Radiographic intensifying screen

Details Radiographic intensifying screen Radiographic intensifying screen

1998-06-19

2001-02-13

Hannaher, Constantine (Department: 2878)

Radiant energy

Invisible radiation responsive nonelectric signalling

Luminescent device

Reexamination Certificate

active

06188073

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a radiographic intensifying screen employable for radiography.
BACKGROUND OF THE INVENTION
In a variety of radiography such as medical radiography for diagnosis, a radiographic intensifying screen is generally used in combination with a radiographic film. The radiographic intensifying screen generally comprises a support, a phosphor layer and a surface protective layer overlaid in order. Since the surface protective layer is provided to keep the phosphor layer from chemical and physical deterioration, the protective layer must have a thickness enough to protect the phosphor layer. However, if the surface protective layer is too thick, the sensitivity lowers and further the resultant image is liable to shows poor sharpness. In order to solve this problem, many studies have been done.
In a generally employable radiographic intensifying screen, a typical material for the surface protective layer is a polyethylene terephthalate film having a haze of 5 to 10.
German Patent Publication No. 3,111,831 discloses a surface protective layer containing &ggr;-alumina particles in an amount of less than 0.1 wt. %.
Japanese Patent Publication No. 60-34720 discloses a surface protective layer wherein an organic matting agent is introduced to improve slip property of its surface.
Japanese Patent Provisional Publication No. 62-137599 discloses a surface protective layer in which polymer fine particles are introduced so as to improve slip property of its surface.
Japanese Patent Provisional Publication No. H3-28798 discloses a radiographic intensifying screen which comprises a protective layer having a great number of very small convexes or concaves on its surface.
Japanese Patent Provisional Publication No. 51-127688 discloses a radiographic intensifying screen which comprises a protective layer having a great number of very small convexes of matting agent.
Japanese Patent Provisional Publication No. 53-66392 discloses that a light-scattering layer is provided between the phosphor layer and a silver halide emulsion layer so that production of black spots by radioactive isotope can be prevented.
Japanese Patent Provisional Publication No. 58-58500 discloses a radiographic intensifying screen which has a white light-scattering layer provided on the phosphor layer, and a transparent protective layer provided on the light-scattering layer.
Japanese Patent Provisional Publication No. H3-255400 discloses a radiographic intensifying screen in which metal oxides are provided between the phosphor layer and the surface protective layer so that the screen can have electroconductivity.
The known surface protective layers such as described above have been developed in consideration of protection against chemical and physical deterioration (e.g., scratch resistance, stain resistance and abrasion resistance), as well as sharpness of the resultant radiation image. However, although these known surface protective layers are improved to a certain extent, their properties are still unsatisfactory.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a radiographic intensifying screen which has good surface durability such as high stain resistance and high abrasion resistance and which gives a radiation image of high sharpness with high sensitivity.
The present invention resides in a radiographic intensifying screen comprising a support, a phosphor layer containing phosphor and a surface protective layer over-laid in order, wherein the surface protective layer shows a scattering length in the range of 5 to 80 &mgr;m, said scattering length being measured at main wavelength of light emitted from the phosphor.
The scattering length is used to mean an average distance in which light travels straight until it is scattered, and hence a small value means that the light is highly scattered. In accordance with Kubeluka-Munk theory, the scattering length can be calculated from the date obtained in the following measurement.
First, three film samples are prepared. All film samples have a thickness differing from each other, but each consists of the same components as the target surface protective layer. The thickness (&mgr;m) and the diffuse transmittance (%) of each sample are then measured. The diffuse transmittance (%) can be measured by means of a spectrophotometer equipped with an integrating sphere. In the below-described examples of the present specification, an automatic recording spectrophotometer (U-3210, manufactured by HITACHI, Ltd.) equipped with an integrating sphere of 150 &phgr; (150-0910) was used. The diffuse transmittance must be measured at a wavelength corresponding to the main peak of the luminescence (light) emitted from phosphor contained in the phosphor layer on which the target surface protective layer is provided.
From the thickness (&mgr;m) and the diffuse transmittance (%) obtained in the above measurement, the scattering length is calculated in accordance with the following formula (A) derived from Kubeluka-Munk theory. The following formula (A) can be easily derived, under the boundary condition giving the diffuse transmittance (%), from the formulas 5.1.12 to 5.1.15 described in “Keikotai Handbook [Japanese, Handbook of Phosphor]”, published by Ohm-sha, 1987, pp.403.
Formula (A):
T/
100=4&bgr;/[(1+&bgr;)
2
·exp(&agr;
d
)−(1−&bgr;)
2
·exp(−&agr;
d
)]
in which T represents the diffuse transmittance (%), d represents the thickness (&mgr;m), and &agr; and &bgr; are defined by the formulas: &agr;=[K·(K+2S)]
½
and &bgr;=[K/(K+2S)]
½
, respectively.
The formula (A) is applied to the measured T (diffuse transmittance) and d (thickness) of each film sample, and thereby the values of K and S are determined. The scattering length (&mgr;m) and the absorption length (&mgr;m) described below are values defined by 1/S and 1/K, respectively.
Preferred embodiments of the present invention are as follows.
(1) The scattering length is in the range of 10 to 70 &mgr;m, particularly 10 to 60 &mgr;m.
(2) The surface protective layer contains light-scattering fine particles having a grain size of 0.1 to 1 &mgr;m and s refractive index of more than 1.6.
(3) The surface protective layer contains light-scattering fine particles having s grain size of 0.1 to 1 &mgr;m and a refractive index of not less than 1.9.
(4) The surface protective layer contains light-scattering fine particles comprising at least one material selected from the group consisting of zinc oxide, zinc sulfide, titanium dioxide (particularly, anatase type titanium dioxide), and lead carbonate; and the particles have a mean grain size of 0.1 to 1 &mgr;m.
(5) The surface protective layer comprises a binder containing fluorocarbon resin or polyester resin and light-scattering fine particles dispersed therein.
(6) The surface protective layer has the thickness of 2 to 12 &mgr;m, particularly 3 to 9 &mgr;m.
(7) The phosphor contained in the phosphor layer is represented by the following formula:
M
2
O
2
X:Tb
in which M is at least one element selected from the group consisting of Y, Gd and Lu; and X is at least one element selected from the group consisting of S, Se and Te.
(8) The phosphor layer exhibits a scattering length of 5 to 50 &mgr;m, particularly 7 to 30 &mgr;m.
(9) A light-reflecting layer is provided between the support and the phosphor layer.
(10) The phosphor layer comprises a binder and the phosphor dispersed therein, and the weight ratio of the binder to the phosphor is in the range of 1/12 to 1/200 , particularly 1/16 to 1/100.
DETAILED DESCRIPTION OF THE INVENTION
The radiographic intensifying screen of the invention is now described in detail.
The radiographic intensifying screen of the invention has the same structure as the known intensifying screen comprising a support, a phosphor layer and a surface protective layer overlaid in this order.
The support employed in the invention can be optionally selected from those employed in the conventional radi

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