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-/spl mu/s-microstructure due to the accelerator radiofrequency (RF), are highly corrupted by random coincidences arising from prompt /spl gamma/-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 (/spl sim/2-4 s) between the beam macropulses (/spl les/2 s). We have successfully tested at GSI two methods for suppressing, during beam extraction, the micropulse-induced random coincidences. Image statistics increased /spl sim/90%. Both methods rely on the synchronization of the /spl gamma//spl 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 /spl beta//sup +/-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.