colon converge in the left lower quadrant. Figure 11.7 Cecal Volvulus. Abdominal radiograph demonstrates a massively dilated cecum folded over into the left upper quadrant with distended small bowel. Figure 11.8 Limitations of Plain Films. CT of the same patient in Figure 11.I- B-2 shows the cause of the large bowel obstruction is an annular constricting sigmoid carcinoma. A dilated loop of small bowel is seen adjacent to the sigmoid colon. lines formed by the lateral walls of the loop and the summation of the two opposed medial walls of the loop usually converge inferiorly into the left iliac fossa (Figure 11.6). Cecal volvulus on plain films is characterized by a massively dilated cecum folded over into the left upper quadrant (Figure 11.7), usually with distended small bowel.(6) Confirmation of a volvulus can be obtained with CT or contrast enema (See CT scan [sigmoid volvulus] and fluoroscopy [water soluble enemas]). Benefits and Limitations of Acute Abdominal Series (AAS) Plain Films AAS are relatively inexpensive, and can be performed with porta- ble equipment. However, AAS is insensitive, results are often not specific, and other imaging modalities (e.g. CT) (Figure 11.8) are often needed for definitive evaluation.(2–5) COMPUTED TOMOGRAPHY (CT SCAN) Unenhanced Multidector Computed Tomography vs. Plain Films Unenhanced spiral/helical Multidetector Computed Tomography (MDCT) is an accurate technique in the evaluation of patients with traumatic or nontraumatic abdominal pain and should be considered as an alternative to plain films as the initial imaging modality.(7–10) MDCT is more expensive and exposes the patient to more radiation. However, given the poor sensitivity of plain films; the radiation, time, and money spent pursuing a plain film examination in all patients may be unnecessary.(10) Additionally, MDCT provides more information to the surgeon for preopera- tive planning. With the advent of the new 16, 32, and 64 MDCT fast speed scanners, a high quality MDCT can be obtained almost as rapidly as a plain film, but it is not portable. 1 improved outcomes in colon and rectal surgery Contrast Enhanced Multidector Computed Tomography If the patient has no contraindications to oral and IV contrast (allergy, renal insufficiency), a contrast enhanced MDCT has been shown to be sensitive in diagnosing patients with bowel obstruction; as well as inflammatory, infectious, and neoplas- tic processes.(9, 11–27) Low osmolar, “nonionic,” iodine-based agents are more expensive than the older “ionic” contrast agents, but provide a decreased risk of adverse reactions and renal dam- age. If the patient has a simple allergy to IV contrast, such as hives; a low osmolar, “nonionic” contrast should be used and the patient can be premedicated with steroids and diphenhydramine (Table 11.1). Prior anaphylaxis to IV contrast is a complete con- traindication. Orally administered water soluble contrast and rec- tal contrast add significant additional diagnostic information as accurate interpretation requires optimal opacification of the GI tract. However, to allow appropriate intraluminal distention with contrast, the patient has to drink contrast 1–2 hours before the CT scan can be performed. Diluted 2% barium mixture can also be used for oral contrast, but if bowel perforation is suspected, barium should be avoided as it can cause severe peritonitis. Pneumatosis, Pneumoperitoneum, and Bowel Trauma Pneumatosis (air in the bowel wall due to ischemia) and pneu- moperitoneum from a traumatic or nontraumatic perforated This section will focus on abdominal trauma to the bowel and mesentery that a colorectal surgeon may encounter. Trauma to the other intraabdominal organs (e.g. liver, spleen, bones) will be omitted. Figure 11.9 Pneumoperitoneum. When viewed on lung windows, free air can be seen in the subdiaphragmatic area, along the anterior peritoneal surfaces of the liver. Figure 11.10 CT Signs of Bowel Injury. MDCT axial section demonstrates pneumoperitoneum, free intraabdominal fluid, an air fluid level, and extravasation of oral contrast material in a patient with blunt abdominal trauma. Table 11.1 Premedication for Contrast Allergy. Oral Premedication for Contrast Allergy Prednisone 50 mg orally at 13, 6, and 1 hour prior to IV contrast administration Diphenhydramine 50 mg orally 1 hour prior to IV contrast administration. Emergency Stat Premedication for Contrast Allergy 30 minutes prior to IV contrast: Hydrocortisone 100 mg IV or Solucortef 200 mg IV and Diphenhydramine 50 mg IV. abdominal viscus, can be identified on MDCT when examined on “lung windows” (window level – 400 to -600 H; window width 1000 to 2000 H). Pneumoperitoneum can be seen in the subdia- phragmatic area, along the anterior surfaces of the liver (Figure 11.9). The accuracy of CT in the diagnosis of blunt abdominal trauma has been reported to be as high as 97%.