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DK1212_C006 - 6 Nuclear Imaging Nuclear imaging portrays...

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245 6 Nuclear Imaging Nuclear imaging portrays distribution of radionuclides inside the patient’s body by external measurement of g -rays emanating from the body; this gave the modality its alternative generic name of g -imaging or gammagraphy (the latter is usually used only for planar imaging). Radioactive substances of short half-lives in the range of minutes to weeks are administered to patients intravenously, orally, or inhaled. The radiopharmaceuticals then circulate in the organism and concen- trate gradually in diagnosed regions, from where they are consequently excreted depending on the activities of the observed regions. The infor- mation to be obtained by an external measurement consists of generally time-dependent spatial distribution of the radioactive substance. This way, not only the shape of organs or lesions can be estimated, but also the local activity-dependent time-course of radioactivity density. Of the radioactive decay products, only high-energy photons form- ing the g -component of radiation (ranging in energy from about 50 keV to over 500 keV) are capable of penetrating the surrounding tissue and reaching the external detectors; the a and b radiation only contributes to the patient dose and should be avoided as much as possible. Obvi- ously, the g -photon energy range is about the same as that of diagnostic © 2006 by Taylor & Francis Group, LLC
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246 Jan x-rays; physically, the photons are not distinguishable. Nevertheless, the measured intensities of g -rays, given by the low density of radio- active substances, are generally very weak, being limited by the max- imum allowable dose to patients. Thus, the only difference in the character of rays, besides the different mechanism of generation, is in the photon flux density, which is several orders lower in nuclear imag- ing than in x-ray imaging. At these intensities, the rays do not consti- tute a continuous flux, but rather they consist of discrete photons that must be individually detected; the intensities are expressed by the counts of detected photons that are stochastically generated. This prob- abilistic character of the measurements causes problems with a high relative variance of counts and, consequently, a low signal-to-noise ratio (SNR). The photons are generated in all directions, of which only a very small part, on the order of 10 –3 to 10 –4 , can be measured in any projec- tion acquisition (see below); this contributes substantially to the very low detected intensities. It is clear that the photons are subject to the same quantum mechanisms of attenuation and scatter as described in Section 3.1.3. This contributes to complications of the measurement evaluation too: besides the required local density of the applied radionuclide, the mea- surement is also influenced by the unknown attenuation and scatter- ing, which influences the photon on its way from the point of generation to the detector. It should be understood that the attenuation means a loss of a certain percentage of photons due to interactions with the matter, but the g
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