Small Bowel: CT Evaluation of the Small Bowel: Current Status 2001
Elliot K. Fishman, M.D.
Introduction
The evaluation of the small bowel has always been one of the most challenging examinations for the radiologist. Although our ability to image mucosal detail has improved with double contrast barium studies and enteroclysis, these studies are both time-consuming and experience-intensive and are used judiciously in a few select institutions. The standard small bowel series is also valuable but has limitations, particularly for detecting and defining extra-mucosal disease. In recent years, computed tomography (CT) has made great strides and is currently the prime imaging modality for the evaluation and staging of small bowel pathology. This lecture will discuss the current role of spiral CT in the evaluation of the small bowel with an emphasis on some of the key features that lead to an accurate differential diagnosis.
I. Study Design and Protocols
The key to the successful examination of the small bowel begins and ends with technique. The optimal CT exam preparation results in total bowel opacification with an oral contrast media. The ideal contrast should therefore be well tolerated by the patient and provide consistent and homogeneous bowel opacification. Our current technique uses 1000 cc of oral Hypaque (diatrizoate sodium, Nycomed Amersham) at a solution strength of 3%. Other oral contrast agents, whether barium-based or gastrografin, can be used but in our experience these agents are not as well tolerated. In cases where oral contrast cannot be given (i.e., patient is NPO or very nauseous), a CT scan may be done without any oral contrast as the fluid in bowel will be an excellent marker of bowel distension and its cause. The use of intravenous contrast is not always mandatory although its delivery via a rapid injection (100-120 cc of Omnipaque 350 (Nycomed Amersham) injected at 2-3cc/sec) is often helpful for defining bowel pathology as well as in evaluating other organs like the liver and kidneys. Intravenous contrast should be avoided in those cases where suspected enterovesicle fistulae is a possibility. We have also found that water can be used as an oral contrast agent especially when rapid infusion of IV contrast is used. We have found this technique especially valuable when multiplanar reconstruction (MPR) or 3D imaging is used. Although specific scan parameters will vary depending on the scanner, several general rules can be noted. With a standard spiral CT scanner we routinely scan from the diaphragm to the symphysis pubis with 5-mm collimation at 8-mm intervals. All patients are scanned in the supine position in suspended inspiration. Additional scans can be obtained as needed and in select cases delayed scans may be required if transit time is slow because of either physiologic or functional problems. With Spiral CT we will use .75-1 second scans at 250-300 mAs and 120 kVp. A 30-40 second Spiral acquisition with a single breathhold is usually needed. With multidetector CT (MDCT) we use a 2.5 mm collimator with data reconstructed at 5mm slice thickness at 5 mm intervals. When evaluating a patient with suspected intestinal ischemia, a modification of technique is necessary. The examination with focus on 2 areas: imaging the mesenteric arteries and veins, and imaging the bowel. For accurate mesenteric vessel evaluation, a rapid IV contrast bolus is required. We routinely administer 120 cc of non-ionic contrast through a peripheral catheter at a rate of 3 cc/sec. Since both the arteries and veins can be involved, dual phase imaging is obtained. We acquire images at 25 second (arterial) and 50 seconds (venous) after the start of the injection. Using our MDCT scanner (Siemens, VolumeZoom), we utilized the 4 X 1mm collimator setting to obtain 1.25mm slices. We routinely perform 3 D imaging on these patients, since it greatly improves visualization and identification of branches of the mesenteric arteries and veins. In addition to evaluation of the mesenteric vasculature, it is important to examine the intestines and bowel wall. The use of water as oral contrast will allow excellent visualization of the bowel wall and will not interfere with 3D imaging of the vasculature. We typically ask patients to drink 750 cc of water 20 minutes prior to the study and an additional 250 cc of water immediately prior to the study, in order to distend the stomach and duodenum.
II. Normal Small Bowel and Mesentery
One of the most reliable CT markers of bowel pathology is an increase in the bowel wall thickness. The normal small bowel wall thickness is 3 mm or less. This means that a bowel loop opacified with contrast has essentially no definable wall. The presence of wall thickening, although not a specific marker for a disease entity, is diagnostic of bowel pathology. The mesentery is normally similar in attenuation to fat, often with small nodes in the mesentery in the range of 3-4 mm. With the submillimeter resolution of the newer CT scanners, we also can easily see the key mesenteric vessels and their branching patterns. Abnormalities in bowel rotation can be inferred from the location of the SMA and SMV orientations because mesenteric vascular inversion (i.e., the SMA is to the right of the SMV) is associated with bowel malrotation; although not pathognomonic for it. The distribution of the small bowel within the abdominal cavity is also fairly constant and any change in orientation may suggest bowel pathology.
