EP0003454B1 - X-ray tube comprising a device for reducing the divergence of its useful beam - Google Patents

Description

The present invention relates to an X-ray tube comprising a device for reducing the divergence of its useful beam and fitted in particular to tomography devices. These X-ray tubes make it possible to generate in conjunction with collimation means, such as a slit diaphragm, a flat X-ray beam, of constant thickness, and in a fan shape with a large opening with a substantially uniform distribution of energy. radiant in a plane and in all directions inside this opening.

To obtain such bundles, different types of tubes have been developed. They consist of a cathode emitting an electron beam of rectangular section and an anode, fixed or rotating, bombarded by this beam, both being placed in the same vacuum-tight glass enclosure.

The bombarded surface of the anode, called the focal point or real focal surface, emits an X-ray beam which, using collimation means such as a slit diaphragm external to the tube, is flat and fan-shaped.

The uniform distribution of radiant energy is related to the shape of the anode.

To reduce the divergence of an X-ray beam, it has been proposed, in French Patent No. 1,051,495 filed December 17, 1951 in the name of Compagnie Générale de Radiologie, a frame diaphragm, placed outside the tube, the walls of which consist of oblique pyramidal cells formed from thin blades, opaque to X-rays. This device made it possible to capture X-rays, not coming from the focal point, which constitute the divergent radiation of the useful beam.

This diaphragm is unsuitable for providing a flat beam of radiation since the raster blades lining the tube are focused on the focal point of radiation. In addition, being placed outside the tube, this weft diaphragm does not pick up the X-rays that come out of the X-ray tube out of the mouth of the diaphragm.

French Patent No. 2,304,320 describes an X-ray collimator adapted to give the radiation a flat fan shape and in which the X-ray passage slot is materialized by two flat plates of X-ray absorbing material, parallel to the plane desired range and limiting the radius to it. The thickness of the fan beam therefore depends in principle on the spacing of the plates forming two walls of the collimator. Oblique rays which risk irradiating (in the case of use in tomography) parts of the body outside the cutting plane, meet said plates and are absorbed. However, the effectiveness of such a system is limited and a certain divergent radiation rate cannot be prevented because the oblique rays which are the most slightly diverging from the plane of the fan can pass through the space defined between the two plates without meet them.

The object of the invention is to further reduce the rate of divergent radiation in such a type of collimator used in connection with an X-ray tube.

• - le diaphragme se trouve à l'intérieur de l'enceinte de verre et a la forme d'une couronne ou d'un secteur de couronne, creusé d'une fente en éventail,
• - le diaphragme est placé à proximité immédiate du foyer émetteur du rayon X,
• - des lames opaques sont logées dans la fente du diaphragme,
• - les tranches des lames opaques sont cachées par des feuilles minces d'un matériau ayant pour propriété d'absorber les rayonnements X de faible énergie par rapport à l'énergie du faisceau utile.

This object is achieved, according to the invention, by an X-ray tube comprising an anode and a cathode placed in a vacuum-tight glass enclosure and a diaphragm giving the X-ray beam a fan shape and comprising at least two walls opaque to radiation and parallel to the desired plane of the fan, characterized in that

• - the diaphragm is inside the glass enclosure and has the shape of a crown or a sector of a crown, hollowed out by a fan-shaped slot,
• - the diaphragm is placed in the immediate vicinity of the focal point of the X-ray,
• - opaque blades are housed in the diaphragm slot,
• - The edges of the opaque plates are hidden by thin sheets of a material having the property of absorbing X-rays of low energy compared to the energy of the useful beam.

In this type of device, the two aforementioned opaque walls are materialized by two opposite surfaces of the slot defined in the thickness of said crown or of said crown sector, which slot houses said opaque blades.

As mentioned previously, the main application of the invention is a so-called rotation type X-ray apparatus consisting of a source and a multi-detector; it requires a source emitting a flat fan beam, the opening of which makes it possible to cover all the detectors placed opposite. To limit the irradiation of the patient, it is highly desirable that only the section to be examined is crossed by the X-rays. It is therefore necessary to also absorb the diverging rays out of the flat fan beam, in the thickness direction. of the cup. It is a feature of the present invention to make it possible to provide a flat fan beam free of most of its divergent radiation outside the thickness.

The slot of this diaphragm in the form of a crown or crown sector faces the focal point of the anode, so that the plane of the range of the beam perpendicular to the focal point passes through it.

This slot therefore has in the plane of the range of the beam a section in the form of an opening ring sector identical to that of the desired beam and in the plane normal to the plane of the range a rectangular section of height identical to the thickness of the desired beam.

