FR2600204A1 - Device for modulating an X-ray beam and its use in medical imaging for the production of space filters - Google Patents

Abstract

Device for modulating an X-ray beam 2 by changing the thickness of an absorbing screen, characterised in that the screen consists of at least one shutter 1 in the form of a thin absorbent plate which can be deformed in the opening of a window 11 inserted into the beam, the shutter 1 being capable of occupying an open position, of zero inclination, by placing the plane of the plate parallel to the direction of propagation of the beam, and a closed position for completely shutting off the opening and obliging all the rays of the beam to pass through it, and in that it is coupled to control levers for placing the shutter in an open or closed position and changing the attenuation of the rays of the beam by deformation of the plate when the shutter is in a closed position. Application: medical imaging.

Description

X-ray beam modulator device and its use
in medical imaging for the production of spatial filters
The present invention relates to an X-ray beam modulator device.

 It applies in particular in the field of medical imaging to the production of spatial filters where, to obtain local modulations of the radiation beam, the attenuation of each pixel of the filter is varied, this attenuation depending directly on the amount of material. absorbent crossing.

 This attenuation is obtained in known modulating devices by circulating a flow of absorbent material between a reservoir and the pixels of the filter. The pixels are filled either by keeping the filter permanently in its normal operating position in front of the beam, or even after having moved the filter from its normal operating position to a position away from the beam.

 The circuits housing the material movements of the first arrangement create defects on the image, and the second arrangement which requires replacing the filter in front of the beam once it is composed, requires precise positioning mechanisms and disconnectable material intake circuits.

 The object of the invention is to overcome the aforementioned drawbacks.

 To this end, the subject of the invention is a device for modulating an X-ray beam by varying the thickness of the eyelash, an absorbent screen, characterized in that the screen consists of at least one flap having the shape of thin absorbent plate deformable in the opening of a window interposed in the beam, the flap being able to occupy an open position, of zero inclination, by placing the plane of the plate parallel to the direction of propagation of the beam, and a closed position to completely obstruct the opening and force all the rays of the beam to pass through it and in that it is coupled to control means for placing the shutter in the open or closed position and varying the attenuation of the rays of the beam by deformation of the plate when the shutter is in the closed position.

 The main advantage of the invention is that it makes it possible to carry out global modulations on all of the rays of a beam as well as local modulations on elementary portions of the latter. In particular in the latter case, the device according to the invention can be advantageously used to produce spatial X-ray filters with several individually controllable flaps. It also has the advantages that eUe allows, in the field of medical imaging, the production of spatial X-ray filters which can remain continuously, without risk of notable deterioration, in the beam in course of composition of the filter, and which 'it allows modifications to the composition of the filter in very short times compared to the times required by the prior devices to effect these modifications.

Other characteristics and advantages of the invention will become apparent from the following description given with reference to the appended drawings which represent:
- Figure 1, the principle implemented by the invention to produce a beam modulator device.

 - Figure 2 an application of the modulation principle shown in Figure 1 to the realization of a modulator allowing a global modulation of all the rays of a beam.

 - The figures. 3A and 3B of the flaps of the device according to the invention in the open and closed position inside an opening.

 - Figures 4A, 4B and 4C of the embodiments with one or more components making it possible to obtain modulations with different attenuation levels.

 - Figure 5 a method of addressing a matrix organization of components according to the invention.

 - Figure 6 a principle of organization of a device according to the invention comprising several matrix planes.

 - Figure 7 the addressing principle of 6 aligned flaps allowing 26 levels of attenuation to be obtained.

 - Figures 8A to 8D a configuration of flaps for obtaining a memory effect.

 - Figure 9 a shutter surface engraved in the form of a comb making it possible to limit deformations when the shutter is polarized.

 - Figures 10A to 10D a flap structure for addressing it in a row and in a column in a matrix.

 - Figure 11 an alternative embodiment of a matrix component.

