Suppression of random coincidences during in-beam PET measurements at ion beam radiotherapy facilities

In-beam positron emission tomography (PET) is currently the only method for an in-situ monitoring of charged hadron therapy. However, in-beam PET data, measured at beams with a sub-mu s-microstructure due to the accelerator radio frequency (RF), are highly corrupted by random coincidences arising from prompt gamma rays following nuclear reactions as the projectiles penetrate the tissue. Since random-correction techniques from conventional PET cannot be applied, the clinical in-beam PET at the therapy facility at the Gesellschaft fur Schwerionenforschung (GSI), Darmstadt, Germany, merely reconstructs events registered in the pauses (similar to 2-4 s) between the beam macropulses (<= 2 s). We have successfully tested two methods for suppressing the micropulse-induced random coincidences during beam extraction. Image statistics can be increased by about 90%. Both methods rely on the synchronization of the gamma gamma coincidences measured by the positron camera with the time microstructure of the beam, either by using the RF signal from the accelerator or the signal of a thin diamond detector placed in the beam path in front of the target. Energy and triple-coincidence time-correlated spectra first measured during beam extraction, combined with the corresponding tomographic images of the beta+ activity induced by the beam in a plastic phantom, clearly confirm the feasibility of the proposed random suppression methods. These methods provide the solution for applying in-beam PET at synchrotron and cyclotron radiotherapy facilities with optimal use of the annihilation photon flux.