X-ray detector array maintained in isothermal condition

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.

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.

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 12a, but on the side of gantry plate
 12 which is opposite to region 12a. Region 12a 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 42a and 42b,
 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 42a and 42b, 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 42a and 42b 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 42a and 42b. 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
 42a and 42b 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 42a and 42b 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 48a 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 46a along
 device 46 is at a higher temperature than a location 46b spaced apart
 therefrom, heat is inputted through conduit 48 into the interior thereof,
 proximate to location 46a. As a result, fluid 52 is vaporized in passage
 54 proximate to location 46a. This creates a pressure gradient in passage
 54, between a region proximate to location 46a and a cooler region
 proximate to location 46b. 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 46a, 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
 46b to location 46a. 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.sup.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 42a and
 42b, 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 42a, 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 42a 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 42b. 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.degree. 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.