III. Inflammatory Bowel Disease
• Crohns Disease Crohns disease is the prototype for inflammatory bowel disease on CT because there is both bowel and mesentery involvement as well as extracolonic manifestations. Although a carefully performed small bowel series has a high sensitivity for detecting subtle mucosal changes in the bowel, barium studies are limited in detecting extracolonic manifestations of the disease process. In addition, when barium studies suggest an extracolonic manifestation of disease it is through secondary signs (i.e., mass effect or displacement) that may either overestimate or underestimate disease extent. Goldberg, et al. correlated the CT findings in 28 patients with Crohn disease with barium studies, sinograms, and surgical findings. CT proved superior in demonstrating the extent of disease including bowel wall thickening (82%), fibrofatty proliferation of mesenteric fat (39%), mesenteric abscess (25%), inflammatory reaction of the mesentery (14%), and mesenteric adenopathy (18%). On CT the typical appearance of a diseased loop of small bowel is wall thickening of 7-11 mm. The wall thickening is usually symmetrical although ulcerations may be seen, particularly if images are filmed on bone window settings. Although most disease will involve the distal ileum, involvement of other loops of bowel, including the duodenum, are not uncommon. Skipped segments of bowel are also frequent. In many cases a low-density zone, often referred to as the halo sign, will be seen in the submucosa of bowel. Although not specific for Crohns disease, the halo sign is indeed a marker for bowel pathology and may be seen in cases of radiation enteritis, ischemic bowel, infection and graft versus host disease. In our experience, the halo sign is more commonly seen in patients with long-standing disease. The mesentery is commonly involved in the Crohns patient and this is usually due to focal inflammation. The CT appearance reflects an increase in the density of the mesentery, although mesenteric masses representing abscesses or matted nodes may be seen. A mesenteric fatty mass may occasionally be seen and represents the "creeping fat" of long-standing Crohns disease. These mesenteric fatty masses can occasionally simulate abdominal abscesses on small bowel series or plain radiographs. Increased mesenteric or pericolonic fat is not uncommon in patients with long-standing Crohns disease. Mesenteric or interloop abscesses may also occur and care should be taken not to confuse them with unopacified bowel loops. Extracolonic manifestations of Crohns disease can be divided into those processes with an acute presentation and those of a more chronic nature. Acute processes include abscesses and fistulae formation. Abscess formation may occur anywhere in the abdominal cavity but is most common in the right lower quadrant and in the pelvis. CT can be used to determine whether or not the abscess is amenable to percutaneous drainage or if surgical intervention is necessary. Fistulous tract formation is not uncommon in the patient with Crohns disease. Fistulaes may extend from bowel to adjacent bowel loops (small bowel or colon), adjacent organs (bladder, kidney), and muscle or bony structures. CT is excellent at defining both the presence of the fistulous tract as well as its extent. For example, in the evaluation of enterovesical fistulae conventional studies (barium enema, small bowel series, cystoscopy, colonoscopy) detect less than 50% of cases. CT has been shown to have a better than 90% success rate in the detection and definition of fistulae. Unusual sites of fistulae, including the stomach or pancreas, can also be defined with CT. CT can be used to help plan interventional approaches in select patients. There is a small but definite increase in the frequency of small bowel tumors in the patient with Crohn’s disease. These include small bowel adenocarcinoma and lymphoma. In the patient with Crohns disease and enlarged abdominal nodes, care should be taken to rule out an underlying neoplasm. This is also true if the bowel thickening is greater than 20 mm or appears to be mass-like. Although most patients with Crohns disease are adults, the disease also involves patients in the pediatric age group. Jabra, et al. reviewed the CT scans of 25 patients aged 10-18 and concluded that CT should be the initial imaging study in children with known Crohns disease and a changing pattern of clinical symptoms. We have also begun to use CT angiography for the evaluation of disease activity in Crohn’s disease. The 3D findings include increased size and number of mesenteric branch vessels to the diseased bowel loop as well as increased and persistent enhancement of the bowel loop. Further research into the true sensitivity and specificity of these findings is the subject of ongoing research. • Ischemic Bowel The early diagnosis of bowel ischemia is a difficult one from both a clinical and radiologic perspective. Patients typically are older and the signs and symptoms at presentation are extremely variable and may be similar to other pathologies of the acute