Abstract:
A cooling oil circuit (D) circulates cooling oil over an x-ray tube absorbing its waste heat. A refrigeration circuit (E) then cools the cooling oil. A heat buffer ( 52 ) absorbing peak heat loads from the cooling fluid when the x-ray tube is generating x-rays. Valves ( 58, 60 ) regulate a relative amount of cooling oil entering the heat buffer to increase heat transfer efficiency. The heat buffer enables the system to handle peak heat loads with a smaller, more condensed refrigeration system, by absorbing heat during operation of the x-ray tube and releasing heat between operations.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the radiographic arts. It finds particular application in conjunction with computerized tomographic (CT) scanners and will be described with particular reference thereto. However, it is to be appreciated that the present invention will also be amenable to other diagnostic x-ray applications. 
     Generally, CT scanners have included a floor-mounted frame assembly which remains stationary during a scan and a rotatable frame assembly mounted therein. An x-ray tube is mounted to the rotatable frame assembly which rotates around a patient receiving examination region during the scan. Radiation from the x-ray tube traverses the patient receiving region and impinges upon an array of radiation detectors. Using the position of the x-ray tube during each sampling, a tomographic image of one or more slices through the patient is reconstructed. 
     The x-ray tube assembly includes a housing within which a rotating anode x-ray tube is mounted. High voltage and control leads pass through the housing to the tube. During x-ray generation, electrons are emitted from a heated filament in the cathode and accelerated to a focal spot area on the anode. Upon striking the anode, the focal spot is heated white hot to excite the emission of x-rays. Some portion of the electrons, or secondary electrons, strike the surrounding housing and are converted into undesirable waste heat. In fact, most of the energy applied to an x-ray tube is converted to heat. One of the persistent problems in CT scanners and other radiographic apparatus is effectively and efficiently dissipating the waste heat created while generating x-rays. 
     In order to remove the waste heat, a cooling oil is circulated between the housing and the x-ray tube. The oil is typically drawn from an output aperture located at one end of the housing, circulated through a heat exchanger on the rotating gantry and returned to an inlet aperture in the opposite end of the housing. The returned, cooled fluid flows axially through the housing toward the outlet aperture, absorbing heat from the x-ray tube. Transferring the heat removed by the heat exchanger from the rotating gantry is logistically difficult. The cooling of the x-ray tube is crucial to the life and quality of the tube. With the increasing demand of higher power CT x-ray tubes, the issue of cooling has become even more important and more difficult. 
     The power applied to an x-ray tube generally follows a designated duty cycle. As a result, the amount of the heat dissipation rate from the x-ray tube changes cyclically. To ensure sufficient cooling, an x-ray tube cooling system is generally designed based on the peak value of the heat dissipation received by the system. Thus, the volume of the cooling system may be unnecessarily large, but permitting the x-ray tube to become too hot during operation can irreversibly damage an expensive x-ray tube. 
     The present invention provides a new and improved cooling system for overcoming the above-reference drawbacks and others. 
     SUMMARY OF THE INVENTION 
     The present invention relates to an improved cooling system and method for effective and efficient removal of waste heat from a CT scanner. 
     In accordance with one aspect of the present invention, a diagnostic imaging system comprises an x-ray tube, an x-ray detector disposed across an imaging region from the x-ray tube, a cooling oil circuit which circulates cooling oil over the x-ray tube to remove heat form the x-ray tube, and a second cooling circuit which removes heat form the cooling oil circuit at a heat removal rate that is less than a heat generation rate of the x-ray tube. 
     In accordance with another aspect of the present invention, a cooling system for an x-ray tube of a diagnostic scanner is provided. The system comprises a cooling fluid that is in thermal contact with an x-ray tube and absorbs heat from the x-ray tube. The system also comprises a heat buffer which receives the cooling fluid after absorbing heat from the x-ray tube and a refrigeration system in thermal contact with the cooling fluid. The heat buffer contains a high heat capacity material in thermal contact with the cooling fluid passing therethrough. The refrigeration system removes heat from the cooling fluid before the cooling fluid returns to the x-ray tube. 
     In accordance with another aspect of the present invention, a radiographic cooling method is provided. The x-ray tube is intermittently operated to generate x-rays and heat. The heat generated by the x-ray tube is absorbed with a cooling fluid. A portion of the heat from the cooling fluid is absorbed in a heat buffer while the x-ray tube is generating x-rays and heat. The heated cooling fluid is cooled and the cooled cooling fluid is recirculated to the x-ray tube. 
     One advantage of the present invention resides in its ability to handle peak heat loads during the generation of x-rays, yet reduce the size of the heat retraction system needed to cool the cooling fluid. 
