FR2548447A1 - X-ray tube with high-intensity focus - Google Patents

Abstract

The invention relates to an X-ray tube with high-intensity focus, able to produce a beam of X-rays (FX) capable of much greater emission than that from a prior art tube. The tube according to the invention includes, on an anode 5, a first target 18 associated with at least one thin target (Cm), such that the X-ray radiation produced by each target (18, Cm) is compounded along a principal axis 27.

Description

HIGH INTENSITY FIREPLACE XA TUBE
The invention relates to a high intensity focus X-ray tube, applicable in the fields of radiology and more particularly in the field of radiodiagnostics and computed tomography.

 In X-ray tubes, X-ray radiation is commonly obtained by electron bombardment concentrated on a small surface, called focal point, of a target that has the anode, this focal point becoming the source of X-ray radiation, and constituting in rotation a crown. focal (case of a rotating anode).

 A small part, about 895, of the electrical energy expended to accelerate the electrons, is transformed into X-rays; the rest of this energy is dissipated as heat. This heat, most of which is evacuated by radiation, can lead to deterioration of the anode and particularly of the focal ring and constitutes a limit to the increase in intensity of X-ray radiation.

 It should be noted that the hearth must comprise, on the one hand for the fineness of the radiological image, an area as small as possible, and on the other hand with respect to thermal stresses, an area large enough to avoid deterioration of the target on which it is built. These two contradictory requirements are partial: conciliated by giving an inclination to the plane of the target with respect to the direction of the useful beam produced by the X-ray; the apparent surface of the hearth in this direction, then being less than its real surface, the latter can be increased to increase the possibilities of X emission.

 A defect called heel effect, brought by this inclination of the target plane, resides in that the illumination in the field covered by the useful beam is not uniform, this defect manifesting itself mainly in the cathode-anode direction.

 Other arrangements also make it possible to increase the possibilities of X emission, such as in particular: by scrolling the target under the electron beam (in the case of rotating anodes), and by using targets made of refractory material with high power of thermal radiation.

The present invention relates to a high intensity focus X-ray tube capable of constituting a tube with a fixed anode or with a rotating anode, and capable of producing a beam of radiation.
X with a possibility of emission much greater than that of a tube according to the prior art, without increasing the apparent surface of the hearth.

The arrangement of the X-ray tube according to the invention also makes it possible to eliminate the aforementioned defects such as for example the heel effect.

 According to the invention, a high intensity X-ray tube with focus, producing an X-ray beam used along a main axis, comprising at least one cathode providing at least one electron beam intended to bombard at least a first target, an anode provided with this first target, which first target produces X-radiation by forward scattering, is characterized in that the anode further comprises at least one thin target producing X-radiation by transmission, these two targets being arranged relative to the electron beam so that a first and a second focus, produced respectively at the first target and the thin target, are aligned with the main axis of the X-ray beam used.

 The invention will be better understood thanks to the description which follows, and to the five appended figures among which: - Figure 1 schematically shows a first embodiment of an X-ray tube according to the invention; - Figures 2, 3, 4 and 5 show a first, second, third and fourth variant of the arrangement of characteristic parts of the X-ray tube according to the invention.

 FIG. 1 shows an X-ray tube 1 according to the invention, the representation of which is limited to elements essential for understanding the invention.

 The tube 1 comprises, in an envelope 2, a cathode 3 held in a conventional manner by a support means 4, and an anode 5.

In the nonlimiting example described the anode 5 is a rotating anode having the shape of a disc, but may also, within the framework of the invention, be a fixed anode (not shown); the anode 5 is linked via an axis 6, and by fixing means 7 to a rotor 8, by which it is rotated about a longitudinal axis 9 of this rotor. In the nonlimiting example of the description, the cathode 3 comprises a first and a second filament 11, 12 supplied in a manner known per se and not shown, so as to generate respectively a first and a second beam of electrons F1, F2, these electron beams being concentrated on parts of the anode 3 capable of generating X-rays.

 To this end, the anode 5 comprises in the vicinity of its periphery, on its upper face 13 for example, a first circular groove 14 in the form of a V, the walls 1S, 16 respectively constitute a first and a second slope, opposite l 'from one another in the nonlimiting example described.

