Abstract
The radioactive decay of many radioisotopes generates penetrating photons capable of escaping outside the matter in which the isotopes are located. From this radiation it is possible to image the spatial distribution of such isotopes inside an object. However, by itself the detection of a single photon outside the body of a patient carries minimal information on the location of its origin, unless some device capable of connecting the detection with the emission location is used. These devices are the optics of the imaging instrument and they identify, in combination with a position sensitive radiation detector, a line in space (the line of response, LOR) along which the photon must have originated (Fig. 8.1A,B). The LOR data are manipulated in reconstruction software to produce three-dimensional (3D) images of the activity distribution. When imaging humans, it is necessary to use photons capable of escaping undeflected from a few centimeters of tissue. The energy of these photons is such that their path cannot be bent by reflection (mirrors), refraction (lenses), or diffraction as in visible light optics. Nuclear scintigraphy and single photon emission computed tomography (SPECT) instrumentation resort to absorptive collimation, in which photons are selectively passed or absorbed depending on their emission location and angle of incidence on the optics. The drawback of this approach is that the wide majority of photons are lost before image reconstruction. For example, typical parallel-hole collimators [low energy-technetium-99m (99mTc; 140keV); general purpose] pass on the order of 1 in 10,000 (10-4) photons, but sensitivity is even lower for high-resolution and high-energy collimators, which need lower acceptance angles and thicker septa, respectively. Although sensitivity can be recouped by trading off resolution (as with high-sensitivity collimators) or field-of-view (as with converging collimators), it is the concept of absorptive collimation itself that implies an inefficient use of emitted photons.
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Accorsi, R., Surti, S., Karp, J.S. (2006). Physics and Instrumentation in PET. In: Charron, M. (eds) Pediatric PET Imaging. Springer, New York, NY. https://doi.org/10.1007/0-387-34641-4_8
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DOI: https://doi.org/10.1007/0-387-34641-4_8
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