 - Figures 12A to 12C a structure of a parallel bimorph flap allowing matrix addressing on two faces.

 - Figures 13A and 13B a structure of a series bimorph flap allowing matrix addressing on two faces.

 - Figures 14A and 14B of the flaps cut from strips of piezoelectric material covered with electrodes.

 - Figure 15 an assembly mode of the bands of Figures 14A and 14B to achieve a matrix plane.

 - Figures 16A and 16B another embodiment of shutters cut from strips.

 - Figure 17 an assembly mode of the bands of Figures 16A and 16B.

- Figures 18A and 18B an embodiment of a flap hinged on two edges
- Figures -19A and 19B of the flaps of the type shown in Figures 18A and 18B cut in strips.

 - Figure 20 a sectional view of an embodiment of a hinged flap on these two edges.

 - Figure 21 an embodiment of a matrix of flaps articulated on two edges using a continuous strip in which the flaps are cut.

 - Figure 22 an embodiment sectional view of a double flap.

The beam modulator which is represented according to the schematic diagram of FIG. 1 consists of a plate or shutter 1 immersed in a beam of radiation 2 and inclined by a variable angle G relative to the direction L of propagation of this- this. The plate 1 has a first face 3 which is exposed to the rays of the incident radiation beam 2 and a second face 4 parallel to the first face 3 through which the attenuated radiation beam emerges at 5 according to the relationship I = Io exp sing (I )
where p denotes the attenuation coefficient of the material constituting the plate 1, e is the thickness of the plate, and lo is the intensity of the beam 2.

According to this principle, a 7u thick gold plate immersed in an X-ray beam, provides II ~ 95% attenuation for G = 1
o degree and attenuation 11 ~ 5% when Q = 72.8
10 !, 8 degrees.

 When the cross section of the X-ray beam is large, the device which has just been described can still be used although it involves at grazing angles a long length of flap.

However, in this case it will still be possible to fragment the flap by folding it several times on itself to form several elementary flaps 61 ... 6n, of roughly rectangular dimensions as shown in Figure 2. The flap is then free to deform in the direction of the folds, for example, along two slides 7 and 8 in front of the radiation beam 9 emitted by a source 10. By compressing the folds 61
... 6n of the shutter 1 along the slides 7 and 8, these approach each other which increases the thickness of material traversed by the radiation beam 9, while stretching the ends of the shutter 1 in a reverse movement, the folds move apart, which decreases the thickness of material traversed by the radiation. In this movement, modulation is obtained by moving back and forth from the elementary flaps 61 to 6n along the slides 7 and 8.

 An embodiment of a shutter operating according to the principle of the plate of FIG. 1 is illustrated in FIGS. 3A and 3B which represent a shutter 1 in the respectively open (fig. 3A) and closed (fig. 3B) positions. inside a conduit or window 11 inside which the radiation beam 2 passes. The shutter 1 is constituted in the manner of a piezoelectric bimorph by two layers 12 and 13 of PVF2 polarized at 1800 and glued together on a strip absorbent gold 14, PVF2 denoting a polyvinylidene fluoride of formula (CH2 - CF2) n. The layers 12 and 13 are bonded so that under the action of an electric field, the layer 12 is restricted and the layer 13 expands. When a sufficient difference in electrical potential + V is applied between the outer faces of the two layers 12 and 13, this causes the shutter 1 to move until the duct 11 closes. This arrangement is taken advantage of by the invention for modulating each pixel of a spatial X-ray filter, because it constitutes an excellent engine which is both compact and transparent to radiation.

 However, for the application to the spatial filtering targeted by the invention, it is desirable that the flap shown in FIGS. 3A and 3B cannot take intermediate inclinations between the open position, where it only has its thickness, and the closed position where it is present over the entire surface of the pixel. Otherwise, due to the absence of X-ray scattering and diffraction, a lamellar structure, similar to that of the shade of a blind, would be visible on the image, for all the intermediate positions. In other words, the device which has just been described cannot be effectively used to modulate the transmission of a pixel by modulating the size of its transparent surface.

