Curved slit rotating collimator

Abstract

Rotating collimator with curved slit Collimation device (5) for an X-ray detection system (1), the collimation device comprising a substantially planar support (51) made of a material with partial or zero radio transparency, the support being intended to be rotated about an axis of rotation (52) perpendicular to the plane in which the support extends, said support being provided with a slit (54) totally transparent to X-rays and passing through the thickness of the support to create a flux of X-rays when the support is exposed to an X-ray source, the shape of the slit in projection in the plane of the support being a curve. Figure for abstract: figure 2

Description

Title of the invention: Rotating collimator with curved slit Technical field of the invention

[0001] The present application relates to the field of X-ray detection systems, and more particularly concerns the field of spatial localization of elements in an X-ray detection system.

[0002] The present application also applies to X-ray imaging, in particular so-called interventional imaging in which the aim is to precisely determine the positioning of a tool at the same time as image acquisition is carried out, for example during a surgical intervention or an operation carried out using this tool. State of the prior art

[0003] Document FR2841118A1 presents an X-ray imaging system comprising a means for determining the positioning of an X-ray device using a target fixed on an object of which an image is acquired. In such a system, the image of the target interferes with that of the object that one wishes to study.

[0004] Document US6490475B1 presents a fluoroscopy system in which the positioning of elements of the system is determined using markers. Digital processing is then carried out to be able to subsequently remove the imprint of these markers from the final image.

[0005] Document US7065393B2 relates to a method for calibrating an X-ray imaging system using inertial sensors arranged on different elements of the system. This type of sensor often experiences a large accumulation of errors, low accuracy and can therefore pose problems with regard to the reliable determination of the position of an element provided with sensors in an X-ray imaging system.

[0006] Document FR3042881A1 presents a device for X-ray imaging comprising a rotating collimator, comprising two slits. Such a device makes it possible to determine the position in a given frame of reference of an element provided with X-ray sensors intended to receive X-ray beams from the slits of the collimator. Such a device nevertheless has a blind zone in which said element is undetectable. In addition, such a device is relatively complex and has limited precision.

[0007] Document EP4010739A1 discloses a rotating collimation device for an X-ray detection system comprising a substantially flat support made of a material with partial or zero radio transparency. The support is rotatable. around an axis of rotation passing through the support and perpendicular to a first face of the support. The support is equipped with a single rectilinear slit, transparent to X-rays, extending at a non-zero distance from the axis of rotation of the support. Such a device also has a blind zone around the axis of rotation. In addition, such a device is also complex and has limited accuracy. In particular, when the element to be detected is far from the axis of rotation of the support, its position is calculated with too great an uncertainty. Presentation of the invention

[0008] The present Application aims in particular to make the device disclosed in document EP4010739A1 more precise.

[0009] More specifically, a first object of the invention is a collimation device making it possible to detect with great precision the position of an object equipped with an X-ray sensor, while limiting the exposure to X-rays of the environment of this object. Summary of the invention

[0010] The invention relates to a collimation device for an X-ray detection system, the collimation device comprising a substantially planar support made of a material with partial or zero radio transparency, the support being intended to be driven in rotation about an axis of rotation perpendicular to the plane in which the support extends, said support being provided with a slot which is totally transparent to X-rays and passes through the thickness of the support to create a flux of X-rays when the support is exposed to an X-ray source, the slot comprising a shape which further satisfies the following properties: * the shape of the slot in projection in the plane of the support is a curve, * the shape of the slot is composed of a first half-curve and a second half-curve, each half-curve having a unique intersection with any virtual circle of radius r centered on the axis of rotation, the radius r being defined in the interval [Rmin, Rmax], where Rmin denotes a minimum radius and Rmax denotes a maximum radius, * the function F defined by F(r) = al(r) - a2(r) is a bijective function, where: - r denotes the distance between a point on the first half-curve or the second half-curve and the axis of rotation of the support, r being defined between the minimum radius Rmin and the maximum radius Rmax, - al(r) denotes the angle in polar coordinates of a point on the first half-curve, this point being positioned at a distance r from the axis of rotation, and - a2(r) denotes the angle in polar coordinates of a point on the second half-curve, this point being positioned at a distance r from the axis of rotation.

[0011] According to one embodiment, the value of the function F(r) is strictly less than the angle Pi, whatever the value of r included in the interval [Rmin, Rmax].

[0012] According to one embodiment, said curve passes through the axis of rotation of the support.

[0013] According to one embodiment, the function F(r) is a strictly increasing function or a strictly decreasing function.

[0014] According to one embodiment, said curve has at least one axis of symmetry.

[0015] According to one embodiment, said curve is configured so that there is a single point of intersection between said curve and an image of said curve obtained by rotation of any angle of said curve around the axis of rotation, said point of intersection being distinct from the axis of rotation, a tangent to said curve at the level of the point of intersection forming with a tangent of said image of the curve by rotation an angle greater than or equal to 45° if the distance between the point of intersection and the axis of rotation is greater than or equal to Rmax / 2, Rmax designating the maximum radius.

