EP0540392B1 - X-ray image intensifier tube housing - Google Patents

Description

The invention relates to the sheaths of radiological image intensifier tubes and the means used in these sheaths in particular to provide protection against X-radiation.

The radiological image intensifier tubes (abbreviated as "IIR tube") are vacuum tubes comprising an input screen towards the front of the tube, an electronic optical system, and an image observation screen. visible on the back of the tube.

Figure 1 schematically shows such an IIR tube. The tube comprises a glass vacuum enclosure 2, one end of which at the front of the tube, is closed by an inlet window 3 exposed to X-ray photon radiation.

The second end of the enclosure 2 forming the rear of the tube is closed by an outlet 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 emits electrons. These electrons are accelerated towards the exit window 4 using different electrodes 7 arranged along a longitudinal axis 9 of the tube, and which form the electronic optical system.

In the example shown, the exit window 4 carries a cathodoluminescent screen 10, made of phosphors for example. The impact of the electrons on the cathodoluminescent screen 10 makes it possible to reconstruct an image formed at the start on the photocathode 6.

• 1) protection de l'optique électronique 7, du tube contre les champs magnétiques parasites extérieurs ;
• 2) protection des personnes contre le rayonnement X, aussi bien le rayonnement X direct non absorbé par le scintillateur 5, que le rayonnement X diffusé engendré par le tube lui-même ;
• 3) protection mécanique de l'enceinte à vide 2 vis-à-vis de l'environnement extérieur ;
• 4) former l'interface de fixation, avec d'équipement radiologique (non représenté) sur lequel le tube doit être monté, ainsi qu'avec les accessoires d'utilisation de cet équipement : elle doit comporter par exemple des trous filetés pour la fixation et des faces ou plans de référence.

The IIR tube is contained in a protective enclosure called sheath 15, which in particular performs the following functions:

• 1) protection of the electronic optics 7, of the tube against external parasitic magnetic fields;
• 2) protection of persons against X-rays, both direct X-rays not absorbed by the scintillator 5, and the scattered X-rays generated by the tube itself;
• 3) mechanical protection of the vacuum chamber 2 from the external environment;
• 4) form the attachment interface, with radiological equipment (not shown) on which the tube is to be mounted, as well as with the accessories for using this equipment: it must for example have threaded holes for fixing and reference faces or planes.

To this end, the wall of a conventional sheath 15 for an IIR tube generally comprises several superposed layers 16, 17, 18.

The outermost layer 16 forms an envelope 16 which is the mechanical framework of the sheath 15. It is an envelope which may be of the type for example; molded metal alloy; either pushed back or stamped; or mechanically welded from metal sheets.

The outer layer 16 or envelope 16 provides mechanical protection of the vacuum enclosure, and particularly in its rear part 13 located towards the outlet window 4 of the tube, it provides the attachment interface. The image produced by the cathodoluminescent screen 10 is visible outside the sheath thanks to an opening 14 made in the rear part 13.

An inner layer 17, arranged between the outer layer 16 and the enclosure 2 of the tube, constitutes a shielding which ensures the protection of the electronic optical system against external magnetic fields.

This inner layer or shield 17 is made of a material having a high magnetic permeability, such as, for example, soft iron, or an iron-based alloy such as example the "permalloy" (iron-nickel alloy), or even metal, etc ...

It should be noted that the inner layer 17 forming a shield against magnetic fields, is produced from sheets of these materials with high magnetic permeability.

A second inner layer 18, disposed between the enclosure 2 of the tube and the first inner layer 17, constitutes a screen whose function is to absorb X-rays; the aim with the aid of this screen 18 is to attenuate the X-ray radiation which leaves the sheath 15.

The materials used to constitute this second inner layer 18 are also generally available in the form of sheets, so that the production of this second inner layer or screen 18 (but also of the first inner layer 17) calls on the techniques of boilermaking: partial stamping, embossing, rolling, mechanical welding. These techniques result in long and costly operations, which also result in high geometric tolerances.

To make the second inner layer or screen 18, it is common to use materials with a high atomic number, and more particularly lead, which has the advantage of being of low cost and of strongly absorbing X-radiation. .

However, lead is a difficult material to process because of its great malleability or ductility. In addition, the work of lead is regulated and the personnel who perform this work are subject to medical surveillance constraints.

Another point which poses a problem in the manufacture of IIR tube sheaths comes from the difficulty which there is in securing to one another the different layers 16, 17, 18.

They are often joined by gluing, which constitutes an operation that is hardly compatible with requirements industrial and which results in particular in high tolerances on wall thicknesses.

The purpose of absorbing X-rays is to attenuate the X-rays leaving the sheath, up to a value compatible with regulations. As the intensities of this incident X-ray are not equal at all points of the sheath 15, the absorption of the radiation does not necessarily have to be equal at all these points, and it suffices to confer on the wall of the sheath in these different points, with an appropriate absorption coefficient to obtain the desired attenuation.

As shown in Figure 2, we can define in an IIR tube sheath several main areas a , b , c , d , e , which extend from before 21 of the sheath on the side of the inlet window 3 , to the rear 13 of the sheath on the exit window side 4. Each of these zones may require attenuation of the X-ray different from that of a neighboring zone. For example zone b where the tube at its largest diameter generally requires less attenuation of the X-ray radiation than zone c where this diameter is decreasing.

In each of the zones a to e , the thickness of the screen or absorbent layer 18 of the Xs is calculated to reduce to the regulatory value the highest incident X-ray level existing in each zone.

But in the same area, there may be significant differences in the level of X-radiation, and the thickness of the layer or screen 18 is chosen for the attenuation of the X-radiation having the highest intensity.

As a result, the quantity of material constituting the screen 18 is superabundant in many points of the latter, which leads to increases in weight and cost.

Thus, for example, it is common to confer on the different zones a , b , c , d , e of the screen 18, if it is made up of lead sheets, thicknesses respectively of about 1.7 mm, 1, 2 mm, 2.3 mm, 1.2 mm and 2.7 mm, while for zone b for example the major part of this area could be only 0.6mm thick.

The subject of the invention is an IIR tube sheath, the design of which makes it possible to avoid the above-mentioned drawbacks.

The sheath according to the invention makes it possible to provide the various necessary protections, while having a lower weight than in the prior art, as well as greater ease of manufacture and therefore lower cost.

According to the invention, a sheath of a radiological image intensifier tube, comprising an envelope, an X-ray absorbing screen, a shielding against external magnetic fields, is characterized in that the envelope is at least partially consisting of a thermoplastic resin loaded with a powder of an X-ray absorbing material, so that said screen is at least partially constituted by the envelope.

By the expression "material absorbing X˝ radiation we mean defining a material (used pure or alloyed or else a compound of this material) whose atomic number is sufficiently high (for example equal to or greater than 70) to ensure significant absorption X-rays passing through it, such as lead or oxide, lead for example. Such materials are commonly used for protection against X-rays, in particular in the field of radiology.

We can cite for example in addition to lead and lead oxide: Ta, Bi, W, etc ..., used pure or alloyed, or in the form of oxides.

One of the advantages of such an arrangement is not only that it makes it possible to remove the layer made of lead sheets, or of another material with a high atomic number, but also that the envelope constitutes a particularly advantageous means for support and hang shielding.

• la figure 1 déjà décrite montre une gaine de tube IIR suivant l'art connu ;
• la figure 2 montre schématiquement une gaine de tube IIR conforme à l'invention suivant une forme de réalisation préférée ;
• la figure 3 illustre une variante de réalisation d'une gaine conforme à l'invention.

The invention will be better understood and other advantages which it provides will become apparent on reading the description which follows, given with reference to the appended figures in which:

• Figure 1 already described shows an IIR tube sheath according to the prior art;
• FIG. 2 schematically shows an IIR tube sheath according to the invention according to a preferred embodiment;
• Figure 3 illustrates an alternative embodiment of a sheath according to the invention.

FIG. 2 shows a sheath 20 according to the invention, containing an image intensifier tube. This image intensifier tube being for example of a type similar to that shown in FIG. 1, it is represented in FIG. 2 only by its enclosure 2. The sheath 20 has a general shape similar to that of FIG. 1, that is to say, it has a large opening 21 called the inlet opening located on the side of the inlet window 3 of the tube, and an outlet opening 22 on the side of the outlet window 4.

