which radionuclide is used, it will travel a few millimeters to centimeters before annihilating with an atomic electron. As such, the site of annihilation is not the site of emission, resulting in a limitation when defining the origin of the decay. Another limitation is the fact that the positron-electron pair is not at rest when the annihilation occurs; thus by virtue of the conservation of momentum, the two photons are not exactly collinear. Although the lack of collinearity becomes increasingly important with greater detector separation, this effect is ignored, for the most part, in existing tomographs because the detector ring diameter is less than a meter, at which distance the deviation from 180° is a fraction of a millimeter.

image

FIGURE PET 2 The three panels from left to right show a combined FDG PET/computed tomography (CT) image in transaxial, saggital, and coronal views. The colored hot metal image is the PET image and the gray image is from the CT camera. The combined image enables physicians to determine the precise location of abnormal function (high uptake in the mass visible on the chest wall in the CT image in this case). In addition, a metastatic tumor is visible in the pelvic region. SOURCE: T. Ruth, 2011, The uses of radiotracers in the life sciences, Reports on Progress in Physics 72: 016701. Photo courtesy of British Columbia Cancer Agency and reprinted with permission from IOP Publishing Ltd.

Because diagnostic imaging is driven by a digital approach (present/absent, yes/no), the desire to have uncluttered images resulting from PET is very important. Nevertheless, the true power of PET lies in its ability to track the distribution of a tracer over time and to extract detailed kinetic data, as in a physical chemistry experiment where rate constants are determined. So, the conflict between using PET technology for clinical diagnosis and using it as an in vivo biochemistry tool will not be easily resolved, nor should it be.

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1The information in this vignette is adapted from T.J. Ruth, 2009, The uses of radiotracers in the life sciences, Report on Progress in Physics 72: 016701. Permission granted by IOP Publishing, Ltd.



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