EP1984926B1

EP1984926B1 - A radiation detection device comprising two slit plates

Info

Publication number: EP1984926B1
Application number: EP07747308.0A
Authority: EP
Inventors: Frederik Johannes Beekman
Original assignee: Milabs BV
Current assignee: Milabs BV
Priority date: 2006-05-11
Filing date: 2007-05-10
Publication date: 2017-12-27
Anticipated expiration: 2027-05-10
Also published as: WO2007133068A3; WO2007133068A2; NL1031800C2; EP1984926A2

Description

The present invention relates to a radiation detection system, comprising a detector plate that is sensitive to radiation, and an imaging system that is arranged to form an image of an object to be examined on the detector plate and that comprises a first slit plate with at least a first slit, and a second slit plate that is positioned between the first slit plate and the detector plate and with a second slit that makes a non-zero angle with the first slit, wherein the first and second slits together define a pinhole passage.
Such radiation detection systems are often used to examine objects, such as test animals, among which also human beings, that have been treated with radioactive preparations. Therein, a radioactive substance (isotope) is introduced in a test animal, and the test animal is positioned in or in front of a radiation detection system such as the present one. The radioactive radiation, often gamma radiation, is then used to determine the distribution of the radioactive substance in the test animal, from which desired information may be obtained.
From the article 'An analytical algorithm for skew-slit imaging geometry with nonuniform attenuation correction' (Med. Phys. 33 (4), April 2006) there is known a system in which an image of an object, that is made radioactive, is made through two plates. A first one, positioned close to the object, has a first slit, and a plate positioned behind it has three slits, perpendicular to the first slit. As a net result, these slits image the object three times on a detector that is positioned behind the second plate.
A disadvantage of the known system is that it has only a limited efficiency to image objects, or parts thereof, that have different dimensions. In particular with radioactive detection, it is important to have a detection system that is as efficient as possible, since it is desirable to have a radiation exposure of the object to be examined that is as small as possible.
An object of the present invention Is to provide a more efficient detection system.
Another object of the invention is to provide a more flexible detection system.
At least a part of the above objects is achieved with the detection system according to claim 1.
With the radiation detection system according to the invention, it is achieved that many more settings of the device for examination with radiation become possible. Some of the possibilities, elucidated hereinbelow, are different "zoom" settings, whereby smaller and bigger objects, or smaller and bigger parts of one object, may be examined with one and the same device. Furthermore, the sensitivity of the device may also be adjusted in a simple way. This offers big advantages if the object may only be subjected to a small radiation exposure, or if the resolution need not be high but measuring speed is important. With the present device, use is made of the Insight that the slits and their properties, such as position and width, determine the properties of the radiation detection system as a whole.
Here it is noted that with 'slit', there is intended here a slit having a width of an ordinary pinhole passage, i.e. between 0.05 and 5 mm, and with a length-to-width ratio of the slit of at least 3, but often at least 10 up to 50.
At least one of the slit plates comprises at least two slit
plate parts, that are moveable with respect to each other, such that the width of at least one of the slits in the slit plate is adjustable. This offers an elegant and efficient way to adjust the sensitivity of the device. After all, the sensitivity is approximately proportional to the slit width. Although it holds that the resolution decreases for a larger slit width, in some cases the advantages of a higher sensitivity, such as a shorter measuring time and better possibilities for noise reduction, do outweigh that.
At least one of the first and second slit plates comprises a
slit plate displacement means for adjusting the position of that slit plate with respect to the other slit plate and/or the detector plate and/or the object to be examined. One can contemplate changing the distance, which brings about a kind of "zoom" effect through a different magnification factor. Also, one slit plate may be displaced parallel to itself, so that the location of the pinholes, i.e. the positions of net radiation passage, is changed. This latter may be favourable in order to examine the object under a different angle or to change the field-of-view, such as to make it larger. In general it holds that information from more angles also provides a better reconstruction of the isotope distribution, and hence a better examination result. The slit plate displacement means may incidentally be selected from any of the displacement means known in the art, such as mechanical transmissions, (piezo)electric motors, pneumatic or hydraulic actuators and so on. Furthermore, it is also possible to provide only passive means, such as a guide rail or the like. In case a part of the slit plate displacement means is located in a transmitted radiation beam, it is preferable if that part is to a large extent transmissive to radiation.
