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Scintillation Techniques used up to now in Nuclear Medicine could only provide, at the detector plane, some projected images of the distribution of the radionuclide concentration in the examined organ or tissue. Now, the use of coded-aperture principle, instead of the conventional collimators in the scintillation cameras allows to get true three-dimensional information and consequently to have some maps of the distribution of the radionuclide along slices of the organ or tissue. The process is realized in two steps. In a first step, an encoding lead mask is placed between the radioactivated object and the detector surface on which the coded-imacTe is formed. From this coded image one can reconstruct, in the second step, a certain number of slices chosen into the object ; these reconstructed slices are obtained by an analogical or digital decoding process of the coded image. In fact, the information corresponding to a given slice, at a given depth determined by its distance to the encoding mask, is obtained by using only some numerical methods ; never-theless these methods are rather complicated and necessitate a large number of operation and calculus. After a quick survey of previous papers on the coded-aperture work in Nuclear Medicine, we present the first results we have obtained with a code originated from the Fresnel zoned array - the. Fresnel zoned linear array - the numerical simulation of the diffraction phenomena being used as the reconstruction method. In the case of the circular Fresnel zoned array the numerical reconstruction necessitates a two-dimensional Fourier-transform. On the contrary the Fresnel linear array necessitates a Fourier-transform in one dimension only ; so it becomes possible to use this method with the processing equipment frequently available in the Nuclear Medicine facilities. The experimental results obtained at the CHR - Toulouse have allowed to verify the focusing-properties of this code (tomographic effect) and also to show that the values obtained by numerical reconstruction agree with those given by the theory of Optics.
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