Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent application No. 11-243856, filed Aug. 30, 1999, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to an X-ray computed tomography apparatus in which an X-ray tube is cooled by coolant, for example oil or cooling water. This will be explained using the most popular oil as an example. 
     FIG. 1 shows a structure of a rotation section inside a gantry of a conventional X-ray computed tomography apparatus. A substantially annular rotor  77  is rotatably supported by a fixing section (not shown). This rotor  77  has X-ray tube unit  71  and an X-ray detector  76  mounted thereon. This X-ray tube utilizes a braking X-ray that is accelerated by a high voltage applied between a cathode and an anode and generated by causing collision with the anode at a very high speed. As well known, the conversion efficiency of the X-ray energy against electric energy is very low, and 99% or more of the electric energy is converted into a heat. When a focal face of the anode is excessively high, the anode material is fused, and cracks, resulting in shorter service life of the X-ray tube. In order to increase a heat capacity, an apparatus of such type housing an X-ray tube in a container together with insulation oil is mainly used at present. In addition, there is employed an apparatus of such type improving a cooling effect by forcibly circulating oil between the X-ray tube unit  71  and a radiator (core)  73  of a radiator unit  72 . 
     Further, in helical scan which is significantly popular recently, it is required to general an X-ray within a comparatively long time and continuously. In addition, X-ray strength per a unit time is likely to increase in order to suppress lower sensitivity due to a higher rotation speed. In order to process the thus increased heat rate, it is required to provide a fan  74  for forcibly cooling oil. The ventilation capability of this fan  74  is very highly designed based on the maximum heat rate of the X-ray tube simulated under a severe scan condition. 
     Thus, an excessive cooling state can occur under a normal scan condition. This excessive cooling provides an environment in which arcing is likely to occur with thermal electrons inside of the X-ray tube. 
     In addition, the fan  74  having its very high cooling capability generates a very large operating noise. This operating noise not only causes discomfort to a patient and an operator, but also interferes voice communication between the patient and the operator. 
     BRIEF SUMMARY OF THE INVENTION 
     An object of the present invention is to reduce noise in an X-ray computed tomography apparatus, while ensuring the cooling effect of the X-ray tube. 
     According to the present invention, this apparatus comprises: a rotor rotated in a predetermined direction; an X-ray tube unit mounted on the rotor; an X-ray detector opposed to the X-ray tube unit to detect X-rays transmitted through a subject; a radiator mounted on the rotor; a circulating system configured to circulate and a fluid between the X-ray tube unit and the radiator. The radiator is disposed in a direction in which the radiator is subjected to the air moved by the rotation of the rotor at its front face. 
     According to the present invention, when the rotor is rotated, the radiator is subjected to the resultant air at its front face, whereby the fluid can be efficiently cooled. In addition, an air cooling fan is eliminated or the operation frequency of the air cooling fan can be reduced, and thus, noise can be significantly reduced. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention. 
     FIG. 1 is a structural view showing a rotation section inside of a gantry of a conventional X-ray computed tomography apparatus; 
     FIG. 2A is an external view showing a gantry of an X-ray computed tomography apparatus according to a first embodiment of the present invention; 
     FIG. 2B is a sectional view taken along the line B—B of FIG. 2A; 
     FIG. 3 is a structural view showing a rotation section inside of the gantry shown in FIG. 2A; 
     FIG. 4 is a block diagram showing a ventilation control system in the first embodiment; 
     FIG. 5 is a view showing a control operation (open and close operation of an exhaust port and activation/deactivation of a ventilation fan) using a ventilation controller shown in FIG. 4; 
     FIG. 6 is a structural view showing a rotation section inside a gantry of the X-ray computed tomography apparatus according to a second embodiment of the present invention; 
     FIG. 7 is a block diagram depicting a radiator unit controlling system in the second embodiment; 
     FIG. 8 is a view showing the steps of controlling the radiator unit controller shown in FIG. 7; 
     FIG. 9 is a view showing a control operation of the radiator unit controller (opening and closing of an radiator air exit and activation/deactivation of the fan) shown in FIG. 7 based on an output of the X-ray tube temperature sensor shown in FIG. 7; 
     FIG. 10 is a view showing a control operation of the radiator unit controller (opening and closing of an radiator air exit and activation/deactivation of the fan) shown in FIG. 7 based on an output of the oil temperature sensor shown in FIG. 7; 
     FIG. 11 is a view showing the steps of controlling the radiator unit controller based on both of the output of the X-ray tube temperature sensor shown in FIG.  7  and the output of the oil temperature sensor. 
