FIGURE 3.1 PET is a powerful tool to probe the functions of the brain. In these images of the brain, the radionuclide is fluorine-18 while the molecules for each image obviously have different biodistributions. The left-hand figure shows fluorodopa (to probe dopamine integrity) while the right-hand figure shows fluorodeoxyglucose (to probe sugar metabolism). SOURCE: Courtesy of Don Wilson, British Columbia Cancer Agency.

activity with no long-term residual activity in the body. However, the short lifetime means that fluorine-18 and 18F-FDG have to be produced very near to where the procedures are to be performed. This often requires in situ small-scale particle accelerators, another capability developed by nuclear physicists, to produce the isotope.

Radionuclides that emit gamma-rays have a long history as imaging tools in the diagnosis of cancer. SPECT has been built around the gamma-ray associated with the decay of molybdenum-99. Molybdenum-99 decays (t1/2 = 66 hours) into an isomer of technetium-99m (m indicating metastable), which in turn decays (t1/2 = 6 hours) by emitting a 140-keV gamma-ray. The cameras for this imaging technique are typically made with a cluster of photomultipliers coupled to a large NaI crystal. In recent years, the semiconductor material CdZnTe (CZT) has gained favor because of its higher energy resolution. Having this type of capability means that multiple tracers can be imaged simultaneously through the use of different energy windows.

In North America, the main radioisotopes needed for imaging and treatment are produced by the Isotope Development & Production for Research and Applications (IDPRA) program, in the Nuclear Physics Program of the Department of

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