I. Introduction
POSITRON Emission Tomography (PET) is a non-invasive medical imaging technique. A β+ emitter is used to mark a tracer which is injected into a living organism. The two 511 keV photons, produced by the annihilation inside the organism, are detected in coincidence and their line of response is identified. The reconstruction of several lines of response allows to retrieve a functional image of the tissue in which the tracer is absorbed. The energy and the time resolution of the used gamma-detectors are two important parameters which determine the overall performance of the PET scanner. A good energy resolution is crucial in order to reject photons which have undergone Compton scattering inside the organism and thus changed their direction. The time resolution of the gamma-detectors determines the minimal width of the coincidence window and thus the amount of background arising from random coincidences. Furthermore, the time information can be used to directly improve the spatial resolution as done in the time-of-flight PET. Apart from the requirements on the energy and time resolution a small pixel size of the PET camera is required since this mainly determines the spatial resolution. In the present work it will be demonstrated that a gamma-detector consisting of a fast scintillator crystal (Lutetium Fine Silicate [2]) read out by a Multi-Pixel Photon Counter from Hamamatsu well meets the mentioned requirements. The MPPC is a very compact device with excellent photon counting capability which can be operated at low voltage and which is insensitive to magnetic fields. It shows a high sensitivity in the 420 nm spectral region. For more details see [1]. This matches the emission spectra of fast crystal scintillators which are peaked in the blue and ultra-violet spectral region, yielding a high photon-detection efficiency. The outline of this article is as follows.
Experimental setup.