Abstract:
X-ray detector apparatus is provided for use in a CT imaging system having a rotatable gantry. The apparatus comprises a selected number of X-ray detector cells and two curved rails, which hold the detector cells in an array comprising an arcuate configuration and mount them onto the gantry for rotation therewith. Conduit segments are distributed along the rails, each conduit segment being proximate to a corresponding group of X-ray detector cells, and a quantity of selected working fluid and a porous wick structure is sealably enclosed in each conduit segment. The fluid is disposed to move along a conduit segment in gaseous form by means of convection, and to move in the opposing direction through the wick structure, in liquid form, by means of capillary action Heat is thereby transferred along a conduit segment to maintain a substantially isothermal condition among the detector cells proximate thereto.

Description:
BACKGROUND OF THE INVENTION 
     The invention disclosed and claimed herein is generally directed to an array of X-ray detector cells, such as are used in a computed tomography (CT) imaging system, wherein the cells of the array are maintained in a substantially isothermal, or constant temperature, condition. More particularly, the invention pertains to a detector array of the above type which is provided with means for rapidly transferring heat from higher temperature locations to lower temperature locations, in order to acheive the isothermal condition. Even more particularly, the invention pertains to a detector array of the above type wherein the heat transfer means are comparatively simple and inexpensive, and operate with a high degree of efficiency. 
     In a CT imaging system or scanner, a gantry ring rotates an X-ray tube around a patient or other object of scanning. X-radiation projected by the tube, which is not absorbed by intervening patient body structure, is sensed by respective detectors of a detector array which is also mounted to the gantry ring. In certain classes of CT products, such as those manufactured by the General Electric Company, the assignee herein, the array comprises solid state detectors or detector cells which generate electric signals corresponding to the sensed radiation. The signals are coupled to a data acquisition system, and data acquired thereby is in turn coupled to an image processor which reconstructs an image of patient body structure or other object of interest. In a common arrangement, the detector cells are mounted to the gantry ring or plate by means of two curved rails, which trap or hold respective detector cells between them to form an array having an arcuate configuration. 
     In the design of a solid state CT detector array, it is essential to maintain respective X-ray detectors of the array at a fixed temperature, in order to maintain a constant gain at the detectors. If the temperature of the detectors changes, their respective electric signal outputs, for a given dose of X-radiation, will also change. In addition, it is very desirable to hold all the detectors at substantially the same temperature (within a few degrees) in order to prevent movement of adjacent wires or other structure, which could partially block some of the detectors from receiving X-rays. Moreover, the detector mounting rails also support a pair of collimator plates for each detector cell. The collimator plates of a given detector cell are selectively spaced apart, to determine the incident X-ray radiation received thereby. If the detector mounting rails experience thermal deflection, i.e., motion or flexure caused by a temperature gradient along the rails, the spacing between some of the collimator plates may change. This, in turn, will effect the amounts of radiation received by the corresponding detectors. 
     In view of the problems caused by temperature variations, efforts have been made in the past to maintain an X-ray detector array in an isothermal condition, that is, to maintain a substantially constant temperature at all detectors of the array and along the rails thereof. To this end, heating elements have been placed at selected locations with respect to the rails, and heating strips are placed along the rails to distribute heat. However, it has been found that even with these arrangements, holding a uniform temperature on the rails, under all scanning conditions, tends to be very difficult. The rails rely on thermal conduction to move heat from one region to another, since the heating elements do not supply the appropriately distributed heat load for all possible detector operating conditions. Heat transfer in currently used rail designs requires that a temperature gradient be developed, and may proceed too slowly for present operational needs. Moreover, the temperature gradient in the detector mounting rails can change under different scanning configurations. In addition, the rails can be deflected by thermal gradients that are developed in the gantry plate to which the rails are attached. This plate currently is not thermally controlled. The gantry plate has power supplies mounted to it that can produce large thermal gradients, and these gradients may change as the gantry plate rotates during scanning. 
     SUMMARY OF THE INVENTION 
     The invention is generally directed to apparatus for detecting X-rays, projected by an X-ray tube or the like, and comprises a selected number of X-ray detector cells and a frame disposed to join the detector cells together to form an array. The frame also orients the detector cells to collectively receive the projected X-rays. The apparatus further comprises a selected number of conduit segments, each conduit segment being joined to the detector array proximate to a corresponding group of X-ray detector cells. A quantity of selected working fluid is sealably contained in respective conduits, and means are positioned within each conduit segment for enabling bidirectional flow of the fluid therein, in order to transfer heat between first and second conduit locations, and to thereby maintain a substantially isothermal condition amongst all the detector cells which are proximate to the conduit segment. 
