Abstract
SPECT (Single Photon Emission Computed Tomography) and PET (Positron Emission Tomography) are nuclear medicine imaging techniques which can examine metabolic functions of the body in three dimensions. Both methods use pharmaceutical agents labeled by radioisotopes (RIs), called tracers. In SPECT scans, RIs emit gamma radiation directly measured by gamma cameras, which rotate around the patient to acquire tomography images. Many radiopharmaceuticals are developed for different kinds of examinations; for example, 99mTc-HMPAO (hexamethyl-propyleneamine oxime) and 99mTc-ECD (ethyl cysteinate dimer) for cerebral blood flow studies, 99mTc-HMDP (hydroxylmethylene diaphosphonate) for bone scans, and 99mTc-TF (tetrofosmin) for myocardial perfusion imaging. In contrast, a positron emitted by PET tracer annihilates with an electron, producing a pair of gamma rays at almost 180 degrees to each other. These two gamma rays are detected coincidently by scintillators and photomultipliers at two different locations in a PET scanner, enabling localization of the tracers within the body. PET tracers, for example 11C, 13N, 15O, and 18F, have very short half lives and need to be produced by a cyclotron installed in the hospital. SPECT and PET tracers are biologically active and their concentrations in tissue reflect the behavior of the molecules of interest. Recent advances in this field are PET-MRI and micro PET for small animals. PET-MRI is a combination of a C-shaped MRI scanner and a PET camera, with connection between the scintillators within the MRI gantry and the photodetectors of the PET module established by multiple optical fibers. Both PET and MR images can be obtained simultaneously with better soft-tissue contrast than PET-CT images. Micro PET camera for small animals has a very high spatial resolution of about 1mm, and it is possible to study the brain activities of mice in vivo.
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