[0016] According to one embodiment, said curve is defined at least approximately by one of the following polar functions: * r(a) = lcd 2R / Pi, where: - a denotes the angle in polar coordinates of a point on the curve, a being defined in the interval ] -pi / 2,pi / 2[, and - r(a) denotes the distance between a point on the curve and the axis of rotation of the support, this point being positioned at angle a, *r(a) = R exp(lal - Pi / 2), where: - a denotes the angle in polar coordinates of a point on the curve, a being defined in the interval ] -pi / 2,pi / 2[, and - r(a) denotes the distance between a point on the curve and the axis of rotation of the support, this point being positioned at angle a, * r(a) = (a + Pi / 2) R / Pi, the curve also including a rectilinear part between the points with coordinates (0,0) and (0,R), where: - a denotes the angle in polar coordinates of a point on the curve, a being defined in the interval ]-pi / 2,pi / 2[, and - r(a) denotes the distance between a point on the curve and the axis of rotation of the support, this point being positioned at angle a r(a) = Rmin + - a denotes the angle in polar coordinates of a point on the curve, a being defined in the interval [-pi / 2,pi / 2], - r(a) denotes the distance between a point on the curve and the axis of rotation of the support, this point being positioned at angle a, and - Rmin and ai are coefficients defined so that r(Pi / 2) = Rmax.

[0017] The invention also relates to an X-ray detection system, the detection system comprising: - an X-ray source configured to emit an X-ray beam, - a collimation device as defined above, the collimation device being intended to be exposed to the X-ray beam, - a drive device configured to drive the support of the collimation device in rotation about its axis of rotation, - an X-ray detection element positioned in a field of view opposite the support relative to the X-ray source, the detection element being configured to detect X-rays passing through the slot of the support, and - an angular position sensor adapted to determine the angular position of the collimation device support.

[0018] The detection system may be configured to detect X-rays passing through the slit at two successive times while the support is rotating about its axis of rotation, a first time corresponding to a first angular position of the slit and a second time corresponding to a second angular position of the slit, the first and second angular positions defining in the plane of the support a single point of intersection corresponding to the projected position in the plane of the support of the detection element, the detection system further comprising a processing unit configured to determine the projected position in the plane of the support of the detection element from the first and second angular positions.

[0019] The invention also relates to a method for locating a detection element of an X-ray detection system as defined previously, the method comprising the following steps: - rotation of the collimation device support around its axis of rotation, - exposure of the support to a beam of X-rays from the X-ray source while the support rotates, then - detection by the X-ray detection element passing through the slit of the support at two successive times, a first time corresponding to a first angular position of the slit and a second time corresponding to a second angular position of the slit, - determination by the processing unit of the projected position in the plane of the support of the detection element on the basis of the first and second angular positions. Presentation of figures

[0020] These objects, characteristics and advantages of the present invention will be explained in detail in the following description of a particular embodiment made without limitation in relation to the attached figures among which:

[0021] [Fig.l] is a schematic view of an X-ray detection system according to one embodiment of the invention.

[0022] [Fig.2] is a schematic front view of a support for a collimation device of the detection system of [Fig.l].

[0023] [Fig. 3] is a schematic view of an embodiment of an angular position sensor of the support of the collimation device.

[0024] [Fig.4] is a schematic front view of a second support of the collimation device.

[0025] [Fig.5] is a schematic front view of the support of [Fig.2] at an initial instant of a detection method according to an embodiment of the invention.

[0026] [Fig.6] is a schematic front view of the support of [Fig.2] at a first instant t1 of the detection process.

[0027] [Fig.7] is a schematic front view of the support of [Fig.2] at a second time t2 of the detection process.

[0028] [Fig.8] is a superimposed view of the support at time t1 and of the support at time t2.

[0029] [Fig.9] is a schematic front view of a support for a collimation device according to a second embodiment of the invention.

[0030] [Fig. 10] is a schematic front view of a support of a collimation device according to a third embodiment of the invention.

[0031] [Fig. 11] is a schematic front view of a support of a collimation device according to a fourth embodiment of the invention. Detailed description

[0032] [Fig.l] schematically illustrates an X-ray detection system 1 according to an embodiment of the invention. The detection system 1 is intended to locate an object 2 on which a detection element 3 is fixed. The detection system 1 can for example be used in the context of a medical intervention to locate a tool, for example a catheter, inside the body of a patient. The detection system 1 can thus be integrated into an X-ray medical imaging system. The detection system 1 is intended to provide a location of said object in projection in a plane, that is to say that the detection system 1 is intended to determine the position of said object according to at least two dimensions of space.