     FIG. 12 is a view showing a control operation of the radiator unit controller (opening and closing of the radiator air exit and activation and deactivation of the fan) shown in FIG. 7 based on both of the output of the X-ray tube temperature sensor and the output of the oil temperature sensor shown in FIG. 7; 
     FIG. 13 is a structural view showing a rotation section inside of a gantry of an X-ray computed tomography apparatus according to a third embodiment of the present invention; 
     FIG. 14 is a view showing the steps of controlling a radiator unit controller according to the third embodiment; and 
     FIG. 15 is a view showing a control operation of the radiator unit controller according to the third embodiment (opening and closing of the radiator air exit, activation/deactivation of the fan, and activation/deactivation of a cooler). 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, the present invention will be described in detail with reference to the accompanying drawings by way of preferred embodiments. Computed tomography apparatuses are classified into various types of apparatuses such as a rotate/rotate-type apparatus for integrally rotating an X-ray tube and X-ray detector around a subject and a stationary/rotate-type apparatus for rotating only an X-ray tube around a subject while a large number of detection elements are arranged in a ring. The present invention is applicable to any type and will be explained using the most popular rotate/rate-type apparatus as an example. 
     (First Embodiment) 
     FIG. 2A shows an appearance of a gantry of an X-ray computed tomography apparatus according to a first embodiment. FIG. 2B is a sectional view taken along the line B—B shown in FIG. 2A. A gantry  1  houses a number of parts in a cabinet  2 . A rotor  17  is rotatably supported by a fixing frame  18 . A direct drive motor, for example, is employed in order to rotate this rotor  17  at a high speed. An X-ray tube unit  11  for generating X-rays and an X-ray detector  16  for converting the X-rays transmitted through the subject examined into an electric signal are mounted on the rotor  17 . The X-ray tube unit  11  is such type housing the X-ray tube together with coolant in the X-ray tube container. This invention will be explained using the most popular insulation oil as an example of the coolant. 
     In order to prevent the heat generated by the X-ray tube unit  11  from being accumulated inside of the cabinet  2 , an exhaust port  3  is provided at the upper part of the cabinet  2 , and a air intake port  7  is provided at the lower part of the cabinet  2 . An electrically driven opening and closing mechanism  4  is mounted on this exhaust port  3  so that the exhaust port  3  can be opened and closed as required. When the exhaust port  3  is opened, the inside warmed up air is discharged from the exhaust port  3  to the outside. Instead, a new air is entered from the air intake port  7 . In order to improve this ventilation efficiency, a ventilation fan unit  5  is mounted inside of the opening and closing mechanism  4 . As will be described later in detail, activation of this fan unit  5  and opening and closing of the exhaust port  3  are controlled based on activation/deactivation of the rotor detected by a rotary encoder  10  mounted on a fixing frame  18 . 
     FIG. 3 is a front view showing the rotor  17  shown in FIG.  2 B. At the rotor  17 , the radiator unit  12  is mounted in addition to the X-ray tube unit  11  and the X-ray detector  16 . This radiator unit  12  is disposed in the vicinity of the X-ray tube unit  11  and at a position which is more frontal than the X-ray tube unit  11  in the rotational direction of the rotor  17 . A cabinet  9  of the radiator unit  12  is cylindrical, and a substantially flat shaped radiator (core)  13  provided with a heat radiating fin is engaged with an opening which is frontal thereof. An oil hose  15  is coupled between this radiator  13  and the X-ray tube unit  11 , and the oil is circulated between the radiator and the X-ray tube unit  11  by means of a circulation pump  8 . 
