A Practical Gamma-Ray Camera System Using High-Purity Germanium

A prototype gamma camera system has been constructed which is based on a high purity germanium detector fabricated with orthogonal strip electrodes. In this device, position sensitivity is obtained by connecting each contact strip on the detector to a charge-dividing resistor network. Excellent energy and spatial resolution have been achieved by cooling the resistor network to 77°K and by proper selection of noise filtering parameters in the pulse shaping amplifier circuitry. The significant advantage of employing this charge-splitting detector in a semiconductor gamma camera system is its electronic-readout simplicity, requiring only three amplifier channels to measure the energy and two-dimensional location of gamma ray events. A complete discussion of our investigation of the charge-splitting detector concept is presented with special reference to its potential use in the construction of a high resolution gamma imaging system having sufficient field of view and sensitivity for clinical utilization. Several orthogonal strip-electrode germanium detectors have been fabricated and evaluated experimentally in our laboratory. The most recent of these measures 2 cm × 2 cm × 5 mm thick and incorporates 10 contact strips on each surface which are spaced on 2 mm centers. The measured FWHM energy and spatial resolutions were 5.5 keV and 1.66 mm, respectively. Theoretical calculation of the magnitude of noise in the energy channel and comparison of these values to measured data shows that correlated noise cancellation significantly enhances the energy resolution in this type of charge splitting device.