[0033] The detection system 1 comprises an X-ray source 4, a collimation device 5, a drive device 6 for the collimation device 5, and a unit information processing 8.

[0034] The X-ray source 4 is configured to emit a beam F of X-rays. The source 4 can be considered as a point source and the beam F can be a diverging beam. The X-ray beam produced by the source 4 is for example of the “cone-beam” type. The beam F is centered on an emission axis. The X-axis is defined as an axis parallel to the emission axis. The X-ray source 4 comprises, for example, an X-ray tube operating at voltages between electrodes lying between 40 and 125 kV.

[0035] The collimation device 5 is intended to be exposed to the X-ray beam generated by the source 4. The collimation device 5 (or collimator) is configured to spatially limit an incident X-ray beam. The collimation device 5 is interposed between the X-ray source 4 and the detection element 3.

[0036] The collimation device 5 comprises a substantially planar support 51 made of a material with partial or zero radio transparency. By “partial radio transparency” is meant a non-zero percentage of attenuation of a flux of mono-energetic photons, for example between 1% and 99%, preferably greater than or equal to 10%, or even greater than or equal to 50%, or even greater than or equal to 80%. This percentage depends on the material used for the support 51 and the thickness of this support. The material of the support 51 may be, for example, chosen from the materials NaI, CsI, CdWO4, BGO, LSO, PreLude® or any other equivalent material. The thickness of the support 51 may be, for example, between 0.5mm and 5mm inclusive. For example, a support made of 0.9 mm thick CdW04 material can attenuate 95% of the flux of a beam of mono-energetic photons with an energy of 100 keV. By "zero radio transparency" we mean total opacity to X-rays.More precisely, by "zero radio transparency" we mean a percentage attenuation of a monoenergetic photon flux of more than 99%.

[0037] The support 51 extends in a plane perpendicular to the axis X. The support 51 is intended to be driven in rotation around an axis of rotation 52 parallel to the axis X. For this purpose, the support 51 is mounted to be mobile in rotation around a shaft 53 aligned with the axis of rotation 52.

[0038] The Y and Z axes are defined as two axes perpendicular to each other and perpendicular to the X axis. The Y and Z axes are fixed in the terrestrial reference frame. The support therefore extends in a plane parallel to the Y and Z axes. The Y' and Z' axes are also defined as two axes in the reference frame of the support 51, perpendicular to each other and perpendicular to the X axis. The Y' and Z' axes are therefore respectively coincident with the Y and Z axes for a given orientation of the support 51 around the rotation axis 52. The angle [3] is defined as the angle formed between the Y and Y' axes (this angle being equal to the angle formed between the Z and Z' axes). The angle [3] is an oriented angle. the origin O as the point of intersection of the Y and Z axes, which also corresponds to the point of intersection of the Y' and Z' axes. The origin O also corresponds to the point of intersection of the axis of rotation 52 with a surface of the support 51.

[0039] Preferably the support 51 has a circular outer contour and the axis of rotation 52 passes through the center of the circular shape. The support is thus well balanced and can be rotated at relatively high speed without generating vibration. Alternatively, the support could take any other two-dimensional shape.

[0040] [Fig. 2] illustrates in front view the support 51 according to a first embodiment. The support 51 is provided with a slit 54 which is totally transparent to X-rays and passes through the entire thickness of the support to create a flux of X-rays when the support is exposed to the X-ray source 4. By "totally transparent to X-rays" is meant that the slit 54 has no absorption or no significant absorption of the X-ray beam. For example, the thickness or the material, or a combination of these two parameters, of the slit defined in the support 51 can be chosen to obtain a slit 54 which is totally transparent to X-rays. This will be the case, for example, if the slit corresponds to an opening in the support (i.e. if it is filled with air).In all cases, the percentage of attenuation of a flow of mono-energetic photons by the slit 54 is strictly less than the percentage of attenuation of a flow of mono-energetic photons by the support 51.

[0041] When the slit 54 is observed in section parallel to the axis of rotation 52 in the thickness of the support 51, the slit 54 may have a rectangular shape or a trapezoidal shape adapted to the divergence of the incident X-ray beam. The width of the slit 54, that is to say the dimension of the slit 54 between two opposite edges of the slit perpendicular to the direction in which the slit 54 extends, may be a constant width throughout the slit 54. Alternatively, the slit 54 could also have a variable width. The width of the slit 54 may possibly be optimized to improve a signal-to-noise ratio when implementing the detection method, and / or to homogenize the distribution of the X-rays.

[0042] The X-ray beam downstream of the collimation device 5 therefore extends along a surface or sheet whose shape is given by the shape of the slit 54. When the support 51 rotates around its axis of rotation 52, the collimated beam at the output of the collimation device 5 scans a part of the space, called the observation field. As we will see later, this scanning allows the determination of the position of the detection element 3 within this observation field.

