X-Ray Tube Having a Variable Focus Which Is Self-Adapted to the Load

1. Field of the Invention

The present invention relates to an x-ray tube having in particular a variable focus which is self-adapted to the load, for use in the medical field. The main characteristics of these tubes relate to the resistance to drift of their emission characteristics as a function of their temperature as well as homogeneity of the x-ray illumination produced by all the points of their focus. The aim of the invention is to improve such tubes while guarding against any danger of destruction under the action of overheating of their anode.

2. Description of the Prior Art

In general terms, x-rays are produced by electron bombardment, within a vacuum enclosure, of a target fabricated from material having a high atomic number. The electrons which are necessary for bombardment of said target are liberated by thermoelectronic effect, usually in a helical filament of tungsten, of a cathode placed with precision within a concentration component. The concentration component performs a focusing function at the same time as a Wehnelt function. The target is constituted by the anode of the x-ray tube. In this very conventional type of configuration, the initial velocities of the electrons at the level of the emitter are highly dispersed. The electron trajectory therefore has a disordered structure and the focusing system provides a correcting function but does not usually have sufficiently high performance characteristics. In consequence, instead of an impact of bombardment electrons on the target, there is obtained a fairly complicated entanglement of trajectories. This provides the thermal focus of the x-rays with an energy profile which is hardly compatible with good quality of the image.

In recent developments, for example in those described in European patent Application No. 85 106753.8 filed on May 31st 1985, reference is made to a cathode which is no longer constituted by a filament but is constituted by a portion of strip provided for emission of the electrons with a flat surface located opposite to the anode. The advantage of employing a flat electron emitter has already been presented prior to this Application. It consists in maintaining a certain cohesion of the electronic charges during their trajectory towards the target. Experience has in fact shown that there is obtained in this case a distribution of electrostatic potential which is favorable to better focusing of the electric charges. The x-ray focus thus obtained then has a practically homogeneous energy profile which is conducive to good quality of the image. The scientific literature records certain experiments which are based on this general principle and in which use is always made of an emitter constructed in the form of a tungsten strip.

However, these strips are systematically attended by problems of thermomechanical strength. It was in fact with a view to solving such problems that the invention corresponding to European patent Application cited above was conceived. In particular, in spite of all the care and attention devoted to rolling of the strips, these strips are subjected to differential stress phenomena and, as a result of successive heating and cooling within the tube, acquire a so-called corrugated-sheet appearance. The advantages arising from the use of a flat emitter are then lost.

In addition to these defects, flat emitters or even filament emitters have a disadvantage in that the shape of the energy profile of the focus varies in an uncontrolled manner with the load on the tube. The tube load corresponds to the x-ray delivery. This delivery is related to the magnitude of the thermoelectronic effect in the cathode and this latter is related to the temperature to which said cathode is heated. In point of fact, more and more radiology devices are provided with regulating circuits for regulating the tube load. Taking into account the x-ray absorption coefficient of a given patient to be examined, this regulation has the effect of ensuring that the radiation which passes through the patient is limited to a minimum. As can be understood, this regulation acts on the heating circuit of the cathode. The technique whereby regulation tends to produce action on the high voltage between anode and cathode has been abandoned in view of the fact that, in this technique, the hardness of the x-radiation employed has to be modified while examination is in progress.

Modification of the tube load is not without effect on the energy distribution of the focus. This has a number of consequences. Particularly in certain situations, by reason of the modification of said tube load, it is possible that the energy densities attained at certain points of the anode may exceed the thermal densities which are acceptable for this anode, in which case the anode is liable to be destroyed. The phenomena of expansion and compression of useful surfaces of the thermal focus are due to the existence of the space charge transported by the electrons before impinging upon the target. It is also necessary to relate the magnitude of said space charge itself to the high voltage which is necessary for detachment of the electrodes from the cathode.

Consideration could also be given to the possibility of modifying the function of the focusing member as a function of the space charge in order to limit, for example, the destructive effects of an excessively abrupt increase in thermal density of the focus. Apart from the complexity of a control system of this type which cannot be contemplated in the present state of the art, it would be necessary to ensure in addition that this control system is capable of anticipating thermal drift in the thermal density of the focus.

This solution is not possible. In consequence, in the present state of the art, regulation applied to the tube load automatically produces a variation in the x-ray illumination and therefore affects the quality of the resultant images. In the final analysis, the heterogeneous character of the combined effects of the space charge and of the high voltage (of the tube load) does not make it possible to obtain tubes in which at least certain emission characteristics would be controlled irrespective of the load.

