X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
1999-10-28
2001-06-26
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C250S367000, C250S370090, C250S370110
Reexamination Certificate
active
06252927
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method of manufacturing a scintillator layer for a detector for the detection of electromagnetic radiation transmitted by an object, in which a scintillator layer for converting the radiation of a first energy level into radiation of a second energy level is provided on a photosensor layer for converting such radiation into an electric current, and in which the scintillator layer, comprising a plurality of scintillator elements, is provided with intermediate layers which extend in the vertical direction along the side faces of the scintillator elements.
2. Description of Related Art
Scintillator layers of detectors which are used, for example in computer tomographs, customarily consist of cadmium tungstate [CWO] (CdWO
4
), of yttrium gadolinium oxide [YGO] (Y,Gd)
2
O
3
:E) or of gadolinium oxysulphides [GOS] (Gd
2
O
2
S:Pr). Such materials provide conversion of radiation of a first energy level into radiation of a second energy level, for example, the conversion of X-rays into visible light in the case of computer tomographs.
The international patent application WO 95/04289 describes a detector with a scintillator layer which consists of a two-dimensional array of scintillator elements, i.e. a plurality of rows of scintillator elements arranged parallel to one another. The scintillator elements are formed by monocrystals. Between the scintillator elements there are provided optical separation layers which extend along the side faces of the scintillator elements. These layers are provided with a thickness of from 0.05 to 1 &mgr;m by metal deposition. The materials used for these layers are aluminium, tungsten, molybdenum, iron, chromium, nickel, gold, silver or copper.
Since recently it is attempted to enhance the resolution, and hence the image quality, of X-ray examination devices, notably computer tomographs, by utilizing detectors comprising a larger number of detector elements or scintillator elements. This gain in respect of optical resolution, however, is accompanied by the drawback of increased crosstalk between neighboring scintillator elements, which increase is due to the crossing over of photons and X-ray quanta.
Citation of a reference herein, or throughout this specification, is not to construed as an admission that such reference is prior art to the Applicant's invention of the invention subsequently claimed.
SUMMARY OF THE INVENTION
It is an object of the present invention to propose a method of manufacturing a scintillator layer for a detector which has a higher resolution and in which the disturbing interaction between the scintillator elements is only insignificant.
This object is achieved by means of the method which is characterized as described in the claims as well as by means of the scintillator layer which is characterized as described in the claims.
The basic idea of the invention is to pour in a molten mass of a radiation-absorbing metal, having a melting point below 350° C., into the intermediate spaces between neighboring scintillator elements.
The molten mass may consist of pure radiation-absorbing metals, preferably lead, bismuth and mercury which have melting points of 327.5, 271.3 and 33.4° C., respectively. Molten masses of lead and bismuth solidify at room temperature. Mercury is liquid at room temperature. The metal is retained by layers which enclose the scintillator layer as a whole.
Preferably, use is made of metal alloys whose components can be selected from the following group of metals: bismuth and/or lead and/or zinc and/or tin and/or cadmium and/or mercury. Preferably, compositions of the components which correspond to the eutectic compounds are selected.
The described choice of metals and metal alloys offers a particularly attractive low melting point in combination with a high absorptivity. The pouring method enables the formation of thin intermediate layers which do not unnecessarily reduce the active surface area of the scintillator element. Because of the choice of metals having melting points below 350° C., chemical reactions with the scintillator crystals or damage are avoided.
The radiation-absorbing layers have two functions. First of all, they serve as an optical separation layer in that they reflect photons arising during the conversion (optical crosstalk) back to the individual scintillator elements, so that they increase the signal strength. The intermediate layers extend in the vertical direction, i.e. transversely to or perpendicularly to the surface of the scintillator layer.
On the other hand, such layers serve for the absorption of K fluorescence X-ray quanta (X-ray crosstalk). K fluorescence X-ray quanta or secondary X-ray quanta arise when the energy of the electromagnetic radiation or X-rays is not fully taken up by the scintillator elements. In the case of the described scintillator metals GOS and CWO such K fluorescence X-ray quanta amount to from 40 to 50% of the primary absorbed radiation. This is because the energy of the absorbed X-ray quanta exceeds the so-al led K-edge energy of the scintillator crystal. Only a part of the K fluorescence X-ray quanta then arising can be absorbed in the same scintillator element; a further part is emitted and a third part is absorbed by neighboring crystals. The proposed poured absorption layers prevent such X-ray crosstalk between neighboring scintillator elements.
Known layers of materials such as molybdenum offer only an unsatisfactory optical separation function of this kind in conjunction with a high absorptivity for secondary X-rays. The same also holds for known optical separation layers of titanium dioxide embedded in epoxy resin. Therefore, such materials can be used only for one-dimensional detectors with a low spatial resolution; such one-dimensional detectors involve less crosstalk of X-ray quanta in comparison with two-dimensional detectors.
The proposed pouring method enables optimum filling of the gaps between the scintillator elements. Fillings with a width of preferably 100 &mgr;m can be realized in a fast, easily reproducible and inexpensive manner. The method is, therefore, very suitable for the manufacture of scintillators composed of a large number of scintillator elements in a flat arrangement.
The pouring process is preferably performed in vacuum or in an inert gas atmosphere. This has a positive effect on the fluidity of the molten metal.
The preparation of gaps along the side faces of the scintillator elements, and hence of a pattern of recesses to be filled with molten metal, is realized on the one hand by arranging monocrystals in such a manner that a minimum distance is maintained. On the other hand, a pattern of recesses can be mechanically formed in a scintillator layer, for example by sawing by means of an appropriate tool. This method enables the formation of a large number of individual scintillator elements. After the filling of the gaps or the pattern of recesses with the molten metal, a remaining non-filled edge zone of the scintillator layer can be mechanically removed.
The proposed method of manufacturing a scintillator layer for a detector is particularly suitable for a two-dimensional scintillator layer, i.e. an array of scintillator elements with n rows and m columns, where n, m are numbers larger than
1
. The method, however, is also suitable for the manufacture of the scintillator layer of one-dimensional or linear detectors.
Assuming a two-dimensional detector or a cone beam detector provided with a scintillator array, a version of the method according to the invention is proposed in which the radiation-absorbing layers are poured into the gaps between the rows of scintillator elements whereas the radiation-absorbing layers are inserted as preformed thin layers or foils in the gaps between the columns of scintillator elements. The reverse situation is also feasible. The preformed metal layers are preferably made of lead, tantalum, tungsten or gadolinium. Whereas the insertion of preformed metal layers into the gaps
Lauter Josef
Schneider Stefan
Wieczorek Herfried K.
Dunn Drew A.
Kim Robert H.
U.S. Philips Corporation
Vodopia John F.
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