Compton camera system and method for detecting gamma radiation

Abstract

A Compton camera system and method for detecting gamma radiation, comprising a gamma radiation source, at least one fast scintillator plate P1 of which the rise time to peak light is less than 1 ns, having a thickness greater than or equal to 5 mm, equipped with an array of segmented photodetectors (5) and a dedicated fast-reading microelectronic means. The system is characterised in that it is capable of measuring the spatial and temporal coordinates (X, Y, Z, T) and energy E at at least two successive positions of a gamma photon when said photon undergoes Compton scattering at a first point A before being absorbed at a second point B, by recognising circles of non-scattered photons corresponding to each scintillation interaction. The system has a module for estimating a valid Compton event. The detection system has two scintillator plates P1 and P2.

Claims

1. A detection system for detecting gamma radiation, of Compton camera type, comprising: a source of gamma radiation, at least one scintillator plate of a scintillator crystal, emitting photons when exposed to said source of gamma radiation, with a rise time to light peak shorter than 1 ns and having a thickness greater than or equal to 5 mm, an array of segmented photodetectors for detection of said photons, and a dedicated rapid reading microelectronics, for reading the signal emitted by said segmented photodetectors at said detections, wherein the system is configured for measuring the times of said detections with a time resolution smaller than said rise time to light peak, and thereby perform a discrimination, among the detected photons, between the unscattered photons and the scattered photons, based on their times of detections, wherein the system is further configured for: measuring the spatiotemporal coordinates (X, Y, Z, T) and the energy E in at least two successive positions of a gamma photon when said photon undergoes Compton deviation at a first point A before being absorbed at a second point B, by recognizing circles of unscattered photons corresponding to each scintillation interaction, A and B, according to said discrimination, an identification of Compton deviation events, by selecting the couples of detections having a delay between said scintillation interactions A and B which is lower than a predetermined threshold, so as to allow a reconstitution of a Compton path within said at least one scintillator plate.

2. The detection system according to claim 1, wherein said at least one scintillator plate is a single scintillator plate having a thickness greater than or equal to an average free path, within said scintillator crystal, of gamma rays emitted by said source and wherein said identification of Compton deviation events is performed for such events within said single scintillator plate.

3. The detection system according to claim 2, further comprising two photodetector arrays each disposed on an input face and an output face of said scintillator plate, respectively, to improve precision of said reconstitution of the Compton path within the scintillator plate.

4. The detection system according to claim 3, wherein the input face and the output face of the scintillator plate are polished and coupled to the photodetector array by a media of index n less than 1.5 to arrange a total reflection angle.

5. The detection system according to claim 1, wherein the lateral faces and an input face of said scintillator plate which are not coupled to a photodetector array are rough or treated such that the absorption of incident photons or the diffuse reflection of photons are limited.

6. The detection system according to claim 1 wherein the input face of said scintillator plate which is not coupled to a photodetector array is painted black to limit the reflection on said input face.

7. The detection system according to claim 1, wherein the lateral faces and an input face of said scintillator plate, which are not coupled to a photodetector array, are coated with a white reflector with an air gap between this reflector and said scintillator plate.

8. The detection system according to claim 2 wherein said identification of Compton deviation events, by selecting interactions A and B separated by a delay lower than the predetermined threshold, is performed by module of the system identifying at least one first and one second extremum in the distribution of light inside said plate, said second extremum appearing while the difference between the arrival time Ta of photons at A, and the arrival time Tb of photons at B is less than said threshold which corresponds to three times a transfer time Tt of the light in the plate, where Tt=nH/c with H the height of the plate.

9. The detection system according to claim 1, wherein: the system further comprises a second scintillator plate, which has a thickness thinner than the thickness of the first scintillator plate and which is disposed between said source of gamma radiation and the first scintillator plate, such that the gamma rays emitted by said source may undergo Compton deviation at a point A in said second scintillator plate, while said first scintillator plate has a thickness for absorbing at least 50% of the energy of the gamma radiation, said second scintillator plate being separated from said first scintillator plate by a distance D of at least 10 mm, preferably greater than the thickness of the thickest plate, said identification of Compton deviation events, by selecting interactions A and B separated by a delay lower than the predetermined threshold, is performed by a module for estimating valid Compton deviation events, measuring a coincidence trigger between said second scintillator plate and said first scintillator plate in a time window smaller than said threshold which corresponds to the maximum transfer time of light between said second scintillator plate and said first scintillator plate.

