CODED APERTURES DERIVED FROM THE FRESNEL ZONE PLATE.

C. Mertz and N. O. Young introduced the idea of using a Fresnel zone plate as a shadow-casting reticle, or coded aperture, in x-ray astronomy. More recently, considerable progress has been made toward using the zone-plate aperture for gamma-ray imaging in nuclear medicine. The most successful configuration has used an off-axis section of a zone plate in conjunction with a halftone screen. In this paper a variety of closely related coded apertures is discussed, including an annulus, an inverted zone plate, a spiral zone plate, and the Girard grill. In most cases, the technique of grid-coded subtraction is used to suppress the zero-order (DC) background light usually associated with zone-plate imaging.