Abstract:
A CT scanner includes a stationary gantry (C) having an examination region (12) centrally therein. A rotating frame or gantry (C) which is mounted for rotation about the examination region carries an x-ray tube assembly (B), a liquid-to-liquid heat exchanger (34), and a liquid-to-air heat exchanger (48) around a peripheral edge thereof. A first closed loop (30) carries a first cooling fluid, particularly oil, between a housing (22) which surrounds an x-ray tube (68) and the liquid-to-liquid heat exchanger to remove heat from the x-ray tube. A second closed loop (40) conveys a second cooling fluid, particularly water, between the liquid-to-liquid heat exchanger and the peripheral liquid-to-air heat exchanger. The second closed loop includes a reservoir (44) for storing a substantial volume of water such that a significant portion of the heat generated during an x-ray examination can be stored by the water in the reservoir. After the x-ray examination while the patient is being repositioned or removed, the water continues to be circulated to the liquid-to-air heat exchanger cooling the water from the reservoir.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the radiography art. 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 may also find application in conjunction with other radiation treatment apparatus and imaging apparatus. 
     Heretofore, tomographic scanners have commonly included a floor-mounted frame assembly which remains stationary during a scan. An x-ray tube is mounted to a 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. From the radiation data sampled by the detectors and the position of the x-ray tube during each sampling, a tomographic image of one or more slices through the patient is reconstructed. 
     An x-ray tube generates x-rays by directing a high energy electron bean against a tungsten target. One of the persistent problems in CT scanners and other radiographic apparatus is dissipating the waste heat created while generating x-rays. In higher powered x-ray tubes, the anode rotates so that the high energy electron beam only dwells a fraction of a second at a time on any point on the anode. The x-ray tube is jacketed with a lead lined housing. A cooling oil is circulated between a glass vacuum envelope of the x-ray tube and the lead-lined housing to remove waste heat. 
     In some scanners, the x-ray tube rotates in one direction during a scan and returns in the other direction for the next scan. Such scanners are normally limited to about 360° of rotation. The single rotation enables the hot cooling oil to be conveyed from the rotating frame by flexible hoses to a non-rotating heat exchanger. Accommodating the cooling oil-carrying hoses is a space consumptive handling problem. Limiting a scanner to about 360° of rotation makes it unable to perform many common diagnostic procedures. 
     In other CT scanners, the cooling oil is circulated to a radiator or other air-oil heat exchanger that is mounted on the rotating frame portion. This alleviates the hose handling problems and enables the x-ray tube to rotate a plurality of times, e.g., a continuous rotate scanner. However, accommodating the size and weight of the heat exchanger in the tight space constraints of the rotating frame is difficult. As the x-ray tube and rotating frame portion rotate, air passes through the heat exchanger cooling the oil. Limited space on the rotating gantry limits the surface area of the radiator, limiting cooling. In other CT scanners, hot oil or other hot fluid is conveyed to a fluid slip ring. The fluid slip ring is an annular structure that surrounds the patient bore. One part of the slip ring rotates with the rotating gantry and the other part is connected to the stationary gantry. An annular fluid passage is defined between the rotating and stationary slip ring halves. In one prior art design, the hot fluid circulated to a radiator immersed in the fluid in the slip ring. In another design, the hot fluid emptied into the fluid slip ring. In both designs, hot fluid from the slip ring was conveyed to a chiller. Surrounding the patient with a hot fluid carrying slip ring raises serious patient safety concerns. Leaking or failure of the slip ring seals could cause serious burns to the patient. 
     One of the limiting factors on the speed of a CT scan is the amount of x-rays produced by the x-ray tube. The tube must irradiate each detector for a sufficient duration that each detector receives the minimum total flux needed to reconstruct a good contrast image. Lower power tubes require the tube to dwell or focus longer on each detector. Larger, more powerful x-ray tubes supply the minimum flux more quickly, allowing the speed of x-ray tube rotation to be increased, hence the scan time decreased. However, as the x-ray tubes become more powerful, more heat is generated. More heat is also generated in continuous rotate scanners in which the tube remains &#34;on&#34; during several consecutive rotations for multi-slice imaging. 
     Larger x-ray tubes, such as seven inch anode x-ray tubes, generate so much heat that the prior art heat dissipation techniques are taxed. The limited air volume in the interior of a CT scanner limits the effectiveness of the rotating oil-air heat exchanger. Space constraints prevent larger heat exchangers from being accommodated on the rotating frame. 
