System and method for radiological image formation

DESCRIPTION
1. FIELD OF THE INVENTION
This invention relates to a system for radiological image formation by means of a suitable film material in operative association with an intensifying screen and a method for image formation.
2. BACKGROUND OF THE INVENTION
During the last decade there is an ever lasting demand in medical diagnosis to get an image in quite a short time after the patient has been exposed to X-rays with preferably minimum radiation doses. An important step having a determining influence on the total time between exposure of the patient and examination by the radiologist is the processing time. Materials coated from emulsions having crystals rich in silver chloride are advantageous with respect to rapid processing (shorter developing times as well as fixation times for the said crystals) if compared with those coated from emulsions rich in silver bromide or silver bromoiodide (see e.g. EP-A 0 712 036 which is illustrative for mammographic applications) as has been demonstrated e.g. in EP-A 0 794 456. These references are illustrative for the feasibility in diverse applications of using small cubic grains rich in silver chloride having a crystal diameter of less than 0.65 .mu.m.
When images are generated from silver halide photographic film materials exposed to appropriate visible light for which the film materials are made sensitive, wherein said visible light is generated by conversion of X-ray irradiated intensifying screens held in intimate contact with said screens, subsequently followed by processing of the said film materials, then it is a stringent requirement to obtain a high covering power and a good image tone (color hue) of the developed silver, preferably a purely black image, in the already mentioned rapid processing conditions. The said image tone is closely related with the crystal size of cubic silver halide emulsions as becomes clear from e.g. EP-A 0 555 897. So it is well-known that in order to get a suitable "pure black" image tone, cubic grains having a lower sphere equivalent diameter are preferred.
It is further clear that in wet processing conditions chemical waste after processing of the said materials should preferably be reduced to minimum amounts. Therefore it is recommended to reduce replenishing amounts of developer and fixer. Especially when silver halide photographic materials are strongly hardened cross-over of liquid processing solutions and of rinsing water is reduced to a minimum and the drying time of the processed material is considerably reduced.
Otherwise as there is a demand for environmental friendly or ecologically justified systems for image formation in order to minimize the load of the environment at the level of the customer low coating amounts of silver halide in the silver halide material are preferred. However the said low coating amounts may lay burden on the preferred high covering power of the developed crystals. In EP-A 0 709 730 and in EP-Application No. 96203728, filed Dec. 30, 1996, it has further been shown that in a developer having an adapted chemical composition, covering power is advantageously increased.
Strongly hardened silver halide photographic materials moreover provide the advantage to use concentrated developing and fixing solutions free from hardening agents as has been set forth e.g. in U.S. Pat. No. 5,296,342, which again is in favour of ecology.
From the side of the manufacturer of silver halide photographic film materials, it is thus of utmost importance to provide strongly hardened films coated from low amounts of silver halide in favour of consumption of low amounts of chemicals, wherein said films can be processed in a processing cycle wherein hardener free processing solutions are used, without loosing speed or covering power in short (rapid) processing times. Efficient cross-linking of the gelatin chains of the photographic material indeed reduces the amount of water absorption in the processing cycle of the said material comprising cubic emulsion crystals having a grain diameter of less than 0.65 .mu.m.
An ever lasting demand however will remain to further improve the sensitivity (speed) of the said crystals from the side of crystal habit and/or composition on one hand and chemical and/or spectral sensitization at the other hand, especially under the severe limiting circumstances described above.
In order to get high speed it is well-known that use of crystals rich in silver bromide is feasible. Indeed from a more fundamental point of view silver bromide and silver chloride strongly differ in the energetical position of the highest valence energy level. Particularly silver halide crystals rich in chloride, spectrally sensitized with a green, red or even infrared dye, show a limited quantum efficiency due to instable dye holes (electron vacancies) on the silver halide crystal surface, leading to recombination between photo-electrons and dye positive holes. Especially for high coverage degrees of one or more dyes, it is required to enhance absorption of light emitted from phosphors present in intensifying screens at the crystal surface as "desensitization" is a factor limiting sensitivity. In order to overcome recombination the most suitable way consists in adding of a reducing agent, also called "supersensitizer", which may however become a competitive adsorbing compound for the sensitizing dye, limiting thereby its "maximum dye coverage". Moreover the presence of such organic dye compounds in amounts of mmoles per mole of silver is disadvantageous from the point of view of residual coloration of the processed material and coloration of processing liquids. Another disadvantage introduced by addition of such reducing agent is the generation of fog and poor preservability of the coated materials.
3. OBJECTS OF THE INVENTION
Therefore it is an object of the present invention to provide a system for radiological image formation having a so-called "400" speed (high speed), offered by means of a suitable double-side coated or duplitized silver halide photographic film material in operative association with an intensifying screen, wherein after exposure to X-ray irradiation an image is formed in hardener free processing, wherein use is made of minimum replenishing amounts of chemicals, in favour of ecology, within a dry-to-dry cycle time of less than 90 seconds, offering besides said "400" speed a suitable black image tone.
Particularly it is an object of the present invention to reach the required high speed, without fog generation or preservation problems for the film-screen system wherein emulsion crystals rich in silver chloride are predominantly used.
4. SUMMARY OF THE INVENTION
According to the present invention, an image-forming system for radiological imaging is disclosed said system consisting of an intensifying screen comprising on a support at least one layer of a green-light emitting phosphor and, in operative association therewith, a prehardened light-sensitive photographic silver halide film material, comprising a support and on both sides thereof one or more hydrophilic colloid layers, said layers being hardened to such an extent that their swelling degree is reduced to less than 200% after immersing said material for 2 minutes in demineralized water of 35.degree. C.; further comprising in at least one of said hydrophilic layers chemically ripened, monodisperse essentially cubic silver chloroiodide grains having a mean crystal diameter of from 0.40 .mu.m up to 0.65 .mu.m; wherein said grains have been spectrally sensitized in a wavelength range between 520 and 580 nm, have a maximum absorption between 540 and 500 nm and have been coated in a total amount of silver per sq.m. of from 6 g up to 8 g, wherein said amount is expressed as an equivalent amount of silver nitrate per sq.m.; said image-forming system being characterized in that said silver chloroiodide grains have been doped with a complex ion compound satisfying the formula
[ML.sub.6 ].sup.n- (I)
wherein
M represents a filled frontier orbital polyvalent metal ion;
L.sub.6 represents six coordination complex ligands which are independently selected, provided that at least three, more preferred four, of the said ligands are more electronegative than any halide ligand and at least four of the said ligands are anionic ligands; n=1, 2, 3 or 4;
and wherein an energetic distance between filled higher occupied (HOMO) and lower unoccupied (LUMO) molecular orbital is more than 27000 cm.sup.-1.
More preferably said grains have further been spectrally sensitized with a combination of imidacarbocyanine and oxacarbocyanine dyes.
Moreover a method of image formation is provided by means of the said image-forming system wherein said image formation comprises the step of processing said film material after exposure with light emitted by a green-light emitting phosphor of an intensifying screen after conversion of X-rays having an energy from 60 to 150 kVp, wherein said step of processing proceeds in an automatic processor.
In a preferred embodiment said processing is performed within a total processing cycle of less than 90 seconds comprising the steps of developing in a developing solution comprising (iso)ascorbic acid, 1-ascorbic acid, reductic acid, salts and/or derivatives thereof; fixing in a fixer solution free from aluminum salts; rinsing and drying.
Replenishing of said developing and fixer solution preferably proceeds with amounts of replenisher in the range from 100 up to 200 ml/m.sup.2 and from 50 up to 150 ml/m.sup.2 respectively.
5. DETAILED DESCRIPTION
Quite unexpectedly it has become clear from our experiments that even when a light-sensitive emulsion layer from the material of the image-forming system according to the present invention comprises relatively small cubic silver chloroiodide crystals having an average grain size of from 0.40 up to 0.65 .mu.m and a monodisperse grain size distribution a sufficient speed was attained without deterioration of image tone, in that no shift to brown colored silver after development was observed, provided that one or more complex ion compounds according to the general formula (I) represented hereinbefore was(were) added as a dopant to the silver chloroiodide emulsion crystals during precipitation, thereby forming shallow electron traps.
Presence of the said dopants forming shallow electron traps (SETs) in cubic silver chloroiodide crystals makes the magnitude of the electron paramagnetic resonance signal at g=1.88.+-.0.01 increase by at least 20%; following the procedure described in Research Disclosure 36736 (November 1994). Therein an EPR-test has been run by cooling three different samples of each emulsion to 20, 40 and 60.degree. K respectively, exposing each sample to the filtered output of a 200 W Hg lamp at a wavelength of 365 nm, further measuring the EPR signal during exposure. If, at any of the selected observation temperatures, the intensity of the electron EPR signal is significantly enhanced (by at least 20%) when a comparison is made between two emulsion samples being identical, except for the presence or the absence of complexion compounds according to the general formula (I) given hereinbefore, then the energetic distance between filled higher occupied (HOMO) and lower unoccupied (LUMO) molecular orbital is more than 27000 cm.sup.-1.
The LUMO is defined as the "lowest unoccupied molecular orbital" of the related complex, whereas the HOMO is the "highest occupied molecular orbital" as has been described by D. F. Shriver, P. W. Atkins and C. H. Langford in "Inorganic Chemistry"-Oxford Univ. Press (1990), Oxford-Melbourne-Tokyo.
