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-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. Therefore, since random-correction techniques from conventional PET cannot be applied, at the clinical in-beam PET at the therapy facility at the Gesellschaft fuer Schwerionenforschung (GSI) Darmstadt merely events registered in the pauses (2-3s) between the beam macropulses (< 2s) are reconstructed. Image statistics may increase up to 50% if coincidences acquired during the macropulse, but out of the micropulses, are taken into account. Two methods for suppressing the micropulse-induced random coincidences have been successfully tested at GSI with carbon ion beams. They 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. Time and energy correlated spectra, first-measured during the macropulses, together with the corresponding tomographic images of the beta⁺ activity induced by the beam in a phantom, clearly confirm the feasibility of the proposed random supression methods.