The aim of the present invention is to overcome this drawback by proposing a flat emitter device having the advantage of offering a degree of mechanical strength which makes it possible to remove the problems of corrugation mentioned earlier. The solution to the problems of variation in thermal density along the focus or of variation in dimension of said focus as a function of the load on the tube may accordingly be provided by the installation of said flat cathode within a so-called stair-step focusing member. It has in fact been discovered that self-regulation of the characteristics of said focus takes place in this case. It is thus possible in respect of a particular geometry of the stair-step focusing member to obtain a thermal density of the focus which is always constant. The advantage of this solution is that it is applicable over a wide range of high voltage between the anode and the cathode so that one and the same tube can serve for a number of different applications.

The advantage of providing controlled thermal density and therefore a focus whose dimension is variable with the load enables the user to employ with the same tube a large number of foci having different dimensions. In fact, the photographic images formed with a fine focus can be produced at a lower x-ray delivery and, when the user operates with a larger focus, he needs higher power. The invention accordingly puts at the user's disposal a tube with a focus of constantly variable dimension and adjustable and having a constant thermal flux on the target. Control of utilization is thus ensured in a simple manner.

The invention is therefore directed to an x-ray tube provided with a cathode and an anode opposite to the cathode for emitting x-radiation, as distinguished by the fact that

the cathode is a flat cathode,

said cathode being placed at the base of a stair-step focusing device.

FIG. 1 is a schematic sectional view of an x-ray tube in accordance with the invention.

FIG. 2 is an energy diagram for the tube of FIG. 1.

FIG. 3 is a view in perspective showing an example of a flat cathode in accordance with the invention.

FIG. 4 is a sectional view of the cathode of FIG. 3.

FIG. 1 shows diagrammatically an x-ray tube in accordance with the invention. Said x-ray tube is provided within a vacuum enclosure (not shown) with a cathode 1 located opposite to an anode 2. The anode receives an electron radiation 3 on its focus 4 and re-emits an x-radiation 5 which is directed in particular to a utilization window 6. The utilization window forms part of the tube envelope. In accordance with the invention, a distinctive feature of the cathode lies in the fact that a flat face 7 is located opposite to the anode 2. Another feature is that said cathode is inserted in a so-called stair-step optical focusing device 8. The object of this stair-step optical device is to produce a distribution of the electric field between the anode and the cathode such that the electron radiation 3 is of the convergent type. Two types of convergent radiation are distinguished. In a first type shown in FIG. 1, the point of convergence of the electrons is located behind the plane of the anode and is virtual. In this case, the radiation is known as direct. In a second type of radiation or so-called crossed radiation, the point of convergence of the electrons is located in the intermediate position between the cathode 7 and the anode 2 and is real.

Although the focusing device 8 can consist of a single step, it has been found more advantageous in this case to provide a double step. The focusing member 8 has a prismatic shape, the right section plane of which is shown in of FIG. 1. The member 8 has two stair-steps designated respectively by the references 9 and 10 and distributed symmetrically at 9' and 10' on each side of the cathode 1. Each stair-step has a top face or "tread" 91 or 101 and a riser 92 or 102 (respectively 91', 92', 101', 102'). In a preferred example of construction, the plane 7 of the cathode 1 is located at a distance of approximately 7.5 mm from the anode 2. The treads 91 and 91' of the steps 9 and 9' are located at a distance of approximately 7 mm from the anode. The treads 101 and 101' are located at a distance of approximately 6 mm from the plane of the anode 2. The width of the cathode 1 as measured in the right section plane of the prismatic focusing member 8 has a value of 2 mm. The width of a housing 11 in which said cathode is placed within the focusing member 8 has a value of 2.2 mm. The distance between the risers 92 and 92' is 4 mm whilst the distance between the risers 101 and 102' is 5 mm. It is possible to consider that the risers are thus applied against parallelepipedal cylinders (considered in the theoretical sense of the term) having respective widths of 4 mm and 5 mm. Preferably, the device has a symmetrical shape with respect to a plane which passes through the radiation axis 12 at right angles to the plane of the figure. By way of alternative, however, instead of being prismatic, the assembly can be circular and the axis 12 serves as an axis of revolution for the cathode as well as for the focusing member. The anode 2 may possibly be an anode of the rotating type and may even have a face which is inclined to the axis 12. In this case, the distances indicated are rather the distances measured on said axis 12 between the plane 7 of the cathode and the trace of the axis 12 on the anode 2.

