Hepatic Imaging: Positron Emission Tomography, Digital Angiography, and Nuclear Magnetic Resonance Imaging

Abstract

Major advances in diagnostic imaging of the human body have been made in recent years. Positron emission tomography, a technique founded on advances in radiopharmaceuticals and radionuclide imaging apparatus, permits imaging regional metabolism, metabolite distribution, and flow. Thus far, its major applications have been in the study of the brain, and to a lesser extent, the heart; however, it is also finding a role in the study of the liver. Digital angiography is being applied to fluoroscopic systems which permits visualization of relatively low doses of intravascular iodinated radiographic contrast media. Imaging of the major arteries is possible using simple intravenously administered contrast media; digital techniques also increase the diagnostic yield in intraarterial studies. Nuclear magnetic resonance (NMR) imaging is an outgrowth of laboratory NMR spectroscopy; the interaction of radiofrequency signals with nuclei in strong magnetic fields permits imaging the distribution and certain chemical properties of various isotopes. Hydrogen NMR imaging is proving most useful for clinical diagnosis. The techniques used to image hydrogen are also being applied to NMR spectroscopy in viuo of various isotopes, including carbon-13, phosphorus-3 1, and hydrogen-1.Medical imaging began shortly after discovery of Xrays by Roentgen in 1895. Until the past few decades, radiography of the abdomen was limited to the familiar abdominal X-ray or "flat-plate." Such films are projectional images; the three-dimensional structure of the abdomen is projected onto a two-dimensional film with consequent superposition of overlying structures. Both the projectional nature and poor contrast resolution of plain films limit their diagnostic potential for evaluating an organ such as the liver; the overall size is easily determined but, with few exceptions (such as air in the biliary tree), little can be said about internal architecture. The development of ultrasound, radionuclide imaging, and computed tomography greatly enhanced our ability to image the liver, and to diagnose hepatic disease. This paper will review mechanisms, applications, and other features of the newest imaging modalities, [viz. positron emission tomography, digital angiography, and nuclear magnetic resonance imaging (NMR)]. Theoretical and Address reprints requests to: