US20020131547A1

US20020131547A1 - Scintillator arrays for radiation detectors and methods for making same

Info

Publication number: US20020131547A1
Application number: US09/681,301
Authority: US
Inventors: Robert Riedner; David Hoffman; Richard Ruzga; Eti Ganin
Original assignee: Individual
Current assignee: GE Medical Systems Global Technology Co LLC
Priority date: 2001-03-15
Filing date: 2001-03-15
Publication date: 2002-09-19
Anticipated expiration: 2021-03-15
Also published as: US6448566B1

Abstract

To provide simpler, more efficient methods for making scintillator arrays, one embodiment of the present invention is a method for making a scintillator array. The method includes extruding a mixture of a scintillator powder and a binder into rods; laminating the extruded rods with a sinterable reflector material; and sintering the laminated rods and reflector material into a scintillator block. Scintillator array embodiments of the present invention are useful in many types of pixelated radiation detectors, such as those used in computed tomography systems.

Description

BACKGROUND OF INVENTION

This invention relates generally to methods for making scintillator arrays used in radiation detectors, and to the scintillator arrays made from these methods.

In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the “imaging plane”. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.

In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units”, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.

Detectors of CT and other types of x-ray imaging systems utilize scintillation detectors having pixelated scintillator arrays. It would therefore be desirable to provide simplified, inexpensive methods for making such arrays, and to provide inexpensive, pixelated scintillator arrays for CT and other imaging applications.

SUMMARY OF INVENTION

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a pictorial view of a CT imaging system. [0006]
FIG. 2 is a block schematic diagram of the system illustrated in FIG. 1. [0007]
FIG. 3 is a flow chart illustrating two different method embodiments of the present inventive method for making a scintillator array. [0008]
FIG. 4 is a flow chart illustrating another embodiment of the present invention for making a scintillator array. [0009]
FIG. 5 is a simplified perspective representation of a scintillator block of the present invention during one stage of its fabrication. [0010]

