BACKGROUND OF THE INVENTION:
FIELD OF THE INVENTION:
[0001] The present invention relates to a radiographic image conversion screen. More particularly,
it relates to a radiographic image conversion screen, i.e. a radiographic intensifying
screen (hereinafter referred to simply as "intensifying screen") or a fluorescent
screen, which comprises double phosphor layers i.e. a green emitting rare earth phosphor
layer and a blue emitting phosphor layer and which has a high speed and exhibits superior
image forming characteristics (in this specification, the "radiographic image conversion
screen" includes the intensifying screen and the fluorescent screen).
DESCRIPTION OF THE PRIOR ART:
[0002] As is well known, a radiographic image conversion screen is used for medical diagnosis
and non-destructive inspection of industrial products, and it emits an ultraviolet
ray or a visible ray upon absorption of radiation passed through an object, and thus
converts a radiographic image to an ultraviolet image or a visible image.
[0003] - When the radiographic image conversion screen is used as an intensifying screen
for radiography, it is fit on a radiographic film (hereinafter referred to simply
as "film") so that a radiation image will be converted on the fluorescent surface
of the intensifying screen to an ultraviolet image or a visible image which will then
be recorded on the film. On the other hand, when it is used as a fluorescent screen,
the radiation image of the object converted on the fluorescent surface of the fluorescent
screen to a visible image may be photographed by a photographic camera or may be projected
on a television screen by means of a televison camera tube, or the visible image thus
formed may be observed by naked eyes.
[0004] Basically, the radiographic image conversion screen comprises a support made of e.g.
paper or a plastic sheet and a fluorescent layer formed on the support. The fluorescent
layer is composed of a binder and a phosphor dispersed in the binder and being capable
of efficiently emitting light when excited by the radiation of e.g. X-rays, and the
surface of the fluorescent layer is usually protected by a transparent protective
layer.
[0005] For medical diagnosis by means of radiography, a high speed radiographic system (i.e.
a combination of a film and an intensifying screen) is desired to minimize the patients'
dosage of radioactivity. At the same time, a radiographic system is desired which
is capable of providing good image quality (i.e. sharpness, granularity and contrast)
suitable for diagnosis by clinical photography. Accordingly, the intensifying screen
is desired to have a high speed and to provide superior sharpness, granularity and
contrast. Likewise in the case of a fluorescent screen, it is desired to have a high
speed and to provide particularly good contrast so that it is thereby possible to
minimize the patients' dosage of radioactivity and at the same time to obtain an image
having good image quality.
[0006] As high speed radiographic image conversion screens, there have been developed radiographic
image conversion screens comprising a rare earth oxysulfide phosphor, such as one
wherein a terbium-activated rare earth oxysulfide phosphor which is a green emitting
phosphor and represented by the formula (Ln, Tb) 2° 2S where Ln is at least one selected
from lanthanum, gadolinium and lutetium, is used (US Patent No. 3,725,704), and one
wherein a terbium-activated yttrium oxysulfide which is a blue emitting phosphor and
represented by the formula (Y, Tb)
2 0
25, is used (US Patent No. 3,738,856). Among them, intensifying screens using a green
emitting rare earth phosphor, particularly, a rare earth oxysulfide phosphor such
as a terbium-activated gadolinium oxysulfide phosphor represented by the formula (Gd,
Tb)
2O
2S or a terbium-activated lanthanum oxysulfide phosphor represented by the formula
(La, Tb)
2O
2S, have a speed several times higher than the speed of commonly used conventional
intensifying screens using a calcium tungstate phosphor represented by the formula
CaWO
4 and they have relatively good granularity as compared to other high speed intensifying
screens. -Therefore, they are utilized in high speed radiographic systems in combination
with an orthochromatic-type (hereinafter referred to simply as "ortho-type") film
having a wide spectral sensitivity ranging from a blue region to a green region. Meanwhile,
in the recent high speed radiographic systems based on a combination of a green emitting
rare earth intensifying screen and an ortho-type film, there is a tendency to use
a low speed ortho-type film utilizing fine silver halide grains in order to minimize
the amount of silver used for the film and to improve the image quality, particularly
the granularity, at a high speed level. It is therefore strongly desired to further
improve the speed of the intensifying screen with a view to reduction of the patients'
dosage of radioactivity and at the same time to improve the sharpness of the intensifying
screen, which tends to be reduced with an increase of the speed.
[0007] Among the green emitting phosphors, a gadolinium oxysulfide phosphor is paticularly
preferably used for a high speed intensifying screen. However, it has a K absorption
edge at 50.2 KeV, and accordingly, the intensifying screen using it has drawbacks
that the contrast thereby obtainable within the X-ray tube voltage range commonly
used for medical diagnosis (i.e. from 60 to 140 KVp) is inferior due to the X-ray
absorbing characteristics of such a phosphor, and the change of the speed of the intensifying
screen depending on a change of the tube voltage tends to be great, thus leading to
difficulties in setting the condition of radiography.
SUMMARY OF THE INVENTION :
[0008] It is an object of the present invention to overcome the above mentioned difficulties
in the conventional radiographic diagnosis systems wherein radiographic image conversion
screens are used, and to provide a radiographic image conversion screen which, when
used as an intensifying screen in combination with an ortho-type film, has a speed
at least equal to the speed of the conventional intensifying screens using a green
emitting rare earth phosphor and is capable of providing an image having superior
image quality, particularly superior sharpness and contrast without degradation of
the granularity, and which is less dependent in its speed on the X-ray tube voltage
as compared with the conventional intensifying screens.