abdomen. CT is commonly used for diagnosis in patients presenting with an acute abdomen, thus the specific appearance of the CT scan in this setting is important. The major CT findings in bowel ischemia and/or infarction include diffuse or focal bowel wall thickening, focally dilated loops of bowel, intramural gas, mesenteric or portal venous gas, and ascites. Using these signs, Smerud et al. retrospectively evaluated 23 patients with proven ischemic bowel and found 9 patients (39%) with CT signs consistent with the diagnosis. In this series, CT was superior to plain radiographs in detecting venous air. Several authors have shown that CT is more sensitive for the detection of small bowel air than plain radiographs. In these cases the use of lung windows on the abdominal images (window width 1650, window center -650) is helpful for detecting subtle air in the bowel wall. Although intramural gas and/or gas in the splanchnic or portal vasculature is characteristic of infarcted bowel, we have seen several cases where both pneumatosis and portal venous air was present yet no ischemic or infarcted bowel was seen at surgery. It should be emphasized that a normal CT scan does not exclude the diagnosis of ischemic bowel although it does make it less likely. There has been increased interest recently in using Spiral CT with rapid contrast injection for determining the presence of bowel ischemia. Persistent bowel wall enhancement is suggested as one of the signs of bowel wall ischemia. Recent work with multidetector CT scanners suggest that a combination of measuring bowel enhancement and the creation of CT angiograms may be ideal for the early detection of bowel ischemia. A detailed analysis of the role of CT in mesenteric ischemia was published by Horton and Fishman and several of the findings include: Mesenteric ischemia can result from a variety of conditions, all of which result in compromised blood flow to the gut. The condition can be organized into acute mesenteric ischemia and chronic mesenteric ischemia.
- Acute Mesenteric Ischemia Acute mesenteric ischemia results from decreased blood flow to the intestines. This can be a result of (1) arterial embolism or thrombosis, (2) venous occlusion or (3) low flow states compromising the splanchnic circulation. Patients with acute mesenteric ischemia usually present with severe abdominal pain. The exact clinical presentation will depend on the etiology. The most common etiology of acute mesenteric ischemia is an embolism to the SMA. Patients with mesenteric ischemia as a result of an SMA embolism will present with sudden onset of pain, typically in the periumbilical region or right lower quadrant. Classically the pain is out of proportion to the physical findings. The patient may also report nausea, vomiting or diarrhea. Laboratory studies may reveal en elevated white blood cell count and in cases of intestinal infarction, there may also be acidosis, hypovolemia and hemoconcentration. Most emboli originate in the heart. Therapy may involve systemic anticoagulation or angiography with intra-arterial thrombolysis. In severe cases, surgery may be necessary to reestablish blood flow and to resect infarcted bowel. Patients with thrombosis of the superior mesenteric artery usually present with a more gradual onset of abdominal pain. Often these patients will have a history consistent with chronic mesenteric ischemia and will have underlying atherosclerotic disease effecting the mesenteric arteries. A thrombosis then forms on a pre-existing plaque, resulting in acute symptoms. This is estimated to account for up to 25% of cases of acute mesenteric ischemia. Again the patient’s complaints may be significant, although the physical exam findings are not impressive; i.e. the abdomen is usually soft and non-tender. Laboratory findings may be aid in the diagnosis, but are usually not significantly elevated until infarction has occurred. These patients may require emergent systemic anticoagulation or intra-arterial thrombolysis. Once they are stabilized, surgery will often be performed in order to bypass the vessels compromised by atherosclerotic plaque. Non-occlusive intestinal ischemia accounts for up to a third of cases of acute mesenteric ischemia and occurs in patients with systemic shock, severe dehydration, hypovolemia from trauma, decreased cardiac output, or drug-induced vasospasm. Patients will present with vague abdominal pain, which is often severe. The clinical history is most helpful in identifying these patients. Treatment usually consists of selective arterial administration of vasodilators and fluid resuscitation in an effort to restore adequate blood flow to the intestines. Patients with peritoneal signs will require surgery. The least common cause of acute mesenteric ischemia is venous thrombosis. Thrombus can form in the superior mesenteric vein in patients with hypercoaguable states, post-operative patients, patients with trauma or patients with portal hypertension, pancreatitis, or diverticulitis. Treatment usually consists of anticoagulation and treatment of the underlying disorder.
- Chronic Mesenteric Ischemia
Chronic mesenteric ischemia typically has a more insidious course with patients complaining of repeated episodes of abdominal pain, frequently occurring after a large meal. As a result patients may develop sitophobia (fear of eating) and involuntary weight loss. Most patients are over the age of 50 and have widespread atherosclerotic disease. Almost 95% of cases of chronic mesenteric ischemia result from stenosis of the superior mesenteric artery by atherosclerotic plaque. The severity and frequency of symptoms depend on the availability of adequate collaterals. On physical exam, the patients are usually malnourished and when suffering from an acute attack, complain of sever abdominal pain. Typically there will be no rebound or guarding on physical examination. An abdominal bruit is occasionally present. Treatment may include long acting nitrates, which result in mesenteric vasodilatation. However, surgery is usually required. Surgical treatment consists of arterial reconstruction or aortomesenteric bypass. Recently percutaneous transluminal angioplasty and endovascular stent have been attempted in this patients population.
- CT Findings
- Acute Mesenteric Ischemia When imaging a patient for acute possible mesenteric ischemia, it is important to evaluate the mesenteric vasculature, both the mesenteric arteries (superior mesenteric artery, inferior mesenteric artery) and mesenteric veins (superior mesenteric vein, inferior mesenteric vein). Patients with acute ischemia may demonstrate thrombus within the mesenteric arteries or veins. This will typically appear as low density clot within the proximal portion of the vessels, although distal thrombi can also occur, especially if related to emboli from the heart. These distal thrombi may not be directly visualized since they are within distal branches, but segmental areas of wall thickening and /or pneumatosis may be seen. The thrombus can be occlusive or non-occlusive. Tumors with mesenteric vessel encasement such as pancreatic cancer and carcinoid tumors or conditions such as sclerosing mesenteritis can also result in acute mesenteric ischemia, depending on the availability and adequacy of collaterals. In these cases the mesenteric vessels will appear narrowed or even occluded due to local tumor invasion. Acute mesenteric ischemia can also be caused by low-flow states. This can be s result of hypovolemia after trauma or from decreased cardiac output or vasospasm, often related to drugs. On CT, the mesenteric arteries may appear small in caliber with decreased visualization of distal branches. 3D reconstruction of the data is very helpful in this setting. In addition to abnormalities in the mesenteric vasculature, patients with acute mesenteric ischemia can demonstrate changes in the affected bowel loops. If the proximal SMA or SMV is involved, the entire small bowel, right colon and transverse colon may be involved. Involvement of smaller branches of the mesenteric vessel as occurs is patients with embolic branches of the arteries, will only affect the supplied small bowel segments. Segmental ischemia can also occur due to mechanical obstruction, such as a hernia or small bowel volvulus. The most common findings in patients with acute mesenteric ischemia, is nonspecific small bowel thickening. The affected loops will appear thickened and there is often associated mesenteric stranding and fluid. The bowel wall may appear low in density compatible with submucosal edema. Ischemia can also result in submucosal hemorrhage. This can be easily detected if non-contrast scans are obtained. The affected loops may also be dilated, most likely due to disruption of its normal peristaltic activity, similar to ileus. In addition, changes in small bowel enhancement have also been described and include, decreased enhancement, delayed enhancement or rarely lack of enhancement. Pneumatosis (air within the bowel wall) is usually a late finding and indicates irreversible disease, i.e. infarction. At this point, surgical resection will be necessary. In addition to the presence of pneumatosis in cases of advanced ischemia and infraction, air can also be occasionally detected in the mesenteric veins or portal vein. This is an ominous finding and is associated with a high mortality rate.
