Abstract:
A multi-source computed tomography system has a first x-ray source and a second x-ray source that are respectively optimized for different imaging procedures and can be used simultaneously in the multi-source CT system. The first x-ray source can be optimized for higher power short-term operation and the second x-ray source can be optimized for lower power, longer term operation.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to a multi-source CT system for the generation of X-ray images of an object under examination of the type having at least one first and one second X-ray radiation source mounted in a rotatable manner about a rotational axis, each emitting a radiation beam and at least one X-ray radiation detector that is struck by the radiation beams. 
         [0003]    2. Description of the Prior Art 
         [0004]    A significant future requirement of computer tomography (CT) scanners is ever greater rotational frequencies of gantries. A faster rotation of the gantry around the patient shortens the recording (data acquisition) time, which in turn results in higher temporal resolution of the recordings. This is of great importance in cardiology in particular, as here a high temporal resolution at the beating heart also influences spatial resolution and thus the possibility of detecting arteriosclerosis at an early stage. In the case of higher rotational frequencies, however, the mAs requirement must also be met in order to reach a certain image quality. Ever greater radiation power must be delivered, so that these X-ray tubes must be specially designed for the requisite short-term load of typically 1 s. 
         [0005]    An increase in the rotational frequency, however, also increases the strain on the components of the gantry as a result of greater centrifugal force, so that the rotational frequency is currently limited to 3 to 4 Hz. In order nonetheless to increase the number of projections achieved per unit of time, the use of dual- or multi-tube CT systems is known, in which two or more X-ray emitter-detector units are arranged, offset at an angle in a gantry housing. Furthermore, a known method for better differentiation of a variety of tissue types is to scan the area under investigation simultaneously with X-ray radiation of different spectral distribution, that is to say with different maximum X-ray energy or a different spectral centroid. 
         [0006]    A dual-source CT system for examination of a patient is known from DE 10 2007 024 158 A1, having a gantry housing, in which two X-ray tubes are arranged offset an at angle with oppositely arranged detector systems, which rotate about a system axis for scanning of the patient, while the patient is conveyed along the system axis through the measuring area of the CT system by means of the controllable patient couch. During a dual-energy CT examination, the two similar X-ray tubes are operated with different acceleration voltages, so that the two X-ray spectra used differ greatly and also deliver different absorption values in the assigned detectors upon irradiation of the patient. 
         [0007]    In addition to short-duration scans with a maximum of a few seconds duration, CT systems are also used for whole-body scans and perfusion measurements. In such cases the X-ray emitter must be designed for recording times of up to one minute. The X-ray emitter that has been designed so as to be able to be used in short-term scans is subjected to extreme stresses in such longer term operation, which in particular impacts its useful life. 
       SUMMARY OF THE INVENTION 
       [0008]    An object of the present invention is to optimize a multi-source CT system in relation to the requirements for different types of recording. 
         [0009]    The basic idea of the invention consists, in the case of a multi-source CT system with at least two X-ray radiation sources, of providing different X-ray radiation sources, which differ significantly in particular with respect to their technical structure and the maximum X-ray radiation power achievable for a particular recording duration. At least one of the X-ray radiation sources is optimized for a first mode of operation and at least one second X-ray radiation source for a second mode of operation of the CT system. In particular the inventive CT system preferably comprises just one technically highly complex X-ray radiation source, which can generate an extremely high X-ray radiation power for a short recording duration. The second X-ray radiation source, on the other hand, is designed for continuous output. The second X-ray radiation source is preferably used exclusively for whole-body or perfusion measurements. 
         [0010]    By means of the invention it is possible, in the case of a multi-source CT system with a multiplicity of X-ray radiation sources, to optimize the latter in each case for their special purpose. This results in a reduction in costs for the multi-source CT system concerned. As the X-ray radiation sources used are at least predominantly only operated in the mode of operation for which they have been optimized, their useful life is also prolonged thereby. 
         [0011]    According to the invention the first and second X-ray radiation sources differ by a factor at least in respect of the maximum X-ray radiation power that they can generate for a particular recording duration. This factor preferably lies in the range 1.3 to 3. 
         [0012]    In a preferred embodiment of the invention, the first X-ray radiation source has a magnetic bearing for the anode plate. Rotational frequencies of the rotating anode of up to 1000 Hz are thereby enabled. Furthermore, the first X-ray radiation source advantageously has a steel plate thermally coupled with the anode plate for heat dissipation purposes. The first X-ray radiation source is here designed and optimized for recording times of less than 5 s. 
