Gamma ray detection with gamma cameras for nuclear imaging is a multiple stage process. First, the incident gamma photon is converted to a scintillation light flash (i.e. a scintillation photon). Then, each of the visible light photons from this flash (sometimes ultraviolet or other photons may be generated) is captured with a photodetector and a spatial signal distribution is obtained, which is then used to estimate the energy of the incident gamma photon and the two- or three-dimensional position of the gamma ray photo conversion. This determined position then forms the basis for the estimation of a line of response (LOR) and the image reconstruction from a nuclear decay process.
While the energy of the gamma photon is proportional to the number of detected visible light photons, the photo conversion position (i.e. the position where the gamma ray interacts with the scintillator and the light flash is created) may be extracted from the spatial distribution of the visible light photons. One possibility is to couple each element of the scintillator to a single photodetector element. All the light is collected by this single photodetector element and the position of the gamma ray impact is given by the position of the photodetector. This type of readout is generally called one-to-one coupling.
Because one-to-one coupling usually requires a large number of digitization channels, light-sharing is often used, which refers to a light-guide interposed in between the scintillator and the photodetector. In this way, the scintillation light from the photo conversion is distributed over several photodetector elements and the number of required digitization channels is reduced considerably. After digitization of the charge pattern, i.e. based on the spatial signal distribution, energy and position are computed from the values. The most widely used method is centroid computation (also called anger method or centre of gravity (COG)). Alternatively, maximum likelihood (MLE) estimation can be used (Lerche, C. W. et al.; “Maximum likelihood based positioning and energy correction for pixelated solid-state PET detectors”; Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC), 2011 IEEE). This method is based on the calculation of the likelihood of a photo conversion position in the scintillator for a given observed charge pattern at the photodetector. The position with the highest likelihood value is chosen.
Both, COG positioning and MLE positioning work fine and result in reasonable position estimates, as long as the observed gamma radiation events all result from single gamma ray impacts. However, if Gamma radiation events result from the impact of multiple gamma rays at the same time, i.e. within one sampling interval in a digital photodetector or within the integration time in an analog charge converter, (pile-up events) or from inter-crystal Compton scatter (inter-crystal Compton scatter events), the detected position might be erroneous.
Further, the recognition of gamma radiation events resulting from pile-up events or from inter-crystal Compton scatter events is difficult during normal data acquisition. One approach is to analyze the spatial position of the centroid of the detected spatial signal distribution, the rising edge of the temporal charge pulse (pulse shape discrimination) or the energy of the event.
In U.S. Pat. No. 5,293,044 (A) a method for rapid localization of a scintillation event in a gamma camera using a maximum likelihood estimator is disclosed. The method is applied in a process for localizing a scintillation event in a gamma camera having a plurality of photomultipliers forming a camera surface. Each photomultiplier generates an output signal in response to the scintillation event. According to the disclosed method, a plurality of comparative signal sets are generated from output signals of the photomultiplier corresponding to respective scintillation events of known location. Then, a location-dependent probability function is formed based on a comparison of the outputs of the photomultipliers for a scintillation event of unknown location with the comparative signal sets, and the location of the scintillation event of unknown location is defined as the location corresponding to the maximum of the probability function. Further, the speed with which the localization is accomplished is increased by initially defining a portion of the total gamma camera surface in which there is a high probability that the location of the scintillation event of unknown origin lies, and limiting the investigation for the location of the scintillation event of unknown location to that portion of the camera surface, with increasing precision.