GB2056112A - Intensifying Screens in Transaxial Tomography - Google Patents

Abstract

Method for the production of transaxial tomographs, a combination of materials therefor and X-ray intensifying screens incorporating at least one reflecting element for use in transaxial tomography, wherein the exposure of a photographic silver halide emulsion material proceeds at an angle within the range of 2 DEG to 10 DEG in conjunction with an X-ray fluorescent intensifying screen including an ultra-violet and/or visible radiation reflective coating or sheet to increase the radiation output of the screen and to reduce the exposure time and radiation dose e.g. in medical X-ray applications.

Description

SPECIFICATION Method for the Production of Transaxial Tomographs, a Combination of Materials Therefor and X-ray intensifying Phosphor Screens Incorporating at Least One Reflecting Element for Use in Transaxial Tomography The present invention relates to a method for the production of transaxial tomographs with an Xray intensifying phosphor screen incorporating at least one reflective element. It also relates to such phosphor screens and to the combination of such a phosphor screen with a photographic silver halide material for use in the method of transaxial tomography.

Transaxial tomography is a radiographic technique that provides an image of a selected twodimensional section of a three-dimensional object. The basic principles of this technique are reviewed by Harrison H. Barrett and William Swindell in Proceedings of the IEEE, Vol. 65, No. 1, January 1977, P. 89-107.

One of the earliest descriptions of a transaxial recording system is based on the summation method with fan-shaped X-ray beam as described in the U.S. Patent Specification 2,196,618. In that method a fan-shaped X-ray beam is projected under a small angle onto a photographic film. The film being placed on a rotatable film table and the object to be radiographed are rotated synchronously so that back projections are continuously formed on the film thus producing a summation image.

In an alternative embodiment resulting likewise in a summation image the object is kept stationary and the film and X-ray source are rotated synchronously.

According to A. Lindegaard-Andersen and G. Thuesen in J. Phys. E.: Sci. Instrum., Vol. 1 1. 1978 p.

811 transaxial tomography is of good performance and low initial and running expenses and may prove to be of interest in industrial radiography, whereas the exposure time and radiation dose needed to achieve a satisfying quality probably make the system in its present form unsuitable for medical use.

Fluorescent intensifying screens are widely used in common radiographic work. It is general practice to use such screens for obtaining optimum definition with the fluorescent layer in direct contact with the silver halide emulsion layer and having the support of the screen directed towards the X-ray source. Further, when in normal radiography a lesser-dosage, higher speed image is desired i.e.

an image wherein a larger percentage of X-ray photons are utilized, thick screens are used (ref. US-P 4,101,781).

However, when in transaxial tomography a fluorescent screen is used, the object-wise modulated X-rays striking the fluorescent screen under a very small angle, usually between 20 and 100 have to travel a long way in the screen and thus undergo considerable attenuation. By attenuation is meant that the X-ray beam intensity expressed in W.cm-2 becomes reduced by travelling of the beam through an X-ray radiation absorptive medium. Moreover, we have established experimentally that thick phosphor layers, under the circumstances of the small angle irradiation yield lower fluorescent light output than a thin phosphor layer because of the higher internal light absorption.

The present invention provides a method of producing a transaxial tomograph by transaxially tomographically X-ray exposing at an angle within the range of 20 to 100 a photographic silver halide emulsion layer material, characterized in that a combination of the said photographic material and an X-ray fluorescent intensifying screen is used whereby the screen upon being struck by X-rays emits fluorescent light that exposes the silver halide emulsion layer, the said screen including (1) an ultraviolet and/or visible radiation reflective coating or sheet containing one or more elements of atomic number less than 30 (or compounds of such elements) and having a thickness such that an X-ray beam generated at 80 kV with a tungsten cathode and striking that coating or sheet at an angle of 50 is attenuated by not more than 20%, and (2) a fluorescent layer containing one or more phosphors which emit ultra-violet and/or visible radiation when struck by X-rays, and having a coverage of said phosphor less than 500 g of phosphor per sq.m to provide the said screen with respect to the said photographic silver halide emulsion layer an intensification factor of at least 20 at 80 kV.

The "intensification factor" is hereby defined as the value obtained by dividing the radiation dose of X-rays incident and necessary for producing a silver image of given optical density D(D:1 .00) without the intensifying screen, by the radiation dose required to produce a silver image of the same density with said screen, under the same conditions.

A reflective layer or sheet with a thickness as small as possible but still having a reflection capacity of more than 50% of incident ultraviolet radiation and/or visible light is preferred, because otherwise in transaxial tomography the X-rays striking the reflective coating or sheet under the small angle used have to travel a long way through said element before reaching the phosphor layer and can be still considerably attenuated even when elements with the low atomic number of less than 30 (or a compound of such elements) is used as a reflective medium in the invention.

The use of phosphors with high X-ray absorption and high conversion efficiency in a thin phosphor layer obliquely struck by the X-rays enhances the speed of the screen-photographic film system but gives rise to pronounced mottle images.

Mottle is a phenomenon that disturbs image detail retrieval and is composed of quantum mottle and screen mottle. Quantum mottle comes from "noise" due to the random fluctuations of the spatial distribution of the absorbed X-ray quanta, and screen mottle is produced by inhomogeneities in the structure of the phosphor coating.

In the invention the disturbing appearance of the mottle can be reduced by separating the silver halide emulsion layer from the phosphor layer by a 20 to 300 ym thick transparent layer or sheet e.g. a transparent organic resin support, which acts to some extent as a light diffusor.

The present invention therefore includes a combination of materials suitable for axial tomographic radiography with reduced mottle which combination comprises (i) a silver halide photographic material comprising a support and at least one silver halide emulsion layer thereon, and (ii) an X-ray fluorescent intensifying screen which comprises (1) an ultra-violet and/or visible radiation reflective coating or sheet containing one or more elements of atomic number less than 30 (or compounds of such elements) and having a thickness such that an X-ray beam generated at 80 kV with a tungsten cathode and striking that coating or sheet at an angle of 50 is attenuated by not more than 20%, and in direct contact with the said coating or sheet or separated therefrom by a transparent subbing layer.

(2) a fluorescent layer containing one or more phosphors which emit ultra-violet and/or visible radiation when struck by X-rays, and having a coverage of said phosphor less than 500 g of phosphor per sq.m. to provide the said screen with respect to the said photographic silver halide emulsion layer an intensification factor (as hereinbefore defined) of at least 20 at 80 kV, and (3) a transparent layer or sheet permanently or separatably united with the said fluorescent layer separating the said silver halide emulsion layer from the said fluorescent layer and having a thickness of 20 to 300 ym.

Preferably the thickness of the said transparent layer or sheet is from 100 to 300 lem when the said sheet acts as a support for the fluorescent layer.