     The above radiator  13  is disposed so as to be substantially parallel to a tangent line at a position of the radiator  13  in a circle whose center is a rotary shaft of the rotor  17 . By disposing the radiator  13  in such orientation, the air moved by rotation of the rotor  17  is subjected to the radiator  13  at its front, and the oil can be efficiently cooled. Therefore, an air cooling fan is eliminated or the operating frequency of the air cooling fan can be reduced, and thus, noise can be significantly reduced. 
     A radiator air exit  18  for exhausting the warm air passing through the radiator  13  is opened at the rear of the cabinet  9  of this radiator unit  12 . This radiator air exit  18  is opened laterally rather than backwardly so as not to directly subject the warmed air through the radiator  13  to the X-ray tube unit  11 . An air filter for removing dust or the like generated from a slip ring or the like is engaged with this radiator air exit  18 . 
     FIG. 4 shows a control system for controlling opening and closing of an exhaust port  3  and activation/deactivation of a ventilation fan unit  5 . A ventilation controller  19  controls opening and closing of the exhaust port  3  and activation/deactivation of the ventilation fan unit  5 . This controlling is performed based on the cabinet inside temperature detected by the temperature sensor  6  as described above; and activation/deactivation of the rotor  17  detected by the rotary encoder  10 . 
     As shown in FIG. 5, the rotor  17  is rotated intermittently together with scan execution and stoppage. In addition, the temperature inside of the cabinet of the gantry fluctuates due to a variety of factors such as X-ray exposure frequency. The ventilation controller  19  supervises the temperature inside of the cabinet based on the output of the temperature sensor  6 . When the cabinet inside temperature exceeds a predetermined threshold value TH, the exhaust port  3  is opened. In this manner, the air inside of the cabinet is ventilated, and the temperature inside of the cabinet is lowered. On the other hand, when the cabinet inside temperature is lowered not more than a predetermined threshold TH, the exhaust port  3  is closed. In this manner, when the ventilation of the air inside of the cabinet is stopped, the lowering of the temperature inside of the cabinet is suppressed. Through such opening and closing control, the fluctuation of the temperature inside of the cabinet can be suppressed within a comparatively narrower range around the threshold value TH. In general, a semiconductor device such as photo diode of the X-ray detector  16  and electric circuit of a data acquisition unit (DAS) is sensitive to a temperature change. Functional degradation may occur if a temperature is too high or too low. As in the present embodiment, the internal temperature is not only lowered, but also is prevented from being excessively lowered, whereby the semiconductor device can be preferably operated. 
     In addition, the ventilation controller  19  supervises the temperature inside of the cabinet based on an output of a temperature sensor  6 , and supervises activation/deactivation of the rotor  17  based on an output of a rotary encoder  10 . Based on this supervision result, the activation/deactivation of the ventilation fan unit  5  is switched. 
     Specifically, when the temperature inside of the cabinet exceeds a predetermined threshold value TH, and moreover, the rotor  17  is deactivated, the ventilation fan unit  5  is operated. Then, the air inside of the cabinet is forcibly ventilated. On the other hand, when the rotor  17  is operated, even if the temperature inside of the cabinet exceeds a predetermined threshold value TH, the ventilation fan unit is deactivated. In addition, even when the rotor  17  is deactivated, when the temperature inside of the cabinet is lowered than the predetermined threshold value TH, the ventilation fan unit  5  is deactivated. 
     Namely, when the rotor  17  is rotated, or, when a scan (X-ray radiation and acquisition of projection data) is executed, the ventilation fan unit  5  is always deactivated. Forcible ventilation is performed only when the rotor  17  is deactivated or X-ray radiation is stopped, and moreover, the temperature inside of the cabinet exceeds the predetermined threshold value TH. Therefore, noise during scanning can be reduced to the minimum. 
     (Second Embodiment) 
     Now, a second embodiment of the present invention will be described here. Hereinafter, the points different from the first embodiment will be primarily described. 
