A brain imager includes a compact ring-like static PET imager mounted in a helmet-like structure. The closure devices include a proximal anchor member, a distal anchor member, and at least one flexible center joint connecting the two anchor members. In the heterostructure, the ungraded layers are strained layers that comprise at least one strained smoothing layer of a semiconductor material having in a relaxed state a third in-plane lattice parameter which has a value between the first and the second lattice parameter. The granules of the thermoplastic elastomer are preferably angular granules or multi-facet granules.