Abstract:
A method and device that provides independent temperature control of x-ray detector crystals, either singly or in small groups. In addition to a thermal control network for the crystals, electronic devices are associated with each detector crystal and are independently cooled using Peltier devices so that lifetime and reliability are maximized. In most operating environments the ambient temperature is less than the operating temperature of the detector crystals. In these situations, the heat removed from the electronics can be used to heat the detector crystals, resulting in efficient operation.

Description:
FIELD OF THE INVENTION 
     The present invention relates to x-ray detector crystals and, more particularly, to a method and device that provides independent temperature control of x-ray detector crystals, either singly or in small groups. 
     BACKGROUND OF THE INVENTION 
     It is well-known that temperature has a significant effect on the performance of pixilated semiconductor and semi-insulator materials used for photon counting in CT machines. In particular, CdTe, CdTe:Cl, CdZnTe, HgI 2 , and other semi-insulators exhibit poor carrier transport properties so that significant space charge can build within the material during use. This space charge affects the induced signal, resulting in tailing of the spectrum (i.e., spreading of the lower energy tail), spectral broadening, as characterized by FWHM measurements, and spectral shifts. The counter can become highly non-linear and, in extreme cases, even cease to function. 
     At low flux, space charge has very little effect. In certain medical applications a higher flux is used so that exposure time can be minimized. Inspection applications, for example in luggage scanning, require a high flux so that penetration can be maximized. In both cases sufficient photons must be collected to ensure statistical significance; it is inevitable that some level of space charge will build up during use. 
     The energy-discriminating systems currently being developed for medical and inspection applications use spectral integration to determine attenuation over specific energy ranges. Repeatable, non-linear effects can be removed mathematically from the data, provided that the response of the detector crystal does not vary with time. Controlling the temporal variation of space charge is thus critical to advance this technology. 
     The existing prior art, which deals mostly with scintillator/silicon devices, teaches that the temperature of detector material must be maintained within a narrow range to prevent significant changes in spectral response. Thermo-mechanical stability has also been identified as an important factor in CT applications. 
     U.S. Pat. No. 6,249,563 for X-RAY DETECTOR ARRAY MAINTAINED IN ISOTHERMAL CONDITION by Snyder et al. teaches the use of heat pipes to reduce temperature gradients within the detector arm of a CT scanner. Related U.S. Pat. No. 6,459,757 for X-RAY DETECTOR ARRAY WITH PHASE CHANGE MATERIAL HEAT SYSTEM by Joseph Lacey teaches the use of a phase change material and a sensor to measure and control the temperature within a detector array. However, this reference fails to control the temperature through the use of a feedback network. 
     U.S. Pat. No. 6,370,881 for X-RAY IMAGER COOLING DEVICE by Fyodor Maydanich teaches the use of a planar heat spreader disposed directly beneath an array of silicon detectors to maintain a nearly uniform temperature. The heat spreader is cooled with a thermoelectric device. 
     U.S. Pat. No. 6,931,092 for SYSTEM AND METHOD FOR THERMAL MANAGEMENT OF CT DETECTOR CIRCUITRY by Joshi et al. teaches the use of a heat sink to cool chips used for data acquisition within a CT machine. However, these chips are not located near the scintillator array and are more akin to the cooling schemes that are widely used in computers. 
     U.S. Pat. No. 7,135,687 for THERMOELECTRICALLY CONTROLLED X-RAY DETECTOR ARRAY STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH by Lacey et al. teaches the use of a thermoelectric system to control the temperature gradient across a large array of detectors. It cannot, however, adequately control a large array of detectors which will be subjected to very different thermal environments due to local conduction and convection effects. It also fails to address the need to reduce space charge at high flux levels, but seeks only to minimize spectral and special shifts with operating temperature. 
     Finally, U.S. Pat. No. 7,161,157 for SELF REGULATING DETECTOR RAIL HEATER FOR COMPUTED TOMOGRAPHY IMAGING SYSTEMS by Joseph Lacey teaches the use of PTC (Positive Temperature Coefficient) heaters within a detector array to maintain a uniform temperature. Since the current flowing through the heaters is a strong function of temperature, the devices become self-regulating. There is, however, a dependence on ambient temperature, air-flow and other variables. This scheme is therefore unlikely to provide the temperature stability needed to control variations in crystal response arising from the build-up of space charge. 
     In energy-discriminating CT systems, the performance of pixilated semiconductor and semi-insulator materials is improved when the temperature is raised above 40° C. due to increased carrier mobility. 
