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-/spl mu/s-microstructure due to the accelerator radio frequency (RF), are highly corrupted by random coincidences arising from prompt /spl 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 (/spl sim/2--4 s) between the beam macropulses (/spl les/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 /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 synchrotron and cyclotron radiotherapy facilities with optimal use of the annihilation photon flux.