 On the first slope 15 is reported a refractory material of high atomic number, such as tungsten for example, having a thickness e close to or greater than 0.5 mm, and which constitutes a first target 18 forming a thick target; The anode 5 itself being made of a material with high thermal radiation power, and having a low absorption by X-rays, such as graphite for example. On the second slope 16 is arranged a deposit, of the same material as that of the first target 18 for example, but of very small thickness (not shown), of the order of 1 to 5 microns, constituting a thin target Cm. The first electron beam F1 bombards the first target 18, part of which is delimited by the limits 20 of this first electron beam, constitutes a first focus 22; in the same way, the second electron beam F2 bombards the thin target Cm, part of which delimited by the limits 21 of this second electron beam constitutes a second focus 23. In the nonlimiting example described, the beams d electrons F1, F2 are shown separate, but their neighboring limits 20, 21 can be confused with an electron radiation axis 40, and the neighboring borders of the foci 22, 23 can be confused at the junction 41 of the targets 18, Cm .

 The first focal point 22 generates X-ray radiation (not shown) by front scattering, the used part of this radiation leaving through the front face 24 of the first target 18, that is to say its bombarded side, and advancing in the direction shown by arrow 30; the X-ray used (not shown) generated by the second focal point 23 and propagating in the same direction 30, leaves by transmission, that is to say by the face 25 of the thin target Cm, opposite the bombarded side.

 Thus on the main axis 27 of a beam of X-ray radiation FX used, leaving the anode 5 by an output edge 31 and produced on the one hand, by the X-ray radiation diffused in front which is generated by the first focus 22, and on the other hand by the X-ray outgoing by transmission which is generated by the second focal point 23, there are two focal points 22, 23 aligned on this main axis 27 of the beam FX; the first target 18 being the furthest from the exit edge 31.

The total intensity provided by these foci 22, 23, in a way superimposed, is the sum of the intensity of the radiation of the first target 18 attenuated by the filtration of the thin target Cm, and the intensity of the radiation emitted by transmission by the thin target Cm; the absorption due to the material of the anode 5 itself, being negligible.

 For a thin target Cm whose thickness (not shown) has been optimized for determined operating conditions, it is possible to obtain as much X-radiation by transmission as by front scattering with a first target 18. In total, the luminance available on such a compound target, constituted by the first target 18 and the thin target Cm, can reach 1.5 times that of a conventional tube with a single target. This advantage, constituting a first object of the invention, is obtained in particular because the electron bombardment produced by the first and the second electron beam F1, F2 having in the nonlimiting example described axes 32, 33 parallel, determines the source of the X-ray beam FX formed by the two foci 22, 23 aligned with the main axis 27; the axes 32, 33 of the electron beams F1, F2 intersecting the main axis 27 at the level of the first target 18 and of the thin target Cm respectively.

 Another advantage of such an arrangement, compared to a tube according to the prior art provided with a single slope anode and a single target, is that it makes it possible to standardize the illumination of the field covered in compensating for the heel effect. This heel effect resides in the variation of an emission indicator in the anode-cathode direction (more radiation towards the cathode and less towards Panode) 1 as is shown by a first curve 35 in dotted lines representing the indicator emitting the first target 18; due to the arrangement of the targets 18, Cm, in a tube according to the invention, the emission indicator for the thin target Cm, represented by a second curve 36 in dotted lines, is practically symmetrical with respect to the first. The total indicator is the sum of the two .ndXcatrices, and leads to a better distribution of the illumination of the covered field, formed between a first and a second limit 37, 38 of the X-ray beam FX used; these limits 37, 38 of the beam FX used can be determined in a conventional manner, by a collimator device not shown.

 This uniformity of illumination is mainly due to the fact that the first target 18 and the thin target Cm are attached to the walls 15, 16 and also form opposite and equal slopes. The front face 24 of the thick target 18 is symmetrical to the thin target Cm with respect to the electron radiation axis 40 or bombardment axis, with which they respectively form a first and a second angle 1> 1 '2 of same value; this electron radiation axis 40 passing at the junction 41 of these two targets 18, Cm.

 A third important advantage of this configuration is that it provides a better distribution of the resolution power in the image field. it is known that a defect in a conventional configuration is, that from one end to the other of the covered field (represented in the figure by the limits 37, 38 of the beam of radiation X FX), one of the dimensions of the focal point varies with the angle formed between the axis of the X-ray beam and the axis of vision of the source: taking for example the focal point 22 formed at the level of the first target 18, this focal point appears longer seen near the second limit 38 of the beam FX, and appears shorter seen near the first limit 37. But in the case of the tube 1 according to the invention, this effect also exists, but in the opposite direction for the focal point 23 formed at the level of the thin target Cm.

