Patent Number: 
Section: description

Reference will now be made to figures wherein like structures will be provided with like reference designations. It is to be understood that the drawings are diagrammatic and schematic representations of various embodiments of the invention, and are not to be construed as limiting the present invention, nor are the drawings necessarily drawn to scale. In general, the present invention relates to cooling systems for use in any type of x-ray tube environment requiring improved cooling. FIGS. 1 through 7 indicate various embodiments of a cooling system conforming to the teachings of the invention. Reference is first made to FIG. 1, which depicts an x-ray device indicated generally at 100. X-ray device 100 includes an x-ray tube 102 having a vacuum enclosure 104, inside of which are disposed an electron source 106 and a target anode 108. In operation, power is applied to electron source 106, which causes a beam of electrons e1 to be emitted by thermionic emission. A potential difference is applied between the electron source 106 and target anode 108, which causes the electrons e1 to accelerate, and then impinge upon a focal spot 110 location on the target anode 108. A portion of the resulting kinetic energy is released as x-rays, indicated at xe2x80x9cXxe2x80x9d, which are then collimated and emitted through window 200 and into, for example, the body of a patient. As is represented in FIG. 1, some of the electrons strike and then rebound from the surface of target anode 106, and then strike other xe2x80x9cnon-targetxe2x80x9d areas, such as the window 200, and/or other areas within vacuum enclosure 104. As discussed elsewhere, the kinetic energy of these secondary electron e2 collisions generates extremely high temperatures. Moreover, since a relatively large number of the rebounding electrons strike the window 110 and the adjacent structure of the vacuum enclosure 104, a significant amount of heat is created in those areas. This heat must be reliably and continuously removed. With continuing reference to FIG. 1, x-ray tube 100 also includes a cooling system 300 having a reservoir 302 containing a volume of liquid coolant 304 in which x-ray tube 102 is at least partially immersed. In a preferred embodiment, liquid coolant 304 is a dielectric oil, but can be any appropriate fluid that is capable of functioning as a heat transfer medium. One such fluid is Syltherm. An external cooling unit 306 is connected to reservoir 302 by way of fluid conduits 308A and 308B, as shown in FIG. 1. In a preferred embodiment, external cooling unit 306 comprises a fluid pump, as well as a heat exchanger, or the like, that is configured to remove heat from liquid coolant 304. Fluid conduits 308A and 308B preferably comprise hoses or the like. In general, liquid coolant 304 circulates through reservoir 302 and absorbs at least some of the heat generated by x-ray tube 102, including heat present in window 200 and the adjacent vacuum enclosure 104 structure. Liquid coolant 304 then exits reservoir 302 via fluid conduit 308A and enters external cooling unit 306 where at least some heat is removed from liquid coolant 304. Thus cooled, liquid coolant 304 then re-enters reservoir 302 by way of fluid conduit 308B to repeat the cycle. As discussed in further detail below, a portion of the liquid coolant 304 exiting external cooling unit 306 is preferably diverted to a cooling plenum 310 substantially enclosing window 200, so as to provide for a relative increase in the rate at which heat is removed from window 200 and the adjacent structure of vacuum enclosure 104. A fluid conduit 312 connects external cooling unit 110 and cooling plenum 310. Turning now to FIG. 2, additional details regarding the construction and operation of cooling plenum 310, and window 200, are provided. Cooling plenum 310 is preferably integral with vacuum enclosure 104. Alternatively however, cooling plenum 310 may be formed separately from vacuum enclosure 104 and attached thereto by a joining process such as welding, brazing, or the like. Cooling plenum 310 preferably includes a compensating window 314. In a preferred embodiment, cooling plenum 310 is made of copper. However, any other cooling plenum material providing the functionality of copper, as disclosed herein, is contemplated as being within the scope of the present invention. With reference now to FIG. 3A, and with continuing reference to FIG. 2, cooling plenum 310 further comprises a fluid inlet connection 316 and a fluid outlet connection 318, both of which are in fluid communication with a fluid passageway 320 which is cooperatively defined by cooling plenum 310, compensating window 314, and window 200. As further suggested in FIG. 3A, an important feature of x-ray tube cooling system 300 relates to the construction of window 200. In particular, window 200 comprises a body 201 having a plurality of extended surfaces 202 attached thereto. As a consequence of the disposition of extended surfaces 202 on body 201 of window 200, a plurality of slots 204 are necessarily defined, a slot 204 being interposed between succeeding extended surfaces 202, as indicated in FIG. 3A. Extended surfaces 