In-beam PET at clinical proton beams with pile-up rejection

Positron emission tomography (PET) is a means of imaging the β+-activity produced by the radiation field in ion beam therapy and therefore for treatment verification. Prompt γ-rays that are emitted during beam application challenge the detectors and electronics of PET systems, since those are designed for low and medium count rates. Typical PET detectors operated according to a modified Anger principle suffer from multiple events at high rates. Therefore, in-beam PET systems using such detectors rely on a synchronization of beam status and measurement to reject deteriorated data. In this work, a method for pile-up rejection is applied to conventional Anger logic block detectors. It allows for an in-beam data acquisition without further synchronization. Though cyclotrons produce a continuous wave beam, the radiation field shaping technique introduces breaks in the application. Time regimes mimicking synchrotrons as well as cyclotron based ones using double-scattering or pencil beam scanning field shaping at dose rates of 0.5, 1.0 and 2.0Gy/min were investigated. Two types of inhomogeneous phantoms were imaged. The first one simulates cavity structures, the other one mimics a static lung irradiation. It could be shown that, depending on the dose rate and the beam time structure, in-beam measurement including a few seconds decay time only, yield images which revealed all inhomogeneities in the phantoms. This technique can be the basis for the development of an in-beam PET system with traditional detectors and off-the-shelf electronics. Die Positronen-Emissions-Tomographie (PET) ist eine Methode zur Darstellung der β+-Aktivitätsverteilung, die durch das Strahlenfeld in der Ionenstrahltherapie erzeugt wird. Damit ist eine Verifikation der Bestrahlung möglich. Prompt emittierte γ-Strahlen, die während der Strahlapplikation entstehen, belasten die Detektoren und die Elektronik von PET-Systemen, welche für niedrige und mittlere Zählraten entwickelt wurden. Typische PET-Detektoren, die nach einem modifizierten Anger-Prinzip betrieben werden, leiden unter multiplen Events bei hohen Raten. Daher werden PET-Systeme mit solchen Detektoren mit dem Strahlstatus synchronisiert, um korrupte Daten zu verwerfen. In dieser Arbeit wird eine Methode zur Verwerfung von Pile-up-Ereignissen auf konventionelle Anger-Block-Detektoren angewendet. Sie erlaubt die Erfassung von In-beam-Daten ohne Synchronisation von PET und Bestrahlungssystem. Obwohl Zyklotrons einen kontinuierlichen S