(28–33) The routine use of oral contrast in CT examination of abdomi- nal trauma is controversial. Oral contrast material can aid in the identification of bowel loops, and differentiation of bowel from hematoma or hemorrhage. Disadvantages of oral contrast include risk of aspiration, and additional time requirements which may delay diagnosis.(33–35) Posttraumatic abdomi- nal CT examinations should be performed using IV contrast, which maximizes the difference between contrast-enhancing bowel and nonenhancing hematomas.(36) CT signs of bowel and mesenteric injury include: (1) pneumoperitoneum; (2) extravasation of oral contrast material; (3) free intraabdominal fluid (Figure 11.10); (4) intramural hemorrhage, manifested as luminal narrowing with thickened and/or discontinuous bowel Table 11.2 Bowel wall thickening. Benign Neoplastic Circumferential thickening Eccentric thickening Symmetric thickening Asymmetric thickening Thickening <1–2 cm Thickening >2 cm Segmental or diffuse involvement Focal soft tissue mass Mesenteric fat thickening Long segments of involvement Abrupt transition and lobulated contour Short segments of involvement except lymphoma Wall is homogeneous soft tissue density Spiculated outer contour Stratified enhancement “double halo” or “target” appearance Luminal narrowing, adenopathy, or liver metastasis. 11 limitations of colorectal imaging studies wall; (5) intense enhancement or high-attenuation clot (senti- nel clot) adjacent to the involved bowel (30, 37–48). Bowel Wall Thickening When fully distended the bowel wall is 1–2 mm in thickness, and the collapsed bowel wall should not exceed 3–4 mm. MDCT can often differentiate benign from malignant wall thickening (Table 11.2). Benign, pathologic wall thickening (Figure 11.11) is seen in infectious, inflammatory, and ischemic processes; usually does not exceed 1–2 cm; is homogenous in attenuation; and is circum- ferential, symmetric, and segmental in distribution.(49) A strati- fied enhancement pattern in a thickened segment of bowel wall is used to exclude malignant conditions. Such a pattern may have a “double halo” or “target” appearance of the intestine in cross section which is caused by inflammation, edema, and hyperemia (Figure 11.12). Neoplastic wall thickening is thicker (2–3 cm), asymmetric, nodular, lobulated, or spiculated in contour and tends to narrow the intestinal lumen (Figure 11.13). The extent or length of bowel wall involvement aids in narrowing the differ- ential diagnosis. With few exceptions (mainly lymphoma), long segments of involvement indicate a benign condition. When the perienteric fat adjacent to a thickened bowel segment is normal, Figure 11.11 Benign, Pathologic Wall Thickening. MDCT coronal reformatted image demonstrates colonic wall thickening (<1 cm) that is homogenous, circum- ferential, symmetric, and segmental in distribution. The long segment of involvement suggests a benign condition. Figure 11.12 “Double Halo” or “Target” Sign. MDCT axial image of the same patient in Figure 11.II-D-1, demonstrates a stratified enhancement pattern in a thickened segment of descending colon bowel wall. Figure 11.13 Neoplastic Wall Thickening. MDCT axial image demonstrates abnormal colon wall thickening (>3 cm) that is asymmetric, nodular, and lobulated in contour with narrowing of the intestinal lumen. The combination of a paucity of pericolonic fat stranding with the short segment of involvement is worrisome for malignancy. Surgery confirmed adenocarcinoma. Table 11.3 Ulcerative colitis versus Crohn Colitis. Ulcerative Colitis Crohn Colitis Circumferential disease Eccentric disease Continuous disease Skip lesions Predominantly left-sided Predominantly right-sided Rectum usually involved Rectum normal in 50% Confluent shallow ulcers