[0043] The detection element 3 is therefore positioned in the observation field opposite the support 51 relative to the X-ray source 4. The detection element 3 is configured to detect X-rays passing through the slot 54 of the support 51. The detection element 3 is configured to emit a signal to the processing unit information 8 when it is reached by X-rays. For this purpose, the detection element 3 may be based on gallium nitride (GaN).

[0044] The drive device 6 is configured to drive the support 51 of the collimation device 5 in rotation about its axis of rotation 52. The drive device 6 may for example comprise an electric motor. An output shaft 61 of the electric motor may be mechanically connected to the shaft 53 of the support, for example by means of a belt 62 and / or gears. Alternatively, the drive device 6 could also cooperate with an outer periphery of the support to drive it in rotation. Preferably, the drive device 6 is configured to drive the support 51 in rotation with a fixed angular speed, for example an angular speed greater than or equal to 1000 revolutions per minute.

[0045] The detection system 1 further comprises at least one angular position sensor 7, configured to determine the angular position of the support 51 of the collimation device around the axis of rotation 52. The angular position sensor 7 may for example be configured to calculate the value of the angle [3 as defined previously. The angular position sensor 7 may for example be integrated into the collimation device 5, or alternatively be integrated into the drive device 6.

[0046] According to an embodiment illustrated in Figures 3 and 4, the angular position sensor 7 may comprise an optical fork 71 cooperating with a second support 51b extending parallel to the support 51. The second support 51b may be integral or form an integral part of the support 51. This second support 51b may comprise a series of N transparent openings or windows in the visible or near infrared ranges, uniformly distributed angularly on a circular periphery centered on a point corresponding to the intersection between the second support 51b and the axis of rotation 52. The N windows are indexed relative to a reference window, which may correspond to a wider (or respectively narrower) window than the other windows. The optical fork 71 is provided with a transmitter 72 and a receiver opposite each other.The transmitter 72 emits a light wave, which, when this light wave passes through one of the windows, is picked up by the receiver. An electrical pulse is produced by the receiver when a window lets the signal from the transmitter 72 pass. The electrical pulse obtained for the reference window will be wider (or respectively narrower) than the electrical pulse received for the other windows if this reference window is wider (or respectively narrower). The electrical pulses emitted by the angular position sensor 7 can be processed by the information processing unit 8 to determine the angular position of the support 51 at any time.

[0047] The information processing unit 8, for example a computer, comprises a memory 81, a microprocessor 82, and a communication interface 83 capable of receive signals supplied by the detection element 3 and by the angular position sensor 7. The memory 81 is a data recording medium on which is recorded a computer program comprising program code instructions for implementing a detection method according to one embodiment of the invention. The microprocessor 82 is capable of executing this computer program. The information processing unit 8 is therefore configured to implement a method for locating the detection element 3.

[0048] The shape of the slit 54 of the collimation device 5 is now described in more detail with reference to [Fig. 2]. The shape of the slit in projection in the plane of the support follows a curve Cx. Since the slit 54 has a given width, it can be defined by convention that the curve Cx corresponds to the median line extending equidistant from the two opposite edges of the slit 54. By “curved” shape, we mean a non-rectilinear shape. The curve Cx therefore has at least locally a radius of curvature less than or equal to a predefined value. In addition, it is therefore possible to find at least two points on the curve Cx whose tangents are not parallel to each other. The curve Cx may nevertheless comprise one or more rectilinear portions. The curve Cx may in particular be described by a function expressed in polar coordinates. This function is advantageously a continuous and differentiable function. The derivative of this function may itself be a continuous function.

[0049] The curve Cx can be decomposed into a first half-curve Cl and a second half-curve C2. Each half-curve Cl and C2 is defined between a minimum radius Rmin and a maximum radius Rmax. The radius Rmax corresponds to the maximum radius of the slot 54. The maximum radius Rmax can correspond to the radius of a functional surface of the support 51. The minimum radius Rmin corresponds to the minimum radius of the slot 54. The minimum radius Rmin can be equal to 0, in this case each half-curve Cl, C2 joins the origin O. According to the example illustrated in [Fig.2], the two half-curves Cl and C2 are symmetrical with respect to each other along the Y' axis. The two half-curves join only at the origin O. Neither of the two half-curves includes an inflection point.

[0050] The curve Cx also comprises certain geometric properties which can be described as follows. According to a first property of the curve Cx, each half-curve C1 and C2 has a unique intersection with any virtual circle of radius r centered on the origin O (i.e. centered on the axis of rotation 52 since the axis of rotation 52 passes through the origin O), the radius r of the virtual circle being defined in the interval [Rmin, Rmax]. Such a virtual circle CV is represented by dotted lines in [Fig. 2]. In other words, each point of a half-curve comprises a unique distance from the origin O. This first property makes it possible to avoid significant ambiguities in the implementation of a method for locating the detection element 3 at means of the collimation device 5.