The plates opaque to X-rays (by example in Tantalum) allow the apparent focal point of the anode to be divided into several small focal points at the level of the slit diaphragm. It is as if these small apparent foci generated very weak diverging fan-shaped X-ray beams.

The beam of X-rays emitted from the focus is therefore divided into a number of very thin beams of fan-shaped X-rays equal to the number of plates plus one.

This phenomenon takes place directly at the outlet of the diaphragm and is due to the shadow of the opaque blades with respect to the X-rays.

However, the divergence of the beams, small but existing, and the large distance from the diaphragm to the object to be irradiated, due to its position inside the glass enclosure of the tube, will blur these shadow phenomena. Indeed, thanks to the large distance separating the diaphragm from the object to be irradiated, the latter is crossed by a reconstituted beam, flat and fan-shaped, with divergence identical to the divergence of a beam coming from the space between two blades and consequently with very small divergence.

The edges of the opaque blades are hidden by thin metallic sheets in order to avoid their harmful field effects vis-à-vis the glassware of the tube on the one hand and vis-à-vis residual extra-focal phenomena on the other part, in the case of an X-ray tube with a cylindrical rotating anode. These thin metallic sheets, for example nickel, also have a filtering role, because they have the property of absorbing low energy X-rays. They therefore make it possible to filter the residual extra-focal radiation which is of very low energy compared to that of the useful beam as a fan.

• La figure 1, une coupe axiale d'un mode de réalisation du dispositif suivant l'invention dans un tube radiogène à anode tournante cylindrique.
• La figure 2, une coupe transversale du mode de réalisation de la figure 1.
• La figure 3, une coupe axiale d'un mode de réalisation du dispositif suivant l'invention dans un tube radiogène à anode fixe.
• La figure 4, une coupe transversale d'un second mode de réalisation du dispositif suivant l'invention dans un tube radiogène à anode fixe.

The following description of the invention is given by way of nonlimiting example and illustrated by the appended figures which represent:

• Figure 1, an axial section of an embodiment of the device according to the invention in an X-ray tube with a cylindrical rotating anode.
• Figure 2, a cross section of the embodiment of Figure 1.
• Figure 3, an axial section of an embodiment of the device according to the invention in an X-ray tube with fixed anode.
• Figure 4, a cross section of a second embodiment of the device according to the invention in an X-ray tube with a fixed anode.

Figures 1 and 2 show an embodiment of the device according to the invention in an X-ray tube with a cylindrical rotating anode, respectively in axial and transverse section. The tube comprises a glass envelope 1 of cylindrical shape with an axis of revolution. XX ', the ends of which are united in a sealed manner to the ultrahigh vacuum, to a cathode base 2 on the one hand and to a metal disc 3, anode support on the other hand. These unions are provided in known manner, by annular parts 4 and 5 of a metal alloy having a coefficient of thermal expansion close to that of glass.

The rotating anode 6 has the shape of a flat cylinder whose cylindrical surface is made of X-ray emitting material (for example tungsten) and is connected to a rotor 7 whose axis of rotation yy 'is off-center with respect to the axis xx 'of the X-ray tube. The vacuum-tight junction of the rotor 7 and the metal disc 3 is provided by a thin metallic rotor neck 8 as described in French patent application No. 77/23444, requested in the name of “Compagnie Générale de Radiologie on July 29, 1977 , publication no.2.399.124.

This rotor 7 is arranged in a rotating field generated by a stator 9 at the same potential as the anode which can be either grounded or at high positive voltage as described in the patent application cited above.

An anti-extra-focal device 12, which has the shape of a crown sector centered on the axis of rotation of the anode 6, is placed very close to the cylindrical surface of this anode. It is integral with the metal disc 3 and is maintained at the same potential as the anode. It is composed of two layers A and B and is hollowed out in its center so as to allow the free passage of the electron beams 15 and 15 'on the one hand, as well as the free passage of the beam of energy radiating from the focus d 'somewhere else. Its layer A, made of a light material such as graphite, titanium or any other suitable material, absorbs by braking the secondary electrons which, re-accelerated, would bombard the anode at points other than the focus and cause extra-focal radiation. Its layer B, made of a material of high atomic mass such as tungsten, attached to layer A, absorbs the extra focal radiation emitted at other points of the anode than the hearth.

The dimension of this anti-extra-focal device is such that it covers the projection onto itself of the interval delimited by the two tangents aa 'and bb' at the anode 6. The only possible source of X-rays is limited to dimension 1.