10. The detection system according to claim 1, wherein the measuring of the energy is done conventionally by collecting via diffuse reflection a maximum of photons emitted on at least one of the two plates.

11. The detection system according to claim 1, wherein the fragmented photodetector array is of analog SI-PM type associated with an analog ASIC or of digital SI-PM type.

12. The detection system according to claim 1, wherein the scintillator plates and are of lutetium silicate and/or lanthanum halide type.

13. A process for determining spatiotemporal coordinates (X, Y, Z, T) and the energy E in at least two successive positions of a gamma photon having undergone Compton deviation performed in a system according to claim 1 comprising: detection of an arrival time Ta of the unscattered photons emitted by the Compton deviation at a first point A; detection of an arrival time Tb of the unscattered photons emitted at a second point B by the total absorption of the gamma photon; determination of a circle C.sub.A corresponding to the unscattered photons emitted by the Compton deviation of the gamma radiation at point A, the diameter of the circle C.sub.A measuring Xa, Ya and Za; determination of a circle C.sub.B corresponding to the unscattered photons emitted by the total absorption of the gamma photon at point B, the diameter of the circle C.sub.B measuring Xb, Yb and Zb; wherein: when the photons emitted during the Compton scattering at A and the total absorption at B remain in the same light cone of the unscattered photons emitted at A, the angle C<C where C is the Compton deviation and C is the critical angle of total reflection and the circle C.sub.B is included in the circle C.sub.A, the process further comprises: calculation of the diameters of said circles C.sub.A and C.sub.B to measure (Xa, Ya, Za) and (Xb, Yb, Zb); enumeration of the numbers of photons in said registered circles C.sub.A and C.sub.B; definition of the energy of a gamma photon, said energies Ea and Eb being proportional to the number of photons counted inside said circles C.sub.A and C.sub.B; or when the photon having undergone Compton deviation exits from the light cone, C<C, the distance between the points A and B is large and the circles C.sub.A and C.sub.B are distinct from each other, the process further comprises: determining a first event A corresponding to the strongest energy; measuring coordinates (Xa, Ya, Za, Ta) of said event at A and its energy Ea; determining a second event B corresponding to the lowest energy; measuring coordinates of the event (Xb, Yb, Zb, Tb) and its energy Eb; measuring the initial energy of the gamma photon equivalent to the sum of the energies Ea+Eb; determining a Compton angle of deviation by reconstituting the position of the two interactions; deducing the arrival direction of the gamma photon, from the position of the point A (Xa, Ya, Za), the position of the point B (Xb, Yb, Zb) and the energies Ea and Eb; or when the photon having undergone Compton deviation exits from the light cone C<C, the distance between the points A and B is small and the circles C.sub.A and C.sub.B are joined, the process can comprise the following steps: adjusting the distribution of light by an ellipsis of center A, the point B occupies one of the focuses, the semi minor axis corresponds to the radius RA of the circle C.sub.A and the semi major axis corresponds to the distance AB+RB, where RB is the radius of the circle C.sub.B; determining the coordinates Xa, Ya of the point A given by the center of the ellipsis; determining the depth of interaction Za at A which is given by the semi major axis of the ellipsis RA; calculating the time Ta by correcting the times measured with Za; determining the coordinates Xb, Yb of the point B which is given by the focus of the ellipsis; determining the depth of interaction Zb at B which is given by RB calculated from the semi major axis of the ellipsis: by Distance (AB)+RB; calculating the time Tb by correcting the times measured with Zb; measuring the total energy Ea+Eb by integrating the photons over the whole of said ellipsis; measuring the barycenter of the distribution of the photons in the ellipsis; determining the initial point of interaction A or B, said initial point is that which is the closest to the barycenter; determining the Compton angle of deviation C by reconstituting the position of the two interactions at A and at B.