     In a preferred embodiment, each conduit segment is provided with an inner wall which encloses an interior space, and the working fluid comprises water. The means for enabling bidirectional flow through each conduit segment comprises a porous material, such as, a material comprising small copper beads or pellets, which are sintered to hold them together. The porous material is attached to the inner wall of a conduit segment, and configured to define a passage through the enclosed space thereof that extends along its length. The porous material is selected in relation to the working fluid so that the fluid, when in liquid form, tends to move through the porous material by means of capillary action. Thus, when a first location along a conduit segment is at a selectively higher temperature than a second location thereof, fluid proximate to the first location is vaporized into gaseous form, and then moves along the conduit passage by means of convection, to the second location. At the second location the fluid is condensed into liquid form, and then flows back toward the first location through the porous material. 
     Preferably, the frame for the apparatus comprises a rotatable gantry disposed for use with a CT imaging system. Two selectively curved rails, which are fixed in spaced-apart parallel relationship with one another and fixably hold respective detector cells therebetween, mount the detector cells on the gantry, in a selected arcuate configuration, for rotation therewith. In one useful mode, the conduit segments comprise a plurality of linear conduit segments, which are distributed along each of the curved rails. Each of the linear conduit segments is selectively oriented, with respect to the arcuate configuration of detector cells, so that forces generated by acceleration of the rotatable gantry and applied to respective linear segments have directions which are substantially orthogonal thereto. In an alternative mode, only one conduit segment is joined to each of the rails, each conduit being curved to match the curvature of its adjoining rail, and extending along its adjoining rail from one of the ends thereof to the other. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view showing a generalized CT imaging system. 
     FIG. 2 is a perspective view showing the system of FIG. 1, wherein the gantry shielding has been partly removed to reveal certain system components including the gantry and an X-ray detector array provided with an embodiment of the invention. 
     FIG. 3 is an exploded perspective view showing selected components of the embodiment of FIG. 2 in greater detail. 
     FIG. 4 is a side view showing the detector array of FIG. 2 in greater detail. 
     FIG. 5 is an overhead view taken along lines  5 — 5  of FIG.  4 . 
     FIG. 6 is a perspective view of a heat transfer device, with a section broken away, for the embodiment of FIG.  2 . 
     FIG. 7 shows a modification of the invention. 
     FIG. 8 shows a further modification of the invention 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, there is shown a CT system  10  which includes a gantry frame (shown in FIG. 2) and an annular gantry ring or plate member  12  which is journaled on the frame, or mounted for rotation by means of suitable bearings (not shown). The gantry frame and rotatable gantry plate  12  are contained within a shroud or gantry shielding structure  14 . 
     Referring further to FIG. 1, there is shown an X-ray tube  16  and an X-ray detector array  18  mounted on the rotatable gantry plate  12  for rotation therewith, on opposing sides of a bore  20 . A patient  22 , positioned on a patient support  24 , can be moved along the axis of bore  20  by sliding the support  24  along the direction shown in FIG. 1 by the arrow Z, relative to a base  26 . A region or section  28  of the patient  22  may thereby be positioned within the bore  20 . Thereupon, gantry plate  12  is driven to rotate tube  16  and detector  18  to acquire CT scan data of the patient section  28 , in accordance with conventional practice. The data is then employed to construct an image of the scanned section, likewise in accordance with conventional techniques. 
     Referring to FIG. 2, there is shown gantry  30 , including rotatable plate  12  and gantry frame  32 , positioned within an enclosure  34  formed by the shielding  14 . FIG. 2 further shows a power supply  36  mounted on rotatable gantry plate  12 , proximate to detector array  18 , for furnishing power to the data acquisition system or other conventional CT electronic components (not shown) which are likewise mounted on rotatable gantry plate  12 . Detector  18 , comprising an array of solid state detector cells, usefully comprises a product of assignee General Electric Company. As is well known in the art, respective cells of detector  18  produce electric signals representing X-ray radiation respectively received or sensed thereby. The electric cells are processed in accordance with techniques well known in the art, for use by a system image processor (not shown) to construct a desired CT image. 
