Abstract:
A method for preventing overheating of an X-ray apparatus. The method includes controlling an X-ray tube and an X-ray detector which are opposed to each other with a subject between them so as to acquire projection data concerning the subject, estimating quantities of heat dissipated from the X-ray tube and a high-voltage generator that supplies power to the X-ray tube during the acquisition, and optimizing a control parameter, which is used to control the X-ray tube and the high-voltage generator, on the basis of estimates of the quantities of heat dissipated during the acquisition so as to prevent overheat of the X-ray tube and the high-voltage generator.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a thermal generator assembly including heat dissipators such as an X-ray tube and a high-voltage generator that supplies power to the X-ray tube, an X-ray imaging system, and an X-ray apparatus overheat preventing method.  
         [0002]     In recent years, X-ray imaging systems including an X-ray computed-tomography (CT) system have employed a high-power X-ray tube. Consequently, a large exposure is used to produce high-quality images or continuous X-irradiation is performed to acquire image information from a wider radiographic range.  
         [0003]     On the other hand, as more and more X-ray tubes generate higher power, a quantity of heat dissipated from an X-ray tube has increased. Along with the heat dissipation, the X-ray tube may be overheated and deteriorated. In order to prevent deterioration, before radiography is performed, a quantity of heat dissipated from the X-ray tube for the radiography is estimated. If the quantity of dissipated heat exceeds a permissible range, radiography is stopped or the conditions for radiography are reviewed (refer to, for example, Patent Document 1).  
         [0004]     [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2001-231775 (P.2 to P.3, FIG. 6 and FIG. 7).  
         [0005]     However, according to the foregoing background technology, a quantity of heat dissipated from a high-voltage generator that supplies power to an X-ray tube is not estimated. Therefore, the conditions for radiography are not reviewed based on the information on the quantity of dissipated heat. In other words, every time high-power radiography is repeated, the high-voltage generator is overheated to deteriorate or have the reliability thereof degraded.  
         [0006]     In particular, the power generated by an X-ray tube has drastically increased in recent years. A load the high-voltage generator incurs in supplying power to the X-ray tube has also increased. These increases become factors causing the X-ray high-voltage generator to overheat and to eventually deteriorate or have the reliability thereof degraded.  
         [0007]     Consequently, it is important how to realize a thermal generator assembly that optimizes quantities of heat dissipated from an X-ray tube and a high-voltage generator which supplies power to the X-ray tube, an X-ray imaging system, and an X-ray apparatus overheat preventing method.  
       SUMMARY OF THE INVENTION  
       [0008]     Therefore, an object of the present invention is to provide a thermal generator assembly that optimizes quantities of heat dissipated from an X-ray tube and a high-voltage generator which supplies power to the X-ray tube, an X-ray imaging system, and an X-ray apparatus overheat preventing method.  
         [0009]     In order to solve the above problem and accomplish the object, according to the first aspect of the present invention, there is provided a thermal generator assembly comprising: a plurality of heat dissipators that dissipates heat; a voltage generator that supplies power to the heat dissipators; estimating means for estimating quantities of heat dissipated from the heat dissipators and from the voltage generator; and a control processing unit for performing optimization on the basis of estimates of the quantities of dissipated heat so as to prevent overheat of the heat dissipators and the voltage generator.  
         [0010]     According to the first aspect of the present invention, the plurality of heat dissipators dissipates heat, and the voltage generator supplies power to the heat dissipators. The estimating means estimates the quantities of heat dissipated from the heat dissipators and from the voltage generator. Based on the estimates of the quantities of dissipated heat, the control processing unit performs optimization so as to prevent overheat of the heat dissipators and the voltage generator. Even if one of the heat dissipators and the voltage generator overheats, the quantities of dissipated heat are estimated, and overheat is prevented based on the estimates. Eventually, deterioration of the heat dissipators and voltage generator is prevented, and highly reliably operation is ensured.  
