EP0540391B1 - Radiological image intensifier tube - Google Patents

Description

The invention relates to an image intensifier tube of the radiological type, more particularly means for improving the image resolution of this tube and a production method.

Image intensifier tubes are vacuum tubes comprising an input screen, located at the front of the tube, an electronic optical system, and a visible image observation screen located at the rear of the tube, on the side of an exit window of the latter.

In the X-ray image intensifier tubes or abbreviated to "IIR tube", the input screen further includes a scintillator screen which converts the incident X photons into visible photons.

Figure 1 schematically shows such an image intensifier tube of the radiological type.

The IIR tube 1 comprises a glass envelope 2, one end of which, at the front of the tube, comprises an entry screen 9. This end is closed by an entry window 3 exposed to X-ray radiation.

The second end of the envelope forming the rear of the tube is closed by an exit window 4 transparent to light.

The X-rays are converted into light rays by a scintillator screen 5. The light rays excite a photocathode 6 which in response produces electrons.

The electrons produced by photocathode 5 are accelerated towards the exit window 4 using different electrodes 7, and an anode 8, arranged along a longitudinal axis 13 of the tube and which form the optical system. electronic.

The exit window 4 is formed by a transparent piece of glass which, in the example shown, carries a cathodoluminescent screen or exit screen 10, made of phosphors for example.

The impact of the electrons on the cathodoluminescent screen or output screen 10 makes it possible to reconstruct an image (amplified in luminance) which at the start was formed on the surface of photocathode 6.

The image displayed by the exit screen 10 is visible through the glass piece which constitutes the exit window 4. Generally optical sensor devices (not shown) are arranged outside the tube near the output window 4 to capture this image through it and allow its observation.

In the most recent embodiments, the input screen 9 comprises an aluminum substrate covered by the scintillator 5, itself covered by an electrically conductive and transparent layer 11, for example made of indium oxide. The photocathode is deposited on this transparent layer 11.

X-rays strike the entrance screen on the side of the aluminum substrate and pass through this substrate to then reach the material constituting the scintillator.

The light photons produced by the scintillator are emitted a little in all directions. However, to increase the resolution of the tube, a substance such as cesium iodide (C _s I) is generally chosen as scintillating material, which has the property of growing in the form of crystals perpendicular to the surface on which they are deposited. The needle crystals thus deposited tend to guide the light perpendicular to the surface, which is favorable for good image resolution.

Patent application FR-A-2 634 562 filed on July 22, 1988 describes how to improve this resolution, by reducing the average section of the crystals in scintillator needles, thanks to the surface condition of the layer on which the scintillator.

Image resolution can also be degraded due to the fact that bright photons generated in the scintillator leave towards the arrival side of the X-rays. These photons strike the aluminum substrate, with an incidence which is random. They are reflected by the aluminum substrate towards the front, therefore towards the photocathode, but the path of these photons is such that there is a loss of resolution: for the same incidence of X photons, we can lead to creation of electrons in the photocathode at different points.

Figure 2 shows in more detail the input screen 9 and illustrates this loss of resolution by showing side by side the different paths followed by two light photons PL1, PL2 resulting from the impact of a photon X on the scintillator 5 , resulting in the formation of electrons at different points in the photocathode. The entry window 3 through which the X-rays arrive, constitutes the aluminum substrate carrying the scintillator 5 made of cesium iodide whose crystals 5a perpendicular to the surface tend to channel the light photons; the transparent conductive sublayer marked 11 is placed between the scintillator 5 and the photocathode 6.

In the example shown in FIG. 2, the light photon PL2 is emitted towards the rear, that is to say towards the substrate 3, with an incidence such that it is reflected by the latter towards the photocathode 6 in taking as a path, in the scintillator 5, a needle crystal different from that in which it was generated, which illustrates the loss of resolution.

The document Patent Abstracts of Japan, Vol 6, Num 528, JP-A-56165251 shows an input screen for an image intensifier tube corresponding to the preamble of claim 1 comprising a substrate carrying a scintillator and a layer of alumina covered with an absorbent substance between the substrate and the scintillator. The pores of the alumina layer are blocked before covering it with the absorbent substance.