     The present invention provides a new and improved cooling system which overcomes the above-referenced problems and others. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, oil is circulated through a first fluid carrying passage peripherally around the rotating gantry of a CT scanner. The first fluid carrying passage is thermally connected to the x-ray tube and the circulating oil absorbs x-ray tube heat to cool the x-ray tube. Water is circulated through a second fluid carrying passage peripherally around the rotating gantry. A first heat exchanger thermally connects the first and second fluid carrying passages so that heat from the circulating oil is transferred to the circulating water. A second heat exchanger thermally connects the second fluid carrying passage and an air flow passage. Room air is circulated through the air flow passage and heat from the circulating water is transferred to the circulating air in the second heat exchanger. The heated air is then released into the room. 
     In accordance with a more limited aspect of the present invention, the x-ray tube is mounted on a rotatable frame that is continuously rotatable. An array of x-ray detectors are mounted to receive radiation from the x-ray tube that has traversed a patient examination region. An image reconstruction processor reconstructs an image representation from data received by the radiation detector array. 
     In accordance with another aspect of the present invention, an oil reservoir is connected with the first fluid carrying passage and an oil circulator circulates the oil through the first fluid carrying passage. A water reservoir is connected to the second fluid carrying passage and a water circulator circulates the water through the second fluid carrying passage. 
     In accordance with a yet more limited aspect of the present invention, at least one cold plate is connected to the second fluid carrying passage designed to liquid cool components of the scanner. 
     One advantage of the present invention is that it effectively cools large x-ray tubes and other large thermal loads. 
     Another advantage of the present invention is that it permits the x-ray tube and all associated oil conveying circuitry to be replaced as a unit to prevent cooling oil contamination. 
     Another advantage of the present invention is that it enables existing gantry configurations to carry a large heat exchanger that uses ambient room air to cool the circulating fluid. Additional refrigeration devices are not required. 
     Another advantage of the present invention is that it takes advantage of the normal duty cycle of a CT scanner x-ray tube. 
     Another advantage of the present invention is that a water coolant reservoir stores heat when the x-ray tube is on and the heat is dissipated between scans. 
     Another advantage of the present invention is that blowers run at a minimal speeds to minimize air movement noise. 
     Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments. 
    
    
     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 drawings are only for purposes of illustrating a preferred embodiment and are not to be construed as limiting the invention. 
     FIG. 1 is a diagrammatic illustration of a CT scanner in accordance with the present invention; 
     FIG. 2 is a perspective view of the rotating gantry of FIG. 1 with the front panel removed; 
     FIG. 3 is a schematic of the cooling system in accordance with the present invention; 
     FIG. 4 is an illustration in partial section of the second heat exchanger of the rotating gantry of FIG. 2 and a lower portion of the stationary gantry; 
     FIG. 5A is a perspective view in partial section of one embodiment of the x-ray tube; 
     FIG. 5B illustrates an oil cooling arrangement in accordance with the embodiment of FIG. 5A of the x-ray tube; 
     FIG. 6A is a perspective view in partial section of an alternative embodiment of the x-ray tube housing; and, 
     FIG. 6B is a perspective view of an oil cooling system in accordance with the alternative embodiment of FIG. 6A. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1, a CT scanner includes a floor mounted or stationary gantry A whose position remains fixed during data collection. An x-ray tube B is rotatably mounted on a rotating gantry C. The stationary gantry A includes a cylinder 10 that defines a patient receiving examination region 12. An array of radiation detectors 14 are disposed concentrically around the patient receiving region. In the illustrated embodiment, the x-ray detectors are mounted on the stationary gantry portion such that an arc segment of the detectors receives radiation from the x-ray tube B which has traversed the examination region 12. Alternately, an arc segment of radiation detectors can be mounted to the rotating gantry C to rotate with the x-ray tube. 
