Abstract:
Passive heat transfer apparatus is provided for an X-ray imaging system used in connection with mammaography, to rapidly conduct heat away from the system X-ray tube. The apparatus comprises a thermally conductive support plate located in the tube housing, in spaced apart relationship with the X-ray tube, and further comprises an elongated device for transferring heat by convection, such as a heat pipe. The heat transfer device has a first end joined to the tube, and a second end joined to the support plate. A quantity of selected working fluid sealably contained in the heat transfer device is disposed to transfer heat along the length thereof, from the tube to the support plate, and cooling fins extending through the housing from the support plate dissipate the heat into the surrounding environment. A layer of sound absorbing material is usefully positioned to surround the X-ray tube within the housing, to provide acoustic damping and substantially reduce the level of noise resulting from X-ray tube operation.

Description:
BACKGROUND OF THE INVENTION 
     The invention disclosed and claimed herein generally pertains to passive heat transfer apparatus for an X-ray imaging system having a rotating anode X-ray tube, wherein the heat transfer apparatus is disposed to conduct heat away from regions proximate to the tube. More particularly, the invention pertains to apparatus of the above type wherein the head of an imaging subject is typically positioned so that noises generated proximate to an X-ray tube are particularly disturbing. Even more particularly, the invention pertains to apparatus of the above type which is very useful in connection with X-ray systems used for mammography, and which provides means for reducing acoustic disturbance. 
     In a rotating anode X-ray tube, a beam of electrons is directed through a vacuum, across very high voltage, from a cathode to a focal spot position on an anode. X-rays are produced as electrons strike the anode, comprising a refractory metal track, such as tungsten, molybdenum or rhodium, which is rotated at high speed. However, the conversion efficiency of X-ray tubes is quite low, typically less than 1% of the total power input. The remainder, in excess of 99% of the input electron beam power, is converted to thermal energy or heat. Accordingly, heat removal, or other effective procedure for managing heat, tends to be a major concern in the design and operation of an X-ray tube. Frequently, fans or the like are employed to circulate air to cool the tube. 
     In an X-ray imaging system designed for mammography, the patient is usually positioned so that her ears are very close to the X-ray tube, that is, within two or three centimeters. Typically, two significant sources of noise are located proximate to the tube. One source is the bearings contained within the tube casing, to support the rotary anode. The bearings produce an unpleasant high frequency noise as the anode rotates during X-ray generation. The other noise source is an arrangement of fans, which are typically located in a housing which also contains the tube, the fans being operated to circulate a stream of cooling air around the tube. Noise generated by both sources tends to be very disturbing to a mammography patient. 
     In the past, efforts have been made to reduce noise levels by surrounding the X-ray tube and the fans with sound absorbing material. However, materials commonly used for this purpose also tend to be thermally insulating. Thus, this approach to solving the noise problem prevents dissipation of heat away from the tube, so that the temperature of the tube may be quickly driven above the tube temperature limit. 
     SUMMARY OF THE INVENTION 
     The invention is directed to passive heat transfer apparatus for an X-ray imaging system provided with a rotating anode X-ray tube, wherein the heat transfer apparatus is disposed to rapidly conduct heat away from the tube and dissipate it into the surrounding environment. The apparatus may also be adapted to provide acoustic damping, or to reduce noise levels, and is particularly well suited for use in connection with X-ray equipment designed for mammography applications. However, the invention is by no means limited thereto. The invention generally comprises a thermally conductive plate or other support member, which is located in an X-ray tube housing in spaced apart relationship with the casing of the X-ray tube, also located in the housing. The invention further comprises an elongated heat transfer device having a first end which is proximate to the tube casing and a second end which is proximate to the support plate. A quantity of selected working fluid is sealably contained in the heat transfer device, the working fluid being disposed for bi-directional movement along the device to transfer heat from the first end of the transfer device to the second end thereof. Thus, the heat transfer device is passive and comprises a convective device, that is, employs fluid to move heat along its length. A heat dissipation device is provided to extend through the housing proximate to the support plate, for transferring heat from the support plate to air external to and surrounding the housing. 
     In a preferred embodiment of the invention, the heat transfer device comprises a conduit segment of selected length, the conduit segment having an inner wall in adjacent relationship with a sealed interior space. A selected porous material is attached to the inner wall and configured to define a passage through the sealed interior space that extends along the length of the conduit segment, the porous material being selected in relation to the working fluid so that the fluid, when in liquid form, is disposed for movement through the porous material by means of capillary action. When the first end of the transfer device is at a selectively higher temperature than the second end, the fluid proximate to the first end is vaporized into gaseous form, moved along the passage by means of convection to the second end, and then condensed into liquid form. Preferably, a layer of sound absorbing material is placed around the X-ray tube within the housing, to serve as a barrier to noise generated by anode rotation within the tube. Preferably also, the heat dissipation device comprises a number of cooling fins which are thermally joined to the support plate, and extend through the wall of the housing into the surrounding air or environment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view showing an X-ray imaging system provided with an embodiment of the invention. 
