Abstract:
In a method and x-ray apparatus for exposure of a patient who can be placed at a variable distance relative to an x-ray source, the dose rate emitted by the x-ray source is reduced given reduction of the distance and/or increased given increase of the distance.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention concerns a method and an x-ray apparatus for radiation exposure of a patient who can be placed at a variable distance relative to an x-ray source. 
     2. Description of the Prior Art 
     X-ray methods and x-ray apparatuses have been used for many decades for medical imaging, in which x-rays are directed through the body of a patient in the form of a living person or animal. Despite this large usage in medicine, such ionizing radiation is not free of health risks for the patient. The use of x-ray radiation on living patients is therefore heavily legally regulated. In particular, maximal allowable limit values exist for the dose rate of x-ray radiation on the body surface of a patient. 21 C.F.R. (Code of Federal Regulations) of the Department of Health and Human Services is an example. 21 C.F.R. §1020.32e specifies what is known as the maximum allowed input exposure rate, thus the entrance skin dose rate in mGy/s on the side of a patient facing towards the radiator. 
     The dose rate radiated by an x-ray source onto a given monitoring surface decreases quadratically with the distance of the monitoring surface from the x-ray source. A minimum distance that must be maintained between a biological tissue and the focus of an x-ray source or the x-ray tube in order to not exceed the prescribed limit dose rate (thus the entrance exposure rate) is calculated from the quadratic distance rule and a predetermined maximum value (limit value). 
     This minimum distance is primarily dependent on the primary dose rate radiated by the x-ray source and thus is dependent on the limit energy of the x-ray radiation as well as on the maximum achievable beam current. 
     In specific x-ray applications, the distance between the x-ray radiation for us and the patient is always fixed. The x-ray radiation is then set (by the design of the x-ray tube or by the voltage/current generator or by side filters placed in the beam path) such that no patient is irradiated with an unacceptably high dose rate. 
     The behavior is different in x-ray systems of the type used, for example, in urology. In arrangements known as under-table fluoroscopy workstations, the x-ray radiator is located under a patient table. The patient table is adjustable in terms of height, meaning that the distance between a patient reclining on the table, or the table surface and the x-ray radiator or its focus is variable. Table positions of different heights are desirable so that a urologist working at the table or on the patient located thereon can move the table into an ergonomically-advantageous working position. 
     For the most part, the patient table can be moved toward the x-ray radiator so far that, upon operation of the x-ray radiator, the patient lying on the table would be irradiated with an unacceptably high dose rate. If the urologist is standing, the table is generally upwardly by a relatively large distance, causing the patient to be positioned outside of the critical dose rate. An x-ray fluoroscopy of the patient is then possible without danger. For longer endo-urological procedures under x-ray monitoring, however, the urologist usually prefers to be seated, so the patient is moved downwardly, thus very close to the x-ray radiator. The minimum distance may then no longer be adhered to, which (given an activated x-ray source) would lead to an unacceptably high entrance dose rate at the patient. 
     Various technical possibilities are known in order to always prevent exposure of the patient with a dose rate that is too high. In the designer installation of an x-ray source, fixed primary aluminum filters can be installed in the x-ray radiator in the beam path of the x-ray system between the x-ray source and the patient in order to decrease the maximal primary dose rate that can be emitted by the x-ray source and that can actually strike the patient. The safety distance between the x-ray source and the patient, thus the minimum allowable distance, is thus smaller than without a primary aluminum filter, so that (for example) a urological work table can be lowered further. The radiation, however, is hardened by the primary aluminum filter, causing the image noise in the x-ray image to increase in a disruptive manner, and in fact at distances between the patient and the x-ray source that would be non-critical given unattenuated x-ray radiation (without an aluminum filter). Metal filters (also made from copper) filter the soft radiation portion from the x-ray radiation; so the generated image is thus also higher-contrast, but the image brightness remains. 
     Furthermore, it is known to mechanically limit the movement path of the patient table in a fixed manner such that thus cannot be moved closer to the x-ray source than is prescribed by the safety interval. This limits the ergonomics of the arrangement since the table is precluded from being lowered when the urologist would like to execute a procedure without x-ray exposure. 
     It is also known to deactivate the x-ray source or to block triggering thereof if and when the safety interval is not adhered to. In such a table position, however, x-ray fluoroscopy of the patient is no longer possible at all. In such a situation, the doctor must refrain from moving the x-ray table into a position that is ergonomically advantageous for him or her, or must move it upwardly from a convenient working position in order to obtain a fluoroscopy of the patient. 
