A Rotational Modulator (RM) gamma ray imager, consisting of a single grid of lead slats rotating above an array of detectors with diameter equal to the slat spacing, has the capability of providing angular resolution significantly better than the geometric resolution (i.e., the ratio of detector diameter to mask/detector separation). The sensitivity, weight, and angular resolution are comparable to that of a coded aperture device, but with significantly less complexity. As the grid rotates, the transmission from a source is modulated on each detector between 0 and 100%. The count profile is cross-correlated with precalculated modulation profiles to produce an approximate source image. Deconvolution of this image with the known imager response can accurately resolve point sources and complex emissions. The appropriate deconvolution technique can achieve angular resolution better than the basic geometrical resolution of the instrument. A prototype RM developed at Louisiana State University features high sensitivity and energy resolution, functional angular resolution of 15, and a simple readout system. The detector array consists of 19 1.5 × 1 thick cerium-doped lanthanum bromide (LaBr3:Ce) crystals. LaBr3 produces significantly more light than other common scintillators, offering < 3% FWHM energy resolution at 662 keV. A grid spaced ~1.2 m from the detection plane with slat width 1.5 offers a 13.8° field of view. We present our reconstruction technique, deconvolution algorithms, and simulated and experimental imaging results.
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