     Referring to FIG. 3, there is shown a heater or heating element  38  joined to and extending along a side of detector array  18 . A similar heating element (not shown) extends along the opposing side of detector array  18 . Thus, heating elements  38 , which respectively comprise controllable conventional devices, serve as heat sources which may be operated to provide heat to detector array  18 , in an effort to maintain respective detectors of the array at a constant temperature. However, as stated above, prior art arrangements using heaters such as heating elements  38  are frequently unable to effectively distribute heat among the respective detector cells of an array, in order to maintain all the detector cells in an isothermal condition, i.e., within a specified temperature range. More particularly, such arrangements tend to be controlled by a single sensor element  39  located at the midpoint of the detector array. Sensor element  39  operates to turn the heating elements  38  on, when the temperature proximate to the location of sensor  39  drops below a lower temperature limit, and to turn the heating elements off when such temperature rises above an upper temperature limit. The single sensor  39  may thus be able to maintain nearby detector cells at temperatures within a specified narrow range. However, the single sensor will not be very responsive to detector cells at the ends of detector array  18 . Accordingly, it will not be effective in maintaining a uniform or even substantially uniform temperature throughout the array. This would require multiple sensors distributed along the detector array, and generally would require a much more complicated arrangement. 
     Referring further to FIG. 3, there is shown power supply  36  joined to gantry plate  12  proximate to a region  12   a , but on the side of gantry plate  12  which is opposite to region  12   a . Region  12   a  comprises the portion of gantry plate  12  which is contacted by detector array  18  when the detector array is joined thereto. Because of its location, power supply  36  functions as an uncontrolled source of heat which tends to affect the end of the detector array which is adjacent thereto much more than the opposing end. Such placement of power supply  36  has been found to have a number of design advantages. However, the heat generated thereby further complicates the task of maintaining respective detectors of array  18  in an isothermal condition. 
     Referring to FIGS. 4 and 5 together, there is shown X-ray detector array  18  comprising a pair of selectively curved rails or rail members  42   a  and  42   b , which are fixed in closely-spaced parallel relationship by means of spacers  44  or the like. As best shown by FIG. 5, a large number of X-ray detector cells  40 , typically in excess of  100 , is held or trapped between rails  42   a  and  42   b , in an arcuate configuration determined by the curvature of the rails. 
     Usefully, each detector cell  40  comprises a solid state X-ray detector, as is described for example, in commonly assigned U.S. Pat. No. 5,521,387, issued to Riedner et al. In accordance therewith, a solid state detector cell  40  comprises a scintillator body formed of a polycrystalline ceramic scintillator material, such as a product sold by the General Electric Company under the trademark Lumex. When X-radiation is incident upon the scintillator body, the body scintillates at a predetermined wavelength, thereby converting the incident X-radiation into lower energy radiation in the visible or near-visible spectrum, i.e., into light energy. Accordingly, the X-ray detector cell  40  further comprises a device (not shown) for detecting the visible spectrum or scintillator energy which is produced by the scintillator body. The photo diode device is coupled to the scintillator body to produce an electric signal which is proportional to or otherwise represents the intensity of the X-radiation received by the scintillator body. 
     As stated above, it is essential to maintain each of the detector cells  40  and the rails  42   a  and  42   b  in an isothermal condition, that is, at a uniform temperature to within a specified number of degrees. This is necessary to insure that the detectors  40  operate with maximum accuracy, as well as to minimize deflections of the rails supporting the detectors  40 , which can be caused by thermal gradients in the rails and in the gantry plate  12  adjacent thereto. Thus, in accordance with the invention, a number of heat transfer devices  46 , respectively comprising fluid filled linear conduit segments, are distributed along one or preferably both of the detector support rails  42   a  and  42   b . Each of the heat transfer devices  46  comprises a heat pipe or like device of extremely high conductivity, as described hereinafter in further detail in connection with FIG.  6 . FIGS. 4 and 5 show each of the linear heat transfer devices  46  positioned in proximate relationship with a corresponding group of X-ray detector cells  40 . Thus, if one location in a group of detectors is at a different temperature than another location therein, the devices  46  proximate to the group will act to rapidly transfer heat from the location of higher temperature to the location of lower temperature, until both locations are at the same temperature. FIGS. 4 and 5 further show the ends of adjacent devices  46  positioned along a rail to be in closely spaced relationship, to enhance heat distribution along the entire length of each rail. 
     As shown by FIG. 3, the heat transfer devices  46  are mounted on the rails  42   a  and  42   b  proximate to the heating elements  38 . Thus, heat may readily be transferred from a hotter region of the detector rails to a cooler region even in the almost complete absence of a thermal gradient. Moreover, the heating elements  38  in this arrangement do not require any more sensors than the single sensor  39 . The heat transfer devices  46  are passive, acoustically silent, have extremely high reliability, and are relatively inexpensive. 
