Radiant energy – Invisible radiation responsive nonelectric signalling – Luminescent device
Reexamination Certificate
1999-11-08
2002-01-15
Epps, Georgia (Department: 2873)
Radiant energy
Invisible radiation responsive nonelectric signalling
Luminescent device
C250S487100, C250S483100
Reexamination Certificate
active
06339224
ABSTRACT:
TECHNICAL FIELD
The present invention relates to intensifying screens employed in X-ray radiography or the like, radiation receptors therewith, and radiation inspection devices therewith.
BACKGROUND ART
In X-ray radiography employed in medical diagnosis and non-destructive inspection for industrial purpose, in general intensifying screens are used in combination with X-ray film to enhance system sensitivity. An intensifying screen is generally formed by sequentially forming a phosphor layer and a relatively thin protective film on a support consisting of paper or plastic.
In recent years, reduction of subject's exposure to radiation in medical diagnosis or the like is strongly demanded. In order to cope with this demand, in X-ray radiography, high-speed X-ray films or high-speed X-ray intensifying screens are used to reduce subject's exposure. In order to enhance sensitivity of X-ray film, high speed X-ray films are generally used. In order to enhance sensitivity of intensifying screens, phosphors of high emission efficiency are employed.
When X-ray films or intensifying screens are made highly sensitive, there occur the following problems. That is, when the high-speed X-ray films are employed, though lowering of sharpness is small, granularity is deteriorated. By contrast, when the high-speed intensifying screens are employed, there also occurs deterioration of granularity. Recognizability of a subject in X-ray radiography involves both of granularity and sharpness. Deterioration of granularity deteriorates in particular the recognizability of subjects of low contrast.
From the above, with an object to improve image quality of intensifying screens, various improvements of phosphor layers have been attempted. For instance, when a phosphor layer is produced by the use of a kind of settling method named “Ryuen Hou” in Japanese, a phosphor layer of which particle size distribution becomes smaller from the protective film side toward the support side, a structure in which particle size is graded can be obtained (Japanese Patent Publications (KOKOKU) No. Sho 55-33560 and No. Hei 1-57758). This kind of structure of phosphor layer can enhance speed and sharpness of intensifying screens.
However, the aforementioned intensifying screens of structure of graded particle size distribution are produced by drying solvent while letting settle phosphor particles in phosphor slurry by the use of gravity. Accordingly, it takes long time for produce to result in pushing up the production cost. In Japanese Patent Publications (KOKOKU) No. Sho 55-33560 and No. Hei 1-57758, a structure of multi-layers of phosphors of different particle sizes is disclosed. These patent publications disclose only examples of the structure of graded particle size distribution but does not disclose detailed conditions of each phosphor layer or the like.
By contrast, Japanese Patent Laid-open Publication (KOKAI) No. Sho 58-71500 discloses an intensifying screen in which the surface side of a phosphor layer thereof is constituted of larger phosphor particles of an average particle diameter of 7 to 20 &mgr;m, and interstices of the larger phosphor particles and support side thereof are constituted of phosphor particles of an average particle diameter of 4 &mgr;m or less. According to such an intensifying screen, sensitivity and sharpness can be improved by some degree. However, granularity can not be sufficiently improved.
In Japanese Patent Laid-open Publication No. (KOKAI) Hei 8-313699, there is disclosed an intensifying screen having a plurality of phosphor layers the support side of which layers is composed of phosphor particles of smaller average particle diameter. Each phosphor layer of this intensifying screen, when each average particle diameter of phosphor particles constituting each phosphor layer is R and particle size distribution thereof is &sgr;, satisfies a relation of 0<&sgr;/R≦0.5, respectively. Furthermore, in this patent publication, among the plurality of phosphor layers, the phosphor layer of the protective layer side has an average particle diameter of 10 to 20 &mgr;m and the phosphor layer of the support side has an average particle diameter of 1 to 5 &mgr;m.
Thus, in an intensifying screen having a plurality of phosphor layers, when particle size diameters of phosphor particles constituting the respective phosphor layers are stipulated similarly, sufficient improvement of sharpness and granularity is not necessarily obtained. By the experiments carried out by the inventors, it has been found that when a plurality of phosphor layers is composed of a plurality of phosphor particles of different average particle diameters, according to average particle diameters of the respective phosphor layers, various kinds of conditions have to be set.
As mentioned above, high speed intensifying screens due to the use of phosphors of high emission efficiency can be effective in reduction of subject's exposure and in improvement of sharpness, however, cause a problem of deterioration of granularity. On the contrary, when phosphors of low emission efficiency are used, the granularity can be improved but the sharpness deteriorates. Thus, there is a certain degree of reciprocity between radiographic performance.
As to such problems, existing intensifying screens having a structure composed of single phosphor layer can not satisfy both of granularity and sharpness. The intensifying screens having a structure of graded particle diameter distribution are relatively satisfactory with respect to speed and sharpness. However, it takes longer time for formation of phosphor layer to result in pushing-up of manufacturing cost and at the same time due to fluctuation of manufacturing conditions, large performance variation is invited. Further, in the existing intensifying screens having a plurality of phosphor layers of different average particle diameters, the sharpness and granularity have not been sufficiently improved.
In contrast, radiation is used not only for radiography of medical diagnosis but also for treatment of subjects. A device for radiotherapy is one in which a high energy X-ray beam of approximately 4 MV obtained from a linear accelerator called linac is irradiated to an subject to cure. Before beginning treatment with a device for radiotherapy, in order to confirm reproducibility of a portion being exposed that is set by treatment program, radiography or TV imaging is carried out with the beam being used for treatment.
However, there is a problem that in the aforementioned high energy X-rays, when an X-ray image is taken with an ordinary intensifying screen after transmission of X-rays of a subject, sufficient contrast can not be obtained. To this end, so far, a fluorometallic screen that is composed of integration or superposition of an ordinary intensifying screen and a metallic plate such as lead alloy foil or copper plate, and medical X-ray film or industrial X-ray film are combined to employ. Silver halide in film emulsion has the maximum of spectral sensitivity at 45 kV. Accordingly, a high energy X-ray beam of 1 MV or more is absorbed less to result in poor efficiency. This is the reason why the fluorometallic screen has been employed.
A fluorometallic screen is composed of a phosphor layer of such as CaWO
4
in contact with a lead alloy foil, for instance. In such a fluorometallic screen, after appropriate absorption of a high energy X-ray beam at the lead alloy foil, a sensitizing effect due to emission of phosphor, an elimination effect of scattered X-rays due to the metallic foil, a sensitizing effect of phosphor due to secondary electrons due to Compton scattering or the like can be obtained.
However, there is a problem from an environment point of view as to handling of foils of lead alloy. Other than this, plate of heavy metal such as tungsten has been taken up. However, tungsten plate is much expensive that there is a problem when being put in practice. In contrast, a fluorometallic screen employing copper plate is small in X-ray absorption, that is, insufficient in absorpti
Oyaizu Eiji
Saito Akihisa
Takahara Takeshi
Epps Georgia
Hanig Richard
Kabushiki Kaisha Toshiba
LandOfFree
Radiation intensifying screen and radiation receptor and... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Radiation intensifying screen and radiation receptor and..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Radiation intensifying screen and radiation receptor and... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2845954