     Another advantage of the present invention is that it increases the efficiency of the system. 
     Another advantage of the present invention resides in its compactness, freeing valuable space on the rotating gantry. 
     Still other advantages and benefits of the invention will become apparent to those skilled in the art upon a reading and understanding of the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawing is only for purposes of illustrating a preferred embodiment and is not to be construed as limiting the invention. 
     FIG. 1 is a diagrammatic illustration of a CT scanner in accordance with the present invention; and 
     FIG. 2 is a cooling system schematic for the removal of heat from an x-ray tube of a CT scanner. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIG. 1, a CT scanner includes a floor mounted or stationary frame portion A whose position remains fixed during data collection. An x-ray tube assembly B is mounted on a rotating frame C rotatably mounted within the stationary frame portion A. The stationary frame portion A includes a cylinder  10  that defines a patient receiving examination region  12  therein. An array of radiation detectors  14  are disposed concentrically around the patient receiving region  12 . The stationary frame A with the rotating frame C can be canted or tipped to scan slices at selectable angles. 
     A control console  16  contains an image reconstructing processor  18  for reconstructing an image representation from output signals from the detector array  14 . A monitor  20  converts the reconstructed image representation into a human readable display. The console  16  also includes appropriate digital recording media for archiving image representations, performing image enhancements, and the like. Various control functions, such as initiating a scan, selecting among different types of scans, calibrating the system, and the like are also performed at the control console  16 . 
     The x-ray tube assembly B includes a housing  22  having an x-ray permeable window  24  directed toward the, patient receiving region  12 . A rotating anode x-ray tube is mounted in the housing  22 . High voltages, on the order of 150 kV and higher applied between the rotating anode and a cathode accelerate electrons from the cathode to the anode. The energy from the electrons striking the anode generates x-rays and a large amount of heat. 
     The x-rays pass through the x-ray permeable window  24  and across the patient receiving region  12 . Appropriate x-ray collimators focus the radiation into one or more planar beams which span the examination region  12 , as is conventional in the art. Other equipment associated with the x-ray tube B, such as a high voltage power supply  26 , are also mounted on the rotating frame C. The high voltage power supply  26  provides the necessary high voltages to the anode and the cathode. 
     With particular reference to FIG. 2, the undesirable heat generated by the x-ray tube B is removed by circulating a cooling fluid, such as oil, water, sulphur, hexafluoride and other liquids and gasses, through the housing  22  around the x-ray tube. More specifically, cooling fluid enters the housing  22  through an inlet aperture, absorbs heat from the x-ray tube, and the heated cooling fluid exits the housing  22  through an outlet aperture. A cooling system  50  is used to recirculate and continuously provide the cooling fluid at a desired temperature to the housing  22 . 
     The cooling system  50  comprises a cooling oil or fluid loop D for circulating cooling fluid at a desired temperature to the x-ray tube B and a refrigeration loop E for maintaining the cooling fluid of the cooling fluid loop D at the desired temperature. The cooling fluid loop D includes a heat buffer  52 , a precooler/superheater  54 , an evaporator  56 , heat buffer valve  58 , a bypass valve  60 , and an cooling fluid pump  62 . 
     From the outlet aperture on the x-ray tube assembly, the cooling fluid enters a outlet conduit  64  which splits into heat buffer conduit  66  and the bypass conduit  68 . The heat buffer conduit  66  fluidly communicates with the heat buffer  52  and has heat buffer valve  58  disposed therein. The bypass conduit  68  includes the bypass valve  60  disposed therein. Any fluid allowed to pass through the valves  58 ,  60  eventually flows into a merging conduit  70 . Thus, if both the valves  58 ,  60  are open, one stream of cooling fluid flows through the heat buffer valve  58  and the heat buffer  52  into merging conduit  70  and the other fluid stream flows through the bypass valve  60  into merging conduit  70 . 
     The precooler/superheater  54  is in fluid communication between the merging conduit  70  and the evaporator  56 . Specifically, the precooler/superheater  54  is located downstream of the merging conduit  70  and upstream of the evaporator  56 . The evaporator  56  is upstream of, and in fluid communication with, the pump  62  which fluidly communicates with the x-ray tube housing through the inlet aperture. 
     The heat buffer  52  includes a cavity containing a high heat capacity fluid such as water, liquid metal, or other suitable heat sink. A tubular passage through the cavity of the heat buffer  52  has a large surface area to allow the cooling fluid of the cooling fluid loop D to transfer heat readily to and from the heat buffer  52 . Preferably, parallel tubes include a plurality of fins disposed about their peripheral surfaces. Elongated tubes and other tortuous paths are also contemplated. The heat buffer  52  operates by allowing the high heat capacity fluid to absorb heat from the cooling fluid flowing through the tubes. The fins on the tubes enhance the amount of heat transferred from the cooling fluid. 