Confluent deep ulcers No aphthous ulcers Aphthous ulcer early Collar button ulcers Transverse and longitudinal ulcers Terminal ileum usually normal Terminal ileum usually diseased Terminal ileum patulous in backwash ileitis Terminal ileum narrowed No pseudodiverticula Pseudodiverticula Inflammatory polyps and pseudopolyps No polyps or pseudopolyps No fistulas Fistulas common High risk of cancer Low risk of cancer Risk of toxic megacolon No toxic megacolon 1 improved outcomes in colon and rectal surgery an acute inflammatory condition is less likely. Fat stranding that is disproportionately more severe than the degree of wall thick- ening suggests an infectious or inflammatory process.(50) Inflammatory Bowel Disease (IBD) Benign, circumferential thickening of the bowel wall is a hallmark of Crohn’s Disease (Table 11.3). Wall thickening (Figure 11.14) can result in strictures that narrow the bowel lumen in advanced disease.(51–52) CT is excellent in documenting the extralumi- nal manifestations of the disease such as the “comb sign” (Figure 11.15), which is produced by hyperemic thickening of the vasa recta due to active disease. The swollen blood vessels produce an appearance like the teeth of a comb extending from the thickened bowel wall into the mesenteric fat.(53) “Skip areas” of normal bowel intervened between diseased segments are characteristic. Mesenteric abscesses are characteristic and may can contain fluid, air, or contrast material (54–57) (Figure 11.16). Crohn’s Colitis is characterized by transmural inflammation that usually affects the terminal ileum (80%) and proximal colon (50%).(51–52) Bowel wall thickening in Crohn’s colitis is typically 10 to 20 mm com- pared with the 7 to 8 mm for Ulcerative Colitis (UC). With Crohn’s colitis (Figure 11.15), the outer wall is irregular, whereas with UC the outer wall is smooth.(58) Acute active disease shows layering of the colon wall (“target and halo” signs), whereas chronic dis- ease with fibrosis shows homogeneous enhancement of the colon wall. Fibrous and fat proliferation in the mesentery (“creeping Figure 11.14 Crohn’s Disease. Axial MDCT demonstrates circumferential wall thickening of the terminal ileum that results in narrowing of the bowel lumen and formation of a stricture. Figure 11.15 “Comb Sign”. Swollen blood vessels produce an appearance like the teeth of a comb extending from the thickened bowel wall into the mesenteric fat. Note the irregular outer wall of the cecum indicating Crohn’s colitis. Figure 11.16 Crohn’s Abscess. Axial MDCT image demonstrates extraluminal abscess in this patient with Crohn’s disease/colitis. Figure 11.17 Ulcerative Colitis. Coronal reformatted CT image of the abdomen and pelvis shows wall thickening and marked irregularity of the remaining mucosa in the ascending and descending colon (arrows). 1 limitations of colorectal imaging studies fat”) separates bowel loops with extensive fat-containing fibrous strands. Sinus tracts between bowel loops, enterocutaneous fis- tulas, and enterovesicular fistulas are also characteristic findings. Ulcerative Colitis (UC) is characterized by inflammation and dif- fuse ulceration of the colon mucosa (Figure 11.17). The disease starts in the rectum and extends proximally to involve part or the entire colon. Wall thickening with luminal narrowing and the inflammatory pseudopolyps, that result from mucosal ulceration, are sometimes seen on CT (Figure 11.18). Narrowing of the rectal lumen with thickening of the rectal wall and widening of the pre- sacral space are often seen (51–53, 58). Similar to Crohn’s disease, mesenteric adenopathy can be seen, but it is not specific for IBD. Other Colitides Psuedomembranous colitis results from overgrowth of Clostri- dium difficile and its enterotoxin, as a complication of antibiotic therapy. A pancolitis with irregular wall thickening (up to 30 mm) Figure 11.18 Ulcerative Colitis. Coronal reformatted CT image of the abdomen and pelvis shows pseudopolyps (arrow) that extend into the lumen of the transverse colon. Figure 11.19 Psuedomembranous Colitis. Axial MDCT demonstrates pancolitis with irregular wall thickening and submucoal edema resulting in a characteristic “accordion pattern” of disease. Figure 11.20 Typhilitis. MDCT axial image demonstrates marked wall thickening, low-density edema within the cecal wall, and pericecal fluid and inflammation in a patient with HIV. The wall thickening and inflammation extended from the cecum to the hepatic flexure. Figure 11.21 Radiation Colitis. Axial MDCT image demonstrates mucosal thickening with prominent stranding in the expanded pericolic fat confined to the radiation port distribution. A lymphomatous mass is noted adjacent to the radiation colitis (large arrow). 