  The main challenges posed by the design of future Positron Emission
Tomography machines are the improvement of the spatial and timing resolution
and the combined operation with magnetic resonance. The Micro Pixel Photon
Counter by Hamamatsu is a good candidate for this application. Its small size
(down to 1 mm2) and the high photo-detection efficiency in the blue spectral
region allow the direct readout of a highly segmented scintillator matrix
improving the spatial resolution of the machine. Furthermore, this
photo-detector is insensitive to static magnetic fields up to 5 T, which makes
it a possible candidate for applications in a magnetic resonance environment,
though tests in a fast changing gradient field need still to be performed.
  The aim of this study is the characterization of a system composed by a
scintillator crystal readout via a MPPC. Crystals of 1x1x15 mm3 and 3x3x15 mm3
are directly coupled to a MPPC of the same size active area and the energy
resolution at 511 keV is measured. The coincidence time resolution of two so
assembled detector units is measured. A first comparison of the performances of
LSO and LFS crystals is given.