The invention also provides an X-ray fluorescent intensifying screen comprising (1) an ultra-violet and/or visible radiation reflective coating or sheet containing one or more elements of atomic number less than 30 or compounds of such elements and having a thickness such that an X-ray beam generated at 80 kV with a tungsten cathode and striking that coating or sheet at an angle of 50 is attenuated for not more than 20%, and in direct contact with the said coating or sheet or separated therefrom by a transparent subbing layer, (2) a fluorescent layer containing one or more phosphors which emit ultra-violet and/or visible radiation when struck by X-rays, and having a coverage of said phosphors less than 500 g of phosphor per sq.m to provide the said screen with respect to the said photographic silver halide emulsion layer an intensification factor (as hereinbefore defined) of at least 20 at 80 kV, and (3) a transparent layer or sheet permanently or separatably united with the said fluorescent layer separating said silver halide emulsion layer from the said fluorescent layer and having a thickness of 20 to 300 ,um.

Such fluorescent intensifying screen can comprise in consecutive order: (1) the said ultra-violet and/or visible radiation reflective coating, (2) the said fluorescent layer in direct permanent contact with the said reflective coating, or through the intermediary of a smooth transparent interlayer of thickness up to 50 ym, and (3) as a support for the said fluorescent layer a transparent sheet having a thickness between 100 and 300 rim, Preferably the said support for the fluorescent layer is an organic resin sheet.

Such intensifying screens preferably comprise in consecutive order: (1) a resin film support having a thickness such that an X-ray beam generated at 80 kV with a tungsten cathode and striking that support at an angle of 50 is attenuated by not more than 10%, (2) permanently coated onto said support the said ultra-violet and/or visible radiation reflective coating, (3) the said fluorescent layer, and (4) permanently united with the said fluorescent layer a transparent layer or sheet having a thickness of 20 to 300 ym.

According to still another embodiment of the X-ray fluorescent intensifying screen for use according to the present invention consists in consecutive order of (1) a metal sheet of metal of atomic number less than 30 acting as a support and as ultra-violet and/or visible radiation reflective member, (2) the said fluorescent layer, and (3) permanently united with the said fluorescent layer a transparent layer or sheet having a thickness of 20 to 300,am.

Preferably the transparent layer or sheet is a resin sheet of polyethylene-terephthalate, which is for example applied by lamination by the intermediary of a transparent adhesive layer.

In a preferred embodiment of the present invention the intensifying screen is a separately constituted element, which can be separated from the photographic material. However it is within the scope of the invention to use an intensifying screen incorporating a phosphor layer coated on a support forming an integral part of a single element that also incorporates a light-sensitive silver halide emulsion layer or such layers. It is advantageous to use separatable intensifying screen and silver halide emulsion materials to enable the intensifying screen to be re-used since the screens normally contain costly phosphor particles.

In order to avoid, during the X-ray exposure, the photographic material being exposed to ambient light e.g. daylight, the fluorescent screen and photographic silver halide emulsion layer material may be shielded from that light by a light-tight envelope e.g. cassette or bag, the X-ray entrance wall of which when irradiated at an angle of 50 with an X-ray beam generated at 80 kV with tungsten cathode reduces the intensity of said beam for not more than 20%, preferably for not more than 10%.

Examples of metals of atomic number less than 30 are aluminium, titanium, chromium and nickel. Compounds of low atomic number metals which may be used in the invention are magnesium oxide, magnesium carbonate, aluminium oxide and titanium(lV) oxide.

In the accompanying drawings illustrations of transaxial tomography and a screen-film combination for use therein are provided. In particular, Figures 1 and 2 represent schematic views of embodiments of transaxial tomography, and Figure 3 represents in a sectional view a combination of an X-ray intensifying screen and silver halide film material that is suited for use according to the present invention.

Fig. 1 shows a motion-tomography system that yields a transverse-tomographic section obtained by an embodiment of transaxial tomography wherein the X-ray source 1 is kept stationary. The film 4 arranged in a cassette or light-tight bag (not shown) with an intensifying screen as exemplified in fig. 3 is rotated on a film table simultaneously with the object 3. The X-ray beam obtains a fan-shaped structure by passing through a collimator 2.The exposure angle, i.e. the angle the fan-shaped beam makes with the film plane, is in the range of 2 to 100 e.g. SO. In order to keep the X-ray absorption as low as possible the cassette or bag has a light-tight window of a material or materials having an atomic number smaller than 20 e.g. has a window made of a carbon fibre plate, light-tight resin containing dispersed carbon black; beryllium metal or aluminium metal.

Light-tight resin bags in a metal frame from which the air can be evacuated for keeping the screen and film material close together are likewise very useful. Cassettes for mammography as described in the published German Patent Specification (DT-OS) 2,727,005 are likewise particularly suited for use in transaxial tomography.

Figure 2 shows a system of transaxial tomography wherein the object 9 is kept stationary on a floor 10 and the X-ray source 11 is fixed to a rotatable arm 12. The arm 12 is provided with a central bearing 13 and shaft 14 located in the ceiling 1 5 of a building. At the end of the arm 12 opposite to the X-ray source 11 a motor 1 6 is mounted. The shaft 1 7 of the motor 1 6 is drivingly connected to a film table 1 8 to which a film cassette 19 is fixed. The film and screen system (not shown) in the cassette is obliquely irradiated with a fan-shaped X-ray beam 20 while the X-ray source 11 and the film table actuated by the motor 16 are rotated with the same angular speed but in opposite sense. By mechanisms (not shown) the X-ray source and film table 18 can be moved upwardly or downwardly to provide consecutive summation images of different sections of the object.

Fig. 3 shows a sectional view of a separatable film 30 and screen material 20 for use according to the present invention.

The screen material 20 contains on a transparent resin support 24 a calendered and therefore smooth phosphor layer 23 coated with a reflective metal layer 22, e.g. a layer of vapour-deposited aluminium. The support of the screen has a thickness not larger than 0.25 mm. The film material 30 is a conventional double-coated silver halide emulsion layer film comprising silver halide emulsion layers 25 and 26 coated on a transparent resin support 27. The arrow X indicates the angle (less than 100) under which the screen material 20 is exposed.

Because of the low atomic number of the elements (or compounds thereof) of the reflective layer 22 and its small thickness, the X-rays irradiate practically without attenuation the underlying phosphor particles 28, which expose with fluorescent light the silver halide grains 29 of the silver halide emulsion layers 25 and 26. The reflective layer 22 markedly increases the input of fluorescent light into said silver halide emulsion layers.

Reflective layers or sheets utilizing aluminium, chromium and nickel are preferred, particularly vapour deposited layers of these metals in the thickness range of 0.01 to O. mm. The phosphor layer may however be in contact with a reflective layer provided by a reflective white pigment composed of low atomic number elements e.g. magnesium carbonate or titanium dioxide, dispersed in an organic polymeric binder. The thickness of such layers is, e.g., in the range of 0.1 to 0.25 mm.

The use of vapour-deposited aluminium on top of a phosphor coating is known from the U.S.