[0009] Another object of the present invention is to provide a radiographic image conversion
screen which, when used as a fluorescent screen in association with a photographic
camera or an X-ray television system, has a speed at least equal to the speed of a
conventional fluorescent screen using a green emitting rare earth phosphor and is
capable of providing an image having an improved contrast over the conventional fluorescent
screen.
[0010] As a result of extensive studies on various phosphors used for the fluorescent layers
of the radiographic image conversion screens and various combinations thereof, the
present inventors have found that the above objects can be accomplished by using a
combination of a rare earth phosphor capable of emitting green light upon exposure
to radiation and a phosphor capable of emitting blue light upon exposure to radiation
in such a manner as to form a double layer structure wherein a fluorescent layer composed
of the green emitting rare earth phosphor is disposed on the surface side (i.e. the
output side of the emitted light) and a fluorescent layer composed of the blue emitting
phosphor is disposed on the side facing a support.
[0011] Thus, the present invention provides a radiographic image conversion screen which
comprises a support, a first fluorescent layer formed on the support and consisting
essentially of a blue emitting phosphor and a second fluorescent layer formed on the
first fluorescent layer and consisting essentially of a green emitting rare earth
phosphor.
[0012] The radiographic image conversion screen of the present invention has a fluorescent
layer composed essentially of a blue emitting phosphor interposed between the support
and the fluorescent layer composed essentially of a green emitting rare earth phosphor,
and thus is capable of emitting blue and green lights, and it has a speed at least
equal to the speed of the conventional radiographic image conversion screens comprising
only the green emitting rare earth phosphor layer. Further, it provides an image having
superior image quality, particularly superior contrast, as compared with the conventional
radiographic image conversion screens, and when used as an intensifying screen in
combination with an ortho-type film, it provides improved sharpness over the conventional
intensifying screens and the dependability of its speed against the X-ray tube voltage
is thereby improved.
BRIEF DESCRIPTION OF THE DRAWINGS :
[0013]
Figures 1 and 2 are diagrammatic cross sectional views of the radiographic image conversion
screens of the present invention.
Figure 3 is a graph illustrating an emission spectrum according to a conventional
radiographic image conversion screen.
Figures 4 and 5 are graphs illustrating emission spectra according to the radiographic
image conversion screens of the present invention.
Figures 6 and 7 are graphs illustrating the relative speed and relative sharpness,
respectively, dependent on the proportion of the blue emitting phosphor in the radiographic
image conversion screens of the present invention.
Figure 8 is a graph illustrating the relative speeds of the radiographic image conversion
screens of the present invention and the conventional radiographic image conversion
screen, dependent on the X-ray tube voltage.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS:
[0014] Now, the present invention will be described in detail.
[0015] The radiographic image conversion screen of the present invention can be prepared
in the following manner.
[0016] Firstly, suitable amounts of the blue emitting phosphor and a binder resin such as
nitrocellulose' are mixed, and a suitable amount of a solvent is added to the mixture
to obtain a coating dispersion of the phosphor having an optimum viscosity. The coating
dispersion of the phosphor is applied onto a support made of e.g. paper or plastic
by means of a doctor blade, roll coater or knife coater. In some intensifying screens,
a reflective layer such as a white pigment layer, an absorptive layer such as a black
pigment layer or a metal foil layer is interposed between the fluorescent layer and
the support. Likewise, when the radiographic image conversion screen of the present
invention is to be used as an intensifying screen, a reflective layer, an absorptive
layer or a metal foil layer maybe preliminarily formed on a support and then a blue
emitting phosphor layer may be formed thereon in the above mentioned manner. Then,
a coating dispersion comprising a green emitting rare earth phosphor and a binder
resin such as nitrocellulose, is prepared in the same manner as described above, and
the coating dispersion thus prepared is applied onto the blue emitting phosphor layer
to form a fluorescent layer composed essentially of the green emitting rare earth
phosphor. The support thus coated with the two phosphor layers capable of emitting
lights of different colours, is then subjected to drying to obtain a radiographic
image conversion screen of the present invention. In most cases, radiographic image
conversion screens are usually provided with a transparent protective layer on the
fluorescent layer. It is preferred also in the radiographic image conversion screens
of the present invention to provide a transparent protective layer on the fluorescent
layer composed essentially of the green emitting phosphor.
