Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

FIGURE 21-7. Ways that x- and gamma rays interact with a scintillation camera.All of these, other than the ones depicted in the upper left, cause a loss of contrastand spatial resolution. However, interactions by photons that have scatteredthough large angles and many coincident interactions are rejected bypulse height discrimination circuits.for two photons simultaneously to interact with the crystal; if the energy (Z) signalfrom the coincident interactions is within the energy window of the energy discriminationcircuit, the result will be a single count mispositioned in the image.The fraction of simultaneous interactions increases with the interaction rate of thecamera.Interactions in the crystal of photons that have been scattered in the patient,photons that have penetrated the collimator septa, photons that undergo one ormore scatters in the crystal, and coincident interactions all reduce the spatial resolutionand image contrast. The function of the camera's energy discrimination circuits(also known as pulse height analyzers) is to reduce this loss of resolution andcontrast by rejecting photons that scatter in the patient or result in coincident interactions.Unfortunately, the limited energy resolution of the camera causes a widephoto peak, and low-energy photons can scatter through large angles with only asmall energy loss. For example, a 140-keV photon scattering 45 degrees will onlylose 7.4% of its energy. An energy window that encompasses most of the photopeakwill unfortunately still accept a significant fraction of the scattered photons andcoincident interactions.It is instructive to compare single photon emission imaging with x-ray transmissionimaging (Table 21-0. In x-ray transmission imaging, including radiog-