Patent Specification 3,278,326 for the manufacture of an electron discharge tube. When the phosphor coating in said tube is excited by incident electrons, some light is emitted from the phosphor screen in the direction away from the observer. The aluminium backing reflects such light back to the direction in which the screen is viewed. Provided that the aluminium layer is in intimate contact with the phosphor layer, little light dispersion occurs and image definition is but little impaired. In commercial electron discharge tubes the electrons hit the phosphor coating under an angle substantially larger than 1 00.

In the present invention an aluminium reflective coating is preferred'not only because of the low atomic number of aluminium but also because of its high reflectance of the light of the whole visible spectrum and of ultraviolet radiation so that it is very advantageously used in combination with a phosphor layer having a substantial portion of light emitted below the 400 nm wavelength range.

In the X-ray fluorescent intensifying screens used in the present invention, preferably a phosphor offering a high conversion of X-rays into visible and/or ultra-violet radiation is used. The X-ray absorption or stopping power of the phosphor is not of primary importance because with the small angle irradiation a much longer way has to be travelled by said rays through the phosphor layer than with irradiation in perpendicular direction.

Phosphor layers that are unsuitable for use in normal screen-film radiography with perpendicular irradiation because of their rather low X-ray stopping power come into account for use in transaxial tomography if their X-ray conversion factor is high enough.

Examples of such phosphors are zinc sulphide and zinc cadmium sulphide phosphors doped with copper or silver which phosphors are normally used in fluoroscopy (see e.g. Medical X-Ray Technique, Principles and Applications by G. J. Van der Plaats, Philips Technical Library (1959) p. 141-143).

Since these phosphors emit yellowish green light the silver halide emulsion material has to be spectrally sensitized.

Other intensifying screens with high conversion efficiency contain a phosphor layer that incorporates fluorescent compounds comprising as host metal an element with atomic numbers 39 or 57 to 71, e.g. yttrium, gadolinium and/or lanthanum. Particularly suitable are the rare-earth oxysuiphide and oxyhalide fluorescent materials activated with one or more other rare-earth metals e.g.

lanthanum and gadolinium oxysulphides activated with terbium or europium or a mixture of europium and samarium, and the lanthanum or gadolinium oxyhalides e.g. oxybromides activated with terbium, cerium, thullium or dysprosium and mixtures of terbium and ytterbium. These rare-earth fluorescent materials have been extensively described in recent literature, e.g. in German Patent Specification 1,282,819, French Patent Specifications 1,540,341, 1 580,544 and 2,02 1;397, French Patent of Addition 94,579 to 1,473,531, U.K. Patent Specification 1,247,602 and U.S. Patent Specifications 3,546,128 and 3,795,814, by K. A.Wickersheim et al in "Rare-Earth Oxysulphide X-ray Phosphors", IEEE Nucíetar Science Symposium, San Francisco, October 29-31, 1969 and by-R. A. Buchanan in IEEE Transactions on Nuclear Science, February 1972, pages 81~83. These rare-earth fluorescent materials, especially the gadolinium and lanthanum oxysulphides and oxyhalides, have a high X-ray "stopping power", a high specific weight and a high conversion efficiency of X-rays into ultraviolet radiation and/or visible light so that they are particularly suited for the production of a phosphor layer having the already specified characteristics.

Phosphors suitable for use in an X-ray intensifying screen applied in the method of the present invention correspond to the following general formula: M(W-n) . Mlnowx wherein: M is at least one of the metals yttrium, lanthanum, or gadolinium, M' is at least one of the rare-earth metals dysprosium, erbium, europium, holmium, neodymium, praseodymium, samarium, terbium, thulium, or ytterbium, X is sulphur or halogen, n is 0.0002 to 0.2, and w is 1, when X is halogen, or is 2, when X is sulphur.

The preparation of fluorescent substances falling within the scope of said general formula has been described, e.g., in the French Patent Specification 1,580,544, already mentioned hereinbefore, and in the U.S. Patent Specifications 3,418,246 by Madin R. Royce and 3,418,247 by Perry N. Yocom, both issued December 24, 1 968.

Particularly useful phosphors or phosphor mixtures consist wholly or mainly of a rare-earth metalactivated lanthanum oxyhalide, said phosphor or phosphor mixture having more than half of its spectral emission above 410 nm, more than half of its spectral emission of visible light between 400 and 500 nm, and its maximum of emission in the wavelength range of 400~450 nm. Preferred phosphors of this class correspond to one of the following general formulae: La (1#fl)Tbfl3+0X wherein X is halogen such as e.g. chlorine, bromine, or fluorine, and n is from 0.006 to 0.0001, the halogen preferably being present in an amount ranging between about the stoichiometric amount and about 2.5 percent deviating thereof; or La,,~,~,,OX : TbwYbv wherein X is chlorine or bromine w is from 0.0005 to 0.006 mole of the oxyhalide, amd y is from 0.00005 to 0.005 per mole of the oxyhalide.

Cerium may replace lanthanum in an amount described in the U.K. Patent Specification 1,247,602 filed October 9, 1 969 by General Electric Co.

Other useful lanthanum oxychloride or bromide phosphors mainly emitting ultraviolet radiation are activated with thulium and are described in the U.S. Patent Specification 3,795,814 by Jacob G.

Rabatin issued March 5. 1974.

Other useful phosphors are barium fluoride halide phosphors strongly emitting ultraviolet radiation and activated with europium (II) which have been described in French Patent Specification 2,185,667 filed May 23, 1973 by Philips' Glceilampenfabrieken N.V., the published German Patent Application 2,614,305 filed April 2, 1976 by Ciba Geigy AG and U.S. Patent Specification 4,057,508.

A particularly useful barium fluoride bromide phosphor emitting ultraviolet light has been described in the U.S. Patent Specification 4,028,550.

The preparation of terbium-activated lanthanum oxychloride and lanthanum oxybromide phosphors has been described e.g. in U.K. Patent Specification 1,247,602, already mentioned hereinbefore, French Patent Specifications 2,021,398 and 2,021,399 both filed October 23, 1 969 by General Electric Co., and published German Patent Applications (DOS) 1,952,812 filed October 21, 1969 and 2,161,958 filed December 14, 1971 both by General Electric Co. Suitable lanthanum oxychloridefluoride phosphors have been described in the published German Patent Application (DOS) 2,329,396 filed June 8. 1973 by Siemens A.G.

The preparation of lanthanum oxyhalides activated with terbium and ytterbium has been described, e.g., in the published German Patent Application (DOS) 2,161,958, already mentioned hereinbefore.

According to a preferred embodiment in order to provide protection against the influence of moisture and loss of fluorescence power the rare earth oxyhalide phosphors are used in admixture with a metal organic compound, preferably an organotin compound as described in the published German Patent Application DE-OS 2,71 0,497. For the same purpose, these phosphors may be used in combination with a mineral salt which provides sulphate ions as described in Belgian Patent 861,763.

Preferably magnesium sulphate is used in admixture with said phosphors and at least one of said organic metal compounds e.g. dibutyltin mercapto propionate.