In an expedient embodiment, at least one of the slit plates comprises a plurality of slits, and preferably both slit plates comprise a plurality of slits. With a plurality of slits per slit plate also a plurality of pinholes are created, viz. the product of the number of slits in the first and in the second slit plate. This can provide a favourable amount of angular information with a device that is easy to make. For with another category of detection devices, the "true" pinhole camera, a number of tiny holes or pinholes is made in a thick slab of radiation absorbent material. It will be clear that this is technically more difficult to achieve and less flexible than producing a number of slits having a more or less arbitrary length.
In a particular embodiment, at least one of the slit plates comprises a plurality of partial slits having a length that is smaller than the length of the slit plate, as measured in the direction of the partial slit. This provides the possibility to set the width of the slit differently along the length of the slit.
In a further particular embodiment, the partial slits, as seen along said direction, are provided in a staggered fashion. By means of staggered partial slits, it is again possible to increase the number of angles under which the object may be viewed, since, so to say, each pinhole can look at the object at a different angle with respect to the transverse plane. Because of the staggering, a more favourable, efficient distribution of the images onto the detector plate is possible, in order to have as little overlap as possible.
A special embodiment further comprises a framing device, at least comprising a plate part and having at least one framing opening therein that is formed such that the radiation in the direction of the detector plate can only reach a predetermined section of the detector plate. A framing device prevents overlap of different, neighbouring images on the detector plate, which would cause a loss of information. Without a framing device, there may arise overlap since the pinholes, i.e. in this case the slits, mostly transmit radiation at such an angle that the boundaries of the transmitted beams will intersect at a certain distance from the slits.
In an advantageous embodiment, the framing device is located between the second slit plate and the detector plate. Although in principle it is possible to frame between the object and the first or second slit plate, it is advantageous to do that in the said advantageous set-up, because then the slits are located closest to the object, and a maximum magnification factor may be provided.
Preferably, the framing device comprises a plurality of mutually parallel framing strips in a first direction, as well as a plurality of mutually parallel second framing strips, wherein at least one of the first and second framings strips is displaceable with respect to another of the first and second framing strips. With this embodiment, the framing device may easily be adapted to the images that are desired on the detector plate, without having to exchange the framing device.
A special embodiment further comprises framing strip displacement means for displacing the at least one displaceable framing strip. Once again, such framing strip displacement means may in principle be selected freely from active (i.e. motorized) and passive means (i.e. means along which the framing strips may be displaced). In a favourable way, adapting the framing device may be automated thereby.
In a special embodiment, at least one framing strip has an adjustable width. In cases in which the position of the images on the detector plate does not change, but their dimensions do, it is advantageous if the framing device changes along with it. For example, the borders around each framing opening will be made wider for smaller images, in order to shield radiation optimally.
Advantageously, at least one framing strip comprises at least two framing strip parts that are movable with respect to each other. Advantageously, the framing strip parts are displaceable in a direction that is substantially perpendicular to the radiation to be shielded. Herein, the framing strip parts should overlap at least somewhat, at least in a narrow setting of the framing strip. Of course, it is advantageous if the non-overlapping parts by themselves have sufficient radiation shielding properties. However, it is also possible to provide a variable width to the framing strips in a different way, e.g. by making them tiltable. After all, for a non-circular cross-section, the perpendicular projection will change during tilting, and hence the width. In particular an elliptical cross-section offers advantages here, because of the favourable cut-off of the radiation under all circumstances. It is further remarked here that also tiltable slit plate parts may have a width that is adjustable in that way. However, by tilting, the orientation of the edges with respect to the radiation beam also changes. In order to correct that effect, it is possible to provide the framing strip parts and or slit plate parts with upright protrusions, e.g. having a triangular cross-section. On tilting such a strip, the new edge orientation is no longer determined only by the original edge of the strip or the plate part, but in particular by the upright protrusion. For example, the framing strip part may be turned so far that one side of the upright triangle runs parallel to the edge of the radiation beam. That provides a good edge definition. Alternatively, it is possible e.g. to give the framing strip part of the slit plate part an elliptical cross-section. Such a cross-section provides an edge definition that is almost the same in very many positions, while yet the width may be varied widely. It is noted that the edge definition is not completely equivalent to that of a part, the edge of which runs parallel to the edge of the radiation beam.
Advantageously, the at least two framing strip parts are rotatable with respect to each other. Then to that end, there are advantageously provided two axes of rotation. Rotating two framing strip parts has an additional advantage, in that the framing strip parts may be rotated such that the edges of the transmitted radiation beam and the edges of the framing strip parts, that are adjacent to the beam, can be made as parallel as possible, or at least more so. This improves the edge definition.