1 improved outcomes in colon and rectal surgery and submucoal edema results in the “accordion pattern” of dis- ease that is characteristic.(58) The edema and thickening of the colon may be greater than that seen with other colitides, and a pancolitis suggests pseudomembranous colitis (Figure 11.19). Typhilitis or neutropenic colitis refers to a potentially fatal infection of the cecum and ascending colon in patients who are immuno- compromised. CT is the study of choice for making the diagno- sis. Typhilitis is characterized by marked wall thickening (10–30 mm), low-density edema in the cecal wall, pericecal fluid, and inflammation (Figure 11.20), which may be confused with the reactive changes of appendicitis.(58–59) The length of the cecum and right colon involved is generally much greater with typhlitis, and the thickening is more asymmetric in appendicitis.(60) The presence of known risk factors favors the diagnosis of typhilitis. Ischemic colitis features include benign, pathologic wall thicken- ing in a vascular distribution watershed segment of colon, usually at the splenic flexure (Figure 11.12). Radiation colitis in the acute phase demonstrates mild wall thickening and pericolic fat strand- ing confined to the radiation port area. Chronic radiation injury 6 to 24 months after treatment appears as mucosal thickening with prominent stranding in expanded pericolic fat (Figure 11.21). Infectious colitis differentiation is based on clinical findings because CT findings are nonspecific. Infectious terminal ileitis is usually caused by Yersinia, Tuberculosis, Campylobacter, or Salmonella organisms.(61) The diagnosis is made clinically with stool cultures. CT features are benign wall thickening of the ter- minal ileum and cecum, and moderate or marked enlargement of the mesenteric lymph nodes in the right lower quadrant. (62) Although a Meckel’s diverticulum occurs at some distance (60–100 cm) from the cecum, it may cause complications such as inflammation, whose differential diagnosis includes appendicitis and IBD.(63) The CT diagnosis of an inflamed Meckel’s diver- ticulum relies on the identification of a blind-ending, tubular, or round pouch-like structure in the right lower quadrant attached to the small intestine with surrounding inflammation. Small intestinal obstruction and visualization of a normal appendix aid in diagnosis.(63–64) Diverticulitis Acute diverticulitis is characterized by benign wall thickening, hyperemic contrast enhancement, inflammatory pericolic fat stranding, and diverticula in the involved segment (Figure 11.22). Perforation or abscess may form, and CT is better suited to dem- onstrate extraluminal disease than barium enema (Figure 11.23). Sinus tracts and fistulas may extend to adjacent organs or the skin and are represented by linear fluid or air collections. Air in the bladder suggests a colovesicular fistula, unless the patient has had Figure 11.22 Diverticulitis. Axial MDCT demonstrates benign wall thickening, hyperemic contrast enhancement, and inflammatory change that extend into the pericolic fat, with diverticula in the involved segment. Figure 11.24 Colon Cancer. Axial MDCT demonstrates a large colonic mass in the descending colon that narrows the lumen. CT cannot differentiate tumor extension through the wall from pericolonic edema or desmoplastic reaction. Figure 11.23 Perforated Diverticulitis. Axial MDCT image demonstrates colon wall thickening and extravasation of oral contrast (small arrows), as well as free peritoneal air anteriorly (large arrow). 1 limitations of colorectal imaging studies bladder catheterization. Obstruction of the colon or urinary tract from the inflammatory process can be seen on CT. Diverticulitis of the right colon may be confused with acute appendicitis or IBD. Visualization of a normal appendix or inflammatory changes involving the ascending colon distal to the ileocecal valve favor the diagnosis of diverticulitis over appendicitis.