The particle size of the phosphors used in the present invention is preferably between 0.1 ym and about 20 ,um, more preferably between 1 Hm and 12 cm. This range embodies about 80% by volume of the phosphor present in the intensifying screen.

The thickness of a supported fluorescent layer may vary within a broad range but is preferably in the range of 0.03 to 0.20 mm.

The coverage of the phosphors is, e.g., in the range of about 100 to 500 g/sq.m and preferably about 150 to 300 g/sq.m.

To provide high X-ray efficiency it is preferably that a minimum amount of binder be employed in the fluorescent layer. However, the less binding agent the more brittle the layer, so that a compromise has to be made. Suitable binders for use in the preparation of the fluorescent layers are, e.g., cellulose acetate butyrate, polyalkyl acrylates, polyalkyl methacrylates (e.g. polymethyl methacrylate), a polyvinyl-n-butyral, a copoly(vinyl acetate/vinylchloride) and a copoly(acrylonitrile/butadiene/styrene) or a copoly(vinyl chloride/vinyl acetate/vinyl alcohol) or mixtures thereof.

In order to obtain a smooth-surface-reflective layer the substrate of said layer i.e. the phosphor layer, a transparent interlayer or support is preferably itself as smooth as possible. For that purpose when the reflective layer is directly applied to the phosphor layer the phosphor layer is calendered or coated or laminated with a smooth resin layer or film sheet having a thickness preferably not larger than 0.10 mm.

To the reflective layer of fig. 3 a protective coating may be applied preferably having a thickness in the range of 5 to 25 Mm and being composed of any film-forming polymeric material.

Polymeric materials suitable for that purpose include, e.g., cellulose derivatives (e.g. cellulose nitrate, cellulose triacetate, cellulose acetate proprionate, cellulose acetate butyrate), polyamides, polystyrene, polyvinyl acetate, polyvinyl chloride, silicone resins, polyacrylic ester- and polymethacrylic ester resins, fluorinated hydrocarbon resins, and mixtures of all these materials.Representative examples of various individual members of these binder materials include the following resinous materials: polymethyl methacrylate, poly(n-butyl methacrylate), polyisobutyl methacrylate, copolymers of n-butyl methacrylate and isobutyl methacrylate, copolymers of vinylidene fluoride and hexafluoropropylene, copolymers of vinylidene fluoride and trifluorochloroethylene, copolymers of vinylidene fluoride and tetrafluoroethylene, terpolymers of vinylidene fluoride, hexafluoropropylene and tetrafluoroethylene, and polyvinylidene fluoride.

Where in the fluorescent intensifying screen suited for use in the present invention a transparent support is applied preference is given to organic resin supports for their low X-ray absorption power, flexibility, mechanical strength and optical clearness. In that respect preference is given to a polyester resin support such as a polyethyleneterephthalate support.

The silver halide material for use according to the invention may be a single side coated silver halide emulsion layer material but for obtaining higher spectral densities preferably a dual silver halide emulsion layer material, i.e. a photographic material having a silver halide emulsion layer on each side of a transparent support is used. The silver halide emulsion layers are preferably coated with a thin e.g.

2 jum to 5 ym thick anti-abrasion layer.

In the method of transaxial tomography according to the present invention wherein a photographic material is used with a silver halide emulsion layer on either of the support, normally only one fluorescent intensifying screen is used as is illustrated in fig. 3 at the side of the photographic material facing the X-ray radiation contrary to common practice with such double-coated photographic materials in common radiography where either screens are used at both sides of the photographic material or only at the side of the photographic material not facing the X-ray radiation source.

in order to match with the fluorescent light of particular phosphors, e.g. those emitting green light, the spectral sensitivity of the silver halide emulsions may be enlarged or improved by spectral sensitization in the wavelength range of 400 to 500 nm with common spectrally sensitizing dyes used in silver halide emulsions, which include cyanine dyes and merocyanine dyes as well as other dyes as described by F. H. Hamer in "The cyanine dyes and related compounds", Interscience Publishers (1964). These dyes are preferably used in an amount in the range of 20 mg to 250 mg per mole of silver halide.

The silver halide in the emulsion layer(s) may comprise varying amounts of silver chloride, silver iodide, silver bromide, silver chlorobromide, silver bromoiodide, and the like, but when coated must be capable, after exposure and processing, of producing a negative silver image. Particularly good results are obtained with silver bromoiodide emulsions in which the average grain size of the silver bromoiodide crystals is in the range of about 0.1 to about 3,us, preferably to about 0.8 ym.

According to a special embodiment in the radiographic transaxial tomographic recording method of the present invention a silver halide emulsion material suited for use in the silver complex diffusion transfer process as described e.g. in GB-P 1,226,954 and US-P 3,749,912 is used. Such silver halide emulsion material normally contains a silver halide emulsion layer comprising silver bromide and a minor amount of silver iodide.

The image-forming silver halide emulsions may be chemically sensitized by any of the known procedures. They may be digested with naturally active gelatin or with small amounts of suiphurcontaining compounds such as allyl thiocyanate, allylthiourea, sodium thiosulphate, etc. They may be sensitized likewise by means of reductors, e.g. tin compounds as described in the U.K. Patent Specification 789,823 filed April 29, 1 955 by Gevaert Photo-Producten N.V., polyamines e.g.

diethyltriamine, and small amounts of noble metal compounds such as of gold, platinum, palladium, iridium, ruthenium, and rhodium as described by R. Koslowsky, Z. Wiss, Photogr. Phosphys. Photochem.

46,67~72 (1951). Representative examples of noble metal compounds are ammonium chloropalladate, potassium chloroplatinate, potassium chloroaurate and potassium aurithiocyanate.

Emulsion stabilizers and antifoggants may be added to the silver halide emulsion before or after admixture of the low-speed emulsion, e.g., the known sulphinic and selenic acids or salts thereof, aliphatic, aromatic or heterocyclic mercapto compounds or disulphides, e.g. those described and claimed in published German Patent Application 2,100,622 filed January 8, 1971 by Agfa-Gevaert A.G., preferably comprising sulpho groups or carboxyl groups, compounds of the type described in British Patents 1,209,813 and 1,239,01 7 comprising solubilizing groups e.g. sulphogroups or such compounds not containing a solubilizing group, mercury compounds, e.g. those described in Belgian Patent Specifications 524,121 filed November 7, 1953 by Kodak Ltd., 677,337 filed March 4, 1966, 707,386 filed December 1, 1 967 and 709,195 filed January 11, 1 968 all by Gevaert-Agfa N.V., and tetraazaindenes as described by Birr in Z. Wiss. Photogr. Photophys. Photochem. 47, 2~58 (1952), e.g.

the hydroxytetra-azaindenes of the following general formula:

wherein: each of R1 and R2 represents hydrogen, an alkyl, an aralkyl or an aryl group, and R3 represents hydrogen, an alkyl, a carboxyl, or an alkoxycarbonyl group, such as 5-methyl-7 hydroxy-s-triazolo [ 1 ,5-a ] -pyrimidine.

