Computer tomography apparatus
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is directed to a computer tomography apparatus, and in particular to such an apparatus including means for identifying a change in the focus position of the x-ray beam and means for taking this change into account in constructing the image for display. 
     2. Description of the Prior Art 
     Computer tomography devices are known which generally include a measuring unit having an x-ray source and a radiation receiver. The x-ray source emits a fan-shaped radiation beam in which a patient is disposed, with the radiation attenuated by the patient being incident on a radiation receiver consisting of a row of individual detectors. Each detector forms an electrical signal corresponding to the radiation intensity incident thereon. The x-ray source and radiation detector are rotated around the patient for transirradiating a measuring field from different directions. The output signals from the individual detectors at the different transirradiation directions are supplied to a computer which constructs an image therefrom for visual display. 
     In computer tomography devices of this type, a condition for obtaining an image free of artifacts is that the focus be at a predetermined known position with respect to the radiation receiver. In practice, however, the focus will migrate or change position slightly relative to the known or rated position, particularly in the longitudinal direction of the patient support (z-direction). This is primarily due to heating of the x-ray tube, in particular the anode thereof, during use. 
     One proposed solution to compensate for such changes in focus position is described in U.S. Pat. No. 4,559,639, wherein a plurality of detectors are provided to measure the change in focus position by direct sensing thereof. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a computer tomography apparatus of the type described above wherein a correction for the change of the focus position can be undertaken without the need for direct measurement thereof, thus eliminating the need for the presence of measuring detectors. 
     The above object is achieved in accordance with the principles of the present invention in a computer tomography apparatus wherein a focus position computer is provided which receives signals from the high voltage supply which feeds the x-ray source. The signals from the high voltage supply identify the operating values for the x-ray source, such as voltage, current and duration of use. From these values, the temperature of the rotating anode of the x-ray source can be calculated, and a known functional relationship exists between this operating temperature and the focus position at any given time, particularly in the z-direction. The focus position computer accordingly generates a signal identifying the current position of the focus, and supplies this signal to the image construction computer. The image construction computer takes the change of focus position into account in constructing the image for visual display. The resulting image is thus free of image artifacts caused by the assumption that the position of the focus is unchanged. 
     The functional relationship between the temperature which is calculated from the operating values of the high voltage supply and the position of the focus in the z-direction is measured once for a typical arrangement, and is stored in the focus position computer. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of a computer tomography apparatus constructed in accordance with the principles of the present invention. 
     FIG. 2 is a schematic view taken in the direction of the arrow indicated at II in FIG. 1 for explaining the operation of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The basic components of a computer tomography apparatus constructed in accordance with the principles of the present invention are schematically shown in FIG. 1. The apparatus includes a measuring unit consisting of an x-ray source 1 which emits a fan-shaped x-ray beam 2 and a radiation receiver 3, which consists of a row of individual detectors, for example, 512 individual detectors. A patient 4 to be examined rests on a patient support table 5. For scanning the patient 4, the measuring unit consisting of the x-ray source 1 and the radiation receiver 3 is rotated through 360° around a measuring field 9 in which the patient 4 is disposed. The rotational axis is referenced at 10. The x-ray source 1 is fed by a high voltage supply 6, and may be pulsed or continuously operated. Given predetermined angular positions of the x-ray source 1 and the radiation detector 3, sets of data are generated which are supplied from the receiver 3 to an image construction computer 7, which calculates the attenuation coefficients of predetermined picture elements from the generated data sets, and provides a pictorial reproduction of a slice of the patient 4 on a display 8. 
     The change in the direction of the x-ray beam 2 is achieved by rotating a live ring 14, on which the x-ray source 1 and the radiation receiver 3 are mounted, by a rotational drive unit 15. 
     As shown in FIG. 2, the focus 11 of the x-ray source 1 can migrate or change position slightly in the z-direction during operation due to heating of the x-ray source 1, particularly the anode thereof. A fixed functional relationship between the various focus positions and the temperature of the x-ray source 1, particularly the rotating anode thereof, is established for a specific computer tomography apparatus. This functional relationship is stored in a focus position computer 16. The focus position computer 16 receives input signals on a line 17 from the high voltage supply 6 identifying various operating values of the x-ray source 1. From the signals supplied on line 17, the focus position computer 16 calculates the current temperature of the x-ray source 1, and using the aforementioned fixed relationship then calculates the respective focus position from the calculated temperature. A signal corresponding to the current focus position is supplied form the focus position computer 16 to the image construction computer 7 on line 18. The image construction computer 7 can then replace the known or rated position of the focus 11 with the updated focus position obtained from the focus position computer 16, or undertake any suitable correction instead of making an actual substitution, in constructing the image for display. The image on the display 8 is thus free of artifacts whch would otherwise be caused by changes in the focus position due to heating of the x-ray source 1. 
     Although modifications and changes may be suggested by those skilled in the art it is the intention of the inventor to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of his contribution to the art.