         [0011]     Moreover, according to the second aspect of the present invention, there is provided a thermal generator assembly in which when the estimates exceed permissible ranges of values of the overheat, the control processing unit optimizes a control parameter, which is used to control the power, so that the estimates of the quantities of heat dissipated from the heat dissipators and voltage generator will fall within the permissible ranges if the estimates exceed the permissible range of the overheat.  
         [0012]     According to the second aspect of the present invention, even if one of the heat dissipators and voltage generator overheats, since the quantities of dissipated heat are estimated, the control parameter is optimized in advance. Consequently, overheat is prevented.  
         [0013]     According to the third aspect of the present invention, there is provided an X-ray imaging system comprising: an X-ray tube that generates an X-ray beam; a high-voltage generator that supplies power, which is needed to generate the X-ray beam, to the X-ray tube; an X-ray detector that detects the X-ray beam; a data acquisition unit that controls the X-ray tube and X-ray detector which are opposed to each other with a subject between them so as to acquire projection data concerning the subject; estimating means for estimating quantities of heat dissipated from the X-ray tube and the high-voltage generator during the acquisition; and a control processing unit that optimizes a control parameter, which is used to control the X-ray tube and the high-voltage generator, on the basis of estimates of the quantities of heat dissipated during the acquisition so as to prevent overheat of the X-ray tube and the high-voltage generator.  
         [0014]     According to the third aspect of the present invention, the X-ray tube generates an X-ray beam, and the high-voltage generator supplies power, which is needed to generate the X-ray beam, to the X-ray tube. The X-ray detector detects the X-ray beam. The data acquisition unit acquires projection data concerning a subject from the X-ray tube and X-ray detector that are opposed to each other with the subject between them. The estimating means estimate the quantities of heat dissipated from the X-ray tube and high-voltage generator during acquisition. The control processing unit optimizes a control parameter, which is used to control the X-ray tube and high-voltage generator, on the basis of the estimates of the quantities of heat dissipated during acquisition so as to prevent overheat of the X-ray tube and high-voltage generator. Consequently, even if one of the X-ray tube and high-voltage generator overheats, since the quantities of dissipated heat are estimated, the control parameter is optimized in advance in order to prevent overheat. Eventually, deterioration of the X-ray tube and high-voltage generator is prevented, and highly reliable radiography is ensured.  
         [0015]     Moreover, an X-ray imaging system in accordance with the fourth aspect of the present invention is an X-ray CT system.  
         [0016]     According to the fourth aspect of the present invention, tomographic images are produced through image reconstruction performed based on projection data.  
         [0017]     An X-ray imaging system in accordance with the fifth aspect of the present invention uses the control processing unit to disable acquisition when the estimates exceed the permissible ranges of values of the overheat.  
         [0018]     According to the fifth aspect of the present invention, when the estimates exceed the permissible ranges, data acquisition is not performed in order to prevent deterioration or breakdown of the X-ray tube and high-voltage generator.  
         [0019]     An X-ray imaging system in accordance with the sixth aspect of the present invention uses the control processing unit to perform optimization at a step preceding a step of acquisition when the quantities of dissipated heat exceed the permissible ranges of values of the overheat.  
         [0020]     According to the sixth aspect of the present invention, an optimized control parameter is obtained prior to acquisition.  
         [0021]     In an X-ray imaging system in accordance with the seventh aspect of the present invention, when the estimates are expressed with functions of the control parameter, inverse functions of the functions or binary search is used in the optimization to calculate a control parameter that causes the estimates to agree with upper limits of the permissible ranges.  
         [0022]     According to the seventh aspect, the optimal value of the control parameter can be calculated quickly and easily.  
         [0023]     In an X-ray imaging system in accordance with the eighth aspect of the present invention, the control parameter is at least one of a tube current and a tube voltage that are supplied from the high-voltage generator to the X-ray tube.  
