Imaging Efficiency of Ge and NaI(Tl) Gamma-Ray Detectors

A figure of merit is derived for the efficiency with which a detector images a γ-ray emitting object using the signal-to-noise ratio as the criterion. Photons scattered in the detector that ultimately culminate in photoelectric absorption produce an undesirable background when imaging with position-sensitive detectors as in cameras. When imaging with detectors that are not position sensitive as in scanners, these photons contribute to the image of the object. In comparison with NaI(Tl), Ge detectors have lower photoelectric efficiency and higher detector scatter. However, these are offset by much higher energy resolution and the consequent near complete tissue-scatter rejection. As a result, the imaging efficiency for a brain phantom at 150 keV is shown to be comparable for Ge and NaI(Tl) cameras. In the case of scanners, the imaging efficiency of Ge was calculated to be 40-75% higher for a brain phantom with a one γ-ray emitting radionuclide such as 99mTc, and effectively 2-2.5 times higher for a three γ-ray emitting radionuclide such as 67Ga. The imaging efficiency, which depends on absorption characteristics and energy resolution, is a measure of the total performance of the detector. It is shown that in addition to being useful for comparing different types of detectors, this figure of merit also provides means for optimizing imaging parameters such as detector thickness.