The energetic distance between HOMO and LUMO of the compounds is experimentally attainable from spectroscopic measurements. Dissolved in an appropriate solvent and measured by UV-VIS absorption spectroscopy e.g. in tranmission, no absorption may be detected lower than 27000 cm.sup.-1 for the compounds satisfying the requirements of the present invention.
Useful examples of such ligands are e.g. ligands selected from the group consisting of cyanide, cyanate, isocyanate, thiocyanate, carbonyl, nitrosyl and thionitrosyl.
Moreover in a more preferred embodiment spectral sensitization should be performed with a suitable combination of oxacarbocyanines and an imidacarbocyanines as spectral sensitizers.
Metal ions or metal ion complexes also called dopants, are commonly added in significantly lower amounts to the cubic silver chloroiodide crystals coated in emulsion form in one or more hydrophilic layers of the material used in the image-forming system according to the present invention than e.g. spectral sensitisers and are thereby causing much less chemical load of the image-forming system of the present invention. Said dopants can in principle be added in whatever a stage of the emulsion preparation. Generally dopants have little influence on crystal distributions in the emulsions but they may be added to cause advantageous effects with respect to reciprocity, pressure sentisitization, etc.
In a preferred embodiment the cubic silver chloroiodide grains present in coated light-sensitive layers of the material used in the image-forming system of the present invention have been doped with at least one complex ion compound according to the formula (I) wherein M represents Ru.sup.2+, Os.sup.2+, Rh.sup.3+, Cr.sup.3+ or Pt.sup.2+.
Especially complex ion compounds of ruthenium are preferred, and more preferably hexacyano-ruthenium salts as dopant(s) for the said cubic silver chloroiodide grain emulsions in the image-forming system of the present invention: at least one complex ion compound being [Ru(CN).sub.6 ].sup.4- is present in order to reach the objects of the present invention by modification of the crystal structure, thereby further influencing the physical and chemical properties of the emulsion crystals as well.
As a function of the choice of the metal dopant and the complex ion form (e.g. in form of a coordination complex or oligomeric coordination complex) in which it is introduced in the preparation of silver chloroiodide crystals, and further as a function of its concentration, its valency and its location in the crystal parameters like sensitivity (speed), gradation, stability, developability, pressure sensitization, dye (de)sensitization and reciprocity are influenced.
It is common practice not to divide the dopant homogeneously over the cystal volume of the cubic silver chloroiodide crystals: a non-uniform distributrion plays a dominant role as has e.g. been demonstrated in EP-A's 0 423 765 and 0 697 619; in U.S. Pat. No. 5,051,344 and in JP-A 6-222487. Addition of dopants in order to get them in the outer shell of core-shell grains or at the grain surface or even in epitaxial protrusions may lead to interactions between additives added during chemical ripening, spectral sensitization or during addition of coating solutions, thereby influencing e.g. preservation properties.
In the image-forming system according to the present invention it is preferred that the dopants are added after the end of the nucleation step, more preferably after addition of more than 50% of the total amount of silver (thus during the growth step) and before addition of the last 5% of silver chloride in order to complete crystal growth of silver chlor(oiod)ide crystals. In a more preferred embodiment it is recommended to add the complex metal ion compound(s) between addition of from 75 to 95% of the total amount of silver during precipitation, and it is still more preferred to add the said compound(s) between addition of from 80 to 90% of the said total amount of silver.
More details about grain modifying conditions and adjustments during silver halide precipitation and specifically with respect to the introduction of dopants can be found in Section I-D of Research Disclosure 38957, published September 1996.
It is clear that special attention should be paid to the way in which dopants are introduced during crystal growth as the stability of the dopants in complex compounds may be limited, depending on the solutions wherein the said compounds are available. Therefore said solution can be introduced by means of a so-called "third-jet" in order to introduce the dopant in the reaction vessel where rapid incorporation in the growing crystals is performed. In a preferred embodiment e.g. said third-jet is adding the dopant solution in the vicinity of the stirrer, where also new rapidly dissolving fine grain nuclei are formed which are precipitated further onto stable growing silver chloride or chloroiodide cubic crystals.
Dopants used in the preparation of silver halide crystals rich in silver chloride have e.g. been described (preferred elements between brackets) in U.S. Pat. No. 4,269,927 (Cd,Pb,Zn); U.S. Pat. No. 4,835,093 (Re-complexes); U.S. Pat. No. 4,981,781 (Ru,Fe,Re,Os,Mn,Mo,Cr,W); U.S. Pat. No. 5,024,931 (Ru,Rh,Os,Ir,Pd,Pt); U.S. Pat. No. 5,252,456 (Pt,Ir) and U.S. Pat. No. 5,360,712 (groups 8 and 9 from periods IV, V and VI); EP-A's 0 017 148 (Cd,Pb,Zn,Cu); 0 336 426 (Ru,Re,Os); 0 336 427 (Ru,Os); 0 415 481 (Rh,Ir,Os,Ru,Fe,Co); 0 658 802 (Cr); JP-A 04-009939 and WO 90/016014 (group VIb). A survey of dopants for use in cubic silver chloroiodide grains has further been given in EP-A's 0 718 677 and 0 718 678. For use in tabular {111} grains a survey has been given in U.S. Pat. No. 5,503,971.
Most frequently occurring dopants in literature are ruthenium, rhodium and iridium. Combinations of one or more dopant(s) may be added, in the same or different preparation steps of silver halide crystals rich in silver chloride having a cubic habit. Just as halide ions the said dopants can be divided homogeneously or heterogeneously over the total crystal volume. So in the core or in the shell or even at the crystal surface, as is e.g. the case when conversion techniques are applied, the said halide ions and/or the said dopants may be concentrated.
It has further been established that addition of small amounts of e.g. iridium compounds during and/or at the end of the precipitation step and/or in the chemical ripening step is highly preferred and leads to a positive effect on processing latitude, in that less differences in sensitivity and gradation are observed after fluctuations in developing time within a range of about 5 seconds and in processing temperature within a range of about 10.degree. C., if compared with iridium doped silver bromide and silver bromoiodide crystals. Preferred amounts of a compound as e.g. potassium hexachloroiridate(III), added during chemical ripening, are from 0.5 to 20 .mu.mole per mole of silver and more preferred from 1 to 5 .mu.mole per mole as has been described in EP-A 0 794 456.
In the preparation step of the doped silver chloroiodide crystals coated in emulsion form in materials used in the image-forming system according to the present invention, the precipitation conditions are chosen such that said emulsions are emulsions having an essentially cubic crystal habit. The precipitation of such cubic crystals is principally performed by at least one double jet step; but more preferably it consists of a sequence of consecutive double jet steps comprising a nucleation step and at least one growth step. The different steps of the precipitation are commonly alternated by physical ripening steps. In order to get reproducible emulsion grain distributions said different steps preferably proceed under controlled conditions of pH, pAg, temperature, stirring velocity and addition rates, wherein said addition rates may be held constant or may be increased as precipitation proceeds in order to reduce the total time thereof. However care should be taken in order to avoid renucleation. During the precipitation a crystal growth accelerator may be added, in favour of crystal growth, further avoiding said renucleation. Preferred examples of growth accelerators are thioether compounds as e.g. methionine, 1,8-dihydroxy-3,6-dithiooctane, etc., or polyoxyalkylenes although care should be taken with respect to fog formation.
Crystals having an essentially cubic habit, dispersed in an emulsion coated in one or more hydrophilic layers of the material used in the image-forming system of the present invention have an average crystal diameter of from 0.40 up to 0.65 .mu.m, with a high degree of homogeneity: a variation coefficient on the grain size distribution of less than 0.25, more preferred between 0.10 and 0.20, contributes to the desired sensitometry and image quality. Mixtures of emulsions having grains with homogenous or monodisperse grain size distributions may be useful.
Iodide ions should be provided, in order to get a silver chloroiodide composition for the cubic grains, by using aqueous solutions of inorganic salts thereof as e.g. potassium iodide, sodium iodide or ammonium iodide. Iodide ions can however also be provided by organic compounds releasing iodide ions as has e.g. been described in EP-A's 0 561 415, 0 563 701, 0 563 708, 0 649 052 and 0 651 284 and in WO 96/13759. Although preferred with respect to intrinsic and to spectral sensitivity it is recommended to limit average iodide concentrations to up to 3 mole % and even more preferably to limit them from 0.1 mole % to 1.0 mole %, based on the total silver amount as higher concentrations retard development and lead to unsatisfactory sensitivities. Moreover the velocity of fixation can be disturbed in that case and as a consequence residual coloration may be unavoidable. An excessive amount of iodide can be provided by conversion at the end of precipitation and thus at the end of the last growth step in order to have a concentration of iodide corresponding with the objectives set forth before. It is however not excluded to prepare doped pure silver chloride crystals and to add iodide, in whatever condition as described hereinbefore, just before or after washing and/or redispersing, and in a more preferred embodiment even just before or after addition of the chemical ripening agents. Even when e.g. only 0.1 mole % of silver iodide is added after redispersion, before chemical and/or spectral sensitization the cubic grains used in the context of the present invention should be considered as having a "silver chloroiodide" composition.