The present invention also proposes to improve the image resolution, by reducing the quantity of light photons reflected by the aluminum substrate after being emitted towards the rear.

To this end, the invention shows how to interpose between the aluminum substrate and the scintillator, a screen absorbing at least partially the light produced in the scintillator, this screen being made of tinted porous alumina.

According to the invention defined in the appended claims, it is proposed to produce a radiological image intensifier tube in which the input screen comprises, between the scintillator and the substrate carrying this scintillator, a layer of porous alumina tinted with a absorbing substance at the wavelength emitted by the scintillator, so that light photons emitted by the scintillator in the direction of the substrate are at least partially absorbed in this layer of tinted porous alumina.

By absorbing at least a portion of the light photons emitted towards the rear, the proportion of these photons is reduced which, after reflection by the substrate, strike the photocathode at points very different from the light photons emitted towards the front and generated by the same X photons.

By the expression "tinted by an absorbent substance at the wavelength emitted by the scintillator", we want to define a substance capable of opacifying the alumina which contains it or which is impregnated with it, that is to say to reduce its transmission, and that at least for the wavelength emitted by the scintillator. Consequently the term "tinted" also applies to a neutral or gray tint capable of absorbing a wider range of wavelengths.

In the most common case where the substrate is aluminum and the scintillator is made of cesium iodide, the tinted alumina layer also has a very important advantage which is to promote the adhesion of the scintillator to the aluminum substrate.

In addition, the advantage of such a solution is that it remains compatible with the reduction in cross section of the crystals into needles of the scintillator.

The tinted alumina layer can be produced by anodizing the substrate; the anodization of the substrate can be accomplished by an appropriate method to make the porous alumina layer, and the anodization is followed by a step of filling the pores with an absorbent substance at the wavelength emitted by the scintillator.

This absorbent substance can be deposited on the interior walls of the pores by a soaking method, in a solution suitable to provide the coloring suitable for absorbing the light produced by the scintillator.

The absorption coefficient conferred on the alumina layer can be controlled for example by the concentration of the solution in colored product, and / or by the degree of porosity of the alumina layer.

In addition, by modifying the absorption coefficient of the tinted alumina layer, between its edges and the center, this variation of the absorption coefficient can be given an appropriate law, for example to correct the brightness curve of the IIR tube. .

It should also be noted that by controlling the degree of porosity of the alumina layer, it can be given a structure better suited to carrying the scintillating layer and to withstanding the effects of the differences in coefficient of thermal expansion which it exhibits with the latter.

• la figure 1 déjà décrite, montre schématiquement une coupe d'un tube intensificateur d'image classique ;
• la figure 2 déjà décrite montre schématiquement en coupe des détails d'une partie d'un écran d'entrée montré à la figure 1 ;
• la figure 3 représente schématiquement en coupe un écran d'entrée d'un tube IIR selon l'invention.
• la figure 4 montre des courbes de brillance mesurées en sortie d'un tube IIR ;
• la figure 5 montre schématiquement en coupe des détails d'une partie d'une couche d'alumine montrée à la figure 3 ;
• la figure 6 montre schématiquement une autre forme de réalisation de la couche d'alumine montrée à la figure 3 ;
• la figure 7 illustre de façon schématique comment obtenir un gradient de la porosité de la couche d'alumine montrée à la figure 3.

The invention will be better understood on reading the description which follows, made with reference to the appended figures in which:

• Figure 1 already described, schematically shows a section of a conventional image intensifier tube;
• Figure 2 already described schematically shows in section details of part of an input screen shown in Figure 1;
• Figure 3 shows schematically in section an inlet screen of an IIR tube according to the invention.
• FIG. 4 shows gloss curves measured at the outlet of an IIR tube;
• Figure 5 shows schematically in section details of part of an alumina layer shown in Figure 3;
• Figure 6 schematically shows another embodiment of the alumina layer shown in Figure 3;
• FIG. 7 schematically illustrates how to obtain a gradient of the porosity of the alumina layer shown in FIG. 3.

FIG. 3 shows, by a view similar to that of FIG. 1, an IIR tube comprising an inlet screen 15 according to the invention, the IIR tube being for the rest similar to that shown in FIG. 1.

