A Method for System Matrix Construction and Processing for Reconstruction of In-Beam PET Data

At the experimental heavy ion therapy facility (Gesellschaft für Schwerionenforschung Darmstadt, Germany) an in-beam PET scanner is operated for quality assurance monitoring simultaneously to the therapeutic irradiation. The fixed dual-head PET scanner, which is completely integrated into the treatment facility, registers the annihilation γ-rays following the decay of minor amounts of β+-radioactive nuclei produced via nuclear reactions between the ions of the therapeutic beam and the atomic nuclei of the irradiated tissue. From a comparison of the reconstructed activity distributions with those predicted from the treatment plan, deviations between the prescribed and the applied dose distributions can be detected. The quality assurance monitoring of the therapeutic irradiation could profit significantly of a very fast (ideally, real time) reconstruction algorithm and, therefore, an accurate knowledge of the system response function (system matrix) is required. These requirements lead to the development of a dedicated method of system matrix calculation which has to model the system response precisely and deliver system matrix values rapidly. By exploiting the system symmetries, in combination with an optimized interpolation and approximation strategies, the proposed method delivers the system response function in a continuous 3D image space with a high accuracy.