When several gray levels are necessary, the duration of the closed position of the flap must be modulated over the entire surface of the pixel, or the flap must be given a variable inclination in the closed position as shown in FIGS. 4A and 4B, or cascade several flaps 11 ... 13 operating by all or nothing as shown in Figure 4C, with different nominal modulations due to their thickness or their inclination.

By means of a suitable layout of the polarization electrodes of the piezoelectric bimorphs, the shutter structures which have just been described have the advantage of being well suited to realizations of the matrix type where the shutters are located at the crossings for example, of N1 lines and N2 columns as shown in FIG. 5, the N1 rows and the N2 columns which form in FIG. 5 a matrix referenced 15 being addressed by address decoders 16 and 17. By stacking several matrix planes one can then obtain a ray absorber of the type which is represented in FIG. 6, comprising for example a set of m matrix planes of N1 rows and N2 columns. According to this latter organization the m flaps located in each of the planes at the crossing of the same line and d 'the same column are placed one behind the other with respect to the incident X-ray beam and their control can be obtained as shown in Figure 7 using a converter analog-digital issuer providing from a voltage c a voltage gradient Ui = a. with each component, i being on the
i jVc figure 7 an integer between 1 and 7.

 To make this type of organization feasible, it is necessary to conform the upper electrode of the piezoelectric bimetallic strip of each of the flaps to feed it indifferently by line excitation and column excitation and to obtain a behavior of the bimetallic strip such that when only the line excitation or column excitation is present the bimetallic strip deforms very little, that is to say insufficiently to take the closed position and that when the two excitations are present simultaneously, the flap tilts to its closed position where it comes into contact with a pad ensuring the maintenance in the closed position once the row and column excitations disappear.

 Several embodiments of shutters shaped according to these principles are shown in FIGS. 8A to 13B where elements similar to those illustrated in FIG. 1 are represented with the same references. FIG. 8A represents a flap 1 in profile view and FIG. 8B the same flap I in top view. The shutter 1 is fixed by one end 1a to the wall of the tube 11 and its opposite end is free to move between a retaining stud 18 and the wall of the tube 11 on which the end 1 is fixed.

at
Two conductive electrodes 19 and 20 are etched on the outer face of the piezoelectric layer 12 not in contact with the layer 13 along the line shown in FIG. 8B. The electrode 19 in the form of
U occupies by its central bar the free end of the shutter and its two lateral bars are adjacent to the lateral edges of the shutter. The electrode 20 is shaped like a T, the rod of the T being placed in the middle of the outer face of the layer 12 between the two side bars of the U formed by the electrode 19 and its arms occupy the end not free of the flap. To be brought into the position of FIG. 8A, the flap is biased between the electrode 20 and the gold layer 14 by a control voltage V, in this movement the electrode 19 comes into contact with the stud 18 which applies a suitable voltage to the electrode 19 to maintain the shutter in the closed position when the voltage V is zero.

 Given the spatial location of the electric field on the electrodes 19 and 20, the flap 1 can deform in the section planes aa 'and bb' as shown in FIGS. 8C and 8D.

 These deformations may possibly be limited by shaping the electrodes 19 and 20 in the form of a comb and by intersecting them according for example to the representation in FIG. 9.

 The shutter structures which have just been described solve in a simple and effective manner the problem of the memory effect, they however lead to somewhat complicated embodiments of the shutter control circuits of a matrix of shutters which do not can be controlled only individually, since there can only be one control voltage per shutter for the configurations described.

 This problem is solved by configuring the flaps in one of the ways shown in FIGS. 10A to 10D; 11; 12A to 12C, and 13A, 13B.

These configurations allow both to obtain a memory effect and sufficient shutter movement only when it is addressed along the line and column to which it belongs in the matrix organization.