     FIG. 2 is a sectional view, taken along lines  2 — 2  of FIG. 1, which shows an embodiment of the invention. 
     FIG. 3 is a perspective view having a section broken away showing a heat transfer device which may be used in the embodiment of FIG.  2 . 
     FIG. 4 is a sectional view taken along lines  4 — 4  of FIG.  2 . 
     FIG. 5 is a cross-sectional view taken along lines  2 — 2  of FIG. 1, which shows another embodiment of the present invention. 
     FIG. 6 is a perspective view of a heat transfer device according to the embodiment of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, there is shown an X-ray imaging system  10  of a type which is commonly used for mammography. In operation, a female patient (not shown) is positioned to stand close to system  10 , so that her breast is placed upon support  12  for imaging. In such position, the patient&#39;s head and ears are adjacent to housing  14  of system  10 , which contains an X-ray tube for producing X-rays required for imaging. Thus, it is highly desirable to limit, as much as possible, the level of noise emanating from housing  14 . At the same time, means must be provided for effective heat removal, as described above. 
     Referring to FIG. 2, there is shown an X-ray tube  16  within housing  14 . In accordance with conventional practice, tube  16  generally includes a metal casing  18  which supports other X-ray tube components including a cathode  20 , and which also provides a protective vacuum enclosure therefor. Cathode  20  directs a high energy beam of electrons  22  onto a target track  24  of an anode  26 , which consists of a refractory metal disk and is continually rotated by means of a conventional mounting and drive mechanism  28 . Target track  24  has an annular or ring-shape configuration and typcially comprises a tungsten, molybdenum or rhodium based alloy integrally bonded to the anode disk  26 . As anode  26  rotates, the electron beam from cathode  20  impinges upon a continually changing portion of target track  24  to generate X-rays, at a focal spot position. A beam of X-rays  30  generated thereby is projected from the anode focal spot through an X-ray transmissive window  32  provided in the side of housing  18 , and is further projected through a plastic cover  34  positioned below housing  14 . In order to produce X-rays as described above, there must be a potential difference on the order of 25-140 kilovolts between cathode  20  and anode  26 . 
     In accordance with the invention, it has been recognized that it would be highly advantageous to quietly, passively and efficiently remove heat, generated by production of X-rays, from regions proximate to tube casing  18 . Thus, FIG. 2 further shows an elongated heat transfer device  40 , for conducting heat by convection, extending from tube casing  18  to a support plate  36  spaced apart from the tube. The left end of device  40 , as viewed in FIG. 2, is fixably joined to casing  18 , such as by brazing, and the right end is fixably joined to and supported by plate  36 . Plate  36  is formed of copper or other thermally conductive material, and is joined by screws  38  or the like to another plate  42 , likewise formed of copper or other thermally conductive material. Plate  42  is fixably attached to an inner side or wall of housing  14  by means of bolts  44  and complementary nuts  46 . 
     Referring to FIG. 3, there is shown a heat transfer device which may usefully be employed as transfer device  40 . The device  40  of FIG. 3 comprises a length of copper tubing or conduit  48 , which is tightly closed or sealed at its ends to form a vacuum tight vessel. The vacuum tight vessel provided by conduit  48  is evacuated and partially filled with a working fluid  52 , such as water, and is usefully of circular cross section. FIG. 3 further shows a porous metal wicking structure  50 , which is joined to the inner wall or surface  48   a  of copper conduit  48 . Wicking structure  50  is usefully formed of a porous material, such as a material comprising small copper pellets or beads which are sintered together. Wick structure  50  is configured to surround or define a passage  54  which extends along the length of transfer device  40 . 
     By providing a heat transfer device  40  with the construction shown in FIG. 3, such device is enabled to transfer heat by respective evaporation and condensation of working fluid  52 . More particularly, if point  40   a  along device  40  is at a higher temperature than a location  40   b  spaced apart therefrom, heat is inputted through conduit  48  into the interior thereof, proximate to location  40   a.  As a result, the fluid  52  is vaporized in passage  54  proximate to location  40   a.  This creates a pressure gradient in passage  54 , between a region proximate to location  40   a  and a cooler region proximate to location  40   b.  This pressure gradient forces the vaporized fluid to flow along passage  54  to the cooler region, where it condenses to a liquid and gives up its latent heat of vaporization. The working fluid  52 , now in liquid form, then flows in the opposite direction along device  40 , back toward location  40   a,  through the porous wick structure  50 . Such fluid motion is caused by capillary action in the wick structure, or by gravity if device  40  is oriented to decline downwardly from location  40   b  to location  40   a.  Usefully, a heat transfer device  40  comprises a device which is similar to a product sold by Thermacore Inc. and referred to commercially thereby as a heat pipe. Devices of such type may have an effective thermal conductivity which exceeds the thermal conductivity of copper by more than 10 3 . Moreover, such devices are silent and totally passive, that is, they do not require power sources for their operation, in contrast to fans or like cooling devices. 