The dimensions given in the foregoing have an advantage in that the thermal flux FT is in this case substantially constant in respect of a given utilization high voltage, as a function of the load D on the tube. In fact, the diagram of FIG. 2 shows three curves 13 to 15 respectively having high voltage parameters of 20 KV, 40 KV or 50 KV indicating a substantially flat course within a utilization load range located between 150 milliamperes and 500 milliamperes. The thermal flux is expressed in KW per mm.sup.2. In the example considered, the thermal flux is always less than 50 KW per mm.sup.2, even at the highest utilization high voltage. The flat appearance of said thermal flux as a function of the load means quite simply that the dimension 16 of the thermal focus varies linearly with the load. In fact, if the load increases, for example to double the value, the dimension 16 increases and the x-ray power also increases to double the value. This does not produce any abnormal local thermal stresses on the anode since the thermal flux remains the same. This increase in load causes a relative outward displacement of the lateral directions of the electron beam 3 in the direction of the arrows 17 and 18. The beam becomes more and more direct.

Although the dimension of the focus changes when the load changes, the advantage of the solution considered is related to the fact that a focus of predetermined dimension is thus made available in a simple manner. In fact, the curves 13 to 15 are regular curves without undulation. In consequence, in particular in metrology, when the problem of dose rate is a crucial point or even in medicine when the limits of irradiation are not overstepped, it is possible to choose a desired dimension of focus as a function of sharpness of detail of the image to be produced. A simple means has thus been presented for adjusting the dimension of said focus to a suitable value.

In a preferred example, the cathode 1 has the appearance of a beam as shown in perspective in FIG. 3. This beam is prismatic, hollow, and has substantially the shape of a house. The house in this case is presented as if it were laid flat on one of its walls. The base of the house constitutes the emissive face 7 of the cathode, the walls of the house such as the wall 23 have windows such as the window 24. The advantage of constructing a hollow beam lies in the reduction of the quantity of metal to be heated. Moreover, the beam structure endows said cathode with mechanical strength which guards against corrugated sheet phenomena. Since the quantity of metal to be heated is smaller, the thermal inertia of the cathode is lower and starting of the x-ray tube can be faster. Moreover, the consumption of heating power supplied to the cathode can be reduced, which is an advantage when considering the insulation problems which have to be faced in cathode heating circuits.

Although it is possible to contemplate direct heating of this cathode by passing an electric current directly through this latter, it is preferred to employ a heating filament 25, for example of the same type as a heating element employed in the present state of the technique as an emitter. This filament 25 is itself negatively polarized (several thousand volts) with respect to the cathode 1. In a preferred example, the beam cathode is made of tungsten. In order to ensure in addition that the quantity of thermal energy to be delivered for heating the cathode is limited, the ceiling 26 and the interior of the walls of said cathode are provided with a mattress 27 of heat-insulating fibers. This concentrates the heating on the emissive portion of the cathode. In one example, the fibers are ceramic fibers which permit good insulation of the lateral internal walls of the house. Accordingly, the electrons emitted by the heating filament bombard only the rear portion of the cathode 7 in a pattern represented by the electric field curves 28. This bombardment is limited to the front wall.

Moreover, said front wall has a concave profile. In a preferred example, this profile is even concave to such an extent that wings 29 and 30 respectively of said cathode have internal faces 31 and 32 respectively which are closer to the filament 25 than the internal face of the cathode 7 at its midpoint 33. Thus the wings which are both of greater thickness and which would be more difficult to heat are nevertheless heated to a greater extent so as to ensure that the top of the beam is brought to a substantially constant temperature at all points. In this manner, the required radiation of electrons is emitted at a substantially constant rate.

Although the beam in accordance with the invention now offers an advantage in that its emissive face 7 is no longer subject to distortion under the action of overheating, the beam is nevertheless subject to expansions which have to be guided without restraining them. To this end, the cathode is attached by means of a single lug 34 which virtually constitutes the chimney of the house. The mode of attachment is preferably obtained by locking said lug 34 between two clamping screws 35 and 36 respectively. This assembly with a single point of attachment has the advantage of providing the cathode with all the degrees of freedom which may be desired. It is preferable in particular to a two-point mode of attachment which would be attended by a disadvantage in that the reactions between the two points would inevitably produce harmful effects on the flatness of the emissive surface 7. In order to guide the displacements of the cathode with the temperature, the walls of said cathode are maintained within the focusing member 8 by ceramic studs such as the studs 37 and 38 which are applied against said member on each side. This serves to guard against any phenomenon of bending or vibration which would have an unfavorable effect on accurate positioning of the emitter within the focusing member. The studs permit thermal expansion of the emitter along its greatest length while maintaining it laterally in its reference position. In practice, the supply of electric power to the cathode can be obtained by passing the high voltage through the screws 35 or 36. If necessary, the focusing member can be decoupled electrically from the cathode.