Other additives may be present in one or more of the hydrophilic colloid layers of the radiationsensitive silver halide elements of the present invention, e.g. hardening agents such as formaldehyde, dialdehydes, hydroxyaldehydes, mucochioric and mucochioric acid, acrolein, and glyoxal, mordanting agents for anoinic colour couplers or dyes formed therefrom, plasticizers and coating aids e.g. saponin, e.g. dialkylsulphosuccinic acid salts such as sodium diisooctylsulphosuccinate, alkylarylpolyether sulphuric acids, alkylarylpolyethyl ether sulphonic acids, carboxyalkylated polyethyleneglycol ethers or esters as described in French Patent Specification 1,537,417 filed September 18, 1967 by Agfa Gevaert N.V. such as iso-C8H17~CsH4(OCH2CH2)8OCH2COONa, fluorinated surfactants e.g. those described in Belgian Patent Specification 742,680 filed December 5, 1 969 by Gevaert-Agfa N.V. and the published German Patent Applications 1,950,121 filed October 4,1969 by du Pont de Nemours and 1,942,665 filed August 1969 by Ciba A.G., inert particles such as silicon dioxide, glass, starch and polymethyl methacrylate particles. These particles are normally present in the gelatin antistress layer coated over the silver halide emulsion layer of the radiographic material. The gelatin antistress layer of radiographic material can be treated in order to reduce the friction coefficient and tackiness with so-called Werner-complexes which are chromium, aluminium, zirconium or vanadium complexes of fatty acids including perfluorinated fatty acids e.g.Quilon (trade mark of du Pont for a chromium complex of stearic acid) and Scotchban Paper Size-FC 805 (trade name of 3M for a chromium complex of a perfluorocarboxylic acid). The Werner-complexes can be added to the coating composition of the antistress layer but they are preferably applied to the coated antistress layer via an aqueous solution.

For the purpose of accelerating the development, the exposed photographic material is developed preferably in the presence of development accelerators. These development accelerators can be used either in the silver halide emulsion, in adjacent layer(s) or in the developing bath. They include alkylene oxide compounds of various types, e.g. alkylene oxide condensation products or polymers as described in U.S. Patent Specifications 1,970,578 of Conrad Schoeller and Max Wittner issued August 21, 1 934, 2,240,472 of Donald R.Swan issued April 29, 1941, 2,423,549 of Ralph Kingsley Blake William Alexander Stanton and Ferdinand Schulze issued July 8, 1947, 2,441,389 of Ralph Kingsley Blake issued May 11, 1948, 2,531,832 of William Alexander Stanton issued November 1950 and 2,533,990 of Ralph Kingsley Blake issued December 1 2, 1 950 and in U.K. Patent Specifications 920,637 filed May 7, 1959, 940,051 filed November 1, 1961, 945,340 filed October 1961, all by Gevaert Photo-Producten N.V., 991,608 filed June 14, 1961 by Kodak Ltd. and 1,015,023 filed December 24, 1 962 by Gevaert Photo-Producten N.V.Other development accelerating compounds are onium and polyonium compounds preferably of the ammonium, phosphonium, and sulphonium type, e.g. trialkyl sulphonium salts such as dimethyl-n-nonyl sulphonium p-toluene sulphonate, tetraalkyl ammonium salts such as dodecyl trimethyl ammonium o-toluenesulphonate, alkyl pyridinium and alkyl quinolinium salts such as 1 -m-nitrobenzylquinolinium chloride and 1-dodecylpyridinium chloride, bis alkylenepyridinium salts such as N,N'-tetramethylene bispyridinium chloride, quaternary ammonium and phosphonium polyoxyalkylene salts especially polyoxyalkylene bispyridinium salts, examples of which can be found in U.S. Patent Specification 2,944,900 of Burt H. Carroll, Hubert S. Elins, James L. Graham and Charles V.Wilson issued July 12, 1 960, etc.

After radiographic exposure the radiographic silver halide elements are developed, preferably in an energetic surface developer. The high energy is required in order to allow the development to proceed quickly and may be obtained by properly alkalizing the developing liquid (pH 9~12), by using high-energy developing substances or a combination of developing substances, which as a consequence of their superadditive action is very energetic.

Economy on the silver halide in the emulsion may be realized by building up the image density partly with dyes. Such may proceed by introducing (a) colour coupler(s) into the emulsion, which at least at the stage of development form(s) (a) dye(s) with the oxidation product of an aromatic primary amino developing agent, e.g. of the p-phenylenediamine type, which dye(s) absorb(s) in the visible part of the spectrum.

Further it is known that a relatively high maximum density and contrast can be obtained even with a low amount of silver halide content per unit of surface when a colour image is produced together with a silver image as is described, e.g., in the published German Patent Application (DE-OS) 1,946,652 filed September 1 5, 1969 by Agfa-Gevaert A.G. The image contrast of such combined image is increased by viewing with light absorbed by the colour image e.g. yellow light.

When a colour development is applied, preferably so-called 2-equivalent couplers are used to further reduce the consumption of silver. Thus only 2 instead of 4 molecules of exposed silver halide are necessary for the production of 1 dye molecule. Such couplers contain in the coupling position a group that is split off, e.g. a halogen atom such as iodine, bromine, or chlorine (see e.g. the U.S. Patent Specification 3,006,759 of Anthony Loria, Warren A. Reckhow and limari F. Salminen issued October 31, 1 961). The density of the image is thus realized by addition of the densities of the silver image(s) combined with the dye image(s).

For improving the information content retrieval phenol or a-naphthol type colour couplers are particularly suitable that on colour development of the silver halide with an aromatic primary amino developing agent form a quinoneimine dye mainly absorbing in the red and also absorbing in the green and having an absorption maximum in the spectral wavelength range of 550 to 700 (ref. e.g. to the published German Patent Application (DE-OS) 1,946,652 as mentioned hereinbefore).

Phenol couplers suited for that purpose correspond, e.g., to the following general formula:

wherein: each of R1 and R2 represents a carboxylic acid acyl or sulphonic acid acyl group including said groups in substituted state, e.g. an aliphatic carboxylic acid acyl group, an aromatic carboxylic acid acyl group, an heterocyclic carboxylic acid acyl group, e.g. a 2-furoyl group or a 2-thienoyl group, an aliphatic sulphonic acid acyl group, an aromatic sulphonic acid acyl group, a sulphonyl thienyl group, an aryloxy-substituted aliphatic carboxylic acid acyl group, a phenyl carbamyl aliphatic carboxylic acid acyl group, or a tolyl carboxylic acid acyl group.

For such types of phenol colour couplers and their preparation reference may be made to U.S.

Patent Specifications 2,772,162 of Ilmari F. Salminen and Charles R. Barr issued November 1 956 and 3,222,176 of Jan Jaeken issued December 7, 1965 and to U.K. Patent Specification 975,773 filed September 4, 1961 by Gevaert Photo-Producten N.V.

