Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

FIGURE 20-22. Pulse pileup.The dashed lines represent thesignals produced by two individualinteractions in the detectorthat occur at almost thesame time. The solid linedepicts the actual pulse producedby the detector. This singlepulse is the sum of the signalsfrom the two interactions.For example, the energy resolution of a 5-cm-diameter and 5-cm-thick NaI(Tl)crystal, coupled to a PMT and exposed to the 662-keV gamma rays of Cs-137, istypically about 7% to 8%.In pulse height spectroscopy, count-rate effects are best understood as pulse pileup.Fig. 20-22 depicts the signal from a detector in which two interactions occur, separatedby a very short time interval. The detector produces a single pulse, which isthe sum of the individual signals from the two interactions, having a higher amplitudethan the signal from either individual interaction. Because of this effect, operatinga pulse height spectrometer at a high count rate causes loss of counts and misplacementof counts in the spectrum.20.6 NONIMAGING DETECTORAPPLICATIONSSodium Iodide Thyroid Probe and Well CounterThyroidProbeMost nuclear medicine departments have a thyroid probe for measuring theuptake of 1-123 or 1-131 by the thyroid glands of patients and for monitoringthe activities of 1-131 in the thyroids of staff members who handle large activitiesof 1-131. A thyroid probe, as shown in Figs. 20-23 and 20-24, usually consistsof a 5.1-cm (2-inch) diameter and 5. I-em-thick cylindrical NaI(Tl) crystalcoupled to a PMT which in turn is connected to a preamplifier. The probe isshielded on the sides and back with lead and is equipped with a collimator sothat it detects photons only from a limited portion of the patient. The thyroidprobe is connected to a high-voltage power supply and either an SCA or anMCA system.