202 are preferably integrally formed with body 201 and are in substantial contact with liquid coolant 304 as liquid coolant 304 flows through fluid passageway 320. While extended surfaces 202 are preferably integral with body 201 of window 200, it will be appreciated that extended surfaces 202 could be separately formed, either individually or collectively, and then attached to body 201 so as to provide the functionality disclosed herein. In a preferred embodiment, extended surfaces 202 comprise fins, or the like. However, it will be appreciated that a wide variety of extended surface types, and/or combinations thereof, could be employed to provide the functionality disclosed herein. To the extent such extended surface types conform to the requirements outlined elsewhere herein for extended surfaces 202, they are contemplated as being within the scope of the present invention. Such other extended surface types include, but are not limited to, rectangular protrusions, pyramidal protrusions, cylindrical protrusions, and the like. In general, extended surfaces 202 serve to increase the overall surface area of window 200. As is well known, the rate of heat transfer from a body is directly proportional to the surface area of that body that is in contact with the cooling medium. Accordingly, extended surfaces 202 serve to facilitate a relative increase in the rate at which heat dissipated by window 200 is absorbed by the liquid coolant 304 as liquid coolant 304 flows through fluid passageway 320. Because extended surfaces 202 permit window 200 to dissipate heat at a relatively higher rate than otherwise possible, the functionality of x-ray device 100 is significantly improved. That is, the higher rate of heat dissipation from window 200 allows x-ray device 100 to operate at relatively higher power levels and/or for relatively longer periods of time, such as are required for helical exposures or other similar evolutions. As previously noted, compensating window 314 is preferably disposed in relatively close proximity to window 200 so as to cooperate with window 200 to define a fluid passageway 320 having a cross-section with a relatively small perimeter, indicated in FIG. 3A. In particular, compensating window 314 includes extended surfaces 314A and slots 314B which are disposed opposite slots 204 and extended surfaces 202, respectively, of window 200. As compensating window 314 and window 200 are brought together, their respective extended surfaces and slots cooperate with each other to define fluid passageway 320. Note that, in addition to facilitating cooling of window 200, compensating window 314 possesses other valuable features, at least some of which relate to x-ray intensity attenuation, discussed in detail elsewhere herein. In similar fashion to that just described, cooling plenum 310 preferably comprises extended surfaces 310A and 310B that cooperate with, respectively, slots 310C and 310D, to form a fluid passageway 320A in communication with fluid passageway 320. It will be appreciated that various other configurations could be profitably employed as well. Such other configurations include, but are not limited to, one where only fluid passageway 320 is defined. Generally however, any configuration providing the functionality disclosed herein is contemplated as being within the scope of the present invention. Directing further attention now to fluid passageway 320, it is well known that, for a given flow rate, the velocity of a fluid through a passageway increases as the cross-section area of that passageway decreases. It is also well known that the rate of heat absorption by a flowing coolant is directly proportional to the velocity of the coolant so that relatively higher velocities produce relatively higher rates of heat absorption by the coolant. Thus, a fluid passageway 320 with a relatively small cross-sectional area translates to an increased rate of heat absorption by liquid coolant 304, for a given flow rate of liquid coolant 304. It will be appreciated that the distance between compensating window 314 and window 200 may thus be varied so as to achieve a desired cooling effect. Likewise, the flow rate of liquid coolant 304 may be varied to the same end. The forced convective heat transfer thus facilitated by the flow of liquid coolant 304 through fluid passageway 320 desirably augments the convective heat transfer effect achieved by virtue of the contact between exterior surfaces 322 (see FIG. 2) of cooling plenum 310 and liquid coolant 304 so as to enable a relatively higher rate of heat transfer from window 200 and from the adjacent vacuum enclosure 104 structure than would otherwise be possible. As a result of the relatively higher rate of heat transfer from these areas, the functionality of x-ray device 100 is greatly enhanced. In particular, x-ray device 100 is able to operate at relatively higher temperatures and/or for relatively longer periods of time without compromising the operational or structural integrity of window 200 and/or that of adjacent vacuum enclosure 104 structure. In view of the foregoing discussion, it will be appreciated that the affects achieved by the various extended surfaces and