Suppression of random coincidences during in-beam PET measurements

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-μs-microstructure due to the accelerator radiofrequency (RF), are highly corrupted by random coincidences arising from prompt γ-rays following nuclear reactions as the projectiles penetrate the tissue. Therefore, 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 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, during beam extraction, the micropulse-induced random coincidences. Image statistics increased ∼90%. Both methods rely on the synchronization of the γγ-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 β⁺-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 future synchrotron and even cyclotron facilities with reduced beam acceleration (delivery pause) times.