EP0003454A1 - X-ray tube comprising a device for reducing the divergence of its useful beam - Google Patents

Abstract

The invention relates to an X-ray tube comprising a device for reducing the divergence of its useful beam. The device consists of a slit diaphragm (13) provided with opaque x-ray blades (18), placed very close to the focal point of the anode, inside the glass enclosure of the tube. The opaque blades (18) divide the focal point into several small apparent focal points generating beams of X-rays with low divergence, which group together, at a certain distance from the device, to form a flat beam in a fan with very low divergence. The beam obtained by such a device allows the irradiation of an object on a very precise slice, which recommends its use for transverse axial tomography devices.

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.

Uniform energy distribution radiating is linked to the shape of the anode. In the case, for example, of an X-ray tube with a cylindrical rotating anode, the X-ray beam in the form of a flat fan with a substantially uniform energy distribution is obtained by the bombardment of a rectangular electron beam on the surface. cylindrical anode.

The cathode is placed so as to clear the space facing the rectangular focal point located on a generator of the cylindrical surface of the anode and so that the axis of the X-ray beam is normal to this cylindrical surface at the focal point .

In addition, a diaphragm outside the tube is used as collimator, comprising a rectangular slot oriented so as to select the rays emitted at the focal point in order to obtain a flat fan-shaped radiation, as described in French patent application no. 77.02456 requested in the name of "Compagnie Générale de Radiologie" on January 28, 1977 and published under the number 2.379 158.

The radiogenic tube with a rotary anode can also be provided with an anti-extra-focal device placed very close, parallel to the cylindrical surface of the anode and composed of two layers of different materials, one of which absorbs by braking on its external face the secondary electrons which rebound from the hearth and which, re-accelerated, would risk to cause in other points of the anode a radiation called extra-focal.

The second layer, is the closest to the cylindrical surface of the anode and aims to absorb the extra-focal radiation from the node at points other than the hearth.

In the case of an X-ray tube with a fixed anode, the X-ray emitting surface is placed in a well dug in the anode of a material such as copper, so as to serve as a target for an electron beam from 'a cathode and so that the resulting X-ray beam can be emitted through a second well connected to the previous one and dug perpendicular to it. In this type of tube, the extra-focal radiation is very limited by the fact that the focus of the anode is enclosed in a very thick copper well. Slit diaphragms outside the tube are also used in this case in order to obtain a flat and fan-shaped X-ray beam.

A radiology device (computed tomography) comprising one of these x-ray tubes associated with X-ray detectors, makes it possible to measure the absorption coefficients of a body placed between this tube and the detectors and therefore its internal observation on a certain slice.

In this case the ideal would be to irradiate the body only in the desired area so that the detectors receive all of the radiation attenuated by it, that is to say by a useful beam of radiant energy, flat, fan of constant thickness.

Such a beam, of strictly constant thickness, represents in the current state of the art only the ideal beam which must be imitated as best as possible. Indeed, the slit diaphragms which are placed close to the object to be observed, do not sufficiently eliminate the phenomena of divergence of radiation and allow only beams in range with non-parallel directions.

The radiation zones located on either side of the theoretical parallel directions cause an unnecessary irradiation of the object in zones which are not to be observed, as well as a harmful increase in the scattered radiation.

For the same X-ray emission, the area to be observed is therefore irradiated by radiation lower than that which would be used in the theoretical case of the fan beam of strictly flat shape.

In order to compensate for this difference, for good observation, it is necessary to increase the amount of radiation emitted by the tube. An increased dose of X-rays is therefore applied to the object to be observed. Since CT scanners are used to observe the human body, this increase is most harmful and requires greater protection against unwanted radiation.

The present invention relates to a device for reducing the divergence of the useful beam as a fan of an X-ray tube.

According to the invention a tent diaphragm provided with parallel blades is placed inside the glass enclosure, vacuum tight, of the tube, near the focus of the anode. This tube can be a fixed anode or a cylindrical rotating anode.

This diaphragm consists of a support in the form of a crown sector pierced with a slot facing 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 beam range 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 fan a rectangular section of height identical to the thickness of the desired beam.

X-ray opaque blades (for example in Tantalum) are placed in this slot parallel to each other and to its section in the shape of a crown sector. These blades make it possible to split the apparent focus of the anode into more. several small foci at the level of the slit diaphragm. It is as if these small apparent foci generated fan-shaped yon beams with very little divergence.

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, it is crossed by a reconstructed beam, flat and fan-shaped, with divergence identical to the divergence of a beam 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 with respect to the glassware of the tube on the one hand and with respect to residual extra-focal phenomena on the other hand, in the case of an X-ray tube with a cylindrical rotating anode than. These thin metallic sheets, for example nickel, also have a filtration 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 radiogèneà anode fixe.
• Les figures 1 et 2 représentent un mode de réalisation du dispositif suivant l'invention dans un tube radiogène à anode tournante cylindrique, respectivement en coupe axiale et transversale.

