METHOD FOR OPTIMIZING SIDE SHIELDING IN POSITRON-EMISSION TOMOGRAPHS AND FOR COMPARING DETECTOR MATERIALS

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  • 1 January 1980
    • journal article
    • research article
    • Vol. 21  (10) , 971-977
Abstract
Analytical formulas for the image-forming and background event rates seen by circular positron-emission tomographs with parallel side shielding are presented. These formulas include deadtime losses, detector efficiency, coincidence resolving time, amount of activity, patient-port diameter, shielding gap and shielding depth. A figure of merit, defined by these quantities, describes the signal-to-noise ratio in the reconstructed image of a 20-cm cylinder of water with uniformly dispersed activity. For 1-cm-wide NaI(TI) [TI drifted NaI] detectors with a 50-cm patient port, an activity of 200 .mu.Ci/axial cm and a shielding gap of 2 cm, the optimum shielding depth is 20 cm, which requires a detector circle diameter of 90 cm. For a 25-cm patient port and other conditions as above, the optimum shielding depth is 14 cm. NaI(TI), BGO, CsF plastic and Ge(Li) scintillators, and wire chambers with Pb converters are reported. BGO provided the best signal-to-noise for activity levels below 1000 .mu.Ci/cm and CsF was best for higher activity levels.