[0051] Then we can define a mathematical function F in the following way: F(r) = al(r) - a2(r), where: - r denotes the distance between a point on the first half-curve or the second half-curve and the axis of rotation of the support, r being defined between the minimum radius Rmin and the maximum radius Rmax, - al(r) denotes the angle in polar coordinates of a point PI of the first half-curve, this point being positioned at a distance r from the axis of rotation, and - a2(r) denotes the angle in polar coordinates of a point P2 of the second half-curve, this point being positioned at a distance r from the axis of rotation. According to a second property of the curve Cx, the function F is a bijective function, that is to say that each element of the image set of the function F has a unique antecedent in the definition set of the function F. In particular, this bijective function can for example be a strictly increasing function or a strictly decreasing function. For example according to the embodiment illustrated in [Fig.2], the function F increases from 0 to Pi over the interval [0; Rmax]. This second property makes it possible to determine the distance between the projection of the detection means 3 in the plane of the support and the axis of rotation.

[0052] It has been observed that the choice of a curved shape respecting the first property and the second property stated above allows the implementation of a method for detecting the position of the detection element 3 that is very precise compared to the use of a support equipped with a rectilinear slot. Different shapes of curves respecting these properties can be envisaged and various examples will be presented subsequently. It can be determined mathematically that the positioning error obtained with a support equipped with a rectilinear slot increases linearly as a function of the distance between the projection of the detection element in the plane of the support and the origin O as long as this distance is less than or equal to "^2xD, then as the square of this distance as soon as this distance is greater than or equal to "^2xD, D designating the distance between the rectilinear slot and the origin O.Furthermore, this error is all the more significant as the surface area of ​​the central blind zone is small. Thus, with a support according to the state of the art, one is obliged to make a compromise between the surface area of ​​the central blind zone (which one wishes to be as small as possible) and the positioning error that one undergoes (which one also wishes to be as small as possible). On the contrary, by using a curved slot as defined above, such a compromise is no longer necessary. In particular, one can obtain a support without a central blind zone and a positioning error which, in the vicinity of the origin O, is minimal and increases only proportionally to the distance between the projection of the detection element in the plane of the support and the origin O, which corresponds to a . lowest possible positioning error.

[0053] Furthermore, when the curve passes through the rotation axis 52, i.e. through the origin O, the detection system is free of blind zone around the rotation axis of the support. Consequently, whatever the position of the observation element in the observation field, the position of the detection element can be determined.

[0054] According to another aspect of the invention, a third mathematical condition can be defined for the curve Cx, consisting of imposing that the function F(r) is strictly less than the angle Pi, whatever the value of r included in the interval [Rmin, Rmax]. In other words, the curve Cx is contained in a hemi-disc, that is to say in a half of the support 51 delimited by a diameter of the support. This condition makes it possible to ensure that, when the curve Cx(tl) is superimposed with a curve Cx(t2) obtained by any rotation of the curve Cx(tl) around the axis of rotation 52 (as can be seen for example in [Fig.8]), a single point of intersection Px is always obtained between the curves Cx(tl) and Cx(t2). This property of the Cx curve thus makes it possible to obtain a unique location of the detection element 3 whatever the position of the detection element in the observation field.Alternatively, in the event that the function F would be greater than or equal to Pi for radius values ​​r included in an interval [RI; R2], the implementation of the detection method could lead to the identification of two potential positions of the detection element 3 if the projection of the detection element in the plane of the support 51 is actually positioned at a distance r from the origin O included in the interval [RI; R2]. In such an eventuality, one of these two positions could then be excluded, for example by relying on previous detected positions of the detection element 3 and / or by relying on plausibility criteria.

[0055] According to another aspect of the invention, a fourth mathematical condition can be defined for the curve Cx, consisting of imposing that the curve C has an axis of symmetry. The use of a curve Cx having an axis of symmetry facilitates the calculations implemented in the execution of the detection method.

[0056] An embodiment of a method for detecting the position of the detection element 3 by means of the detection system 1 is now described, with reference to FIGS. 5 to 8. When implementing the detection method, the detection element 3 is considered to be stationary in the terrestrial frame of reference or mobile at a speed that can be neglected given the speed of execution of the detection method according to the invention.

[0057] [Fig. 5] represents the collimation device 5 at an initial instant t0. It is assumed that the detection element 3 is positioned at an arbitrary point in the observation field. This point comprises a projection 3' in the plane of the support 51 parallel to the direction of propagation of the X-rays. This projection is identified by a cross in [Fig. 5]. The X-ray source 4 is activated and emits X-rays towards the collimation device 5. Only the X-rays incident at the slit 54 propagate in the observation field. The other X-rays are generally blocked by the material with partial or zero radio transparency of the support 51. The projection 3' does not pass through the slit 54. Thus, the detection element 3 receives no X-rays and therefore emits no signal to the information processing unit 8.