DETAILED DESCRIPTION

Referring to FIGS. 1 and 2, a computed tomograph (CT) [0011] imaging system 10 is shown as including a gantry 12 representative of a “third generation” CT scanner. Gantry 12 has an x-ray source 14 that projects a beam of x-rays 16 toward a radiation detector array 18 on the opposite side of gantry 12. Detector array 18 is formed by detector elements 20 which together sense the projected x-rays that pass through an object 22, for example a medical patient. Detector array 18 may be fabricated in a single slice or multi-slice configuration. In one embodiment of the present invention, and as described below, detector elements 20 comprise sintered scintillator elements. Each scintillator element produces light in response to x-ray radiation, which is converted to an electrical signal by a sensing region of a semiconductor array optically coupled thereto. Each detector element 20 produces an electrical signal that represents the intensity of an impinging x-ray beam on that detector element and hence the attenuation of the beam as it passes through patient 22 at a corresponding angle. During a scan to acquire x-ray projection data, gantry 12 and the components mounted thereon rotate about a center of rotation 24.
Rotation of [0012] gantry 12 and the operation of x-ray source 14 are governed by a control mechanism 26 of CT system 10. Control mechanism 26 includes an x-ray controller 28 that provides power and timing signals to x-ray source 14 and a gantry motor controller 30 that controls the rotational speed and position of gantry 12. A data acquisition system (DAS) 32 in control mechanism 26 samples analog data from detector elements 20 and converts the data to digital signals for subsequent processing. An image reconstructor 34 receives sampled and digitized x-ray data from DAS 32 and performs high speed image reconstruction. The reconstructed image is applied as an input to a computer 36 which stores the image in a mass storage device 38.
[0013] Computer 36 also receives commands and scanning parameters from an operator via console 40 that has a keyboard. An associated cathode ray tube display 42 allows the operator to observe the reconstructed image and other data from computer 36. The operator supplied commands and parameters are used by computer 36 to provide control signals and information to DAS 32, x-ray controller 28 and gantry motor controller 30. In addition, computer 36 operates a table motor controller 44 which controls a motorized table 46 to position patient 22 in gantry 12. Particularly, table 46 moves portions of patient 22 through gantry opening 48.
In one embodiment of the present invention and referring to FIG. 3, a scintillator precursor is prepared by mixing [0014] 10 a temporary organic binder or gel with a scintillator powder. Suitable organic binders include organic binders or gels used in ceramic molding, such as polyethylene glycol, methylcellulose, ethylhydroxy ethylcellulose, hydroxybutyl methylcellulose, hydroxymethylcellulose, hydroxypropyl methylcellulose, hydroxyethyl methylcellulose, hydroxybutylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, sodium carboxy methylcellulose, and mixtures thereof. Suitable scintillator powders include gadolinium oxysulfide (GdOS), Lumex ((YGd)₂O₃), cadmium tungstate (CdWO₄), GGG (Gd₃Ga₅O₁₂garnet), bismuth germanate (BGO, Bi₄Ge₃O₁₂) and mixtures thereof.
The scintillator precursor is partially solidified or dried to make a flexible “cake,” that is extruded [0015] 12 through a die or multiple dies to make a square or round rod. The rods are then cut to a length dependent upon the embodiment. The cut parts are then assembled 14 into an array with sheets, laminates or layers that comprise a low melting point or easily dissolvable sacrificial material. Suitable materials for such sheets, laminates or layers include any low melting point polymer sheet coated with adhesive, such as polyester films, MYLAR® and polycarbonate films. The sheet, laminate, or layer forms a separator between individual scintillator elements or pixels. In one embodiment, the entire separator is made of a sacrificial material.
In one embodiment, the scintillator structure is also supported [0016] 16, for example, by bonding conforming plates at cut ends or faces of the scintillators to hold extrusions in place during subsequent operations. After building the structure, the sacrificial layer is removed 18, for example, by heating the structure to a temperature at which the sacrificial layer material melts away. In one embodiment, the sacrificial layer is entirely burned out. In another embodiment, the sacrificial layer is removed by a solvent. The precursor material that remains is sintered 20 at an appropriate sintering temperature to form a scintillator array.
After sintering and removal of the sacrificial layer, the gap left by the sacrifical layer is filled [0017] 22 with a reflective material which separates each scintillator array into individual channels or pixels. Examples of suitable reflective materials include titanium dioxide (TiO₂), aluminum oxide (Al₂O₃), and barium sulfate (BaSO₄) powders, and mixtures thereof. The conforming plates are then removed 24 and the long scintillator array is diced or cut 26 into thinner arrays suitable for the desired application.
