Radionuclide imaging, including planar scintigraphy, single-photon emission computed tomography (SPECT) and positron emission tomography (PET), relies on the tracer principle, in which a minute quantity of a radiopharmaceutical in introduced into the body to monitor the patient's physiological function. In a clinical environment, radionuclide images are interpreted visually to assess the physiological function of tissues, organs, and organ systems, or can be evaluated quantitatively to measure biochemical and physiological processes of importance in both research and clinical applications. Nuclear medicine relies on non-invasive measurements performed with external (rather than internal) radiation sources and detectors in a way that does not allow the radionuclide measurement to be isolated from surrounding body tissues or cross-talk from radionuclide uptake in non-target regions.

Within the spectrum of macroscopic medical imaging (Figure 1), sensitivity ranges from the detection of millimolar concentrations of contrast media with CT and MRI, to picomolar concentrations in PET: a 109 difference.1 With CT and MRI, contrast is produced by detecting differences in tissue density and water content; however, with radionuclide imaging, contrast is conferred by detection of a clearly identified molecule labelled with a radioactive isotope of one of its natural constituent elements. Signal sensitivity is a prerequisite for studies of biological pathways and binding sites which function at less than the micromolar level. It also is important to avoid the pharmacological effects of administering a labelled molecule to study its inherent biodistribution. The sensitivity of in vivo tracer studies is achieved par excellence with PET, which uses electronic collimation and thereby operates with a wide acceptance angle

*PD Dr H. Zaidi, Geneva University Hospital, Division of Nuclear Medicine, CH-1211 Geneva, Switzerland yProf. B.H. Hasegawa, Department of Radiology, University of California, San Francisco, CA, USA

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^single photon scintigraphy internal source — internal tracer positron emission tomography thermography \ ECG, EEG map macroscopic medical imaging

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