- Chronic Mesenteric Ischemia Patients with chronic mesenteric ischemia will typically have atherosclerotic plaque in the proximal portion of the celiac axis or superior mesenteric artery. The inferior mesenteric artery can occasionally also be involved. Unlike acute mesenteric ischemia, the mesenteric veins are typically not involved in patients with chronic mesenteric ischemia, unless there are superimposed acute symptoms. Atherosclerotic plaque is often calcified and can therefore be easily be identified on CT as focal calcification near the origin of the mesenteric arteries. The amount of plaque will determine the degree of vessel narrowing /stenosis and it is important not only to detect the presence of plaque but also to determine the degree of vessel compromise. It is also important to realize that atherosclerotic disease of the mesenteric vessels is a common finding in older individuals, most of whom will be asymptomatic. Therefore the presence of atherosclerotic plaque involving the mesenteric arteries needs to be correlated with the patients history and symptoms, and does not in itself indicate the presence of ischemia. Since atherosclerotic disease occurs gradually overtime, collateral vessels will form to maintain adequate blood flow to the gut. These collaterals can also be visualized on CT and are important to identify prior to surgical planning. Surgeons will need an accurate road map of the mesenteric vessels and collaterals before attempting arterial reconstruction of aorta-mesenteric bypass. Unlike acute mesenteric ischemia, patients with chronic mesenteric ischemia will not usually demonstrates changes in the bowel wall thickness or enhancement, unless there is superimposed acute thrombus or emboli.
- Complications of Oncologic Therapies CT scanning is commonly used in the oncologic patient for detection of disease, staging of disease, and for monitoring response to therapy. Although most patients will have an uncomplicated course, others may have significant complications that will vary depending on the type of therapeutic intervention. The most common complications involving small bowel imaged with CT are due to radiation therapy, chemotherapy, and post-bone marrow transplant. These complications are usually mild but may be potentially life-threatening in certain patients.
- Radiation Therapy The most commonly recognized complication of radiation therapy in the abdomen is radiation enteritis. The frequency and severity of involvement is related to total dose, treatment volume, and fractionization of the dose. Patients with prior surgery in the area of the radiated tumor bed tend to have an increased frequency of complications due to adhesions of bowel locally or to the abdominal wall. If a dose of 5000 rads is given over 6 weeks the incidence of small bowel complications requiring operative intervention is approximately 10%. With doses of 6000 rads, complications are more frequent. Although clinical symptoms, such as abdominal pain, nausea, vomiting, and diarrhea often occur, many patients are asymptomatic despite abnormal small bowel radiologic studies. The mucosa and submucosa are most prone to injury. The submucosa is particularly vulnerable and an obliterative endarteritis resulting in tissue ischemia may occur. The damaged bowel appears shortened, with thickening of the bowel wall. Inflammation of the mesentery is also common. The CT findings of radiation enteritis closely match these pathologic changes. The bowel wall is thickened with a submucosal "halo" or low density zone because of inflammation. The bowel loops have a serpentine appearance due to wall thickening, adhesions of adjacent bowel loops, and shortening of the mesentery. Radiation enteritis can simulate a recurrent tumor mass, particularly if the bowel loops involved are not opacified by contrast. Careful attention to scanning technique is therefore particularly critical in these patients. In addition, consultation with the radiation therapist for definition of the therapy port and dosage can help prevent potential misdiagnosis. • Chemotherapy Gastrointestinal complications of chemotherapy include neutropenic enterocolitis (typhitis), acute ischemic colitis, infectious enteritis, and necrotizing enterocolitis. Several of the chemotherapy agents that cause these processes include fluorouracil, especially when combined with leucovorin. Grem et al. found that high dose leucovorin plus 5-FU caused severe or lethal diarrhea in 25% of patients. The CT appearance of chemotherapeutic-induced enteritis may be similar to ischemic enteritis. There is bowel wall thickening with dilated folds and ascites is frequently observed. The detection or suggestion of chemotherapeutic complications is important because early detection can indicate early discontinuation of the chemotherapeutic agent and hydration can reverse the toxicity. • Bone Marrow Transplantation Bone marrow transplantation is currently used for a wide range of conditions including aplastic anemia, acute leukemia, lymphoma, and breast cancer. In patients with allogenic donors a graft versus host (GVHD) response may develop because of the donor’s lymphoid tissue attacking the host tissue. Prerequisites for the development of GVHD are histoincompatibility of host and donor effector cells, immunocompetence of the host, and competence of the donor effector cells. Target organs include the skin, liver, and gastrointestinal tract. GVHD may occur soon after transplant or within several months after transplant. Up to 70% of allogenic transplant patients may develop some form of GVHD. Radiographic features of GVHD include small bowel fold