It is noted here that, both for the slit plates and the framing device and the detector plate, it holds that these may also be composed of a plurality of partial plates, that the plate need not be flat but may also be curved, and so on. Such things will be elucidated here below. It is furthermore remarked here that the displaceability of the parts of the device also has an effect on the projections, i.e. the apparent dimensions, in particular the width. For, a framing device or a slit (plate), or part thereof, will appear bigger and wider in comparison to all other parts that are located "downstream" in the beam, if that framing device etc. is moved towards the object, and vice versa.
In a special embodiment, the first slit plate comprises a plurality of first partial slit plates arranged around an object space. This not only has the advantage that, thereby, more information, such as angular information, may be collected at the same time, without having to turn the object. It is further well possible thereby to restrict radioactive radiation, undesirable exposure to which should be avoided as much as possible. Herein, it is advantageous when at least one of the first or second slit plate, framing device or detector plate extends closed around the object space in at least two dimensions. However, if desired, a separate housing may also be provided, in which one or more slit plates, framing devices and/or detector plates extend at least partly around the object space.
Advantageously, the second slit plate comprises a plurality of second partial slit plates. For this, the same advantages hold as mentioned above.
In particular, the slits in at least one, and preferably both, of the slit plates make a non-zero sharp angle with a direction of a longitudinal axis of the object space. In this way, the net created "pinholes" will make more different angles with the object, therefore allowing to obtain more angular information and a better reconstruction. Herein, the longitudinal axis is the direction parallel to the detectors around the object space, or at least an average position of those detector plates in the case that detector plates have been provided "tilted" with respect to the radiation incident thereon. In case there is provided only one flat detector, the longitudinal axis is thus not always defined, unless the detector rotates around an axis. Then, the axis of rotation is the longitudinal direction. In the other cases, in which there is provided at least a curved detector, or at least two mutually non-parallel detectors, the longitudinal axis is formed by the line parallel to each of those detectors. Herein, it is assumed that the object space has a constant cross-section. A different definition of the direction of the longitudinal axis could then be the direction perpendicular to a plane of (smallest) cross-sectional area. In practice, however, there is often an object space with a certain symmetry, such as a cylindrical or triangular object space. In such a case, the longitudinal axis will be easy to determine. Often, this longitudinal axis is called the z-axis, and often it coincides with an direction for inserting objects. Slits that are parallel to a longitudinal axis may also be provided staggered, or slanted, with the same effect of providing staggered "pinholes".
Advantageously, at least the first slit plate forms part of a cylinder. With a cylinder, it is easily possible to position each slit equidistantly from the object or a part thereof, which ensures an equivalent sensitivity, magnification and resolution. Furthermore, it is then easy to turn the object with respect to the cylinder and gain additional information for a better reconstruction, for the distances remain the same. Therein, the first slit plate may form a complete cylinder or a part thereof. In the latter case, it is also possible to provide a plurality of slit plates, that together form a complete cylinder. The second slit plate may, e.g. be built up as a collection of rings, with the second slits there between. Of course, other arrangements with a cylinder shape as a whole, are possible. Polygons, such as triangles to hexagons, are also possible.
In particular, the first and second slit plates form concentric cylinders. Of course, this gives good conditions for equivalent sensitivity, magnification and resolution.
In a special embodiment, the slits define at least locally a helical shape, in at least one of the, and preferably both, slit plates. This could also be a plurality of parallel helices. For the purpose of the strength of the slit plates, the slits may also be made up of partial slits that are mutually aligned. With the helical shape, it is again possible, in an elegant way, to achieve that the "pinholes" thus created are located at many different angles. Incidentally, it is mainly of importance that the slits lie staggered with respect to each other, as to their angles with the transverse plane, since more angular information becomes available thereby. It is definitely not necessary that, together, the slits form a helix, although when staggering in a regular fashion, this is brought about automatically. In fact it suffices when the slits could also lie slanted with respect to the longitudinal axis of the cylinder.