     Furthermore, it is known to permanently operate the x-ray radiator below its maximum dose rate, thus with a lower product made up of beam current and acceleration energy than the x-ray source could actually maximally emit. This likewise leads to a permanent decrease of the maximum safety distance. As in the case of a permanently-installed filter, however, the image quality of the x-ray system is generally degraded since the x-ray source is never operated at its optimal operating point. This also applies for table positions that are actually situated above the safety distance and would otherwise allow a higher image quality. The decrease of the generator power leads to a decrease of the number of generated x-ray quanta and an increase of the quantum noise. The x-ray image is coarser, appears noisy and is no longer regarded as being acceptable. 
     All known solutions are thus compromises between the x-ray contrast or the x-ray image quality and the ergonomics of the overall arrangement. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a method and a corresponding x-ray apparatus for fluoroscopy of a patient who can be placed at a variable distance relative to an x-ray source wherein the aforementioned problems are avoided or made less severe. 
     This object is achieved in accordance with the invention by method for fluoroscopy of a patient who can be placed at a variable distance relative to an x-ray source, wherein the dose rate emitted by the x-ray source is reduced given a reduction of the distance and/or increased given an increase of the distance. 
     The dose rate that can be emitted by the x-ray source thus is no longer the same for all distances between the patient and the x-ray source, but rather is variable. If the patient is farther removed from the x-ray source, the x-ray source thus can be operated with a higher (for example the maximum or system-optimal) dose rate, which leads to high-quality x-ray imaging. Moving the patient closer to the x-ray source is possible until a minimum distance allowable for the currently-used dose rate is achieved. Upon reaching the minimum distance or beforehand, the dose rate emitted by the x-ray source is inventively reduced, and thus there is now a new maximum distance between the patient and the x-ray source. The emitted dose rate is thus always dimensioned such that the minimum distance resulting therefrom is smaller than or equal to the actual distance of the patient from the x-ray source. The patient may then be moved a bit closer to the x-ray source. Upon the patient being moved away from the x-ray source, the dose rate of the x-ray source can be increased again until the minimum distance resulting therefrom is smaller than or equal to the actual distance of the patient from the x-ray source. 
     This method therefore ensures that the patient is never exposed with a dose rate that is too high. The x-ray imaging can also be continued at distances between the patient and the x-ray source that lie below the safe distance for the maximum output capacity of the x-ray source, without exceeding the limit dose rate, i.e., the maximum allowed dose rate that strikes the patient. Although the image quality is then possibly limited, imaging still occurs without having to move the patient again away from the x-ray source and position that is ergonomic for the doctor can be achieved. 
     The variation of the dose rate can be implemented before or during a treatment on the patient. The doctor thus can select arbitrary distances that are optimal for him or her between the x-ray source and the patient. The possible degradation of the image quality of the x-ray images that is involved with a reduction of the dose rate is, under the circumstances, less important for the doctor than the achieved better ergonomics. By the inventive method, an optimal compromise of image quality and patient position relative to the x-ray source can always be found. 
     The variation of dose rate that can be emitted by the x-ray source can be undertaken in steps. For example, the dose rate arriving at the patient could be held constant when the dose rate that can be emitted by the x-ray source is varied approximately proportional to the distance between the patient and the x-ray source. The patient thus would always be exposed with the maximum allowable dose rate, which always leads to the best possible image quality of the x-ray images at a given patient distance from the x-ray source. 
     Alternatively, the dose rate can be varied in steps given specific interval limits of the distance between the patient and the x-ray source. For example, an interval limit exists that is associated with the maximal emittable power of the x-ray source. For intervals (spacings) above this interval limit, the patient is in no way irradiated with a dose rate that is too high, even when the x-ray source radiates with its system-dependent maximum dose rate. Moreover, further interval limits can be predetermined that are respectively associated other reduced output capacities of the x-ray source. The output power radiated by the source is then constant between two interval limits such that, for example, the remaining components of the imaging system can likewise be adjusted in steps to the respective source characteristic. Complicated, continuous updating tracking of the component properties is thus avoided. 
     The variation of the dose rate upon the distance (the current distance) approaching an interval limit can be terminated beforehand. A person implementing the method thus has discretion to leave a patient (for example) slightly above or slightly below a specific interval limit in order to avoid a reduction of the dose rate emitted by the x-ray source, so as to not impair the image quality of x-ray exposures. A possible decisive advantage in image quality can thus be achieved through a slight loss in ergonomic comfort. 