When colour images are prepared together with silver images normally aromatic primary amino colour developing agents are used, e.g. N,N-dialkyl-p-phenylenediamines and derivatives thereof, e.g.

N,N-diethyl-p-phenylenediamine, N-butyl-N-sulphobutyl-p-phenylenediamine, 2-amino-S- diethylaminotoluene hydrochloride, 4-a mino-N-ethyl-N(-P-methane sulphonamidoethyl)-m-tol uidine sesquisulphate monohydrate and N-hydrnxy-ethyl-N-ethyl-p-phenylenediamine. The colour developer may be used together with black-and-white developing agents, e.g. 1 -phenyl-3-pyrazolidinone and pmonomethylaminophenol, which are known to have a super-additive effect on colour development (see L. F. A. Mason, J. Phot.Sci. 11(1963)136-139), and other p-aminophenol derivatives, e. g. those according to French Patent Specification 1,283,420 filed February 16, 1961 by liford Ltd. such as 3 methyl-4-hydroxy-N,N-diethyl-ani line, 3-methyl-4-hydrnxy-N-ethyl-N-#-hydrnxyethyl-aniline, 1 methyl- or 1 -hydroxyethyl-6-hydroxy- 1 ,2,3,4-tetrahydroquinoline, 1 -P-hydroxyethyi-6-hydroxy- 1 ,2,3,4-tetrahydroquinoline and N-(4-hydroxy-3'-methylphenyl)-pyrrolidine. It is also possible to use combinations of aromatic primary amino colour developing agents to obtain an increased rate of colour development (see e.g.German Patent Specification 954,311 filed December 5, 1 953 by Agfa A.G. and French Patent Specification 1,299,899 filed September 8, 1961 by Agfa At.); favourable effects are obtained, e.g., by the use of N-ethyl-N-2-hydroxyethyl-p-phenylenediamine together with N-butyl-N sulphobutyl-p-phenylenediamine, 2-amino-5-diethylamino-toluene hydrochloride or N,N-diethyl-p phenylenediamine hydrochioride.

The developing solutions may also comprise any of the usual additional ingredients, e.g. sodium sulphite and hydroxylamine or derivatives thereof, hardening agents, antifoggants, e.g. benzotriazole, 5-nitro-benzimidazole, 5-nitro-indazole, halides such as potassium bromide, silver halide solvents, toning and intensifying compounds, solvents, e.g. dimethylformamide, dimethylacetamide and nmethylpyrrolidone for chemical ingredients that are difficult to dissolve in the preparation of the developing solutions or that tend to precipitate upon standing.

The radiation-sensitive emulsions for use in the present invention may be coated on a wide variety of supports, e.g. films of cellulose nitrate, cellulose esters, polyvinylacetal, polystyrene, polyethylene terephthalate and other polyester materials as well as cg-olefin-coated papers, e.g. paper coated with polyethylene or polypropylene.

Preferred supports comprise a linear condensation polymer, blue coloured polyethylene terephthalate being an example thereof.

The supports used in the present recording materials may be coated with subbing layers for improving the adhesion of (a) gelatino-silver halide emulsion layer(s) thereto.

The mechanical strength of melt-extruded supports of the polyester type can be improved by stretching. In some cases as described in the U. K. Patent Specification 1,234,755 filed September 28, 1967 by Gevaert-Agfa N.V. the support may carry a subbing layer in the stretching stage.

Suited subbing layers are known to those skilled in the art of silver halide photography. With regard to the use of hydrophobic film supports reference is made to the composition of subbing layers described in the latter U.K. Patent Specification.

According to said Patent Specification a hydrophobic film support has 1) a layer that is directly adherent to said hydrophobic film support and comprises a copolymer formed from 45 to 99.5% by weight of at least one of the chlorine-containing monomers vinylidene chloride and vinyl chloride, from 0.5 to 10% by weight of at least one ethylenically unsaturated hydrophilic monomer, and from 0 to 54.5% by weight of at least one other copolymerisable ethylenically unsaturated monomer, and 2) a layer comprising in a ratio of 1:3 to 1:0.5 by weight a mixture of gelatin and a copolymer of 30 to 70% by weight of butadiene with at least one copolymerisable ethylenically unsaturated monomer. The first layer can be applied before or after longitudinal stretching and the second layer can be applied before or after transversal stretching.

The exposed radiographic elements of the present invention are preferably processed in an automatic processing apparatus for X-ray films in which the photographic material may be guided automatically and at a constant speed from one processing unit to the other, but it will be understood by those skilled in the art that the radiographic image recording elements disclosed herein can also be processed apart from the above mentioned automatic processing apparatus in a variety of ways, such as by using the manual conventional multi-tank methods well known in the art.

For common emulsion preparation processes, the use of particular emulsion ingredients and processing with respect to the light-sensitive material used in the present invention reference is made in general to the Research Disclosure-issue of 1978, No. 31,332, in which the following terms are dealt with in more details: I/ll Emulsion preparation and types/emulsion washing Ill Chemical sensitization IV Spectral sensitization V Brighteners Vl Antifoggants and stabilizers VIII Absorbing and scattering materials IX Vehicles X Hardeners Xl Coating Aids XII Plasticizers and lubricants XIII Antistatic layers XIV Methods of Addition XV Coating and drying procedures XVI Matting Agents XVII Supports XIX Processing XX Developing Agents XXI Development modifiers, and XXII Physical development systems.

The following examples illustrate the present invention.

Example 1 Preparation of the Fluorescent Screen Material.

The fluorescent screen used according to the invention was prepared as follows: 89 g of LaOBr : 0.002 Tb : 0.0005 Yb phosphor particles prepared according to the method described in the published German Patent Application 2,161,958, were dispersed in a solution of 11 g of Elvacite 2044 (trade name of du Pont de Nemours, Wilmington, Del., USA, for a high-molecular weight poly(n-butyl methacrylate) in 25 g of toluene.

The obtained dispersion was filtered through a filter having pores with a mean diameter of 75 ym and was deaerated by subjecting it to a pressure of 100 mbar (100 cm of water). The average grain size of the phosphor particles was 5 ym.

The dispersion was coated onto a subbed polyethylene terephthalate resin support of 100 4m.

The subbing layer was produced from a latex on the basis of a copolymer of vinyl chloride, vinylidene chloride, n-butyl acrylate and itaconic acid (weight ratio : 63/30/5/2).

The coating of the phosphor dispersion was effected in such a way that 200 g of phosphor were applied per sq.m.

The phosphor coating was dried and after calendering the screen was put in a vacuum chamber wherein under reduced pressure an aluminium film of 0.07 mm was vapour-deposited on the phosphor coating.