Grain distributions of silver chloroiodide crystals over the light-sensitive emulsion are homogeneous or monodisperse by controlling the precipitation methods used. Therefore it is very important in the precipitation conditions of cubic silver chloride cystals to carefully control pAg, temperature, dilution of the reaction vessel, presence of growth restrainers or growth accelerators, addition rate of added aqueous soluble silver salt and halide solutions during different precipitation steps (especially during the nucleation step during which e.g. less than 10% of the total amount of silver salt available is consumed and further during the at least one growth step during which at least 90% of the said silver salt is consumed), way of mixing and mixing or stirring rate in the reaction vessel during the different precipitation steps. Homogeneous crystal size distributions having variation coefficients (defined as ratio between standard deviation and average diameter) of not more than 0.10 to 0.20 instead of the normally occurring variation coefficients between 0.20 and 0.30 are thus obtained. Although depending on the precipitation conditions more heterogeneous distributions may be obtained and may even be more advantageous e.g. from the point of view of exposure latitude a far more better way to obtain the same effect of e.g. an increasing exposure latitude is reached by making mixtures of different homogeneous emulsions having very low variation coefficients e.g. in the range from 0.05 to 0.15. This may lead to even more advantageous sensitometric characteristics (e.g. increased contrast) or image quality (e.g. granularity and/or sharpness) as has been illustrated e.g. in U.S. Pat. No. 4,446,228 and in EP-A 0 555 897.
To silver chloroiodide emulsion grains, wherein after precipitation and redispersion said grains are also called "primitive" or "unripened" as long as no chemical sensitizer(s) is(are) added, addition of chemical ripening agents is further required in order to make them, in a first step after precipitation, washing and redispersing procedures sensitive to irradiation exposure.
With respect to the chemical ripening process besides the commonly used sulphur and gold compounds the use of selenium sensitizers has been promoted, especially during the last decade. Patent literature related with the chemical ripening of emulsion grains rich in silver chloride can be found e.g. in EP-A's 0 443 453, 0 454 278; 0 458 278; 0 513 748; 0 590 593; 0 661 589 and 0 718 674 and in U.S. Pat. Nos. 4,810,626; 5,306,613 and 5,348,850, wherein said selenium sensitizers are normally used together with other sensitizers as at least gold and optionally sulphur.
As set forth in the invention described in EP-Application No. 97200590.4, concurrently filed with the present invention on the same day, preferred unstable selenium compounds should however generate silver selenide in an emulsion comprising said grains at a temperature of from 45.degree. C. up to 70.degree. C. and at an electrical potential difference between a silver electrode and a saturated silver/silver chloride reference electrode of from 100 up to 200 mV only. So could it e.g. be proved that triphenylphosphorselenide was not a suitable chemical sensitizer for silver chloroiodide emulsion crystals coated in materials used in the image-forming system of that invention, opposite to those selenium compounds selected from the group of compounds consisting of substituted selenoureum, substituted triphenylphosphine selenide and substituted and unsubstituted triphenylorthophosphate selenide compounds. As already mentioned hereinbefore said selenium sensitizers are preferably used together with other sensitizers as at least gold and optionally sulphur. In a preferred embodiment said monodisperse cubic silver chloroiodide grains should, besides being chemically ripened with one or more selenium compound(s) mentioned hereinbefore, further be chemically ripened with one or more sulphur and/or gold compounds. Especially useful labile compounds providing sulphur are therefore e.g. the preferred tetramethylthiodithioacetic acid diamide and further dimethylaminodithiomercaptane, thiosulphates or thiosulphonates. Other useful compounds which may be applied are those as described e.g. in "Chimie et Physique Photographique" by P. Glafkides, in "Photographic Emulsion Chemistry" by G. F. Duffin, in "Making and Coating Photographic Emulsion" by V. L. Zelikman et al, and in "Die Grundlagen der Photographischen Prozesse mit Silberhalogeniden" edited by H. Frieser and published by Akademische Verlagsgesellschaft (1968).
As described in said literature chemical sensitization can be carried out by effecting the ripening in the presence of small amounts of compounds containing sulphur as e.g. thiosulphate, thiocyanate, thioureas; sulphites, mercapto compounds, rhodamines etc., wherein combinations of gold-sulphur ripeners together with the required selenium sensitizers are the most preferred. According to the present invention chemically ripened monodisperse cubic silver chloroiodide grains are chemically ripened with one or more sulphur and/or selenium compound(s)and one or more gold compound(s). Addition of tellurium compounds as e.g. tellurosulphate, tellurocyanate, telluroureas in very small amounts is thereby however not excluded. Further reductors as e.g. tin compounds as described in GB-Patent 789,823, amines, hydrazine derivatives, formamidine-sulphinic acids, and silane compounds may be used, although care should be taken in order to prevent the emulsion from fog formation in an uncontrollable way.
Normal amounts of selenium compounds are in the range from 1.times.10.sup.-5 to 1.times.10.sup.-7 moles per mole of silver, whereas normal amounts of gold compounds (as gold chloride or gold thiocyanate) are in the range from 1.times.10.sup.-5 to 2.5.times.10.sup.-5 moles per mole of silver.
As has already been suggested hereinbefore the use of reducing agents in the chemical ripening of silver halide emulsion crystals rich in chloride is not preferred, but not excluded either as, depending upon the circumstances, it may be recommended to use small amounts in order to counterbalance the restraining actions from spectral sensitizers, fog-restrainers or stabilizers as e.g. substituted heterocyclic mercapto compounds described in U.S. Pat. No. 5,242,791. Silver solvents may have a regulating role therein as e.g. thiocyanate ions. It is a common method to add chemical sensitizers after redispersion. Before starting chemical sensitization the surface of the silver chloroioide grains may be treated with slightly oxidizing compounds as e.g. toluene thiosulphonic acid and/or corresponding salts thereof in order to reduce small silver specks to grow to fog centers in an uncontrolled manner.
In general terms light-sensitive cubic silver chloroiodide grains may be spectrally sensitized with methine dyes such as those described by F. M. Hamer in "The Cyanine Dyes and Related Compounds", 1964, John Wiley & Sons. Dyes that may be used for the purpose of spectral sensitization include cyanine dyes, merocyanine dyes, complex cyanine dyes, complex merocyanine dyes, homopolar cyanine dyes, hemicyanine dyes, styryl dyes and hemioxonol dyes.
In the image-forming system according to the present invention the doped silver chloroiodide grains having a regular, essentially cubic habit are, opposite to the grains described in EP-A 0 336 426, spectrally sensitized with compounds selected from the group consisting of benzimidazoles, benzoxazoles, or a combination thereof. As already set forth hereinbefore in order to further improve speed for the complex metal ion doped cubic silver chloroiodide emulsion crystals according to the present invention especially spectral sensitization with a combination of benzimidazolo- and benzoxazolo-carbocyanines is preferred provided that absorption characteristics of the said combination of those green light-absorbing spectral sensitizers are such that spectral sensitization occurs in the preferred wavelength range between 520 and 580 nm, with a maximum absorption between 540 and 500 nm.
An example of a useful spectral sensitizer of the oxacarbocyanine type is anhydro-5,5'-dichloro-3,3'-bis(n-sulphobutyl)-9-ethyloxacarbocyanine hydroxide or anhydro-5,5'-dichloro-3,3'-bis(n-sulphopropyl)-9-ethyloxacarbocyanine hydroxide.
A very suitable mixture or combination of spectral sensitizers of the oxa- and imidacarbocyanine type is anhydro-5,5'-dichloro-3,3'-bis(n-sulphobutyl)-9-ethyl oxacarbocyanine hydroxide or anhydro-5,5'-di-chloro-3,3'-bis(n-sulphopropyl)-9-ethyloxacarbocyanine hydroxide together with anhydro-5,5'-dicyano-1,1'-diethyl-3,3'-di(2-acetoxy-ethyl)ethyl-imidacarbocyanine bromide.
Specific combinations of imidacarbocyanines and oxacarbocyanines as spectral sensitizers added to emulsions have e.g. been described in EP-A's 0 443 453 and 0 608 955 and in U.S. Pat. Nos. 5,296,345 and 5,338,655. Cubic crystals rich in chloride may also be spectrally sensitized with one or more spectral sensitizers, not only in favour of sensitometry but also in favour of decolorizing properties.
Specific sensitizations with green-sensitizing imida-oxacarbocyanines have e.g. been described in U.S. Pat. Nos. 4,701,405; 5,219,723; 5,376,523; 5,462,850 and JP-B 95-013732. Spectral sensitizers having asymmetrical heterocycles may be useful and may even be preferred with respect to improvements in residual coloration after processing. Unsymmetrically chain substituted oxacarbocyanine dyes and/or imidacarbocyanine dyes suitable to improve d ye stain and spectral sensitivity in the green short wavelength region have e.g. been given in JP-A 03-048235.Supersensitization with a symmetrical oxacarbocyanine dye in combination with a carbocyanine dye of e.g. the oxazole-imidazole type has been disclosed in U.S. Pat. Nos. 4,594,317 and 4,659,654. Specifically spectral sensitization with N-fluoroalkyl substituted imidacarbocyanine dyes has been described in U.S. Pat. Nos. 4,675,279; 5,196,299; 5,210,014; and 5,466,822.