[0017] In a case where the green emitting rare earth phosphor to be used has a mean grain
size or specific gravity substantially greater than the mean grain size or specific
gravity of the blue emitting phosphor to be used, the process may advantageously be
modified in such a manner that firstly a protective layer is formed on a flat substrate
such as a glass plate or a plastic sheet, and then a coating dispersion composed of
a mixture comprising the green emitting rare earth phosphor, the blue emitting phosphor
and a binder resin, is coated on the protective layer and gradually dried at room
temperature while controlling the ambient atmosphere. During this step of drying the
coating dispersion, the green emitting rare earth phosphor grains having a greater
mean grain size or specific gravity will settle to form an under layer while the blue
emitting phosphor grains having a smaller mean grain size or specific gravity are
pushed upwardly to form a top layer, whereby two separate fluorescent layers, i.e.
a top layer composed essentially of the blue emitting phosphor and an under layer
composed essentially of the green emitting rare earth phosphor, are obtainable. Then,
the integrally formed protective and fluorescent layers are peeled -off from the substrate,
and placed on a support so that the top layer composed essentially of the blue emitting
phosphor is brought in contact with and fixed to the support, whereby a radiographic
image conversion screen of the present invention, is obtainable. In this case, the
separation between the green emitting rare earth phosphor grains and the blue emitting
phosphor grains may not be complete, i.e. a certain minor amount of the green emitting
rare earth phosphor grains may be present in the fluorescent layer composed essentially
of the blue emitting phosphor and likewise a certain minor amount of the blue emitting
phosphor grains may be present in the fluorescent layer composed essentially of the
green emitting rare earth phosphor. It has been confirmed that so long as the first
fluorescent layer, i.e. the layer adjacent to the support, is composed essentially
of the blue emitting phosphor and the second fluorescent layer, i.e. the layer on
the surface side (i.e. the emission output side) is composed essentially of the green
emitting rare earth phosphor, the radiographic image conversion screen thereby obtainable
has characteristics substantially equal to the characteristics of the above mentioned
radiographic image conversion screen obtained by separately coating the blue emitting
phosphor layer and the green emitting rare earth layer on the support.
[0018] Figure 1 shows a diagrammatic cross sectional view of a radiographic image conversion
screen of the present invention prepared in the above mentioned manners. A first fluorescent
layer 12 consisting essentially of a blue emitting phosphor is provided on a support
11, and a second fluorescent layer 13 consisting essentially of a green emitting rare
earth phosphor is formed on the first fluorescent layer 12. Reference numeral 14 designates
a transparent protective layer formed on the surface of the second fluorescent layer
13.
[0019] Further, the blue emitting phosphor layer of the radiographic image conversion screen
of the present invention may be formed in such a manner that firstly the blue emitting
phosphor grains are classified into a plurality of groups having different mean grain
sizes by means of a proper phosphor grain separation means such as levigation, and
the groups of the phosphor grains thus classified are respectively dispersed in a
proper binder resin and sequentially applied onto the support and dried so that the
phosphor grains having a smaller mean grains are coated first, whereby the blue emitting
phosphor layer is formed to have a grain size distribution of the phosphor grains
such that the grain size becomes smaller gradually from the side facing the green
emitting rare earth phosphor layer to the side facing the support.
[0020] Figure 2 shows a diagrammatic cross sectional view of a radiographic image conversion
screen of the present invention prepared in the above mentioned manner. A first fluorescent
layer 22 composed essentially of a blue emitting phosphor, a second fluorescent layer
23 composed essentially of a green emitting rare earth phosphor and a transparent
protective layer 24 are laminated in this order on a support 21. The blue emitting
phosphor grains in the first layer 22 are arranged in such a manner that the phosphor
grain size becomes smaller gradually from the side facing the green emitting phosphor
layer 23 toward the side facing the support 21. Such a radiographic image conversion
screen provides substantially improved sharpness over the radiographic image conversion
screen illustrated in Figure 1.
[0021] The green emitting rare earth phosphors which may be used in the radiographic image
conversion screens of the present invention, include a phosphor composed of a terbium-activated
rare earth oxysulfide of at least one rare earth element selected from yttrium, lanthanum,
gadolinium and lutetium, a phosphor composed of an oxyhalide of the above rare earth
elements (provided that the phosphor contains at least 0.01 mole of terbium per mole
of the phosphor), a phosphor composed of a borate of the above rare earth elements,
a phosphor composed of a phosphate of the above rare earth elements and a phosphor
composed of a tantalate of the above rare earth elements. Thus, the green emitting
rare earth phosphors contain at least one lanthanide element or yttrium as the host
material of the phosphors and are capable of emitting green light with high efficiency
when excited by the X-rays. Particularly preferred among them in view of the emission
efficiency and granularity, are a terbium activated or terbium-thulium activated rare
earth oxysulfide phosphor represented by the formula (LN
1-a-b, Tb
a, Tm
b)
20
2S where Ln is at least one selected from lanthanum, gadolinium and lutetium, and a
and b are numbers meeting the conditions of 0.0005 ≦ a ≦0.09 and 0 s b ≦0.01, respectively,
and a terbium activated or terbium-thulium activated rare earth oxysulfide phosphor
represented by the formula (Y
1-i-a-b, Ln., Tb Tm
b)
2O
2S where Ln is at least one selected from lanthanum, gadolinium and lutetium, and i,
a and b are numbers meeting the conditions of 0.65 ≦ i ≦0.95, 0.0005 ≦ a ≦0.09 and
0 < b ≦0.01.
