US2904689A - Fluorescent x-ray screens - Google Patents

Description

Sept. 15, 1959 PIGMENT ED LIGHT-REFLECTING LAYER METALIZED COATING PIGMENTED LIGHT-REFLECTING LAYER METALIZED COATING RESINOUS COATING METALIZED, PIGM ENTED LIGHT-REFLECTING LAYER METALIC FOIL METALIZED, PIGMENTED LlGHT-REFLECTING LAYER METALIZED COATING METALIZED, PIGMENTED LIGHT- REFLECTING LAYER METALIZED COATING F. J. MAsl ETAL FLUORESCENT X-RAY SCREENS Filed June 1, 1956 FIG. I

FIG. 2

FIG. 3

Ki u 9 Rmm (II-$ M- INVENTORS FRANK J. MASI ANDREW F. MILLETT United States Patent 6 FLUORESCENT" X-RAY SCREENS Franlin II Masi and Andrew FLMillett, .Morristown N.J., assignors to: United: States Radium-- Corporation-,. New York-,;N=Y-.,. azcorporationz of Delaware Application June 1', 1956, .Serial'No. 588,723

4 Claims; ((31.1 2503-80.)

This invention relates. to. fluorescent.screensernployed to-intensify the image produced by X'-rays.-on..a.photositive film or plate. More especially the invention relates to the reduction of undesired exposure of the photosensitive emulsion by' local static discharges between such screens and" the film or plate;

Intensifying'screens are employed-'to'increase-theexposure-ofthe' sensitized plate or film (hereinafter referred to. as a"negative) without increasing theexposure time or the X-ray concentration; These screens are customarily'inserted on bothsides of thenegative; and the resulting sandwich issecured in" a light-tight holder linownasacassette; In exposing the negative, the X-rays pass through one-side ofthecassette, through one entire intensifying (front) screen, through the: sensitizednegativezand thence into the fluorescent;layer-oflthe second intensifying- (back) screen. This causes'both screens to fluoresc'e, and" the resulting 'light'combineswith the X'-rays in1exposing theznegative. I Radiologists are' aware of'the fact that',,in addition to intentional exposure of the. negative by the process above summarized; X ray negatives. frequently are mottled by spurious exposures which have been found'to be due to local static discharges. These l discharges are more prevalent in dry climates. and they appear also to be related to the materials employed in the negative and in. the intensifying screen, some materials being. more proneth'an; others to the generation of static charges.

Explanations of static charges. andldischarges and the relation of the characteristics of materials thereto, especially dielectric' characteristics, are to be found in chapters onstatic electricity'in'. text books on physics. In general; two types ofstatic electricity have been observed in connection with.X'-ray screens andnegatives. or films: (1) friction static. which results from relative movement ("usually sliding) between the negative and the adjacent screen, and ('2)' pressure static which results when the screenis pressed tightly against'the negative. Both types of static are discharged when the negative is separated from the screen, because thework in so doing induces ahigh potential". difference. Thefriction type ofstati'c produces dark. streaks or. crow-foot designs on the developed negative and the pressurestatic produces dark spots which are usually denser than. the first type, but both interfere with an accurate reading and. interpretation of the radiograph. Heretofore, no successful means. has been proposed. to. overcome the; undesired exposure; by static. discharge.

We have discovered. that this undesired: exposure can be substantially reduced by" introducing; into; the. intensifying, screen structure one or more discrete layers. of suitable material which is. electrically conductive in the manner below described, and suificiently transparent to Xn'ays's It has been found that satisfactory results can beachieved by interposing separate conductive layers or by modifying previously employed layers so" as tobe sufficientl y conductive. I w I 'ice The invention will be more fully understood from the foll'owing description considered in connection with the accompanying drawings in which:

Fig. 1. illustrates in crosssection, an embodiment of the. inventionwhich is generally preferred because. it is of. simple. construction and is especially effective in. minimizing static discharges;

Fig. 2. is. an alternative, construction. of an intensifying screenin which a. conductive. layer is interposed near the center;

Fig. 3 shows another alternative in which the pig; mented light-reflecting layer is modified, to comprise. a conducting. layer;

Fig.v 4 shows a third alternative embodiment employing two layers of. metallic foil; and

Fig. 5 illustrates, an embodiment, in. which all of the layers except the. backing. sheet. are conductive. The various layers. arev not. drawn to scale in any of, the. figures.