14. The process according to claim 13, wherein, when the circles C.sub.A and C.sub.B are joined and C<C wherein the process further comprises: adjusting the overall distribution of light by a composition of two circles C.sub.A and C.sub.B; determining the coordinates Xa, Ya and Xb, Yb of the interactions at A and at B, respectively, said positions are given by the center of each circle C.sub.A and C.sub.B; determining the depth of the interactions Za and Zb, by determining the diameter of the circles C.sub.A and C.sub.B; measuring the total energy Ea+Eb by integrating the photons over the whole of said composition; determining the barycenter of the overall distribution of light of photons in the composition of two circles; determining the initial point of interaction A or B, said initial point is that which is the closest to the barycenter of the overall distribution of light; determining the Compton angle of deviation C by reconstituting the position of the two interactions at A and at B.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) Other characteristics, details and advantages of the invention will emerge from the following description in reference to the appended figures in which:

(2) FIG. 1 shows the detection system according to the invention in the case of a plate P1 and when the photon having undergone Compton scattering of angle .sub.c remains in the same light cone (.sub.c less than .sub.c);

(3) FIG. 2 shows the detection system according to the invention in the case of a plate P1 when the photon having undergone a Compton scattering of angle .sub.c exits from the light cone (deviation .sub.c greater than .sub.c), existence of two separate circles;

(4) FIG. 3 shows the detection system according to the invention in the event where (deviation .sub.c greater than .sub.c), existence of two joined circles, the distribution of light being adjusted by an ellipsis;

(5) FIG. 4 shows the principle of FIG. 3, existence of two joined circles, the distribution of light being adjusted by a composition of two circles;

(6) FIG. 5 shows the detection system according to the invention with the plate P1 fitted with two photodetector arrays respectively on an input face and an output face;

(7) FIG. 6 shows an embodiment of the system with two scintillator plates P1 and P2.

DETAILED DESCRIPTION

(8) The present invention uses a time camera capable of measuring at the same time the position in space, in time and the energy of each gamma photon. The principle of a time camera is taught in patent applications No. 1260596 FR and No. 1454417 FR by the same applicant.

(9) In this type of time camera, for each scintillation event (photo-electric effect or Compton scattering) a circle corresponding to the unscattered photons which are the first detected is identified.

(10) The unscattered photons are distributed in a cone whereof the apex is the place of interaction (X,Y,Z,T) and whereof the angle of opening is the total reflection angle on the output face.

(11) When the gamma photon undergoes a photoelectric effect, there is one circle only. Hereinbelow the aim is to characterize the position and diameter of a circle and not the barycenter of distribution of light.

(12) If the gamma photon undergoes Compton scattering then a photoelectric effect, there are two circles which appear almost coincident on the plane of the detectors. A good criterion experimentally for validating the presence or not of Compton scattering is that the difference between the times of arrival TaTb of the two maximums of light must be under three times the transfer time of the light in the crystal.

(13) In the case where the gamma photon undergoes Compton deviation at point A (Xa, Ya, Za, Ta, Ea) before being absorbed at point B (Xb, Yb, Zb, Tb, Eb), the following three cases will be considered:

(14) FIG. 1 shows the detection system according to the invention in a first case where the photon undergoes a Compton scattering of angle .sub.c remaining in the same light cone as the unscattered photons. The system comprises a plate P1, fitted with a photodetector array 5 associated with micro-electronic components 6. The plate P1 has a thickness greater than or equal to 10 mm. On this plate P1 the arrival time Ta of unscattered photons emitted by Compton scattering is detected at a first point A; then the arrival time Tb of photons having undergone a total absorption at a second point B is detected; and a circle C.sub.A corresponding to the photons emitted during Compton deviation and a circle C.sub.B corresponding to the photons emitted during complete absorption of the gamma photon are determined. In this first case, the Compton deviation of angle .sub.c is less than the angle .sub.c where .sub.C is the critical angle of total reflection. In this case which is the commonest, the unscattered photons emitted by the interaction all remain in the same light cone, but their distribution can have asymmetry i.e. the circle C.sub.B (corresponding to point B) included in the circle C.sub.A corresponding to point A. However, in the same manner as a photoelectric event, the diameter of the circle C.sub.A measures Xa, Ya and Za. Thus, the diameter of the circle C.sub.B measures Xb, Yb and Zb. Reconstitution of the interaction measures Ta and Tb. Enumeration of the number of photons in the circles C.sub.A and C.sub.B estimates the relative energy deposited at points A and B. In a simplified version of the processing only the biggest circle is considered (circle C.sub.A). It is evident that the Compton effect n.sub.e does not modify the precision of the measuring of spatiotemporal coordinates of the point A. As shown in FIG. 2, the case where the photon having undergone a Compton scattering of angle .sub.c exits from the light cone is also considered. In this case the deviation .sub.c is greater than .sub.c. This gives separate events, i.e., the existence of two circles quasi-simultaneous in time, as compared to the case of the pile-up. Here too, there are two cases: either the two circles are joined (Distance AB<Ra+Rb), or the two circles are separate (Distance AB>Ra+Rb). FIG. 2 shows the case where the two circles C.sub.A and C.sub.B are separate. It is evident that this is Compton scattering if TbTa is adjacent to Distance (AB)/C. In this case each event is processed independently. The first event detected and/or according to the geometry the one corresponding to the most energy deposited is the initial event. The energy of the initial photon is equal to EA+EB. The position of the two points measures .sub.c.