[0022] Any blue emitting phosphor may be used for the radiographic image conversion screens
of the present invention so long as it is a phosphor capable of emitting blue light
with high efficiency when excited by radiation such as X-ray radiation. In practice,
however, in view of the speed of the obtainable radiographic image conversion screen
and the sharpness of the image thereby obtainable, it is preferred to use at least
one selected from the group consisting of a yttrium or yttrium-gadolinium oxysulfide
phosphor represented by the formula (Y
1-c-d-e, Gd
c, Tb
d, Tm
e)
2O
2S where e, d and e are numbers meeting the conditions of 0 ≦ c ≦0.60, 0.0005 ≦ d ≦0.02
and 0 ≦ e ≦0.01, respectively; an alkaline earth metal complex halide phosphor represented
by the formula MeF
2 · pMe'X
2 · qKX' . rMe"SO
4 : mEu
2+, nTb
3+ where Me is at least one selected from magnesium, calcium, strontium and barium,
each of Me' and Me" is at least one selected from calcium, strontium and barium, each
of X and X' is at least one selected from chlorine and bromine, and p, q, r, m and
n are numbers meeting the conditions of 0.80 ≦ p ≦1.5, 0 ≦ q ≦2.0, 0 ≦ r ≦1.0, 0.001
≦ m ≦0.10 and 0 ≦ n ≦0.05, respectively; a rare earth oxyhalide phosphor represented
by the formula (Ln'
1-x-y-z, Tb
x, Tm
y Yb
z)OX where Ln' is at least one selected from lanthanum and gadolinium, X is at least
one selected from chlorine and bromine, and x, y and z are numbers meeting the conditions
of 0 < x ≦0.01, 0 ≦ y ≦0.01, 0 ≦ z ≦0.005 and 0 < x + y; a divalent metal tungstate
phosphor represented by the fromula M
IIWO
4 where M
II is at least one selected from magnesium, calcium, zinc and cadmium; a zinc sulfide
or zinc-cadmium sulfide phosphor represented by the formula (Zn
1-j, Cd.)S:Ag where j is a number meeting the condition of 0 ≦ j ≦ 0.4; and a rare earth
tantalate or tantalum-niobate phosphor represented by the formula(Ln"
1-v, Tm
v) (Ta
1-w, Nb
w)O
4 where Ln" is at least one selected from lanthanum, yttrium, gadolinium and lutetium,
and v and w are numbers meeting the conditions of 0 ≦ v ≦0.1 and 0 < w ≦0.3, respectively.
[0023] In the radiographic image conversion screens of the present invention, in view of
the speed of the obtainable radiographic image conversion screen and the sharpness
of the image thereby obtainable, the phosphor to be used for the blue emitting phosphor
layer, preferably has a mean grain size of from 2 to 10
11, more preferably from 3 to 6µ, and a standard deviation of from 0.20 to 0.50, more
preferably from 0.30 to 0.45, as represented by the quartile deviation, and the phosphor
to be used for the green emitting phosphor layer preferably has a mean grain size
of from 5 to 20µ, more preferably from 6 to 12µ and a standard deviation of from 0.15
to 0.40, more preferably from 0.20 to 0.35, as represented by the quartile deviation.
Likewise in view of the speed of the obtainable radiographic image conversion screen
and the sharpness of the image thereby obtainable, the coating weight of the phosphor
in the blue emitting phosphor layer and the coating weight of the phosphor in the
green emitting phosphor layer are preferably from 2 to 100 mg/cm and from 5 to 2 2
100 mg/cm
2, respectively and more preferably from 3 to 50 mg/cm and from 20 to 80 mg/cm
2, respectively. In view of the sharpness of the image obtainable, it is preferred
that the mean grain size of the phosphor grains in the blue emitting phosphor layer
is smaller than the mean grain size of the phosphor grains in the green emitting rare
earth phosphor layer.
[0024] Figure 3 shows an emission spectrum according to a conventional radiographic image
conversion screen comprising a single fluorescent layer composed solely of (Gd
0.995, Tb
0.005)
2O
2S phosphor as one of green emitting rare earth phosphors. Figures 4 and 5 show emission
spectra obtained by the radiographic image conversion screens of the present invention.
In the radiographic image conversion screen illustrated in Figure 4, the blue emitting
phosphor layer (the coating weight of the phosphor: 20 mg/cm
2) is composed of (Y
0.998, Tb
0.002)
2O
2S phosphor and the green emitting phosphor layer (the coating weight of the phosphor:
30 mg/cm
2) is composed of (GdO.
995, Tb
0.05)
2O
2S phosphor. Whereas, in the radiographic image conversion screen illustrated in Figure
5, the blue emitting phosphor layer (the coating weight of the phosphor: 15 mg/cm
2) is composed of BaF
2 · BaCl
2 · 0.1 KCl · 0.1 BaS0
4:
0.06 Eu
2+ phosphor, and the green emitting phosphor layer (the coating weight of the phosphor:
35 mg/cm
2) is composed of (Gd
0.995, Tb
0.005)
2O
2S phosphor. In each of Figures 3 to 5, the broken line and the alternate long and
short dash line indicate a spectral sensitivity curve of an ortho-type film and a
spectral sensitivity curve of an image tube, respectively. It is apparent from the
comparison of Figure 3 with Figure 4 or 5, that the radiographic image conversion
screen of the present invention has a wide emission distribution ranging from the
green region to the blue region or the near ultraviolet region and better matches
the spectral sensitivities of the ortho-type film and the photocathode of the image
tube than the conventional radiographic image conversion screen comprising a single
fluorescent layer composed solely of the green emitting rare earth phosphor, and it
is advantageous particularly in view of its high speed.