     The dose rate can be automatically varied. The implementing person thus does not have to attend to the adherence to safety intervals or the like since, for example, the dose rate is automatically adapted given free selection of the patient distance from the x-ray radiator or a suitable patient distance is set given prior selection of a specific image quality. For example, in a critical phase of an operation the doctor can thus automatically attain an x-ray image with improved image quality simply by raising the treatment table. 
     Various alternatives alone or in combination are possible in order to reduce the dose rate that the patient experiences from the x-ray source. The location of the dose reduction is unimportant. This can already ensue in the x-ray source itself as an upstream measure at the x-ray source, or proximal to the patient. Only the arrangement of the attenuation in the beam path between source and patient is important. 
     To reduce the dose rate, a filter can be brought between the x-ray source and the patient. This is brought into the beam path of the x-ray radiation between the x-ray source and the patient or removed again from this beam path dependent on the interval. In contrast to known permanently-installed filters, this filter is inventively only pivoted into the beam dependent on the interval, thus is present between x-ray source and patient only when the dose rate at the patient would otherwise be too high. Given increase of the distance between patient and x-ray source, the filter can be pivoted out of the beam again, so the full dose rate and full image quality of the x-ray imaging is produced again. A metal filter (for example a copper filter) can be used as such a filter. Such copper filters are already present in many x-ray sources, but must be manually pivoted by the doctor before the beginning of the treatment or the imaging. Interaction with the minimum safe distance is not provided. 
     The method can be executed particularly simply when the filter is brought between the x-ray source and the patient in a motorized manner. The entire method thus can be automated with no personnel for manual pivoting being necessary. 
     If the maximum dose rate of the x-ray source is predetermined by a power-limited current-voltage characteristic curve (the term “curve” encompassing a linear relationship), a characteristic curve with lower maximum dose rate thus can be used to reduce the dose rate. The output dose rate is likewise reduced by the use of different characteristic curve in the x-ray source or the associated activation, which likewise allows an approach of the patient towards the x-ray source without exceeding the maximum entrance dose rate. Such a decrease of the dose rate is achieved purely electronically, meaning that no moving parts must be pivoted into the x-ray beam path as above. The dose rate reduction by selection of a different characteristic curve can be used alternatively or additionally with pivoting the aforementioned radiation or dose filter. 
     As alternative characteristic curves to the characteristic curve of maximum dose rate, characteristic curves with limited beam currents and voltages varying mutually therewith can be used. The beam characteristic of the beam source thus is retained; only the output power is reduced. Subsequent image processing or other measures do not have to be adapted to a new characteristic curve of the x-ray source. Moreover, the control mode of the dose rate regulation is not significantly changed. 
     As an alternative or in addition to the aforementioned alternatives, the x-ray source can be deactivated to reduce the dose rate. The x-ray irradiation onto the patient can be completely stopped in the event that, given the current distance between patient and x-ray source), the dose rate emitted by the x-ray source cannot otherwise be decreased sufficiently so that the patient is irradiated without danger. X-ray imaging is, however, then no longer possible. 
     The dose rate can be varied (decreased or raised) in a multi-step adjustment procedure. For example, given approach of the patient toward the x-ray source the dose rate is hereby successively decreased in a number of adjustment steps such that the critical minimum interval of patient and x-ray source always drops with the approach of the patient toward the source, such that the patient never reaches this critical minimum distance. Given distancing of the patient from the source, the dose rate of the x-ray source can be successively increased again as soon as the patient has passed the safety interval of the next-highest possible dose rate. 
     The aforementioned object of the invention also is achieved by an x-ray apparatus for fluoroscopy of a patient who can be placed at a variable distance relative to an x-ray source, with a device for automatically varying the dose rate that can be emitted by the x-ray source, dependent on the distance. 
     The inventive x-ray apparatus explained above is operable in accordance with all of the embodiments of the inventive method. 
     The aforementioned filters that can be pivoted into the x-ray path between x-ray source and patient to reduce the dose rate (for example metal filters such as copper filters) thus can be used in the inventive apparatus as the device for variation of the dose rate dependent on the distance. 