[0058] The support 51 is then rotated around its axis of rotation 52. After a time tl, the support 51 has rotated through an angle [31 and has reached the position as illustrated in [Fig. 6]. A first half-slit, corresponding to the first half-curve Cl, intersects the projection 3' of the detection element. The detection element 3 therefore receives X-rays and emits a first signal to the information processing unit 8.The value of the angle [31, at the instant when the information processing unit 8 receives the first signal from the detection element 3, can be determined or approximated by means of the angular position sensor 7. The information processing unit is therefore able to determine the orientation of the slit 54 at the instant tl when the detection element received X-rays. At this instant tl, it can therefore already be established that the projection 3' is located at a point on the curve Cx(tl) as it is oriented at the instant tl.

[0059] Then, the support 51 continues its rotation and the projection 3' leaves the slot 54. The detection element 3 therefore no longer receives any X-rays and therefore no longer emits any signals to the information processing unit 8.

[0060] Then, after a time t2, the support 51a has rotated through an angle [32 and has reached the position as illustrated in [Fig.7]. A second half-slit, corresponding to the first half-curve C2, intersects the projection 3' of the detection element. The detection element 3 again receives X-rays and emits a second signal to the information processing unit 8. The value of the angle [32, at the time when the information processing unit 8 receives the second signal from the detection element 3, can be determined or approximated by means of the angular position sensor 7. The information processing unit is therefore able to determine the orientation of the slit at time t2. At time t2, it can therefore be established that the projection 3' is located at a point on the curve Cx(t2) as it is oriented at time t2.

[0061] [Fig.8] illustrates a superposition of the two curves C at times tl and t2. These two curves, referenced Cx(tl) and Cx(t2), are in contact at only two points: at the origin O and at an intersection point Px. We can therefore deduce that the projection 3' is located either at the origin O or at the intersection point Px. The case where the projection 3' is located exactly at the origin O can be easily identified because in this case, the detection element 3 is constantly exposed to X-rays and therefore produces a continuous signal. If such a continuous signal is not observed by the information processing unit 8, then it can be deduced that the projection 3' of the detection element is located at the intersection point Px. It can thus be established that the detection element 3 is positioned on an axis passing through the X-ray source 4 and through the intersection point Px.

[0062] The resolution of the calculation of the intersection point Px of the two curves Cx(tl) and Cx(t2) respectively at times tl and t2 can be carried out by the information processing unit 8 for example by a graphical method, an empirical method or even by an analytical method. For a resolution by an empirical method, it is possible for example to provide a correspondence table indicating the coordinates of the intersection point Px previously observed as a function of angle values ​​[31 and [32. In the hypothesis of an analytical resolution, the coordinates of the intersection point Px can be calculated by solving equations. In this case, the presence of an axis of symmetry in the curve Cx reduces the complexity of the calculations to be carried out.

[0063] An analytical resolution method is proposed below. The estimator £ of the intersection point Px can be calculated by the formula in which: and are the measurements of the angles [31 and [32 provided by the position sensor AA angular 7, and r(a) is the polar function describing the curve Cx.

[0064] It is observed that the tangents Tal and Ta2, respectively to the curves Cx(tl) and Cx(t2), at the intersection point Px form an angle Ax close to 90°. The curve Cx can advantageously be determined so that the angle Ax is greater than or equal to a given value regardless of the position of the projection 3' on the support 51. This angle Ax is thus preferably greater than or equal to 45°, or even greater than or equal to 60° regardless of the position of the projection 3' on the support 51, or at least when the distance between the projection 3' and the origin O is greater than or equal to a threshold given, for example a threshold equal to Rmax / 2. Thus, even if there is an inaccuracy in measuring the angle [31 and / or [32, this inaccuracy only results in a slight displacement of the intersection point Px. We therefore maintain a high degree of precision in calculating the position of the detection element 3 despite a certain inaccuracy in measuring the angle [31 and / or [32.

[0065] As a note, the curve Cx illustrated in Figures 2 and 5 to 8 can be defined at least approximately by the polar function r(a) = lcd 2Rmax / Pi, where: - a denotes the angle in polar coordinates of a point on the curve, a being defined in the interval ] -pi / 2,pi / 2[, and - r(a) denotes the distance between a point on the curve and the axis of rotation of the support, this point being positioned at angle a. By "approximately" we understand that this definition encompasses any curve which would be inscribed in a surface defined by r(a) plus or minus 10%, or even plus or minus 20%.

[0066] Finally, it is possible to position the detection element 3 while limiting the exposure of the environment of the detection element to X-rays since only the X-rays incident at the level of the slit 54 are likely to irradiate this environment.

