Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

DetectorelementsSeptalcollimatorringsFIGURE 22-19. Side view of PET scanner performingtwo-dimensiona I data acquisition,showing lines of response for a single slice.Cross-ring coincidences have been added tothose occurring within the ring of detector elements.This increases the number of coincidencesdetected, but causes a slight loss of axialspatial resolution that increases with distancefrom the axis of the scanner.halfway between these rings. For greater sensitivity, coincidences between rings N -1 and N + 2 can be added to these data. However, increasing the number of adjacentrings used in two-dimensional acquisition reduces the axial spatial resolution.In three-dimensional (volume) data acquisition, axial collimators are not usedand coincidences are detected between many or all detector rings (Fig. 22-20).Three-dimensional acquisition greatly increases the number of true coincidencesdetected and may permit smaller activities to be administered to patients. There aredisadvantages to three-dimensional data acquisition. For the same administeredactivity, the greatly increased interaction rate increases the random coincidence fractionand the dead-time count losses. Thus, three-dimensional acquisition mayUllIIIIIIIIIIII Detector elementsIcollimatorIS~ptalrings removedFIGURE 22-20. Side view of PET scanner illustrating threedimensionaldata acquisition. Without axial collimator rings,interactions from activity outside the field of view (A) andscattered photons (8) are greatly increased, increasing thedead time, random coincidence fraction, and scatter coincidencefraction. However, the number of valid photon pairs(C) detected is also greatly increased.