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-microsecond-microstructure due to the accelerator radiofrequency (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 fuer Schwerionenforschung (GSI) Darmstadt merely reconstructs events registered in the
pauses (~ 2 - 4 s) between the beam macropulses (< 2 s). We have successfully tested at GSI two methods for suppressing
the micropulse-induced random coincidences during beam extraction. Image statistics 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 utilization of the annihilation photon flux.