 Thus by traversing the field covered from the first to the second limit 37, 38 for example, a dimension 13 visible in the plane of the figure, of visible or optical foci 22A, 22B of the focal point 22 of the first target 18, increases . In contrast, a dimension 14 of the apparent focal point 23A, 23B of the focal point 23 of the thin target Cm decreases, thus allowing a homogenization of the resolving power in the image field; the homes 22, 23 having dimensions 11, 12 equal, apparent homes 22C, 23C which they determine on the main axis 27, are combined.

 The X-ray tube according to the invention can be installed on any already existing radiology installation. In most cases, the dissipable power on the first target 18 and on the thin target Cm can be the same; these two targets 18, Cm can then be bombarded by the same electron beam (not shown), symmetrical with respect to the electron radiation axis 40, and having an opening sufficient to bombard the two targets 18, Cm. But if for reasons of maximum optimization, account is taken of the differences in permissible powers on these two targets, two filaments must be available as shown in FIG. 1, and a separate heating control for filaments (not shown), in order to obtain different electronic bit rates on each of the targets 18, Cm; the existence of two filament heating circuits being a frequent case in a radiological installation.

 The X-ray tube according to the invention may also include an anode 5 provided, in addition to the first target 18 and the thin target Cm, with a plurality of additional thin targets.

 This case is shown in FIG. 2 in which the anode 5, constituted as in the case of FIG. 1 in a material of low atomic number (graphite for example), is partially represented; the anode 5 comprises in the first groove 14 the first target 18 and the thin target Cm, and n additional thin targets Cml, Cm2, n being in the example equal to 2.

The cathode 3 comprises, in the nonlimiting example described, as many additional filaments 50, 51 as electron beam generators F3, F4, as there are additional thin targets Cml,
Cm2. These additional filaments 50, 51 and the first and second filaments 11, 12 being, in this example, arranged in planes perpendicular to the figure, in order to allow a small distance d between grooves 14, 52, 53 containing the targets 18, Cm, Cml, Cm2; additional grooves 52, 53 each containing an additional thin target Cml, Cm2.

 As previously explained, the first target 18 provides scattered X-rays which emerge from the front, passing through the thin target Cm, then the succession of additional thin targets Cml, Cm2. Each thin target Cm, Cml, Cm2 provides, as previously explained, X-rays exiting forward by transmission. The total intensity of X-ray radiation then being linked to the radiation intensity of a target, the number of targets, and the transmission rate of a thin target.

This X-ray radiation by transmission and by forward diffusion makes it possible, as has been explained above, to obtain a beam of X-ray radiation FX used, leaving the anode 5 by the output edge 31, along the main axis. 27. Targets 18, Cm, Cml,
Cm2 are relatively close together and are aligned on this main axis, the thin targets Cm, Cml, Cm2-being parallel; this non-limiting configuration is favorable to the production of an FX beam in the form of a fine brush, delimited for example by conventional means (not shown). In order to favor the bombardment of the first target 18 and of the thin target Cm, the axes 32, 33 of the electron beams F1, F2 are crossed.

 FIG. 3 shows a variant of the X-ray tube 1 according to the invention, in which the anode 5 is preferably made of a material of low atomic number, such as carbon or titanium for example, and in which the first target 18 and the thin target Cm are parallel.

 The first and second walls 15, 16, of the groove 14, formed in the upper face 13 of the anode S, are parallel in this version, and are provided as in the previous examples with the first target 18 and the target thin Cm; the distance dl between these targets, which can vary from a few tenths of a millimeter to a few millimeters.

 In this version of the invention, the axes 32, 33 of the first and second electron beam F1, F2 can be crossed as in the example described, so as to allow better bombardment of each of these targets, account given their small spacing; X-ray by diffusion and X-ray by transmission being produced in the same manner as previously described.

 It is possible to add, as in the nonlimiting example described, a fixed locator screen 60, making it possible to locate the X-ray radiation beam FX very effectively, since it is very close to the X emission sources that constitute the foci 22, 23, thus helping to reduce the extra-focal radiation (not shown). This locator screen 60, having a high absorption coefficient with respect to X-radiation, is brought to the same potential (not shown) as the anode 5, and is held in a fixed position above it, thanks to a central fixed shaft 61, located along the longitudinal axis of the rotor 8. In this configuration, the locator screen 60 has a first opening 62 allowing the passage of electron beams, F1, F2 and a second opening 63 used to delimit the X-ray beam of FX FX; this screen can advantageously be used to determine a fan-shaped X-ray X-ray beam, having a thickness E and whose opening angle (not shown) is perpendicular to the plane of the figure. Such a locator screen 60 is applicable to all versions of the X-ray tube according to the invention.