 In FIGS. 10A to 10D, for example, the piezoelectric layer 12 is covered by two separate electrodes 19 and 20 to which two polarization potentials are applied leading, as a function of the polarities thereof, to positions of the shutter 1 different from the type of those illustrated in FIGS. 1QB to lOD, the "closed shutter" position represented in FIG. lOD being obtained when the two electrodes 19 and 20 are brought to the same potential + V.

 We note in FIG. 11 that an equivalent result can be obtained by depositing on the layer 12 two electrodes 19 and 20 serving as column and holding electrode and a line electrode 21 intermediate between the electrodes 19 and 20.

 It can also be noted that the addressing can also take place by polarizing the electrodes of the flaps on their two faces as shown for example in FIGS. 12A to 12C, on the one hand, and in FIGS. 13A, 13B, on the other hand . In the side view and in section of FIG. 12A, an electrode 22a is interposed between the layers 12 and 13 and an electrode 22b is deposited below the flap, on the absorbent part 14 to allow excitation of the flap under two different voltages V1 and V2. The asymmetry of the line electrodes, in the top view of the shutter in FIG. 12B, and of the column electrodes, in the bottom view of the shutter in FIG. 12C, reduces the deformation of the closed shutter when the voltage comes from the memory electrode 19.

In FIGS. 13A and 13B, the column excitation is applied directly to the absorbent layer 14 and the line excitation is applied to a T-shaped electrode 20 deposited on the layer 12. These two electrodes are each polarized as a function of the position of the component sought by two tensions
Naturally the adoption of a polarization system among those described above is a question of choice which can only be defined according to the applications according to the topology of the modulator device selected.

 To facilitate manufacture, the flaps belonging to the same column of a matrix can be cut from a strip 23 of PVF2 comprising notches 24 perpendicular to two opposite opposite edges 25 and 26 of the strip, as shown in FIGS. 14A and 14B. Each component in these figures is delimited by the space between two successive notches on the same edge. The strips 23 are metallized on each of their faces to conform the polarization electrodes of each of the flaps. For example, in FIG. 14A the face of the strip 23 shown is covered in its middle part by two metallized lines 27 and 28 parallel to the direction of the edges 25 and 26, to which perpendicular electrodes 20 branch and in FIG. 14B which represents the opposite face of the strip 23, metal lines 29 are deposited in the longitudinal direction of the flaps approximately in half. type of embodiment, where lines 27 and 28 can be considered as forming the column electrodes of the shutters and lines 29 as forming the line electrodes, appears more particularly suitable for forming the matrix planes of a beam modulator of the type shown in Figure 15 which shows two strips 23 folded in a U shape on themselves, their line electrodes 29 being in contact respectively with a line co nductrice 30 of the plane of the matrix and their column electrodes being in contact respectively with a conductive column 31 of the plane of the matrix. Conductive holding bars 32, having the same holding function as the studs 18 described above, are arranged in parallel to the columns 31 at a sufficient distance in front of a branch of the U formed by the strip 23 so that each flap of a branch, when in the open position, can apply its electrode 19 against the retaining bar 32 opposite. The branches of each U formed in a strip 23 enclose an insulating separating plate 33. In this arrangement, the plane of the matrix of flaps thus formed is placed perpendicular to the incident X-rays, these can thus be obscured by any flap when this is controlled in the closed position by the voltages applied to both the conductive line and the column with which its line 29 and column 20 electrodes are in contact.

 Another embodiment of a matrix of shutters is described below with the aid of FIGS. 16A, 16B and 17. This embodiment also uses strips 23 of PVF2 from which these shutters are cut.

