Two cylindrical interruptions of the SIGMA telescope passive shield act as collimators projecting arc-shaped images on the detector. The field of view (FOV) of these collimators produces significant sidelobes in the telescope sensitivity diagram. The imaging properties of SIGMA exploiting these sidelobes enable spatial analysis of the diffuse galactic continuum in the 35–600 keV energy range by means of model fitting. A model, based on the spatial distribution of atomic and molecular gas of the Galaxy, is considered. The obtained spectrum of the diffuse γ-ray emission is then compared to theoretical predictions of various contributions in which orthopositronium and cosmic-ray electrons are involved.