US 3394261 A
Description (OCR text may contain errors)
ly 1968 B. w MANLEY ETAL 3,394,
ELECTRONIC INTENSIF'IER DEVICE FOR PRODUCING A VISIBLE IMAGE FROM AN X-RAY IMAGE Filed May 25, 1965 FIGJ INVENTORJ BRIAN WILLIAM MANLEY BY JOHN 40mg 2! K AGENT United States Patent 3,394,261 ELECTRONIC INTENSIFIER DEVICE FOR PRO- DUCING A VISIBLE IMAGE FROM AN X-RAY IMAGE Brian William Manley, Burgess Hill, and John Adams, East Grinstead, England, assignors to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed May 25, 1965, Ser. No. 458,734 Claims priority, application Great Britain, May 29, 1964, 22,471/64 Claims. (Cl. 250213) ABSTRACT OF THE DISCLOSURE An electronic image intensifier device for producing a visible image of an X-ray image employing a body of insulating material having channels therein extending between opposite faces which are provided with conductive layers thereon. X-rays enter one face and are absorbed in the material liberating photoelectrons which enter the channels and strike the walls thereof liberating secondary electrons which emerge from the channels and strike a luminescent screen producing a visible image.
This invention relates to an electronic device for producing a visible image of an X-ray image. More particularly, the invention relates to a device employing a body of insulating material having opposite major faces provided with conductive layers and channels therein extending between those faces. X-rays entering one face liberate photoelectrons which enter and strike the walls of the channels liberating secondary electrons which are converted to a visible image by a luminescent screen.
It is known to intensify electron currents by secondaryemission. In those cases in which secondary-emission intensification takes place of concentrated electron currents, increase of the beam cross-section can be avoided by using narrow tubes which internally have secondary-emissive properties. As such may be considered also the electron current of a photo-electric cathode. For secondary-emission intensification of such a beam may be used a great number of closely arranged channels which are provided in a flat plate or are obtained by combining the required thin tubes. In addition to having a secondary emissive surface, the boundary walls of the channel destined for passing the electrons should be electrically conductive to some extent and may therefore be coated with resistance material or made of a suitably chosen wall material. In order that the secondary electrons follow the original direction of movement and retain sufficient energy for repeatedly contributing to the intensification, the surfaces in which the channels open onto are each lined with an electrically conductive coating for setting up an electric accelerating voltage.
Certain electron tubes, which have a photoelectric cathode and a fluorescent screen are termed image intensifiers because the photoelectric current traverses an accelerating electric field and often a reduced image of the photo cathode image is formed on the fluorescent screen. X-ray images are reproduced in a corresponding manner with an X-ray image intensifier. In this case the photo electric cathode is combined with a layer of fluorescent 3,394,261 Patented July 23, 1968 material which is struck by the incident X-rays, the light produced therein causing the photo-electric emission.
For the conversion into light in the fluorescent layer the X-rays are only partly used because the thickness of this layer is small as compared with the thickness which would be required for completely absorbing the X-rays. The largest possible thickness is determined by the permissible lack of definition of the image which is the result of the dilfuse scattering of the luminescent light. In addition, the manufacture of a photo-electric cathode combined with a layer of fluorescent material is difficult to make and require particular methods which have to be performed accurately and with a great amount of skill.
The invention relates to an X-ray image intensifier in which the X-rays are used in a maner other than the commonly used manner to obtain a fluorescent image. According to the invention the X-ray image intensifier comprises an evacuated envelope containing a secondaryemissive intensification screen of narrow channels which are arranged in a material which has the property of being activated to the emission of photo-electrons by X- ray absorption, the said electrons being increased in number by secondary emission in the channels and impinging from the channels upon a fluorescent viewing screen on the side remote from the side where the X-rays impinge under the influence of an electric voltage.
The lack of definition of the image reproduced on the screen is now substantially given by the fineness of distribution of the secondary-emission channels in the carrier material of the intensification screen and independent of the density of this material in which the X-rays are absorbed.
