A method and apparatus for using a tomography to facilitate the placement of an instrument in a vessel or in an organ of a patients body wherein a specific or selected region of the patient is defined. X-ray radiation is projected onto the selected region and the X-ray radiation being emitted by the X-ray source of the apparatus. The radiation transmitted to a detector of the apparatus is measured and the detector being positioned in line with the X-ray source. The signals of the selected region are measured by the detector and transmitted for acquisition that stores the image. The acquired signals for the image are transmitted to a display in order to display a projected image of the selected region of the patient in a plane parallel to the longitudinal axis of the patient and such that the time interval between measuring the radiation and displaying the radioscopic image is short enough to allow the instrument to be guided in the patients body in real-time. The method is repeated in order to acquire new projected images of the selected region.
