EP1077019B1 - X-ray source and its use in radiography - Google Patents

Abstract

An X-ray emission device having a microwave source and a resonant chamber. The resonant chamber contains a hermedically sealed volume of gas, a magnetic structure defining a geometrical electron-confinement zone in which electrons move at high speed. At least one target is placed in an electron path in order to emit X-rays. Each target is offset with respect to a mid-region of the geometrical conbinement zone.

Description

The present invention relates to ray emission devices X. More specifically, the invention relates to a new source of X-rays for applications in radiology, in particular medical.

Current X-ray sources use tubes containing a gas under very low pressure (of the order of 10 ^-9 Torr; 1 Torr = 133.32 Pa), in which a potential difference of the order of 50 to 100 kilovolts between a cathode and a target makes it possible to generate a flow of electrons moving at high speed to strike the target and emit X-rays. Their operation requiring a high voltage supply, these installations are by nature fixed, heavy and bulky. In addition, the presence of the transformers necessary for the high voltage supply makes the total cost of the installations high, and imposes particularly restrictive operating and maintenance protocols.

We know a principle of X-ray emission by RCE (Resonance Cyclotronique Electronique), different from that used by installations with existing tubes, the implementation of which does not require supply high tension. This principle, which was described in an article published in 1990 by MM. Garner et al. In Review of Scientific Instrumentation, is schematically the following: electrons emitted by a heated filament into a cavity, exposed to microwave radiation, enter certain resonant conditions to create a plasma in which, having reaches high energy levels, said electrons move at great speed. If we place on the trajectory of these energetic electrons a solid target, it can emit X-rays. By creating, using two magnets placed symmetrically with respect to the cavity, a field magnetic whose median plane intersects the volume of the cavity, we observes trajectories of energetic electrons having the general shape a ring substantially inscribed in said median plane. So he is possible to place the target in this plane, in order to intercept electrons energy with the target to give rise to an X-ray emission. The emitted X-rays are then filtered through a thin window made in a material such as glass, capable of guaranteeing the tightness of the cavity gas, but letting X-rays pass.

The implementation of this principle would have very important compared to current tube sources: it would no longer necessary to provide a high voltage electrical power source, this which would make the installation safer, easier to use, more economical and considerably more compact.

Attempts have been made to develop X-ray sources based on the principle of issue by RCE. These attempts, described in particular in patents US 5323442, US 5282899 and US 5327475, have certainly made it possible to produce X-rays, but in quantity and at energy levels that seem insufficient to consider use operational in particular in medical radiology. In particular, some medical radiology applications require producing with a sufficient density an X-ray flux with an energy of around 100 keV, and the above-mentioned attempts do not seem capable of producing such X-ray fluxes.

Also known from US Patent 5,355,399 is a device operating on the same principle as the patent devices cited above and emitting X-rays, and a method associated therewith to produce radiographic images. But in this last device, the target located in the median plane of the magnetic field disturbs the generation of energetic electrons, which leads to limiting the flow of X-rays emitted by the device. This has a direct influence on the operation of the device and on the quality of the images, because a bit rate of weak x-rays implies exposure time of the subject to be x-rayed long, which on the one hand is restrictive in practice and, on the other hand, decreases image resolution due to subject movement during exposure time. This important limitation prohibits a operational operation of the device. So we don't have currently from X-ray source by RCE really operational, whereas such a source could, thanks to its compactness, be transportable, and thus considerably widen the field of application of X-ray imaging devices (used in medical imaging in environments that are difficult to access, for example on the site of a accident, or in industrial imagery, for example in submarines, planes or factories).

An object of the present invention is to allow a radiology device according to the principle of X-ray emission by RCE, so as to have a sufficient X-ray flow, in particular for produce images of quality at least equal to that of current shots produced by tube devices, using an X-ray source compact and transportable, not using high power voltage.

A second object of the invention is to allow a X-ray source whose response time to a control signal is short enough to produce fixed x-ray images or animated,

A third object of the invention is to make it possible to carry out stereoradiography images and produce radiographic images in relief.

In order to achieve these aims, the present invention proposes, according to a first aspect an X-ray emission device comprising a source of a microwave, a resonance chamber containing a volume of gas hermetically confined, a magnetic structure defining an area geometric electron confinement in which electrons are move at high speed and at least one target placed on a path of electrons to emit X-rays, characterized in that the or each targets is offset from a middle region of the geometric area containment.