According to a characteristic of the invention, the sheath 20 comprises a wall 25 forming an envelope, made of a composite material allowing this wall to fulfill both the function of mechanical framework and screen absorbing X-radiation. composite material may consist of an injectable thermoplastic material loaded with a material (in powder form) with a high atomic number in order to absorb the X-ray in a significant manner, as has already been explained in the preamble.

The thermoplastic material or base material can be for example: ABS (acrylonitrile-butadiene-styrene) such as for example the product called "RONFALIN" produced by DSM (Company in Germany, producer of pastic materials); or even high density polypropylene; these two products have been tested with satisfaction, but there are many others which can be used.

With regard to the material forming the charge, that is to say the material (in the form of powder) absorbing the X-ray, interesting results have been obtained by using a lead oxide PbO more particularly a litharge, the others lead oxides being less advantageous because their lead content is lower. Of course, other materials or other heavy metal oxides can also be used to absorb X-radiation as explained above, since these materials are in the form of powder to charge the base material.

The above-mentioned lead oxide (litharge) was used with a particle size of less than 45 μm, for charging "RONFALIN" produced by DSM with a charge rate of the order of 35% by volume. The 35% loading rate can be considered to be substantially a maximum above which problems arise in the injection molding technique.

It should be noted that the production of plastic objects loaded with PbO is a process already used for small objects for very young children. In this case the charge rate is much lower and does not exceed 5%; in the event of accidental ingestion, the presence of Pb allows easy localization on radiography.

JP-A-63 293 500 describes an X-ray absorbing screen made of a plastic material charged with a powder of lead oxide.

The material loaded with heavy material, intended to constitute the wall 25 of the sheath 15 can be used by injection molding techniques which in themselves are conventional. For the production of a sheath 20, an advantage of injection molding techniques is that they make it possible to obtain relatively simple parts having complex geometric shapes. In addition, in the case of large series, the cost of the parts obtained by molding is low while having good reproducibility of these parts, and the tolerances on the dimensions are lower than in the case of the sheaths produced according to the techniques of the prior art.

These advantages are to be considered in addition to the considerable simplification of the structure of the sheath 20, a simplification which resides in the fact that the wall 25 provides both mechanical rigidity and the screen function for absorbing X-radiation.

Another important advantage which the realization by molding of the wall 25 brings, is that it makes it possible to confer, at all points on this wall, the thickness E1, E2, E3 strictly necessary to absorb the X-ray radiation existing at each of these points. , resulting in a reduction in size and weight as well as a saving in material.

In the nonlimiting example shown in FIG. 2, it can be seen that the different zones a , b , c , d , e do not all have a constant thickness, in order to adapt it to what is strictly necessary: by example zone b has a greater thickness E1 of the order of 5 mm towards the front 21, and a smaller thickness E2 of the order of 2.5 mm towards the rear 13.

It should be noted that to obtain an attenuation of X-rays (with the charged thermoplastic material) equivalent to that of lead, a length approximately 4 times greater than with lead is required: for example, it would be necessary to interpose in zone b a lead thickness of 1.2 mm to achieve the attenuation which is obtained with the charged thermoplastic material having the thickness E1, that is to say 5 mm.

The sheath 15 comprises a layer 24 of a material with high magnetic permeability forming a shielding 24 against external magnetic fields, and at least partially surrounding the enclosure 2 of the tube. In the example shown in FIG. 2, the shield 24 is disposed against the wall 25 inside the sheath 20. The shield 24 can be produced from sheets of a material with high magnetic permeability, and these sheets can be secured to the wall 25 by gluing for example. But this can be done in a much simpler way, by inserting these sheets in the mold in order to constitute the layer 24 before injecting the material serving to constitute the wall 25. The sheets can line the bottom of the mold, before the injection of material, and they are made integral with the wall 25 by adhesion of the material on the layer 24 forming a shield, during the cooling of this material. It is also possible according to a technique in itself known to provide hooking elements (not shown) in the shielding layer 24.