     Alternatively or additionally, the device can be a device for selection of various characteristic curves respectively having various maximum dose rates, when the maximum dose rate of the x-ray source is predetermined by a power-limited current-voltage characteristic curve. This can ensue, for example, by a suitably-adapted activation of the x-ray source or x-ray tube, by a generator equipped with such a set of characteristic curves. 
     The device can also be a device for blocking activation or triggering of the x-ray source. If and when maximum distance is not adhered to, the x-ray source can then no longer be activated or an x-ray exposure shot can no longer be initiated. This can be realized, for example, by a simple mechanical switch. 
     Naturally, if the method is automated the x-ray apparatus must possess components such as a distance sensor for the distance or the interval of the patient from the x-ray source, a corresponding workflow controller (for example by hardware or software in the x-ray system) and further self-evident system components. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a urological under-table fluoroscopy workstation with patient and standing urologist. 
         FIG. 2  shows the workstation of  FIG. 1  with a seated urologist and lowered patient. 
         FIG. 3  shows the x-ray source of  FIG. 1  in an enlarged representation. 
         FIG. 4  shows a characteristic curve set associated with the x-ray source of  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a urological workstation  2  with a patient  4  and a doctor  6 . The workstation  2  has an x-ray C-arm unit  8  and a patient table  10  with a table plate  12  on which the patient  4  lies. The table plate  12  is supported on a base  14  such that it can be adjusted in terms of height in the direction of the double arrow  16 . The height adjustment ensues by actuation of a control (operating) lever  42  in a manner that is not necessary to explain in detail. 
     The x-ray C-arm unit  8  has a C-shaped supporting arm  18  at the ends of which an x-ray source  20  and an x-ray image intensifier  22  are mounted, respectively. In the x-ray source  20  (more precisely, the x-ray tube contained therein and not shown), x-ray radiation  26  (represented in  FIG. 1  by a beam cone) is emitted from the focus  24  along a central ray  28  toward the x-ray image intensifier  22 . The x-ray radiation  26  penetrates the patient  4  in order to generate an x-ray image  30  of the inside of the body in a known manner, this x-ray image  30  is displayed to the doctor  6  on a monitor  32 . Since the x-ray source  20  is arranged below the table plate  12 , in the workstation  2  is known as an under-table arrangement, but the invention can naturally be transferred to all other arrangements (for example above-table). 
     Since the patient  4  lies on the table plate  12 , the top side  34  of the table plate  12  is the lowest possible position at which the patient  4 , or his or her body parts (at the back  36 ) rests. This means that the smallest possible distance between the focus  24  and the body of the patient  4  is the distance between the focus  24  and the top side  34  of the table plate  12 . In  FIG. 1 , this distance is indicated as a double arrow representing the actual current distance  38   a  between the patient  4  and the focus  24 . Given a height adjustment of the patient table  10  by adjusting the height of the table plate  12 , this current distance  38   a  changes. For example, upon lowering of the table plate  12  in the direction of the x-ray source  20  the distance becomes smaller. 
     Dependent on its design or due to its activation, each x-ray source  20  has a maximum output dose rate P max  of x-ray radiation  26  which, in the extreme case, is completely incident on the body of the patient  4 . Given a designation or fixed output power of the x-ray source  20 , the dose rate (known as the entrance skin dose rate Pin) exposing the patient  4  at the side of the entrance of the radiation into the body of the patient  4  then depends on the distance  38   a  and P max  according to a quadratic distance rule. For example, legal prescribed limit values P limit  which cannot be exceeded exists for this entrance skin dose Pin reaching the patient. A minimum distance  40   a  of the patient  4  from the focus  24  thus can be calculated from the beam geometry of the x-ray radiation  26  and the distance  38   a . At least this minimum distance  40   a  must be adhered to so that the patient  4  is exposed only with the maximum allowable entrance skin dose P in ≦P limit . 
     As long as the top side  34  of the table plate  12  in  FIG. 1  is located in the interval range A, meaning that the current distance  38   a  is greater than the minimum distance  40   a  (which applies in  FIG. 1 ), even given a maximal output power P max  of the x-ray source  20  it is always ensured that the patient  4  is not exposed with an unacceptably-high entrance skin dose P in &gt;P limit  of the x-ray radiation  26 . The x-ray source is therefore operated with the output power P A =P max  in the entire interval range A. 
     In the entire interval range A, the doctor can therefore adjust (displace) the table plate  12  via the control lever  42  until a treatment position or working position that is ergonomically convenient for the doctor has been found in order to implement a task on the patient  4 . For example, in  FIG. 1  the doctor  6  (a urologist) implements auxiliary measures on the patient  4  under x-ray monitoring (i.e. given activated x-ray radiation  26 ) while standing. 