[0067] More generally, the polar function defining a curve Cx according to the invention can be defined by a polynomial function of degree n. In this case, the polar function can be written in the following form: r(a) =Rmin+ or' - a denotes the angle in polar coordinates of a point on the curve, a being defined in the interval [-pi / 2,pi / 2], and - r(a) denotes the distance between a point on the curve and the axis of rotation of the support, this point being positioned at angle a - Rmin and ai are coefficients defined so that r(Pi / 2) = Rmax

[0068] Furthermore, in one or more embodiments, the X-ray source 4 may be a pulsed source configured to emit X-rays in emission cycles composed of two modes used alternately, one being an X-ray emission mode, the other being a non-X-ray emission mode. The detection element 3 is then configured to detect X-rays passing through the slit 54 in a manner synchronized with the X-ray emission cycles. In this case, the rotation speed of the support 51 may in particular be chosen so that the support makes one revolution over the duration of the firing of the pulsed source, i.e. the duration of the X-ray emission mode. This emission duration may last from a few milliseconds to a few tens of milliseconds. This makes it possible to further reduce the exposure of the environment to X-rays.

[0069] Furthermore, the information processing unit 8 can also be configured to determine the distance L1 between the detection element 3 and the X-ray source 4. Indeed, by assuming that the X-ray source 4 is divergent, it can be demonstrated that the further the detection element 3 is from the X-ray source 4, the longer the irradiation time of the detection element 3. The distance L1 can in particular be calculated from the formula L1 = Ti. r . V . L2 / L3 , where: - Ti denotes the irradiation time of the detection element 3 (which can be estimated by measuring the duration of the first signal or the second signal), - r denotes the distance between the projected position 3' in the support plane P and the origin O, - V denotes the angular rotation speed of the support (which can be calculated on the basis of the data provided by the angular position sensor 7), - L2 denotes the distance between the X-ray source and the support 51 (given by construction), and - L3 denotes the width of the slot 54 (given by construction). Determining the distance L1 makes it possible to locate the detection element 3 in three dimensions in space.

[0070] [Fig.9] illustrates a second embodiment of the collimation device in which the slit describes a curve Cx2. According to this second embodiment, the curve Cx2 is defined at least approximately by the polar function r(a) = Rmax exp(lal - Pi / 2), where: - a denotes the angle in polar coordinates of a point on the curve, a being defined in the interval ] -pi / 2,pi / 2[, and - r(a) denotes the distance between a point on the curve and the axis of rotation of the support, this point being positioned at angle a. The curve Cx2 is thus defined for values ​​of r between Rmin and Rmax, with Rmin strictly greater than 0. Consequently, the collimation device whose slit 54 is defined by this function comprises a blind zone ZA circumscribed by a dotted circle of radius Rmin.

[0071] [Fig. 10] illustrates a third embodiment of the collimation device in which the slit describes a curve Cx3. According to this third embodiment, the curve Cx3 is defined at least approximately by the polar function r(a) = (a + Pi / 2) Rmax / Pi, the curve further comprising a rectilinear part PR between the coordinate points (0,0) and (0,R), where: - a denotes the angle in polar coordinates of a point on the curve, a being defined in the interval ]-pi / 2,pi / 2[, and - r(a) denotes the distance between a point on the curve and the axis of rotation of the support, this point being positioned at angle a.

[0072] [Fig. 11] illustrates a fourth embodiment of the collimation device in which the slit describes a Cx4 curve having the properties of the invention. Of course, still other examples of curves respecting the properties mentioned above could still be proposed.

[0073] Generally speaking, one can advantageously choose a curve Cx whose polar function satisfies the following first condition and second condition, the first condition being formulated in the following manner: Rmin < max(max(r(a), r'(a)) < 107?mtu.andthesecondcondition^ œfO.f] formulated as follows: p \°ù: Rmax- ^nm J0maxr'(a)) da <W (Rmax- r(a) is the polar function describing the curve Cx, r'(a) is the derivative of the polar function r(a), Rmin is the minimum slit radius, and Rmax is the maximum radius of the slit. A Cx curve respecting these two conditions makes it possible to achieve a particularly low positioning error of the detection element 3.

Claims

Claims

1. Collimation device (5) for an X-ray detection system (1), the collimation device comprising a substantially planar support (51) made of a material with partial or zero radio transparency, the support being intended to be rotated about an axis of rotation (52) perpendicular to the plane in which the support extends, said support being provided with a slot (54) totally transparent to X-rays and passing through the thickness of the support to create a flux of X-rays when the support is exposed to an X-ray source, characterized in that: • the shape of the slot in projection in the plane of the support is a curve (Cx), • the shape of the slot is composed of a first half-curve (Cl) and a second half-curve (C2), each half-curve having a unique intersection with any virtual circle (CV) of radius r centered on the axis of rotation, the radius r being defined in the interval [Rmin, Rmax],where Rmin denotes a minimum radius and Rmax denotes a maximum radius, • the function F defined by F(r) = al(r) - a2(r) is a bijective function, where: - r denotes the distance between a point of the first half-curve or the second half-curve and the axis of rotation of the support, r being defined between the minimum radius Rmin and the maximum radius Rmax, - al(r) denotes the angle in polar coordinates of a point (PI) of the first half-curve, this point being positioned at a distance r from the axis of rotation, and - a2(r) denotes the angle in polar coordinates of a point (P2) of the second half-curve, this point being positioned at a distance r from the axis of rotation.,

2. Collimation device (5) according to the preceding claim, characterized in that the value of the function F(r) is strictly less than the angle Pi, whatever the value of r included in the interval [Rmin, Rmax].