thickening, separation of bowel loops, "ribbon bowel", and diarrhea. On CT, the bowel wall thickening can be diffuse or focal and may be associated with a submucosal zone of low attenuation. In addition, we have observed an unusual coating abnormality consisting of prolonged adherence of orally administered barium to the luminal contour, resulting in either circles outlining the lumen when seen en face, or in double tracking parallel lines of contrast outlining the narrowed lumen within the thickened wall. The cause of this coating is thought to be adherence of the barium to mucosal inflammation or following sloughing of the mucosa to the submucosa. In GVH disease long segments of the bowel are commonly involved and CT scans may even define areas of ulceration in select cases. • Hemorrhage Small bowel hemorrhage can result from a wide range of conditions including trauma, anticoagulant therapy, underlying bleeding diasthesis, or underlying tumor. Although in many cases the diagnosis can be made directly from the clinical history, in other cases the history is vague or confusing. It is often on a CT scan being done for a history of acute abdomen that the radiologist is the first to suggest the diagnosis of intestinal hemorrhage. As described by Balthazar et al. the CT findings include symmetrical wall thickening, a narrowed bowel lumen and occasionally increased density to the bowel wall. The length of involved segment may be focal or diffuse in extent. Intestinal hemorrhage may result in intussusception of the small bowel. Intestinal hemorrhage may be one of the most confusing diagnoses on CT because if the hemorrhage is focal it can simulate a primary small bowel tumor such as lymphoma. Intraluminal bleeding may also be suggested on a CT scan based on high intraluminal CT attenuation. This may be the result of hemorrhage secondary to a tumor or even erosion by an aneurysm fistualizing into the bowel.
IV. Small Bowel Tumors
• Introduction Small bowel tumors may present with a variety of clinical presentations. These include chronic symptoms of anemia, weight loss and general fatigue, or acute symptoms of acute abdominal pain, obstruction, or GI bleeding. Obstruction can be attributed to numerous causes, including intussusception. CT provides an excellent means of detecting the presence of an intussusception as well as determining its cause. Patients with multiple intussusceptions are also easily identified with CT even if there is proximal obstruction and contrast can not opacify the loop in question. With CT, as the bowel invaginates the mesentery it is carried forward and is caught between the overlapping layers of bowel. Three different patterns have been described reflecting the severity and duration of the intussusception. These are: 1) the target sign; 2) a sausage-shaped mass with alternating layers of low and high attenuation; and 3) a reniform mass. Pathophysiologically, the target sign represents the earliest stage of intussusception. As the process progresses a layering pattern, caused by alternating low attenuation (mesenteric fat) and high attenuation (bowel wall), develops. If untreated, thickening and edema of the bowel wall will increase. When the appearance is reniform it corresponds to severe edema and vascular compromise of the small bowel. Entero-enteric intussusception comprises approximately 40% of intussusceptions in adults. The most common causes of adult small bowel intussusceptions include benign or malignant neoplasms, Meckel's diverticulum, celiac disease, Crohns disease, and idiopathic causes. Approximately 20% of intussusceptions in adults are idiopathic in nature. • Benign Small Bowel Tumors Benign small bowel tumors are uncommon and are typically detected on CT as incidental findings. The most common of these tumors are leiomyomas and lipomas, although neurofibromas, hamartomas, or angiomas may occasionally be seen. The typical leiomyoma is a smooth, homogeneous submucosal mass measuring from 1-4 cm. These lesions, as documented by angiography, are typically vascular and will enhance up to 1.5 times baseline following dynamic injection of intravenous contrast. In other cases, leiomyomas may appear ulcerated or necrotic because they may bleed. Occasionally, leiomyomas may become the lead point in an intussusception. Lipomas may occur anywhere in the GI tract and are easily recognized by their fat attenuation values, which typically are in the range of -90 to -120 HU. Most are 1-2cm in size and are incidental findings. However, in other cases the lesion may form the lead point for an intussusception. This may be an entero-entero or enterocolic intussusception.
• Malignant Tumors- Adenocarcinoma The duodenum is the most common site of small bowel adenocarcinoma (40%), with the majority of lesions in the second and third portion of the duodenum. On CT the lesions have a variable appearance, including focal thickening of small bowel, a discrete tumor mass (which may lead to intussusception) and an ulcerating lesion. Although associated enlarged lymph nodes are uncommon, tumor implants on the mesentery or omentum can occur. Liver metastases are not uncommon and are usually hypovascular. Adenocarcinoma of the small bowel may present clinically with abdominal pain (intussusception), GI bleeding (ulceration), or obstruction. Farah found CT to be accurate in staging carcinoma of the duodenum. There can be significant overlap in appearance with lymphoma and, in the larger lesions, with leiomyosarcoma (GIST tumors). As noted previously, adenocarcinoma may occur more frequently in the patient with Crohns disease as well as in patients with sprue.
