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| Fig. 1: PET scan of a normal and Alzheimer's brain. (Source: Wikimedia Commons) |
Conventional planar imaging techniques, like X-rays, allow medical practitioners to obtain information for the purposes of analyzing anatomy and structure of the human body. The incorporation of radiopharmaceuticals in imaging techniques has allowed health care providers to obtain information on bodily function as well as disease physiology, location, treatment, and more. The chemist George de Hevesy is regarded as the father of nuclear medicine for his work on radioactive labels in tracing the behavior of elements: the beginning of what would become modern nuclear imaging techniques. [1] Nuclear imaging systems often use gamma cameras, which utilized the gamma rays released by nuclides, for imaging.
Unlike other imaging techniques, like CT or X-rays, that form images by applying radiation to the body, nuclear imaging creates images by utilizing radiation emitted from within the body using injectable radionuclides.
After the injection of a radiopharmaceutical into the body, one must elucidate the radioactivity in the various tissue that the emitted positron passes through. This is accomplished using a component called a collimator. The collimator is a multi-hole component designed to help accurately localize the photon and allows for the released photons to interact with the next component: the scintillating crystal. A scintillating crystal in the gamma camera converts gamma ray photons into visible light photons, or scintillations. An array of photomultiplier (PM) Tubes detect these scintillations and amplify them to create a detectable electronic signal. [2] These signals then go on to be further refined and amplified by a preamplifier and amplifier. The signal then undergoes pulse height analysis and is displayed as an image, often by a cathode ray tube. [3]
Please note that this is a basic, high-level description of how gamma cameras work. More modern approaches involve sending the position signals to a computer for storage, where they can be further manipulated and displayed on a computer screen. Single Photon Emission Computed Tomography (SPECT), for example, uses gamma cameras to obtain images of the patient at various angles. A type of processing, called image reconstruction, can then create image slices of the patient. [2,3]
Another imaging technique that can utilize gamma cameras is positron emission tomography, or PET. Once a positron released by a radionuclide makes contact with an electron in the body, two two rays are emitted at 180° to each other - this process is called annihilation. PET takes advantage of this phenomenon by detecting these gamma rays with various detectors that surround the patient. Because annihilation results in back-to-back gamma ray emission, the detectors are able to recognize the ray in pairs in a process called coincidence detection. Unlike SPECT that scans incrementally, PET is able to simultaneously obtain data because of its design. [2,3] Fig. 1 is an example of a PET scan of the brain.
© Aaron Rios. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] G. Klaris, "The Future of Nuclear Medicine," Physics 241, Stanford University, Winter 2016.
[2] F. A. Mettler, Jr. and M. J. Guiberteau, Eds., Essentials of Nuclear Medicine Imaging, 5th Ed. (Saunders, 2005).
[3] M. M. Khalil, Basic Sciences of Nuclear Medicine (Springer, 2010).
