Patent Number: 059404698
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is directed to a radiation source for generating multi-chromatic, particularly di-chromatic x-radiation, having at least one cathode and an anode for generating x-ray bremsstrahlung and a target surrounded by the anode for converting the x-ray bremsstrahlung incident onto the target into fluorescence radiation. 2. Description of the Prior Art X-ray sources that generate a continuous x-ray spectrum are almost exclusively employed in medical diagnostics. For a number of applications, however, a monochromatic x-ray spectrum would be advantageous since additional material properties such as, for example, the discontinuous rise in the intensity attenuation at the absorption edges, could thereby be exploited. Although German OS 42 09 226 discloses a monochromatic x-ray source of the type initially described, this known x-ray source only generates x-radiation at a wavelength defined by the selection of the fluorescence target. Different wavelengths are required, however, for producing images using subtraction techniques. This subtraction method was therefore previously utilized in x-ray diagnostics by employing either x-radiators with continuous bremsstrahlung spectrum and two different acceleration voltages, or a di-chromatic synchrotron source. In the first method with two continuous bremsstrahl spectra shifted relative to one another, however, the absorption edge of the contrast agent is inadequately used. The second approach can be utilized only in conjunction with accelerator rings, and thus not at all in normal hospital use, and moreover requires the use of two-line detectors. SUMMARY OF THE INVENTION An object of the present invention is to provide a compact x-ray source having a number of rapidly switchable x-ray frequencies, preferably two rapidly switchable x-ray frequencies. This object is achieved in accordance with the invention in an x-ray source having a target composed in sections of different materials and wherein the sections can be optionally irradiated with the x-ray bremsstrahlung. In a first embodiment of the inventive radiation source, a needle-shaped target is employed which is divided in a longitudinal middle plane, and the two halves are composed of the aforementioned different materials, and the cathode is likewise divided and its sections are separately driveable so as to selectively irradiate two sections (maximally in the shape of a half-ring) of the anode lying opposite one another with electrons, from which x-rays only reach one target half. Dependent on which of the sub-sections of the anode is driven at the moment, thus, x-ray bremsstrahlung is generated only on one part of the anode surface, this is in turn irradiating only one target half composed of different materials, so that only the characteristic fluorescence radiation of this target half can be produced. The switching from one cathode section to the other is possible practically without inertia and without delay, so that the change between the two monochromatic x-ray frequencies can ensue very rapidly. Such a di-chromatic x-ray source is thus excellently suited for the subtraction techniques mentioned earlier, wherein one x-ray frequency lies somewhat above and the other x-ray frequency lies somewhat below the absorption edge of the material of interest. In a second embodiment of the invention, the target is divided in the middle plane of the anode ring perpendicularly to the ring axis; and the anode ring is provided with an inwardly projecting wedge ring symmetrical to the middle plane. Respective ring cathodes, each with a focusing arrangement, are arranged above and under the middle plane. These rings respectively irradiate only the upper or the lower ring surface of the wedge ring with electrons. By bringing the tip of the wedge ring close to the target and/or by fashioning the wedge angle smaller than or equal to twice the heel angle of the wedge ring, it can be assured that radiation from one of the ring surfaces of the wedge ring can exclusively reach either the upper half or the lower half of the target, so that an exactly monochromatic fluorescence radiation is generated dependent on whether the upper or the lower ring cathode is employed. In a further embodiment of the invention a fluid guide surface through which coolant flows is disposed in the hollow ring. This surface is preferably likewise wedge-shaped and inwardly projects into the hollow wedge ring and is spaced therefrom. The coolant also flows through the hollow wedge ring in which, of course, the principal heat quantity is generated due to the incidence of the electrons, and must also be removed therefrom. It also within the scope of the invention to provide a central x-ray exit window in a bottom plate of the anode ring lying opposite the target tip. The x-ray exit window is preferably arranged on a carrying pipe for the lower ring cathode projecting inwardly from a base opening and on the focusing coils thereof. Of course, the different divisions of the target could also be combined with one another, so that the target could be composed of four different sections in order to create a quadro-chromatic radiation source with four x-ray frequencies selectable optionally and in rapid sequence. Such an x-ray source with four different frequencies can be very advantageously utilized for other diagnostic purposes. For x-ray diagnostics systems making use of the subtraction method, however, a di-chromatic x-ray source is sufficient, i.e. only a single partition of the target into two sections.