(15) In the case where the circles C.sub.A and C.sub.B are joined, it is evident that this is Compton scattering if TbTa is adjacent to AB/C. In this case, as shown in FIG. 3, the distribution of light can be adjusted by an ellipsis whereof the semi minor axis (b) corresponds to the radius of the first circle RA and semi major axis (a) corresponds to Distance AB+RB. In this configuration, the essential precision in the estimation of coordinates Xa, Ya and Za is retained, despite the Compton scattering. The position of the point A (Xa, Ya) is given by the center of the ellipsis. The depth of interaction at A is given by the semi minor axis of the ellipsis, which calculates Za. Ta can then be calculated by correcting the times measured with Za. So, the position of the point B is given by the focus of the ellipsis (Xb, Yb). The depth of interaction at B is given by the semi major axis of the ellipsis (AB+RB). Given AB, RB is found, which calculates Zb. Tb can then be calculated by correcting the times measured with Zb. The energy E is measured by integrating the photons over the whole of said ellipsis. The total energy EA+EB is obtained. Calculating the barycenter of the distribution of the photons in the ellipsis can find the initial point of interactions A or B which is the closer to the barycenter.

(16) Another method in the event where the two circles C.sub.A and C.sub.B are joined is to adjust the distribution of light by a composition of two circles such as shown in FIG. 4 in dotted lines. In this case the center of each circle gives the position of the respective interactions Xa, Ya at A and Xb, Yb at B. The diameters of the circles C.sub.A and C.sub.A give the depth of interaction Za and Zb. The energy is measured by integrating the photons over the whole of said composition. The initial point of interaction A or B is that which is the closer to the barycenter of the overall distribution of light. Also, the reconstitution of the position of the two interactions estimates the Compton angle of deviation .sub.c. The ratio EB/EA must verify the laws of the Compton scattering.

(17) FIG. 5 shows the same case as FIG. 1, except that the input face 1 and the output face 3 of the plate P1 are covered in photodetectors segmented with a maximum filling density. In this case, the photons which have undergone Compton deviation of angle of deviation .sub.c are absorbed by the photodetector. The circle of the unscattered photons is seen on each side of the plate and exactly the same treatments described previously are carried out. But propagation of the gamma ray of the input face 1 towards the output face 3 introduces asymmetry between the two faces. In a certain way the image on the input face 1 is the inverse of the image on the output face 3.

(18) The advantage of covering each side of the plate P1 of photodetectors 5 is obtaining two independent estimations of the coordinates of each event (X, Y, Z, T, E). Also, comparison of the distributions of light on each side of the plate dispels any ambiguities in reconstitution if the photon has undergone at least one Compton scattering. Also, the number of photons used for reconstruction is doubled and this improves the energy resolution for each photon since the energy resolution grows with the number of photons collected.

(19) The drawback to the configuration hereinabove is in the cost of the micro-electronic components used on each plane of photodetectors. In fact, in this case the price of the electronics doubles. This configuration is interesting essentially on a thick plate and above all in the case of a single-plate Compton camera where the aim is to reconstitute the path of the gamma photon in the crystal. Also, this configuration fails to reject noise whether intrinsic to the scintillator or not. All events are analyzed whether emanating from the source or not.

(20) For photons of known energy (PET) or known arrival direction (SPECT), the counting of events can be simplified. In this case in point, the sole aim is to determine the initial point of impact of the photon (XA, YA, ZA), its arrival time TA and the overall energy of the interaction EA+EB. In this case the double reading is advantageous.