         [0024]     According to the eighth aspect of the present invention, the quantity of heat dissipated from the X-ray tube is controlled with an increase or decrease in a tube current or a tube voltage.  
         [0025]     In an X-ray imaging system in accordance with the ninth aspect of the present invention, the control parameter is a cooling time during which the tube current that is supplied intermittently does not flow.  
         [0026]     According to the ninth aspect of the present invention, the quantities of heat dissipated from the X-ray tube and high-voltage generator are controlled with the length of the cooling time.  
         [0027]     In an X-ray imaging system in accordance with the tenth aspect of the present invention, the control parameter is a scan time elapsing from a start of the acquisition to an end thereof.  
         [0028]     According to the tenth aspect of the present invention, the quantities of heat dissipated from the X-ray tube and high-voltage generator are controlled with the length of the scan time.  
         [0029]     An X-ray imaging system in accordance with the eleventh aspect of the present invention further comprises display means on which information related to the acquisition is displayed.  
         [0030]     According to the eleventh aspect of the present invention, the display means enable an operator to discern acquisition-related information.  
         [0031]     In an X-ray imaging system in accordance with the twelfth aspect, when the acquisition is disabled, information that acquisition is disabled is displayed on the display means.  
         [0032]     According to the twelfth aspect of the present invention, an operator can discern the acquisition-disabled state of the X-ray imaging system.  
         [0033]     In an X-ray imaging system in accordance with the thirteenth aspect, a value of the optimized control parameter is displayed on the display means.  
         [0034]     According to the thirteenth aspect of the present invention, an operator checks the validity of the optimized parameter.  
         [0035]     An X-ray imaging system in accordance with the fourteenth aspect further comprises operating means for use in entering the acquisition-related information.  
         [0036]     According to the fourteenth aspect, the operating means are used to enter acquisition-related information. An operator can determine various settings.  
         [0037]     In an X-ray imaging system in accordance with the fifteenth aspect of the present invention, the operating means comprise selecting means that are used to select a control parameter for the optimization.  
         [0038]     According to the fifteenth aspect of the present invention, the selecting means included in the operating means are used to select a control parameter for optimization. An operator&#39;s most preferable control parameter can be used for optimization.  
         [0039]     In an X-ray imaging system in accordance with the sixteenth aspect of the present invention, the estimating means estimate the quantity of heat dissipated from the data acquisition unit.  
         [0040]     According to the sixteenth aspect of the present invention, the quantity of heat dissipated from the data acquisition unit is recognized in advance.  
         [0041]     In an X-ray imaging system in accordance with the seventeenth aspect of the present invention, the control processing unit performs optimization on the basis of the estimate of the quantity of dissipated heat so as to prevent overheat of the data acquisition unit.  
         [0042]     According to the seventeenth aspect of the present invention, the quantity of heat dissipated from the data acquisition unit is determined so that overheat will not occur.  
         [0043]     In an X-ray imaging system in accordance with the eighteenth aspect of the present invention, the estimating means and control processing unit adopt a temperature as a physical quantity indicating the quantity of dissipated heat.  
         [0044]     According to the eighteenth aspect of the present invention, a rise in a temperature caused by heat dissipation is used as an index to verify overheat and perform optimization.  
         [0045]     An X-ray apparatus overheat preventing method in accordance with the nineteenth aspect of the present invention comprises the steps of: controlling an X-ray tube and an X-ray detector which are opposed to each other with a subject between them so as to acquire projection data concerning the subject; estimating quantities of heat dissipated from the X-ray tube and a high-voltage generator that supplies power to the X-ray tube during the acquisition; and optimizing a control parameter, which is used to control the X-ray tube and high-voltage generator, on the basis of estimates of the quantities of heat dissipated during the acquisition so as to prevent overheat of the X-ray tube and high-voltage generator.  
         [0046]     According to the nineteenth aspect of the present invention, even if either of the X-ray tube and high-voltage generator overheats, since the quantities of dissipated heat are estimated, the control parameter is optimized in advance in order to prevent overheat. Eventually, deterioration of the X-ray tube and high-voltage generator is prevented, and highly reliable radiography is ensured.  