The input screen 15 comprises a scintillating layer 16 carried by a support or substrate 17. The substrate 17 preferably consists of an aluminum sheet, but it can also be an aluminum-based alloy. Its thickness (of the order of, for example, half a millimeter) gives it good transparency in X-rays.

The scintillator layer 16 is itself conventional, for example in cesium iodide of a few hundred microns (of the order of 400 microns). Cesium iodide is doped, for example, with Sodium, so that it emits at a wavelength of around 4,300 angstroms (blue light), a wavelength which can vary with the doping of iodide. It should be noted that the proportion between the dimensions of the various elements shown in FIG. 3 is not respected to make the latter clearer.

The input screen 15 also comprises, in the traditional way, a transparent layer 11 conducting electricity, carried by the scintillating layer 16 opposite the substrate 17, as well as a layer forming the photocathode 6 and which is deposited on the transparent layer 11.

According to a characteristic of the invention, the entry screen 15 comprises a tinted alumina layer 20, interposed between the scintillator layer 16 and the substrate 17.

The purpose of the tinted alumina layer 20 is in particular to constitute an absorbent screen at the wavelength emitted by the scintillator 16, with a view to absorbing at least partially the light photons emitted by the scintillator 16 towards the rear, that is to say towards the substrate 17. For this purpose, the alumina layer 20 is tinted by a substance capable of absorbing at least the light emitted by the scintillator 16, the blue light in the example.

The absorption coefficient by the alumina layer 20 or conversely its transmission coefficient, at the wavelength emitted by the scintillator layer 16, depends on the quantity and the concentration of the absorbent substance contained in this alumina layer. Generally, the absorption produced by the alumina layer 20 must be a compromise between the acceptable loss of light energy (light produced by the scintillator 16) and therefore the sensitivity and the level of image resolution sought.

According to another characteristic of the invention, the absorption coefficient (at the wavelength emitted by the scintillator 16) of the tinted alumina layer 20 varies between the outer edges 21 and the central zone 22 of the latter, that is to say along a diameter D1 common to this layer 20, and to the whole of the input screen 15.

By giving the alumina layer 20 an increasing absorption coefficient, from the edges 21 to the center 22, this alumina layer 20 simultaneously achieves an improvement in the resolution d image and compensation of the brightness curve measured along a diameter D (shown in FIG. 1) of the output screen 10 of an IIR tube: the brightness curve represents the light intensity at each point of the output screen diameter.

For reasons of electronic optics, the surface of an IIR tube entry screen is not flat but curved; it can be parabolic or hyperbolic (for large screens) or more generally in the form of a spherical cap.

It follows from this curvature of the screen that if one illuminates the input screen with a uniform X-ray beam, the electronic density generated by the screen is not uniform. If we measure the brightness curve along the diameter D of the output screen 10 (Figure 1), we see that this curve is not horizontal; it is generally in the form of an arc of a circle slightly flattened in the center; the brightness of the output screen is maximum towards the center but decreases markedly as one approaches the edges. For small tubes (entry screen 15 cm in diameter for example) the reduction in gloss on the edges relative to the center is around 25%. For larger screens (diameter 30 centimeters for example) the reduction reaches 35%.

An input screen 15 according to the invention, as shown in FIG. 3, makes it possible to improve the uniformity of the gloss, by imparting a non-homogeneous distribution to the absorption achieved by the tinted alumina layer 20 , the wavelength emitted by the scintillator.

FIG. 4 represents a first and a second curve 30, 40 of IIR tube gloss, taken along a diameter of the output screen: they represent the brightness of a line of dots of the image visible on the screen of output as a function of the distance of these points from the center of the screen, assuming uniform illumination of the input screen.

On the abscissa we therefore plotted the radial distance from the center, and on the ordinate the brightness of the visible output image.

The first gloss curve 30 in dotted lines is a conventional gloss curve obtained with a conventional IIR tube.

It can be seen that this first gloss curve 30 is not at all a horizontal straight line or almost as it might be theoretically desirable; it is rather a kind of arc of a circle flattened towards the center. The difference in gloss between the center and the edges ranges from 25% to 35% depending on the types of tubes and according to their diameter. In reality, some difference in gloss may be desirable, but not as high as that.