• le champ magnétique est symétrique et la région médiane de la zone géométrique de confinement est un plan,
• la structure magnétique comprend au moins une paire d'aimants permanents placés de part et d'autre de la chambre de résonance,
• la structure magnétique comprend des bobines placées de part et d'autre de la chambre de résonance,
• il est prévu des moyens pour faire varier l'intensité du courant dans les bobines,
• les moyens pour faire varier l'intensité du courant dans les bobines sont aptes à effectuer la variation d'intensité assez lentement pour permettre aux électrons de conserver sensiblement invariant le rapport B/γ, où B est la valeur du champ magnétique et γ^-2=1-v²/c², v étant la vitesse de l'électron, c la vitesse de la lumière et γ le facteur relativiste de Lorentz,
• le dispositif comprend des moyens pour ajuster la configuration de la structure magnétique de manière à faire varier l'énergie des rayons X émis,
• le dispositif comprend des moyens pour ajuster la position de la ou des cible(s) de manière à faire varier l'énergie des rayons X émis,
• la source de micro-ondes comprend des moyens pour moduler l'émission de micro-ondes,
• le dispositif comprend deux cibles,
• les positions des deux cibles sont symétriques par rapport à la région centrale de la zone géométrique de confinement,

Preferred, but non-limiting aspects of the device according to the invention are the following:

• the magnetic field is symmetrical and the middle region of the geometric confinement zone is a plane,
• the magnetic structure comprises at least one pair of permanent magnets placed on either side of the resonance chamber,
• the magnetic structure comprises coils placed on either side of the resonance chamber,
• means are provided for varying the intensity of the current in the coils,
• the means for varying the intensity of the current in the coils are capable of carrying out the variation in intensity slowly enough to allow the electrons to keep substantially invariant the ratio B / γ, where B is the value of the magnetic field and γ ^-2 = 1-v ² / c ² , v being the speed of the electron, c the speed of light and γ the relativistic Lorentz factor,
• the device comprises means for adjusting the configuration of the magnetic structure so as to vary the energy of the X-rays emitted,
• the device comprises means for adjusting the position of the target (s) so as to vary the energy of the X-rays emitted,
• the microwave source comprises means for modulating the emission of microwaves,
• the device comprises two targets,
• the positions of the two targets are symmetrical with respect to the central region of the geometric confinement zone,

According to a second aspect, the invention also proposes a radiography installation comprising an X-ray emission device according to one of the aspects described above and comprising two targets, means for forming two radiographic images of the same object according to two different angles and means to reconstruct an image of stereoradiography of said object.

La figure 1 est une vue schématique d'une source XRCE d'un type connu,

La figure 2 est une vue schématique de face d'une source XRCE d'un type connu,

la figure 3 est une vue schématique de la trajectoire des électrons énergétiques dans la région médiane du'champ magnétique d'une source XRCE,

la figure 4 est un graphe montrant la variation de l'intensité du rayonnement X émis en fonction de la position de la cible par rapport au plan médian du champ magnétique,

les figures 5a et 5b montrent des trajectoires d'électrons énergétiques dans une source XRCE, obtenues par modélisation pour une distance entre les aimants de 6,2 cm. L'énergie des électrons est de 36 à 38 keV (figure 5a) et de 40 keV (figure 5b),

les figures 6a, 6b et 6c sont des représentations schématiques de la position d'une cible dans un dispositif selon l'invention,

la figure 7 est un graphe montrant l'évolution de l'intensité et de l'énergie du spectre de rayons X émis par une source XRCE, en fonction de la distance entre les aimants. La courbe inférieure correspond à une distance de 6,3 cm et la courbe supérieure à une distance de 9,1 cm,

la figure 8 est un graphe montrant la variation de l'intensité et de l'énergie du spectre de rayons X émis par une source XRCE, en fonction de la distance entre les aimants,

la figure 9 est un graphe montrant la variation du débit de dose d'une source XRCE avec cible en fonction de la pression, pour une puissance micro-ondes de 200W et une distance entre les aimants de 6,2 cm,

la figure 10 est un graphe montrant la variation du débit de dose d'une source XRCE avec cible en fonction de la puissance micro-ondes, pour une distance entre les aimants de 6,2 cm. La courbe du bas correspond à une pression de 2,3 10^-5 Torr (1 Torr = 133,32 Pa) et la courbe du haut correspond à une pression de 6,2 10^-5 Torr,

la figure 11 est une vue schématique d'une source XRCE selon l'invention,

la figure 12 est un exemple d'image obtenue grâce au dispositif selon l'invention,

la figure 13 est une représentation schématique d'un mode de réalisation de cible utilisable dans le dispositif selon l'invention,

la figure 14 est une vue schématique de dessus d'une variante de réalisation de l'invention.