A sheath 20 produced in accordance with the invention makes it possible to use the technique of "inserts". This technique consists in placing objects in the mold before injecting the material to be molded, so as to at least partially drown them in this material and thus making them mechanically integral with the solidified material. It is thus possible, for example, to constitute nuts 30, 31 "in insert" (embedded in the wall 25) and / or studs not shown, in order to ensure the various fastenings.

In the example shown in Figure 2, on the one hand, nuts 30 "insert" are arranged in a front portion 27 of the sheath 20 (located on the side of the inlet opening 21) to allow the fixing the tube 1 to the equipment (not shown) with which it is associated; and on the other hand other screens 31 in "insert" are arranged on a bottom part 28 (in which the outlet opening 22 is made), to allow the attachment of accessories (not shown).

It should be noted that the shield 24, in addition to its shielding function, reinforces the mechanical rigidity of the wall 25, and in the case of particularly heavy loads to be fixed, the shield 24 avoids having to give the wall 25, locally, a thickness larger than that which is strictly necessary to absorb the X-radiation.

The wall 25 of thermoplastic material, even when charged, constitutes an electrical insulator which can locally accumulate electrical charges, all the more so since very often the outlet window 4 of the tube comprises metal parts brought to the potential of the high voltage supply of the tube. The shield 24 being an electrically conductive surface, it can in this case be brought to a reference potential, ground for example, so as to constitute a screen on an electrical plane, between the outlet window 4 and the envelope 25 of the sheath 20.

To this end, it suffices for example to provide an electrical conductor wire (not shown) connected to the shield 24, or even to extend, for example, a nut 31 as an "insert" to bring it into contact with the shield 24.

The variations in thickness E1, E2 of the wall 25 are effected from the shield 24, the latter being inside the sheath 20 in the example shown in FIG. 2.

FIG. 3 illustrates another embodiment which differs from that shown in FIG. 2 in that the shield 24 is secured to the wall 25 outside the sheath 20, the variations in thickness E1, E2 having an impact on inside the sheath 20.

One of the advantage of this arrangement is that it makes it possible to achieve total protection of people and equipment against electrical voltages and currents, if the shield is brought to the reference potential such as ground.

The shield 24 can be constituted in the same way as in the case of the example of FIG. 2, from sheets (not shown) of a metallic material of high magnetic permeability; these sheets can in this case also be placed as an "insert" in the mold, in order to be outside the wall 25, that is to say outside the sheath 20. The nuts 30, 31 can be placed as an "insert" as in the example in FIG. 2, passing for example through the shielding 24.

Both in the example of FIG. 2 and in that of FIG. 3, the front part 21 is shown attached to the wall 25. It is indeed necessary to separate the part before 21 forming a cover, of the body, that is to say of the rest of the sheath 20 for introducing the IIR tube into the sheath.

The assembly of the front part 21 with the body of the sheath is shown in the form of the adhesive joint 35, but of course other means can be used for this purpose, such as for example, screwing, snap-fastening, etc. ..

Claims (9)

1. An X-ray image intensifier housing comprising a casing (25) which acts as mechanical frame, a screen for absorbing X-rays and a shield (24) against outer magnetic fields, characterized in that the casing is at least partially made from thermoplastic material charged with a powder material for absorbing X-rays so that the screen for absorbing the X-rays is at least partially constituted by said casing (25).
2. A housing according to claim 1, characterized in that the shield (24) is disposed inside the casing (25) and in contact with the latter.
3. A housing according to one of the preceding claims, characterized in that the casing (25) presents variations in thickness (El, E2).
4. A housing according to any one of claims 2 or 3, characterized in that it comprises means (31) for electrically connecting the shield (24) to the outside of the casing (25).
5. A housing according to claim 1, characterized in that the shield (24) is disposed outside of the casing (25) and in contact with the latter.
6. A housing according to claim 5, characterized in that the casing (25) presents variations in thickness (E1, E2).
7. A housing according to one of the preceding claims, characterized in that the thermoplastic material is a material which can be injected into a mould.
8. A housing according to claim 7, characterized in that the thermoplastic material is charged with litharge.
9. A housing according to one of the preceding claims, characterized in that the shield (24) is attached to the casing (25) in order to contribute to the mechanical stiffness of the latter.

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