In the preferred embodiment of'Fig. 1 an intensifying screen is shownwhich comprises abacking and supporting sheet 4. which is coated with four different layers; Layer 2. includes a phosphor material (indicated by an.- gular. particles) whichv fluoresces when, energized by. X- rays. Layer 3 isa: pigmented light-reflecting layer. Layer 1 comprisesa thin. protective covering layer which is substantially transparent to visible light in both. direc.. tions and. is arbitrarily represented by the symbol for; plastic. Layer 5 comprises a metallized coating preterably of aluminum: flakes secured in a binder, and is repre sented; by stippling. The materials employed in the various mentioned layers and their relative locationsare important inrconnection, withthe present invention. Consequently, the nature of the mentioned layers and the principles by which the desired results. canbe achieved Willnow bedescr-ibed;

The. material of which thesupporting. or backing sheet 4; ismade can; bechosenfrom several; substances-having: various characteristics. The better kno n; materials are paper and. certain plastics. Some. plastic; materials: are, nonsuitable, for intensifying screen-s; because they; absorb too; muchlxrray energy. We presently-prefer thick paperg as. a; backing material because it has. the required mechanical char eristi bu it bly: flexible, is; comparatively cheap and; is; highly permeable. to: X-rays. For the. present purpose; we employ paper; of; thickness in they range between; 0.005 inch. and: @020; inch, the preferable thickness; beingfrom 0.01:4 inch. to 0:016. inch; Substantially the; only disadvantage of paper isthat itis subject: to absorption of moisture which. tends to alter its dimensions. and reduces its: dielectric qualities. However, when coated in. accordance. with the present invention, the. paperbacking sheet is. substantially urn. affected by moisture conditions usually encountered;

Among the plastic sheet materials. suitable as a back-ing support are cellulose acetate or nitrate, polystyrene, polymethracrylates, rubber, the polyesters and cloth of vege table or glass fibercoated or impregnated with any or? these materials. The; thickness required would depend}. of course, upon. the characteristics of the particular material, but, in general, thicknesses of the order mentioned in connection with paper are useful. One plastic material' which wehave used with good success forbadeing sheets comprises white-pigmented polyvinyl acetatepolyvinyl chloride copol-ymer sheet, 0.015 inch thick;

On topof the backing layer is a pigmented reflectinglayer Swhich is employed; as shown inthisembodiment, for the purpose of reflecting visible light toward the photographic negative. Although some intensifying screens presently employed in the art do not include such a reflecting layer, we prefer to employ it; because itimproves the overall efiici'ency of the screen. Its X-rayabsorption is negligible, but its reflection of light on the negative is considerable. This pigmented layer comprises titanium dioxide stirred with a small amount of binder and solvent. Such a layer has high dielectric properties.

- The fluorescent layer 2 may comprise any of the phosphor materials commonly employed for the purpose. Obviously, the material should be one which actively fluoresces when energized by X-rays, and should have lightemitting properties which match as closely as possible the light-sensitivity characteristics of the photographic emulsion employed in the negative. These characteristics are now well known in the art and need not be discussed here. Usually the phosphor employed comprises calcium tungstate in finely divided form and applied as a suspension in a solution of a suitable binder such as cellulose nitrate or acetate in a suitable solvent. To provide high X-ray efficiency it is preferable that a minimum amount of binder be employed, preferably less than 5% by weight calculated on the mixture. However, the less the proportion of binder material the more brittle is the layer. Consequently, there is an optimum relationship, readily ascertained, between the amount of binder required and the stiffness of the backing sheet 4. In general, flexibility is an advantage.

The cover layer 1 is for the purpose of protecting the fluorescent layer. Such a coating is customarily employed in intensifying screens and, like the fluorescent layer itself, need not be described in detail. It should be as transparent as possible to visible light, as well as X- rays, and should have a hard surface and be moisture resistant. Customarily such a layer is comprised of a solution of cellulose acetate in any suitable solvent in sufficient quantity to produce a very thin film when dry. This coating protects the phosphor surface against abrasion, dirt and moisture, and it comprises a dielectric ma terial.

In accordance with the invention, a conductive layer 5 is applied to the rear surface of the backing sheet 4. Although many different conductive materials of various physical forms maybe employed in this layer, we have found that aluminum appears to be the most satisfactory because, in the thickness required to produce adequate conductivity, its X-ray absorption is tolerable, it is readily available in a variety of forms, and is inexpensive.