Other aspects, objects and advantages of the present invention will appear better on reading the following detailed description of a preferred embodiment thereof, given by way of example and made with reference to the accompanying drawings, in which :

FIG. 1 is a schematic view of an XRCE source of a known type,

FIG. 2 is a schematic front view of an XRCE source of a known type,

FIG. 3 is a schematic view of the trajectory of energetic electrons in the middle region of the magnetic field of an XRCE source,

FIG. 4 is a graph showing the variation of the intensity of the X-ray emitted as a function of the position of the target relative to the median plane of the magnetic field,

FIGS. 5a and 5b show trajectories of energetic electrons in an XRCE source, obtained by modeling for a distance between the magnets of 6.2 cm. The energy of the electrons is 36 to 38 keV (Figure 5a) and 40 keV (Figure 5b),

FIGS. 6a, 6b and 6c are schematic representations of the position of a target in a device according to the invention,

Figure 7 is a graph showing the evolution of the intensity and energy of the X-ray spectrum emitted by an XRCE source, as a function of the distance between the magnets. The lower curve corresponds to a distance of 6.3 cm and the upper curve to a distance of 9.1 cm,

FIG. 8 is a graph showing the variation of the intensity and of the energy of the X-ray spectrum emitted by an XRCE source, as a function of the distance between the magnets,

FIG. 9 is a graph showing the variation of the dose rate of an XRCE source with target as a function of the pressure, for a microwave power of 200W and a distance between the magnets of 6.2 cm,

FIG. 10 is a graph showing the variation of the dose rate of an XRCE source with target as a function of the microwave power, for a distance between the magnets of 6.2 cm. The bottom curve corresponds to a pressure of 2.3 10 ^-5 Torr (1 Torr = 133.32 Pa) and the top curve corresponds to a pressure of 6.2 10 ^-5 Torr,

FIG. 11 is a schematic view of an XRCE source according to the invention,

FIG. 12 is an example of an image obtained using the device according to the invention,

FIG. 13 is a schematic representation of an embodiment of a target usable in the device according to the invention,

Figure 14 is a schematic top view of an alternative embodiment of the invention.

It is specified, prior to the following description, that the elements identical or equivalent of the devices described are to the extent of possible indexed in the same way with reference to figures different.

Referring first to Figure 1, we will describe an X-ray source by RCE (XRCE source) of a type already known per se. An aluminum enclosure 10 is hermetically closed at its two ends by a window 20 which can be made of a material such as Teflon (registered trademark) or quartz, and by an aluminum window 30, respectively. The enclosure 10 thus closed by the windows 20 and 30 defines a cavity 40 filled with argon, the pressure of which is maintained at a set value which can be between 10 ^-6 and 10 ^-4 Torr (1 Torr = 133.32 Pa), thanks to a pressure regulator of a type known per se and not shown in the figures. On either side of the cavity 40 are two permanent magnets 50 and 51, identical and coaxial, inducing a magnetic field B of axis Z. This magnetic field is symmetrical with respect to a median plane M containing a main direction of the cavity 40 and having an intersection with said cavity. A microwave source 60 facing the window 20 is capable of injecting through said window microwaves of frequency F typically equal to 2.45 Gigahertz, to excite the electrons contained in the cavity 40. A target 90 in tungsten is carried by a fixed support 80, so as to be in the median plane M. A receiving plate carrying a photosensitive film 100, of a known type and usually used in radiology, is located opposite the aluminum window 30.

