According to one embodiment, the invention relates to a light guide comprising a first surface which receives light, a second surface which emits light, wherein the second surface is parallel to the first surface and the second surface has a smaller area than the first surface, at least one edge surface which extends between the first surface and the second surface, and a light barrier which extends between the first surface and the second surface, wherein the light barrier divides the light guide into separate regions and reduces the propagation of light between the separate regions. The light guide can be used in a positron emission tomography scanner.