slots indicated in FIG. 3A can be readily obtained with a variety of other geometries as well. Two examples of such alternative geometries are indicated in FIGS. 3B and 3C. With reference first to FIG. 3B, extended surfaces 202 and 314A are generally triangular in cross section, and, in similar fashion, slots 314B and 204 have an analogous shape designed to cooperate with extended surfaces 202 and 314A, respectively, so as to define fluid passageway 320. It will be appreciated that similar configurations could be effectively employed for extended surfaces 310A and 310B and slots 310C and 310D of cooling plenum 310 (see FIG. 3A). Finally with reference to FIG. 3C, yet another alternative geometry is indicated. In particular, extended surfaces 202 and 314A have a continuous wave shaped cross section, and, in similar fashion, slots 314B and 204 have an analogous shape designed to cooperate with extended surfaces 202 and 314A so as to define fluid passageway 320. As with the embodiment depicted in FIG. 3B, it will be appreciated that similar configurations could be effectively employed for extended surfaces 310A and 310B and slots 310C and 310D of cooling plenum 310 (see FIG. 3A). In an alternative embodiment of cooling system 300, no cooling plenum or compensating window is employed. Rather, convective cooling of window 200 is facilitated by virtue of the direct contact between extended surfaces 202 and slots 204 of window 200, and liquid coolant 304 disposed in reservoir 302. This embodiment may, or may not, employ an external cooling unit 306. In the aforementioned embodiment, a pump or the like may additionally be employed so as to enhance the circulation of liquid coolant 304 within reservoir 302. The fluid motion induced by the pump produces a forced convection cooling effect. As liquid coolant 304 flows over extended surfaces 202, it absorbs at least some of the heat dissipated by window 200 and adjacent vacuum enclosure 104 structure. Preferably, the flow produced by the pump is directed primarily over extended surfaces 202 and slots 204. Because the rate of heat transfer is directly proportional to the velocity of liquid coolant 304, the motion imposed by the pump induces a forced convection cooling effect that augments the convective cooling effect realized as a result of the direct contact between extended surfaces 202 and slots 204, and liquid coolant 304 disposed in reservoir 302. Directing attention now to FIG. 4, additional details regarding the construction and operation of window 200 are indicated. In a preferred embodiment, window 200 comprises beryllium. However, it will be appreciated that various other materials may be selected for window 200 so as to produce a desired effect with respect to the cooling of window 200, and/or with respect to the x-rays produced by x-ray device 100. Such other window materials include, but are not limited to, titanium, nickel, carbon, silicon, and aluminum. Window 200 is brazed to vacuum enclosure 104, which is preferably made of copper. Note however, that this invention contemplates as within its scope any other joining method that would provide the functionality if the brazed joint disclosed herein. Such joining methods include, but are not limited to, welding processes and the like. As noted elsewhere, a plurality of extended surfaces 202 are disposed on body 201 of window 200. Because the intensity of x-rays passing through window 200 is at least partially a function of the window geometry and window material, extended surfaces 202 must be arranged so that they do not materially interfere with the diagnostic imaging quality of x-ray device 100. In particular, extended surfaces 202 are preferably disposed in a plane which is oriented so as to be substantially parallel to the plane of a CT slice 400. When thus oriented, extended surfaces 202 serve to desirably increase the heat transfer area of window 200 without compromising the diagnostic imaging quality of x-ray device 100. In the embodiment of window 200 disclosed in FIG. 4, extended surfaces 202 are not only disposed in a plane parallel to CT slice 400, but may themselves be parallel to CT slice 400. However, as further suggested by FIG. 4, extended surfaces perpendicular to CT slice 400, but nevertheless disposed in a plane parallel to CT slice 400, would be equally effective in providing the functionality disclosed herein. Accordingly, various orientations of extended surfaces 202 are contemplated as being within the scope of the present invention, at least to the extent that those extended surfaces are disposed within a plane parallel to CT slice 400. Directing attention now to FIGS. 5A through 5D, various embodiments of a window 200 employing different arrangements of extended surfaces 202 and slots 204, are depicted. Note that the embodiments depicted are representative only and are not intended to limit in any way the scope of the present invention. With reference first to FIG. 5A, one embodiment of window 200 comprises a plurality of extended surfaces 202 and slots 204 distributed substantially uniformly across window 200. Alternatively, an arrangement is contemplated where a window 200 having a plurality of extended