(58) The extrin- sic inflammation from a tubo-ovarian abscess may cause serosal edema and mural thickening of the cecum or sigmoid colon wall. Recognizing that the inflammation is centered in the adnexa and appropriate distention of the colon with oral and rectal contrast agents assist in making the correct diagnosis.(65–66) Colorectal Cancer Detection and Differentiation from Diverticulitis The CT appearance of diverticulitis overlaps that of colon cancer, as bowel wall thickening and pericolonic fat stranding occur in both. Fluid in the sigmoid mesentery and engorgement of mes- enteric vessels favors diverticulitis (Table 11.2). Enlarged lymph nodes, asymmetric wall thickening, and the presence of an intralu- minal mass (Figure 11.24) favors cancer.(67–73) Prior recent films showing no colonic wall thickening can aid in diagnosis. A biopsy may be required in equivocal cases. Colorectal Cancer Staging Preoperative CT for staging colorectal cancer is performed to detect invasion of adjacent organs, enlargement of local nodes, or evidence of distant metastases. The Tumor, Nodes, Metastasis (TNM) and Dukes classification are both used for preoperative staging (Table 11.4). Squamous cell carcinoma can occur in the anal canal, and it does not have any discriminating imaging fea- tures from adenocarcinoma. The primary adenocarcinoma may be seen as a polyp larger than 1 cm, or a soft-tissue cancer mass that narrows the lumen of the colon.(74) Flat lesions appear as focal, lobulated thickening of the bowel wall (>3 mm). “Apple core” lesions demonstrate irregular bulky circumferential wall thickening with marked and irregular narrowing of the bowel lumen.(74–75) Adequate luminal distention is essential, and may be achieved with oral and rectal water soluble contrast or water as a negative contrast agent.(76–77) The accuracy of CT in preoper- ative staging varies from 48% to 77%, and this variability depends on the actual stage of the cancer.(78–82) The accuracy of CT stag- ing ranges from 17% for early lesions (Dukes stage B) to 81% for advanced lesions (Dukes stage D) (75, 80, 83). Inaccuracies arise from the inability to distinguish tumor from peritumoral desmo- plastic reaction or edema, and the inability to detect microscopic extramural tumor extension (Figure 11.24). A major advantage of CT is the ability to demonstrate the local extent of tumor and involvement of adjacent organs, such as the bladder, vagina, peri- toneum, and abdominal or pelvic musculature.(84) Also, MDCT can reliably detect enlarged lymph nodes for staging of colorectal cancer.(83) Although the presence of lymph nodes larger than 1–1.5 cm in short-axis diameter is considered pathologic, not all enlarged nodes contain tumor. Conversely, normal-sized nodes may have microscopic tumor involvement.(80) Another factor that poses problems in staging is ischemic bowel associated with an obstructing colon cancer.(85–87) Detecting ischemic change proximal to colonic carcinoma is important because 25% of cases Table 11.4 TNM compared to modified Dukes staging for colorectal carcinoma. TNM Stage Modified Dukes Stage I: Submucosa T1, Muscularis, T2 N0M0 A: Limited to wall mucosa or submucosa II: Muscularis T3, Perirectal T4 N0M0 B: Extension into or through serosa IIIA: T1–4N1M0 IIIB: T1–4N2–3M0 C: Cancer that extends to lymph nodes IV: T1–4 N1–3 M1 D: Distant Metastasis Figure 11.25 Colon cancer liver metastasis. MDCT performed with IV contrast material during the portal vein phase of enhancement shows heterogeneous, ring- enhancing metastases that are hypodense to the liver. Figure 11.26 Mucinous Adenocarcinoma Metastatic Disease. MDCT axial image shows necrosis and calcification of meatastatic mucinous adenocarcinoma to the liver. 1 improved outcomes in colon and rectal surgery with proximal ischemic colitis have been reported to cause post- operative complications such as suture-line disruption.(86–89) Also, an ischemic segment in colonic carcinoma may give a false radiologic impression regarding tumor length or depth of tumor invasion.(85) Ischemia can develop at sites remote from the obstructing cancer, such as the terminal ileum, mimicking syn- chronous lesions.