An embodiment of an X-ray image intensifier according to the invention is diagrammatically shown in crosssection in FIGURE 1 of the drawing, while FIGURE 2 serves for further explaining the manner in which the ray conversion takes place.
In an evacuated envelope 1, which is manufactured from insulating material, for example, glass or ceramic or from metal, for example, aluminum, the flat wall part on the left-hand side is permeable to X-rays and the flat wall on the right-hand side is transparent so that in certain cases these parts of the wall have to be provided with windows. In the drawing, the shape which these parts of the wall must have in that they are sufficiently strong is not shown because this is a known structural problem. The envelope is provided with lead-in conductors 2, 3 and 4 for electric voltages.
In the space enclosed by the envelope the secondaryemission intensifier 5 and a fluorescent screen 6 are arranged. It is noted in this connection that the simplest possible arrangement is shown. It is normal that in X- ray intensifiers a reduced image is reproduced on the viewing screen of the photo-cathode image by electron optical projection. Further means required for that purpose may be provided in the present image intensifier without departing thereby from the way shown of converting the X- rays into radiation for the image observation.
The secondary-emission intensifier is lined on both sides with a conducting layer, the coating 7 being connected to the supply conductor 2 and the coating 8 to the supply conductor 3. The fluorescent screen is connected to the supply conductor 4. Increasing voltages are supplied from the two voltage sources 9 and 10 with the supply conductors 2, 3 and 4.
FIGURE 2 shows a small portion of the secondaryemission intensifier 5, namely two joints 11 and 12 between successive channels 13, 14 and 15. The whole intensifier consists of channels spaced apart in a similar manner.
X-rays impinging upon the solid material are indicated by the waving lines X and X The ray X has penetrated to the point D where a photo-electron is liberated. This electron substantially moves at right angles to the direction of the incident ray along the line Ph 1. The point where this electron is generated is so near to the surface of the solid substance that there is no chance for it to generate any further secondary electrons in the substance. Thus the photo-electron enters the channel 14 and crosses to the opposite side where one or more secondary electrons are liberated at the point where it impinges. Under the influence of the electric field produced by the applied votlages, the movement of these electrons is directed in the longitudinal direction of the channel. Under the influence of transverse components of the speed which are also operative, the electrons will repeatedly strike the wall of the channel as a result of which their number increases continuously.
Of the ray X it is assumed that it penetrates to D at which point a photo-electron is liberated. The longer travel of the electron in the soild substance presents the possiblity of liberating further electrons Sa, Sb and Sc which, together with the first electron, move along the indicated tracks in which their number also increases. As in the case with the electrons indicated by Sa, these electrons are partly braked in the solid substance and are lost. Othere electrons, for example, S and s reach a channel along a curved travel, since their energy is less than that of the photo-electron P122. The electrons together contribute to the formation of electron currents from each of the channels, the cross-section of the channel being decisive of the image definition.
A method of manufacturing such channel intensifier-s may consist in the assembly of drawn tubes to a bundle which are sealed together and then form a body of a solid substance which is provided with channels. The tubes have a wall thickness such that in the body the solid substance between two channels constitutes approximately double the thickness of the wall thickness of the tubes. The term wall thickness will be used hereinafter to refer to the said thickness of the tubes.
It will be discussed hereinafter, with reference to the description of an embodiment, which are the restrictions of a channel intensifier in the material of which photoelectrons are excited by X-rays. When comparatively wide channels are used having a diameter of approximately 200 microns and a wall thickness of about 30 microns a matrix is formed in which the conversion of X-rays into photo-electrons, which take part in the further intensification, takes place to a slight extent. The cause hereof is associated with the known small freedom of movement of photo-electrons which in most materials experience no deviation larger than approximately microns, so that most photo-electrons created in the material of the wall are lost. Thus effectively only the 10 micron thick layer of the wall adjacent the passageways contributes photo-electrons to the intensification.
