This paper presents an evaluation of two types of Geiger-mode avalanche photodiodes (G-APDs) for their potential to be used in a positron emission tomography (PET) detector. While the MPPC G-APD had only 3600 cells, the solid state photomultiplier (SSPM)-type G-APD had 8100 cells. In a single-channel G-APD/LSO setup, the energy resolution (DeltaEpsilon/Epsilon) of the SSPM at 511 keV was 25%, while the (DeltaEpsilon/Epsilon) of the MPPC was 13.5% (FWHM). No influences were observed while the detectors were inside a 7 T magnetic resonance (MR) scanner. A time resolution of 2.7 ns (FWHM) was measured for the LSO/SSPM and 0.9 ns for the LSO/MPPC detector setup. Although the linearity was superior for the SSPM in the single detector readout, the inferior energy and time resolution excluded them to be used for the block detector readout. All 12 x 12 LSO crystals of the block could be resolved in a crystal map using a 3 x 3 MPPC G-APD array. The time resolution of the block detector was 950 ps. While the energy spectra for the MPPC-based single-channel setup were nonlinear, they reached linearity better than 5% in the block detector. A high number of G-APD cells provide a linear signal in a single-channel detector setup, but not necessarily a good timing or (DeltaEpsilon/Epsilon) due to a larger inactive surface resulting in lower photon detection efficiency. G-APDs with a low number of cells provide a good timing and (DeltaEpsilon/Epsilon) and linear signals in block detector designs, where the scintillation light is shared over many G-APDs.