Preparation of a Dual-coated Silver Halide Film A silver bromoiodide X-ray emulsion (2.0 mole % of silver iodide) was prepared in such a way that it contained silver halide grains with an average grain size of 0.75 ym and comprised per kg 80 g of gelatin and an amount of silver halide corresponding to 190 g of silver nitrate. As stabilizing agents the emulsion contained per kg 545 mg of 5-methyl-7-hydroxy-s-triazolo [ 1 ,5-a ] pyrimidine, 6.5 g of 1phenyl-5-mercapto-tetrazole and 0.45 mg of mercurycyanide.

The above emulsion was coated in such a way on either side of a polyethylene terephthalate support subbed on both sides that on either side a silver halide emulsion layer was obtained containing an amount of silver halide equivalent to 11 g of silver nitrate per sq.m.

Exposure Procedure The silver halide film was arranged with one of its silver halide emulsion layers in contact with the support of the phosphor layer of the above screen and put into a cassette having an X-ray transparent black resin window as described in the published German Patent Application 2,727,005. The radiographic screen-film system was exposed under an angle of 50 on a rotating turntable to 80 kV X ray radiation through a synchronously rotating test object being a polymethylmetacrylate resin cylinder of 30 cm diameter in order to determine the system speed. The exposure proceeded as illustrated in Fig. 1 with the screen and film arranged as illustrated in Fig. 3.

After removal of the fluorescent screen the radiographic film material was processed in an automatic 90 seconds processing machine; the development occurred for 23 s at 350C in Agfa Gevaert's hardening developer G 138, which comprises hydroquinone and 1 -phenyl-3-pyrazolidinone as developing agents and glutardialdehyde as a hardener.

It was established that the combined use of the above fluorescent screen with the above radiographic silver halide material yielded a photographic speed being a factor 82 larger than that of the same radiographic film element used without said screen and that the photographic speed was 1.86 times as high as that of the same screen-film combination but containing no reflective aluminium coating.

Example 2 Preparation of the Fluorescent Screen Material.

Onto a polyethylene terephthalate resin support of 1 80 #m an aluminium coating of 0.07 mm was vapour-deposited.

Onto the aluminium layer a phosphor dispersion prepared as follows was coated in such a way that 200 g of phosphor were applied per sq.m.

89 g of LaOBr : 0.002 Tb :0.0005 Yb phosphor particles prepared according to the method described in the published German Patent Application 2,161,958, were dispersed in a solution of 11 g of Elvacite 2044 (trade name of du Pont de Nemours, Wilmington, Del., USA, for a high-molecular weight poly(n-butyl methacrylate) in 25 g of toluene.

The obtained dispersion was filtered through a filter having pores with a mean diameter of 75 #m and was deaerated by subjecting it to a pressure of 100 mbar (100 cm of water). The average grain size of the phosphor particles was 5,us.

Preparation of a Dual-coated Silver Halide Film A silver bromoiodide X-ray emulsion (2.0 mole % of silver iodide) was prepared in such a way that it contained silver halide grains with an average grain size of 0.75 #m and comprised per kg 80 g of gelatin and an amount of silver halide corresponding to 190 g of silver nitrate. As stabilizing agents the emulsion contained per kg 545 mg of 5-methyl-7-hydroxy-s-triazolo [ l ,5-a ] pyrimidine, 6.5 g of 1phenyl-5-mercapto-tetrazole and 0.45 mg of mercurycyanide.

The above emulsion was coated in such a way on either side ot a polyethylene terephtnalate support subbed on both sides that on either side a silver halide emulsion layer was obtained containing an amount of silver halide equivalent to 11 g of silver nitrate per sq.m.

Exposure Procedure The sandwich of silver halide film and the above fluoresent screen having between one of the emulsion layers and the fluorescent layer of the screen a removable polyethylene terephthalate film sheet of a thickness of 100 ym, was put into a flat black plastic bag from which the air was removed for obtaining a good contact between the elements.

The flat bag containing said combination of elements was put on a rotating turntable with the screen material directed towards the X-ray source and exposed under an angle of 50 to 80 kV X-rays radiation as illustrated in Fig. 1, the test object being a synchronously rotating polymethylacrylate resin cylinder of 30 cm diameter.

After removal of the fluorescent screen and of the intermediary resin film sheet the radiographic film material was processed in an autometic 90 seconds processing machine; the development occurred for 23 s at 350C in Agfa-Gevaert's hardening developer G 138, which comprised hydroquinone and 1 -phenyl-3-pyrazolidinone as developing agents and glutardialdehyde as a hardener.

It was established that the described use of the above fluorescent screen with the above radiographic silver halide material yielded a photographic speed being a factor 80 larger that that of the same radiographic film element used without said screen and that the photographic speed was 1.86 times as high as that of the same screen-film combination but containing no reflective aluminium coating.

Claims (26)

Claims

1. A method of producing a transaxial tomograph by transaxially tomographically X-ray exposing at an angle within the range of 20 to 100 a photographic silver halide emulsion layer material, characterized in that a combination of the said photographic material and an X-ray fluorescent intensifying screen is used whereby the screen upon being struck by X-rays emits fluorescent light that exposes the silver halide emulsion layer, the said screen including (1) an ultra-violet and/or visible radiation reflective coating or sheet containing one or more elements of atomic number less than 30 (or compounds of such elements) and having a thickness such that an X-ray beam generated at 80 kV with a tungsten cathode and striking that coating or sheet at an angle of 50 is attenuated by not more than 20%, and (2) a fluorescent layer containing one or more phosphors which emit ultra-violet and/or visible radiation when struck by X-rays, and having a coverage of said phosphor less than 500 g of phosphor per sq.m to provide the said screen with respect to the said photographic silver halide emulsion layer an intensification factor of at least 20 at 80 kV, the intensification factor being a value obtained by dividing the radiation dose of X-rays incident and necessary for producing a silver image of given optical density D(D:1.00) without the intensifying screen, by the radiation dose required to produce a silver image of the same density with said screen under the same conditions.

2. A method according to claim 1, wherein the said reflective layer or sheet has the capacity of reflecting more than 50% of incident ultra-violet and/or visible radiation.

3. A method according to claim 1 or 2, wherein the said combination of the silver halide emulsion layer material and fluorescent screen comprises (i) the said silver halide photographic material including a support therefor, and (ii) the said X-ray fluorescent intensifying screen which comprises the said ultra-violet and/or visible radiation reflective coating or sheet, the said fluorescent layer being in direct contact therewith or separated therefrom by a transparent subbing layer, and a transparent layer or sheet permanently or separatably united with said fluorescent layer separating the said silver halide emulsion layer from the said fluorescent layer and having a thickness of 20 to 300,am.

4. A method according to any of claims 1 to 3, wherein the said X-ray fluorescent intensifying screen consists in consecutive order of (1) the said ultra-violet and/or visible radiation reflective coating, (2) the said fluorescent layer in direct permanent contact with said reflective coating or separated therefrom by a smooth transparent interlayer of up to 50 #m thickness, and t3) as a support for the said fluorescent layer a transparent sheet having a thickness of 100 to 300 him.