In classical emulsion preparation spectral sensitization traditionally follows the completion of chemical sensitization. It can however be advantageous to add a small amount of spectral sensitizing dye to the emulsion crystals just before cooling of the dispersion at the end of the growth stage, but in principle the addition of said dye may be performed at any stage of the precipitation, during or after redispersing or before, during or after chemical ripening. The addition can further be performed in one or more portions. So in U.S. Pat. No. 5,286,621 it has been shown that spectral sensitizer is added in amounts ranging from 10.sup.-5 to 5.times.10.sup.-3 moles per mole of silver halide as a whole after completion of the precipitation or in several fractions during and after precipitation.
An important factor influencing growth of silver nuclei in the preparation of silver chloroiodide grains is the choice of and the amount of protective colloid present in the reaction vessel or added simultaneously with one of the solutions added thereto during nucleation and further, eventually, after nucleation, during physical ripening before and/or during growth of the nuclei formed.
The most well-known and practically used hydrophilic colloidal binder during precipitation of silver chloroiodide crystals is gelatin. The preparation of conventional lime-treated or acid treated gelatin has been described in e.g. "The Science and Technology of Gelatin", edited by A. G. Ward and A. Courts, Academic Press 1977, page 295 and next pages. The gelatin can also be enzyme-treated as described in Bull. Soc. Sci. Phot. Japan, N.sup.o 16, page 30 (1966). A preparation method of tabular grain emulsions wherein in the grain growth process use is made of gelatin derivatives with chemically modified NH.sub.2 -groups and wherein said gelatin has a specific methionine content has been described in e.g. EP-A 0 697 618.
Gelatin may, however, be replaced in part or integrally by synthetic, semi-synthetic, or natural polymers. Synthetic substitutes for gelatin are e.g. polyvinyl alcohol, poly-N-vinyl pyrrolidone, polyvinyl imidazole, polyvinyl pyrazole, polyacrylamide, polyacrylic acid, and derivatives thereof, in particular copolymers thereof. Natural substitutes for gelatin are e.g. other proteins such as zein, albumin and casein, cellulose, saccharides, starch, and alginates. In general, the semi-synthetic substitutes for gelatin are modified natural products e.g. gelatin derivatives obtained by conversion of gelatin with alkylating or acylating agents, by grafting of polymerizable monomers on gelatin or prehardened gelatins with blocked functional groups as a consequence of this prehardening treatment, cellulose derivatives such as hydroxyalkyl cellulose, carboxymethyl cellulose, phthaloyl cellulose, and cellulose sulphates and even potato starch.
Further synthetic high molecular compounds described in JP-B-52-16365, Journal of The Society of Photographic Science and Technology of Japan, Vol. 29(1), 17, 22(1966), ibid., Vol. 30(1), 10, 19(1967), ibid., Vol. 30(2), 17(1967), and ibid., Vol. 33(3), 24(1967) may be used as a dispersion medium. Also the crystal habit restraining agent described in EP-A 0 534 395 may be used.
Part of gelatin may further be replaced with a synthetic or natural high-molecular material.
An interesting substitute for gelatin may be silica as has been described in the published EP-A's 0 392 092, 0 517 961, 0 528 476 and 0 649 051 and 0 704 749. As has been set forth in EP-A 0 528 476 a method of preparing a silver halide light-sensitive photographic material incorporating layers of silver halide precipitated in colloidal silica serving as a protective colloid is given. In this document the silver halides are prepared in colloidal silica, leading to emulsion crystals that are stable at the end of the precipitation, without however having a predictable mean crystal diameter and crystal size distribution. These problems have been overcome as has been described in U.S. Pat. No. 5,543,284, for the preparation of crystals rich in silver chloride, wherein clearly defined circumstances wherein such crystals can be prepared: during the precipitation stage of regular silver chloroiodide crystals amounts of silica sol and of stabilizing onium compounds), should be optimized in order to avoid uncontrolled formation and growth of aggregates.
At the end of the precipitation, following all possible physical ripening steps, the emulsion mixture is normally cooled to about 40.degree. C., before or after adding a flocculate being a polymeric compound as e.g. polystyrene sulphonic acid, providing as a anionic polymer a behaviour depending on pH. Under carefully controlled conditions of addition and stirring rate the pH of the said dispersing medium is adjusted with an acid to a value in order to get a qualitatively good flocculate. Said flocculate may become decanted and washed with demineralized water in order to remove the soluble salts and the development inhibiting crystal habit modifier e.g. adenine to an allowable residual amount (preferably at most 0.3 mg/g of gelatin) or applying an ultrafiltration washing procedure as disclosed e.g. in Research Disclosure, Vol. 102, October 1972, Item 10208, Research Disclosure Vol. 131, March, Item 13122 and Mignot U.S. Pat. No. 4,334,012. Said ultrafiltration technique may be applied on-line during the whole precipitation, in order to reduce the increasing amount of water, thus avoiding dilution of the reaction vessel and increasing amounts of soluble salts like the mainly occurring potassium nitrate. Examples thereof have been described e.g. in EP-A 0 577 886. When the emulsion after precipitation is washed by diafiltration by means of a semipermeable membrane, a technique also called ultrafiltration, it is not necessary to use polymeric flocculating agents that may disturb the coating composition stability before, during or after the coating procedure. Such procedures are disclosed e.g. in Research Disclosure Vol. 102, October 1972, Item 10208, Research Disclosure Vol. 131, March, Item 13122 and U.S. Pat. No. 4,334,012. Redispersion may further be performed by addition of extra hydrophilic colloid. As a consequence values of gesi and/or sisi may be enhanced up to values desired in order to prepare stable coating solutions. It is clear however that any useful protective colloid cited hereinbefore as an alternative of gelatin or gelatin in modified form may be used.
As already set forth additional gelatin or another hydrophilic colloid, suitable as a binder material can be added at a later stage of the emulsion preparation e.g. after washing, to establish optimal coating conditions and/or to establish the required thickness of the coated emulsion layer. Preferably a gelatin to silver halide ratio, silver halide being expressed as the equivalent amount of silver nitrate, ranging from 0.3 to 1.0 is then obtained. Another binder may also be added instead of or in addition to gelatin. Useful vehicles, vehicle extenders, vehicle-like addenda and vehicle related addenda have been described e.g. in Research Disclosure N.sup.o 38957 (1996), Chapter II.
Prior to coating any thickening agent may be used in order to regulate the viscosity of the coating solution, provided that they do not particularly affect the photographic characteristics of the silver chloroiodide emulsion in the coated photographic material. Preferred thickening agents include aqueous polymers such as polystyrene sulphonic acid, dextran, sulphuric acid esters, polysaccharides, polymers having a sulphonic acid group, a carboxylic acid group or a phosphoric acid group as well as colloidal silica. Polymeric thickeners well-known from the literature resulting in thickening of the coating solution may even be used in combination with colloidal silica. Patents concerning thickening agents are e.g. U.S. Pat. No. 3,167,410; Belgian Patent No. 558.143 and JP-A's 53-18687 and 58-36768. Negative effects on physical stability possibly resulting from the addition of polymeric compounds can be avoided by exclusion of those compounds and by restricting extra additions of colloidal silica. In order to coat hydrophilic colloidal layer compositions on a support by slide-hopper or curtain-coating techniques, wherein said compositions have gelatin in low amounts in order to provide a ratio by weight of gelatin to silver halide expressed as an equivalent amount of silver nitrate in the range from 0.05 to 0.4, thickening agents composed of synthetic clay and anionic macromolecular polyelectrolytes wherein said synthetic clay is present in an amount of at least 85% by weight versus the total amount of thickening agents are recommended as has been disclosed in EP-A 0 813 105.
Photographic material having thin emulsion layers e.g. layers with a layer thickness of not more than 6 .mu.m, containing at most 6 g of gelatin, more preferably from about 2 to 6 g/m.sup.2 and even more preferably to about 3.5 g/m.sup.2 of gelatin offer the advantage that besides rapid processing applicability and the rapid drying of the wet processed material an improvement in sharpness is observed. Since the drying characteristics in the processor are mainly determined by the water absorption of the hydrophilic layers of the photographic material, and since the water absorption is directly proportional to the gelatin content of the layers and inversely proportional to the amount of hardener, added to the layer, its composition is optimized with a low gelatin content and a high hardening degree in order to allow hardener free processing within a total processing time cycle from 30 to at most 60 seconds dry-to-dry.
In order to reach a high hardening degree the layer binder should of course dispose of an acceptably high number of functional groups, which by reaction with an appropriate hardening agent can provide a sufficiently resistant layer. Such functional groups are especially the amino groups, but also carboxylic groups, hydroxy groups, and active methylene groups. Hardeners may be added to the antistress layer, covering one or more light-sensitive silver halide emulsion layers rich in chloride before or during the coating procedure, or to one or more of the said emulsion layers. The binders of the photographic element, especially when the binder used is gelatin, can be hardened with appropriate hardening agents such as those of the epoxide type, those of the ethylenimine type, those of the vinylsulfone type as e.g. 1,3-vinylsulphonyl-2-propanol, chromium salts as e.g. chromium acetate and chromium alum, aldehydes as e.g. formaldehyde, glyoxal, and glutaraldehyde, N-methylol compounds as e.g. dimethylolurea and methyloldimethylhydantoin, dioxan derivatives as e.g. 2,3-dihydro-xy-dioxan, active vinyl compounds as e.g. 1,3,5-triacryloyl-hexa-hydro-s-triazine, active halogen compounds as e.g. 2,4-dichloro-6-hydroxy-s-triazine, and mucohalogenic acids as e.g. mucochloric acid and mucophenoxy-chloric acid. These hardeners can be used alone or in combination. The binders can also be hardened with fast-reacting hardeners such as carbamoylpyridinium salts. Formaldehyde and phloroglucinol can e.g. be added to the protective layer(s) and to the emulsion layer(s) respectively. Preferred hardening agents however in the context of the present invention are bis-(vinyl-sulphonyl)-methane (BVSME) and ethylene bis-(vinyl-sulphone).
