Radiant energy – Source with recording detector – Using a stimulable phosphor
Reexamination Certificate
2001-03-01
2004-03-16
Hannaher, Constantine (Department: 2878)
Radiant energy
Source with recording detector
Using a stimulable phosphor
C250S484400
Reexamination Certificate
active
06707057
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to storage phosphor screens and to a method for recording and reproducing X-ray images by the use thereof.
BACKGROUND OF THE INVENTION
A well-known use of storage phosphors is in the production of X-ray images. In U.S. Pat. No. 3,859,527 method for producing X-ray images with a photostimulable phosphor, which being incorporated in a panel is disclosed. The panel is exposed to incident pattern-wise modulated X-ray beam and as a result thereof the phosphor temporarily stores energy contained in the X-ray radiation pattern. At some interval after the exposure, a beam of visible or infra-red light scans the panel to stimulate the release of stored energy as light that is detected and converted to sequential electrical signals which are can be processed to produce a visible image. For this purpose, the phosphor should store as much as possible of the incident X-ray energy and emit as little as possible of the stored energy until stimulated by the scanning beam. This is called “digital radiography” or “computed radiography”.
The image quality that is produced by any radiographic system using phosphor screen, thus also in a digital radiographic system, depends largely on the construction of the phosphor screen.
Several ways and means to provide storage phosphor screens—and/or methods for using them—combining high speed with high resolution and low noise have been proposed. In, e.g., U.S. Pat. No. 4,585,944 it is disclosed to provide a storage phosphor screen that carries, on the side of the screen where the stimulating rays are entering the screen, one or more layers having an average thickness (d
av
) such that, for each of the layers present, the product of d
av
with the refractive index of the layer is larger than 1.05 times the wavelength of the stimulating rays. It is shown in that disclosure that especially, when a transparent protective layer is adhered to the phosphor layer by means of an adhesive layer, the thickness of the adhesive layer should also fulfil the condition mentioned above.
In, e.g., EP-A-233 497 a radiation image storage panel is disclosed, comprising a phosphor layer which contains a stimulable phosphor, characterised in that one surface of said phosphor layer is provided with a mullet-layer optical filter which has a transmittance of not less than 70% for the light of the stimulation wavelength of said stimulable phosphor and at an incident angle in the range of 0-5 degrees and has a reflectance of not less than 60% for the light of said stimulation wavelength and at an incident angle of not smaller than 30 degrees.
In EP-A-440 853 a luminescent storage screen is disclosed for storing latent x-ray images, said storage screen being read-out by excitation with stimulating radiation having a first wavelength, said storage screen comprising
a stimulable phosphor in which said x-ray image is latently stored which is reactive to said radiation of first wavelength to emit radiation of a second wavelength, and
at least one optical layer coating a surface of said stimulable phosphor layer for reducing reflections at least of said radiation having said first wavelength and being highly transmissive at least for said radiation having said first wavelength.
In U.S. Pat. No. 5,877,508 a radiation image storage panel is disclosed, comprising a substrate and a stimulable phosphor layer, which is overlaid on the substrate constituted of a material, which transmits the light emitted by the stimulable phosphor layer and absorbs and/or scatters light having wavelengths falling within a stimulation wavelength range for the stimulable phosphor layer. Stimulation rays are shown to impinge on the screen at the side opposite to the surface and the emitted light is captured on the side whereon the stimulating rays impinge as well as on the side opposite to said first side. The light having wavelengths falling within the stimulation wavelength range is thus prevented from passing or propagating through the substrate so that it does not disturb the reading of the stimulated light on that side of the screen, and the signal-to-noise ratio of an image signal detected from the radiation image storage panel is thereby prevented from becoming low.
In EP-A-021 174 it is disclosed that the sharpness of the image produced by a storage phosphor screen comprising a support, a phosphor layer and a protective film can be enhanced when at least one of the support, phosphor layer or protective film comprises a colorant absorbing stimulating light.
In EP-A-158 862 it is disclosed that the sharpness of a phosphor screen could be further enhanced when the protective layer is adhered to the phosphor layer by an adhesive layer which comprises a colorant absorbing stimulating light.
The use of storage phosphor screens according to the disclosures above does indeed provide the possibility of creating sharp x-ray images, but it is, in X-ray imaging, an everlasting desire to further increase the sharpness of an image.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the invention to provide a method for recording and reproducing images made by penetrating radiation that have high resolution, especially in the higher frequencies.
It is a further object of the invention to provide a storage phosphor screen for use in a method for recording and reproducing images made by penetrating radiation that have high resolution, especially in the higher frequencies.
The object of the invention is realised by providing a method for reading a radiation image stored in a photostimulable storage phosphor screen
stimulating said photostimulable phosphor screen to release stimulated light and
collecting said stimulated light characterised in that:
said phosphor screen has a transparent outermost layer with a thickness, d, higher than 150 &mgr;m and
both said stimulating and said collecting proceed through said transparent outermost layer.
The further object of the invention is realised by providing a stimulable phosphor screen having a transparent outermost layer with a thickness, d, higher than 150 &mgr;m.
Preferably said thickness of said outermost layer is such that 150<d≦4000 &mgr;m.
More preferably said outermost layer comprises a colorant absorbing the stimulating light.
Further advantages and embodiments of the present invention will become apparent from the following description.
DETAILED DESCRIPTION OF THE INVENTION
In this document the term “penetrating radiation” is used to include i.a. radiation originating from a radioisotope (e.g. a Co60 source), radiation created by an X-ray generator of any type, radiation and high energy particles created by a high energy radiation generator (e.g. Betatron), radiation from a sample labelled with a radioisotope as is the case in e.g. autoradiography.
It was now found that the sharpness of an image—produced by image wise absorbing penetrating radiation in a storage phosphor screen and stimulating the screen for releasing the stored energy—could be enhanced, especially in the portions with higher spatial frequency (i.e. >2 lp/mm) when the stimulation of the energy stored in the storage screen and the reading of the stimulated light proceeded via a thick outermost layer, i.e. a transparent layer with a thickness, d, larger than 150 &mgr;m. Preferably said thickness is such that 150 &mgr;m<d≦4000 &mgr;m. Even more preferably, said thick outermost layer has a thickness, d, such that 150 &mgr;m<d≦2500 &mgr;m. Even when the stimulation of the stored energy and reading the amount of stimulated light proceeds through a thick (thickness 180 &mgr;m), non-coloured layer, the beneficial effect on sharpness in the region of higher frequencies is seen. In that case the square wave response was higher from a spatial frequency of 4 lp/mm on, when compared to the stimulating and reading through a thin (thickness 10 &mgr;m) non-coloured transparent layer, at frequencies lower than 4 lp/mm the square wave response when stimulating and reading through the thick layer was lower.
Preferably the thick outermost layer, through
Cresens Marc
Willems Peter
Agfa-Gevaert
Breiner & Breiner L.L.C.
Hannaher Constantine
Lee Shun
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