[0025] Figure 6 illustrates a relation between the ratio (represented by percentage) of
the coating weight of the phosphor in the blue emitting phosphor layer to the coating
weight of the total phosphor in the entire fluorescent layers in the radiographic
image conversion screens of the invention and the speed of the radiographic image
conversion screens thereby obtained. The relative speed on the vertical axis indicates
the speed obtained in combination with an ortho-type film, in a relative value based
on the speed of the screen having no blue emitting phosphor layer (i.e. comprising
only the green emitting rare earth phosphor layer) where the latter speed is set to
be 100. The curves a, b, c, d, e and f represent the cases where the blue emitting
phosphor layer is composed of (Y
0.998, Tb
0.002)
2O
sS phosphor, (Gd
0.5, Y
0.495, Tb
0.003, Tm
0.002)
2O
2S phosphor, BaF2. BaCl
2 · 0.1 KCl · 0.1 BaSO
4: 0.06 Eu
2+ phosphor, (La
0.997, Tb
0,003)OBr phosphor, CdWO
4 phosphor, and CaWO
4 phosphor, respectively. In each case, the total coating weight of the fluorescent
layers is 50 mg/cm
2, and the green emitting rare earth phosphor layer is composed of (Gd
0.995, Tb
0·005)
2O
2S phosphor.
[0026] It is apparent from Figure 6 that the optimum ratio of the coating weight of the
blue emitting phosphor layer to the total coating weight of the phosphors varies depending
upon the type of the blue emitting phosphor used. However, by providing a blue emitting
phosphor layer beneath the green emitting phosphor layer composed of (Gd, Tb)
2O
2S phosphor, it is possible to obtain a radiographic image conversion screen having
a speed at least equal to the speed of the conventional radiographic image conversion
screen comprising a single fluorescent layer composed solely of (Gd, Tb)
2O
2S phosphor (i.e. comprising only the green emitting phosphor layer).
[0027] Figure 7 illustrates a relation between the ratio (represented by percentage) of
the coating weight of the phosphor in the blue emitting phosphor layer to the total
coating weight of the phosphors in the entire fluorescent layers of the radiographic
image conversion screens of the present invention and the sharpness of the radiographic
image conversion screen. In Figure 7, curves a, b, c, d, e and f represent the cases
where the blue emitting phosphor layer is composed of (Y
0·998, Tb
0.002)
2 2 O
2S phosphor, (Gd
0.5, Y
0.495, Tb
0.003, Tm
0.002)
2O
2S phosphor, BaF
2 · BaCl
2 · 0. 1 KCl · 0. 1 BaSO
4: 0.06 Eu
2+ phosphor, (La
0.997, Tb
0.003)OBr phosphor, CdWO
4 phosphor and CaWO
4 phosphor, respectively. In each case, the total coating weight of the fluorescent
layers is
50 mg/cm
2 and the green emitting rare earth phosphor layer is composed of (Gd
0.995, Tb
0,005)
2O
2S phosphor. The sharpness of each radiographic image conversion screen is determined
by obtaining a MTF value at a film density of 1.5 and spatial frequency of 2 lines/mm,
and the MTF value is indicated in a relative value based on the MTF value of the radiographic
image conversion screen having no blue emitting phosphor layer (i. e. comprising only
the green emitting rare earth phosphor layer) where the latter MTF value is set to
be 100.
[0028] It is apparent from Figure 7 that the radiographic conversion screens of the present
invention provided with a blue emitting phosphor layer beneath the green emitting
phosphor layer have improved sharpness over the conventional screen having no such
a blue emitting phosphor layer.
[0029] Figure 8 is a graph illustrating the dependency of the speeds of the radiographic
image conversion screens of the present invention and the conventional radiographic
image conversion screen, on the X-ray tube voltage. In Figure 8, curves a, b, c, d
and e represent the speeds of the radiographic image conversion screens of the present
invention in which the blue emitting phosphor layer is composed of (Y
0,998, Tb
0.002)
2O
2S phosphor, BaF
2 · BaCl
2 · 0.1 KCl · 0.1 BaSO
4: 0.06 Eu
2+ phosphor, (LaO.
997, Tb
0.003)OBr phosphor, CdWO
4 phosphor and CaWO
4 phosphor, respectively, and the green emitting phosphor layer is (Gd
0.995, Tb
0.005)
2 0
2S phosphor in each case. In each case, the coating weight of the green emitting phosphor
is 30 mg/cm and the coating weight of the blue emitting phosphor is 20 mg/cm
2. Curve f represents the speed of the conventional radiographic image conversion screen
wherein the fluorescent layer is composed solely of (Gd
0.995, Tb
0,005)
2O
2S and the coating weight of the phosphor is 50 mg/cm
2. The vertical axis of Figure 8 indicates the speed obtained by a combination of each
radiographic image conversion screen with an ortho-type film, as a relative value
against the speed of the radiographic conversion screen comprising a single fluorescent
layer of CaWO
4 phosphor (as combined with a regular-type film). The relative value is spotted for
every X-ray tube voltage.
[0030] It is seen from Figure 8 that in the radiographic image conversion screens of the
present invention, the change of the speed due to the variation of the X-ray tube
voltage is less as compared with the conventional radiographic image conversion screen
comprising a single fluorescent layer composed of (Gd, Tb)
2O
2S phosphor, within the X-ray tube voltage range of from 60 to 140 KVp which is commonly
used in the radiography for medical diagnosis.
