Prompt Gamma Imaging
An ion beam travelling through matter undergoes nuclear reactions with the target nuclei. Besides the build up of β+ emitting nuclides, that are the basis of the PET technologyfor in-vivo dosimetry, nuclei are excited. These excited statesrelax by emission of gamma radiation. Since this process happens within the first 10^-11 seconds after the collision, these gamma quanta are referred to as prompt gammas. This radiation component is suitable for a verification of dose delivery.
However, these gamma rays show a broad and high energetic spectrum. Therefore, the detection of them is a quite complex and challenging task. The application of an Anger camera, a system which is frequently used in nuclear medicine for SPECT, is not feasible. In our group, two different approaches are under investigation.
First, a passively collimating system, a so called slit-camera is investigated. This camera has been developed by the Ion Beam Applications (IBA), Louvain-la-Neuve, Belgium. At the OncoRay center, it is planned to apply this camera during irradiations of patients in near future. For this purpose a comprehensive testing at realistic phantoms, and a calibration of the detectors is required. Furthermore, the hard- and software has to be optimized for clinical application and a quality assurance procedure has to be established.
Secondly, an electronically collimating system, a so called Compton-Camera is under investigation. This type of camera is based upon a combination of two detectors. In the first, the so called scatterer, the incoming photon is scattered incoherently at a quasi-free shell electron of an atom. In the second detector, referred to as absorber, the photon is completely absorbed. From the energy deposition in the two
detectors, the possible source volumes can be estimated. These volumes have the shape of cones.
By means of iterative reconstruction algorithms, the source distribution can be calculated from many single events. This is achieved by superimposition of all the calculated cones. The reconstruction algorithm has to consider the physical processes and their probabilities.
For the optimization of such a camera, Monte-Carlo simulations are essential. The toolkit GEANT4 allows for a complete investigation of all photon interactions, the creation of secondary particles as well as the deposited energy in the detectors. Therefore, conclusions can be drawn about the the efficiency, the background and the optimum design of the camera. Furthermore, these simulations play an important role
during the preparation and evaluation of experiments.