Materials used in the image-forming system according to the present invention commonly have a hardening degree corresponding with a swelling degree of the layers of the material of less than 200% and even more preferably of not more than 150% as can be measured from thickness ratios of the layers of the material before and after immersion in demineralized water of 25.degree. C. for 3 minutes.
A lot of other ingredients are further required in order get suitable sensitometric properties, as e.g. sensitivity (also called speed), gradation (also called contrast and specified in the toe, the linear part and/or the shoulder of the characteristic curve), fog and maximum density in preferred rapid processing conditions for the materials coated from silver chloroiodide emulsions used in the image-forming system according to the present invention.
Therefore compounds preventing the formation of fog or stabilizing the photographic characteristics during the production or storage of the photographic elements or during the photographic treatment thereof are required and are in most cases already present during emulsion precipitation and/or (spectral and/or chemical) sensitization. Many known compounds can be added as fog-inhibiting agent or stabilizer to the silver halide emulsion layer or to other coating layers in water-permeable relationship therewith such as an undercoat or a protective layer. Suitable examples are e.g. those described in Research Disclosure (RD) N.sup.o 17643 (1978), Chapter VI and in RD N.sup.o 38957 (1996), Chapter VII.
The photographic element may further comprise various kinds of coating physical property modifying addenda as described in RD N.sup.o 38957 (1996), Chapter IX, wherein coating aids, plasticizers and lubricants, antistats and matting agents have been described.
Development acceleration can be accomplished by incorporating in emulsion layer(s) or adjacent layers various compounds, preferably polyalkylene derivatives having a molecular weight of at least 400 such as those described in e.g. U.S. Pat. Nos. 3,038,805; 4,038,075 and 4,292,400 as well as in EP-A's 0 634 688 and 0 674 215.
The photographic element may further comprise various other additives such as e.g. compounds improving the dimensional stability of the photographic element, ultraviolet absorbers and spacing agents. Suitable additives for improving the dimensional stability of the photographic element are e.g. dispersions of a water-soluble or hardly soluble synthetic polymer e.g. polymers of alkyl(meth)acrylates, alkoxy(meth)acrylates, glycidyl (meth)acrylates, (meth)acrylamides, vinyl esters, acrylonitriles, olefins, and styrenes, or copolymers of the above with acrylic acids, methacrylic acids, .alpha.-.beta.-unsaturated dicarboxylic acids, hydroxyalkyl (meth)acrylates, sulphoalkyl (meth)acrylates, and styrene sulphonic acids.
Suitable UV-absorbers are e.g. aryl-substituted benzotriazole compounds as described in U.S. Pat. No. 3,533,794, 4-thiazolidone compounds as described in U.S. Pat. Nos. 3,314,794 and 3,352,681, benzophenone compounds as described in JP-A 56-2784, cinnamic ester compounds as described in U.S. Pat. Nos. 3,705,805 and 3,707,375, butadiene compounds as described in U.S. Pat. No. 4,045,229, and benzoxazole compounds as described in U.S. Pat. No. 3,700,455 and those described in RD N.sup.o 38957 (1996), Chapter VI, wherein also suitable optical brighteners are mentioned.
Spacing agents may be present of which, in general, the average particle size is comprised between 0.2 and 10 .mu.m. Spacing agents can be soluble or insoluble in alkali. Alkali-insoluble spacing agents usually remain permanently in the photographic element, whereas alkali-soluble spacing agents usually are removed therefrom in an alkaline processing bath. Suitable spacing agents can be made e.g. of polymethyl methacrylate, of copolymers of acrylic acid and methyl methacrylate, and of hydroxypropylmethyl cellulose hexahydrophtha-late. Other suitable spacing agents have been described in U.S. Pat. No. 4,614,708.
In X-ray photography a material with a single or a duplitized emulsion layer coated on one (single-side coated) or both sides (double-side coated) of the support. This invention is related with double-side coated materials comprising cubic silver chloroiodide emulsion cystals as discussed hereinbefore.
A mixture of two or more emulsions having silver chloroiodide crystals with the same or different crystal sizes, the same or a different crystal habit, a different or the same chemical ripening treatment and/or a different or the same coverage degree with one or more spectral sensitizers being different from each other or the same, as those described hereinbefore may be added to at least one light-sensitive emulsion layer, provided that at least one emulsion is composed of crystals doped with at least one complex metal ion compound corresponding with the general formula (I).
If more than one emulsion layer is coated onto at least one side of the support of the double-side coated radiographic material used in the image-forming system of the present invention, the same or different emulsions or emulsion mixtures may be present in the different layers. If the same emulsion or emulsion mixture is present in different emulsion layers distinct amounts of (same or different) spectral sensitizer may have been added during chemical riping and/or preparation for coating in order to get a broader exposure latitude for the material according to the image-forming method of this invention and less sensitometric fluctuations in the processing of the radiographic material. If more than one spectral sensitizer is used, wherein at least one of them is absorbing to a differing wavelength region, it is preferred to add them to different layers too, and still more preferred to add them to layers situated at different sides of the support as wandering of spectral sensitizers may form a problem. Such arrangement has e.g. been s described in e.g. U.S. Pat. Nos. 4,978,599 and 5,380,636.
Besides the light-sensitive emulsion layer(s) the photographic material may contain several light-insensitive layers at the side of the support carrying said light-sensitive emulsion layer(s), e.g. a protective antistress layer which can be split up into two layers, one of them being an underlying interlayer or an outermost afterlayer coated or sprayed on top of the "basic" protective antistress layer, one or more subbing layers, one or more intermediate layers as e.g. filter layers and even an afterlayer containing e.g. hardening agent(s), antistatic agent(s), filter dyes for safety-light purposes etc.
Protective antistress layers preferably contain coating aids and coating physical property modifying addenda mentioned in RD 38957, published September 1996, Chapter IX. Antistatic properties are especially preferred in order to prevent blackening after processing in form of sparks etc. due to abrupt decharging of electrostatic charges during production and/or handling before exposure and/or processing. It is highly preferred to add antistatic agents to the protective antistress layer or to an afterlayer coated thereupon as has been described e.g. in EP-A's 0 534 006, 0 644 454 and 0 644 456 and U.S. Pat. Nos. 4,670,374 and 4,670,376. Abrasion resistance of these outermost layers may be improved as described in U.S. Pat. Nos. 4,766,059 and 4,820,615. Spraycoating of afterlayers has been disclosed e.g. in U.S. Pat. No. 5,443,640. Non-imagewise blackening occurring as a result of pressure sensitivity of silver halide grains rich in chloride is lowered in the present invention due to the presence of iodide ions at the grain surface of the cubic silver chloroiodide crystals. Measures in order to further suppress pressure sensitivity may be coating of enhanced amounts of binder as e.g. gelatin. This however is disadvantageous with respect to rapid processing and therefore as an alternative silver chloroiodide prepared in silica may offer an alternative as has been disclosed e.g. in EP-A 0 528 476. Moreover with respect to the binder material in the light-sensitve emulsion layer an improvement of pressure sensitivity can be expected if use is made therein from synthetic clays as has been disclosed in U.S. Pat. No. 5,478,709. In the presence however of spectrally sensitized emulsion crystals in the said light-sensitive layers care should be taken in order to select suitable synthetic clays as has been disclosed in EP-A 0 757 285.
Intermediate layers eventually containing filter- or antihalation dyes that absorb scattering light and thus promote the image sharpness have been described in e.g. U.S. Pat. Nos. 4,092,168; 4,311,787; 5,344,749; 5,380,634; 5,474,881; 5,478,708; 5,502,205; in EP-A 0 489 973 and 0 586 748 and in EP-A's 0 786 497 and 0 781 816, in DE 2,453,217, and in GB-Patent 7,907,440. Situated in such an intermediate layer between the emulsion layers and the support there will be only a small neligible loss in sensitivity but rapid processing conditions, although said dyes decolorize very rapidly in alkaline solutions, require minimization of the thickness of the whole coated layer, an item which has already been discussed hereinbefore: multilayer arrangements of thin layers clearly result in shorter drying times after washing in the processing cycle. It is further in favour of decolorizing properties to have said suitable dyes in form of finely dispersed form and more preferred in solid particle dispersed form. Evidence therefore can specifically be found in EP-A 0 724 191 and in a more general way in EP-A 0 756 201.
In addition thereto it is recommended to prepare aqueous solid dispersions in colloidal silica for any photographically useful compound as has been suggested e.g. in EP-A 0 569 074. Advantages with respect to thin layer coating and rapid processing ability can be expected with relation thereto, without enhancing pressure sensitivity of more vulnerable layers.
