Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

trum on the left shows a large photopeak at about 30 keV, whereas the spectrum onthe right shows a peak at about 30 keVand a smaller peak at about 60 keY. The 60-keY peak in the spectrum from the well detector is a sum peak caused by two photonssimultaneously striking the detector. The sum peak is not apparent in the spectrumwith the source 7.5 cm from the detector because the much lower detectionefficiency renders unlikely the simultaneous interaction of two photons with thedetector.Performance CharacteristicsEnergy ResolutionThe energy resolution of a spectrometer is a measure of its ability to differentiatebetween particles or photons of different energies. It can be determined by irradiatingthe detector with monoenergetic particles or photons and measuring the widthof the resultant peak in the pulse height spectrum. Statistical effects in the detectionprocess cause the amplitudes of the pulses from the detector to randomly vary aboutthe mean pulse height, giving the peak a Gaussian shape. (These statistical effectsare one reason why the pulse height spectrum produced by a spectrometer is notidentical to the actual energy spectrum of the radiation.) A wider peak implies apoorer energy resolution. The width is usually measured at half the maximal heightof the peak, as illustrated in Fig. 20-21. This is called the full width at half-maximum(FWHM). The FWHM is then divided by the pulse amplitude (not the peakheight) corresponding to the maximum of the peak:Energy resolution = FWHM X 100%Pulse height at center of peakFull width at 1/2 max(FWHM)FIGURE 20-21. Energy resolution of a pulse height spectrometer. The spectrum shown isthat of cesium137, obtained by an Nal{Tl) scintillator coupled to a photomultiplier tube.