The device of Figure 1 is capable of emitting X-rays according to the following known principle: Subject to microwave radiation from from source 60, the electrons become "energetic", that is to say that their energy increases, and each moves along trajectories special. There is thus between the two magnets an area H called confinement, having substantially the shape of a hyperboloid of revolution of axis Z, in which are registered the trajectories of the electrons entering resonance with microwaves and charging with energy. The energy of electrons increases as long as said electrons are contained in this resonance zone, and part of the trajectories of the electrons energetic is therefore included in the resonance zone H, shown in Figure 2. In the presence of the symmetrical magnetic field B, the majority of the electrons are moreover confined in the median plane M, and their trajectories are close to a circle inscribed in said median plane. According to the state of the art, a tungsten target 90 is placed in the part of the median plane M included in said hyperboloid and is thus on the trajectory of energetic electrons.

The X-rays generated by this bombardment of target 90 by the electrons, are directed towards the aluminum window 30 thanks to a adequate orientation of said target 90, as illustrated in FIG. 3 which schematically represents the envelope of the trajectory T of an electron energy in the median plane M. The aluminum window 30 has two functions: on the one hand, it seals the cavity 40 so as to contain argon under low pressure inside said cavity, on the other apart it must also be thin enough to let the x-rays come out of said cavity in order to strike the film 100. It is possible, thanks to a device for this type make images of an object or part of the body exposed between the aluminum window 30 and the film 100.

But as we said, the pictures obtained with this existing device, in which the target is placed in the midplane M, are of resolution low.

The Applicant has discovered that it is possible to increase substantially the density of X-ray radiation emitted by a source XRCE, by shifting the position of the target outside the median plane M of the field magnetic. To highlight this property, the Applicant has varies the position of the target on either side of the median plane, keeping the radius constant (distance between the target and the Z axis). The results of this experiment, which are shown in Figure 4, show that there is an optimal position of the target relative to the median plane.

To explain this phenomenon, the Applicant has modeled the trajectory of energetic electrons in cavity 40. The model used has been validated experimentally by comparing the energy levels of electrons, as shown in the tables below which show the levels d 'maximum energy Emax of the electrons for different values of the spacing D between the magnets of the source, said maximum energies being obtained on the one hand by modeling and on the other hand experimentally:

The modeling of electron trajectories thus makes it possible to observe in FIG. 5a that the trajectory of the electron intersects the median plane M, which corresponds on the figure to the ordinate axis (Z = 0), according to a circle of constant radius equal to 2.5 cm, while outside this median plane the trajectory rays are scattered.

By placing the target 90 in the median plane as illustrated in the FIG. 6a in which the resonance zone H and the envelope E of the traces of the trajectory of an energy electron in the horizontal plane comprising the Z axis are represented schematically, so we are assured to intercept the electrons from the start of their movement in the resonance zone, whereas said electrons will probably only be not very energetic. Now placing the target on the trajectory of the electron outside the median plane as illustrated in figure 6b, one increases the probability of intercepting the electron only after it has made a number of revolutions around the magnetic axis Z, i.e. that he has reached a significant level of energy; therefore we increase the energy of the X-rays emitted. Figure 6c illustrates a position intermediate of the target on the electron trajectory, between the position central of Figure 6a and the marginal position of Figure 6b.

In order to precisely quantify the phenomenon highlighted and to develop a device making it possible in particular to produce a sufficient flow of X-rays whose energy is close to 100 keV for use in medical radiology, the Applicant has identified parameters influencing the operation of an XRCE source, and characterized the influence of these input parameters of the device on the emission of X-rays, which is itself described by two output parameters. The following table summarizes the input and output parameters of the device:

