X-ray detector array maintained in isothermal condition

X-ray or gamma ray systems or devices – Specific application – Computerized tomography

Reexamination Certificate

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Details

C378S004000

Reexamination Certificate

active

06249563

ABSTRACT:

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.


REFERENCES:
patent: 4283817 (1981-08-01), Cotic
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patent: 5486703 (1996-01-01), Lovin et al.
patent: 5596228 (1997-01-01), Anderton et al.
patent: 0 240 718 A1 (1987-10-01), None
“HP-1 Heat Pipe”, Product Data Guide, Thermacore, Inc., h-1.p.65, Aug. 7, 1998.

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