The second gloss curve 40 is obtained with the tinted alumina layer 20 interposed between the substrate 17 and the scintillator layer 16 (shown in FIG. 3). It is noted that the absorption by the alumina layer 20 is greater towards the center 22 than towards the edges 21, it makes it possible to obtain a much flatter gloss curve 40, in which the difference between the center and the edges is limited to around 10%.

It is clear that by giving the tinted alumina layer 20 the appropriate absorption profile, it is possible to obtain a gloss curve having the desired profile.

It should be noted, however, that the absorption by the tinted alumina layer, that is to say the attenuation on the transmission of blue light effected by this alumina layer, must take into account the fact that the photons light emitted towards the rear, such as the light photon PL2 shown in FIG. 2, undergoes this attenuation twice: a first time to reach the substrate 17, and a second time when they set off forwards.

It should also be noted that a particularly favorable effect for improving the image resolution comes from the fact that the light photons which strike the substrate and return towards the scintillator, undergo a double attenuation all the more strong as they are more inclined relative to normal at their point of incidence on the substrate, because they travel a greater distance in the attenuating medium.

For the improvement of the image resolution, the absorption or attenuation by the tinted alumina layer 20 can be homogeneous along the diameter D1 thereof. But the area of the image in which generally the best resolution is sought is the central area, so that the improvement of the resolution and the compensation of the brightness curve can be obtained simultaneously by the same layer of alumina 20.

The tinted alumina layer 20 can be produced in different ways. It can be carried out for example by a method called co-evaporation under vacuum. In this method, the alumina is evaporated under vacuum, simultaneously, on the one hand (to form the alumina layer) and, on the other hand, the opacifying product responsible for "tinting" the alumina layer, ie that is to give it its absorbing power with respect to the wavelengths emitted by the scintillator 16.

In the case of a scintillator 16 emitting in blue, the opacifying product can be a metallic element such as chromium for example, or a compound body such as for example silicon monoxide.

The technique of vacuum co-evaporation is classic. It is used in particular for producing thin or thick layers of composite materials, for example ceramic compositions having for example electrical or electro-optical characteristics.

Among the drawbacks of this method, it may be mentioned in particular that it does not allow easy control of the surface condition of the layer.

According to a preferred embodiment of the invention, the tinted alumina layer 20 is a porous layer whose pores contain the absorbent substance of the wavelength emitted by the scintillator 16. The tinted alumina layer 20 is then a so-called "thick" layer (of thickness for example between 1 and 15 microns), as opposed to thin and dense layers (thickness less than 1 micron).

The porous alumina layer 20 can be obtained in a simple manner, by anodizing an inner face 30 of the aluminum substrate 17 in an appropriate acid medium. At this stage, the alumina layer 20 is porous and practically transparent, and it must be "tinted" with an opaque substance to acquire its "absorbent" property.

Figure 5 is a schematic sectional view of a portion of the screen 15 showing more particularly the alumina layer obtained by anodizing the substrate 17 in an acid medium according to a method in itself conventional; the scintillator layer 16 not yet being deposited at this stage on the alumina layer 20.

This acid medium can be for example a sulfuric acid solution at about 15% by weight; or a 5% by weight phosphoric acid solution; or a 2% by weight oxalic solution, etc.

The thickness E1 of the porous alumina layer 20 depends conventionally in particular on the density of the anode current, the temperature of the acid bath and the duration of the operation.

The anodic current densities can vary for example between 1 and 2 amperes per dm². These operations are generally carried out at room temperature.

It is simple under these conditions to produce an alumina layer 20 such as that shown in FIG. 5. The alumina layer 20 is formed on the inner face 30 of the aluminum substrate 17, and the layer 16 forming the scintillator ( not shown in FIG. 5) is then deposited on the alumina layer 20.

The alumina layer 20 comprises pores 32 forming channels whose general orientation is substantially perpendicular to the substrate 17. These pores 32 or channels leave from the surface 33 of the layer 20 (on the side intended to receive the scintillator 16), and they have an average depth P1 slightly less than the average thickness E1 of the alumina layer 20: for example an average depth P1 of the order of 7.5 microns for an average thickness E1 of the order of 10 microns, and an average diameter D2 of the order of 0.05 microns.

