Positron emission tomography in the study of hepatic encephalopathy.

Positron-emission tomography (PET) is a quantitative technique that produces images of biological or physiological processes. The nature of the image depends on the tracer used: common tracers used to study HE include 18F-fluordeoxyglucose, a marker of glucose metabolism; 15O-water, a marker of cerebral blood flow; and 13N-ammonia, a marker of ammonia metabolism. Combined blood flow and ammonia metabolism studies can be used to calculate the permeability surface area product for ammonia at the blood brain barrier. To take full advantage of PET, the data should be analyzed using one of the several sophisticated image processing and analysis techniques that are available. Thus, PET is an ideal technique to evaluate ammonia metabolism and, because of a close linkage of blood flow and glucose metabolism with neural activity, to investigate the neural response to drugs and other treatments and to examine neural systems that mediate specific tasks that are impaired in patients with HE.