In one embodiment, after preparing [0018] 10 the precursor material, extruding 12 the cake, the rods are laminated 14 with a material that is not removed (or not completely removed) when the rods are sintered 28. Thus, it is not necessary to use conforming plates to hold the laminated structure together during sintering. After sintering 28, conforming plates are then applied 30 to the ends of the rods and the laminate material is then removed 32. Removal 32 of the laminate forming the sacrificial layers is accomplished, for example, by use of heat or a solvent. The gaps left by removal of the sacrificial laminate material are then filled 22, the conforming plates are removed 24, and the resulting structure cut or sliced into sections 26, as in an embodiment described above[0009] In embodiments in which the separator layer serves as a permanent part of the array, there are no gaps to fill with reflector, so the extruded and sintered array is simply diced into a thickness appropriate for the desired application. More particularly, and referring to FIG. 4, powder and binder are mixed 10 and extruded 12 into rods, as in the embodiments of FIG. 3. The rods are then laminated (i.e., coated and joined) 34 with a sinterable reflector material. Suitable sinterable reflector materials include, for example, titanium dioxide (TiO₂), aluminum oxide (Al₂O₃), and barium sulfate (BaSO₄) powders, and other high temperature inorganic reflectors capable of surviving the scintillator sintering temperature, as well as mixtures of sinterable reflector materials. The rods and reflector material are then heated and sintered 36 in one operation, leaving a sintillator block that can simply be sliced 38 into sections of desired dimensions.
FIG. 5 is a simplified view of one embodiment of a “laminated” [0019] scintillator block 40 comprising a plurality of rods 42 of extruded scintillator material, a sheet of sacrificial laminate material 44, and additional sacrificial material 46. Although FIG. 5 shows only four rods 42, it is illustrative of embodiments having a larger number of rods 42.
[0020] Extruded rods 42 comprising a scintillator powder and an organic binder are assembled with elongate axes parallel to one another. In FIG. 5, rods 42 have a square cross-section transverse to their elongate (extruded) dimension, having been extruded through a square die. However, other embodiments utilize round rods or rods having other geometrical shapes. Laminate 40 is assembled using sacrificial materials 44 and 46. For example, rods 42 are assembled parallel to one another, in layers 48 parallel to one another, using a sheet 44 of laminate material between layers 48. In one embodiment, sheet 44 has adhesive properties (e.g., it is coated with an adhesive) so that rods 42 adhere to sheet 44. In another embodiment, rods 42 are dipped in a sacrificial adhesive (not shown) to adhere rods 42 to sheet 44. A liquid or solid (e.g., powdered) sacrificial material 46 is applied between rods 42 in each layer. Although not shown in FIG. 5, the embodiment described herein is scalable, so that laminated block 40 embodiments of the present invention can comprise any number of layers 48, and a layer 48 can comprise any number of blocks 42. Opposite faces 50, 52 of rods 42 are then joined or bonded to conforming plates 54 (only one of which is shown in FIG. 5). In one embodiment, opposite faces 50 and 52 of rods 48 are flat and parallel to one another, so conforming plates 54 are also flat and parallel to one another.
[0021] Sacrificial laminate sheets 44 and additional sacrificial material 46 are removed by heating or by dissolution in a solvent. However, because rods 42 are bonded to conforming plates 54 at faces 50 and 52, rods 42 maintain their separation from one another, and gaps remain where sacrificial laminate sheets 44 and additional sacrificial material 46 is removed. The luminescent powder comprising rods 42 is then sintered in the rods by further heating. Gaps between rods 42 are then filled with a reflector material. Conforming plates 54 are removed, and the resulting scintillator block 40 is sliced in a direction perpendicular to the length of rods 42 and parallel to faces 50 and 52. Each slice is useful as a scintillator assembly for a detector array.
In one embodiment, a [0022] sacrificial material 46 is applied by dipping each rod 42 into a sacrificial material 46. In this embodiment, no sacrificial laminate material 44 is required. Instead, sacrificial material 46 separates rods 42 both within layers 48 and between layers 48.
In one embodiment, [0023] rods 42 are sintered prior to removal of sacrificial material 46, or 44 and 46. After sintering, sacrificial material 46, or 44 and 46 is removed and the resulting gaps filled with a reflector material (not shown in FIG. 5). Conforming plates 54 are removed and the resulting scintillator assembly 40 is diced into sections as above.
In yet another embodiment, after extrusion of [0024] rods 42, rods 42 are assembled into an array 40 using a sinterable reflector material (not shown in FIG. 5) instead of sacrificial laminate material 44 and additional laminate material 46. For example, the sinterable reflector material is provided in the form of a sheet, a coating (e.g., a liquid), or a powder. Rods 42 of sintillator powder mixture and the reflector material are then sintered together, so that there is no gap filling required, and thus, no conforming plates 54 are required. The resulting sintered assembly 40 is simply sliced into sections using cuts perpendicular to the direction of the rods.
While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims. [0025]

