Patent Grant
6974959
The invention relates to a flat storage element for an X-ray image.
Storage elements of this kind are obtainable commercially as so-called storage films.
With such storage films the storage layer formed by storage particles and a binding agent matrix is optically inhomogeneous, and there occurs as a result of said inhomogeneities a scattering of the activating light, which is used for reading out the latent image, and also a scattering of the measuring light read out. The resolution of the storage element is consequently influenced disadvantageously.
The above-mentioned scatter effects are the stronger the smaller the storage particles are. Small storage particles are conversely advantageous, however, with respect to high resolution of the storage element.
There is therefore to be created by the present invention a storage element which is optically homogeneous, so that no scattering of activating light and measuring light takes place in the storage layer.
This object is achieved according to the invention by a storage element for an X-ray image, with a large number of storage particles which may be placed by means of X-ray light in metastable excitation states that are convertible by irradiation with activating light into an unstable excitation state which is in turn decomposed with the radiation of fluorescent light, and with a transparent binding agent by means of which the storage particles are held together to form a storage layer, wherein the binding agent and the storage particles have substantially the same refractive index.
With the storage element according to the invention the refractive indices of the storage particles on the one hand and of the binding agent on the other are adjusted to one another. The optical inner boundary surfaces at which the scattering of activating light and measuring light takes place therefore disappear. The whole of the storage layer behaves optically like a single-component material.
An improved resolution is thus obtained with the storage element according to the invention.
If different salts crystallising together are used for the storage particles, the refractive index may be adjusted simply within very wide limits. It is possible by corresponding variation of the ratio in which the two salts are provided to cover a wide range of binding agent refractive indices, to attain exactly the refractive index of a predetermined binding agent.
A refractive index of preferably between 1.4 and 1.6 is selected for the binding agent according to a preferred embodiment of the invention. A large number of different salt compositions is then available with which said range of the refractive index may be realised, so that a selection may be made from said large number in terms of other parameters to be included, e.g. the size of the particular unit cell of the salt which influences the preferred excitation wavelength of the colour centres formed.
If an isotropic binding agent and isotropic storage particles are used, this also prevents small residual scattering of the light, such as would be caused by an anisotropic material.
An anit-reflection coating borne by the front side of the storage layer prevents a deterioration in the resolution, such as would be obtained by reflections on the front boundary surface of the storage layer viewed in the direction of motion of the light.
With an absorbing layer arranged on the rear side of the storage layer, reflections of activating light on the rear side of the storage layer are eliminated. A further improved spatial resolution of the X-ray image read out is thereby obtained.
With a storage element the rear side of which is provided with a reflecting layer the yield of fluorescent light is improved, since the light radiated into the rear half-space is reflected towards the front side. The sensitivity of the storage film is improved by a factor of 2 in this way.
A storage element in which a protective layer of material absorbing X-ray beams is arranged behind the storage layer is of advantage in terms of minimising the radiation load on a patient whose teeth are X-rayed with a storage element held behind the jaw.
If such protective layer is firmly connected to the storage layer, this is of advantage in terms of a simple handling of the storage element. The whole of the storage element may thus also be bent without fold formation.
A storage element forming a bendable layered structure may be adapted effectively to curved surfaces, e.g. the curvature of a jaw.
If a storage element is produced by preparing a binding agent in the liquid state, dispersing storage particles in the liquid binding, dispersing the material obtained in this way to form a thin film-type layer and then curing the binding agent, this method ensures that the binding agent also fits exactly positively in microscopic terms around the storage particles. No small air inclusions or cavities therefore arise, which in turn could again represent scatter centres.
The invention will be explained in detail below from embodiments with reference to the drawing. In the latter:
The reflective layer may be formed by a corresponding interference layer. It may also for its part be produced from two sub-layers lying one behind the other, e.g. a front sub-layer, which is responsible for the reflection of the fluorescent light, and a second, rear sub-layer, which absorbs the laser excitation light.
A metal such as aluminium may be used for the reflecting sub-layer. Said layer may then simply be vapour-deposited onto the rear side of the storage layer 12. Instead of this it is also possible to use a diffusely reflecting powder layer as reflecting sub-layer, which consists e.g. of BaSO4 powder. BaSO4 is characterised by a particularly high reflection factor for light of the wavelengths of interest here.
The various layers are connected to form a one-piece layered structure, wherein the connection between the storage layer 12 and the anti-reflection coating 14 or the absorbing layer 16 is obtained by in-situ application of the two last-mentioned layers, e.g. by evaporation or by printing on of a corresponding ink and vaporising of the solvent etc. The lead film 18 may be connected to the rear side of the absorbing layer 16 by a thin layer of adhesive.