     A control console 16 contains an image reconstruction processor 18 for reconstructing an image representation out of signals from the detector array 14. Preferably, the image reconstruction processor reconstructs a volumetric image representation from radiation attenuation data taken along a spiral path through the patient. A video monitor 20 converts selectable portions of the reconstructed volumetric image representation into a two-dimensional human-readable display. The console 16 also includes appropriate tape and disk recording devices for archiving image representations, performing image enhancements, selecting planes, 3D renderings, or color enhancements, and the like. Various scanner 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 B includes an oil filled housing 22 that has an x-ray permeable window directed toward the patient receiving region. An evacuated envelope is disposed within the housing holding a rotating anode, such as a 7 inch anode, and a cathode or other electron source. High voltages, on the order of 150 kV and higher applied between the rotating anode and the cathode, cause the generation of x-rays. The x-rays pass through an x-ray permeable window 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. A shutter under control from the console 16 selectively gates the beam on and off to control patient dosage. Electrical power from the console is conveyed to electrical slip rings for transferring electrical power and control signals between the console and the x-ray tube, shutter and the like. A high voltage power supply 24 is mounted on the rotating gantry C for rotation with the x-ray tube. This permits relatively low voltage to be conveyed across the electrical slip ring and converted to high voltage on the rotating gantry. 
     With particular reference to FIGS. 2 and 3, a closed oil passage loop 30 is disposed on the rotating gantry C. The oil passage loop 30 is connected to the x-ray tube housing 22. An oil circulating pump 32 circulates oil through the oil passage loop 30 into and around the x-ray tube housing 22. As high voltages are applied between the rotating anode of the x-ray tube and the cathode, x-rays are generated along with a substantial amount of heat Q1. The circulating oil absorbs the heat generated from the x-ray tube cooling the x-ray tube. The heated oil then circulates through an oil-water or first heat exchanger 34 which has an effective steady-state counterflow heat capacity UA1. The first heat exchanger 34 includes a plurality of annular loops of tubing that define passages through which the heated oil is circulated. The tubes are interconnected by webs which increase the heat transfer surface area. The oil passage loop 30 has a line disconnect mechanism 36 which allows the x-ray tube B to be removed from the loop for maintenance or replacement. An oil reservoir 38 is connected to the oil passage loop to maintain an oil supply. Alternatively, other working fluids or mediums may be used in place of Oil. 
     The first heat exchanger 34 thermally connects the oil passage loop with a water passage loop 40. The water passage loop is annularly disposed around the gantry forming a closed loop. A water circulating pump 42 circulates water through the water passage. As the water circulates through the first heat exchanger, heat from the heated oil in the oil passage loop is transferred to the circulating water. In this manner, the heated oil is cooled. A 10 gallon water reservoir 44 is connected to the water passage to maintain a supply of water and provide a thermal reservoir. The water reservoir 44 functions as a heat sink by increasing the thermal mass of the water passage. The water reservoir is positioned opposite the x-ray tube B on the gantry and additionally functions as a counter-weight. Other working fluids or mediums may also be used in place of water. Cold plates 46 are connected to the water passage to cool the high voltage generator 24 or other rotating gantry mounted electronic or heat sensitive components. The water from the water pump 42 is pumped through a water-air or second heat exchanger 48. 
     With reference to FIGS. 3 and 4, an air flow passage 50 is defined by stationary ducts disposed around the gantry. The air flow passage is an open-ended loop so that air is drawn from and released into the surrounding room. The second heat exchanger 48 thermally connects the air flow passage and the water passage loop. In the preferred embodiment, the second heat exchanger 48 is a large diameter circular radiator and has an effective steady-state counterflow heat capacity UA2. The water-air heat exchanger includes a series of radial vanes that surround the rotating gantry C. As the rotating gantry rotates, the vanes function as a squirrel cage blower to pump cool air from a plenum 54 through the vanes and discharging hot air to an annular surrounding region 56 of the air passage 50. Variable speed blowers 58 are mounted on the stationary gantry to draw cool ambient room air through the input ducts 60 to provide a positive air flow into the plenum 54. In this manner, heat from the circulating water is transferred to the air, thus cooling the water. Hot air from annular region 56 is discharged into the room or can be connected to ceiling ducts which remove the heated air from the room. It is to be appreciated that excess heat in the room air can be removed in a number of ways as known by those of ordinary skill in the art. 
     Sensors monitor temperature, pressure, and flow of the cooling system. The sensor measurements are inputted to a thermal controller which monitors system status and performance. The thermal controller controls air circulating speed, dampeners, blower speeds, air supply, and exhaust ducts, and produces signals indicating when maintenance is needed. Furthermore, because the oil and water passages are closed loops, volume expansion devices, preferably accumulators (not shown), are connected to the passages to maintain stability. 