     After the implementation of the auxiliary measures, the treatment of the patient  4  begins without an interruption.  FIG. 2  therefore shows the workstation  2  of  FIG. 1  at a later point in time, namely when the doctor  6  implements an endo-urological procedure on the patient  4  under x-ray monitoring. In order to satisfy the ergonomic requirements of the doctor  6  during this treatment, the doctor is seated on a stool  44 . The position of the patient  4  shown in  FIG. 1  would be unsuitable for this. In order to move the patient  4  further into a working position that is ergonomically advantageous for the doctor  6 , the doctor  6  lowers the table plate  12  (using the control lever  42 ) together with the patient  4  in the direction of the x-ray source  20 , thus in the direction of the arrow  46 . 
     The top side  34  of the table plate  12  and thus the back  36  of the patient  4  now exhibits a changed (namely smaller) current distance  38   b  from the focus  24  relative to  FIG. 1 . If the x-ray source  20  were to again radiate with the maximal dose rate PA explained in  FIG. 1 , the back  36  of the patient  4  would be exposed with an unacceptably high entrance skin rate P in &gt;P limit  of x-ray radiation  26 , which represents an unallowable situation. For this reason, the output dose rate of the x-ray source  20  in  FIG. 2  is reduced to a value P C &lt;P A . A minimum distance  40   c  that is reduced relative to the minimum distance  40   a  thus results for the output power P C  of the x-ray source  20  selected in  FIG. 2 . Since the current distance  38   b  of the patient  4  from the focus  24  is greater than the minimum distance  40   c , the patient furthermore experiences only an entrance skin dose rate P in &lt;P limit  via the x-ray radiation  26  which is smaller than the maximum allowed entrance skin dose. 
     Furthermore, from  FIG. 2  it is apparent that if one starts from an initial situation according to  FIG. 1 , upon lowering in the direction of the arrow  46  the top side  34  of the table plate  12  passes through the interval ranges A to F. In each of these interval ranges, the x-ray source  20  is operated with a continuously smaller power P F &lt;P E &lt;P D &lt;P C &lt;P B &lt;P A . Toward the focus  24 , the corresponding interval ranges A-E are therefore respectively limited by the minimum distances  40   a - 40   e  calculated from the respective power. 
     Upon reaching each minimum distance  40   a - 40   e , the power of the x-ray source  20  is reduced successively and in steps such that the effective minimum distance is reduced to the next following minimum distance. Given lowering of the table plate  12  from the position shown in  FIG. 1  (there the minimum distance  40   a  applies), the output power P A  is thus initially reduced to the output power P B  upon reaching the minimum distance  40   a , and the minimum distance  40   b  applies. A further decrease of the table plate  12  through the range B ultimately leads to this reaching the distance limit  40   b , such that the output power of the x-ray source  20  is in turn decreased by a specific value to the output power PC, and the effective minimum distance is reduced to the distance  40   c.    
     This procedure ends when the minimum distance  40   e  is reached after passing through the interval range E. No further reduction of the output power of the x-ray source  20  below the minimum distance  40   e  is possible or reasonable since an effective x-ray imaging in the workstation  2  is then no longer possible. In the interval range F, the x-ray source  20  is therefore completely deactivated, or an activation of the x-ray source  20  for an exposure shot is prevented. 
     Upon raising the patient table  10  counter to the direction of the arrow  46 , the corresponding interval ranges F-A are traversed in the reverse order, and the output power of the x-ray source is successively increased by the aforementioned power steps. If the top side  34  of the table plate  12  reaches the minimum distance  40   a  and the table plate  12  henceforth moves into the interval range A, the x-ray source returns to its maximal output power PA. 
     For the step-by-step attenuation of the output power of the x-ray source, what is known as an escalation method is used whose execution is explained using  FIGS. 3 and 4 . 
       FIG. 3  shows a partial view of the x-ray source  20  of  FIG. 1  and  FIG. 2 , with the focus  24  of the x-ray tube (not shown) being indicated as the source of the x-ray radiation  26 . For global power limitation of the x-ray radiation  26  that can be emitted from the x-ray source  20 , primarily aluminum filters  48  are permanently mounted in the x-ray source  20  (for example screwed into the x-ray source  20 ) in the design or installation of the workstation  2 . The aluminum filters  48  limit the maximum internal x-ray power P intern &gt;P max  of the x-ray tube that can be emitted at the focus  24 , such that only a maximal power P max  can be emitted at the exit aperture  50  of the x-ray source  20 . 
