Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

elatively low conversion efficiency (Table 22-2) ofBCO produces poor energy resolution,limiting the ability of energy discrimination to reject scatter. The fractionof scatter coincidences can also be reduced by two-dimensional data acquisition andaxial collimation, as described in the following section. Approximate methods areavailable for scatter correction.In two-dimensional (slice) data acquisition, coincidences are detected and recordedwithin each detector ring or small groups of adjacent detector rings. PET scannersdesigned for two-dimensional acquisition have thin annular collimators, typicallymade of tungsten, to prevent most radiation emitted by activity outside a transaxialslice from reaching the detector ring for that slice (Fig. 22-18). The fraction ofscatter coincidences is greatly reduced in PET systems using two-dimensional dataacquisition and axial collimation because of the geometry. Consider an annihilationoccurring within a particular detector ring with the initial trajectories of the annihilationphotons toward the detectors. If either of the photons scatters, it is likelythat the new trajectory of the photon will cause it to miss the detector ring, therebypreventing a scatter coincidence. Furthermore, mOst photons from out-oF-sliceactivity are absorbed by the axial collimators.In two-dimensional data acquisition, coincidences within one or more pairs ofadjacent detector rings may be added to improve the sensitivity (Fig. 22-19). Thedata from each pair of detector rings are added to that of the slice midway betweenthe two rings. For example, if N is the number of a particular detector ring, coincidencesbetween rings N - 1 and N + 1 are added to the projection data for ring NFor further sensitivity, coincidences between rings N - 2 and N + 2 can also beadded to those of ring N. After the data from these pairs of detector rings are added,a standard two-dimensional reconstruction is performed to create a transverse imagefor this slice. Similarly, coincidences between two immediately adjacent detectorrings, i.e., between rings Nand N + 1, can be used to generate a transverse image•• Detector elements1"1""""'"11sePta~i~~~matorFIGURE 22-18. Side view of PET scanner illustrating twodimensionaldata acquisition. The collimator rings preventphotons from activity outside the field of view (A) and mostscattered photons (8) from causing counts in the detectors.However, many valid photon pai_rsee) are also absorbed.