     With further reference to FIG. 3, the definitions of the cooling system variables are: 
     To1: Temp of oil entering x-ray tube B; 
     To2: Temp of oil exiting x-ray tube B; 
     Tw1: Temp of water entering first heat exchanger 34; 
     Tw2: Temp of water exiting first heat exchanger 
     Ta1: Temp of air entering second heat exchanger 48; 
     Ta2: Temp of air exiting second heat exchanger 48; 
     Q1: Heat flow from the x-ray tube B; 
     Q2: Heat flow across first heat exchanger 
     Q3: Heat flow across second heat exchanger 48; 
     Q4: Heat flow across the generator 
     Mdo: Oil flow rate; 
     Mdw: Water flow rate; 
     Mda: Air flow rate; 
     UA1: First heat exchanger capacity; and, 
     UA2: Second heat exchanger capacity. 
     The cooling system is designed to move heat Q1 created by the x-ray tube B to the air at Q3. The rate at which the heat is removed from this system is dictated by: 
     (1) overall temperature differential, found by the difference between the hot oil temperature To2 leaving the x-ray tube and the cold air temperature Ta2 entering the second heat exchanger 48; 
     (2) heat exchanger capacities; and, 
     (3) mass flow rates of the oil, water, and air represented by Mdo, Mdw, and Mda. 
     Items (2) and (3) above are limited by packaging restraints. Item (1) is limited by the type of flow control within the tube housings of the fluids. 
     Two flow control embodiments effect the temperature differential of the system. The first is a flow through embodiment shown in FIGS. 5A and 5B, and the second is a counterflow embodiment shown in FIGS. 6A-6B. 
     With reference to FIG. 5A, a cutaway view of housing 22 of x-ray tube B is shown for the flow through embodiment. The x-ray tube housing 22 has an oil inlet port 62 and an oil outlet port 64 which connect to the oil passage loop 30. A motor 66 rotates an x-ray tube evacuated envelope 68 to which the anode is attached. The surface of the anode and housing are channeled or vaned to urge oil flow from the inlet port 62 through apertures in baffles 70 over the surface of the evacuated envelope 70 to the outlet port 64. 
     With reference to FIG. 5B, an oil cooling arrangement for the flow through embodiment is shown. This arrangement corresponds to the embodiment shown in FIG. 2. In the flow through embodiment, the temperature of oil exiting the x-ray tube at outlet 64 is limited by the thermal sensitivity of components located within the x-ray tube housing 22, in the preferred embodiment about 65° C. 
     The cooling system shown in FIG. 2 is sized to match the flow through embodiments requirements. By numerical simulation, the steady-state maximum oil temperature at the end of a 160 second 57 kW, 33% duty cycle run is 75° C. This temperature is determined based on a 24° C. inlet air temperature Ta1 to the second heat exchanger 48. Since the maximum oil temperature of To2 reduces proportionally to the inlet air temperature Ta1, a maximum oil temperature of 65° C. will result if 14° C. inlet air is provided. 
     With reference to FIG. 6A, a cutaway of the x-ray tube housing 22 is shown for the counterflow embodiment. A motor 66 and stationary baffles 70 are mounted on either side of the evacuated envelope 68 of x-ray tube B within the housing 22. The housing 22 has two oil inlet ports 62 and an oil discharge port 72. The anode, on the motor side of the evacuated envelope and in alignment with the discharge port has radial vanes that form a centrifugal pump to pump oil out of the discharge port. The baffles 70 have apertures that are seized such that most of the oil flows over the motor 66 and only a smaller component flows over the evacuated envelope 68. 
     With reference to FIG. 6B, an oil cooling arrangement is shown for the counterflow embodiment. In this embodiment, the primary or oil-water heat exchanger 34 is directly connected with the x-ray tube housing 22. With further reference to FIGS. 6A and 3, the counterflow embodiment keeps the hot oil separate from the heat sensitive components. This allows the oil temperature of inlet oil temperature To1 entering inlet ports 62 to be higher, e.g., 65° C. and the temperature of the oil To2 exiting port 72 to be around 100° C. With a larger temperature differential heat exchanger, components and pump sizes are greatly downsized. Reduction of flow rates reduces the overall size of the cooling system as well as reducing noise. Further reduction of the first heat exchanger capacity results in a reduction in heat exchanger volume as well as a cost savings. Down-sizing of the first heat exchanger further eliminates quick disconnects in the oil loop which aid in the serviceability of the x-ray tube. 
     The invention has been described with reference to the preferred embodiment. Obviously, modifications and alterations will occur to others upon 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.