Radiant energy – Invisible radiation responsive nonelectric signalling – Luminescent device
Patent
1996-12-30
1999-03-23
Hannaher, Constantine
Radiant energy
Invisible radiation responsive nonelectric signalling
Luminescent device
2504861, G21K 400
Patent
active
058863540
DESCRIPTION:
BRIEF SUMMARY
1. FIELD OF THE INVENTION
The present invention relates to a single sheet photostimulable phosphor screen suited for dual energy recording of an X-ray pattern containing X-rays of different energy level.
2. BACKGROUND OF THE INVENTION
Substances showing the phenomenon of luminescence under the influence of X-rays are called phosphors.
According to a classical method of recording and reproducing X-ray images X-ray absorbing phosphor particles are used in phosphor panels or screens to form radiographs in photographic silver halide emulsion materials sensitive to the fluorescent light promptly emitted by the phosphors where struck by X-rays.
In another radiographic method the principles of which are disclosed in U.S. Pat. No. 3,859,527 photostimulable phosphors are used. Said method comprises the exposure to X-rays of a photostimulable phosphor panel which phosphor stores at least part of the energy of the incident X-rays. 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 into sequential electrical signals which are processable to produce a visible image.
According to the periodical Radiology, September 1983, p. 834 the imaging plate containing a photostimulable phosphor can be used repeatedly to store X-ray images simply by flooding it with light in order to erase the residual energy contained.
In the medical X-ray field it is often necessary to provide information on soft tissue structures and bone-like structures simultaneously. For that purpose "dual-energy" techniques operating in a single shot X-ray exposure with a low energy absorbing front screen and a high energy absorbing back screen separated by X-ray beam hardening filter have been used in classical screen-film assemblage exposure as well as in X-ray imaging with photostimulable storage phosphor screens (ref. Dev P. Chakraborty and Gary T. Barnes in Med. Phys. 16 (1). Jan/Feb 1989. p. 7).
The production of low- and high-energy images that are acquired simultaneously avoids image misregistration between the low- and high-energy exposures that could occur by patient-motion.
For the front screen preferentially absorbing the low-energy X-ray photons, useful in the imaging of the structure of soft tissue, low atomic number phosphors are used, whereas the higher energy part of the X-ray beam giving more information on the bone structure is absorbed or stopped in a back screen containing a higher atomic number phosphor.
In the above mentioned periodical Med. Phys. 16 (1). p. 7-8 an energy sensitive cassette for dual-energy mammography has been described wherein the back imaging plate contains a commercial barium fluorobromide phosphor and the front plate contains a similar but no commercially available strontium fluorobromide phosphor. The selection of these phosphors has been made in view of the K edges of Ba and Sr being respectively 37.4 and 16.1. The K edge of Ba ensures high absorption efficiency of the barium fluorobromide phosphor above 38 keV, whereas the strontium fluorobromide is absorbing effectively in the X-ray region with energies from 15 to 25 keV.
In order to make that the lower energy X-ray radiation cannot interfere with the Ba-type phosphor an interscreen filter, also called midfilter in the form of a copper sheet is used (ref. Medical Physics, Vol. 17, No. 4, Jul/Aug 1990, p.667).
The following phosphor combinations for classical screen-film dual energy X-ray recording were considered : SrFBr/BaFBr. Y.sub.2 O.sub.2 S/Gd.sub.2 O.sub.2 S and Y.sub.2 O.sub.2 S/LuTaO.sub.4.
In U.S. Pat. No. 5,051,589 a stimulable phosphor sheet suited for subtraction radiography is described wherein different stimulable phosphors are mixed together or take the form of a layer, wherein at least a first stimulable phosphor is represented by the formula GdOCl:Ce and a second stimulable phosphor is represented by the formula SrFCl:Eu.sup.2+. Said phosphors have different X-ray absorption characteristics which represent different absorp
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AFGA-Gevaert, N.V.
Hannaher Constantine
Jiron Darren M.
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