 Figures 4 and 5 show the association of a first target 18 and a thin target Cn arranged in the first groove 14, this first groove being formed in the edge 70 of the anode disc 5; the anode 5 being partially shown, with its axis of rotation constituted by the longitudinal axis 9, the means necessary for this rotation not being shown.

In the example of FIG. 4, the first target 18 and the thin target Cm are parallel, and are bombarded respectively by the first and the second electron beam 1 F. In the same way as in the previous examples, a x-ray beam
FX (not shown) of main axis 27 is generated; the X-ray exiting from the anode 5 via an output edge 31, located opposite the first target 18.

 FIG. 5 shows the first target 18 and the thin target Cm arranged relative to the cathode 3 according to the same configuration as in FIG. 1, and each bombarded by an electron beam F1, F2; these two electron beams F1, F2 intersect, and as previously explained, the X-ray radiation leaves the anode 5 by an output edge 31, opposite the first target 18, along the main axis 27.

This description of an X-ray tube according to the invention is not limiting, a first target 18 and one or more thin targets
Cm, Cml, Cm2 can be combined in other configurations without departing from the scope of the invention, the main thing being that the X-rays produced by these targets are added, so as to form a beam of X-ray radiation, for which the optical focal points 22C, 23C are combined on the main axis 27 of this beam; it should be noted that the first target 18 can also consist of a thin target Cm.

Claims (17)

 1. X-ray tube with high intensity focus, producing an X-ray beam (FX) along a main axis (27), comprising at least one cathode (3) providing at least one electron beam (F1, F2) intended to bombard at least a first target (18), an anode (S) provided with this first target (18), which first target produces X-radiation by forward scattering, characterized in that the anode (5) further comprises at least one thin target (Cm) producing X-radiation by transmission, these two targets (18, Cm) being arranged with respect to the electron beams (F1, F2) so that a first and a second focus (22, 23), produced respectively at the level of the first target (18) and of the thin target (Cm ), are aligned with the main axis (27) of the X-ray beam (FX) used.  2. X-ray tube according to claim 1, characterized in that the anode (5) is a rotating anode.  3. X-ray tube according to one of the preceding claims, characterized in that the first target (18) is a thick target.  4. X-ray tube according to one of the preceding claims, characterized in that the X-ray used propagates, in the direction (30) of the first target (18) towards an exit edge (31) of the anode (5), and crosses at least one thin target (Cm).  5. X-ray tube according to one of the preceding claims, characterized in that the first target (18) and the thin target (Cm) are arranged on opposite slopes with respect to a radiation axis (40) of the cathode (3) with which they respectively have a first and a second angle (d 1 (2)  6. X-ray tube according to claim 5, characterized in that the first and the second angle (0 <1, 2) 2> are equal.  7. X-ray tube according to one of claims 1 to 4, characterized in that the first target (18) and the thin target (Cm) are parallel.  8. X-ray tube according to one of the preceding claims, characterized in that the homes (22, 23) have the same dimension flî, 12).  9. X-ray tube according to claim 1, characterized in that the anode (5) comprises at least a first circular groove (14) of which the first and the second walls (15, 16) are respectively provided with the first target (18) and the thin target (Cm).  10. X-ray tube according to the preceding claim, characterized in that the first groove is formed in one face (13) of the anode (5).  11. X-ray tube according to claim 9, characterized in that the first groove (14) is formed in the edge (70) of the anode (5).  12. X-ray tube according to claim 1, characterized in that the anode (5) further comprises additional thin targets (Cml Cm2).  13. X-ray tube according to claim 12, characterized in that the additional thin targets (Cml, Cm2) are contained in additional grooves (52, 53) external to the first groove (14).  14. X-ray tube according to claim 12, characterized in that the additional thin targets (Cml, Cm2) are parallel.  15. X-ray tube according to claim 1, characterized in that the cathode (3) generates at least a first and a second beam (F1, F2) of electrons, having axes (32, 33) parallel, serving to bomb respectively the first target (18) and the thin target (Cm).  16. X-ray tube according to claim 1, characterized in that the cathode (3) generates at least a first and a second beam (F1, F2) of electrons, having axes (32, 33) crossed, serving to bomb respectively the first target (18) and the thin target (Cm).  17. X-ray tube according to claim 1, characterized in that it further comprises a fixed locator screen (60) of the X-ray beam (FX).