     None of the aforementioned inventions recognizes that detector crystals should be operated at these temperatures. The prior art is largely confined to scintillator/silicon technology; operating temperatures above 35° C. are not taught. Thus the electronics, which are typically reliable only at operating temperatures of less than 40° C., can operate in the same thermal environment as the detectors. This is no longer the case when crystal temperatures are increased to improve carrier mobility. 
     Further, a complication arises when the counting electronics must be located in proximity to the detector crystal. This requirement arises due to induced noise, low signal levels, and capacitance in the circuitry. It is the proximity of devices, with their conflicting thermal requirements, that has not previously been addressed. 
     Thus, it is desirable to provide independent means of controlling the temperature of semi-insulator, solid-state detector crystals within a narrow range while cooling their associated electronics so that performance, reliability and lifetime are maximized. Clearly a thermal solution is needed, such as this invention which can control and maintain an elevated temperature in detector crystals, either individually or in small groups, while cooling the associated electronics. 
     SUMMARY OF THE INVENTION 
     A method and device provide independent temperature control of x-ray detector crystals, either singly or in small groups. The device includes an insulated chamber with a circulating air stream that encloses the detector crystals, while an air-moving device provides circulation, a heat sink transfers heat into or out of the air stream, a thermoelectric device transfers heat into or out of the heat sink, a housing provides a thermal sink or source for the thermoelectric device, a temperature sensing means either within the air stream or in contact with the detector crystals, and a control network to adjust the temperature according to the temperature sensing means and a predetermined set-point. 
     Leakage current, which creates noise in the detector network, is not critically important in this application because spectral integration reduces its impact. As a result crystals can be operated at temperatures much higher than currently practiced or rated and, for a given photon flux, the space charge is reduced. 
     Peltier devices are used to cool or heat crystals, depending on ambient conditions. The temperature is controlled by a feedback network that can be implemented using either analog or digital control. In a preferred embodiment a digital pass-band is used as the control network. 
     In a preferred embodiment air is directly impinged on the detector crystal in a closed-loop circulation system. Heat removed from the air stream when cooling the crystals is transferred to the outer housing by the Peltier devices. Heat is transferred from the outer housing by the same Peltier devices as the temperature of the crystals is raised. Any crystal or group of crystals can be heated or cooled to the degree necessary to ensure that a uniform temperature, preferably in excess of 40° C., is maintained across the array. 
     In addition to the thermal control network for the crystals, the electronic devices associated with each detector crystal are independently cooled using a second set of Peltier devices so that lifetime and reliability are maximized. These Peltier devices pump heat from the electronics into the outer housing. In most operating environments the ambient temperature is less than the operating temperature of the detector crystals, so heat removed from the electronics can be used to heat the detector crystals, resulting in efficient operation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A complete understanding of the present invention may be obtained by reference to the accompanying drawings, when considered in conjunction with the subsequent, detailed description, in which: 
         FIG. 1  is a sectional view of a first embodiment of the invention, showing the separate thermal environments of the x-ray crystal and the photon counting electronics; airflow is in the viewing plane; 
         FIG. 2  is a detailed view of the heat sink and first set of Peltier devices used to control the temperature of the x-ray detector crystals; 
         FIG. 3  is a sectional view of the first embodiment of the invention, taken perpendicular to the view in  FIG. 1 , with the direction of airflow substantially normal to the viewing plane; 
         FIG. 4  is a detailed view of counting electronics and a second set of Peltier devices, showing the cooling path for the electronics; 
         FIG. 5  is a graph that depicts the temperature as a function of time for the detector crystal under different ambient conditions; and 
         FIG. 6  is a schematic representation of a digital control network. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention is described with reference to the accompanying  FIGS. 1 through 5 , where like reference numbers correspond to like elements. 
       FIGS. 1 through 4  are partial drawings of an x-ray detector assembly. In  FIGS. 1 and 3 , only a portion of the outer enclosure  210  is shown for clarity. It is understood that additional electronics, well known to those versed in the art, have been omitted from the drawings for clarity. 
       FIG. 1  is a cross-sectional view of the invention. Air-flow is substantially two-dimensional and in the cutting plane used to produce this view. The temperature of the package housing the photon counting electronics  110  is cooled while the detector crystals  120  are maintained within a narrow range of temperatures. In one embodiment the temperature of crystals  120  is controlled to within ±2° C. However, tighter control, to within ±0.5° C., is desirable. 
