Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

FIGURE 22-23. Attenuationin PET. The probability thatboth annihilation photonsemitted along a particularline of response (LOR) escapeinteraction in the patient isindependent of the positionon the LOR where the annihilationoccurred.interaction is only about 15%. Thus, attenuation is more severe in PET thanSPECT. The vast majority of the interactions with tissue are Compton scattering.Attenuation causes a loss of information and, because the loss is not the same for alllines of response, causes artifacts in the reconstructed transverse images. The loss ofinformation also contributes to the statistical noise in the images.As in SPECT, both approximate methods and methods using radioactivesources to measure the attenuation are used for attenuation correction in PET. Mostof the approximate methods use a profile of the patient and assume a uniform attenuationcoefficient within the profile.Some PET systems provide one or more retractable positron-emitting sourcesinside the detector ring between the detectors and the patient to measure the transmissionof annihilation photons from the sources through the patient. As mentionedabove, the probability that a single annihilation photon from a source willpass through the patient without interaction is the same as the probability that bothphotons from an annihilation in the patient traveling along the same path throughthe patient will escape interaction. These sources are usually configured as rods andare parallel to the axis of the scanner (Fig. 22-24). The sources revolve around thepatient so that attenuation is measured along all lines of response through thepatient. These sources usually contain Ge 68, which has a half-life of 288 days anddecays to Ga 68, which primarily decays by positron emission. Alternatively, asource containing a gamma-ray-emitting radionuclide, such as Cs 137 (662-keVgamma ray), can be used for attenuation measurements instead of a positron-emittingradionuclide source. When a gamma-ray-emitting radionuclide is used to measurethe attenuation, the known position of the source and the location where agamma ray is detected together determine a line of response for that interaction. Ineither case, the transmission data acquired using the external sources are used to correctthe emission data for attenuation prior to transverse image reconstruction.As in SPECT, the transmission data can be obtained before the PET radiopharmaceuticalis administered to the patient, or can be obtained during emission