The support of the photographic materials comprising silver halide emulsions with cubic silver chloroiodide crystals used for X-ray imaging, may be a transparent resin, preferably a blue colored polyester support like polyethylene terephthalate. The thickness of such organic resin film is preferably about 175 .mu.m. Other hydrophobic resin supports are well known to those skilled in the art and are made e.g. of polystyrene, polyvinyl chloride, polycarbonate and polyethylene naphthalate. The support is further provided with a substrate layer at both sides to have good adhesion properties between the adjacent layers and said support: one or more subbing layers known to those skilled in the art for adhering thereto a hydrophilic colloid layer may be present. Suitable subbing layers for polyethylene terephthalate supports are described e.g. in U.S. Pat. Nos. 3,397,988, 3,649,336, 4,123,278 and 4,478,907. A preferred layer arrangement wherein a subbing layer composition comprising as a latex copolymer vinylidene chloride, methylacrylate and itaconic acid has been covered with hydrophilic layers being at least one gelatinous dye containing layer comprising one or more dyes, at least one silver halide emulsion layer, at least one protective antistress layer, and optionally an afterlayer has been described in EP-A 0 752 617. In that invention said hydrophilic layers have a swelling ratio of not more than 200% and in said hydrophilic layers are coated simultaneously by the slide-hopper coating or by the slide-hopper curtain coating technique. Further information on suitable supports can be found in RD 38957, Chapter XV, published September 1996.
In radiography the interior of objects is reproduced by means of penetrating radiation which is high energy radiation belonging to the class of X-rays, .gamma.-rays and high energy elementary particle radiation, e.g. .beta.-rays, electron beam or neutron radiation. For the conversion of penetrating radiation into visible light and/or ultraviolet radiation luminescent substances are used called phosphors. In a conventional radiographic system an X-ray radiograph is obtained by X-rays transmitted imagewise through an object and converted into light of corresponding intensity in a so-called intensifying screen (X-ray conversion screen) wherein phosphor particles absorb the transmitted X-rays and convert them into visible light and/or ultraviolet radiation whereto a photographic film is made more sensitive: it is clear that spectral sensitizers are chosen as a function of and in order to absorb light of about the same wavelength range as the one emitted by luminescent phosphors coated in phosphor layers of intensifying screens brought into contact with the double-side coated film materials during X-ray exposure. So according to the present invention in the image-forming system silver chloroiodide crystals are spectrally sensitized in the green-wavelength range of the spectrum as has e.g. been described in GB-Patent 1,489,398 and in U.S. Pat. Nos. 4,431,922 and 4,710,637. More particularly silver chloroiodide crystals are spectrally sensitized between 520 and 580 nm, and have a maximum absorption between 540 and 500 nm in order to absorb light emitted from X-ray exposed screens coated from the preferred green-light emitting gadolinium oxisulphide phosphors used in the phosphor screens according to the image-forming system of the present invention. Such phosphors suitable for use in a conventional radiographic system must have a high prompt emission on X-ray irradiation and low afterglow in favour of image-sharpness. Especially terbium activated gadolinium oxisulphide crystals are particularly suitable for use in the image-forming system according to the present invention.
Screen-film systems wherein green-light emitting screens are used in contact with green sensitized silver halide films have been described e.g. in EP-A 0 678 772.
In the practical application wherein the image-forming system according to the present invention is used, an X-ray radiation source is used having an energy of from 60 to 150 kVp, e.g. 80 kVp for the detection of bone.
From the preceding description of the X-ray recording system operating with X-ray conversion phosphor screens in the form of a plate or panel it is clear that said plates or panels only serve as intermediate imaging elements and do not form the final record. The final image is made or reproduced on a separate recording medium or display: for X-ray conversion screens used in the image-forming system of the present invention double-side coated films are the said final record. It is clear that the phosphor plates or sheets can be repeatedly re-used. Since in the above described X-ray recording systems the X-ray conversion screens are used repeatedly, it is important to provide them with an adequate topcoat for protecting the phosphor containing layer from mechanical and chemical damage. Further providing a relief structure that effectively improves manipulation should not be at the cost of image quality as providing protruding particles requires increased thickness of the protective coating and as a consequence reduced image sharpness due to an increased distance between the radiation emitting phosphor layer and the radiation sensitive coating of the photographic film in contact therewith. Not only the increased thickness itself can give rise to increased unsharpness of the emitted light when the refractive indices of phosphor binder and binder of the protective coating differ but also the presence of the particles themselves having different refractive index compared with that of the binder of the protective coating. A good compromise in order to provide a luminescent article, e.g. in the form of a plate, panel or web, comprising a phosphor-binder layer and protective coating applied thereto wherein the protective layer has a relief structure for high ease of manipulation, thereby avoiding sticking, friction and electrostatic attraction with maintenance of an excellent image resolution has been described in EP-A 0 510 754.
From the point of view of the phosphor layer especially an increased thickness itself can give rise to increased unsharpness of the emitted light, this being the more unfavourable if the ratio by weight between the amount of phosphor particles and the amount of binder decreases for the same coating amount of said phosphor particles, also called "pigment".
Enhancing the weight ratio amount of pigment to binder to provide sharper images, by decreasing the amount of binder leads to unacceptable manipulation characteristics of the screen due to e.g. insufficient elasticity and brittleness of the coated phosphor layer in the screen.
One way to get thinner coated phosphor layers in favour of sharpness of the image on the film material in contact therewith during exposure and without changing the coated amounts of pigment and of binder makes use of a method of compressing the coated layer as has been described in EP-A 0 393 662. A much better solution in order to provide a phosphor layer having a ratio by volume of pigment to binder to obtain an excellent image resolution with the maintenance of a high ease of manipulation, thereby providing a good elasticity of the screen, good adhesion properties between the support and the phosphor layer and avoiding brittleness of the said phosphor layer has been described in WO94000531, wherein the binding medium of the phosphor layer substantially consists of one or more rubbery and/or elastomeric polymers, in that the ratio by volume of phosphor to binding medium is at least 70:30 and at most 92:8 and in that the packing ratio is less than 67%. By the choice of the type of binder and the high volume ratio of phosphor to binder it is possible to obtain thin phosphor coatings offering not only high resolution but also high sensitivity without the need for increasing the packing density by compressing so as to reduce the voids as defined in EP-A 0 393 662 to a value of not less than 70%. Moreover the phosphor layer retains high protection against mechanical damage and thus high ease of manipulation.
A practically useful binder medium for the phosphor particles has further been disclosed in WO94000530. Therein the binding medium substantially consists of one or more hydrogenated styrenediene block copolymers, having a saturated rubber block, as rubbery and/or elastomeric polymers. The polymer can be represented by the formula A-B-A (tri-block) or by the formula A-B (di-block), wherein A represents styrene and B represents the hydrogenated diene block e.g. ethylene-butylene or ethylene-propylene.
Screen/film combinations may be symmetric or asymmetric: this means that screens differing in speed and/or radiation emitted therefrom are differing and/or that there is a difference in speed and/or contrast and/or spectral sensitivity at both sides of the film support.
As exposure light is diffracted less by silver halide crystals rich in chloride due to less light absorption, as has e.g. been illustrated in EP-A 0 580 029, a further advance with respect to image sharpness may be expected in comparison with silver halide crystals rich in silver bromide. Further as a result of the better solubility of silver halide crystals rich in silver chloride if compared with crystals rich in bromide, it can be expected that with respect to rapid processing ability materials comprising emulsions having silver halide crystals rich in chloride will be more favourable.
A method of image formation offered as described in the present invention comprises the step of processing said film material after exposure with light emitted by a green-light emitting phosphor of an intensifying screen after conversion of X-rays having an energy from 60 to 150 kVp, wherein said step of processing proceeds in an automatic processor.
The image-forming method of the present invention further comprises the step of processing said film material used in the image-forming system described hereinbefore, wherein said processing comprises the steps of developing in a developing solution comprising (iso)ascorbic acid, 1-ascorbic acid, reductic acid, salts and/or derivatives thereof; fixing in a fixer solution free from aluminum salts; rinsing and drying.
Replenishing of said developing and fixer solution preferably proceeds with amounts of replenisher in the range from 100 up to 200 ml/m.sup.2 and from 50 up to 150 ml/m.sup.2 respectively. For the said processing, preferably an automatically operating apparatus is used, provided with a system for automatic replenishment of the processing solutions. The processing therein proceeds within a short processing time of from 30 up to 60 seconds from dry-to-dry for materials used in the image forming system of the present invention. A normally used configuration in the automatic processing apparatus shows the following consecutive tank units corresponding with, as consecutive solutions: developer-fixer-rinse water.
Recent developments however have shown, that from the viewpoint of ecology and especially with respect to reduction of replenishing amounts, as consecutive solutions the sequence developer-fixer fixer-rinse water-rinse water is preferred. One washing step between developing and fixation and one at the end before drying may also be present.
Instead of or partially substituting (e.g. in a ratio by weight of from 1:1 up to 9:1) the ecologically questionable "hydroquinone" (iso)ascorbic acid, 1-ascorbic acid and tetramethyl reductic acid are preferred as main developing agent in the developer. Said developing agents have been described in EP-A's 0 461 783, 0 498 968, 0 690 343, 0 696 759, 0 704 756, 0 732 619, 0 731 381 and 0 731 382; in U.S. Pat. Nos. 5,474,879 and 5,498,511 and in Research Disclosure No 371052, published Mar. 1, 1995, wherein a more general formula covering the formula of said developing agents has been represented.
