Radiant energy – Source with recording detector – Using a stimulable phosphor
Reexamination Certificate
2000-06-16
2002-12-17
Epps, Georgia (Department: 2873)
Radiant energy
Source with recording detector
Using a stimulable phosphor
C250S581000
Reexamination Certificate
active
06495850
ABSTRACT:
FIELD OF THE INVENTION
BACKGROUND OF THE INVENTION
Radiation image recording systems wherein a radiation image is recorded on a photostimulable phosphor screen by exposing said screen to image-wise modulated penetrating radiation are widely used nowadays.
Screens which are suitable for this application comprise e.g. a BaFX: Eu
2+
phosphor or a divalent europium activated cesium halide phosphor wherein said halide is at least one of chloride and bromide.
The recorded image is reproduced by stimulating the exposed photostimulable phosphor screen by means of stimulating radiation and by detecting the light that is emitted by the phosphor screen upon stimulation and converting the detected light into an electrical signal representation of the radiation image.
In a specific embodiment light emitted by the phosphor screen upon stimulation is detected by means of an array of charge coupled devices. In order to obtain a good collection efficiency the light emitted by the phosphor screen upon stimulation is guided by means of a light guide onto the array of charge coupled devices.
In one embodiment this light guide is implemented in the form of a fibre optic plate. A fibre optic plate consist so a number of of juxtaposed optical fibres that together form a two-dimensional light guiding array. The first dimension of the array corresponds with the length of a scan line on the photostimulable phosphor screen while the second dimension covers the width of the array of transducer elements. In this way the light emitted when stimulating a scan line on the photostimulable phosphor screen is guided onto the array of transducer elements in a point-by-point like fashion.
The light which is used for stimulating the phosphor screen has to be separated from the light emitted by the screen upon stimulation.
An easy way to obtain separation of stimulating light from emission light is to use an optical filter in between the light input face of the fibre optic plate and the phosphor screen.
Coloured glass filters are widely used for this purpose.
Optimal resolution is obtained when the fibre optic plate is in close contact with the phosphor screen.
However, coloured glass filters are rather thick. The provision of a coloured glass filter in between the fibre optic plate and the phosphor screen is incompatible with the requirement of having close contact between the fibre optic plate and screen.
In case a CsBr:Eu phosphor is used, the stimulating light source is a light source emitting in the range of 600 to 800 nm. The filter should absorb the laser light to a large extent while at the same time the absorption of the light emitted upon stimulation, which has a maximum at 440 nm, should be minimal.
Optimally the optical density of the filter at the stimulation wavelength range should be at least 8 while the transmission at the emission wavelength should be higher than 75% resulting in a density equal to or less than 0.12. Reaching an optical density of 8 means that the laser light is attenuated with factor 10
7
(Absorption=99.999999%)
To achieve these specifications by means of a glass filter such as a BG 39 Schott filter, the thickness of the filter should at least be 7 mm.
In case of a read out apparatus as described higher wherein the light emitted by the phosphor screen is guided to the array of transducer elements via a fibre optics plate, the gap between the input face of the fibre optic plate and the phosphor screen can only be approximately 100 micrometer in order to attain high resolution. It is thus clear that a glass filter is unsuitable for this application.
In case the fibre optic plate is replaced by an array of microlenses or a selfoc array this gap would attain a value in the range of 2 to 3 mm. Even in this case the use of a glass filter as described higher would be impossible.
OBJECTS OF THE INVENTION
It is an object of the present invention to provide a compact system for reading a radiation image that has been stored in a photostimulable phosphor screen wherein stimulation light and light emitted by the screen upon stimulation are optically separated.
Further objects of the invention will become apparent from the description hereafter.
SUMMARY OF THE INVENTION
The above mentioned objects are realised by a system for reading a radiation image that has been stored in a photostimulable phosphor screen comprising
at least one source of stimulating radiation,
an array of transducer elements arranged for detecting light emitted upon stimulation and for converting said light into a signal representation of said image,
filtering means for preventing light emitted by said source of stimulation light from being detected by said transducer elements, wherein
said phosphor screen comprises a divalent europium activated cesium halide phosphor wherein said halide is at least one of chloride and bromide, and
said filtering means comprises a dye.
In a specific embodiment said filtering means comprise a dye that has an absorption spectrum having an absorption peak within the range of 600 to 800 nm, the maximum of said peak attaining a value corresponding with at least 99% absorption, and the absorption in the range of 400 to 500 nm being less than 25%.
In a specific embodiment the dye is dissolved in a binder such as a gelatine, a lacquer such as Mowilith CT5 (Hoechst) etc.
A filter based on a dye-gelatine composition is preferred because the filter can be made very thin so that it can be placed in between the light input face of the light guiding means and the phosphor screen. The light input face of the light guiding means is the face where the light emitted by the phosphor screen upon stimulation enters the light guiding means.
The composition of dye and binder can be self-supporting. Otherwise it can be provided on a support such as a PET support.
Alternatively it is also possible to adhere the mixture of gelatine and dye to the light entrance face of the light guiding means.
Still alternatively the composition of dye and gelatine can be coated onto the photostimulable phosphor screen itself.
In on embodiment according to the present invention a divalent europium activated cesium halide phosphor screen wherein said halide is at least one of chloride and bromide is used. Such a phosphor is known in the art and has for example been disclosed in EP-A-174 875 (and U.S. Pat. No. 5,028,509). The phosphor is especially well suited for manufacturing ‘binderless’ phosphor screens. Binderless phosphor screens provide optimal sharpness.
It is advantageous however to use a CsX:Eu phosphor wherein X represents a halide selected from the group consisting of Br and Cl, which is obtained by the following method:
mixing CsX with between 10
−3
and 5 mol % of a Europium compound selected from the group consisting of EuX′
2
, EuX′
3
and EuOX′, X′ being a member selected from the group consisting of F, Cl, Br and I,
firing the mixture at a temperature above 450° C.
cooling said mixture and
recovering the CsX:Eu phosphor.
A phosphor that has been obtained as a result of the above method of preparation has an increased conversion efficiency compared to the state of the art divalent europium activated cesium halide phosphor. The phosphor can be stimulated by means of a lower amount of stimulation energy.
A photostimulable phosphor screen using such a phosphor is preferably obtained by the method of
preparing said CsX:Eu phosphor by firing a mixture of said CsX with between 10
−3
and 5 mol % of an Europium compound selected from the group consisting of EuX′
2
, EuX′
3
and EuOX′, X′ being a halide selected from the group consisting of F, Cl, Br and I and
applying said phosphor on a substrate by a method selected from the group consisting of physical vapor deposition, thermal vapor deposition, chemical vapor deposition, radio frequency deposition and pulsed laser deposition.
This method of preparation is advantageous because it allows to deposit the phosphor in the form of needle-shaped crystals. These needle-shaped phosphor crystals act as light
Leblans Paul
Struye Luc
Agfa-Gevaert
Epps Georgia
Harrington Alicia
Hoffman, Warnick & D'Alessandro
Merecki John A.
LandOfFree
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