Gastrointestinal carcinoid, also called carcinoid tumor, is the most common primary tumor of the small bowel and appendix. Gastrointestinal carcinoid accounts for more than 95% of all carcinoids. The tumors arise from enterochromaffin cells of Kulchitsky, which are considered neural crest cells situated at the base of the crypts of Lieberkühn. Gastrointestinal carcinoids account for 1.5% of all gastrointestinal tumors. The tumors elaborate serotonin and other histaminelike substances that normally are transported to the liver, where they are metabolized.
Carcinoid tumors account for 13-34% of small bowel tumors and 17-46% of malignant tumors of the small bowel. Neuroendocrine tumors have an incidence of 40-50 cases per million, and, because of improved diagnostic techniques, rose from 1.09/100,000 in 1973 to 5.25/100,000 in 2004. 
Characteristics of gastrointestinal cancer are shown in the images below.
Most tumors are clinically silent, but they may cause pain or intestinal obstruction, weight loss, a palpable mass, or, rarely, bowel perforation. Carcinoid syndrome occurs when the humoral load exceeds the capacity of monoamine oxidase (MAO) present in the liver and lung to metabolize serotonin. Most patients with carcinoid syndrome have liver metastases from a bowel carcinoid, although in rare cases, the humoral load from a primary tumor may overwhelm the liver and the capacity of the lungs to metabolize serotonin. Rarer still is carcinoid syndrome that develops in patients with noncarcinoid malignant tumors and dermatomyositis.
Diagnosis is usually achieved by using several complementary imaging techniques.
The most promising imaging technique is somatostatin receptor scintigraphy. [2, 3, 4] The technique can aid diagnosis by localizing primary and metastatic sites of gastro-enteropancreatic endocrine tumors. The degree of radionuclide uptake is related to somatostatin receptor density. In gastrointestinal carcinoids, the concentration at the receptor sites is high (90%).
Plain radiographic findings (eg, soft-tissue mass, punctate calcification within a mass, signs of intestinal obstruction) are not specific for carcinoids. Plain radiographs are usually obtained in an acute setting, being taken, for example, in patients presenting with intestinal obstruction or perforation.
Results of barium studies are nonspecific, and these tests are usually performed in patients presenting with nonspecific gastrointestinal symptoms. Clinical correlation is required.
Ultrasonography is performed to investigate a variety of abdominal complaints. The radiologist must be aware of the ultrasonographic appearances of a carcinoid. Although these are nonspecific, they may lead to the most appropriate investigation and result in a diagnosis.
Computed tomography (CT) scan findings in a malignant carcinoid depend on its size, degree of mesenteric invasion and desmoplastic reaction, and the presence of regional lymph node invasion. In the appropriate setting, the appearance of a malignant carcinoid on a CT scan may be highly suggestive of a diagnosis of carcinoid.
Angiography is essential in interventional procedures involving liver metastases.
Somatostatin receptor scintigraphy and endoscopic ultrasonography have a direct clinical impact, because they influence individual therapeutic strategies.
Limitations of techniques
None of the radiologic techniques can provide a specific diagnosis; therefore, clinical input is obviously important. All of the imaging techniques listed above provide anatomic information, with the exception of radionuclide studies.
A carcinoid is a functioning tumor, which means that clinical correlation is important. Indications for performing various imaging studies are dependent on the initial presentation. Two techniques stand out: CT scanning is better at providing anatomic information than are barium studies, and radionuclide studies add a functional element, which is obviously important in the context of carcinoid syndrome.
Wang et al studied the CT enhancement patterns of 44 patients with gastrointestinal neuroendocrine carcinomas, as confirmed by clinical pathology and immunohistochemistry. Most cases (81.8%) demonstrated moderately or obviously homogeneous enhancement in the arterial phases, with only 17.2% of cases appearing as heterogeneous enhancement. In addition, 86.4% of cases were further enhanced in the venous phase. The CT images also revealed some of the metastases, with some liver metastasis having obvious homogeneous enhancement. The authors concluded that the CT features of gastrointestinal neuroendocrine carcinomas increase diagnostic accuracy, particularly enhancement patterns. CT can provide useful information regarding the location, density, enhancement pattern, and some metastasis of gastrointestinal neuroendocrine carcinomas. 
In a study of 24 patients with gastrointestinal carcinoid tumors (1994 to 2012), endoscopy, endoscopic ultrasonography, and CT were the primary diagnostic approaches. Mean age at presentation was 50 years (range, 7-74 yr), 14 patients were male, and 10 were female. The mean lesion was 8.3 mm (range, 2-20 mm), with 50% being type 1, 12.5% type 2, 29% type 3, and 8.3% type 4. 
