We report laboratory imaging results of a balloon-borne gamma-ray imaging telescope, Máscara Codificada (Portuguese for “coded mask,”) designed to obtain high angular resolution (∼14 arcmin) images of the sky in the 50 keV–1.8 MeV energy range. The instrument incorporates a coded mask with an aperture pattern based on a 19×19-element modified uniformly redundant array (MURA). This pattern belongs to a subclass of MURAs that are almost completely antisymmetrical for 90° rotations with respect to the lower-right corner of the central element, which allows the implementation of an antimask by a single rotation of the whole mask by 90°. The symmetry properties of the MURAs are discussed. The algorithm for determining the position of the gamma-ray interactions on the instrument’s main detector is described and the results of laboratory tests of the imaging system are presented. The mask–antimask subtraction, applied after a flat-fielding procedure, produced a 60% increase in the signal-to-noise ratio of a strong (∼100σ) point source (662 keV photons coming from a 137Cs radioactive source) image by eliminating systematic distortions in the instrumental background measured over the detector plane. © 2002 American Institute of Physics.