Contrary to FIGS. 14A and 14B, the absorbent electrode in gold 14 is deposited not on the face on which the column electrodes rest but on the other face where the line electrodes are engraved and the storage electrodes are no longer engraved as previously on the face comprising the row electrodes but conversely on the opposite face comprising the column electrodes. This arrangement implies, as shown in FIG. 17 where the homologous elements of FIG. 15 are marked with the same references, a different mounting of the strips 23 which are no longer folded as in FIG. 15 on either side of a separating partition 33 between two storage bars 32. In a different way, the branches of the U rest, when the flaps are in the open position, against support partitions 34 arranged perpendicular to the plane of the lines 30 and each storage bar 32 is located at the inside of a U approximately halfway between the two opposite branches, so that in the closed position, each of the memory electrodes 19 of a flap of one of the two branches can be applied against the memory bar 32 facing the two branches. As in the example in FIG. 17, the absorbent electrodes 14 protrude from the piezoelectric layer 13, the partition 34 has grooves 35 in which the absorbent electrodes 14 of the flaps 1 engage in the open position.

 An alternative to the systems described above consists in making the flaps of FIGS. 3A and 3B work, for example, no longer in simple bending in the manner of a beam maintained embedded at one of its ends, but in buckling. This gives a flap which has - the shapes shown in Figures 18A and 18B.

 According to the two configurations shown, the flap can deform under the action of the bias voltages applied to its electrodes with a deflection sufficient to apply its memory electrode 19 against the memory pad 18.

 The fact that the flap is articulated at its two ends makes it possible to envisage manufacturing on a single support band the flaps of the matrix. In FIGS. 19A and 19B the flaps are delimited by horizontal slots regularly spaced along horizontal lines of the support 23 by forming several columns in the vertical direction. The face of the support 23 shown in FIG. 19A supports the gold absorbing electrodes 14 of each flap spaced apart from one another on the same line by the line polarization electrodes 29. The opposite face of the support shown in FIG. 19B supports the column polarization electrodes of each of the flaps.

 By gluing two strips. 23, 23a and 23b on the side which is represented in FIG. 19A so that the line electrodes are in contact, the embodiment seen in section in FIG. 20 is obtained and when the line electrodes are on the side absorbent there is interposed at 37 either, a network of conductive wires or, - a plate metallized by strips. An exemplary embodiment of a corresponding flap matrix is illustrated in FIG. 21. In this figure, the strips 23a and 23b glued to each other by their line electrodes, are shaped according to a serpentine whose alternating loops oscillate. on either side of a plane passing through all of the components. In this configuration, the support bars 32 are placed in each loop at an intermediate distance between the two flap columns placed at the intersection of the loop and the plane passing through all of the flaps. To tension the two strips 23a and 23b an insulating pad as shown at 36 in FIG. 21 can be inserted in the rounding of each loop, the part opposite to the rounding being covered with a metal electrode 37 playing the role of grab bar 32.

 It will also be noted that according to another alternative embodiment, the modulator described above can also be modified in the manner shown in FIG. 22, to return to an operation of the flaps of FIGS. 3A and 3B where each flap works on bending and no longer in compression. This can be obtained simply by cutting off each of the flaps at the point where, in FIGS. 18A and 18B, the deflection is maximum.

This last arrangement can be interesting because it offers compared to the previous one the possibility of obtaining two additional levels of attenuation.

Claims (22)