Cholangiography shows a polypoid filling defect within the biliary tree or an infiltrating form in which duct stenosis is demonstrated.
On barium examination, radiologic features are similar to those of adenocarcinoma and include a filling defect within the colon caused by a sessile polyp, circumferential narrowing (apple-core lesion) associated with mucosal destruction, loss of mucosal folds, and ulceration.
Imaging findings in a malignant carcinoid depend on its size, the degree of mesenteric invasion/desmoplastic reaction, and the presence of regional lymph node invasion. Bowel loops in the right iliac fossa separate and external compression, asymmetric spiculated contour, and kinking of adjacent bowel are demonstrated.
The carcinoid tumor may not be visible because it is usually buried in the adjacent mass. A large-bowel barium series may show compression or infiltration of the cecum at the base of the appendix.
On barium series, duodenal carcinoid tumors appear as intraluminal polypoid lesions or infiltrating lesions that cause an irregular stricture.
Imaging findings are nonspecific and depend on the type of tumor. On barium meal examination, the most common finding is a single, intramural, sharply demarcated defect that is usually 2-3 cm in diameter.
Tumors may be located anywhere in the stomach, although a fundal location is said to be rare. Features may mimic leiomyoma/leiomyosarcoma. Tumors may be ulcerated. Less commonly, tumors may appear as a large ulcer or polypoid mass.
In type I disease, multiple sessile polypoid lesions of varying sizes may be seen arising from the wall of the stomach. Atypical carcinoids may occur in which histology demonstrates a combination of carcinoid and adenocarcinoma. Atypical tumors have a tendency to ulcerate and show more aggressive behavior, with local tumor spread and lymph node metastasis. The prognosis in patients with atypical tumors usually is poor.
A bull's-eye, or target, lesion may be seen. This appears as an ulcer on the apex of a nodule on barium meal examination. When a target lesion is noted, the differential diagnosis includes gastric metastases (melanoma, lymphoma, carcinoma-breast, bronchus, pancreas), leiomyoma, pancreatic rest, and gastric neurofibroma.
Tumors may present as intraluminal, extramucosal filling defects on barium studies.
Plain radiographs may show curvilinear calcification in the region of the pancreatic bed
Tumors are indistinguishable from the more common adenomatous polyps on barium enema. Polyps may be ulcerated. Endorectal ultrasonography and endorectal magnetic resonance imaging (MRI) better demonstrate perirectal infiltration.
Plain abdominal radiographs may reveal curvilinear calcification within the abdomen. These are usually smaller than 15 mm in diameter and result from calcification within the tumor.
On barium studies, findings consist of fairly well-defined, round, intraluminal bowel-filling defects. These may be associated with thickening of the valvulae conniventes resulting from interference of the bowel blood supply by the tumor.
With invasion of the mesentery, the mesenteric mass causes rigidity, displacement/stretching, and fixation of small-bowel loops. Desmoplastic reaction from mesenteric invasion causes sharp angulation of a bowel loop or a stellate or spokelike wheel arrangement of adjacent bowel loops.
The tumor often infiltrates the mesentery, provoking an intense fibrotic reaction that results in kinking of the bowel segments; such kinking may in turn cause intestinal obstruction.
On a small-bowel barium series, kinking of the small-bowel loops is considered the hallmark of a small-bowel carcinoid tumor.
See the images of small-bowel carcinoid below.
Degree of confidence
Plain radiographic findings in gastrointestinal carcinoid are nonspecific and may simply reflect bowel obstruction. The calcification occasionally seen in a pancreatic or bowel carcinoid is nonspecific and has many differential diagnoses; however, kinking of the small bowel on a barium series is considered the hallmark of a small-bowel carcinoid.
Any cause of intestinal obstruction may mimic obstruction resulting from a carcinoid; moreover, on barium series, an intraluminal filling defect found at any location within the bowel, whether benign or malignant, may appear similar to a gastrointestinal carcinoid.
CT scans may demonstrate intrahepatic biliary dilatation associated with an intraductal mass of varying attenuation in the common bile duct. A Klatskin-type tumor representing biliary carcinoid has been reported.
The frequent presence of an extraluminal component can be delineated better on CT scans. Generally, CT scanning is used to stage colon tumors but not to detect them. CT scan findings of colon carcinoids appear similar to those of adenocarcinoma. The tumor may be visualized as a discrete mass or as focal wall thickening.