3. Collimation device (5) according to one of the preceding claims, characterized in that said curve (Cx) passes through the axis of rotation (52) of the support (51).

4. Collimation device (5) according to one of the preceding claims, characterized in that the function F(r) is a strictly increasing function or a strictly decreasing function.

5. Collimation device (5) according to one of the preceding claims, characterized in that said curve (Cx) has at least one axis of symmetry.

6. Collimation device (5) according to one of the preceding claims, characterized in that said curve (Cx) is configured so that there is a single point of intersection (Px) between said curve (Cx) and an image of said curve obtained by rotation of any angle of said curve around the axis of rotation (52), said point of intersection being distinct from the axis of rotation (52), a tangent (Tal) to said curve at the point of intersection (Px) forming with a tangent (Ta2) of said image of the curve by rotation an angle (Ax) greater than or equal to 45° if the distance between the point of intersection (Px) and the axis of rotation (52) is greater than or equal to Rmax / 2, Rmax designating the maximum radius.

7. Collimation device (5) according to one of the preceding claims, characterized in that said curve (Cx) is defined at least approximately by one of the following polar functions: • r(a) = lcd 2R / Pi, where: - a denotes the angle in polar coordinates of a point of the curve, a being defined in the interval ]-pi / 2,pi / 2[, and - r(a) denotes the distance between a point of the curve and the axis of rotation of the support, this point being positioned at the angle a, • r(a) = R exp(lal - Pi / 2), where: - a denotes the angle in polar coordinates of a point of the curve, a being defined in the interval ]-pi / 2,pi / 2[, and - r(a) denotes the distance between a point of the curve and the axis of rotation of the support, this point being positioned at the angle a, • r(a) = (a + Pi / 2) R / Pi, the curve further comprising a rectilinear part between the points with coordinates (0,0) and (0,R), where: - a denotes the angle in polar coordinates of a point on the curve,a being defined in the interval ]-pi / 2,pi / 2[, and - r(a) denotes the distance between a point of the curve and the axis of, rotation of the support, this point being positioned at the angle ar(a) = Rmin + or: - a denotes the angle in polar coordinates of a point of the curve, a being defined in the interval [-pi / 2,pi / 2], - r(a) denotes the distance between a point of the curve and the axis of rotation of the support, this point being positioned at the angle a, and - Rmin and ai are coefficients defined so that r(Pi / 2) = Rmax.

8. An X-ray detection system (1), the detection system comprising: - an X-ray source (4) configured to emit a beam (F) of X-rays, - a collimation device (5) according to one of the preceding claims, the collimation device being intended to be exposed to the X-ray beam, - a drive device (6) configured to drive the support (51) of the collimation device in rotation about its axis of rotation (52), - an X-ray detection element (3) positioned in an observation field opposite the support (51) relative to the X-ray source, the detection element being configured to detect X-rays passing through the slot (54) of the support, and - an angular position sensor (7) adapted to determine the angular position of the support of the collimation device.

9. Detection system (1) according to the preceding claim, wherein the detection element (3) is configured to detect X-rays passing through the slit (54) at two successive instants while the support (51) is rotating about its axis of rotation (52), a first instant (t1) corresponding to a first angular position ([31) of the slit and a second instant (t2) corresponding to a second angular position ([32) of the slit, the first and second angular positions defining in the plane of the support a single point of intersection (Px) corresponding to the projected position (3') in the plane of the support of the detection element (3), the detection system (1) further comprising a processing unit (8) configured to determine ... detection from the first and second angular positions.

10. Method for locating a detection element (3) of a system of X-ray detection (1) according to the preceding claim, the method comprising the following steps: - rotating the support (51) of the collimation device (5) around its axis of rotation (52), - exposure of the support to a beam of X-rays from the X-ray source (4) during rotation of the support, then - detection by the detection element (3) of X-rays passing through the slit (54) of the support at two successive instants, a first instant (tl) corresponding to a first angular position (|31) of the slit and a second instant (t2) corresponding to a second angular position ([32) of the slit, - determination by the processing unit (8) of the projected position (3') in the plane of the support (51) of the detection element (3) on the basis of the first and second angular positions.

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