Technique for balancing anger cameras using an externally irradiated

Abstract

To calibrate a nuclear camera, the scintillation crystal (10) is irradiated with a uniform flood source (34). Scintillation events cause corresponding electrical pulses from photomultiplier tubes (12) that are optically coupled to the scintillation crystal. A selection circuit (40) compares (42) the amplitude or energy of pulses at each coordinate with a median energy and divides the pulses to generate a higher energy image (44) and a lower energy image (46). When the higher and lower energy images are subtracted (50) regions of the difference image (52) with high count densities identify a photomultiplier tube for gain adjustment. A gain adjustment circuit (60) monitors the amplitude of the electrical pulses and the number of pulses with each amplitude from the selected photomultiplier tube to generate a raw data energy distribution curve (FIG. 3). The energy distribution curve is smoothed (70) and the first derivative is taken (72). The curve is filtered (74) to isolate a preselected energy region. The curve is analyzed (92) to determine an energy value corresponding to a centroid (94) of the filtered first derivative curve. The centroid energy value is compared (96) with the centroid energy value from a preprogrammed table (32). The deviation between the calculated and looked-up centroid energy addresses a look-up table (98) which retrieves the appropriate adjustment to the selected photomultiplier tube. Each adjustment of the photomultiplier tube is stored in a gain adjustment history memory (110).