In order to reduce "sludge formation" which is favoured by solubilizing agents like sulphites, present in the developer as preservatives, a particularly suitable developer solution is the one comprising a reduced amount of sulphite and ascorbic acid which acts as a main developer and anti-oxidant as well and which is called "low-sludge", developer.
In favour of ecological fixation the presence of aluminum ions should be reduced, and more preferably, no aluminum ions should be present. This is moreover in favour of the absence of "sludge" formation, a phenomenon which leads to pi-line defects when high amounts of silver are coated in the light-sensitive layers. Measures in order to reduce "sludge-formation" have further been described in U.S. Pat. Nos. 5,447,817; 5,462,831 and 5,518,868. A particularly suitable fixer solution comprises an amount of less than 25 g of potassium sulphite per liter without the presence of acetic acid wherein said fixer has a pH value of at least 4.5, in order to make the fixer solution quasi odorless. The presence of .alpha.-ketocarboxylic acid compounds may be useful as described in EP-A's 0 620 483, 0 726 491 and in RD No. 16768, published March 1978.
It is possible to use sodium thiosulphate as a fixing agent, thus avoiding the ecologically undesired ammonium ions normally used. For low coating amounts of emulsion crystals rich in chloride a fixation time which is reduced to about 2 to 10 seconds can be attained.
The developer solution used in the method according to the present invention should be replenished, not only for decrease of the liquid volume due to cross-over into the next processing solution but also for pH-changes due to oxidation of the developer molecules. This can be done on a regular time interval basis or on the basis of the amount of processed film or on a combination of both. In these circumstances, no dilution and mixing procedures are required before the regeneration bottles are adjusted to the processing unit. Moreover regeneration is kept to a minimum, especially in the processing of materials coated from very low amounts of emulsion crystals rich in silver chloride. Preferred minimum regeneration or replenishment amounts are from 100 to 200 ml/m.sup.2 and more preferably from 100 to 150 ml/m.sup.2 for the developer and from 50 to 150 ml/m.sup.2 and more preferably from 50 to 100 ml/m.sup.2 for the fixer solution. Replenishment of a developer comprising ascorbic acid or derivatives thereof and a 3-pyrazolidone derivative has been described in EP-A 0 573 700, wherein a method is disclosed for processing with constant activity image-wise exposed silver halide photographic material comprising the steps of (a) developing photographic material in a continuous automatic way by means of a developing solution containing an ascorbic acid analogue or derivative and a 3-pyrazoli-done derivative as developing agents; (b) replenishing said developing solution by means of at least one replenishing solution having a higher pH than the developing solution.
Other references related therewith are EP-A 0 552 511 and U.S. Pat. No. 5,503,965 and further EP-A 0 660 175 and EP-Application 96203727, filed Dec. 30, 1996.
Although it is possible to use whatever a processing unit adapted to the requirements described hereinbefore to reach the objectives concerning a perfect link between rapid processing and ecology, the objects of this invention concerning processing have e.g. been realized in the processing unit CURIX HT 530, trade name product marketed by Agfa-Gevaert.
New developments however have become available with respect to processing apparatus. In a conventional processing apparatus the sheet material is transported along a generally horizontal feed path, the sheet material passing from one vessel to another usually via a circuitous feed path passing under the surface of each treatment liquid and over dividing walls between the vessels. However, processing machines having a substantially vertical orientation have also been proposed, in which a plurality of vessels are mounted one above the other, each vessel having an opening at the top acting as a sheet material inlet and an opening at the bottom acting as a sheet material outlet or vice versa. In the present context, the term "substantially vertical" is intended to mean that the sheet material moves along a path from the inlet to the outlet which is either exactly vertical, or which has a vertical component greater than any horizontal component. The use of a vertical orientation for the apparatus leads to a number of advantages. In particular the apparatus occupies only a fraction of the floor space which is occupied by a conventional horizontal arrangement. Furthermore, the sheet transport path in a vertically oriented apparatus may be substantially straight, in contrast to the circuitous feed path which is usual in a horizontally oriented apparatus. The straight path is independent of the stiffness of the sheet material and reduces the risk of scratching compared with a horizontally oriented apparatus. In a vertically oriented apparatus, it is important to avoid, or at least minimize leakage of treatment liquid from one vessel to another and carry-over as the sheet material passes through the apparatus. Furthermore it is desirable that the treatment liquid in one vessel is not contaminated by contents of the adjacent vessels, that is neither by the treatment liquid of the next higher vessel nor by vapours escaping from the next lower vessel. In order to reduce consumption of treatment liquids, it is furthermore desirable to reduce the evaporation, oxidation and carbonization thereof. A solution therefore has been proposed in EP-A 0 744 656, wherein it has been disclosed that contamination and evaporation, oxidation and carbonization can both be reduced in a simple manner by a particular construction of the apparatus for the processing of photographic sheet material comprising a plurality of cells mounted one above the other in a stack in order to define a substantially vertical sheet material path through the apparatus, each cell comprising a housing within which is mounted a rotatable roller biased towards a reaction surface to define a roller nip there-between through which the sheet material path extends and associated sealing means serving to provide a gas- and liquid-tight seal between the roller and reaction surface on the one hand and a wall of the housing on the other. According to a first aspect, that invention is characterized by means for connecting each cell to adjacent cells in the stack in a closed manner and according to a second aspect, that invention is characterized in that the roller is a drive roller.
Particularly the objectives set forth above may be achieved when the developing cell of the apparatus is a closed cell and the developing liquid contains an ascorbic acid developing agent as has been described in EP-Application No. 96201753, filed Jun. 24, 1996.
According to that invention, there is provided a method of processing photographic sheet material by use of an apparatus comprising a plurality of processing cells so arranged in order to define a sheet material path through the apparatus, at least one of the cells constituting a developing cell containing a developing liquid, characterized in that the developing cell is a closed cell and the developing liquid contains an ascorbic acid type developing agent.
With respect to further characteristics of the processing apparatus we refer to EP-Application No. 96202032, filed Jul. 17, 1996, wherein it was an object to provide an apparatus in which operating components can easily be replaced without the need for substantial re-programming of the CPU. This could be achieved when information concerning characteristics of each operating component is stored in separate memory means.
As a rule, a processing apparatus for photographic sheet material comprises several treatment cells, most or all of which are in the form of vessels containing a treatment liquid, such as a developer, a fixer or a rinse liquid. As used herein, the term "sheet material" includes not only photographic material in the form of cut sheets, but also in the form of a web unwound from a roll. The sheet material to be processed is transported along a sheet material path through these vessels in turn, by transport means such as one or more pairs of path-defining drive rollers, and thereafter optionally to a drying unit. The time spent by the sheet material in each vessel is determined by the transport speed and the dimensions of the vessel in the sheet feed path direction.
From time to time it is necessary to clean the processing apparatus, in order to remove debris which may derive from the sheet material itself and deposits derived from the treatment liquids. The usual process for cleaning a processing apparatus, whether of the vertical or horizontal configuration, is to drain the treatment liquids and to flush the apparatus through with cleaning liquid. Water, optionally containing various additives and optionally at an elevated temperature, is the usual cleaning liquid. Therefore it has ever been an object to provide an apparatus in which the path-defining rollers can be separated from each other in the open position, in a simple and convenient manner. The way in which this can be achieved has been described in EP-Application No. 96202164, filed Aug. 31, 1996, wherein the path-defining rollers are supported by bearings carried by eccentric sleeves which are stationary in the closed position, and where means are provided for partly rotating the sleeves thereby to withdraw the path-defining rollers from each other into the open position. A sheet material processing apparatus has thus been provided, comprising at least one treatment cell, a pair of rotatable path-defining rollers defining a sheet material path through the cell, the path-defining rollers having a closed position in which the path-defining rollers are biased into contact with each other to form a nip through which the sheet material path extends and an open position in which the path-defining rollers are spaced from each other, characterized in that the path-defining rollers are supported by bearings carried by eccentric sleeves which are stationary in the closed position, and means are provided for partly rotating the sleeves thereby to withdraw the path-defining rollers from each other into the open position.
It is clear that within the scope of this disclosure any combination of two screens with a double-side coated film may be used, wherein said film comprises cubic silver chloroiodide crystals as defined in the claims and in the description hereinbefore, and, optionally, {111} tabular silver chloroiodide crystals as disclosed e.g. in EP-A 0 678 772, e.g. in a multilayer arrangement as disclosed in EP-A 0 770 909, in combination with a processing unit, provided that with minimum amounts of silver coated a sufficient covering power (see therefore e.g. EP-A 0 709 730) is attained for the film in rapid ecological processing (with e.g. ascorbic acid and/or derivatives thereof as developing agent(s) in a hardener-free developer and an odor-free fixer, free from aluminum ions, thereby reducing sludge; and replenishing amounts for developer and fixer as low as possible) and provided that an optimal relationship is attained between sensitometry and image quality, especially sharpness, partly thanks to low cross-over exposure for said double-side coated films.

EXAMPLES
Example 1
This example demonstrates the advantages of emulsions comprising cubic AgCl(I) crystals doped with a ruthenium compound and having been chemically and spectrally sensitized over other chemically sensitizing compounds.