     FIG. 6 is a frontal view showing a rotor  27  inside of a gantry of an X-ray computed tomography apparatus according to the second embodiment. A radiator unit  22  according to the present embodiment is equipped with a fan unit  24 . When this fan unit  22  operates, the quantity of air passing through the radiator  13  increases. The oil cooling effect is thus improved. 
     In addition, a opening and closing mechanism  29  is provided at the radiator unit  22  so as to enable a radiator air exit  28  to be opened/closed by being electrically driven. By means of the opening and closing mechanism  29 , when the radiator air exit  28  is opened, air passes through the radiator  13  together with rotation of the rotor  17 . When the radiator air exit  28  is closed by means of the opening and closing mechanism  29 , even if the rotor  17  is rotated, the air hardly passes through the radiator  13 . Thus, the oil cooling efficiency is reduced. Moreover, an opening and closing mechanism  30  may be provided at an air entrance or exit of the fan unit  24 . When the mechanisms  29 ,  30  close the air exits, the reduction of the cooling efficiency is facilitated. 
     The fan unit  24  is activated/deactivated, and the radiator air exit  28  is opened or closed by means of the opening and closing mechanism  29 , whereby the quantity of air passing through the radiator  13  can be accurately controlled. 
     The activation/deactivation of the fan unit  24  and the opening and closing of the radiator air exit  28  by means of the opening and closing mechanism  29  are controlled based on at least one of an output of a temperature sensor  31  mounted to the outer surface or the like of the X-ray tube in order to directly detect the temperature of the X-ray tube and an output of the temperature sensor  32  mounted to a hose  15  that circulates oil from the radiator  13  to the X-ray tube unit  11  in order to detect the temperature of the oil immediately after cooling, for example. 
     The temperature sensor  31  has characteristics sensitive to a temperature change in the X-ray tube because it detects the X-ray tube temperature. On the other hand, the temperature sensor  32  is comparatively sensitive to a temperature change in the X-ray tube because it detects a temperature of the circulation oil, but has characteristics sensitive to the cooling effect upon the radiator  13 . 
     FIG. 7 shows a control system of the radiator unit  22 . The radiator controller  33  is provided at one section of the control unit for mainly controlling X-ray generation, the control unit being mounted to a rotor  17 , for example. An output of the temperature sensor  31  and an output of the temperature sensor  32  are acquired by a radiator controller  33 . The radiator controller  33  comprises three types of control modes for activation/deactivation of the fan unit  24  and opening and closing of the radiator air exit  28  by means of the opening and closing mechanism  29 . The operator can select control mode arbitrarily. 
     FIG. 8 shows the steps of controlling first mode using the radiator controller  33 . FIG. 9 shows a change in activation/deactivation of the fan unit  24  relative to a change in X-ray temperature (detection temperature of the sensor  31 ) and a change in opening and closing of the radiator air exit  28 . In general, the upper limit value of the X-ray tube temperature is specified as an interlock level. When the X-ray tube temperature exceeds the interlock level, the supply of power (tube voltage or filament current) to the X-ray tube unit  11  is stopped urgently in order to urgently stop the X-ray generation. An upper threshold value TH upper ( 1 ) is set at a temperature lower than this interlock level. In addition, a lower threshold value TH lower ( 1 ) is set at a temperature slightly higher than a temperature at which the arcing in the X-ray tube is comparatively higher in frequency. 
     In the radiator controller  33 , the X-ray tube temperature detected by the sensor  31  is compared with the lower threshold value TH lower ( 1 ) (S 1 ), and the X-ray tube temperature detected by the sensor  31  is compared with the upper threshold value TH upper ( 1 ) (S 4 ). 
     When the X-ray tube temperature is equal to or smaller than the lower threshold value TH lower ( 1 ), the radiator air exit  28  is closed in order to prevent excessive cooling (S 3 ). On the other hand, when the X-ray tube temperature exceeds the lower temperature TH lower ( 1 ), the radiator air exit  28  is opened in order to improve the cooling effect of the radiator unit  22  (S 2 ). 
