Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

NUCLEAR IMAGING-THESCINTILLATION CAMERANuclear imaging produces images of the distributions of radionuclides in patients.Because charged particles from radioactivity in a patient are almost entirelyabsorbed within the patient, nuclear imaging uses gamma rays, characteristic x-rays(usually from radionuclides that decay by electron capture), or annihilation photons(from positron-emitting radionuclides) to form images.To form a projection image, an imaging system must determine not only thephoton flux density (number of x- or gamma rays per unit area) at each point in theimage plane but also the directions of the detected photons. In x-ray transmissionimaging, the primary photons travel known paths diverging radially from a point(the focal spot of the x-ray tube). In contrast, the x- or gamma rays from theradio nuclide at each portion of a patient are emitted isotropically (equally in alldirections). Nuclear medicine instruments designed to image gamma ray and x-rayemitting radionuclides use collimators that permit photons following certain trajectoriesto reach the detector but absorb most of the rest. A heavy price is paid forusing collimation-the vast majority (typically well over 99.95%) of emitted photonsis wasted. Thus collimation, although essential to image formation, severelylimits the performance of these devices. Instruments for imaging positron (W) emittingradionuclides can avoid collimation by exploiting the unique properties ofannihilation radiation to determine the directions of the photons.The earliest successful nuclear medicine imaging device, the rectilinear scanner,which dominated nuclear imaging from the early 1950s through the late 1960s,used a single moving radiation detector to sample the photon fluence at a smallregion of the image plane at a time. This was improved upon by the use of a largeareaposition-sensitive detector (a detector indicating the location of each interaction)to sample simultaneously the photon fluence over the entire image plane. TheAnger scintillation camera, which currently dominates nuclear imaging, is an exampleof the latter method. The scanning detector system is less expensive, but theposition-sensitive detector system permits more rapid image acquisition and hasreplaced single scanning detector systems.Nuclear imaging devices using gas-filled detectors (such as multiwire proportionalcounters) have been developed. Unfortunately, the low densities of gases,even when pressurized, yield low detection efficiencies for the x- and gamma rayenergies commonly used in nuclear imaging. To obtain a sufficient number of interactionsto form statistically valid images without imparting an excessiveradiation