5. A method according to any of claims 1 to 3, wherein the said X-ray fluorescent intensifying screen consists in consecutive order of: (1) a resin film support having a thickness such that an X-ray beam generated at 80 kV with a tungsten cathode and striking that support at an angle of 50 is attenuated by not more than 10%, (2) permanently coated onto the said resin film the said ultra-violet and/or visible radiation reflective coating, (3) the said fluorescent layer, and (4) permanently in contact with the said fluorescent layer a transparent layer or sheet having a thickness of 20 to 300 Mm.

6. A method according to any of claims 1 to 3, wherein the said X-ray fluorescent intensifying screen consists in consecutive order of (1) a metal sheet of metal of atomic number less than 30 acting as a support and as the said ultra-violet and/or visible radiation reflective member, (2) the said fluorescent layer, and (3) permanently united with said fluorescent layer a transparent layer or sheet having a thickness of20to300,um.

7. A method according to any of claims 1 to 6, wherein the said fluorescent screen and the said photographic silver halide emulsion layer material are shielded from ambient light by a light-tight envelope the X-ray entrance wall of which when irradiated at an angle of 50 with an X-ray beam generated at 80 kV with tungsten cathode reduces the intensity of said beam for not more than 20%.

8. A method according to any of claims 3 to 6, wherein the said transparent layer or sheet is an organic resin layer or sheet.

9. A method according to any of claims 1 to 8, wherein the said metals of atomic number less than 30 are aluminium, chromium or nickel.

10. A method according to claim 9, wherein the said reflective layer is formed from aluminium vapour-deposited in a thickness of 0.01 to 0.1 mm onto a transparent sheet.

11. A method according to any of claims 1 to 10, wherein the said fluorescent layer incorporates fluorescent zinc sulphide or zinc-cadmium sulphide, which sulphides are doped with copper or silver.

12. A method according to any of claims 1 to 10, wherein the said fluorescent layer incorporates fluorescent compounds comprising as host metal an element with atomic numbers 39 or 57 to 71.

13. A method according to claim 11, wherein the said compounds are rare-earth oxysulphide or oxyhalides activated with one or more other rare-earth metals.

14. A method according to claim 13, wherein the said fluorescent layer contains a barium fluoride halide phosphor activated with europium (II).

1 5. A method according to any of claims 1 to 14, wherein the thickness of the said fluorescent layer is in the range 0.03 to 0.200 mm and the coverage of the phosphors in the range 1 50 to 300 g per sq.m.

16. A method according to any of claims 1 to 4, wherein the said phosphor layer is calendered or coated or laminated with a smooth transparent resin layer or film sheet onto which a reflective metal layer is vapour-deposited.

17. A combination of materials suitable for axial tomographic radiography with reduced mottle which combination comprises (i) a silver halide photographic material comprising a support and at least one silver halide emulsion layer thereon, and (ii) an X-ray fluorescent intensifying screen which comprises (1) an ultra-violet and/or visible radiation reflective coating or sheet containing one or more elements of atomic number less than 30 (or compounds of such elements) and having a thickness such that an X-ray beam generated at 80 kV with a tungsten cathode and striking that coating or sheet at an angle of 50 is attenuated by not more than 20%, and in direct contact with the said coating or sheet or separated therefrom by a transparent subbing layer, (2) a fluorescent layer containing one or more phosphors which emit ultra-violet and/or visible radiation when struck by X-rays, and having a coverage of said phosphor less than 500 g of phosphor per sq.m to provide the said screen with respect to the said photographic silver halide emulsion layer an intensification factor of at least 20 at 80 kV, and (3) a transparent layer or shaft permanently or separatably united with the said fluorescent layer separating the said silver halide emulsion layer from the said fluorescent layer and having a thickness of 20 to 300 ym, the intensification factor being a value obtained by dividing the radiation dose of X-rays incident and necessary for producing a silver image of given optical density D(D:1.00) without the intensifying screen, by the radiation dose required to produce a silver image of the same density with said screen under the same conditions.

1 8. An X-ray fluorescent intensifying screen comprising an ultra-violet and/or visible radiation reflective coating or sheet containing one or more elements of atomic number less than 30 or compounds of such elements and having a thickness such that an X-ray beam generated at 80 kV with a tungsten cathode and striking that coating or sheet at an angle of 50 is attenuated for not more than 20%, and in direct contact with the said coating or sheet or separated therefrom by a transparent subbing layer, (2) a fluorescent layer containing one or more phosphors which emit ultra-violet and/or visible radiation when struck by X-rays, and having a coverage of said phosphors less than 500 g of phosphor per sq.m to provide the said screen with respect to the said photographic silver halide emulsion layeran intensification factor of at least 20 at 80 kV, the intensification factor being a value obtained by dividing the radiation dose of X-rays incident and necessary for producing a silver image of given optical density D(D=1.00) without the intensifying screen, by the radiation dose required to produce a silver image of the same density with said screen under the same conditions, and (3) a transparent layer or sheet permanently or separatably united with the said fluorescent layer separating said silver halide emulsion layer from the said fluorescent layer and having a thickness of 20 to 300 ym.

1 9. An X-ray fluorescent intensifying screen according to claim 1 8, wherein it comprises in consecutive order (1) the said ultra-violet and/or visible radiation reflective coating, (2) the said fluorescent layer in direct permanent contact with the said reflective coating, or through the intermediary of a smooth transparent interlayer of thickness up to 50 ym, and (3) as a support for the said fluorescent layer a transparent sheet having a thickness between 100 and 300 cm.

20. An X-ray fluorescent intensifying screen according to claim 1 8, wherein it consists in consecutive order of (1) a resin film support having a thickness such that an X-ray beam generated at 80 kV with a tungsten cathode and striking that support at an angle of 50 is attenuated by not more than 10%, (2) permanently coated onto said support the said ultra-violet and/or visible radiation reflective coating, (3) the said fluorescent layer, and (4) permanently united with the said fluorescent layer a transparent layer or sheet having a thickness of 20 to 300 ym.

21. An X-ray fluorescent intensifying screen according to claim 18, wherein it consists in consecutive order (1) a metal sheet of metal of atomic number less than 30 acting as a support and as ultra-violet and/or visible radiation reflective member, (2) the said fluorescent layer, and (3) permanently united with the said fluorescent layer a transparent layer or sheet having a thickness of 20 to 300 ym.

22. An X-ray fluorescent intensifying screen according to claim 19, wherein the said resin film support is made of polyethylene terephthalate.

23. An X-ray fluorescent intensifying screen according to any of claims 18 to 22, wherein the said reflective coating or member is made of aluminium.

24. A method as claimed in claim 1, substantially as hereinbefore described, with particular reference to the Examples.

25. A combination as claimed in claim 17, substantially as hereinbefore described, with particular reference to the Examples.

26. An X-ray intensifying screen as claimed in claim 18, substantially as hereinbefore described, with particular reference to the Examples.