     Preferably, the high heat capacity fluid should only fill the cavity in the heat buffer  52  approximately three-fourths full. Maintaining the amount of high heat capacity fluid in the cavity at less than full capacity allows agitation action of the high heat capacity fluid as the rotating frame C rotates. Such agitation can further enhance the heat transfer between the cooling fluid and the high heat capacity fluid of the heat buffer  52 . 
     Heat is also removed from the cooling fluid of the cooling fluid loop D by the precooler/superheater  54  and the evaporator  56 . More specifically, the precooler/superheater  54  and the evaporator  56  allow heat transfer between the cooling fluid loop D and the refrigeration loop E. The refrigeration loop E operates in a conventional manner using a refrigerant, preferably a compressible gas, to remove the heat from the cooling fluid passing through the precooler/superheater  54  and the evaporator  56 . 
     The refrigeration loop E includes the precooler/superheater  54 , the evaporator  56  downstream of the precooler/superheater  54  and fluidly connected thereto, a compressor  72  for receiving the refrigerant discharge from the precooler/superheater  54  and fluidly connected thereto, a condenser  74  downstream of the compressor  72  and fluidly connected thereto, and an expansion valve  76  located between the condenser  74  and the evaporator  56  and fluidly connected to the condenser  74  and the evaporator  56 . 
     In operation, the liquid refrigerant of the refrigeration loop E vaporizes in the evaporator  56  by absorbing heat from the cooling fluid of the cooling fluid loop D. The vaporized refrigerant is dried and heated or superheated in the precooler/superheater  54  before being sent to the condenser  74  by the compressor  72 . In the condenser  74 , the vaporized refrigerant dissipates heat to cooling air passing through the condenser  74  and, as a result, becomes liquid refrigerant again. The liquid refrigerant returns to the evaporator  56  through the expansion valve  76  and repeats the aforementioned cycle. 
     The amount of heat generated by the x-ray tube varies over time. When x-rays are being generated, the amount of heat generated tends to be at or near a maximum heat loading rate. In contrast, the amount of heat generated at all other times is relatively lower. The cooling system  50  of the present invention employs the heat buffer  52  to assist in heat removal from the cooling fluid during peak heat load periods. Using the heat buffer  52  requires that the refrigeration loop E be capable of removing only an average rate of heat from the cooling fluid. The heat buffer  52  essentially queues or stores a variable portion of the heat generated by the x-ray tube B during peak loading. When the peak load period ends, the heat buffer  52  is then cooled over time by the cooling fluid in preparation for the next peak load period. 
     In operation during peak heat load periods, the bypass valve  60  is open. The heat buffer valve  58  is open a variable amount dependent of the temperature of the cooling fluid exiting the x-ray tube B which allows heated cooling fluid from the x-ray tube B to enter the heat buffer  52 . Preferably, a thermal sensor  90  senses the cooling oil temperature and a valve controller  92  opens the valve  58  progressively more with rising temperature and progressively closes it with falling temperature. The heat buffer  52  assists the precooler/superheater  54  and the evaporator  56  in removing heat from the cooling fluid which keeps the x-ray tube B from overheating. 
     When the peak load period ends, i.e., the x-ray tube power is turned off, the bypass valve  60  is closed forcing all cooling fluid through the heat buffer  52 . As the temperature of the cooling fluid drops below the temperature of the high heat capacity material in the heat buffer  52 , it begins absorbing heat from the high heat capacity material. When the temperature of the heat buffer  52  returns to a desired temperature, the x-ray tube B may be powered again and the cycle repeated. 
     In an alternate embodiment, the bypass conduit  68 , the bypass valve  60  and the control valve  58  are eliminated. The cooling fluid flows from the x-ray tube B directly through the heat buffer  52  during peak and off-peak heating loads. The heat buffer  52  would continue to operate as discussed above. 
     In yet another alternative embodiment, the precooler/superheater  54  is eliminated. The precooler/superheater  54  serves to enhance the operating efficiency of the system  50  but is not a required component. 
     The refrigeration loop E is sized to remove all of the heat generated by the x-ray tube over a most rapidly cycling mode of operation. The heat buffer  52  is sized to absorb the difference between the heat generated by the x-ray tube and the heat removed by the refrigeration circuit E during the longest duration cycle of the x-ray tube. When sizing the heat buffer  52 , it must be remembered that the heat buffer  52  is not always brought to ambient temperature between operations of the x-ray tube. The heat buffer  52  should be sized to absorb the heat difference even when starting at the elevated temperature of a rapid on-off cycle. 
     The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon a reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.