• Il est possible de faire varier l'énergie et l'intensité des rayons X émis en fonction de la distance entre les aimants qui entourent la cavité. En effet, la Demanderesse a procédé à des mesures de spectres de rayons X émis par une source XRCE. Le graphique de la figure 7 montre qu'en écartant symétriquement les aimants sans modifier la position du plan médian M du champ magnétique, on obtient en sortie une intensité de rayonnement X beaucoup plus importante. De plus, le pic d'énergie du spectre émis se déplace alors vers les énergies croissantes, et l'énergie des rayons émis augmente jusqu'à 80 keV. Dans le cas présenté sur la figure 6, le fait d'écarter les aimants de façon à faire passer leur distance de 6.3 à 9.1 cm permet d'émettre une quantité importante de rayons autour de 30 keV, ce qui permet d'envisager des applications pratiques en radiographie des tissus et plus particulièrement en mammographie.
• Il existe pour les rayons X émis une énergie E et une intensité I maximales, pour une valeur donnée de la distance D entre les aimants. Comme exposé sur la figure 8, la Demanderesse a mis en évidence le fait qu'au delà d'une certaine valeur de D (9.6 cm dans le cas de la figure 8), l'énergie et l'intensité du spectre émis diminuaient rapidement. Ce phénomène est dû au changement de configuration de la zone de résonance H, dont l'axe de révolution bascule au-delà de cette distance D, ce qui a pour effet de modifier le régime de fonctionnement de la source XRCE. Ainsi est-il possible dans une source XRCE d'adapter la distance D entre les aimants pour optimiser les caractéristiques du spectre d'émission des rayons X.
• Il existe une valeur de la pression P de l'argon optimale pour chaque point de fonctionnement (Pµ, D), comme indiqué sur la figure 9. La mise en évidence de cette propriété permet d'ajuster au mieux, grâce au régulateur équipant la source XRCE, la pression P en fonction des valeurs de Pµ et de D.
• Il existe, pour chaque valeur de D, une valeur maximale de l'énergie que peuvent avoir les électrons compris dans la zone de résonance H. Cette énergie maximale est très importante car elfe définit la limite supérieure du niveau d'énergie des rayons X émis par la source XRCE, et par conséquent les applications pratiques envisageables. Les résultats présentés dans les tableaux 1 et 2 de la page 8, obtenus par modélisation et validés par des mesures expérimentales, mettent en évidence l'augmentation de cette énergie maximale avec la distance D. Il est ainsi possible de prévoir l'énergie maximale associée à chaque configuration des aimants. Le modèle dont les valeurs du tableau 1 sont issues permet également de calculer la trajectoire des électrons énergétiques dans la zone de résonance H. Les figures 5a et 5b montrent les trajectoires de deux électrons dont les énergies se situent de part et d'autre de la valeur de l'énergie maximale, pour un écartement des aimants de 6.2 cm. Sur la figure 5a, la trajectoire de l'électron est confinée dans la zone de résonance. Par contre, l'électron de la figure 5b a acquis un niveau d'énergie ne lui permettant pas de rester dans la zone de résonance, et la trajectoire de cet électron diverge loin du plan médian M défini par (Z=0).
• Le débit de dose des rayons X émis dépend de la puissance des micro-ondes provenant de la source 60. La figure 10 montre en effet l'augmentation du débit de dose Dd émis par une cible placée dans une source XRCE, en fonction de la puissance Pµ des micro-ondes . Cette propriété est également importante car le débit de dose est un paramètre qui semble limiter les performances des sources XRCE existantes et interdit actuellement leur exploitation opérationnelle. Parmi les paramètres opérationnels, la puissance Pµ des micro-ondes est donc l'un des facteurs permettant d'atteindre le débit de dose désiré.

Following theoretical calculations and practical experiments, the Applicant has thus demonstrated the properties of X-ray emission by RCE, making it possible to develop an XRCE source comprising at least one target outside the median plane M and whose operation is optimized according to the invention. These properties are:

