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

At the experimental heavy ion therapy facility at the Gesellschaft fur 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 gamma-rays following the decay of minor amounts of beta⁺-radioactive nuclei produced via nuclear reactions between the ions of the therapeutic beam and the atomic nuclei of the irradiated tissue. As the quality assurance monitoring using an in-beam PET can profit significantly from a rapid reconstruction, it is highly desirable to perform one using a fast converging algorithm, like OSEM. An influence of the quality of the system matrix calculation on the reconstruction results has to be evaluated. The method of system matrix calculation currently in use computes the system matrix "on-the-fly" using on-line intra-channel Monte Carlo simulation and delivers a rough approach. A new method for a precise calculation of the system matrix has been developed. 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. The influence of the accuracy of the system matrix calculation on the results of MLEM and OSEM reconstructions has been evaluated for real treatment data.