     In addition, when the X-ray tube temperature exceeds the upper threshold value TH upper ( 1 ), a fan unit  24  is operated in order to improve the cooling capacity. On the other hand, when the X-ray tube temperature is equal to or smaller than the upper threshold value TH upper ( 1 ), the fan unit  24  is deactivated in order to prevent excessive cooling (S 6 ). Such controlling is continued until radiography has been completed (S 7 ). 
     Thus, with respect to a temperature rise, the radiator air exit  28  is first opened. Even in the case where such temperature rise cannot be stopped, the fan unit  24  is operated. In addition, with respect to a temperature fall, the fan unit  24  is deactivated. Even in the case where such temperature fall cannot be stopped, the radiator air exit  28  is closed. 
     When rotation of the rotor  17  is stopped, and when the X-ray tube temperature exceeds the upper threshold value TH upper ( 1 ), the radiator air exit  28  is closed in order to obtain the cooling effect, and the fan unit  24  is operated. 
     In FIG. 10, in a second mode of the radiator controller  33 , there is shown a change in activation/deactivation of the fan unit  24  relevant to a change in oil temperature (temperature detected by the sensor  32 ) and a change in opening and closing of the radiator air exit  28 . The upper threshold value TH upper ( 2 ) relevant to the oil temperature may be set at a temperature lower than the upper threshold value ( 1 ) relevant to the X-ray tube temperature used in a first mode. Similarly, the lower threshold value TH lower ( 2 ) relevant to the oil temperature is set at a temperature higher than the lower threshold value TH lower ( 1 ) relevant to the X-ray tube temperature used in the first mode. 
     In the radiator controller  33 , as in the first mode, the oil temperature detected by the sensor  32  is compared with the lower threshold value TH lower ( 2 ), and the oil temperature detected by the sensor  32  is compared with the upper threshold value TH upper ( 2 ). When the oil temperature is equal to or smaller than the lower threshold value TH lower ( 2 ), the radiator air exit  28  is closed in order to prevent excessive cooling. On the other hand, when the oil temperature exceeds the lower threshold value TH lower ( 2 ), the radiator air exit  28  is opened. In addition, when the oil temperature exceeds the upper threshold value TH upper ( 2 ), the fan unit  24  is operated. On the other hand, when the oil temperature is equal to or smaller than the upper threshold value TH upper ( 2 ), the fan unit  24  is deactivated in order to excessive cooling. 
     When the rotor  17  is deactivated, and when the oil temperature exceeds the upper threshold value TH upper ( 2 ), the radiator air exit  28  is closed in order to obtain the cooling effect, and the fan unit  24  is operated. 
     In a third mode, controlling is performed by using both of two types of sensors  31  and  32 . FIG. 11 shows the steps of controlling the third mode using the radiator controller  33 . FIG. 12 shows a change in activation/deactivation of the fan unit  24  relevant to a change in X-ray tube temperature (temperature detected by the sensor  31 ) and a change in oil temperature (temperature detected by the sensor  32 ); and a change in opening and closing of the radiator air exit. In the radiator controller  33 , the X-ray tube temperature detected by the sensor  31  is compared with the lower threshold value lower ( 1 ), and the oil temperature detected by the sensor  32  is compared with the lower threshold value TH lower ( 2 ) (S 11 ). Here, when at least one of the X-ray tube temperature and the oil temperature exceeds each one of the lower threshold values TH lower ( 1 ) and TH lower ( 2 ), the radiator air exit  28  is opened in order to improve the cooling effect of the radiator unit  22  (S 22 ). On the other hand, when both of the X-ray tube temperature and the oil temperature are equal to or smaller than the respective lower threshold values TH lower ( 1 ) and TH lower ( 2 ), the radiator air exit  28  is closed in order to prevent excessive cooling (S 13 ). 