______________________________________Preparation of Emulsions A and B (comparative emulsions)______________________________________Solution 1 Water* 880 ml Gelatin 46 g Solution 2 Water* 1000 ml Silver nitrate 500 g Solution 3 Water* 1000 ml Sodium chloride 173 g______________________________________ *demineralized water
The UAg value of solution 1 (potential value expressed in mV versus a saturated silver/silver chloride reference electrode) was adjusted at a constant value of +138 mV.+-.2 mV before starting nucleation by dropwise addition of about 7 ml of a solution having 234 grams of sodium chloride after addition of 0.44 ml of a silver nitrate solution having a concentration of 50 g per liter of demineralized water.
During the said nucleation step which was performed at a constant temperature of 60.degree. C., there was simultaneously added to solution 1, while stirring at a stirring rate of 500 rpm, a part of solution 2 and of solution 3 over a period of 5 minutes at a flow rate of 3 ml/min.
After this nucleation step, UAg was readjusted at the same value of +138 mV while solution 2 was added at an increasing flow rate varying from 3 ml per minute to 30 ml per minute simultaneously with solution 3, the flow rate of which was varied in order to maintain the same constant UAg-value over a period of 59 minutes and 42 seconds, meanwhile maintaining UAg at the same constant UAg value of +138 mV.
The emulsion was washed with a solution of demineralized water containing 0.46 g of sodium chloride per liter after flocculation by addition of polystyrene sulphonic acid to the acidified emulsion. To the washed flocculate 130 g of gelatin was added, followed by redispersion.
In this way a cubic silver chloride emulsion having a mean grain size of 0.57 .mu.m having a chloride content of 100 mole % was obtained. The pH-value of the said emulsion was adjusted at 5.20; the pAg-value at +170 mV. To the dispersion obtained as described hereinbefore 3.8 mg of para-toluene thiosulphonate, 1 g of potassium iodide, 12.5 mg of chloro auric acid, 25 mg of ammonium thiocyanate and 25 mg of tetramethylthio-dithiocarboxylic acid diamide were added at 40.degree. C. Addition of iodide before chemically ripening thus introduces iodide in the silver chloride emulsion to a concentration of 0.2 mol % vs. silver. Chemical sensitization was carried out at 52.degree. C. during 150 minutes. The chemically ripened emulsion was divided into two equal parts, called Emulsions A and B respectively, in order to spectrally sensitize said two parts with differing sensitizers.
Spectral sensitization was carried out for part A by means of a mixture of 0.025 mmoles of anhydro-5,5'-diphenyl-3,3'-bis(n-sulphatopropyl)-9-ethyl-oxacarbocyanine hydroxide and 0.8 mmoles of anhydro-5,5'-dichloro-3,3'-bis(n-sulphobutyl)-9-ethyloxacarbocyanine hydroxide per mole of silver.
Spectral sensitization was carried out for part B by means of a mixture of 0.015 mmoles of anhydro-5,5'-dicyano-1,1'-diethyl-3,3'-di (2-acetoxyethyl)ethyl-imidacarbocyanine bromide and 0.8 mmoles of anhydro-5,5'-dichloro-3,3'-bis(n-sulphobutyl)-9-ethyloxacarbocyanine hydroxide per mole of silver.
Further, 40 mg (per mole of Ag) of 1-phenyl-5-mercaptotetrazole and 150 mg (per mole of Ag) of 1-p-carboxy-phenyl-5-mercaptotetrazole were added as stabilizers. Resorcinol was added as a hardener accelerator in an amount of 2.8 g per mole of Ag.
Consecutively 0.5 g of polyglycol (MW=6000) were added as a development accelerator; 20 ml of polyoxyethylene surfactant H.sub.17 C.sub.8 -Phenyl-(O--CH.sub.2 --CH.sub.2).sub.8 --O--CH.sub.2 --COOH and in an amount of 140 mg (per mole of Ag) floroglucinol was added as a hardener stabilizer together with polymethyl acrylate latex (in an amount of 140% by weight, based on the amount of gelatin binder) which was used as a plasticizer. The thus prepared emulsion coating solutions were coated on a polyethylene terephthalate support in such an amount in order to give a coating weight of 3.75 g/m.sup.2 per side in terms of AgNO.sub.3 and 1.9 g of gelatin per m.sup.2 per side.
The following protective layer was coated thereupon (pH value: 6.1)
__________________________________________________________________________Protective layer__________________________________________________________________________ Gelatin 1.1 g/m.sup.2 Polyethyl acrylate latex 500 mg/m.sup.2 Kieselsol 15 mg/m.sup.2 Chromium acetic acid 5.5 mg/m.sup.2 Compound (1) 7.5 mg/m.sup.2 Compound (2) 19 mg/m.sup.2 Mobilcer Q 25 ml/m.sup.2 Compound (3) 8 mg/m.sup.2__________________________________________________________________________ Compound (1) ##STR1## - Compound (2): ##STR2## - Compound (3): ##STR3## Preparation of Emulsions C and D (Inventive Emulsions)
The inventive emulsions C and D were prepared in the same way as the comparative emulsions A and B, except for the presence of a ruthenium dopant in the silver chloroiodide crystals of both emulsions: 13.33 ml of a K.sub.2 Ru(CN).sub.6 solution was added as an aqueous solution having a concentration of 91 mg/liter in order to add ruthenium as a dopant in amounts of 10 ppm vs. silver to the silver chloroiodide crystals from the moment in the growth step that 80% of the total amount of the silver nitrate solution was added, until 90% was precipitated: the said solution was added in 217 seconds at an addition rate of 3.69 ml/min., wherein the addition of the dopant solution was starting 52 minutes 40 seconds after the growth step was started.
After dividing the emulsion in two equal parts C and D, the same washing, redispersing, chemical ripening, spectral sensitizing and coating procedure as set forth hereinbefore for the parts A and B respectively was performed.
Evaluation of the Coated Samples
An X-ray exposure proceeded with 68 kVp X-rays and an ANSI phantom was exposed at the screen-film system wherein the screen was an ORTHO REGULAR NEW screen, trademarked product from Agfa-Gevaert, wherein the film was variable (see Emulsions A to D in Table 1). The density of the curve obtained was plotted versus the corrected logK value, wherein said value is corrected for the air absorption.
The processing of the exposed silver halide emulsion materials A to D proceeded with the following developing liquid INVDEV, followed by fixing in fixing liquid INVFIX and rinsing at the indicated temperature of 35.degree. C. and processing time of 45 seconds.
______________________________________Developer INVDEV______________________________________1-pheny1-4-methyl-4'hydroxymethyl- 2 g/l 3-pyrazo1idine-1-one Sodium EDTA 2 g/l Potassium bromide 3.3 g/l Potassium thiocyanate 1 g/l Potassium sulphite 33 g/l Potassium carbonate 96 g/l Polyglycol (M.W. = ca. 400) 20 ml/l Compound (4) 1 g/l Ascorbic Acid 50 g/l pH ready-for-use 10.0______________________________________ Compound (4): ##STR4##
The developed samples were fixed in fixer INVFIX, followed by rinsing with water.
The composition of the said fixer was as follows:
______________________________________Fixer INVFIX______________________________________Ammonium thiosulphate (60% 710 ml solution, wherein 1 ml comprises 0.778 g) Sodium metabisulphite 80 g Sodium acetate 130 g Acetic acid 31 ml pH ready-for-use (after 4.90 dilution 1 + 3)______________________________________
The results obtained are shown in Table 1.
TABLE 1______________________________________ Ru- Screen Em. cmpd. Fog Speed GG Dmax Speed______________________________________A - 44 186 436 403 0.43 B - 65 157 354 382 0.60 C + 47 182 409 383 0.46 D + 87 147 333 383 0.65______________________________________
The said sensitometric differences are expressed for
fog levels F, determined as minimum densities above support density, wherein densities are multiplied by a factor of 1000;
speed values S, determined at a density of 1.0 above fog level, wherein said values are multiplied by a factor of 100 (a negative difference is indicative for a loss in speed);
gradation levels GG, wherein differences are expressed as a procentual figure: GG-gradation values are determined between a density of 1.0 and 3.0 above fog level;
Dmax: maximum densities obtained, multiplied by a factor of 100.
As can be concluded from the figures given in the Table 1, the presence of a combination of oxacarbocyanine/imidacarbocyanine spectral sensitizer in combination with ruthenium-doped cubic silver chloroiodide crystals gives an opportunity to reach the required film speed, which could neither be attained for films coated from cubic silver chloroiodide crystals without ruthenium dopant, nor for said crystals spectrally sensitized by a combination of e.g. two oxacarbocyanine dyes as has been illustrated in the Examples.
The said speed values obtained for materials coated from the inventive emulsion D, exposed and thereafter processed in the rapid processing cycle presented in the Examples hereinbefore are indicative for the ability to reach a screen speed (or sensitivity) as required for a "400"-system, if compared with the reference screen speed of 0.63 obtained for CURIX HTU film (trademarked product from Agfa-Gevaert Nevada).
Having described in detail preferred embodiments of the current invention, it will now be apparent to those skilled in the art that numerous modifications can be made therein without departing from the scope of the invention as defined in the following claims.

Number	Name	Date
5380636	Malfatto et al.	Jan 1995
5728516	Edwards et al.	Mar 1998
5783379	Wiilems et al.	Jul 1998
5795704	Willems et al.	Aug 1998

System and method for radiological image formation

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