(85, 89–93) CT can usually distinguish an isch- emic segment from a tumoral segment in approximately 75% of the cases by applying the criteria for benign (ischemic) bowel wall thickening, and the “target” or “double halo” sign is the most spe- cific sign.(88–93) This finding must be interpreted with caution, as signet ring cell adenocarcinoma can have low-density mural thickening. Colorectal Cancer Metastasis The liver is the predominant organ to be involved with metas- tases from colorectal cancer, and MDCT has an established role in detection. Hepatic metastases are supplied by the hepatic artery, and two thirds of the liver is supplied by the portal vein. MDCT (performed with IV contrast material during the portal vein phase of enhancement, 60–70 sec after the start of the bolus) typically shows heterogeneous, ring-enhancing metastases that are hypodense to the liver (Figure 11.25). Images obtained after a longer delay may not reveal evidence of disease because lesions become isodense to the liver.(94–95) Small lesions (<1 cm) do not have adequate enhancing properties to be differentiated and are named indeterminate lesions. Necrosis and calcification of metastatic mucinous adenocarcinoma to the liver can be seen on MDCT (Figure 11.26). CT can detect intraperitoneal metastases, which appear as thickening of the peritoneal surfaces or perito- neal nodules (Figure 11.27), but microscopic seeding will not be detected. Careful attention to the peritoneal surfaces and omen- tum is needed in the setting of unexplained ascites to identify peritoneal metastases. CT arterial portography is a very sensitive technique for detec- tion of liver metastasis. This procedure requires fluoroscopic/ angiographic guided catheter placement in the superior mes- enteric artery using the Seldinger technique through the femo- ral artery. Intraarterial injection of contrast material results in intense portal vein enhancement of the normal liver. Both meta- static disease and benign perfusion abnormalities manifest as fill- ing defects on CT portography, and therefore the sensitivity of this test is less than perfect. Likewise, intraarterial hepatic artery injection (hepatic arteriography) can aid in identifying liver metastasis as they will enhance brightly. Some studies have sug- gested that combined CT arterial portography and CT hepatic arteriography significantly improved the detect ability of hepatic metastases.(96) However, these unified CT-angiography sys- tems are not widely available and require an invasive procedure. Furthermore, a recent study (97) showed that the use of portal venous phase enhanced MDCT as the only preoperative imag- ing technique in the assessment of colorectal cancer metastases allowed accurate preoperative staging (sensitivity, 85.1%; positive predictive value, 96.1%). Colorectal Cancer Recurrence Local recurrent tumor usually appears as a soft-tissue mass in or near the surgical site. Due to the often largely extrinsic component of local recurrence, CT is better than colonoscopy at demonstrat- ing the early, mass like tumor recurrence.(98, 99) This appearance can mimic postoperative fibrosis, although fibrosis usually appears more linear without a discrete mass. Following abdominoperineal resection, it is common to see soft tissue density within the pre- sacral space. When this material remains plaque-like, fibrosis is likely. Often, distinction between postoperative fibrosis and recur- rent tumor is not possible unless serial scans are obtained. CT findings clearly indicative of recurrent malignant disease include enlargement of a soft-tissue mass over time, enlarging lymphade- nopathy, and invasion of contiguous structures. CT performed with intravenous contrast material is the imaging modality of choice for detection of recurrent tumor within the liver. CT has been shown to be more helpful in diagnosis of recurrent Figure 11.27 Colon Cancer Peritoneal Metastasis. Axial MDCT demonstrates thickening of the peritoneal surfaces, ascities, and two large peritoneal nodules in a patient with colon cancer. Figure 11.28 Anastomotic Leak. Axial MDCT demonstrates high density ascites representing extravasted oral contrast in this patient status postterminal ileum resection and ileocolic anastomosis. Pneumoperitoneum is also seen. 1 limitations of colorectal imaging studies hepatic metastases than laboratory studies (liver function tests, mea- surement of carcinoembryonic antigen level).