 1. X-ray beam modulator device (2) by varying the thickness of an absorbent screen, characterized in that the screen consists of at least one flap (1) having the shape of a thin absorbent plate deformable in the opening of a window (II) interposed in the beam, the flap (1) being able to occupy an open position, of zero inclination, by placing the plane of the plate parallel to the direction of propagation of the beam and a closed position to completely obstruct the opening and force all the rays of the beam to pass through it and in that it is coupled to control means (16, 17) to place the shutter in the open or closed position and to vary the attenuation of the rays of the beam by deformation of the plate when the shutter is in the closed position.  2. Device according to claim 1, characterized in that the flap (1) is fragmented into several elementary flaps (61 ... 6n), of roughly rectangular dimensions, obtained by successive alternating folding of a flexible sheet of an X-ray absorbing material, each flap forming an elementary flap.  3. Device according to claim 2, characterized in that the flap is free to bend or unfold on slides (7, 8).  4. Device according to claims 1, 2 and 3, characterized in that the screen consists of a matrix of flaps (15) arranged at the intersection of a line and a column.  5. Device characterized in that it comprises several planes (m) of shutter matrix according to claim 4 juxtaposed with each other.  6. Device according to claims 1, 4 and 5, characterized in that each absorbent plate is covered with two juxtaposed lamellae (12, 13) of a piezoelectric material electrically polarizable to allow the deformation of the plate.  7. Device according to claim 6, characterized in that the piezoelectric material is PVF2.  8. Device according to any one of claims 1 to 7, characterized in that each absorbent plate is made of gold.  9. Device according to claims 4 to 9, characterized in that each of the flaps consists of a bimorph piezoelectric material metallized on each of its faces (20, 14) to form two electrodes, one electrode also serving as absorber.  10. Device according to claim 9, characterized in that it comprises means for electrically polarizing the electrodes to control the opening and closing of each of the flaps.  11. Device according to claim 10, characterized in that in the closed position an electrode (19) of a flap is placed in contact with a polarization pad (32) to keep the flap in the closed position when the polarization means cease to polarize the electrodes.  12. Device according to any one of claims 6 to 11, characterized in that the electrodes have the form of crisscrossed combs.  13. Device according to any one of claims 4 to 12, characterized in that the flaps have on one face two metallized parts (19, 20) to allow the addressing in row and in column of each flap in the matrix.  14. Device according to any one of claims 1, 4 to 13, characterized in that the flaps have an approximately rectangular shape and are tiltable around one of their side (la).  15. Device according to claims 4 to 12, characterized in that the flaps are polarized on each of the electrodes (14, 19, 20) covering their two faces to allow the addressing in row and in column of each of the flaps in the matrix .  16. Device according to any one of claims 4 to 15, characterized in that the flaps belonging to two successive columns of the matrix are formed in a PVF2 strip cut on its two parallel edges (25, 26) opposite to form notches (24) of equal lengths and perpendicular to the direction thereof, each flap being delimited by the space between two successive cuts on the same edge.  17. Device according to claim 16, characterized in that the strips are metallized on their two faces (19, 20, 14) to apply the respective polarizations necessary for the selection of each of the shutters by row and column.  18. Device according to claim 17, characterized in that a matrix comprises first conductors (30) arranged in line, and second conductors arranged in columns (31), the metallizations (29) of the opposite flaps and etched on the first faces of the strips being placed in contact respectively with a conductor (30) of a line and the metallizations (20) of the opposite flaps and etched on the second faces of the strips being placed respectively in contact with a conductor of a column (31) ; each strip being folded up in a U shape, the interior space of the U delimited by the second face of a strip being occupied by the conductor (31) of the corresponding column of the matrix, a retaining bar (32) being placed at side of each conductor of a column at a sufficient distance to be able to be brought into contact with the ends (19) of the flaps (1) of two adjacent columns (31) in contact with two different adjacent strips when they are in position closed.  19. Device according to any one of claims 1, 4 to 13, characterized in that the plate constituting a flap is articulated on two opposite end edges.  20. Device according to claim 19, characterized in that the flaps (1) of a matrix are formed by cutting slots arranged in a row and in a column on at least one and the same support strip (23a, 23b), the support strip being shaped according to a coil whose alternating loops oscillate on either side of a plane passing through all of the flaps to form the matrix of flaps.  21. Device according to claim 20, characterized in that holding bars (32) are placed in each loop at an intermediate distance between the columns of flaps located at the intersection of the loop and the plane passing through all of the shutters (1).  22. Use of the device according to any one of claims 1 to 21 for the realization in medical imaging of spatial filters with several individually controllable flaps.