     The detector crystals  120  are isolated within insulating enclosure  210 . In a preferred embodiment this enclosure  210  is completely contained within a housing  100 . However, in another embodiment, enclosure  210  is only partially located within housing  100 . Air is circulated within enclosure  210  by a fan or other air-moving device  130  ( FIG. 3 ). The temperature of the air is monitored by an RTD or thermocouple (not shown) that is part of the control network described in detail below. 
     The flow of heat is described with reference to  FIG. 2 . When the temperature of the circulating air is below the target temperature, a voltage is applied to the first set of Peltier devices  142  in order to transfer heat from housing  100  to the air stream. Peltier device  142  is a widely-used, solid state device that transfers heat when a voltage is applied across it. In general, the amount of heat that can be pumped is a function of the temperature differential across the device, the applied current, and the temperature of the sink. 
     Peltier device  142  is disposed between two layers of a thermally-conductive compound  116   a  and  116   b  that serve to reduce the thermal resistance. An example of such material  116   a ,  116   b  is TIC-1000, manufactured by The Bergquist Company of Chanhassen, Minn. When voltage is applied across Peltier device  142 , heat flows from housing  100  through thermally-conductive layer  116   b , Peltier device  142 , and layer  116   a , to a heat sink  190 . Peltier device  142  can be smaller than the base of heat sink  190 , which acts as a heat-spreader. Any number of Peltier devices  142  can be used with a single heat sink  190 . Although heat sink  190  is shown with a pin-finned design, an extruded, finned heat sink  190  can also be used. 
     Heat removed from housing  100  reduces its temperature below the ambient conditions. It is understood that the reduction in temperature is limited by heat transfer from the environment by free or forced convection via means well known in the art. 
     Referring once again to  FIG. 1 , the flow of air within enclosure  210 , while heating, is now described in detail. The temperature of heat sink  190  is raised as described directly above. Air-moving device  130 , shown as a scroll fan, draws air through heat sink  190 , raising the air&#39;s temperature. Heated air moves vertically through an opening  172  in the lower portion of enclosure  210 , into the intake on the lower surface of fan  130 , and is exhausted horizontally into the upper chamber defined by enclosure  210 . The air then flows through aperture  174  and impinges on detector crystal  120  before being drawn again through heat sink  190 . Aperture  174  constricts the air-flow so that pressure is nearly equal along its length, ensuring that air impinges equally on all crystals  120 . Aperture  170  is provided so that an x-ray fan (not shown) can irradiate the crystals  120 . Aperture  170  is sealed with, for example, a polyimide tape  180  to prevent the escape of air from enclosure  210 . 
       FIG. 3  provides another view of a portion of the circulation path. At first, air drawn through heat sink  190  flows horizontally in order to gain heat, and then flows vertically through openings (not visible) in enclosure  210 , into scroll fans  130 . Impellers  134  force air through openings  132  in the fan housings, and from there through the remainder of the circulation path. 
     Since it is an object of this invention to operate the detector crystals  120  at temperatures above 40° C., it is necessary to provide heating to the detector crystals  120  in most settings. It may, however, be necessary to cool the crystals  120  if the x-ray detector is operated at extremely high ambient conditions. In such instances the airflow is identical to that described above and the voltage applied to the Peltier devices  142  is reversed so that heat is removed from the air stream. 
     The cooling path for the electronic device package  110  is now described with reference to  FIG. 4 . Reliability of electronic components is understood to be a direct function of operating temperature. Thus, heating of device  110  is not required. The electronics typically comprise an Application Specific Integrated Circuit ASIC)  118  that is back-bonded to a circuit card  150 , using a thermally-conductive material  116   c , such as that previously described. Wire-bonds  114  are used to make the electrical connections to circuitry (not shown) on card  150 . An alternative structure uses flip-chip interconnections. All such processes are widely practiced. 
     Heat is generated on the active surface of ASIC  118  and is conducted through the semiconductor material, the conductive layer  116   c , and through card  150  by thermal vias  112 . Another layer of thermally-conductive compound  116   d  conducts heat into the upper layer of Peltier device,  142 . When cooling, heat is pumped across Peltier device  142  from the second thermally-conductive layer  116   d  to a third, similar material  116   e . Finally, the enclosure  100  acts as a heat sink, absorbing the thermal energy transported by Peltier device  142 . 