(80, 100) The impor- tance of abdominal CT is reflected in the surveillance guidelines proposed by the American Society of Clinical Oncology.(101) The guidelines recommend annual CT of the abdomen for 3 years after primary therapy for patients who are at high risk of recurrence. Postoperative Complications MDCT is often used to diagnose postoperative complications such as abscess, anastomotic leak, fistula, small bowel obstruc- tion, ileus, and pulmonary embolus (small bowel obstruction and ileus will be covered later). Before postoperative day five, it is difficult to differentiate normal postoperative intraperitoneal free air and fluid from fluid or air from an anastomotic leak or abscess.(102) Findings suggestive of an anastomotic leak include an inappropriate volume of free air or fluid in the abdomen. The presence of extraluminal oral contrast (Figure 11.28) confirms an anastomotic leak.(102) CT is the most accurate imaging test for diagnosing abscess formation in the postsurgical patient.(103) The combination of water soluble oral contrast and an intravenous contrast agent is essential in differentiating between a fluid-filled bowel loop and an abscess. The CT appearance of an abscess is variable depend- ing on its age and location. Early abscess appears as a mass with an attenuation value near that of soft tissue. As the abscess matures, it undergoes necrosis with a central region of near-water attenuation surrounded by a high attenuation rim that usually enhances.(104) Approximately one third of abscesses contain variable amounts of gas (Figure 11.29).(104–108) Postoperative packing materials used for hemostasis, such as oxidized cellulose and gelatin bioabsorbable sponge, can mimic a gas-containing abscess. Findings that may help differentiate are: linear arrange- ment of tightly packed gas bubbles, an unchanged appearance on subsequent examinations, and lack of either a gas-fluid level or an enhancing wall.(109, 110) A low-density mass containing a high density object suggests a foreign body abscess caused by a retained surgical sponge (gossypiboma).(111) Although CT findings are highly suggestive of abscess, they are not specific. Other masses that can have a central low attenuation include a cyst, pseudo- cyst, hematoma, urinoma, lymphocele, biloma, loculated ascites, thrombosed aneurysm, and necrotic neoplasm. Because a specific diagnosis of abscess based on CT findings alone is not possible, correlation with clinical history is important. Percutaneous nee- dle aspiration may be necessary to make a definitive diagnosis (See interventional radiology). Fistula formation between the bowel and other organs such as the bladder, vagina, or skin can form as complications of surgi- cal or radiation therapy. Fistula can be identified with IV or oral contrast and are demonstrated as extension of contrast from one organ to the other. Oral and IV contrast should not be used simultaneously for a suspected bowel/bladder fistula because contrast in the bladder could be from renal excretion or a bowel/ bladder fistula. Pulmonary embolism can occur in cancer and postoperative patients. Since the introduction of spiral MDCT, CT angiography Figure 11.29 Postoperative Abscess. Axial MDCT demonstrates a large air fluid collection (abscess) in the operative field, in this patient status posttotal colectomy and ileorectal anastomosis. The anastomotic suture line can also be seen. The large amount of gas in the abscess suggests communication with the gastrointestinal tract. Figure 11.30 A and 11.30B Acute Appendicitis. Axial MDCT demonstrates an obstructing calcified appendicolith with an abnormally dilated, enhancing appendix surrounded by inflammatory fat stranding. (a) (b) . remaining mucosa in the ascending and descending colon (arrows). 1 limitations of colorectal imaging studies fat”) separates bowel loops with extensive fat-containing fibrous strands. Sinus. thickening and pericolic fat strand- ing confined to the radiation port area. Chronic radiation injury 6 to 24 months after treatment appears as mucosal thickening with prominent stranding in expanded. Mucinous Adenocarcinoma Metastatic Disease. MDCT axial image shows necrosis and calcification of meatastatic mucinous adenocarcinoma to the liver. 1 improved outcomes in colon and rectal surgery with