• It is possible to vary the energy and intensity of the X-rays emitted as a function of the distance between the magnets which surround the cavity. Indeed, the Applicant has carried out measurements of X-ray spectra emitted by an XRCE source. The graph in FIG. 7 shows that by symmetrically separating the magnets without modifying the position of the median plane M of the magnetic field, a much higher X-ray intensity is obtained at the output. In addition, the energy peak of the emitted spectrum then moves towards increasing energies, and the energy of the emitted rays increases up to 80 keV. In the case presented in FIG. 6, the fact of spreading the magnets so as to increase their distance from 6.3 to 9.1 cm makes it possible to emit a large quantity of rays around 30 keV, which makes it possible to envisage applications practices in tissue radiography and more particularly in mammography.
• There exists for X-rays emitted a maximum energy E and an intensity I, for a given value of the distance D between the magnets. As shown in FIG. 8, the Applicant has highlighted the fact that beyond a certain value of D (9.6 cm in the case of FIG. 8), the energy and the intensity of the spectrum emitted rapidly decrease . This phenomenon is due to the change in configuration of the resonance zone H, the axis of revolution of which tilts beyond this distance D, which has the effect of modifying the operating regime of the source XRCE. Thus it is possible in an XRCE source to adapt the distance D between the magnets to optimize the characteristics of the X-ray emission spectrum.
• There is an optimal argon pressure P value for each operating point (Pµ, D), as shown in Figure 9. Highlighting this property makes it possible to adjust at best, thanks to the regulator fitted to the source XRCE, the pressure P as a function of the values of Pµ and D.
• There exists, for each value of D, a maximum value of the energy that can have the electrons included in the resonance zone H. This maximum energy is very important because elf defines the upper limit of the energy level of the X-rays emitted by the XRCE source, and therefore the possible practical applications. The results presented in tables 1 and 2 on page 8, obtained by modeling and validated by experimental measurements, show the increase in this maximum energy with the distance D. It is thus possible to predict the maximum associated energy at each configuration of the magnets. The model from which the values in Table 1 are derived also makes it possible to calculate the trajectory of the energetic electrons in the resonance zone H. Figures 5a and 5b show the trajectories of two electrons whose energies are on either side of the maximum energy value, for a spacing of the magnets of 6.2 cm. In FIG. 5a, the trajectory of the electron is confined in the resonance zone. On the other hand, the electron of figure 5b acquired an energy level not allowing it to remain in the zone of resonance, and the trajectory of this electron diverges far from the median plane M defined by (Z = 0).
• The dose rate of the X-rays emitted depends on the power of the microwaves coming from the source 60. FIG. 10 indeed shows the increase in the dose rate Dd emitted by a target placed in an XRCE source, as a function of the microwave power Pµ. This property is also important because the dose rate is a parameter which seems to limit the performance of existing XRCE sources and currently prohibits their operational exploitation. Among the operational parameters, the power Pµ of the microwaves is therefore one of the factors making it possible to achieve the desired dose rate.

It thus appears that it is possible to significantly improve the performance of XRCE sources, on the one hand increasing the energy of X-rays emitted by adjusting the distance D between the magnets which create the magnetic field B, on the other hand the dose rate by adjusting the microwave power Pµ and target position outside the plane median M, and finally adjusting the pressure P inside the cavity.

The characteristics of the device according to the invention are shown schematically in Figure 11. This device takes elements from the known device shown in Figure 1, These elements are referenced the same way as in Figure 1, and they will not be described again in the next paragraph.

The XRCE source according to the invention is provided with a device 70 for three-dimensional displacement of the support 80 of the target 90. Said target, which intercepts the electrons when these are on the part of their trajectory located on the side of the microwave source 60, is according to the invention placed by the displacement device 70 in a plane N parallel to the median plane M and offset by a distance Δz relative to said plane M. La target 90 is in this embodiment of the invention consisting of a bevelled surface at the end of a bar, but may consist of a planar surface element of any geometry, which one controls orientation to direct the beam of X-rays emitted. The device displacement 70 allows the target position to be adjusted with precision on the order of a millimeter or better. Such a device is known per se and does not will not be described in more detail in the present description. It is to highlight that the support 80 of the target 90 is made of a material resistant to the impacts of energetic electrons, such as ceramic. On the other hand, the magnets 50 and 51 are not fixed as in the known device of FIG. 1, but are able to move along the magnetic axis Z, to traverse the segments 500 and 510 respectively. The movement of the magnets can be controlled by a positioning system also known per se, not represented in FIG. 11. The microwave source 60 delivers a radiation whose power is adjustable, for example between 0 and 1000 W. It is thus possible according to the invention to adjust the spacing of the magnets, your microwave power, the target position and finally the value of setpoint of the pressure in the cavity 40, in order to optimize the radiation issued. The device according to the invention therefore allows adjustments to be made to adjust the energy of x-rays on the one hand, and their dose rate on the other go. An example of an x-ray image obtained on film 100 is shown in figure 12.

According to a variant of the invention, the microwave source 60 can emit impulsively towards cavity 40. This impulse control generates electrons in the cavity energetic according to an impulse mode, because the times rise and fall of electron energy are extremely short. We thus generate at the source of the ray pulses X whose durations can typically be of the order of nannoseconds. This variant is therefore particularly advantageous for applications which only require reduced exposure times, such as scintigraphy or fluorometry which require exposure times of the order of millisecond. The pulse emission allows for these applications to minimize effective exposure time, compared to installations existing tubes which have rise times (of "heating" of the cathode) important and which unnecessarily expose the subject, causing risks of carcinogenesis.