         [0047]     As described above, according to the present invention, even if one of a heat dissipator such as an X-ray tube and a voltage generator such as a high-voltage generator overheats, since the quantities of heat dissipated from the heat dissipator and voltage generator are estimated in order to optimize a control parameter in advance, overheat of the heat dissipator and voltage generator is prevented. Eventually, deterioration of the X-ray tube and high-voltage generator is prevented, and highly reliable radiography is ensured.  
         [0048]     Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0049]      FIG. 1  is a block diagram showing the overall configuration of an X-ray imaging system.  
         [0050]      FIG. 2  is a flowchart describing actions to be performed by a control processing unit included in an embodiment.  
         [0051]      FIG. 3  is a flowchart describing actions to be performed by an optimizing means included in the present embodiment.  
         [0052]      FIG. 4  shows a pattern indicating actions to be performed according to the binary search in the present embodiment.  
         [0053]      FIG. 5  indicates a cooling time required for an X-ray tube. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0054]     Referring to the appended drawings, the best mode of an X-ray imaging system in accordance with the present invention will be described below.  
         [0055]     To begin with, a description will be made of the overall configuration of an X-ray CT system that is an example of the X-ray imaging system in accordance with an embodiment of the present invention.  FIG. 1  is a block diagram showing the X-ray CT system. As shown in  FIG. 1 , the X-ray CT system comprises a scanner gantry  2 , an operator console  6 , and a high-voltage generator  10 .  
         [0056]     The scanner gantry  2  includes an X-ray tube  20 . The X-ray tube  20  serves as a heat dissipator. X-rays that are not shown and radiated from the X-ray tube  20  are recomposed into, for example, a conical X-ray beam by a collimator, and then radiated to an X-ray detector  24 .  
         [0057]     The high-voltage generator  10  is a voltage generator that applies a high voltage to the X-ray tube  20 . Herein, the high-voltage generator  10  applies a voltage, which generally ranges from 120 kV to 140 kV and brings about 8 to 9 HU (heat unit), to the X-ray tube  20 .  
         [0058]     The X-ray detector  24  includes a plurality of X-ray detection elements arrayed two-dimensionally in a direction in which the conical X-ray beam spreads. In other words, the X-ray detector  24  is a multi-channel detector having the plurality of X-ray detection elements set in array.  
         [0059]     The X-ray detector  24  has an X-ray incidence surface curved like a cylindrical concave surface as a whole. The X-ray detector  24  is formed with a combination of, for example, scintillators and photodiodes. Alternatively, the X-ray detector  24  may comprise semiconductor X-ray detection elements that utilize cadmium telluride (CdTe) or ionization chamber type X-ray detection elements that utilize xenon gas. The X-ray tube  20 , collimator, and X-ray detector  24  constitute an X-irradiation/detection assembly.  
         [0060]     A data acquisition unit  26  is connected to the X-ray detector  24 . The data acquisition unit  26  acquires detection data from each of the X-ray detection elements constituting the X-ray detector  24 . An X-ray controller  28  controls X-irradiation from the X-ray tube  20 . Connection between the X-ray tube  20  and X-ray controller  28  and connection between the X-ray controller  28  and high-voltage generator  10  are not illustrated.  
         [0061]     The foregoing components starting with the X-ray tube and ending with the X-ray controller  28  are incorporated in a rotary unit  34  of the scanner gantry  2 . A subject or a phantom lies down on a cradle in a bore  29  formed in the center of the rotary unit  34 . The rotary unit  34  rotates while being controlled by a rotation controller  36 , and shoots X-rays from the X-ray tube  20 . The X-ray detector  24  detects X-rays transmitted by the subject or phantom as each view of projection data. The illustration of the connective relationship between the rotary unit  34  and rotation controller  36  will be omitted.  