The storage layer 12 comprises a large number of storage particles 20 which are shown simplified in the drawing as small spheres, but in reality have an irregular geometry such as is obtained by the fine grinding of salt. The storage particles 20 are held together by a transparent binding agent 22 which is preferably a transparent organic binding agent that is selected from the group given in Table 1 below:
The refractive index for the above-mentioned plastics for visible light is shown in
In
The storage particles 20 consist of a material in which metastable excited states are generated by interaction with impinging X-ray beams. Said metastable states have typically a life of at least a few minutes. Because activating light is irradiated into the absorption bands of said metastable states, an unstable excited state may be obtained, which then passes into the ground state with the emission of fluorescent light.
Suitable metastable states are based in practice on defects in the crystal lattice, which are formed inter alia by lattice defects or else impurity atoms. Thus in alkali halide crystals, for example, anion defects may store electrons metastably, which are accelerated during the X-ray absorption, and form so-called colour centres. Holes may form metastable states in said metals in V-centres or on impurity atoms.
The capacity to generate a latent X-ray image in the storage layer 12 is attributable to the colour centres of the storage particles 20. The refractive index which the activating light sees or the fluorescent light triggered by the latter sees, depends first and foremost on the macroscopic refracting angle index of the storage particles 20 or of the binding agent 22.
Because the two refractive indexes are adjusted to one another, the scattering of the activating light and of the fluorescent light, which is generated by emptying of a metastable state with the use of activating light, is prevented. The fluorescent light detected with a photodetector, which forms part of a reproduction device for latent X-ray beams, may therefore be correlated precisely with the radiated point-by-point read-out surface of the storage element.
The adjustment of the refractive indices of storage particles 20 and binding agent 22 may in the case of alkali halides be produced within wide limits by specific choice of the basic material for storage particles 20. Table 2 below gives an overview of the refractive indices of pure alkali halides:
Since the alkali halides are all miscible with one another over a wide range (same crystal class), the refractive index of the mixed crystal obtained may be varied within wide limits by the mixing of two different salts. If, for example, a mixed crystal of KCl and RbBr is considered and the composition of the mixed crystal is written as KxRb1-xClyBr1-y, where x and y each lie in the range between 0 and 1, there is obtained with varying of x and y between 0 and 1 a range of adjustment of the refractive index of 1.490 to 1.559.
If defects are formed in said mixed crystal, e.g. by the addition of 0.1 mol % Tl+, because of the small concentration, the doping has only a small effect on the refractive index of the mixed crystal of not more than 0.1%.
A second means of securing the adjustment of the refractive index is the selection of the binding agent, wherein different refractive indices are obtained for different binding agents in accordance with the nature of the monomers. For some of the binding agents the refractive index may again be varied within a range by influencing the chain length and the cross-linking. This is discernible from the representation of the refractive index for various plastics materials which is reproduced in
Typically the diameter of the storage particles comes to about 10 μm, the thickness of the storage layer to 100 μm.
It is further seen from
In terms of the robustness of the storage element and in terms of a manufacturability of the storage elements at not excessively high temperatures, organic binding agents are preferred.
The anti-reflection coating is produced in the conventional manner, e.g. by the evaporation of material with suitable refractive index and in suitable thickness. The absorbing layer 16 is manufactured of a material absorbing the laser light used for the reading out of the latent image and may likewise be vapour-deposited or printed on as ink.
In
Because the refractive index of storage particles 20 and binding agent 22 are adjusted to one another, said phase objects disappear and there is obtained for the storage element the appearance reproduced in
As already mentioned above, the storage particles have in reality the shape of ground material with small bevels. In order also to obtain an embedding of the storage particles in the binding agent which is free of microscopic cavities, the following procedure is adopted during the production of the storage layer 12:
Binding agent 22 is prepared in the liquid state. The storage particles 20 are distributed homogeneously in the liquid binding agent 22. The material obtained in this way is brushed out to a thin layer and the binding agent is then cured, so that a storage film with corresponding thickness is obtained.
The binding agent is further preferably prepared in the highly liquid state, to which end it is diluted and/or heated.
| Number | Date | Country | Kind |
|---|---|---|---|
| 198 59 880 | Dec 1998 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP99/09250 | 11/29/1999 | WO | 00 | 6/22/2001 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO00/39805 | 7/6/2000 | WO | A |
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