A Monte Carlo simulation of coincidence detection and imaging of gamma-ray cascades with a scintillation camera

Several radionuclei used in nuclear medicine decay through the emission of cascaded gamma-rays. These successively-emitted gamma-rays could potentially be detected in coincidence and then be used to create coincident event-based images. While several groups have considered various ways to leverage the gamma-ray cascade to produce images, the concept of producing images using cascaded gamma rays in coincidence with a dual-head gamma camera has not been previously explored in detail. Given that current dual-head gamma cameras cannot be configured in coincidence mode, the purpose of this work was to explore this concept with In-111 using a Monte Carlo simulation. First, GEANT4 was used to model a dual-head gamma camera and to simulate the decay of the nucleus. The model was then validated using planar data from a physical scanner. The validated model was then used to simulate a point source of In-111 located at the camera isocenter to evaluate the true coincidence efficiency with the detectors oriented at 90 degrees. The noise-equivalent-count-rate (NECR) as a function of timing window resolution was computed for different source activities. A simple reconstruction algorithm was then used to produce coincidence-based images from sources in various configurations. The coincident gamma-ray detection efficiency for a single point source in air with the detector heads oriented at 90 degrees was found to be 6 x 10(-8), independent of the source-to-detector distance. The time window resolution for the maximum NECR ranged between 200 and 500 ns, dependent on source activity. The algorithm could produce images over a range of timing window resolutions, with varying levels of reconstruction artifacts. Images produced from a small acrylic phantom showed that scattered events produced small degradations in resolution and contrast-to-noise ratios. This work described the conditions under which images based on coincident events from In-111 could be created using a dual head gamma camera, assuming appropriate electronics modifications.