It appears that in order to minimize the above-described effects of static discharges in undesirably exposing the photographic negative, it is necessary to construct the screen so that in itself it constitutes a condenser, or an aggregation of condensers, of suflicient electrostatic capacity to dissipate the local charges built up on the front surface of the screen due to the pressure or to the relative movement between the screen and the negative with which it is in contact. Consequently, a conductive layer, viz., a layer comprising conductive material, is added to the screen in juxtaposition to the front surface but separated therefrom by dielectric material. The various layers commonly employed in the more recent forms of intensifying screens themselves are of material having reasonably good dielectric properties. It is feasible to provide a conductive layer at various stratum levels in the screen structure either as a discrete conductive layer interposed for the purpose, or as a layer which is made partially conductive by introducing conductive material into a layer also employed for another purpose. Several alternative constructions are therefore possible to provide the anti-static effects in accordance with the invention. By using a suitable metal or non-metallic conductive material of small particle size, mixed with a suitable resin or plastic binder, the layer not only provides the required conductivity but, if applied to the rear surface of the supporting sheet 4, tends to seal it against moisture, thus stabilizing the product as to dimensions and anti-static qualities. For this reason, it is preferable that the edges of the backing material, especially if it is of a fibrous nature, be coated also.

. 'We have found that, in general, material having a H12,904,689. n a

certain degree of conductivity is required to produce the necessary effective electrostatic capacity. However, since the absorption of X-rays increases with increased density of such materials, there is an optimum relationship between the quantity of metal and its physical form required to provide suflicient conductivity with minimum, or at least tolerable, X-ray absorption. In employing aluminum, which we presently prefer, it appears that a given weight in flake form, for example, provides at least as effective anti-static qualities as does the same weight of aluminum employed in the form of thin foil.

The aluminum layer 5 shown in the screen structure of Fig. 1 preferably is formed as follows: Aluminum flake, of which approximately 98% passes through a 325 mesh screen, is dispersed in a resin binder dissolved in methyl ethyl ketone. An example is as follows:

Parts by weight Methyl ethyl ketone 70.0 Vinyl chloride-acetate copolymer 30.0 Aluminum flake 6.0

The aluminum flake is stirred into a binder solution composed of the vinyl chloride-acetate copolymer dissolved in the methyl ethyl ketone. A proportion of aluminum of the mentioned particle size in the range of between approximately 16% and 17% of the total solids appears to provide the most effective anti-static qualities. The dry film thickness of the mentioned coating may, with satisfactory results, vary between 0.0001 inch and 0.002 inch, a coating of 0.0013 inch having been determined to be a satisfactory mean. The linear conductivity is low in such a coating, but the static-reducing effect according to this invention appears not to depend substantially on linear conductivity because the static potentials comprise localized charges which can be dissipated by localized capacitive eflects contributed by individual conductive particles, or by small groups thereof, suitably separated by dielectric material. As above stated, this coating also provides excellent protection against moisture absorption by the backing material and incidentally enhances the appearance of the product.

Other resins which may be used as binding materials for the conductive layer include cellulose esters and ethers, acrylic and methacrylic polymers, and other organic soluble thermo-plastic or thermo-setting resins, if capable of furnishing suitable film-forming properties which are compatible, under the conditions of application with the materials employed in the adjacent layers. Additional binding materials which can be employed for the purpose, although probably not so desirable, include methyl cellulose, hydroxy methyl cellulose, polyvinyl alcohol, alcohol-soluble and water-soluble resins and gums, if capable of furnishing suitable film-forming properties and which can be rendered insoluble in water by special treatment. It is undesirable in screens of the nature herein described to employ substances which in their final condition are hygroscopic.

The proportion of aluminum, or other metal, or nonmetal, powder or flake which will be required to provide the optimum effect will vary to some degree in accordance with the particular binding material employed and to the treatment which it must be given in the course of the fabrication of the conducting layer. However, this proportion can readily be ascertained by a worker in the art once he has made a selection of the specific materials to be used. Likewise it is unnecessary here to specify the exact solvent to be employed with any selected binder because satisfactory solvents for the various binders are well known in the organic chemistry art. g

The electrically conductive material may be any of a variety of substances, including metals and non-metals such as carbon and silicon. Most, if not all of these, are likely to be more expensivethan aluminum, and their characteristics are, for the most part, not useful for the purpose as are those of aluminum. However, other metals including iron, nickel, copper, silver,'tin, lead and bismuth, can be used in back screens, 'viz., behind the X-ray negative. Carbon is less dense :and absorbs X- rays less. If the electrically conductive layeris 'positioned so that'X-rays must 'pass through it in order to penetrate and expose the photographic emulsion on the negative, as in front screens, then it is preferable to use a stable metal of low density, suchas aluminum. Satisfactory X-ray transmission can'be taken to be that which produces an exposed emulsion density within 2% of the control density. However, the X-ray absorption is immaterial if the conductive metal is placed on the back of the screen, as in Fig. 1, and this screen, in turn, is disposed on the far side of the negative away from the X-ray source, as above mentioned.