     Referring again to  FIG. 1 , the operation of the system is described. In most instances the ambient temperature surrounding enclosure  100  is at a temperature below the operating point of the crystals  120 . In such cases, heat pumped from the electronics package  110  by Peltier devices  142  will supply the source for heat pumped to the air-stream by Peltier devices  142 . Such a thermal circuit is extremely efficient. In a particular embodiment of the invention, a set of eight CZT crystals, supplied by eV Products of Saxonburg, Pa., were combined within a single housing. Eight proprietary ASICs were used to read the signal from each of the detector crystals, dissipating a total of 2 W. Two Peltier devices produced by Ferrotec USA Corp. of Bedford, N.H., were attached to a heat sink as described in a preferred embodiment above. 
     At an ambient temperature of 25° C. the target temperatures for the ASICs and the detector crystals were respectively set at 32° and 42° C. The ASIC temperature remained below 32° C. at all times. The detector crystals reached the target temperature within eight minutes and subsequently remained within the target range of ±0.2° C. The temperature of the enclosure was found to average about 0.7° C. above the ambient temperature under free-convection conditions. 
     Target temperatures were also maintained within ±0.2° C. at an ambient temperature of 50° C. In this instance, heat was transferred from both the electronics  110  and the detector crystals  120  to the enclosure  100 . This resulted in a slightly greater difference in temperature between the enclosure  100  and the ambient temperature—about 0.9° C. in this instance. A plot of crystal temperature vs. time at ambient temperatures of 5°, 25° and 50° C. is shown in  FIG. 5 . 
     The operation of an exemplary temperature control network is now explained with reference to  FIGS. 2 and 6 . Many alternate control schemes are possible and can be practiced within the art. While the operation described relates to the detector crystals, it is understood that the network controlling the temperature of the ASICs operates similarly. 
     Three groups of components are shown in the electrical schematic diagram of  FIG. 6 . The first group comprises the temperature comparator (IC 1 ) and its related components above it and to the left. The second group comprises a set of LEDs used to monitor operation, and an AND-gate with opto-isolator outputs. The third group comprises the Peltier driver circuit driven by opto-isolators. 
     The comparator has three inputs: V+, V− and a temperature window voltage Vw. This window defines a deadband whose width is adjusted by R 6 . The negative input, V−, is used as the reference and controls the center value of the temperature window. When at a given temperature the RDT resistance is equal to the series resistance of R 10  and R 9  the voltage at pin  2  of IC 1  is in the center of the window. The output of IC 1  at pin  1  goes high. This lights the green LED, indicating an “at temperature state.” As the temperature at the RDT drops, the input voltage at pin  3 , V 3 , increases. When the resultant volt-age surpasses the upper limit of the temperature window, V 3 &gt;V++[Vw/2], the output at pin  6  is driven high. This lights the red LED and extinguishes the green LED, indicating that the temperature is no longer in the window and a heat cycle is in progress. 
     The current that lights the red LED also drives the opto-isolator  103 , which in turn supplies the bias to turn on Q 2 , supplying a positive current to the Peltier devices  142 . This positive current increases the temperature of heat sink  190 , which in turn raises the temperature of the air-stream, the RDT, and the detector crystals  120 . As the temperature at the RDT increases in response to the heating cycle, the voltage at pin  3  of IC 1  decreases until it again falls within the deadband, turning off the bias to the driver transistor Q 2  and removing the current from the Peltier devices  142 . 
     In the event the RDT temperature exceeds the deadband, the voltage at pin  3  drops so that V 3 &lt;V+−[Vw/2] and the output at pin  6  of IC 1  is driven low. Since neither D 2  nor D 1  are forward biased, no voltage is developed at R 14  to turn on Q 3 . As a result, the blue LED and opto-isolator IC 2  turn on, supplying bias to Q 1 , supplying a negative current to the Peltier devices  142 . This negative current then cools heat sink  190 , cooling the air-stream, the RDT, and the detector crystals  120 . As the RDT senses the decrease in temperature, the voltage at pin  3  of IC 1  increases until the voltage is once again within the deadband, driving pin  1  of IC 1  high, lighting the green LED, forward biasing D 1 , so that the voltage across R 14  in turn shunts IC 2 &#39;s LED and the blue LED L 3 . This removes the bias on Q 1  and the negative current from the Peletier devices  142 . 
     Since other combinations, modifications and changes varied to fit particular operating requirements and environments will be apparent to those skilled in the art, the invention is not considered limited to the chosen preferred embodiments for purposes of this disclosure, but covers all changes and modifications which do not constitute departures from the true spirit and scope of this invention. 
     Having thus described the invention, what is desired to be protected by Letters Patent is presented in the subsequently appended claims.