(21) It is evident that in all cases it is possible to correctly measure the position of initial interaction despite the Compton scattering. Also, it is possible to have an estimation of the vector AB between two successive interactions and the energy EA and EB deposited at each interaction. With such a system it is therefore possible to make a Compton camera with a single scintillator plate. A single-plate system will not be optimal in terms of performance, but it will be very advantageous in terms of cost and in terms of efficiency of detection (high percentage of gamma rays fully absorbed by the detector). If the Compton camera is used in a noisy environment such as LSO detector plates having strong intrinsic radiation and a high radiation rate, it is necessary to use a two-plate system. In fact, the time coincidence between detection on the two plates is an excellent way for countering noise. A valid Compton event must be detected almost simultaneously on both plates. The delay between 2 valid events may not be greater than plate transfer time P1 (c/n)+inter-plate transfer time (c)+plate transfer time P2 (c/n) or a time <1 ns.

(22) In another embodiment of the system, to eliminate reflection of gamma rays on the faces not used for detection (lateral faces and input face if there is a single array of detectors only), the latter are treated such that absorption of incident photons is maximum there. In fact, if the photons are reflected onto these faces they sound detection of the circle of the unscattered. The fact of treating all faces not used for detection, faces called sterile, increases the integration time of time images (i.e. having them go from 750 ps to 1500 ps) by 50 to 100% for a given rate of detected unscattered photons (i.e. 90% of photons detected are unscattered). This mode is possible only if the energy is measured by the temporal method, i.e., for energies >250 KeV in the LSO. The faces not coupled to an array of detectors are rough and treated so as to absorb as far as possible the incident radiation to prevent parasite reflections towards the detectors.

(23) This treatment must avoid reflections on the faces called sterile, especially by an step-index. The treatment can comprise an anti-reflective deposit of any known type, followed by deposit of a layer of absorbent material. It can further be constituted by a deposition of a high-index resin (n>1.5) charged with absorbent material.

(24) If the sterile faces are simply painted black conventionally due to the considerable contrast of index between the crystal (n=1.9) and the paint (n<1.5) most of the photons are reflected towards the interior of the crystal.

(25) The faces coupled to detector arrays are preferably polished. Coupling between these faces and the detectors is achieved by a low-index medium (n<1.5) to create a total reflection angle.

(26) For producing a time camera covering a wide energy spectrum (100 KeV-2 MeV) it can be advantageous for at least one of the two plates to measure the energy of photons by the conventional method (white scattering processing).

(27) Simulation of Three Cases of Compton Effect

(28) Simulation conditions are the following:

(29) LaBr3:Ce crystal of thickness 30 mm of index n=1.9 coupling with the photodetector with lubricant (n=1.4). For each image, the position of the photons detected at a given time is indicated:

(30) 200 ps 700 ps 16000 ps (16 ns)
Each image also shows a simulation of what is seen on each segmented photodetector.

(31) Point A of the Compton scattering positioned at Z=5 mm for the three cases: Case No. 1 (alpha<theta) total absorption point B at Z=15 mm. Case No. 2 (alpha>theta) adjacent Compton: total absorption point B at Z=15 mm. Case No. 3 (alpha=pi/2) disjoined Compton: total absorption point at Z=5 mm.

(32) In current photodetectors, detection of photons is subject to threshold effects. If the aim is to dispense with background noise from the detectors (dark counts) it is necessary to detect 1.5 to 2 photoelectrons. Since the integration time Ti is brief, typically under 2 ns, the number of photons to be detected during Ti can be less than the threshold for the peripheral pixels. The integration time is given by the time on completion of which the number of photons detected outside the circle of the unscattered photons passes a certain threshold. The number of photons emitted by interaction in the angular sector of the unscattered photons is constant. The density of photons/pixels depends on the diameter of the circle. The maximum diameter of the circle depends on the thickness of the crystal. On can therefore play on the density of photons/pixels by playing on the thickness of the crystal scintillator. So, the more finely segmented the detector, the more advantageous it could be to use thin crystals.

(33) Also, since the integration time Ti (less than 2 ns) is brief relative to the possibilities of better current electronics, it is advantageous to search for means for counting photons longer. The integration time given by the time on completion of which the number of photons detected outside the disc of the unscattered photons passes a certain threshold (for example 90%), the passing of this threshold depends essentially on the number of photons scattered on the input face 1 of the crystal, or on the lateral faces for the pixels located at less than a thickness of crystal of the edges. Given that for a Compton imager only the unscattered photons can be used for measuring X, Y, Z, T, E, it can be advantageous to eliminate all the other photons.