The degree of porosity of the alumina layer 20, that is to say the number of pores 32 and therefore the average pitch Pa of these pores, can be controlled in different ways, mainly by the density of the anode current. In the case of the example cited above where the average depth P1 is of the order of 7.5 microns with an average thickness E1 of the order of 10 microns, it is possible to obtain an average distance between two pores on the order of 2 to 3 microns, by acting for example on the current density, the porosity increasing with the current density. Of course, the characteristics of the porosity (number of pore diameters 32) can also be controlled by the nature and the concentration of the acid used.

It is also possible to control the state of the surface 33 of the layer 20 of alumina, and to impart to it a roughness suitable for properly attaching the scintillator layer 16, and for growing the needle crystals which constitute the latter with a section suitable for improving image resolution. This can be obtained by acting for example on the anodizing conditions or on the initial surface condition of the aluminum.

• a) Le trempé peut consister par exemple en un traitement de la couche d'alumine 20 dans une solution d'oxalate ferrique à 20 % en poids. Ce traitement fournit une coloration jaune-orange, apte dans l'exemple à absorber la lumière produite par le scintillateur 16.
• b) Un autre procédé bien connu dans l'art de la décoration de l'aluminium, consiste en un traitement par une solution d'acétate de cobalt d'environ 20 grammes par litre à environ 50°C ; ce traitement étant suivi d'un second traitement par une solution de permanganate de potassium, à raison d'environ 20 grammes par litre. On obtient alors une couleur "bronze".

The porous alumina layer 20 is then easily "tinted" using conventional methods such as those used in particular for decorating aluminum, for example by a quenching process, in order to deposit on the walls of the pores. 32 the absorbent substance shown in the example in the form of a layer 35:

• a) The quenching may consist, for example, of a treatment of the alumina layer 20 in a solution of ferric oxalate at 20% by weight. This treatment provides a yellow-orange coloration, capable in the example of absorbing the light produced by the scintillator 16.
• b) Another process well known in the art of aluminum decoration consists of treatment with a solution of cobalt acetate of approximately 20 grams per liter at approximately 50 ° C; this treatment being followed by a second treatment with a solution of potassium permanganate, at a rate about 20 grams per liter. We then obtain a "bronze" color.

The coloring of the pores 32 results from a phenomenon of fixing of metallic oxide microparticles on the walls of the pores 32, by an ion exchange mechanism. Parameters such as the diameter and the depth of the pores directly influence the intensity of the coloring: the amplitude of the coloring increases when the number of pores 32 increases and / or when the thickness E1 of the layer increases.

Other staining methods can be used, which consist for example of cathodic deposits in an electrolytic medium. The coloring is then specific to the cations used, and there again, the coloring obtained depends on the metal oxides or on the metals deposited.

It may be advantageous (but not compulsory) in certain cases to close, that is to say to seal the pores 32 of the tinted alumina layer 20. For example to better preserve the coloring from chemical attack.

FIG. 6 is a view similar to FIG. 4, and it illustrates the obturation or "clogging" of the pores 32, this "clogging" being obtained by an additional treatment carried out after having carried out the "coloration" of the pores 32. "clogging" can consist, for example, of soaking in a very dilute aqueous solution of nickel and cobalt salt near the boiling point (98 ° C). The pores 32 are "closed" thanks to the growth of an additional layer of alumina 37 on the surface.

As previously mentioned, the degree of porosity can be controlled by the density of the anode current and increases with it. This property can be used to give the tinted alumina layer 20 greater porosity in its central area than towards its edges, in order to give the absorption by the alumina layer 20 the profile suitable for correcting the curve. of gloss, as previously explained. Indeed, the absorbent substance being deposited in the pores 32 if the quantity of these pores 32 is increased in the central zone of the alumina layer 20, or gives a greater absorption coefficient to this central zone relative to the edges.

FIG. 7 schematically shows, by way of nonlimiting example, how to obtain greater porosity in the central zone 22 than towards the edges 21 of the tinted alumina layer 20, by using a geometry electrolysis cell appropriate.