[0031] Further, it has been confirmed that when green emitting rare earth phosphors other
than (Gd
0.995, Tb
0.005)
2O
2S are used for the green emitting phosphor layer, or when blue emitting phosphors
other than (Y
0.998, Tb
0.002)
2O
2S phosphor, BaF
2 · BaCl
2 · 0.1 KCl 0.1 BaSO
4: 0. 06 Eu
2+ phosphor, (La
0.497, Tb
0,003)OBr phosphor, CdWO
4 phosphor and CaWO
4 phosphor are used for the blue emitting phosphor layer, the radiographic image conversion
screens thereby obtainable have a speed at least equal to the speed of the conventional
screen comprising a single fluorescent layer composed solely of the green emitting
rare earth phosphor, so long as the ratio of the coating weight of the phosphor in
the blue emitting phosphor layer to the total coating weight of the entire phosphors
falls within the specific range, as in the case of the radiographic image conversion
screens illustrated in Figure 6, and the sharpness can be improved and the dependency
of the speed on the X-ray tube voltage can be reduced as compared with the conventional
radiographic image conversion screen comprising a single fluorescent layer composed
solely of the green emitting rare earth phosphor, as in the cases of the radiographic
image conversion screens illustrated in Figures 7 and 8.
[0032] It has further been confirmed that the radiographic image conversion screens of the
present invention provides improved contrast as compared with the conventional radiographic
image conversion screen comprising only the green emitting rare earth phosphor layer.
When used as fluorescent screens for X-ray television systems, they exhibit superior
characteristics, especially in their speed and contrast, as compared with conventional
fluorescent screens comprising only the green emitting rare earth phosphor layer.
[0033] Further, with respect of the granularity and sharpness of the obtainable radiographic
image conversion screens, it has been confirmed that better characteristics are obtainable
by providing a plurality of fluorescent layers so that the green emitting rare earth
phosphor and the blue emitting phosphor constitute the respective separate fluorescent
layers as in the radiographic image conversion screens of the present invention rather
than simply mixing the phosphors.
[0034] As mentioned in the foregoing, the radiographic image conversion screens of the present
invention have a speed at least equal to the speed of the conventional radiographic
image conversion screens comprising only a green emitting phosphor layer and they
provide improved sharpness and contrast without degradation of the image quality,
particularly the granularity, and their speed is less dependent on the X-ray tube
voltage and thus provides an advantage that the condition for the operation of radiography
can thereby be simplified. Thus, the radiographic image conversion screens of the
present invention have a high speed and provide an image having superior image quality,
and their industrial value is extremely high.
[0035] Now, the present invention will further be described with reference to Examples.
EXAMPLES 1 to 26:
[0036] Radiographic image conversion screens (1) to (26) were prepared in the following
manner with use of the respective combinations of a green emitting rare earth phosphor
and a blue emitting phosphor, as identified in Table 1 given hereinafter.
[0037] Eight parts by weight of the blue emitting phosphor and one part by weight of nitrocellulose
were mixed with use of a solvent to obtain a coating dispersion of the phosphor. This
coating dispersion of the phosphor was uniformly coated by means of a knife coater,
on a polyethylene terephthalate support provided on its surface with an absorptive
layer of carbon black and having a thickness of 25
01; so that the coating weight of the phosphor became as shown in Table 1 given hereinafter,
whereby a blue emitting phosphor layer was formed.
[0038] Then, 8 parts by weight of a green emitting rare earth phosphor and one part by weight
of nitrocellulose were mixed with use of a solvent to obtain a coating dispersion
of the phosphor. This coating dispersion of the phosphor was uniformly coated by means
of a knife coater on the above mentioned blue emitting phosphor layer so that the
coating weight of the phosphor became as shown in Table 1 given hereinafter, whereby
a green emitting rare earth phosphor layer was formed. Further, nitrocellulose was
uniformly coated on the green emitting rare earth phosphor layer to form a transparent
protective layer having a thickness of about 10u.
EXAMPLE 27:
[0039] (Y
0.998, Tb
0.002)
2O
2S phosphor having a mean grain size of 5µ and a standard deviation (i.e. quartile
deviation) of 0.35 was preliminarily classified by levigation into four grain size
groups, i.e. smaller than 3µ, from 3 to 5µ, from 5 to 7u and larger than 7p. Eight
parts by weight of each group of the phosphor and one part by weight of nitrocellulose
were mixed with use of a solvent to obtain four different coating dispersions of the
phosphor. The coating dispersions were sequentially uniformly coated by a knife coater
and dried on a polyethylene terephthalate support provided on its surface with an
absorptive layer of carbon black and having a thickness of 250µ in such order that
a group of the phosphor grains having smaller grain size was applied first, so that
the coating weight of the phosphor of each group became 5 mg/cm
2, whereby a plurality of fluorescent layers composed of (Y
0.998, Tb
0.002)
2O
2S and having different phosphor grain sizes were formed.