     By incorporating heat transfer device  40  in the arrangement shown in FIG. 2, substantial quantities of heat can be conducted from metal tube casing  18  to support plate  36 , quietly and with a high degree of efficiency. Thus, the heat is removed from regions proximate to tube  16 . Accordingly, the tube can be surrounded with sound absorbing material  56 , to absorb noise produced by the bearings of X-ray tube mounting and drive mechanism  28 , which support annode  26  for rotation. Material  56  usefully comprises a foam material or a foam material with a backing of lead or other mass material, which is commercially available. It is anticipated that use of such material could reduce noise levels by as much as 10 dBA, relative to comparable mammography systems of the prior art. The sound absorbing material  56 , notwithstanding its thermal insulating effects, would not trap heat around tube  16 , since heat is removed by device  40  as stated above. 
     As a further benefit, the arrangement shown in FIG. 2 eliminates the need to use forced convection cooling on the outer surface of tube casing  18 , and therefore eliminates any requirement for fans in mammography imaging system  10 . Thus, a second source of noise is completely removed from the system. Moreover, since the system cooling is totally passive, failure modes associated with fans are also eliminated. 
     Referring to FIG. 4, there are shown cooling fins  58  fixably joined to the side of plate  42  which is opposite the plate  36 , the fins extending through the wall of housing  14  into the air surrounding and external to imaging system  10 . Fins  58  are formed of copper or other thermally conductive material. By providing fins  58 , heat transferred from X-ray tube  16  by device  40  will flow through plates  36  and  42  to the fins  58 , and then be dissipated thereby into the surrounding air by free convection. A protective plastic cover  62 , provided with vents  64 , is usefully placed over fins  58 . A thermal compound could also be added between plates  36  and  42  to enhance heat transfer therebetween. 
     In another embodiment of the invention a single support plate could be substituted for the two plates  36  and  42 , with both heat transfer device  40  and fins  58  being fixed to the single plate. However, the configuration shown in FIG. 4 is considered to be advantageous, particularly in connection with tube replacement. By fixably joining casing  18 , transfer device  40  and support plate  36  together as a unit, the unit can be detached from the wall of housing  14  in the event tube replacement is required, merely by removal of screws  38 . Plate  42  and fins  58  would remain fixably attached to the housing  14  of mammography system  10 . 
     In another embodiment of the invention, an arrangement employing transfer device  40  and insulation material  56  as described above could be employed to maintain tube-generated noise at the level of currently used systems, while the tube anode was operated at a significantly higher rotational speed. 
     If free convection cooling of the fins is inadequate, the fins could also be moved to the upper back surface of the covers and a fan used to cool the fins. The fan would be located away from the patient&#39;s ear, hence minimizing noise heard by the patient. 
     In certain mammography tube designs, the tube  16  is mounted for rotatable or pivotal movement, relative to housing  14 , through a small angle with respect to an axis through the focal spot and orthogonal to the plane of the view shown in FIG.  5 . By means of such movement, the projected X-ray beam  30  can be selectively varied to meet different imaging beam requirements. It will be apparent that the embodiment shown in FIG. 2, wherein one end of transfer device  40  is fixed to tube  16  and the opposing end is fixed to housing  14  by means of plate  36 , could not be used with a tube that had to be movable with respect to the housing. Accordingly, an alternative embodiment is provided, as shown by FIG. 5, which includes a heat transfer device  60  which is similar or identical to device  40  described above. The left end of device  60 , as viewed in FIG. 5, is brazed or otherwise fixably joined to tube casing  18 , also in like manner with device  40 . Thus, heat generated by tube  16  is conducted away by device  60 , from the left end to the right end thereof. However, the right end of device  60  remains unconstrained, so that it can move freely or float within housing  14 . Thus, heat transfer device  60  is able to rotate or pivot together with tube  16 , as shown by the arrow in FIG.  5 . 
     Referring further to FIG. 5, there is shown a set of fins  66  mounted along device  60 , toward the rightward end thereof, for dissipating heat from tube  16  into a region of housing  14  which is spaced apart from the tube. As with the embodiment of FIG. 2, the region is separated from tube  16  by a layer of sound absorbing material  56 . A vent  68  is provided through housing  14 , to enable the heat from fins  66  to readily flow out from the housing. Such heat flow may be assisted by placing a fan  70  proximate to the vent  68 . The fan  70  will be located much farther from the ears of an imaging subject than the fan of a conventional mammography arrangement, as described above, and will thus be much less disturbing. FIG. 5 further shows a bracket  72  provided to support fins  66  and the rightward end of heat transfer device  60 , relative to the tube casing  18 . 
     Referring to FIG. 6, there is shown heat transfer device  60  and fins  66  joined thereto in further detail. 
     Obviously, many other modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the disclosed concept, the invention may be practiced otherwise than as has been specifically described.