An evaluation of germanium detectors employed for the measurement of radionuclides deposited in lungs using an experimental and Monte Carlo approach.

Abstract

A study was undertaken to evaluate the performance of an advanced design broad energy germanium detector for the in vivo measurement of radionuclides in lungs. Relative counting efficiency, background, and sensitivity for lung counting arrays consisting of four, three, and two 80-mm-diameter by 20-mm-thick (80 x 20 mm) broad energy germanium detectors were simulated by collecting spectra with the single 80 x 20 mm broad energy germanium at each of four locations over a humanoid torso phantom. Regions of interest were evaluated for photon energies ranging from 17 to 1,500 keV. The 80 x 20 mm detector arrays were then benchmarked against a standard array of four 70-mm-diameter by 20-mm-thick (70 x 20 mm) broad energy germanium detectors. Since testing new equipment can be an expensive and time consuming process, an alternative approach, using Monte Carlo simulations instead of physical measurements, was also evaluated and compared to experimental data. With this approach, counting efficiency and minimum detectable amount were simulated for two sizes of germanium detectors (70 mm and 80 mm diameter) at four different crystal thicknesses (15, 20, 25, and 30 mm). For the experimental measurements, arrays consisting of three and four 80 x 20 mm broad energy germanium detectors resulted in an increase in counting efficiencies, relative to the standard array, at all photon energies. The greatest relative increase was observed for the four-detector array (24-35%). In contrast, counting efficiency decreased, relative to the standard array, by 24-28% with a two-detector array. Arrays consisting of two and three 80 x 20 mm broad energy germanium detectors resulted in decreased relative background at all photon energies, with the exception of the 946 keV photon for the three-detector array. The most significant decrease in background occurred with the two-detector array (28 to 40%), while background was increased by 18-43% for the four-detector array. Arrays consisting of three and four 80 x 20 mm broad energy germanium detectors resulted in increased relative sensitivity at all photon energies. The three-detector array provided the greatest sensitivity at photon energies below 344 keV. The four-detector array provided slightly better measurement sensitivity at photon energies greater than 344 keV. The two 80 x 20 mm detector array provided sensitivity unexpectedly comparable to the standard array. Monte Carlo predictions on how size affects counting efficiency and minimum detectable amount agreed well with the experimental results. From the Monte Carlo predictions, the effect of detector thickness on counting efficiency was unimportant at photon energies up to 60 keV and independent of detector diameter. At higher photon energies for both detector diameters, the counting efficiency decreased as the thickness decreased. The values of minimum detectable amount for the 70-mm and 80-mm diameter detectors did not differ by more than 15% at 17 keV or 20% at 60 keV when compared to detectors of equivalent thickness. Minimum detectable amount increased slightly at 17 keV and rose by approximately 52% at 660 keV, with decreases in thickness from 30 mm to 15 mm.