Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

X-Ray and Gamma-Ray Spectroscopy with Sodium IodideDetectorsX-ray and gamma-ray spectroscopy is best performed with semiconductor detectorsbecause of their superior energy resolution. However, high detection efficiency ismore important than ultra-high energy resolurion for most nuclear medicine applications,so most spectroscopy systems in nuclear medicine use NaI(TI) crystals coupledto PMTs.There are a number of mechanisms by which an x-ray or gamma ray can depositenergy in the detector, several of which deposit only a fraction of the incident photonenergy. As shown in Fig. 20-17, an incident photon can deposit its full energyby a photoelectric interaction (A) or by one or more Compton scatters followed bya photoelectric interaction (B). However, a photon will deposit only a fraction of itsenergy if it interacts by Compton scattering and the scattered photon escapes thedetector (C). In that case, the energy deposited depends on the scattering angle,with larger angle scatters depositing larger energies. Even if the incident photoninteracts by the photoelectric effect, less than its total energy will be deposited if theinner-shell electron vacancy created by the interaction results in the emission of acharacteristic x-ray that escapes the detector (D).Most detectors are shielded to reduce the effects of natural background radiationand nearby radiation sources. Figure 20-17 shows two ways by which a x-rayor gamma-ray interaction in the shield of the detector can deposit energy in thedetector. The photon may Compton scatter in the shield, with the scattered photonstriking the detector (E), or a characteristic x-ray from the shield may interactwith the detector (F).Most interactions of x-rays and gamma rays with an NaI(Tl) detector arewith iodine atoms, because iodine has a much larger atomic number than sodiumdoes. Although thallium has an even larger atomic number, it is only a traceimpurity.FIGURE 20-17. Interactions of x-rays and gamma rays with anNal(Tl) detector. See text fordescription.