surfaces 202 and slots 204 is joined to a vacuum enclosure 104 having a plurality of extended surfaces 104A and slots 104B, as indicated in FIG. 5B. In this arrangement, extended surfaces 104A and slots 104B of vacuum enclosure 104 serve to increase the surface area of vacuum enclosure 104 in the vicinity of window 200. By increasing the surface area of vacuum enclosure 104, and thus the rate at which heat can be dissipated by vacuum enclosure 104 to liquid coolant 304, extended surfaces 104A and slots 104B serve to desirably augment the cooling effects imparted to window 200 and the adjacent structure by extended surfaces 202 and slots 204. As indicated in FIG. 5C, it is not necessary that extended surfaces 202 and slots 204 be disposed over the entire surface of window 200 in order to achieve the functionality disclosed herein. In particular, one embodiment of window 200 has a substantially clear area 206 through which a useful beam portion, indicated at xe2x80x9cXxe2x80x9d, of the x-rays produced by x-ray device 100 passes. Where such a configuration is desired, various desired cooling effects may nevertheless be achieved by disposing extended surfaces 202 and slots 204 outside of clear area 206, for example, on the sides of clear area 206 of window 200. Finally, FIG. 5D indicates an alternative embodiment of a window 200 having a clear area 206 for passage of useful beam portion xe2x80x9cXxe2x80x9d of x-rays emitted by x-ray device 100. In particular, clear area 206 is embodied as a large extended surface, in contrast to the embodiment depicted in FIG. 5C, where clear area 206 takes the form of a recessed surface. As discussed above, extended surfaces 202 may be disposed in a wide variety of arrangements, but in any event are preferably disposed in a plane which is substantially parallel to the plane of CT slice 400. Another consideration with regard to the various possible configurations of extended surfaces 202 and slots 204 concerns the tendency of at least some configurations to induce local variations in the intensity of x-rays emitted through window 200. In particular, a window configured in a manner such that x-ray intensity varies at different locations on the window is undesirable because it may compromise the quality of the image produced by the x-ray device. Accordingly, extended surfaces 202 and slots 204 are preferably configured in such a way as to substantially foreclose material differences between the intensity of x-rays emitted through extended surfaces 202, and the intensity of x-rays emitted through slots 204. That is, the intensity of x-rays emitted through window 200 is preferably uniform over the entire window, without regard to the particular geometry of window 200 at any given point. Uniformity of the x-ray intensity produced by x-ray device 100, and emitted through window 200, can be achieved in a variety of different ways. Some of the various possible approaches are discussed in detail below. One approach to ensuring uniform x-ray intensity through window 200 relates to the specific geometry and dimensions of extended surfaces 202 and slots 204, and is suggested in FIG. 6. In particular, the thickness of extended surfaces 202, indicated as dimension xe2x80x9cAxe2x80x9d, and/or the width of slots 204, indicated as dimension xe2x80x9cBxe2x80x9d, can be varied so as to produce configurations that will ensure uniform intensity of x-rays emitted through window 200. For example, if x-ray device 100 has a minimum resolving power of 2.0 millimeters (mm) in the xe2x80x9czxe2x80x9d direction, indicated by longitudinal axis 104A of vacuum enclosure 104, then dimensions xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d are preferably made smaller than 0.5 mm, e.g., 0.4 mm. Applying the appropriate formula, i.e., summing dimensions xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d for two extended surfaces 202 and two slots 204, an overall dimension of 1.6 mm is obtained (2 extended surfacesxc3x970.4 mm, and 2 slotsxc3x970.4 mm=an overall dimension of 4xc3x970.4 mm or 1.6 mm). Because 1.6 mm is less than the aforementioned hypothetical minimum resolving power of 2.0 mm, no material variation in x-ray intensity is imposed by extended surfaces 202 and slots 204 of the aforementioned dimensions. Note that dimension xe2x80x9cAxe2x80x9d need not be the same as dimension xe2x80x9cBxe2x80x9d in order to achieve the functionality disclosed herein. As one example, dimension xe2x80x9cAxe2x80x9d could be 0.6 mm and dimension xe2x80x9cBxe2x80x9d could be 0.2 mm, for an overall dimension of 2xc3x970.6+2xc3x970.2=1.6 mm. Alternatively, dimensions xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d may be made greater than the resolving power of x-ray device 100. For example, dimensions xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d could be made 3 mm each. As with the previous example, dimensions xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d need not be equal to each other. Finally, dimensions xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d may desirably be varied so as to accommodate motion of focal spot 110 (see FIG. 1) in the xe2x80x9czxe2x80x9d direction without compromising