Depth-of-Interaction Detection and Correction with a Multi-Scale Algorithm in CCD Based Gamma Cameras

Gamma cameras based on Charge Coupled Devices (CCDs) and micro-columnar CsI scintillators can reach high spatial resolutions. However, the gamma interaction probability of these scintillators; is low (typically < 30% at 141 keV) due to the limited thickness of presently available micro-columnar scintillators. Continuous scintillators can improve the interaction probability but suffer from increased light spread compared to columnar scintillators. In addition, for both types of scintillators gamma photons incident at an oblique angle reduce the spatial resolution due to the variable depth of interaction (DOI). To improve spatial resolution and spectral characteristics of these detectors, we have developed a fast analytic scintillation detection algorithm that makes use of a depth-dependent light spread model and as a result is able to determine the DOI in the scintillator. This algorithm, performing multi-scale frame analysis, was tested for an Electron Multiplying CCD (EM-CCD) optically coupled to CsI(TI) scintillators; of different thicknesses. For a 2.6 mm thick scintillator a spatial resolution of 148 gm Full Width Half Maximum (FWHM) was obtained with an energy resolution of 46% FWHM for gamma photons incident perpendicularly (interaction probability 61% at 141 keV). The multi-scale algorithm improves the spatial resolution up to 11%, and the energy resolution up to 36% compared to a previously implemented algorithm that did not model the depth-dependent light spread. In addition, the multi-scale algorithm can accurately approximate DOI. As a result, degradation of the spatial resolution due to the variable DOI for gamma photons incident at a 47 degrees angle was improved from 2600 gm to 508 gm FWHM for a 3 mm thick scintillator. We conclude that the multi-scale algorithm significantly improves CCD based gamma cameras as can be applied in future SPECT systems.