Techniques and Applications of Compton Imaging for Position-Sensitive Gamma-Ray Detectors

摘要

Three-dimensional position-sensitive CdZnTe gamma ray detectors are capable of achieving excellent energy resolution at room temperature. With this versatile detector technology, many types of imaging techniques and applications become possible. The maximum-likelihood expectation-maximization (MLEM) method estimates thesource distribution for which the set of measured events is most probable. This method is capable of estimating the source image from each energy range as well as the incident spectrum from each direction. The resulting deconvolved incident spectrum, which has the correct branching ratio in the source direction, is useful for identifying the source isotope and estimating the source activity from each direction, as well as estimating the presence, composition and thickness of any shielding material.To use a greater fraction of events, the system-response function is analytically extended to three-interaction events, and then it is extrapolated to events with any number of interactions from an array system. Also, by combining charge-sharing events, and including events with any number of interactions in the system model, imaging makes use of all recorded events, and the angular resolution is improved.Some interesting applications for which our detector and imaging algorithms are shown to be capable include: reconstructing the distribution of the natural radiation background, finding anomalies in a smoothly varying background, and compensating for the known motion of a source. Two methods are proposed for motion compensation. The first method rotates the imaging reference frame to track the movement of the source. Alternatively, the second method extends the imaging space with additional time dependent target bins. Both methods can be applied to simple back-projection as well as the MLEM deconvolution. Also, when imaging directions are desired to be associated with physical objects, an overlaid optical-radiation image can be generated.With the sub-pixel interaction position sensing capability of the new digital ASIC, the image blur associated with the difference between the true Compton scattering location and the measured centroid of the recoil electron cloud can be reduced by altering the back-projection cone angle. Appreciable improvement of the image angular resolution is achieved using this method with both simulated and experimental data.