         [0062]     The operator console  6  includes a control processing unit  60 . The control processing unit  60  is formed with, for example, a computer. A control interface  62  is connected to the control processing unit  60 . Furthermore, the scanner gantry  2  is connected to the control interface  62 . The control processing unit  60  controls the scanner gantry  2  via the control interface  62 .  
         [0063]     The data acquisition unit  26 , X-ray controller  28 , and rotation controller  36  incorporated in the scanner gantry  2  are controlled via the control interface  62 . The illustration of the connections of these components to the control interface  62  will be omitted.  
         [0064]     A display device  68  and an operating device  70  are connected to the control processing unit  60 . Tomographic images and other information provided by the control processing unit  60  are displayed on the display device  68 . An operator handles the operating device  70  so as to enter scan parameters, various directives, or any other information that is transferred to the control processing unit  60 . The operator uses the display device  68  and operating device  70  to interactively operate the X-ray CT system. Incidentally, the scanner gantry  2  and operator console  6  radiographs the subject or phantom so as to produce tomographic images.  
         [0065]     Herein, the control processing unit  60  produces control parameters, which are used to control the scanner gantry  2  and high-voltage generator  10 , from the scan parameters entered by the operator. The control parameters are transmitted to the respective components incorporated in the scanner gantry  2  via the control interface  62 , whereby radiography, that is, scanning is performed. The control processing unit  60  includes an estimating means that infers overheat of the X-ray tube  20  and high-voltage generator  10  from the produced control parameters, and an optimizing means that optimizes the control parameters.  
         [0066]     The control processing unit  60  is connected to a data acquisition buffer  64 . The data acquisition buffer  64  is connected to the data acquisition unit  26  incorporated in the scanner gantry  2 . Projection data acquired by the data acquisition unit  26  is transferred to the control processing unit  60 .  
         [0067]     The control processing unit  60  uses a transmitted X-ray signal, that is, projection data received via the data acquisition buffer  64  to reconstruct images. A storage device  66  is also connected to the control processing unit  60 . Projection data held in the data acquisition buffer  64 , reconstructed tomographic images, and programs that realize the features of the X-ray CT system are stored in the storage device  66 .  
         [0068]     Next, the actions to be performed in the control processing unit  60  will be described.  FIG. 2  is a flowchart describing the actions to be performed in a control processing unit included in the present invention. First, an operator determines scan parameters using the operating device  70  (step S 201 ). As the scan parameters, a scanned range, the number of times of slicing, a slice thickness, a scan mode, and a matrix size for image reconstruction are determined.  
         [0069]     Thereafter, the control processing unit  60  calculates control parameters on the basis of the determined scan parameters (step S 202 ). At this time, the control parameters based on which the scanner gantry is controlled, especially, a tube voltage, a tube current, a scan time, a tube cooling time, the number of times of irradiation, and other parameters are calculated.  
         [0070]     Thereafter, the control processing unit  60  estimates the temperatures T of the X-ray tube  20  and high-voltage generator  10  on the basis of the control parameters (step S 203  to step S 205 ). Herein, the temperature of, for example, the rotating anode of the X-ray tube  20  is estimated based on such control parameters as a tube voltage, a tube current, and an exposure time. The temperature is provided as a function expressed below: 
        T=f(tube current, tube voltage, scan time, etc.)        
 
         [0072]     At the same time, the temperature T′ of the high-voltage generator  10  that is the source of the tube voltage and tube current is estimated as a function g. 
        T′ =g(tube current, tube voltage, scan time, etc.)        
 
         [0074]     Incidentally, the function g of the temperature of the high-voltage generator  10  is different from the function f of the temperature of the X-ray tube  20 . Thus, not only heat dissipation from the X-ray tube  20  that has been inferred in the past but also heat dissipation from the high-voltage generator  10  are inferred.  