In Figure 7 there is shown the aluminum substrate 17 by a sectional view similar to that of Figure 3. The substrate 17 is immersed in an electrochemical solution 40, as previously mentioned, suitable for causing the formation of the layer porous alumina. The substrate is connected to the positive polarity "+" of a current source 41, so as to constitute the positive electrode of an electrochemical anodization system. The negative polarity "-" of the current source 41 is connected to another electrode 42 forming a cathode of small dimensions compared to that of the anode that constitutes the substrate 17. The cathode 42 is placed in the electrochemical solution 40, vis-à-vis the inner face 30 of the substrate 17 (the outer face 50 of the substrate 17 being for example temporarily protected by a varnish).

To obtain a porous alumina layer 20 having a greater porosity at the center 22 than towards its edges 21, the cathode 42 is positioned so that it is closer to the center 20 than to the edges 21. Under these conditions, the the intensity of the electric current is greater between the center 22 and the cathode 42 than between the latter and the edges 21. This results in an increase in the porosity going from the edges 21 towards the center 20, and therefore a greater number of attachment sites for the absorbent substance, towards the center 22.

It should be noted that in the case where the coloring of the alumina layer 20 by the absorbent substance is obtained by a cathodic operation, an equivalent cell geometry can be used (but of course the substrate 17 would in this case form a cathode), in order to deposit more absorbent substance in the center, in order to correct the gloss curve.

Claims (14)

1. An image intensifying tube of the radiological type comprising an input screen (15), which possesses a scintillating layer (16) borne on a substrate (17) and an alumina layer (20) interposed between the substrate (17) and the scintillating layer (16), said alumina layer (20) being "dyed" by a substance absorbing at least at the emission wave length of the scintillating layer (16) in such a manner that the light photons (PL1, PL2) emitted by the scintillating layer toward the substrate (17) are at least partly absorbed in the "dyed" alumina layer, characterized in that the "dyed" alumina layer is porous and in that the absorbing substance is at least partly contained in at least part of the pores (32) of the dyed alumina layer (20).
2. The image intensifying tube as claimed in claim 1, characterized in that the substrate (17) is made of aluminum.
3. The image intensifying tube as claimed in claim 1, characterized in that the alumina layer (20) is "dyed" in such a fashion as to have an absorbing effect which is substantially constant from its edges (21) to its center (22).
4. The image intensifying tube as claimed in any one of the claims 1 through 3, characterized in that the alumina layer (20) possesses a porosity which is substantially constant from its edges (21) to its center (22).
5. The image intensifying tube as claimed in any one of the claims 1 through 4, characterized in that the alumina layer (20) is "dyed" in such a manner as to have a greater absorbing effect at its center (22) than at its edges (21).
6. The image intensifying tube as claimed in any one of the claims 1 through 5, characterized in that the "dyed" alumina layer (20) has a greater porosity at its center (22) than at its edges (21).
7. The image intensifying tube as claimed in any one of the preceding claims, characterized in that the scintillating layer (16) is made of cesium iodide.
8. A method of manufacturing an image intensifying tube as claimed in any one of the claims 1 through 7, wherein the substrate (17) is made of aluminum, characterized in that it consists of producing the alumina layer (20) on the substrate (17) by an electrochemical anodizing method applied to the latter.
9. The method as claimed in claim 8, characterized in that the electrochemical solution employed for such anodizing is an acidic solution selected in order to produce the porosity of the alumina layer (20).
10. The method as claimed in claim 9, characterized in that it consists of endowing the alumina layer with a porosity which is greater at its center (22) than at its edges (21).
11. The method as claimed in claim 10, characterized in that it consists of performing such anodization of the substrate (17) with the aid of a cathode (42) arranged nearer to the center (22) than the edges (21) of the substrate (17).
12. The method as claimed in any one of the preceding claims, characterized in that it consists in "dyeing" the alumina layer (20) with the aid of metallic oxides.
13. The method as claimed in claim 12, characterized in that it consists in utilizing a dipping method in order to "dye" the layer (20) of alumina.
14. The method as claimed in claim 12, characterized in that it consists in utilizing a cathodic depositing method in an electrolytic medium in order to "dye" the alumina layer (20).

Images