[0040] Then, 8 parts by weight of (Gd
0.995, Tb
0.005)
2O
2S phosphor having a mean grain size of 8µ and a standard deviation (i.e. quartile
deviation) of 0.30 and one part by weight of nitrocellulose were mixed with use of
a solvent to obtain a coating dispersion of the phosphor. This coating dispersion
was uniformly coated by a knife coater on the above mentioned (Y
0.998, Tb
0.002)
2O
2S phosphor layer so that the coating weight of the phosphor became 30 mg/cm
2, whereby a (Gd
0.995, Tb
0.005)
2O
2S phosphor layer was formed. Further, nitrocellulose was uniformly coated on the (Gd
0.995, Tb
0.005)
2O
2S phosphor layer and dried to form a transparent protective layer having a thickness
of about 10µ. Thus, a radiographic image conversion screen (27) was prepared.
EXAMPLES 28 to 30:
[0041] Radiographic image conversion screens (28) to (30) were prepared in the following
manner with use of the respective combinations of a green emitting rare earth phosphor
and a blue emitting phosphor, as indicated in Table 1 given hereinafter.
[0042] The green emitting rare earth phosphor and the blue emitting phosphor were preliminarily
mixed in the proportions corresponding to the respective coating weights of the green
emitting rare earth phosphor layer and the blue emitting phosphor layer. Eight parts
of the phosphor mixture and one part of nitrocellulose were mixed together with a
solvent to obtain a coating dispersion of the phosphors.
[0043] On the other hand, a protective layer was coated on a smooth substrate and dried
to have a thickness of 10
11, and the above coating dispersion of the phosphors was then coated on the protective
layer so that the total coating weight of the phosphors became 50 mg/cm
2. The coated phosphor layer was dried by leaving it to stand still at a constant temperature
of 15°C for 10 hours while controlling the replacement of ambient air, whereby the
green emitting phosphor grains and the blue emitting phosphor grains were settled
to separate from one another.
[0044] Thereafter, the phosphor layer having the protective layer was peeled off from the
flat substrate and heat laminated on a support coated with a thermoplastic binder,
whereby a radiographic image conversion screen comprising a double phosphor layer
structure, i.e. a first fluorescent layer composed essentially of the blue emitting
phosphor and a second fluorescent layer composed essentially of the green emitting
phosphor, was obtained.
EXAMPLES 31 to 33:
[0045] Fluorometallic radiographic image conversion screens (31) to (33) were prepared with
use of the respective combinations of a green emitting rare earth phosphor and a blue
emitting phosphor, as indicated in Table 2 given hereinafter, in the same manner as
in Examples 1 to 26 except that a paper support having a thickness of 250
11 and provided on its surface with a lead foil having a thickness of 30
11 was used.
Reference Example R:
[0046] As a reference example, a radiographic image conversion screen (R) was prepared in
the same manner as in Examples 1 to 26 except that (Gd
0.995, Tb
0.005)
2O
2S phosphor having a mean grain size of 8
11, and a standard deviation (i.e. quartile deviation) of 0.30 was used and a single
fluorescent layer having a coating weight of the phosphor of 50 mg/cm was formed on
the support.
Reference Example R':
[0047] A radiographic image conversion screen (R') was prepared in the same manner as in
Examples 31 to 33 except that the same phosphor as used in Refrerence Example R was
used.
[0048] With respect to 30 different kinds of the radiographic image conversion screens (1)
to (30) of the present invention and the radiographic image conversion screen (R)
prepared as a reference example, their speeds, sharpness, granularity and contrast
were investigated as combined with an ortho-type film. The results thereby obtained
are shown in Table 1.
[0049] It is seen that the radiographic image conversion screens of the present invention
are superior to the conventional radiographic image conversion screen (R) in the speed,
sharpness and contrast, and no substantial degradation in their granularity was observed.
[0050] The radiographic image conversion screens (31) to (33) of the present invention and
the radiographic image conversion screen (R') prepared as a reference example, were
used for industrial non-destructive inspection. The results thereby obtained are shown
in Table 2. The radiographic image conversion screens of the invention were found
to be superior to the conventional radiographic image conversion screen (R') in the
speed and penetrameter sensitivity. Further, it has been confirmed that the radiographic
image conversion screens (31) to (33) can effectively used also for high voltage radiography
and cobaltgraphy in medical diagnosis.
With respect to the radiographic image conversion screens (1) to (30) and (R):
[0051] The speed, sharpness, granularity and contrast of each radiographic image conversion
screen listed in the following Table 1 were obtained by radiography conducted with
use of Ortho G Film (manufactured by Eastman Kodak Co.) and the X-rays generated at
the X-ray tube voltage of 80 KVp and passed through a water-phantom having a thickness
of 80 mm. The respective values in the Tables indicate the following values.
[0052] Speed: A relative value based on the speed of a radiographic image conversion screen
comprising a fluorescent layer of CaWO
4 phosphor (KYOKKO FS, manufactured by Kasei Optonix, Ltd.) where the latter speed
is set to be 100.
[0053] Sharpness: A relative value of the MTF value obtained .
[0054] Sharpness: A MTF value was obtained at a spatial frequency of 2 lines/mm, and it
was represented by a relative value based on the MTF value of a radiographic image
conversion screen comprising a single fluorescent layer composed solely of (Gd
0.995, Tb
0.005)
2O
2S phosphor, obtained at the same spatial frequency, where the latter MTF value was
set to be 100.
[0055] Granularity: A RMS value at a film density of 1. 0 and spatial frequency of 0. 5
to 5. 0 lines /mm.