the diagnostic imaging quality of x-ray device 100. Assuming, for example, a focal spot 110 motion of 0.25 mm, dimensions xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d must be at least the same as the sum of the resolving power of the device and focal spot 110 movement. As an example then, for an x-ray device 100 having a focal spot movement of 0.25 mm and a resolution of 2.0 mm, dimensions xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d must be at least 2.25 mm. With reference now to FIG. 7, another way to facilitate uniform x-ray intensity from window 200 is to employ an attenuator 210 in conjunction with extended surfaces 202 and/or slots 204 such that any differences in the intensity of x-rays emitted through extended surfaces 202 and slots 204 can be minimized. It is well known that by changing the material of window 200, the intensity of the x-rays emitted therethrough can desirably be varied. That is, different materials absorb different amounts of x-rays. The tendency of a material to absorb x-rays is often referred to in terms of the absorption coefficient of that material, so that a material with a relatively higher absorption coefficient tends to absorb relatively more x-rays than a material having a relatively lower absorption coefficient. In general then, the intensity of x-rays emitted through a material that absorbs a relatively greater amount of x-rays will be relatively lower than the intensity of x-rays emitted through a material that absorbs relatively fewer x-rays. With continuing reference to FIG. 7, while the intensity I0 of x-rays entering window 200 is typically fairly uniform across window 200, the intensity I1 of x-rays emitted through the end of extended surface 202, as measured at reference plane 208, is different than the intensity I2 of x-rays emitted through slot 204 filled with liquid coolant 304. This difference is due in large part to the fact that liquid coolant 304 absorbs a different amount of x-rays than extended surface 202 does. For example, in the case of a beryllium window 200 in contact with dielectric oil, I1 is greater than I2. In order to ensure that x-rays exiting window 200 are of a substantially uniform intensity, attenuators 210 are added to the end of extended surfaces 202. In general, the effect of attenuator 210 is to attenuate, or reduce, the intensity of x-rays emitted through extended surface 202 to the point such that intensity I3 is substantially equal to the intensity I2 of x-rays emitted through slot 204 and the liquid coolant 304 disposed therein. In a preferred embodiment, attenuator 210 comprises a material, such as copper, that is readily plated can be securely joined to the ends of extended surfaces 202. As noted elsewhere, extended surfaces 202 preferably comprise beryllium. It will be appreciated however, that parameters including, but not limited to, the thickness and/or material composition of attenuator 210, as well as the material composition of extended surfaces 202, may be varied so as to achieve a desired effect on the intensity of the x-rays emitted through window 200. Finally, it will further be appreciated that attenuators 210 may be disposed in slot 204, either alone or in combination with attenuators 210 at the ends of extended surfaces 202, so as to achieve a desired effect on the intensity of the x-rays emitted through window 200. For example, this type of arrangement could be effectively employed in situations where extended surfaces 202 have a greater absorption coefficient than the absorption coefficient of the liquid coolant 304 disposed in slots 204. In such situations, the intensity of x-rays passing through liquid coolant 304 must be attenuated so as to substantially match the intensity level of x-rays emitted from extended surfaces 202, and thereby facilitate the uniform x-ray intensity necessary for high quality diagnostic imaging. As suggested earlier, compensating window 314 (see FIG. 3A) can also serve an attenuation function with respect to x-rays passing through window 200. In particular, because compensating window 314 preferably comprises the same material as window 200, the extended surfaces 314A and slots 314B of compensating window 314 serve to substantially attenuate (in a manner analogous to that described elsewhere herein) any differences in the intensity of x-rays emitted through extended surfaces 202 and through slots 204 of window 200, respectively, so that the intensity of x-rays emitted through compensating window 314 is uniform. In like fashion, extended surfaces 310B and slots 310C of cooling plenum 310 (see FIG. 3A) serve to substantially attenuate any differences in the intensity of x-rays emitted through extended surfaces 310A and through slots 310D, respectively, so that the intensity of x-rays emitted through cooling plenum 310 is uniform. It will be appreciated that the materials of cooling plenum 310, compensating window 314, window 200, and vacuum enclosure 104 may desirably be varied as required to achieve a desired effect with regard to the intensity of x-rays emitted through cooling plenum 310 and/or compensating window 314. The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.