CT scanning may demonstrate thickening of the stomach wall by nodular masses. A large polypoid mass has been reported arising from the lesser curve of the stomach, with areas of low density that were presumed to represent necrosis.
Often multiple, these tumors are hypoattenuating on nonenhanced CT scans and are strongly enhancing on contrast-enhanced CT scans. Cystic degeneration may occur. Calcification within carcinoid metastases is not unusual. (See the images below.)
CT scanning should be able to demonstrate extraluminal extension and metastases.
Ultrasonographic and CT scan findings from these carcinoids are indistinguishable from those of islet cell tumors, but calcification is a clue to the diagnosis. CT scans may show a low-attenuation mass associated with calcification in the pancreas. CT scanning and ultrasonography may demonstrate lymph node and liver metastases. (See the images below.)
CT scanning reveals a mass with soft-tissue attenuation and variable size, with spiculated borders and radiating surrounding strands. Calcification may be noted in the tumor. Linear strands within the mesenteric fat probably are thickened and retracted vascular bundles and represent peritumoral desmoplastic reaction. Lymphadenopathy and liver metastases may be visualized on CT scans. Helical CT enteroclysis has been used to detect small-bowel carcinoids and has been found to be more sensitive than are conventional barium studies.  (See the images below.)
In one study, CT enteroclysis showed 100% sensitivity and 96% sensitivity for small bowel neuroendocrine tumors. Because of higher radiation dose and patient discomfort from nasojejunal intubation, CT enterography has been shown to be a good alternative, identifying 16 of 18 small bowel carcinoids in one study. 
Degree of confidence
When CT scanning reveals a solid mass with spiculated borders and radiating surrounding strands that is associated with linear strands within the mesenteric fat and kinking of the bowel, a diagnosis of gastrointestinal carcinoid can be made fairly confidently. Hypervascular enhancing liver metastases in the setting of a high index of clinical suspicion can also be a clue to the diagnosis. 
Liver and lymph node metastases from an intraluminal bowel mass with mesenteric invasion can mimic carcinoids.
Magnetic Resonance Imaging
Generally, MRI is not used in the diagnosis of gastrointestinal carcinoids. Liver metastases are demonstrated well on MRIs and usually have low signal intensity on T1-weighted images and high signal intensity on T2-weighted images. After the administration of a gadolinium-based contrast agent, liver metastases enhance peripherally in the hepatic arterial phase and appear as hypointense defects against the enhancing normal liver in the portal venous phase (see the images below).
MRI demonstrates biliary carcinoids as being hyperintense relative to the liver on T2-weighted images and hypo-intense relative to the liver on T1-weighted images.
Gadolinium-based contrast agents gadopentetate dimeglumine, gadobenate dimeglumine, gadodiamide, gadoversetamide, and gadoteridol have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see Nephrogenic Fibrosing Dermopathy. The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or magnetic resonance angiography (MRA) scans.
NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness.
Degree of confidence
Sufficient experience has not been gained in the use of MRI to provide an assessment of the degree of confidence. Some studies have reported a sensitivity of 86-94% in diagnosing small bowel tumors, but few of these were carcinoid tumors. Compared with CT, artifacts from bowel motion and lower spatial resolution may be limiting factors, in addition to higher cost. 
Any hypervascular liver metastases can appear similar on MRIs.
Ultrasonography shows evidence of biliary tree dilatation associated with an intraluminal hypoechoic or hyperechoic mass.
A persistent fluid-filled, distended appendix without typical signs of appendicitis has been reported with a carcinoid of the appendix.
One study using high-resolution transabdominal ultrasonography showed a hypoechoic mass lesion arising from the muscular layer of the stomach. 
On ultrasonography, liver metastases vary from hypoechoic to hyperechoic and show strong enhancement with intravenous contrast media. Tumors demonstrate peripheral hypervascularity on color and power Doppler images. (See the images below.)
Ultrasonographic and CT scan appearances are indistinguishable from those of islet cell tumors, but calcification is a clue to the diagnosis. Ultrasonographic findings usually demonstrate a hypoechoic mass, which may be of varying size but tends to be small. Ultrasonography may demonstrate lymph node and liver metastases.
Perirectal infiltration is better demonstrated on endorectal ultrasonography and endorectal MRI.
Ultrasonography of the bowel can depict bowel tumors, with a pseudokidney sign. Associated lymphadenopathy and liver metastases may be demonstrated on ultrasonograms.
Degree of confidence
On ultrasonograms, carcinoid findings are too nonspecific to offer a confident diagnosis.