(34) A known way of achieving this can be to paint the lateral faces and the input face 1 (the faces not used for detection) black to absorb all the photons which exit the crystal. But given that the index of the paint (typically 1.5) is less than the index of the crystal 1.8 to 1.9, most of the photons are reflected by the step-index and will disrupt the signal. A more advantageous way of executing the invention is therefore either to find a black product of index close to that of the scintillator, or to perform anti-reflection treatment by any known means on the lateral faces and the input face 1 of the crystal, then apply a black absorbent deposit to this anti-reflection treatment.

(35) Another way of creating this result can be to deposit on these faces of the crystal a high-index n resin (n>1.5) preferably n greater than 1.7 charged with absorbent particles.

(36) This treatment has the following advantages: considerably decreasing the number of photons detected outside the light cone of the unscattered photons increases the time during which the first photons can be counted for defining the position of the circle.

(37) This system also substantially limits edge effects and therefore exploits the entire detector for imaging.

(38) This anti-reflection treatment can be performed by layers of interferences, photonic crystals or progressive adaptation of index obtained by nanostructuring such as disclosed in European patent application No. 14305365.0 filled on Mar. 13, 2014 Structuration for optimizing the collection of photons in the scintillator crystals and associated technological solutions.

(39) FIG. 6 shows a second embodiment of the system according to the invention comprising two scintillator plates P1 and P2, arrays of photodetector 5 and the associated electronics, not shown, the arrays being stuck to each plate P1 and P2. The plate P1 is finer than the plate P2. On this plate P1 the aim is to obtain Compton scattering at a first point A of coordinates (Xa, Ya, Za, Ta, Ea). The second plate P2 is thicker than the plate P1. The thickness of said second plate P2 absorbs at least 50% of the energy of the gamma ray at a point B of coordinates (Xb, Yb, Zb, Tb, Eb). The second plate P2 is separated from the plate P1 by a distance D of at least 10 mm, preferably 30 mm. The system comprises a module for estimating a valid Compton event. Said module is capable of measuring on the second plate P2 a coincidence trigger Tb in a time window less than 1 ns for identifying the valid Compton events.

(40) Let D be the distance between the 2 plates of the Compton camera, EP1 the thickness of the first plate, EP2 the thickness of the second plate. The maximum transfer time of a photon perpendicular to the detector is:

(41) The time Tmax=EP1*(n/c)+D/c+EP2*(n/c). To simplify, in the case of oblique propagation, T<1.5 Tmax is considered.

(42) The detection times of a Compton event detected on the two plates is to therefore verify: D/c<TBTA<1.5 Tmax. In the case of an LaBr3 system optimized for 511 KeV (EP1=10 mm, D=30 mm, EP2=30 mm). This would give: 100 ps<TBTA<380 ps.

(43) This very strict temporal condition rejects all those events which are not strict Compton scatterings. Such precise time windowing is possible with the electronics developed for temporal cameras and for detectors of digital Si-PM type.

(44) The probability of two coincident events in such a short time (outside Compton) is very low. This windowing therefore enables considerable reduction in the noise of the detectors.

(45) It is therefore evident that the invention enables two types of Compton camera to be made: 1) A single-plate camera having moderated but compact precision and sensitivity and of moderate cost. 2) A highly sensitive multi-plate camera due to rejection of noise by the time windowing, more precise due to better angular definition of the path of the gamma photon, but bulkier and more expensive.

(46) It is clear that in the detection system according to the invention constituted by a single plate P1 or two plates P1 and P2 a good location of each event is maintained in a detector in the event where the photon has undergone a Compton effect and the energy of an event in a detector of time camera type is also measured precisely in the event where the photon has undergone a Compton effect.

(47) Also, an improved Compton camera can be made by combining one or more detectors of time camera type.

(48) Another interest of the system according to the invention is its use in the fields especially of medicine and astronomy. The detection system according to the invention can also be used in the industry for detecting radioactive contamination.

(49) Many combinations can be possible without departing from the scope of the invention; those skilled in the art will select one or the other as a function of economic, ergonomic, dimensional or other restrictions to be respected.