- Lymphoma The small bowel is the second most frequent site of GI tract involvement by lymphoma. Lymphomas constitute approximately one-half of all primary malignant small bowel tumors. The ileum is the most common site of occurrence, while the duodenum is the least frequent. The clinical presentation is variable ranging from bowel obstruction, intussusception or perforation, to a sprue-like syndrome. Simultaneous multifocal sites of involvement are a frequent finding Most cases of small bowel lymphoma are due to non-Hodgkin’s lymphoma. The CT appearance of lymphoma is variable. The typical appearances can be classified as aneurysmal, constrictive, nodular, or ulcerative. Mesenteric involvement by lymphoma may occur by direct extension from bowel, or indirectly by displacement due to mass effect. Although mesenteric involvement may be an isolated finding, it is usually seen as part of a more systematic involvement. Mesenteric involvement can be represented by any of the four general patterns of mesenteric disease: rounded masses; ill-defined masses; cake-like masses; and stellate mesentery. The rounded mass appearance is the most common and is usually seen with non-Hodgkins lymphoma. Discrete mesenteric nodes may directly involve or encase small bowel with a classic "sandwich" appearance. Lymphomatous involvement of the GI tract is more common in the immunocompromised patient who develops lymphoma. Since the initial description of the increased incidence of lymphomas in renal transplant patients there have been numerous reports of this occurrence. Most recently, an increased incidence of GI tract involvement in the AIDS patient has been reported. In nearly all cases the pathology was non-Hodgkins lymphoma. The incidence of non-Hodgkins lymphoma in renal transplant patients is 35 times greater than for the general population. De novo malignancies affect 6% of renal transplant recipients. Non-Hodgkins lymphoma accounts for more than 20% of transplant-related malignancy. The CT appearance of non-Hodgkins lymphoma in the immunocompromised host is indistinguishable from lymphoma in the noncompromised host. However, post-transplant lymphomas involve extranodal sites more frequently, particularly the central nervous system. In patients with renal transplants, the transplanted kidney was the organ most often infiltrated by tumor.
- Leiomyosarcoma or GI Stromal Tumors (GIST) Leiomyosarcomas (now called GIST tumors) are rare but when they occur are typically large (>10 cm) ulcerating masses with poorly defined borders. The lesions are typically nonhomogeneous and may in part enhance with a dynamic bolus of iodinated contrast. GIST’s often appear extraluminal and initially may appear to simulate an abscess or perforation. Metastases to the liver are common and may be cystic with or without mural nodes. Metastases to the mesentery and omentum with tumor nodules (peritoneal seeding) of 1-3 cm are also a frequent finding. GIST’s may calcify with a dystrophic type of calcification. GIST’s may be difficult to distinguish radiologically and pathologically from leiomyomas if the lesions are less than 5 cm in size. The pathologic diagnosis in these cases is crucial and is based on the number of mitoses per high-powered filed (HPF).
- Carcinoid Tumor of the Small Bowel Carcinoid tumor is a neoplasm of enterochromaffin cells that have the potential to secrete hormones such as serotonin and to induce a marked local desmoplastic reaction around the metastases. Carcinoid tumors of the small bowel account for approximately 20% of all carcinoid tumors; of these 90% occur in the ileum. Up to one-third will metastasize, the probability of metastasis being related to the size of the primary tumor. CT findings include a primary mass in the small bowel or cecum, rounded mesenteric masses, and reactive fibrosis with "beading" of the mesentery. Ascites and/or liver metastases may also be seen. Carcinoid metastases to the liver are usually very hypervascular and are best seen on non-contrast CT scans. There are several other pathologic processes that can produce the desmoplastic CT changes similar to that of carcinoid tumors. These include retractile mesenteritis, ovarian carcinoma, non-Hodgkins lymphoma, and Crohn’s disease. In ovarian carcinoma, tumor involvement does not usually include localized mesenteric masses, but typically forms an omental cake. In non-Hodgkins lymphoma, mesenteric masses/adenopathy are not unknown, but the desmoplastic reaction in the mesentery is usually not seen. Finally, Crohns disease may simulate carcinoid tumors of the small bowel, although the clinical history is usually distinctive. Also, the extent of small bowel involvement tends to be greater in Crohns disease. The desmoplastic reaction in carcinoid tumors can lead to bowel ischemia and infarction. This ischemia is usually chronic but may be acute. Care should be taken to evaluate adjacent bowel loops for focal thickening to exclude early changes of intestinal ischemia. Liver metastases from carcinoid tumor are usually vascular and best seen on arterial phase imaging. • Metastases to the Small Bowel Metastases to the small bowel can arise as a result of intraperitoneal seeding, hematogenously disseminated tumor emboli, lymphatic metastases, or direct extension from an adjacent mass. The most common lesions to demonstrate these patterns of spread are ovary, melanoma, testicular, and pancreatic cancer. The CT patterns of small bowel metastases are variable and include:
- (1) implants on bowel surface that may be discrete or diffuse. Associated ascites and mesenteric involvement is not uncommon.