According to another variant of the invention, the XRCE source comprises two targets 91 and 92, mounted on the support 80 as shown in the figure 13. This particular configuration provides two points emission of X-rays. By placing, thanks to the displacement device 70 and as shown in Figure 14, the two targets symmetrically with respect to the median plane M, we intercept with the two targets electrons allowing to emit according to the same spectrum. It is then possible to place a subject S between the XRCE source with two targets according to the invention and the photosensitive film 100, on which two images of the subject will form in 110 and 120. These two images correspond to viewing angles different from the same subject, and it is therefore possible to combine these two images to reconstruct a stereoscopic radiographic image.

According to a third variant of the invention, it is possible to replace the magnets with electromagnetic coils. In this case of figure, the coils can be like the magnets mounted on a displacement system for varying the spacing of said coils. In this variant with coils, it is also possible to use two or more targets mounted on the support 80, and to make vary the power supply of the coils to control the position of the median plane M so that the electrons strike selectively and alternately the different targets of the XRCE source. This last variant makes it possible to significantly lengthen the life of the targets.

According to a fourth variant of the invention, it is possible to use to create the magnetic field B from both permanent magnets and at minus a pair of coils in which we slowly vary the current to generate a variable magnetic field superimposed on the fixed magnetic field B generated by permanent magnets. In this regard, it is known that a variation of the magnetic field, if it extends over a time interval typically greater than one millisecond, allows to keep invariant the ratio B / γ, where B is the value of the field magnetic and γ a value directly related to energy. Such a variation therefore constitutes an additional means of increasing the energy of electrons in the XRCE source.

This description uses for illustrative purposes a XRCE source configuration in which the magnetic field is symmetrical and defines a median plane M in which electrons energy are confined. However, the invention is in no way limited to this particular embodiment. It is indeed possible according to the invention to develop an XRCE source in which the magnetic field is not symmetrical; such an XRCE source will also include an area of confinement of energetic electrons comprising a middle region, equivalent to the midplane M used in this description, the geometry of said middle region may be non-planar.

Finally, it is possible according to the invention to use at least two targets fixed on the support 80, said support being animated by the device 70 of a alternative movement of translation along the Z axis, in order to present alternately each of the targets to the impacts of the electrons. This solution also makes it possible to extend the lifespan of targets.

Claims (12)

1. X-ray emission device comprising a microwave source (60), a resonance chamber (10) containing a hermetically sealed volume of gas, a magnetic structure (50, 51) defining a geometrical electron-confinement zone (H) in which electrons move at high speed and at least one target (90, 91, 92) placed in an electron path in order to emit X-rays, characterized in that the target or each one of the targets is offset with respect to a mid-region (M) of the geometrical confinement zone (H).
2. Device according to claim 1, characterized in that the magnetic field is symmetric and the mid-region of the geometrical confinement zone (H) is a plane (M).
3. Device according to claim 1 or 2, characterized in that the magnetic structure comprises at least one pair of permanent magnets (50, 51) placed on either side of the resonance chamber (10).
4. Device according to any one of claims 1 to 3, characterized in that the magnetic structure comprises coils placed on either side of the resonance chamber (10).
5. Device according to claim 4, characterized in that means to vary the intensity of the current in the coils are provided.
6. Device according to claim 5, characterized in that the means to vary the intensity of the current in the coils are capable of varying the intensity slowly enough to enable the electrons to keep the ratio B/γ almost constant, where B is the value of the magnetic field and γ^-2=1-v²/c², v being the electron speed, c the speed of light and γ the Lorentz relativistic factor.
7. Device according to any one of the preceding claims, characterized in that the device comprises means to alter the configuration of the magnetic structure so as to vary the energy of the X-rays emitted.
8. Device according to any one of the preceding claims, characterized in that the device comprises means (70, 80) to alter the position of the target or targets (90, 91, 92) so as to vary the energy of the X-rays emitted.
9. Device according to any one of the preceding claims, characterized in that the microwave source comprises means to modulate the microwave emission.
10. Device according to any one of the preceding claims, characterized in that the device comprises two targets.
11. Device according to claim 10, characterized in that the positions of the two targets are symmetric with respect to the central region of the geometrical confinement zone (H).
12. Radiography system comprising a device for emitting X-rays according to claim 10 or 11, means (91, 92, 100) to form two X-ray images of the same object (S) from two different angles and means to reconstruct a stereo X-ray image of the said object (S).

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