     Referring further to FIG. 3, there is shown a configuration comprising several more heat transfer devices  46  which are mounted upon gantry plate  12 , very close to power supply  36 . These additional devices  46  act to reduce thermal gradients in gantry plate  12 , in the region thereof at which rails  42   a  and  42   b  are attached, and thereby act to reduce deflections therein. As a further benefit, the additional heat transfer devices reduce temperatures in power supply  36 , leading to improved reliability of electronics associated therewith. 
     Referring to FIG. 6, there is shown a linear heat transfer device  46  comprising a length of copper tubing or conduit  48 , which is tightly closed or sealed at its ends to form a vacuum tight vessel. The vessel is evacuated and partially filled with a working fluid  52 , such as water. Heat transfer device  46  is usefully of circular cross section. FIG. 6 further shows a porous metal wicking structure  50 , which is joined to the inner wall or surface  48   a  of copper conduit  48 . Porous wicking structure  50  is usefully formed of a material such as the material formed of copper pellets, as described above, and is configured to surround or define a passage  54  which extends along the length of transfer device  46 . 
     By providing heat transfer device  46  with the construction shown in FIG. 6, such device is enabled to transfer heat by respective evaporation and condensation of working fluid  52 . More particularly, if point  46   a  along device  46  is at a higher temperature than a location  46   b  spaced apart therefrom, heat is inputted through conduit  48  into the interior thereof, proximate to location  46   a . As a result, fluid  52  is vaporized in passage  54  proximate to location  46   a . This creates a pressure gradient in passage  54 , between a region proximate to location  46   a  and a cooler region proximate to location  46   b . This pressure gradient forces the vaporized fluid to flow along passage  54  to the cooler region, where it condenses into 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  46 , back toward location  46   a , through the porous wick structure  50 . Such fluid motion is caused by capillary action in the wick structure  50 , or by gravity if device  46  is oriented to decline downwardly from location  46   b  to location  46   a . Usefully, each of the heat transfer devices  46  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  times. 
     Referring to FIG. 7, there is shown a modification of the invention, wherein a single heat transfer device  56  is joined to each rail  42   a  and  42   b , rather than a number of linear devices  46  as described above. While each heat transfer device  56  has the same internal construction as a device  46 , it is curved to match the curvature of its adjoining rail, and extends along its adjoining rail in close proximity to each of the detector cells  40  supported thereby. 
     It has been recognized that when the gantry rotates, an acceleration load is developed, which may be applied to the heat transfer devices. It could be very undesirable if a significant component of the acceleration load was directed along the axis of a linear heat transfer device  46 . 
     This acceleration load or force could impede the capillary movement of fluid  52  through porous material  50 , and thereby interfere with the heat transfer process. Accordingly, FIG. 8 shows a second modification of the invention. In FIG. 8, a number of linear heat transfer devices  46  are distributed along detector support rail  42   a , as described above in connection with FIGS. 4 and 5. However, instead of following the curvature of the rail, each of the linear devices  46  is oriented at a selected angle with respect to an axis R, which may be selected to be a line which is tangent to the outer diameter of rail  42   a  at the mid-point P thereof. More particularly, each of the linear heat transfer devices  46  is oriented so that forces generated by acceleration of the rotatable gantry plate  12  and applied to respective linear devices  46  have directions which are substantially orthogonal thereto, as depicted by arrows a in FIG.  8 . It will be appreciated that devices  46  would also be similarly attached along rail  42   b . In FIG. 8, the vector co is the angular velocity of gantry plate  12  and detector  18 . 
     Usefully, if detector array  18  is rotated by gantry plate  12  around a circular path having a center at point C, each of the devices  46  is oriented so that a line r, extending from point C to the midpoint of a device, is at an angle of 90° with the axis thereof. An acceleration force directed through the midpoint of the device will thus be orthogonal thereto, and will not effect fluid flow along the device by capillary action. If an acceleration force is not perfectly orthogonal to the device, the effect on fluid flow will still be negligible, if the device is sufficiently short. However, the greater the departure from being orthogonal, the greater the degradation of capillary action will be, and the shorter the heat pipe will need to be. Generally, in the arrangement of FIG. 8, using a larger number of devices  46 , each of reduced length, will diminish the adverse effects of acceleration forces applied thereto. However, as the number of devices  46  is increased, the number of spaces between adjacent heat transfer devices also increases, which tends to inhibit heat transfer along detector array  18 . It is anticipated that one of skill in the art will be able to determine the proper balance between these two considerations for a particular application. 
     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.