     In addition, in the radiator controller  33 , the X-ray tube temperature is compared with the upper threshold value TH upper ( 1 ), and the oil temperature is compared with the upper threshold value TH upper ( 2 ) (S 14 ). Here, when at least one of the X-ray tube temperature and the oil temperature exceeds each one of the threshold values TH upper ( 1 ) and TH upper ( 2 ), the fan unit  22  is operated (Si 5 ). On the other hand, when both of the X-ray tube temperature and the oil temperature are equal to or smaller than the respective upper threshold values TH upper ( 1 ) and TH upper ( 2 ), the fan unit  22  is deactivated in order to prevent excessive cooling (S 16 ). Such controlling is continued until radiography has been completed (S 17 ). When rotation of the rotor  17  is stopped, and when at least one of the X-ray tube temperature and the oil temperature exceeds the upper threshold value, the radiator air exit  28  is closed in order to obtain the cooling effect, and the fan unit  24  is operated. 
     According to the present embodiment, the temperature can be controlled with higher precision than that in the first embodiment. 
     In the foregoing description, although the activation/deactivation of the fan unit  24  is switched, the quantity of air from the fan unit  24  may be finely adjusted. Namely, a plurality of upper threshold values are set in stepwise manner. At a temperature rise, the air quantity of the fan unit  24  is increased in stepwise manner every time the quantity exceeds each of the upper threshold values. On the other hand, at a temperature fall, the air quantity of the fan unit  24  is reduced in stepwise manner every time the quantity is smaller than each of the upper threshold values. The air quantity may be adjusted by increasing or decreasing the number of fans to be driven, and its output may be changed by changing the power applied to the fan unit  24 . 
     (Third Embodiment) 
     Now, a third embodiment of the present invention will be described here. FIG. 13 is a front view showing a rotor  17  inside of a gantry of an X-ray computed tomography apparatus in the third embodiment. A radiator unit  52  according to the present embodiment is equipped with a cooler (cooling unit)  59  in addition to an arrangement of the radiator unit according to the second embodiment. The cooler  59  has a coolant vaporization—liquefying cycle system, and a vaporizer  60  is disposed in front of the radiator  13 . When the cooler  59  is operated, the air cooled at a temperature less than the internal temperature of the gantry is supplied to the radiator  13  by means of the vaporizer  60 . In this manner, the oil cooling effect is significantly improved. 
     FIG. 14 shows the control steps using the radiator controller. FIG. 15 shows an example when the activation/deactivation of the fan unit  24  is switched relevant to a change in x-ray tube temperature (temperature detected by the sensor  31 ); when the radiator air exit  28  is opened and closed; and the activation/deactivation of the cooler  59  is switched. Although the control operation of the present embodiment will be described here based on the first mode of the second embodiment, it is applicable to the second mode and the third mode. 
     The control operation of the present embodiment is different from that of the second embodiment as follows. That is, the upper limit value TH limit is set between an interlock level and an upper threshold value TH upper; the X-ray tube temperature is compared with the upper limit TH limit (S 21 ); when the X-ray tube temperature exceeds the upper limit TH limit, the cooler  59  is operated in order to cool the oil accurately (S 22 ); and when the X-ray tube temperature is lowered to be equal to or smaller than the upper limit value TH limit, the cooler  59  is deactivated in order to prevent excessive cooling (S 23 ). 
     In this embodiment, although only activation/deactivation of the cooler  59  is switched, the cooling capability of the cooler  59  may be finely adjusted. Namely, a plurality of upper limits TH limit is set in stepwise manner. At a temperature rise, the output of the cooler  59  is increased in stepwise manner every time the output exceeds each of the upper limits. On the other hand, at a temperature fall, the output of the cooler  59  is decreased in stepwise manner every time the output is lower than each of the upper limits. 
     According to the present embodiment, the X-ray tube temperature can be reduced by means of the cooler  59 . Thus, the apparatus is suitable to a case of scanning a large number of persons in which longer X-ray exposure time is required. In addition, in the case where the X-ray tube temperature rises abnormally, and an interlock is provided, the X-ray tube temperature is decreased rapidly, and a state in which X-ray exposure is possible can be restored within a short period of time. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.

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