     Copper filters  52   a - 52   c  are additionally provided in the x-ray source  20 . Each of the copper filters  52   a - 52   c  can be pivoted into and out of the beam path of the x-ray radiation  26  in the direction of the arrow  54 , independently of one another. For this purpose, they are supported on axles  55  (not shown in detail) that are driven by a motor  56 . In contrast to  FIG. 1 , in  FIG. 3  the first copper filter  52   a  is already pivoted into the beam path of the x-ray radiation  26 , causing the exit dose rate of the x-ray source  20  to be reduced from the value PA to the value PB. The configuration of pivoted-in copper filter  52   a  and pivoted-out copper filters  52   b  and  52   c  shown in  FIG. 3  therefore applies for the interval range B. Although the rough contrast in the x-ray image  30  hereby decreases, given a continued imaging further work can still be done below the previous minimum distance  40   a.    
     To achieve a further attenuation of the output power of the x-ray source  20  to the value PC, a further measure is used, which is explained using  FIG. 4 . The characteristic curve set of the x-ray source  20  or of the associated generator  60  (i.e. the generator  60  driving the x-ray tube) is shown in  FIG. 4 . The beam current is plotted on the abscissa and the generator voltage is plotted on the ordinate for two different characteristic curves  62   a  and  62   b  in  FIG. 4 . The maximum output dose rate P max  that can be emitted by the x-ray source  20  is predetermined by the power hyperbola  58  (taking into account the permanently-installed aluminum filters  48 ) that is approached by the characteristic curve  62   a.    
     To decrease the internal dose rate P intern  emitted at the focus  24  by the x-ray tube (not shown), the x-ray source  20  is switched to a second power hyperbola  58   b  by the system controller or the generator  60  in the interval range C, D and E (as, for example, in  FIG. 2 ). It is thus operated with a different characteristic curve  62   b  that is characterized by a maximum output power P intern  of the x-ray tube that is limited relative to the power hyperbola  58   a . An exposure correction (not shown) in the x-ray system compensates (within certain limits) the lower receiver dose; the image  30  behaves noisier, but it can still be used. 
     The dose rate generated at the focus  24  and already reduced according to the characteristic curve  58   b  is thus reduced to the maximum value P C  at the exit aperture  50  by the copper filter  52   a  that is pivoted-in in the interval ranges B, C, D and E. 
     For further reduction of the output power of the x-ray source  26 , the second copper filter  52   b  is additionally pivoted into the x-ray radiation  26  in the interval range D and the third copper filter  52   c  is additionally pivoted into the x-ray radiation  26  in the interval range E. As already noted, the table plate  12  can be moved up to the minimum distance  40   e  relative to the focus  24  with such reduced output power PE given pivoted-in copper filters  52   a - 52   c  and reduced characteristic curve  62   b.    
     Upon the minimum distance  40   e  not being satisfied, the generator  60  deactivates the x-ray tube so that no further x-ray radiation  26  is generated at the focus  4 . The output power PF is thus zero. The generator  60  acts as an activation block of the x-ray source  20 , so to speak. 
     The characteristic curves  62   a  and  62   b  oriented on the power hyperbola  58   a  and  58   b  are correlated such that the radiation characteristic or the operating behavior of the x-ray source  20  does not change when the output power is reduced according to power hyperbola  58   b , which leads to the x-ray behavior of the workstation  2  being identical up to the limit power. Only the image quality changes given each transition between two interval ranges. 
     Both the pivoting-in and pivoting-out of the copper filters  52   a - 52   c  and the switch between the characteristic curves  62   a  and  62   b  ensue wholly automatically depending on the determined distance  38   a ,  38   b , such that the doctor  6  merely needs to concentrate on the lowering or raising of the table plate  12  with the control lever  42 . The doctor  6  is informed on the monitor  32  of the current table position or the attainment of or the distance from various minimum distances  40   a - 40   e . For example, by raising the table  12  by a few cm the doctor  6  can decide to have available x-ray radiation  26  of a higher dose rate when a limit between two interval ranges is exceeded. Given a minimal loss of comfort, this means that an x-ray image  30  of improved image quality is acquired. 
     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 hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.