[0056] Contrast: Photographs were taken through Al having a thickness of 1 mm and Al having
a thickness of 2 mm, and the respective contrasts were obtained from the differences
of the film densities. Each contrast was represented by a relative value based on
the contrast obtained by a radiographic image conversion screen comprising a fluorescent
layer composed of CaWO
4 phosphor (KYOKKO FS, manufactured by Kasei Optonix, Ltd.) where the latter contrast
was set to be 100.
With respect to the radiographic image conversion screens (31) to (33) and (R')
[0057] The speed and penetrameter sensitivity were obtained by radiography conducted with
use of Ortho G Film (manufactured by Eastman Kodak Co.) and a steel plate having a
thickness of 20 mm as the object and with X-rays generated at the X-ray tube voltage
of 200 KVp.
[0058] Speed: A relative value based on the speed of the fluorometalic radiographic image
conversion screen (R') where the latter speed is set to be 100.
1) A radiographic image conversion screen comprising a support, a first fluorescent
layer formed on the support and consisting essentially of a blue emitting phosphor
and a second fluorescent layer formed on the first fluorescent layer-and consisting
essentially of a green emitting rare earth phosphor.
2) The radiographic image conversion screen according to Claim 1 wherein said green
emitting rare earth phosphor is a rare earth oxysulfide phosphor represented by the
formula
where Ln is at least one selected from lanthanum, gadolinium and lutetium, and a and
b are numbers meeting the conditions of 0.0005 ≦ a ≦0.09 and 0 = b ≦0.01, respectively,
or the formula
where Ln is at least one selected from lanthanum, gadolinium and lutetium, and i,
a and b are numbers meeting the conditions of 0.65 ≦ i <0. 95, 0.0005 ≦ a ≦0.09 and
0 ≦ b ≦0.01, respectively.
3) The radiographic image conversion screen according to Claim 1 or 2 wherein said
blue emitting phosphor is at least one selected from the group consisting of
(I) a yttrium or yttrium-gadolinium oxysulfide phosphor represented by the formula
where c, d and e are numbers meeting the conditions of 0 ≦ c <0.60, 0.0005 ≦ d <0.02
and 0 ≦ e ≦0.01, respectively,
(II) an alkaline earth metal complex halide phosphor represented by the formula
where Me is at least one selected from magnesium, calcium, strontium and barium, each
of Me' and Me" is at least one selected from calcium, strontium and barium, each of
X and X' is at least one selected from chlorine and bromine, and p, q, r, m and n
are numbers meeting the conditions of 0.80 ≦ p ≦1.5, 0 ≦ q ≦2.0, 0 ≦ r ≦1.0, 0.001
≦ m ≦0.10 and 0 ≦ n ≦0.05, respectively,
(III) a rare earth oxyhalide phosphor represented by the formula
where Ln' is at least one selected from lanthanum and gadolinium, X is at least one
selected from chlorine and bromine, and x, y and z are numbers meeting the conditions
of 0 ≦ x ≦0.01, 0 < y ≦0.01, 0 ≦ z <0.005 and 0 < x + y,
(IV) a divalent metal tungstate phosphor represented by the formula
where MII is at least one selected from magnesium, calcium, zinc and cadmium,
(V) a zinc sulfide or zinc-cadmium sulfide phosphor represented by the formula
where i is a number meeting the condition of 0 ≦ j ≦0.4, and
(VI) a rare earth tantalate or tantalum-niobate phosphor represented by the formula
where Ln" is at least one selected from lanthanum, yttrium, gadolinium and lutetium,
and v and w are numbers meeting the conditions of 0 ≦ v ≦0.1 and 0 ≦ w <0.3, respectively.
4) The radiographic image conversion screen according to Claim 1, 2 or 3 wherein the
phosphor in the blue emitting phosphor layer has a mean grain size of from 2 to 10µ,
a standard deviation (quartile deviation) of the grain size of from 0.20 to 0.50 and
a coating weight of from 2 to 100 mg/cm , and the phosphor in the green emitting phosphor
layer has a mean grain size of from 5 to 20µ, a standard deviation (quartile deviation)
of the grain size of from 0.15 to 0.40 and a coating weight of from 5 to 100 mg/cm2,
5) The radiographic image conversion screen according to Claim 4 wherein the phosphor
in the blue emitting phosphor layer has a mean grain size of from 3 to 6µ, a standard
deviation (quartile deviation) of the grain size of from 0.30 to 0.45 and a coating
weight of from 3 to 50 mg/cm2, and the phosphor in the green emitting phosphor layer has a mean grain size of from
6 to 12p, a standard deviation (quartile deviation) of the grain size of from 0.20
to 0.35 and a coating weight of from 20 to 80 mg/cm2.
6) The radiographic image conversion screen according to any one of Claims 1 to 5
wherein the blue emitting phosphor layer has a grain size distribution of the phosphor
grains such that the grain size becomes smaller gradually from the side facing the
green emitting rare earth phosphor layer to the side facing the support.
7) The radiographic image conversion screen according to any one of Claims 1 to 6
wherein a reflective layer is interposed between the support and the first fluorescent
layer.
8) The radiographic image conversion screen according to any one of Claims 1 to .6
wherein an absorptive layer is interposed between the support and the first fluorescent
layer.
9) The radiographic image conversion screen according to any one of Claims 1 to 6
wherein a metal foil is interposed between the support and the first fluorescent layer.