On the other hand, a field distributor 11 parallel to the circular faces of the anode is integral with the cathode base 2, and carries perpendicularly to its face opposite the anode, a support 10 consisting of a sector-shaped part 13 crown centered on the axis xx 'of revolution of the glassware 1 of the X-ray tube.

This part 13 is made of metal of very high atomic mass so as to absorb X-rays and assumes both the functions of carrier of the slit diaphragm according to the invention and of carrier of two cathode emitters 16 and 16 '.

These two cathode emitters 16 and 16 'are provided with two concentration pieces 14 and 14' oriented in such a way that the electron beams 15 and 15 ', rectangular and elongated, of almost linear section in the plane perpendicular to the plane of Figure 2, reach the cylindrical surface of the anode in a line which projects at point P of Figure 2. The useful beam of X-rays therefore emerges from the generatrix of the cylindrical surface of the anode containing point P.

The two cathode emitters 16 and 16 'are electrically isolated from the concentration pieces 14 and 14' so as to allow the application of a negative bias voltage to these concentration pieces with respect to the potential of the emitters.

This cathode device, called a “grid”, makes it possible to reduce, depending on the value of the bias voltage, the concentration of the electron beams generating the foci or the electronic blocking of the emitters. The reduction of the beams, therefore of the focal points, is carried out on the smallest dimension of the rectangular section.

The two emitters therefore supply two electron beams allowing a wide range of focal points of different dimensions, starting from the initial dimensions, that is to say without any polarization being applied to the concentration pieces. These initial dimensions may be the same or different.

The two transmitters are never used simultaneously.

On the other hand, these cathode emitters 16 and 16 'provided with the concentration pieces 14 and 14' are placed symmetrically on the part 13 in the form of a crown sector, so as to balance the field lines in the cathode-surface space. cylindrical of the anode, and so as to clear the space opposite the rectangular focal point produced by one of the two electron beams, coinciding with a generatrix of the cylindrical surface of the anode so that the axis of the beam Fan-shaped X-rays thus produced are normal to the cylindrical surface at the focal point.

Part 13 is hollowed out in its center, facing the focal point of the anode so as to allow free passage of the beam of radiant energy of axis zz ′ with an opening angle a and a thickness equal to the length of the foyer.

The slot thus formed therefore also has as its axis of symmetry the axis zz 'and for opening, an angle opening a having the point P at its apex.

Grooves 17 and 17 ′ are machined in the walls parallel to the axis xx ′ delimiting this slot so that blades 18 parallel to one another can be deposited therein. These blades can be parallel to the plane of the range of the X-ray beam normal to the focus of the anode.

This exemplary embodiment is given on a preferential basis. As has been said above, the diaphragm 13 itself being made of a material with high absorbency of X-rays, can also stop divergent rays passing between the pack of parallel blades 18 and the faces of the slit made in the diaphragm.

These blades are made of tantalum or any other material opaque to X-rays and are intended to avoid too great a divergence of the useful beam of X-rays.

The divergence will depend on the spacing of the blades and their length in the direction of propagation of the useful beam, the closer they are, the greater the division into apparent foci and the more the divergence is limited.

By cons, we must compensate by increasing the load of the tube, the fraction of X-radiation directly from the hearth which is stopped by these blades according to their thicknesses, their lengths and their spacings.

In addition, the edges of these X-ray opaque blades are hidden by thin sheets of nickel or any other appropriate material 19 and 20, in order to avoid the field effects harmful to the glassware 1 of the tube and to the anti-extra-focal device. 12.

The position of the concentration pieces 14 and 14 'on the part 13, in the volume delimited by the two planes containing the two circular faces of the anode, is not the only possible.

They can be placed, for example symmetrically with respect to the slot provided with blades as before, but in the plane normal to that used in Figures 1 and 2 and this without affecting the proper functioning of the anti-divergence device according to the invention.

Figures 3 and 4 show two embodiments of the anti-divergence device according to the invention, in an X-ray tube with a fixed anode.

In these figures the same elements as in Figures 1 & 2 have been designated by the same references.

The tube shown in axial section in FIG. 3 comprises a casing 1 of cylindrical shape, of axis of revolution xx ′, the ends of which are united in the same manner as for the tube with the preceding rotating anode, to a cathode 16 d 'on the one hand and to an anode 6 on the other hand.

This anode is dug by two wells, one in the direction xx 'and the other perpendicularly. The intersection of these two wells reveals an inclined surface 6 ′, emitting X-rays, which is bombarded by an electron beam of rectangular section coming from the cathode 16.

