The ClearPEM is a dedicated APD-based PET detector for high-resolution breast cancer imaging. The detector module is composed of 12 LYSO:Ce crystal matrices, each with 4 x 8 individual crystals (2 x 2 x 20 mm(3)) optically coupled on both ends to S8550 Hamamatsu APD arrays for the scintillation light readout. Only 46% of the sensitive area of the detector is constituted by LYSO crystals. The inactive area is due to gaps between APD matrices, encapsulation and BaSO4 reflective walls. To improve the overall sensitivity of the system, a new compact crystal matrix geometry was designed to minimise these inactive parts. In the new crystal matrix geometry, 76% of the sensitive area of the detector is constituted by LYSO:Ce crystals and crystals with three different dimensions are needed. The different matching factors of cross-section between the APDs and the crystals require a study on the effects on the energy and time resolution, optical cross-talk and on depth-of-interaction capability. In this paper, we present an experimental study on the improvement of the sensitivity with this new compact matrix, and a characterisation of its effects on the overall detector performance. (C) 2013 Elsevier B.V. All rights reserved.