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The computerized-aided tomodensitometry by X-rays is a powerful, precise and non invasive tool to investigate the human body as well as different objects and structures. The relatively long examination time, from 2 to 10 sec., which the present state of the art necessitates, limits the field of applications of this tool here to the static on quasistatic organs or objects (e.g. the brain or abdomen in medical diagnosis). When one try to reconstruct images of a dynamic object some errors or artefacts appear. In the case of the heart, in particular, the examination time takes several cardiac periods; so, the reconstructed image is highly blurred and for all purposes unusable. One solution, which can be used with a conventional scanner is the "gating" technique. Taking into account of the repetitive property of the cardiac movement, this technique necessitates to catch the projections in synchronization with the electrocardiogram. Thus a sub-program, controlled by the ECG sianal, has to select a set of projections corresponding to a certain time of the cardiac period. The implementation of this sub-program into the reconstruction code allows to obtain more sharp images of the heart at any given instant of the cardiac period. In this paper we discuss the methodology used to acquire the projections, the problems caused by the small number of projections, some mathematical solutions (extensions of signals and data, interpolations, uses of the redundancy in the information aiven by a FAN-BEAM system). These different possibilities are illustrated by some results we have obtained in our experiments on animals with a FAN-BEAM machine.
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