[en] Recently, particular attention has been paid to the development of small Field of View (FOV) breast imagers for Nuclear Medicine applications provided with YAP :Ce and CsI, both thallium and sodium doped, multipillar matrices. In order to improve their energy and spatial resolution performances, it is necessary to increase the light output of such scintillating matrices. We have developed a MonteCarlo code with the purpose of studying the detection efficiency as well as the light collection efficiency of crystal pillars as a function of their geometry and surfaces treatment. By analysing both the experimental and the simulations results, we have found out that the best choice for this gamma detector is represented by a CsI(Na) matrix made up of orthogonally arranged pillars whose dimensions are 2x2x3 mm³. For this geometry, our simulations have showed a detection efficiency of 62%, a collection efficiency of 80% and an expected light output of 68%