A PET imaging device for observing a brain includes a hollow three-dimensional structure with a shape capable of housing a head. The PET imaging device comprising multiple independent gamma ray detection modules that together form a structure capable of surrounding the head, said detection modules comprise continuous scintillation crystal blocks, wherein “continuous” means that the crystal blocks can be continuous in one or in two directions, each of the continuous scintillation crystal blocks has a polygonal main cross-section, and said structure is an elongated structure having a major axis in a direction corresponding to the front-nape direction and a shorter axis in a direction corresponding to a straight line joining ears on the head. The continuous scintillation crystal blocks are positioned adjacent to fit laterally in an exact manner with each other throughout their entire thickness, building a mosaic, without gaps between adjacent crystal blocks and without overlapping each other.