- (2) nodules may implant on the small bowel ranging in size from 1-10 cm. Large lesions can simulate lymphoma or leiomyosarcoma.
- (3) direct extension from contiguous neoplasms like pancreatic cancer. This is most common in the second and third portions of the duodenum.
- (1) Crohns disease–diffuse thickening of bowel coupled with adenopathy may suggest lymphoma or adenocarcinoma.
- (2) Hemorrhage–long segments of bleeding into the bowel wall (usually due to anticoagulants) may simulate tumor infiltration.
- (3) Leiomyoma–may simulate a malignant lesion especially when it results in an intussusception.
- (4) Unopacified bowel–regardless of quality of exam preparation unopacified bowel can still be confused with tumor. Careful attention to technique is mandatory.
V. Small Bowel Obstruction
The role of CT in the evaluation of small bowel obstruction has recently been a hot topic in the literature with careful attention being paid to its role in patient diagnosis and triage. Although plain radiographs are usually the initial study in the patient with a suspected small bowel obstruction, CT has been shown to be significantly more accurate in determining the presence and location of the obstruction. In addition, CT can also determine the exact etiology of the obstruction and be used for triage to determine whether the patient can be managed conservatively or needs operative intervention. CT is far more accurate in the detection of signs of the acute abdomen including pneumoperitoneum. In a recent study by Earls et al. CT has also been compared to enteroclysis for small bowel obstruction. A blinded retrospective study analyzed 55 patients who had both CT and enteroclysis. Of the 55 patients, nine had no obstruction, 40 patients had obstruction due to adhesions, and 4 had tumor obstruction. CT correctly identified 63% of those with small bowel obstruction and 78% of those without obstruction. CT helped establish a diagnosis in 65% of these cases. When obstruction was classified into low- and high-grade partial obstruction, CT correctly identified 81% of high-grade obstructions and 48% of low-grade obstructions. CT detected the cause and site of obstruction in all 6 tumor-related cases. The authors concluded that CT "will have an important role in the contemporary diagnosis and management of high-grade bowel obstruction in patients in whom immediate surgery is not planned". Ha et al. reviewed the CT scans of 20 patients with small bowel obstruction due to post-operative adhesions. The authors found that in patients with obstruction, a beak-like luminal narrowing was the most common finding. The CT findings that suggested strangulated obstruction were serrated beaks, mesenteric edema, or vascular engorgement, and moderate to severe bowel wall thickening. In simple obstruction the beak was smooth, there were no mesenteric changes, and the bowel wall was normal or minimally thickened. The authors felt that CT was useful in distinguishing patients with strangulation of the small bowel.VI. Summary and Conclusion
CT evaluation of the small bowel is becoming an ever-important part of the radiologist’s diagnostic armamentarium. No longer a secondary imaging study, CT is playing a major role in the detection of small bowel disease as well as in defining its extent. Experience over the years has increased our knowledge base to the point that in many cases we can give a specific diagnosis and be accurate in a high percentage of cases. Similarly, recent articles have shown that CT of the small bowel is far more than just a series of images because the information generated plays a significant role in patient triage and eventual management. One cannot overemphasize the importance of a carefully performed examination from the technical standpoint, as well as the importance of correlating the radiologic findings with all available clinical information and lab results. Only by combining all of our knowledge bases can we optimize our role in the evaluation of the small bowel. In this era of cost-containment and outcome studies, CT of the small bowel will prove to be cost-effective and efficacious if used correctly.