Advantageously, the slit plate displacement means are arranged to rotate at least one slit plate around said longitudinal axis. Preferably, the slit plate displacement means are arranged to rotate the slit plates around said longitudinal axis over equal angles. If desired, this may take place with equal angular velocities, either in opposed directions or in the same. In the latter case, the "pinholes" describe a circle around the axis of rotation, in the other case a line parallel to the axis of rotation, wherein this embodiment also has the advantage that the "stream of pinholes" never stops, but is constantly replenished, without a return movement having to take place. In particular, the framing device is concentric to the slit plates. In particular, the detector plate is concentric to the slit plates. In both cases, image reconstructions are easier, especially if the object and the parts of the detection system are rotated with respect to each other. Furthermore, the 'depth of interaction' problem is much less of a problem with a radiation incidence that is perpendicular as much as possible, which may be provided for in this way. 'Depth of interaction' is uncertainty about the true position of interaction, and thus of the origin of the quantum of radiation, in the case of radiation that is incident slantingly. Often, however, the detector plate comprises one or more flat plates, because of the conventional production techniques. Therefore, a polygonal detector, such as a detector having a tri- to hexagonal cross-section, is also customary. A curved plate is however not excluded. In a special embodiment, the detector plate, and optionally also the framing device, form a polygon around the second slit plate.
Advantageously, the slits in the first and second slit plates have a waisted cross-section, that is limited by bevels that run from the front and the back of the slit plate, and that limit a beam passage. The true slit, that is to say the narrowest part thereof, is then determined by the bevels on the slit plate parts on both sides of the slit. The passage for the beam is also delimited by the bevels. The beam passage is not only a (solid) angle into which incident radiation is transmitted, but it is also a (solid) angle from which radiation is transmitted. Together, they indicate where an object to be examined may be located with respect to the slit and still can form an image on the detector plate positioned behind it. For most objects, the beam passage will generally be larger than necessary for a momentary measurement, because of the possibilities of displacement that are contemplated in the present invention. However, where necessary, this may be corrected for with the framing device. Therefore, there remains a lot of flexibility in the detection device, and both small and large volumes may be examined.
In a particular embodiment, the slits in the first and second slit plates together define a lattice of "pinhole" passages, each with a beam passage having a main direction of passage, wherein the bevels are shaped such that the beam passages at least partially overlap each other on the longitudinal axis, and wherein preferably at least one direction of passage of each beam passage passes through one and the same point. In that way, a focused detection device is obtained, i.e. many, and of course preferably all, "pinholes" look at the same volume to be examined. Thereby, it is not only possible to obtain much angular information, but also a high sensitivity. For thereby the solid angle at which one looks at the volume is increased.
It is also possible to "defocus" the detection device, which means here that a different part of the beam passage is brought onto the detector(s). This may be achieved e.g. by readjusting the framing device by displacing the framing plates etcetera.
If the slit plates would be very thin compared to the pinhole diameter, there can be no talk of radiation framing or selection of the field-of-view by those slit plates, for in principle the beam passage would always be a semi-sphere. However, because the slit plates have to be rather thick because of the radiation shielding properties, the ratio of pinhole diameter to plate thickness is, on the contrary, small. That causes a rather strong directional effect, i.e. a clear and marked transition between directions transmitted and not transmitted. Without special measures, the various beams transmitted by the "pinholes" would overlap unfavourably, or even hardly or not at all, and especially not only in a point Now, by shaping the bevels of the edges of the slit plates around the slits, according to the invention, such that at least one direction of passage, and preferably the direction of main passage, of at least two, and preferably of all transmitted beams converge, a favourable combination of the amount of angular information and sensitivity is obtained. The bevels can then be produced based on very simple geometry. The direction of main passage is determined herein as the direction of maximum transmitted radiation flux, which direction often, but not always, coincides with a geometrically averaged direction of passage.
It is further noted here that with (very) thin slit plates it is still possible to obtain a focusing action if use is also made of a framing device. After all, that framing device also determines the direction from which radiation reaches the detector plate. In fact, one can say that the direction from an edge of a slit in the slit plates to a corresponding edge of a framing orifice in the framing device then forms the bevel.
In a further favourable embodiment, at least one slit plate comprises a material that is leas transmissive to radiation in an area adjacent the slit. This, too, improves the edge definition. Example of favourable materials are lead, tungsten, gold, platinum, osmium, iridium, and depleted uranium, and so on, wherein, from the point of view of costs, in particular gold and platinum are suitable for the areas adjacent the slit.
At least one of the slit plates, and preferably the first slit
plate, comprises a plurality of slit plate parts, that are displaceable by means of stilt plate part displacement means, in such a way that the width of the slits between the slit plate parts is adjustable. Here, the point is that the displacement of both the relevant slit plate parts takes place in slightly divergent directions, for example each in a radial direction. By making the said slit plate parts displaceable in, e.g., a radial direction, by having the slit plate "expand" as a whole, the slits between the parts can easily be made bigger or smaller. For, with a slit plate that as a whole is cylindrical and having 5 slits of 1 mm between as many slit plate parts, the slit width will increase with 0.1 mm, i.e. 10%, if the diameter of the slit plate cylinder increases with 0.16 mm. Of course, this is easy to achieve with simple (displacement) means such as piezo-electrical actuators.