Any bowel mass or hypervascular liver metastases can result in similar findings.
Scintigraphy performed with a somatostatin receptor analogue may prove useful in the treatment of patients with hypergastrinemic states who have increased incidence of gastric carcinoids. In patients with multiple endocrine neoplasia type 1 (MEN-1), localization in the upper abdomen may not be associated with a pancreatic endocrine tumor but rather with a gastric carcinoid.
Somatostatin-analogue scintigraphy has been proven sensitive. However, the findings are nonspecific, because the study may also show positive findings for islet cell tumors.
Somatostatin-receptor scintigraphy performed with indium-111 (111 In) octreotide and111 In pentetreotide is used to image many neuro-endocrine tumors, including carcinoids with somatostatin-binding sites.  Several studies have shown that somatostatin-receptor scintigraphy is a sensitive and noninvasive technique for imaging primary carcinoid tumors and carcinoid metastatic spread. A refinement of the technique that increases sensitivity is the addition of single photon emission CT (SPECT) scanning. (See the images below.)
Scintigraphy performed with iodine-123 (123 I) meta-iodobenzylguanidine demonstrates a 44-63% uptake in gastrointestinal carcinoids.  A higher frequency of radionuclide uptake is found in midgut carcinoids and tumors with elevated serotonin levels.
[Fluorine-18]fluorodopa positron emission tomography (18 F-dopa–PET) scanning has been used to image primary gastrointestinal carcinoid tumors and lymph node and organ metastases with promising results. [12, 13]
Positron emission tomography (PET) scanning with fluorodeoxyglucose (FDG) is now more available and more widely used. In general, FDG-PET scanning is useful in poorly differentiated carcinoids and other neuro-endocrine tumors, but it should not be used as a first-line imaging agent. FDG-PET scanning is primarily useful when the results of somatostatin-receptor scintigraphy are negative. 
See the additional images below.
Degree of confidence
More than 90% of gastrointestinal carcinoids and their metastases are identified using somatostatin receptor scintigraphy, and accumulation is often seen in clinically unsuspected sites not recognized using other imaging techniques.18 F-dopa–PET scanning is a promising procedure that is a useful supplement to morphologic imaging methods. FDG-PET imaging is useful in poorly differentiated carcinoids when the results of somatostatin-receptor scintigraphy are negative.
Somatostatin-receptor scintigraphy is not specific for carcinoids. Uptake occurs in other lesions with a high density of somatostatin receptors; these include the following:
Vasoactive intestinal polypeptide tumors
Neural crest tumors (eg, paragangliomas, medullary thyroid carcinomas, neuroblastomas, pheochromocytomas)
Oat cell lung carcinomas
Lymphoproliferative disease (eg, Hodgkin lymphoma, non-Hodgkin lymphoma)
In addition, the possibility of uptake in areas of lymphocyte concentration in inflammatory states must be kept in mind.
Before the advent of cross-sectional imaging, mesenteric angiography provided useful information regarding characterization of small-bowel carcinoids (see the images below). The angiographic appearances of small-bowel carcinoids encountered on angiograms produced for other indications, such as gastrointestinal bleeding, are worth noting. Foreshortening of the bowel occurring with desmoplastic reaction makes mesenteric arteries tortuous and frequently narrowed; it also draws the arteries into a stellate pattern. The areas involved appear hypervascular, but in reality, the number of arteries in the area does not increase. Instead, the arteries contract into a smaller area as a result of fibrosis. 
An additional arterial change associated with carcinoids is smooth, multifocal stenosis of the mesenteric arteries distant from the tumor. Tumors seldom show capillary blush or demonstrate early or dense venous drainage. Venous occlusion and mesenteric varices also have been reported. These findings are nonspecific and have been reported with sclerosing peritonitis and with a carcinoma of the pancreas invading the mesentery. Selective hepatic angiography can demonstrate hypervascular liver metastases by demonstrating capillary blush in involved areas, highlighting the potential response of tumors to embolization. 
Degree of confidence
Foreshortening of the bowel occurring with desmoplastic reaction makes mesenteric arteries tortuous and frequently narrowed, in addition to drawing the arteries into a stellate pattern. Involved areas appear hypervascular, but in reality, the number of arteries in the area does not increase; instead, arteries contract into a smaller area because of fibrosis. With a high index of clinical suspicion, the degree of confidence in diagnosing gastrointestinal carcinoid is high when these findings are seen in combination with the other imaging features described.
Findings are nonspecific and have been reported with sclerosing peritonitis, desmoid tumors, and a carcinoma of the pancreas invading the mesentery.