X-ray detector module

Details X-ray detector module X-ray detector module

2001-11-26

2004-08-31

Hannaher, Constantine (Department: 2878)

Radiant energy

Invisible radiant energy responsive electric signalling

With or including a luminophor

Reexamination Certificate

active

06784432

ABSTRACT:

The invention relates to an X-ray detector module which includes a carrier that forms cells arranged in the form of a grid and is made of a material that is essentially non-transparent to X-rays. The invention also relates to a method of manufacturing such an X-ray detector module.
X-ray detectors that are used for medical applications and for non-destructive testing of materials generally are manufactured as structured X-ray absorbers and detectors that are associated with the individual pixels of the X-ray absorber. In X-ray absorbers the photons of the X-rays to be detected are absorbed by the scintillator material so as to be re-emitted in the form of visible or ultraviolet light. The photons of this light can then be detected in the detectors and spatially associated with the corresponding pixel of the X-ray absorber. The detectors may be, for example, photodiodes, avalanche diodes or photomultipliers.
Notably in the case of large-format X-ray detectors of the kind set forth, however, the manufacture of the detector field that is subdivided into pixels is very intricate. Depending on the relevant application, the surface area of such an X-ray detector may amount to between 100×3 cm
2
(computed tomography apparatus) or to 30×40 cm
2
(fluoroscopy, radiology). The size of the individual pixels is then in the range of from 0.03 to 30 mm
2
. The number of pixels in each detector is in the range of from one thousand to several millions of pixels.
In the case of a small number of detector pixels it is possible in principle to construct the detector from individual scintillator crystals that act as X-ray absorbers and absorber plates that are provided between the crystals. The absorber plates serve to structure the X-ray detector in individual cells and to prevent optical cross-talk and cross-talk between the cells due to X-ray fluorescence quanta. The cells that are defined by the absorber plates thus define the size of a pixel in the X-ray detector module.
Up to an order of magnitude of some thousands of pixels it is in principle possible to manufacture structured scintillator crystals by cutting large wafers from scintillator crystals and by combining the individual scintillator rods alternately with absorber materials such as, for example, reflecting coated metal foils, so as to form one-dimensionally structured arrays. Two-dimensional structuring is also possible in this manner, be it that a second manufacturing step is then required. The manufacture of such arrays, therefore, is very prone to faults and very expensive. Therefore, this method cannot be used in a practical situation involving a large number of individual pixels of a size that is less than 1 mm.
U.S. Pat. No. 5,981,959 discloses an X-ray detector module in which a special shape is imparted to a mixture of a binder, a solvent and scintillator particles by means of a molding technique, thus forming columns of scintillator material that extend at right angles from a base surface of the material. After absorption of an X-ray quantum in such a column, one or more light quanta of a longer wavelength are isotropically emitted, many of said quanta being conducted, by reflection on the wall of the column, in the direction of a detector element that is associated with the column and in which the quanta are detected. However, considerable deflection and scattering of the light quantum may occur along the path from the column of the scintillator material to the detector element, so that the quantum can be assigned to its origin in space with a poor accuracy only. Furthermore, the manufacture of such detector modules is very intricate and hence expensive because of the complex etching or other methods of forming the columns of the scintillator material.
Considering the foregoing it is an object of the invention to provide an X-ray detector module that can be economically manufactured and enables detection of the X-rays with a suitable spatial resolution.
This object is achieved by means of an X-ray detector module as disclosed in the characterizing part of the claims
1
,
2
and/or
11
, as well as by means of a method as disclosed in the characterizing part of the claim
12
. Advantageous embodiments are disclosed in the dependent claims.
The X-ray detector module thus includes a carrier which forms cells that are arranged in the form of a grid and is made of a material that is essentially non-transparent to X-rays. The cells are adjacently arranged in a row or in several rows and form a one-dimensional or two-dimensional structure of the X-ray detector module. The cells contain a mass that contains a binder in which scintillator particles are embedded, the scintillator particles emitting light in the range of a longer wavelength &lgr; after absorption of an X-ray quantum.
In a first embodiment of the X-ray detector module in accordance with the invention the materials for the binder and for the scintillator particles are chosen to be such that the difference between the optical refractive index n
s
(&lgr;) of the scintillator particles for the wavelength X and the optical refractive index n
B
(&lgr;) of the binder for the wavelength X amounts to less than 20% and preferably less than 10%. Preferably, the refractive indices of the scintillator particles and the binder are essentially the same.
An X-ray detector module of this kind can be economically manufactured and at the same time enables high resolutions to be achieved, that is, smaller pixel sizes. This is possible because of the use of a powder of scintillator particles or of a scintillator liquid whereby the cells that are formed in a carrier can be simply filled. Therefore, it is no longer necessary to cut scintillator crystals into small parts and to enclose such parts individually with an absorber wall. However, it is to be noted that the scintillator particles are capable of scattering the re-emitted light photons, leading to multiple reflection and partial absorption at the scintillator crystals and at the walls of the cell, and hence to erasure of the signal, that is, notably in the case of large slice thicknesses of the scintillator mass. These problems are solved in the described X-ray detector module by limiting the difference between the refractive indices of the scintillator particles and the binder to a maximum of 20%. It has been found that when this limit is observed, said disturbing effects can be kept sufficiently small. The smaller the difference between the refractive indices, the smaller the disturbing effects and the higher the signals of the X-ray detector module can be.
The binder preferably contains a TiO
2
component which is present notably in the form of rutile or anatase and/or a component of ZnO, ZnS, ZrO
2
, BaSO
4
and/or PbCO
3
. The percentage of these substances in the binder material preferably amounts from 50% by weight to 95% by weight. Said substances make it possible to select an organic substance, such as notably a polymer, as the basic substance for the binder and to adjust the refractive index of the resultant mixture (by choosing correspondingly large percentages of the additives such as, for example, of TiO
2
), to be so high that it is as near as possible to the refractive index of the scintillator particles.
In a further embodiment of the X-ray detector module in accordance with the invention, including a carrier that forms cells and a mixture of a binder and scintillator particles that is provided in the cells as described above, the scintillator particles have a grain size of less than 200 nm but preferably less than 100 nm, and in particular preferably less than 50 nm. The grain size of the scintillator particles may notably between 1 and 50 nm. In that case the scintillator particles are so-called nano-particles. The use of such nano-particles makes it possible to keep the disturbing scattering of re-emitted light photons in the scintillator mass of the cells to be kept so small that a suitable signal level is obtained for the X-ray detector module, despite a large layer thickness of the scintillato

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