The embodiment illustrated in Fig. 2 differs from that of Fig. 1 in two respects. First, an aluminized layer or coating 6 is interposed between the back sheet 4 and the pigmented reflecting layer 3, and second, the rear sur face of the backing sheet is coated with a layer 7 Which may comprise any desired moisture-resistant resin, but it may be thicker than layer 1. The object of this layer 7 is, as before, to seal the paper or cardboard or other backing material, if necessary, against the absorption of moisture or other substances. If desired, the aluminized layer of Fig. 1 can also be employed on the rear surface instead of the resinous layer 7.

The interposition of an aluminized layer 6 between layers 3 and 4, as in Fig. 2, is satisfactory in its anti-static effects, especially if either the pigmented layer 3 or the fluorescent layer 2, has good dielectric properties. This layer may be formed of any of the materials or combinations of materials above described in connection with conductive layers.

The screen structure of Fig. 3 differs from those of Figs. 1 and 2 in two novel respects. Instead of employing a pigmented layer such as layer 3 in Figs. 1 and 2, the pigmented reflecting layer 8 in this embodiment is made conductive by the addition of aluminum particles. It appears to make little difference whether the particles are of flake or granular form, if of the same preferred particle size, although the flake is usually less expensive. The results of tests indicate that the addition of the aluminum to the pigmented layer, if in the same quantity as above mentioned, provides substantially the same antistatic qualities in the arrangement of Fig. 3 as it does in the arrangement of Fig. 2. However, the specular lightreflecting properties of the layer 8 are somewhat decreased by the addition of the aluminum, and for that reason a light-reflecting layer Without the metal particles is preferred, if maximum light output be required. Nevertheless, the structure of Fig. 3 is useful and eliminates the additional layer in the embodiment of Fig. 2. In the structure of Fig. 3 the rear surface of the backing sheet 4 is covered with a thin layer 9 of aluminum as illustrated. Foil of a thickness of 0.002 inch cemented to the backing sheet 4 is suitable for the purpose, although, as above stated, it is more readily damaged than is a coating of aluminum flake in a binder, such as 5 in Fig. 1.

The screen structure illustrated in Fig. 4 is similar to that of Fig. 3 except that the conductive foil on the rear surface of backing sheet 4 here comprises a resinous layer containing aluminum particles as described in connection with Fig. 1. Although the light-reflecting prop erties of layer 8 are, as above mentioned in connection with Fig. 3, somewhat decreased by the addition of the aluminum particles, the anti-static properties of the structure are at least as good as those of any of the preceding structures because the dielectric backing sheet has a conductive layer on each side of it. The improved chargeabsorbing characteristics of this structure are believed to result from the presence of a conductive layer 8 below the dielectric layers 1 and 2 and of another conductive layer 5 below the dielectric layer 4.

The intensifying screen illustrated in Fig. 5 is different from all of the others in the respect that aluminum-i other conductive) particles are dispersed in all of the layers, except the backing sheet. Here, the rear surface of the backing .-.s'heet 4.-is coated with analuminized layer 5, as in Fig. .1, and-the front .surfacelof the backing sheet 4 .is coated with a mixture of titanium dioxide and aluminum particles in "a suitable binder forming. a'coating 8 which is the same as that illustrated in Fig. 3. Additionally, in the present embodiment, the fluorescent layer 11 and the protective coating 12 both contain conductive particles, but preferably in lesser quality than in the other layers. The electrical effect of this construction is that the three top layers 8, 11 and 12 act as a single conductor or electrode, and the layer 5 on the rear of dielectric sheet 4 acts as the second conductor, or electrode, of the condenser. Electrically this construction is effective in eliminating the undesirable effects of static, but the addition of conductive particles in the layers 11 and 12 reduces the transmission of light. This embodiment illustrates the fact that substantially any one or a combination of the layers above or below the backing sheet can be made conductive, if separated by a dielectric layer, in order to dissipate the deleterious static charges, as above explained. It is also to be understood that metallic foil, especially if of low X-ray absorption, can be substituted for the conductive particles in any discretely conductive layer.