         [0013]    The second X-ray radiation source of the inventive multi-source CT system is of simpler construction compared with the first X-ray source. In particular this here involves a rotary X-ray source designed for permanent output over durations of far more than 40 s, with a rotating anode mounted by means of a ball or plain bearing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  shows an inventive multi-source CT system. 
           [0015]      FIG. 2  shows the X-ray radiation power dependent upon the operational duration for different X-ray radiation sources. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]    One example of a dual-source CT system  1  according to the invention is shown in  FIG. 1 . This CT system  1  has a gantry housing  6 , in which are two X-ray tubes  2  and  4  arranged offset at an angle, with oppositely located detector systems  3  and  5 , which rotate about a system axis  9  for the scanning of the patient  7 , while the patient  7  is conveyed by means of the controllable patient couch along the system axis  9  through the measuring area of the CT system. A control and arithmetic unit  10  which contains computer programs Prg 1 -Prg n  in its memory that perform the control and reconstruction during operation serves to handle control, reconstruction and execution of the inventive method. 
         [0017]    Dual-energy operation is possible in the case of the CT system  1  for better differentiation of various tissue types in the area of examination of the patient  7 . To this end, the X-ray absorption must be measured at least in relation to two different energy spectra. This takes place with the aid of the dual-energy CT scanner  1 , which permits the reconstruction of two independent images for at least one axial section through the patient  7 , which were generated with different effective X-ray spectra. A simultaneous scan with two different tube voltages preferably takes place. It is important for image generation to determine the different absorption effect on the two different X-ray radiation powers. 
         [0018]    In the example shown here the two X-ray tubes  2  and  4  are operated with different acceleration voltages, so that the two X-ray spectra used differ greatly and also deliver different absorption values in the assigned detectors upon irradiation of the patient  7 . 
         [0019]    According to the invention the two X-ray tubes  2  and  4  differ in their technical aspects. The X-ray tube  2  is capable of emitting a considerably higher X-ray radiation power for a short recording time in the range up to 5 s than the X-ray tube  4 . In dual-energy operation, the X-ray tube  2  is thus operated with the higher acceleration voltage. In contrast to this the X-ray tube  4  is designed for a recording time of 40 s and more. However X-ray radiation power as high as with X-ray tube  2  cannot be achieved in this case, even for a short period. For dual-energy scans the X-ray tube  4  is thus operated with the lower acceleration voltage compared with X-ray tube  2 . It is further used—preferably with the X-ray tube  2  switched off—for whole-body scans and perfusion measurements which require recording times of 10 s or more. 
         [0020]    The X-ray tube  2  takes the form of a technically complex X-ray tube, in which the anode plate is mounted by means of a magnetic bearing and which is also actuated via a magnetic field. For heat dissipation purposes the anode plate is thermally coupled to a radiation surface. The rotational frequency of the anode plate can further amount to 1000 Hz. By means of this construction of the X-ray tube  2 , an extremely high X-ray radiation power can be achieved for a short time. 
         [0021]    By contrast to this, the X-ray tube  4  takes the form of a simply constructed rotary piston tube. Although in this case the maximum achievable X-ray radiation power is significantly lower than with the X-ray tube  2  (for example only 50% compared with the X-ray tube  2 ), a comparatively high X-ray radiation power can however still be achieved for recording times of 1 min and more. 
         [0022]    The diagram according to  FIG. 2  illustrates the relationship between the maximum X-ray radiation power P depending on the recording time s. 
         [0023]    The critical load curve A of the X-ray tube  2  shows a markedly higher X-ray radiation power in the short-term range of up to one second. By contrast, the X-ray tube  4 —illustrated by the critical load curve B—enables a long permanent output of up to 40 s, which lies significantly above the permanent output of X-ray tube  2 . 
         [0024]    Through the use of two technically different X-ray tubes  2  and  4  the inventive dual-source CT system is optimized both for short- and long-term recordings. Dual-energy operation is additionally also possible. As compared with conventional CT systems, the two X-ray tubes  2  and  4  need not be operated at the respective limit for the different types of recording which are thereby possible, the useful life of the X-ray tubes  2  and  4  is significantly increased, and the failure rate reduced. 
         [0025]    In a particular embodiment the “Dual Energy” principle can be attained in such a way that the X-ray tube  2  is operated at a high tube voltage, as less tube current is here required for the same photon flux. The X-ray tube  4  on the other hand is operated with a lower tube voltage and higher current. 
         [0026]    Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted heron all changes and modifications as reasonably and properly come within the scope of their contribution to the art.