Claims

1. A method for making a scintillator array comprising:

extruding a mixture of a scintillator powder and a binder into rods;

laminating the extruded rods with a sinterable reflector material; and

sintering the laminated rods and reflector material into a scintillator block.

2. A method in accordance with claim 1 and further comprising slicing the scintillator block into sections.

3. A method in accordance with claim 1 wherein the sinterable reflector comprises a reflector material powder selected from the group consisting of titanium dioxide, aluminum oxide, barium sulfate, and mixtures thereof.

4. A method in accordance with claim 1 wherein the binder is an organic binder.

5. A method in accordance with claim 1 wherein the binder is selected from the group consisting of polyethylene glycol, methylcellulose, ethylhydroxy ethylcellulose, hydroxybuty methylcellulose, hydroxymethylcellulose, hydroxypropyl methylcellulose, hydroxyethyl methylcellulose, hydroxybutylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, sodium carboxy methylcellulose, and mixtures thereof.

6. A method in accordance with claim 1 wherein the scintillator powder is selected from the group consisting of gadolinium oxysulfide, Lumex ((YGd)₂O₃), cadmium tungstate, GGG (Gd₃Ga₅O₁₂garnet), bismuth germanate and mixtures thereof.

7. A method for making a scintillator array comprising:

extruding a mixture of a scintillator powder and a binder into rods;

laminating the extruded rods with a sacrificial material;

supporting the laminated, extruded rods;

removing the laminated material between the supported rods;

sintering the supported rods; and

filling gaps between the sintered rods with a reflective material to form a scintillator array.

8. A method in accordance with claim 7 wherein the rods have faces at opposite ends, and supporting the laminated, extruded rods comprises bonding conforming plates at opposite ends of the rods.

9. A method in accordance with claim 7 and further comprising slicing the scintillator block into sections.

10. A method in accordance with claim 7 wherein the reflector comprises a powder selected from the group consisting of titanium dioxide, aluminum oxide, barium sulfate, and mixtures thereof.

11. A method in accordance with claim 7 wherein the binder is an organic binder.

12. A method in accordance with claim 7 wherein the binder is selected from the group consisting of polyethylene glycol, methylcellulose, ethylhydroxy ethylcellulose, hydroxybutyl methylcellulose, hydroxymethylcellulose, hydroxypropyl methylcellulose, hydroxyethyl methylcellulose, hydroxybutylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, sodium carboxy methylcellulose, and mixtures thereof.

13. A method in accordance with claim 7 wherein the scintillator powder is selected from the group consisting of gadolinium oxysulfide, Lumex ((YGd)₂O₃), cadmium tungstate, GGG (Gd₃Ga₅O₁₂garnet), bismuth germanate and mixtures thereof.

14. A method for making a scintillator array comprising:

extruding a mixture of a scintillator powder and a binder into rods;

laminating the extruded rods with a sacrificial material;

sintering the laminated, extruded rods;

supporting the sintered, laminated rods;

removing the laminated material between the supported rods; and

filling gaps between the supported rods with reflective material to form a scintillator block.

15. A method in accordance with claim 14 wherein the rods have faces at opposite ends, and supporting the laminated, extruded rods comprises bonding conforming plates at opposite ends of the rods.

16. A method in accordance with claim 14 and further comprising slicing the scintillator block into sections.

17. A method in accordance with claim 14 wherein the reflector comprises a powder selected from the group consisting of titanium dioxide, aluminum oxide, barium sulfate, and mixtures thereof.

18. A method in accordance with claim 14 wherein the binder is an organic binder.

19. A method in accordance with claim 14 wherein the binder is selected from the group consisting of polyethylene glycol, methylcellulose, ethylhydroxy ethylcellulose, hydroxybutyl methylcellulose, hydroxymethylcellulose, hydroxypropyl methylcellulose, hydroxyethyl methylcellulose, hydroxybutylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, sodium carboxy methylcellulose, and mixtures thereof.

20. A method in accordance with claim 14 wherein the scintillator powder is selected from the group consisting of gadolinium oxysulfide, Lumex ((YGd)₂O₃), cadmium tungstate, GGG (Gd₃Ga₅O₁₂garnet), bismuth germanate and mixtures thereof.

21. A scintillator array comprising a plurality of parallel sintered rods and a reflective material disposed between said parallel sintered rods.

22. A scintillator array in accordance with claim 21 wherein said parallel sintered rods comprise a scintillator powder selected from the group consisting of gadolinium oxysulfide, Lumex ((YGd)₂O₃), cadmium tungstate, GGG (Gd₃Ga₅O₁₂garnet), bismuth germanate and mixtures thereof.

23. A scintillator array in accordance with claim 21 wherein said reflective material between said parallel sintered rods is also sintered.

24. A scintillator array in accordance with claim 23 wherein said sintered reflective material comprises a powder selected from the group consisting of titanium dioxide, aluminum oxide, barium sulfate, and mixtures thereof.

25. A computed tomographic imaging system comprising:

a rotating gantry;

a detector array on said rotating gantry; and

an x-ray source on said rotating gantry opposite said detector array and configured to direct a radiation beam through an object towards said detector array;

said detector array comprising a plurality of sintered scintillator elements that produce light in response to x-ray radiation, and said detector array configured to convert said light to electrical signals to represent an intensity of said x-ray beam.