Computed tomography of the hepatobiliary tract
- Umaprasanna S Karnam, MD
Umaprasanna S Karnam, MD
- Jordan Valley Hospital
- West Jordan, UT
- K Rajender Reddy, MD
K Rajender Reddy, MD
- Ruimy Family President's Distinguished Professor of Medicine
- Professor of Medicine in Surgery
- Director of Hepatology
- Director, Viral Hepatitis Center
- Medical Director of Liver Transplantation
- University of Pennsylvania School of Medicine
- Stephan Anderson, MD
Stephan Anderson, MD
- Professor of Radiology
- Boston University School of Medicine
Computed tomography (CT) can be used to image the hepatobiliary system, with perhaps the exception of the gallbladder, which is better imaged with ultrasound. Magnetic resonance cholangiopancreatography (MRCP) is superior to CT for evaluating the biliary tract, but CT is useful for hepatic imaging and in cases where it is not clear whether a problem is originating in the liver, gallbladder, or bile ducts. CT, like magnetic resonance imaging, allows for a more thorough evaluation of the liver and other abdominal structures than ultrasound, and is less dependent upon operator skills. In contrast to ultrasound, successful CT of the liver can be obtained despite obesity, overlying bowel gas, or ascites.
This topic review will discuss some general principles of CT as it applies to imaging of the hepatobiliary system. A more complete discussion of the technique of CT scanning, MRCP, and hepatobiliary ultrasound can be found separately. (See "Principles of computed tomography of the chest" and "Magnetic resonance cholangiopancreatography" and "Ultrasonography of the hepatobiliary tract".)
A CT scan is created by passing fine radiographic beams through the patient, with rotating detectors located on the opposite side of the body to record the amount of radiation not attenuated by the tissues being imaged. This information is then processed by a computer that calculates the attenuation values with reference to a standard water value of 0 Hounsfield units.
Intravenous iodinated contrast material is used to opacify vessels and to determine the vascularity of lesions relative to that of normal liver parenchyma. The timing of image acquisition is crucial in hepatic imaging; images should be obtained during an interval of sustained hepatic enhancement before the equilibrium phase is reached (algorithm 1). At equilibrium, lesions may become isodense with the liver parenchyma, and therefore not visible.
Conventional CT scanners obtain a series of individual scans during suspended respiration. This incremental table-patient transport results in respiratory misregistration due to varying breath-holds during scans. Thus, anatomic fields may be omitted and data lost. Conventional CT scan, while a powerful diagnostic tool, is more expensive than ultrasound and involves the risks associated with radiation exposure and intravenous contrast material. (See "Radiation-related risks of imaging studies" and "Pathogenesis, clinical features, and diagnosis of contrast-induced nephropathy" and "Immediate hypersensitivity reactions to radiocontrast media: Clinical manifestations, diagnosis, and treatment".)
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