It is advantageous therefore to ensure that this 10 micron thick layer is as absorptive of X-rays as possible. One of the means for manufacturing absorbing glass is to make a dense material, for example, lead oxide, one of its major constituents. When, however, more lead oxide is added, the glass is less easy to work and to draw as a result of which the manufacture of the intensifier body from drawn tubes is impeded or no longer possible. It is very well possible to proceed in this manner when the lead content is less than 30% by weight or the glass contains no lead at all and the tubes are internally lined with a layer of glass enamel containing a heavy element, for example, lead or bismuth.
n 0 P D 50 B1203 25 As compared with a material which contains of P 0, such a glass enamel used in the form of a 10 micron thick lining can increase the photo-electric yield by 80% when radiated with X-rays of a hardness commonly used in diagnostic radiology.
In addition, a coating of lead oxide enamel has the additional advantage that the surface can be made somewhat conductive by reduction to lead, as is desired for maintaining an even distribution of the voltage along the surface.
The application of such a layer of enamel having a high lead content may be carried out as follows.
Lead glass of the desired composition is crushed and dry-ground in a mechanical mortar with pestle. On completion of this grinding a small quantity of methylated spirit is added to the powder so as to form a paste which is then rolled in a ball mill. The frit is dried and then mixed with a binder consisting of a solution of nitrocellulose in amyl acetate. The resulting mixture is again rolled in a ball mill to a homogeneous mass of such a viscosity as is required to provide a layer of sufficient thickness.
Coating the wall is carried out by filling the tubes with the liquid enamel, followed by draining and drying. Prior to drawing the tubes the binder must be evaporated, sufficient binder being removed after a few hours heating and drawing can take place. The enamel is hardened as a result of the heating required for this operation.
By drawing, the tubes with a diameter of for example 10 mm. are narrowed. The tubes are coated on the outside with a low-melting-point enamel and may be manufactured, for example, from lead glass. The enamel manufactured with a high content of lead covers the inner surface. Each tube is drawn and narrowed to an inner diameter of 200 microns and cut to pieces, for example, cm. long. A jig which has a hexagonal aperture is filled with such tubes after which the tubes are fused together at a temperature of 600 C. at which the enamel provided on the outer surface melts and the material of the tubes is still solid. Then 1 cm. thick sections are cut from the bundle which are laid side by side in a fiat plane and fused together to a larger cross-section.
The electric conductivity by reduction of the lead oxide is obtained on heating the plate in a hydrogen atmosphere to 340 C. for 2 hours. Naturally, the duration of heating and the temperature depend upon the composition used for the glass enamel which coats the channel walls.
What is claimed is:
1. An electronic intensifier device for producing a visible image of an X-ray image projected onto the input side of a secondary emissive electron multiplier device comprising a body of solid insulating material provided with a plurality of narrow parallel channels extending between opposite major surfaces of said body which constitute the input and output faces respectively of the electron multiplying device, conductive layers on said input face and said output face, a fluorescent viewing screen positioned opposite the output face of said electron multiplying device, said insulating material consisting of a substance which emits photoelectrons by the absorption of X-rays projected onto the input face of the electron multiplying device, said photoelectrons thereafter penetrating into the channels and being increased by secondary emission in the channels.
2. An X-ray image intensifier as claimed in claim 1, characterized in that the solid substance of the secondaryemissive screen consists of glass and the walls of the channels are lined with a thin coating of X-ray absorbing glass.
3. An X-ray image intensifier as claimed in claim 2, characterized in that the coating of the walls of the channels is an enamel which contains lead oxide.
4. An X-ray image intensifier as claimed in claim 3, characterized in that the enamel contains sufiicient reduced lead oxide to obtain a coating of a small electric conductivity.
5. An X-ray image intensifier as claimed in claim 4,
characterized in that the coating of material which absorbs X-rays has a thickness of at most 10 microns.
References Cited UNITED STATES PATENTS 3,128,408 4/1964 Goodrich et alt 250-20'7 RALPH G. NILSON, Primary Examiner.
A. L. BIRCH, Assistant Examiner.