We claim:

1. An X-ray intensifying screen comprising: a backing sheet of cardboard, a moisture resistant and electrically conductive coating of aluminum particles in a resin binder on the back and edges of said sheet, a light-reflecting layer containing titanium dioxide on the front of said sheet, a fluorescent layer on the reflecting layer, and a smooth, thin, transparent, protective dielectric coating fihn over the fluorescent layer, whereby to minimize static discharges when the surface of said protective dielectric coating fihn is pressed against a radiographic film and subsequently removed therefrom.

2. An X-ray intensifying screen comprising in the order named: a translucent, moisture-resistant protective coating film of dielectric material, a fluorescent layer, a dielectric backing sheet and a moisture resistant and electrically conductive coating comprising metallic aluminum particles in a binder on the rear surface of said backing sheet, whereby to minimize static discharges when the surface of said protective coating film is pressed against a radiographic film and subsequently removed therefrom.

3. An X-ray intensifying screen comprising a backing sheet having dielectric properties, a coating between .0001 and .002 inch thick on the rear surface of said sheet comprising aluminum particles substantially of 325 mesh in a binder of vinyl chloride-acetate copolymer in proportion of the order of 1:5 by weight, a pigmented layer on the front of said sheet comprising titanium dioxide, a fluorescent layer on said pigmented layer comprising a phosphor material in a resin binder, and a protective film comprising transparent material having dielectric properties covering said fluorescent layer, whereby to minimize static discharges when the surface of said protective film is pressed against a radiographic film and subsequently removed therefrom.

4. An X-ray intensifying screen comprising a backing sheet of dielectric material, a moisture-resistant coating layer on one side of said sheet, a pigmented light-reflecting layer over the other side of said sheet, a fluorescent layer on said pigmented layer, and a static-forming translucent protective layer including a dielectric resinous material on said fluorescent layer, said moisture-resistant coating layer being characterized in that it contains metallic particles in quantity suflicient to impart electrical conductivity thereto such as to minimize localized static discharges when the face of said screen which bears said 7 8 protective layer is pressed-against a photographienegative 1 2,694,153 Reuter Nov. 9, 1954 and subsequently removed therefrom. 2,740,050 Schultz Mar. 27, 1956 4 2,791,723 Nagy May 7, 1957 References Cited in the file of this patent 2,798,823 Harper July 1957 UNITED STATES PATENTS 5 2,820,146 Kunes Jan. 14, 1958 2,144,040 Wurstlin Jan. 17, 1939 FOREIGN PATENTS 2,188,115 Kallmann Jan. 23, 1940 449,244 Great Britain June 22, 1936 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 2,904,689 September 15, 1959 Frank J Masi et a1,

It is hereby certified that error apgears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should readas corrected below.

Column 1, lines 16 and 17, for "photositiveread photosensitive column 5 line 17, for "back" read backing -g column 6, line ll, for "quality" read quantity Signed and sealed this let day of March 1960.

(SEAL) Attest:

KARL H; AXLINE ROBERT c. WATSON Attesting Officer Commissioner of Patents

Claims (1)

1. 4. AN X-RAY INTENSIFYING SCREEN COMPRISING A BACKING SHEET OF DIELECTRIC MATERIAL, A MOISTURE-RESISTANT COATING LAYER ON ONE SIDE OF SAID SHEET, A PIGMENTED LIGHT-REFLECTING LAYER OVER THE OTHER SIDE OF SAID SHEET, A FLUORESCENT LAYER ON SAID PIGMENTED LAYER, AND A STATIC-FORMING TRANSLUCENT PROTECTIVE LAYER INCLUDING A DDIELECTRIC RESINOUS MATERIAL ON SAID FLUORESCENT LAYER, SAID MOISTURE-RESISTANT COATING LAYER BEING CHARATERIZED IN THAT IT CONTAINS METALLIC PARTICLES IN QUANTITY SUFFICIENT TO IMPART ELECTRICAL CONDUCTIVITY THERETO SUCH AS TO MINIMIZE LOCALIZED STATIC DISCHARGES WHEN THE FACE OF SAID SCREEN WHICH BEARS SAID PROTECTIVE LAYER IS PRESSED AGAINST A PHOTOGRAPHIC NEGATIVE AND SUBSEQUENTLY REMOVED THEREFROM.

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