Technique Selection - Liver, Biliary Tract, and Pancreas

TECHNIQUE SELECTION

Diseases of the liver, biliary system, and pancreas can be con-veniently, if arbitrarily, separated into the following cate-gories to help illustrate the optimal sequences of imaging techniques: diffuse hepatocellular disease, focal hepatic dis-eases, abdominal trauma, inflammatory disease of the biliary tract, and pancreatic inflammation or neoplasm.

Diffuse Hepatocellular Disease

In diffuse hepatocellular disease, CT is probably the first study used to survey the liver because it is moderately sensi-tive to liver lesions and is also helpful for evaluating sur-rounding organs. Ultrasound may have an application unlessfatty liver is present, because fat attenuates the US beam. NM has only infrequent applications. MR imaging may be the most sensitive modality for detecting and characterizing diffuse diseases of the liver, including cirrhosis and he-mochromatosis, especially when combined with contrast agents. Angiography may be used to study collateral forma-tion in cirrhosis.

Focal Hepatic Diseases

In focal diseases of the liver, US is often used first, because it utilizes no radiation, is relatively inexpensive, is widely avail-able, and is moderately sensitive to localized lesions in the ab-sence of preexisting diffuse diseases, such as cirrhosis. It is, however, of limited value in obese patients and whenever air is present, for example when air-filled bowel obscures the liver. CT is a pivotal examination, often employed after US. It is used as a survey of the entire body, is easy to compare in serial stud-ies, and is sensitive to disease. Air and bone do not interfere with CT examinations. Contrast-enhanced MDCT (multide-tector CT) scanners can be used to perform CT angiography, or CTA, which is a noninvasive means of producing images depicting vessels much like conventional angiography. NM techniques may be used to analyze a focal lesion within the liver for possible cavernous hemangioma. MR imaging is usedfrequently to characterize focal lesions within the liver, espe-cially those discovered during survey techniques such as US or CT. NM and MR imaging are considered the optimal means for evaluating the liver for cavernous hemangioma, and both are highly accurate (approximately 95%) in evaluating the liver for cavernous hemangioma. In the opinion of some au-thorities, newer MR pulse sequences, contrast agents, and fast scanning techniques arguably make MR imaging the optimal means for both detection and characterization of focal liver le-sions of all types. Gadoxetate disodium is a newer hepatocyte-specific contrast agent that is taken up by functioning hepatocytes. This can be helpful in evaluating indeterminate liver lesions. Angiography is primarily used to provide a vas-cular road map in planning surgery for focal liver lesions. It can also be used in treatment of cancer, such as with chemoembolization.

Abdominal Trauma

CT is the only commonly accepted means for analyzing ab-dominal trauma, particularly of the liver. CT is reasonably accurate in the detection of trauma-related abnormalities of the liver, biliary system, and pancreas. US may be useful if CT is not available or to quickly identify intraperitoneal hem-orrhage in patients who are in the emergency departmentand are going directly to the operating room. However, it is insensitive for directly identifying solid-organ lacerations. Angiography may be useful to embolize persistently bleed-ing arteries in the liver or spleen when surgery is not possi-ble. Currently, NM and MR imaging have no application in studying the liver, biliary tract, or pancreas in acute trauma. However, they may have some application if there is clinical concern for a bile leak.

Pancreatic Inflammation or Neoplasm

CT is often the initial means to study pancreatic inflamma-tion or neoplasm. It is effective in evaluating the pancreas. Ultrasound may be limited due to bowel gas or patient habi-tus. It is notoriously difficult to evaluate the pancreatic tail with ultrasound. NM has no major current application in studying the pancreas. MR imaging may be useful to study tumors of the pancreas. It is insensitive for small calcifica-tions, as in chronic pancreatitis. Recent advances in MR im-aging, especially MRCP, have brought MR imaging further to the forefront of pancreatic and biliary duct evaluation. This latter technique highlights fluid-containing structures such as biliary or pancreatic ducts and voids nearly all signal in-tensity from background solid structures. Angiography is useful to identify bleeding arteries as a source of hemorrhagic pancreatitis but is occasionally used to identify encasement of arteries in a pancreatic neoplasm.

Patient Preparation for Radiographic Techniques

Generally, these radiographic techniques require little pa-tient preparation. This is convenient, especially in evaluation of trauma. Ideally, a patient should fast after midnight be-fore an US examination. As a minimum, the patient should fast for 6 hours, especially when evaluating the gallbladder. Patients ideally should fast before CT examinations as well, but this requirement is not crucial. Dilute oral contrast medium for CT is administered at least 2 hours in advance and again just before the examination begins. Intravenous contrast material is often given as a bolus by a power injec-tor immediately prior to the study. Proper laboratory evalu-ation of renal function, including serum creatinine below 1.5 mg/dL, is usually required before administering iodi-nated intravenous contrast material because it can be nephrotoxic. Ideally, NM is also performed after fasting. Preparation for angiography again requires fasting and labo-ratory evaluation of renal function and possible coagulopa-thy. Proper preparation of patients for MR imaging is controversial. However, some authorities advise administering an iron-containing oral contrast agent and an agent to relax the bowel, such as glucagon, before scanning. Renal function must be considered when administering gadolinium-based.

Conflicts among Examinations

These examinations may interfere with each another. No bar-ium should be administered before US or CT. Oral contrast agents may generate bowel gas, decompress the gallbladder, and hinder US. The oral contrast agent administered prior to a CT examination interferes with angiography by obscuring the abdomen. Intravenous contrast material interferes with any subsequent NM tests studying iodine metabolism such as those involving the thyroid gland because intravenous con-trast agents contain iodine. Previous angiography usually re-quires that a CT examination be postponed for a day or two so that residual contrast material within the kidneys can be excreted. Usually, there are no conflicts between these exami-nations and NM or MR imaging.