Other characteristics of the invention will emerge from the following description 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.
• FIG. 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 an ultra-vacuum tight manner, to a cathode base 2 on the one hand and to a metal disc 3, support anode on the other hand. These unions are provided in a known manner, by annular parts 4 and 5 made 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 collar 8 as described in French patent application No. 77/23444, requested in the name of "Compagnie Générale de Radiologie" on July 29, 1977.

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 earthed 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 kept at the same potential as the anode. It is made up of these 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 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 X-ray source is limited to the 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 ¹ 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 at point P representing a focal point. The useful X-ray beam therefore emerges from the generator of the cylindrical surface of the anode containing the 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 "grid" allows to reduce according to the value of the bias voltage, the concentration of the electric beams. fires generating homes or electronic blocking of transmitters. 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 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-cylindrical surface space. 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 of rays Fan-shaped X thus produced is 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 with axis zz 'and opening angleo {and thickness the length of the focal point almost linear.

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

Grooves 17 and 17 'are machined in this slot so that one can deposit ser therein blades 18 parallel to each other. These blades can be parallel to the plane of the range of the X-ray beam normal to the focus of the anode.

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 suitable 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 previously, 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 modes for producing 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 15 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 assumes only the role of slit diaphragm conforming to the invention.

To do this, it is hollowed out with a slot so as to allow the 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 may 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 field 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 ramp of detectors, only in the desired zone of my This means that 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 (10)

1. X-ray tube comprising an anode and a cathode placed in a vacuum-tight glass enclosure, characterized in that it includes inside its glass enclosure (l) a diaphragm (13) in the form of crown sector hollowed out with a fan slot, provided with parallel opaque X-ray blades (18), that this slot is placed close enough to the anode (6) facing the X-ray emitting focal point, so that the opaque blades (18) split this focal point into several small visible focal points generating fan-shaped X-ray beams with low divergence and so that at a certain distance from this device, which is a function of the length of the blades (18) defined in the direction of propagation of the beams, their spacing, and the divergence of these beams, these group together to form a single beam of X-rays, flat and fan-shaped with low divergence. 2. X-ray tube according to claim 1, characterized in that the opaque blades (18) parallel, placed in the slot of the diaphragm (13) are parallel to the plane of the range of the useful beam flat and fan. 3. X-ray tube according to one of claims 1 or 2, characterized in that the edges of the opaque blades (18) parallel, placed in the slot of the diaphragm. (13) are hidden by thin sheets (19, 20) of a material having the property of absorbing X-rays of low energy with respect to the energy of the useful beam and this in order to avoid field effects and harmful radiation to the tube itself and to the quality of the useful flat and fan X-ray beam. 4. X-ray tube according to one of claims 1 to 3, the anode of which is a rotary cylindrical anode, characterized in that the divergence reduction device is fixed to the anode support (13) and that the diaphragm (13) in the form of a crown sector is centered on the axis of revolution of the glass envelope (1) of the tube, placed parallel to the cylindrical surface of the anode (6) and which it also serves carrier of two cathode ray emitters (16) (16 ') provided with concentration devices (14) (14') polarizable by a negative voltage, placed symmetrically with respect to the slot provided with blades (18) so as to release the space opposite the focal point produced by the electron beam emitted by one of the emitters (16) (16 ') and in such a way that the axis of the X-ray beam thus produced which is normal to the focal point, is also the axis of symmetry of the slot. 5. X-ray tube according to claim 4, characterized in that the two cathode emitters (16) (16 ') placed symmetrically with respect to the slot provided with blades (18) parallel, are located in the volume delimited by two planes containing the two parallel circular faces of the cylindrical rotating anode (6). 6. X-ray tube according to one of claims 4 or 5, characterized in that the two cathode ray emitters (16) (16 ') provided with concentration devices (14) (14') polarizable by a negative voltage, do not never bombard simultaneously the surface of the anode 6 and produce two electron beams of adjustable size by polarization of the concentration pieces (14) (14 ') thus allowing to have available a range of different foci, for the same tube . 7. X-ray tube according to one of claims 4 to 6, characterized in that the two cathode ray emitters (16) (16 ') provided with concentration devices (14) (14') polarizable by a negative voltage generate for the same bias voltage two beams of different sections, causing two focal points of different size. 8. X-ray tube according to one of claims 4 to 7, characterized in that the two cathode emitters (16) (16 ') are polarized by a negative voltage high enough to cancel the emission of electrons from these emitters (15-16 ') and therefore block the operation of the X-ray tube. 9. X-ray tube according to one of claims 1 to 3, the anode of which is a fixed anode, characterized in that the divergence reduction device is fixed to the anode (6) and is kept at the same potential that the latter, and that the diaphragm (13) with a slit in the form of a crown sector is: centered on the axis of revolution of the glass envelope (1) of the tube; placed parallel to the surface of the anode (6) so that the X-ray beam, emitted in a direction normal to the direction of the electron beam going from the cathode (16) towards the focus of the anode (6 ), crosses the slot provided with the opaque blades (18) parallel. 10. X-ray tube according to claim 9, characterized in that the diaphragm (13) with slot provided with opaque blades (18) in the form of a crown sector has its full part extended so as to take the form of a crown completely surrounding the anode (6) thus making it possible to obtain a good distribution of the field in the glass enclosure (1) of the X-ray tube.

Images