GI Imaging

         RADIOLOGY OF THE GASTROINTESTINAL TRACT

1. Plain radiographs of the abdomen have been replaced by CT, ultrasound, and MRI in patients with acute clinical problems.
2. Plain radiographs are helpful to demonstrate the location of various tubes as well as for evaluation of ICU patients or patients who require serial studies.
3. The first examination the radiologist should obtain for a patient with suspected small bowel obstruction is a CT.
4. Free intraperitoneal gas is detected in a supine plain radiograph in 60% of patients with perforation.
5. Linear pneumatosis is highly suggestive of ischemia.

·         There are two collateral pathways that connect the SMA and IMA: the arc of Riolan proximally and the marginal artery of Drummond distally. The main collateral pathways between the celiac artery and the SMA are through the common hepatic artery, gastroduodenal artery, and pancreatico-duodenal arteries. The arc of Buhler is a direct connection between the SMA and celiac artery.

·         Valvulae conniventes (plicae circulares, folds of Kerckring) are composed of mucosa and submucosa. These folds increase the surface area of the small bowel. The villi of the intestine are just at the radiographic limits of resolution.

·         DIFFERENTIAL DIAGNOSIS OF LIPID-CONTAINING LIVER LESIONS:

1.       Hepatic adenoma

2.       Hepatocellular carcinoma (especially well-differentiated subtype)

3.       Focal steatosis

·         MRI OF THE LIVER:

1. On T2-weighted images, the signal intensity of liver metastases is similar to that of the spleen, whereas liver cysts and hemangiomas are relatively isointense to cerebrospinal fluid.
2. Most liver lesions are hypointense to liver on T1-weighted images. Lesion isointensity to liver on a T1-weighted image suggests that it is hepatocellular in origin.
3. Metastatic disease is rare in a cirrhotic liver. A focal liver lesion in a cirrhotic liver that shows arterial phase enhancement or is isointense to spleen on T2-weighted images should be considered hepatocellular carcinoma until proven otherwise.
4. Chemical shift imaging of the liver is invaluable in diagnosing hepatic steatosis and depicting the presence of lipid within some hepatocellular neoplasms.

·         Hemangiomas have T1 and T2 signal intensity similar to that of cysts. A simplified explanation is that as blood enters the enlarged cavernous vessels of a hemangioma, its velocity decreases such that it mimics stagnant fluid. Thus, hemangiomas and cysts can have similar appearances on T1- and T2-weighted images.

On heavily T2-weighted images, hemangiomas remain hyperintense to liver and spleen but become slightly hypointense relative to relatively simple fluid (e.g., within hepatic cysts or cerebrospinal fluid). On dynamic enhanced CT or MR, hemangiomas show a discontinuous peripheral nodular enhancement that fills in centripetally with time

·         Cirrhosis is defined as the presence of regenerating nodules and surrounding fibrosis. Both CT and MR can show the presence of regenerating nodules. On MR, benign regenerative nodules have relatively high signal intensity on T1-weighted and low signal intensity on T2-weighted images, which is similar to normal liver. Preferential atrophy of the medial segment left lobe and anterior segment right lobe results in an appearance that has been described as the "empty gallbladder fossa sign." Caudate lobe and lateral segment left lobe hypertrophy can also be present in cirrhotic livers. Extrahepatic findings of cirrhosis that reflect the presence of portal hypertension include splenomegaly, varices, and ascites.

·         An insensitive but specific finding of hepatic malignancy on portal or delayed phase imaging has been termed the peripheral washout sign. This refers to the appearance of decreased attenuation (CT) or signal intensity (MRI) within the periphery of a lesion. Peripheral washout is hypothesized to occur because of increased tumor angiogenesis within the growing outer margins of tumors that fosters both increased entry and egress of extracellular contrast agents.

·         During the arterial phase of contrast enhancement the spleen appears as alternating, wavy bands of high and low CT attenuation or MR T1 signal intensity. This has been termed the arciform enhancement pattern and is due to variable rates of flow within the two compartments of the red pulp. Variation from this pattern is suggestive of diffuse splenic disease. After 1 minute, the distribution of contrast within the spleen rapidly equilibrates, and there is homogenous, intense enhancement of the entire spleen.

·         CT AND MRI OF THE SPLEEN:

1.       On postcontrast images, the normal spleen displays alternating bands of high and low attenuation (CT) or signal (MR) in the arterial phase. The spleen appears more homogeneous in a more delayed phase.

2.       Splenic laceration can be differentiated from developmental splenic cleft. Patients with laceration will have a trauma history, display a low-attenuation defect with sharp edges, and have perisplenic hemoperitoneum.

3.       Both MR and CT are less specific in the characterization of splenic lesions than they are in the characterization of liver, adrenal, or renal lesions.

·         The double duct sign refers to obstruction and dilation of both the common bile duct and the pancreatic duct. It is highly suggestive of a pancreatic adenocarcinoma in the head of the pancreas. About 60-70% of adenocarcinomas occur in the head of the pancreas. The double duct sign is not entirely specific for malignancy since focal scarring from chronic pancreatitis can also create this appearance.

·         CT AND MRI OF THE PANCREAS:

1.       Pancreatic adenocarcinoma, which encases the celiac axis, superior mesenteric artery, or portal vein, cannot be surgically resected.

2.       On CT and MRI, pancreatic adenocarcinoma generally enhances less than the remainder of the pancreas.

3.       Pancreatic islet cell tumors are usually hyperintense to the pancreas on T2-weighted images and tend to enhance more than the remainder of the pancreas after administration of CT or MRI contrast agents.