A support 10 connected to the anode has the same potential as the latter and is composed of a part 13 in the form of a crown sector centered on the axis xx 'which only assumes the role of slit diaphragm conforming to the invention.

To do this, it is hollowed out with a slot so as to allow free passage of the useful beam of X-rays coming from the focal point placed on the surface 6 ′ of the anode.

According to the invention, as in the case of the cylindrical rotating anode, and for the same reasons, this slot is placed very close to the focal point of the anode and provided with opaque blades 18, thanks to the grooves 17 and 17 ' .

These blades are parallel to each other, and can be parallel to the plane of the range of the useful beam of X-rays. They have their cutting edges hidden by thin sheets of nickel or any other suitable material 19 and 20, in order to avoid the effects harmful to the glassware of the tube and to absorb extra-focal low energy radiation already very limited in this kind of X-ray tube.

FIG. 4 represents a cross section of a tube with a fixed anode similar to that of FIG. 3, but with a modification in the shape of the part 13. The latter, still hollowed out with a slot provided with opaque blades, surrounds the anode 6 completely. This new shape allows a better distribution of the field in the glass enclosure 1 and in no way affects the proper functioning of the anti-divergence device.

The anti-divergence device according to the invention also has the advantage of being inside the glass enclosure of the X-ray tube, whether it is with a fixed or rotating cylindrical anode. Indeed, one adjusts once and for all its position, and the arrangement of the opaque blades, so as to have the best flat beam with constant thickness and fan possible. The position of the diaphragm therefore remains immutable, unlike slit diaphragms external to the tube, which require renewed adjustments.

X-ray tubes with a fixed or cylindrical rotating anode equipped with the device according to the invention are used in particular in transverse axial tomography devices comprising a ramp composed of numerous radiation detectors, all lit simultaneously by a wide-opening fan beam. The small divergence of the tubes thus equipped makes it possible to irradiate the body to be observed, placed between the tube and the detector ramp, only in the desired area so that the detectors receive almost all of the attenuated direct radiation.

This device therefore improves detection and reduces the harmful effects of irradiating zones due to the divergence of the useful beam of X-rays in a fan.

Claims (8)

1. An x-ray tube comprising an anode (6) and a cathode (16, 16') situated in a vacuum tight glass casing (1) and a diaphragm (13) giving the x-ray beam a fan-shape and comprising at least two walls, opaque to the beam and parallel to the desired plane of the fan, characterized in that

a) the diaphragm (13) is situated within a crown or the sector of a crown, being provided with a fan-shaped slit,

b) the diaphragm is located close to the emitter focus of the x-rays,

c) opaque blades (18) are placed in the slit of the diaphragm,

d) the cut surfaces of the opaque blades (18) are covered by thin strips (19, 20) of a material absorbing x-rays of low energy in comparison with the energy of the useful beam.

2. An x-ray tube according to claim 1, of the type having a cylindrical rotating anode, characterized in that it comprises a field distributor (11) which carries on the one hand the diaphragm (13) in the form of a crown sector centered on the rotation axis of the cylindrical anode (6), and on the other hand two cathode emitters (16, 16') which can be polarized by a negative voltage placed symmetrically on both sides of the slit of the diaphragm (13) furnished with blades (18), and an anti-extra-focal device (12) located between the diaphragm (13) and the surface of the anode (6).

3. An x-ray tube according to claim 2, characterized in that the two cathode emitters (16, 16') are situated in a plane which is orthogonal to the rotation axis of the cylindrical anode (6) passing through the focus center (P).

4. An x-ray tube according to claim 2 or 3, characterized in that the cathode emitters (16, 16') are supplied with concentration devices (14, 14') of distinct characteristics, functioning independently one from the other and alternatively.

5. An x-ray tube according to claim 4, characterized in that the concentration devices (14, 14') are polarized by the same negative voltage.

6. An x-ray tube according to one of the claims 2 to 4, characterized in that each emitter (16, 16') is blocked during the rest period by a sufficiently high negative polarization voltage.

7. An x-ray tube according to claim 1, having a fixed anode, characterized in that the diaphragm (13) is fixed to the anode (6), is held at the same potential as the latter and is centered on the revolution axis of the casing (1) of the tube, and in that the blades (18) are placed parallelly to a plane comprising the central axis of the beam of x-rays.

8. An x-ray tube according to claim 7, characterized in that the crown-shaped diaphragm (13) completely surrounds the anode (6).

Images