         [0075]     Thereafter, the control processing unit  60  compares the temperatures of the X-ray tube  20  and high-voltage generator  10 , which are estimated at step S 203  and step S 205 , with permissible temperatures that do not cause overheat (step S 204  and step S 206 ). The permissible temperatures are read into the control processing unit  60  in advance and regarded as properties inherent to the X-ray tube  20  and high-voltage generator  10  respectively. When the temperatures are exceeded, a fault or a breakdown occurs.  
         [0076]     Thereafter, the control processing unit  60  verifies whether the temperatures compared at step S 204  and S 206  are equal to or lower than the permissible temperatures (step S 207 ). If the both temperatures are equal to or lower than the permissible temperatures (in the affirmative at step S 207 ), control is passed to step S 212 , and scanning is performed.  
         [0077]     If the both temperatures are not equal to or lower than the permissible temperatures (in the negative at step S 207 ), one of the temperatures exceeds the permissible temperature. An indication that scanning is disabled is displayed on the display device  68  (step S 208 ). An operator then uses the optimizing means included in the control processing unit  60  to verify whether any of the control parameters should be optimized (step S 209 ). If none of the control parameters is optimized (in the negative at step S 209 ), control is passed to step S 201 . The scan parameters are redetermined.  
         [0078]     Moreover, if the control parameters are optimized (in the affirmative at step S 209 ), the control processing unit  60  uses the optimizing means to perform optimization (step S 210 ). During the optimization, the control parameter values are changed or set to the largest values that cause the temperatures of the X-ray tube and high-voltage generator  10  to be equal to or lower than the permissible temperatures. The results are displayed on the display device  68 . The optimization will be detailed later.  
         [0079]     Thereafter, the operator verifies whether the optimized control parameter values are valid (step S 211 ). If the parameter values are invalid (in the negative at step S 211 ), control is passed to step S 209 . It is verified whether optimization is resumed. If the control parameter values are valid, scanning is performed in order to acquire projection data (step S 212 ). This process is then terminated.  
         [0080]     The optimization at step S 210  will be described in conjunction with the flowchart of  FIG. 3 .  FIG. 3  is a flowchart describing actions to be performed during optimization. Incidentally, the optimization is based on the binary search. First, an operator selects an optimization parameter P, which is used for optimization, from among the control parameters using the operating device  70  (step S 301 ). As the optimization parameter P, for example, a tube current is selected. The maximum value of a range within which the optimization parameter P is variable shall be maxP, and the minimum value thereof shall be minP. The value maxP is assigned to a variable PH, and the value minP is assigned to a variable PL (step S 302 ). Herein, the domain of variables between the variables PH and PL is sequentially diminished while always containing an optimal value. Finally, the variables PH and PL approximate to the optimal value. When the tube current is adopted as the optimization parameter for optimization, the value maxP indicates the maximum tube current supplied from the high-voltage generator  10 , and the value minP indicates the minimum tube current supplied from the high-voltage generator  10 .  
         [0081]     Thereafter, the optimizing means assigns an intermediate value of the variables PH and PL, (PH+PH)/2, to a variable PM (step S 303 ). Using the intermediate value PM, the temperatures T of the X-ray tube  20  and high-voltage generator  10  are estimated as the functions f and g employed at steps S 203  and S 205  described in  FIG. 2  (step S 304 ).  
         [0082]     Thereafter, the optimizing means verifies whether both the estimated temperatures T fall below the permissible temperatures TO that are the upper limits of permissible ranges (step S 305 ). If the temperatures exceed the permissible temperatures (in the affirmative at step S 305 ), the variable PM is assigned as a new maximum value to the variable PH (step S 307 ). If the temperatures do not exceed the permissible temperatures (in the negative at step S 305 ), the variable PM is assigned as a new minimum value to the variable PL (step S 306 ).  
         [0083]     Thereafter, the optimizing means assigns PH−PL to a difference AP between the variables PM and PL (step S 308 ). The optimizing means then determines whether the difference AP exceeds a set value of a resolution R that is the smallest possible change (step S 309 ). If the tube current is adopted as the optimization parameter, the resolution R is determined with a minimum range of set values of the tube current supplied from the high-voltage generator  10  or an energy resolution of X-rays. If the difference AP exceeds the resolution R (in the affirmative at step S 309 ), control is passed to step S 303 . Processing from step S 303  to step S 308  is then performed. This processing is repeated until the difference AP becomes equal to or smaller than the resolution R.  
         [0084]      FIG. 4  shows a pattern indicating a process for calculating an optimal value by repeating the processing from step S 303  to step S 308 . Referring to  FIG. 4 , the process for calculating an optimal value for the optimization parameter P includes processes  1  to  5 . At the first time, initialization is performed, and the temperatures T estimated using the PM value are higher than the permissible temperatures T0. Therefore, process  2 , the PM value is used as a new PH value, and the same processing is performed. Every time the processing from step S 303  to step S 308  is repeated, the difference ΔP between the variable PM and variable PL is halved. The domain within which an optimal value is present is gradually narrowed.  
         [0085]     Referring back to  FIG. 3 , if the difference ΔP does not exceeds the set value of the resolution R (in the negative at step S 309 ), there is no meaning in repeating the processing from step S 303  to step S 308  so as to make the difference ΔP smaller. The optimizing means therefore adopts the variable PH or PL as the optimization parameter value P (step S 310 ). The optimization parameter value P is then displayed on the display device  68  (step S 311 ). Control is then passed to step  211  in  FIG. 2 .  
         [0086]     As mentioned above, according to the present embodiment, the temperatures of the X-ray tube  20  and high-voltage generator  10  to be attained during scanning are estimated. If the temperatures exceed the permissible temperatures, it means that the temperatures may cause overheat. In this case, an indication that scanning is disabled is displayed. Furthermore, when the optimizing means is selected, an optimization parameter that is a tube current or a tube voltage is optimized according to the binary search and set to a value that causes the temperatures to fall below the permissible temperatures. Therefore, the X-ray tube and high-voltage generator will not overheat but operate with the temperatures thereof retained below the permissible temperatures. Deterioration of the X-ray tube  20  or high-voltage generator  10  is prevented. Eventually, highly reliable scanning can be ensured.  
         [0087]     According to the present embodiment, the temperatures of the X-ray tube  20  and high-voltage generator  10  are controlled. Likewise, an accumulated quantity of heat or any other physical quantity relevant to heat dissipation may be adopted for control as well.  
         [0088]     According to the present embodiment, the tube current of the X-ray tube is optimized. Likewise, the tube voltage may be adopted as an optimization parameter. Furthermore, the cooling time required for the X-ray tube  20  may be adopted as the optimization parameter. The cooling time refers to a time during which no tube current flows as indicated in  FIG. 5 . As the flow of the tube current into the X-ray tube  20  is, as indicated in  FIG. 5 (A), enabled or disabled, the temperature of the X-ray tube  20  rises or drops as indicated in  FIG. 5 (B). When the cooling time is set to a long time, the X-ray tube  20  is cooled so that the temperature of the X-ray tube  20  will be retained at the permissible temperature or lower. The longer the cooling time is, the lower the temperature is. Therefore, the steps S 306  and S 307  described in the flowchart of  FIG. 3  are switched.  
         [0089]     According to the present embodiment, optimization is performed using the binary search. Alternatively, an optimization parameter value may be determined or directly calculated as an inverse function of the function f or g. Otherwise, a high-order search may be adopted for fast search.  
         [0090]     According to the present embodiment, the temperatures of the X-ray tube  20  and high-voltage generator  10  are estimated for optimization. Similarly, the temperature of a data acquisition system (DAS) including the data acquisition unit  26  that is a heat dissipator may be estimated for optimization.  
         [0091]     Many widely different embodiments of the invention may be configured without departing from the spirit and the scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.