The invention will be elucidated further with reference to the embodiments described hereinafter and depicted in the drawings. It is stressed here that the embodiments described and shown are only examples, that are not to be construed as limiting the invention. All measures and embodiments indicated to be advantageous, such as moveable and/or subdivided and/or focusing plates, framing plates and detector plates, may be combined with each other, unless explicitly described to the contrary. In the drawing:

Figure 1 shows a diagrammatic detector according to the prior art,
Figure 2 shows a diagrammatic detector.
Figure 3 diagrammatically shows another detection system embodiment.
Figure 4 shows a detection system, that largely corresponds to Figure 3, in a side elevational view.
Figure 5a diagrammatically provides a detection system, in
which a small area is being imaged. Figure 5b provides an adapted detection system in which a large area is being imaged.
Figure 6 shows a slit plate with first through fifth slit plate parts, for use in a detection system according to the present invention.
Figure 7 diagrammatically shows a framing device for use in the detection system in a plan view.
Figure 8 diagrammatically shows a framing part with a variable width.
Figures 9a and 9b diagrammatically show a framing part with a variable edge angle.
Figure 10 depicts a diagrammatic cross-section of a detection device
Figure 11 gives a diagrammatic cross-section through a focused first slit plate.
Figure 12 diagrammatically shows another example of slit plates, with a helical slit.
Figure 13 diagrammatically shows, in a perspective view, an alternative framing device or slit plate for use in a detection system according to the present invention.
Figure 14 shows a variant having a two-layered structure.
Figure 15 shows an extremely variable and yet relatively easily produceable slit plate for use in a detection system according to the present invention.
Figure 16 shows a very flexible detector system in a diagrammatic perspective view.
Figure 1 shows a diagrammatic detector. An object 1 is
imaged onto a detector 11, via a first slit plate 3 having a first slit 5, and a second slit plate 7 having a second slit 9. Herein, an image 15 is projected via a radiation path 13.

The two slit plates 3 and 7 have mutually perpendicular first and second slits 5 and 9, and are at some mutual distance. Note that a round object gives an elliptical projection.
Figure 2 shows a diagrammatic detector. In it, an object 20 is lying on a support 22 that is connected to support displacement means 24, that can move with respect to a guide 26. A first silt plate 28 having a first slit 30 is also coupled the guide 26, by means of a first slit plate displacement means 32. A second slit plate 34 having a second slit 36 is also coupled to the guide 26, by means of second slit plate displacement means 38. 40 is a detector plate.
The support 22 may comprise e.g. a table or the like. The support displacement means 24 are optional, and arranged to displace the support 22 in at least one of the directions indicated by arrows A. In practice, this latter comes down to a possibility to move along a symmetry axis of the detector as a whole, mostly the longitudinal axis. In the detector according to Figure 2, this could be the direction indicated by arrows B and C, but often a displacement perpendicular thereto is also possible. Note that here the guide 26 is depicted only diagrammatically, and certainly does not limit the freedom of movement of the support to the direction of arrow B. It is stressed here that the displaceability of both the support 22 and the first and second slit plate displacement means 32 and 38 occurs in principle, and advantageously, along three different axes, hence in fact in all directions.
The first slit plate displacement means 32 is arranged to displace the first slit plate 28 with respect to the object 20. In this case, this takes place in the direction of arrows B. By displacing the first slit plate 28, the ratio of the distance between the object 20 and the first slit 30 to the distance between the first slit 30 and the detector plate 40 changes. Thereby, the image ratio of the object 20 onto the detector plate 40 also changes in the direction perpendicular to the first slit 30. Of course, something similar holds in an analogous fashion for the second slit plate 34 with the second slit 36, that may be moved in the direction C by means of the second slit plate displacement means 38. Furthermore, It appears that the imaging ratio can not be the same for both plates, and thus a sphere is always imaged as an ellipse. Incidentally, the various displacement means 24, 32 and 38 may be selected from all means known in the art. Examples are electric motors, piezo-electric motors, mechanical transmissions and so on.
By adjusting the imaging ratios, through changing the respective relative distances, it may be ensured that the detector plate 40 is filled up as much as possible with an image of the desired part of the object. With a sufficiently large magnification, the image of the surroundings of the desired part of the object 20 will not appear on the detector plate 40, but will "fall off", i.e. be outside the edges of it. In this way, the detector "focuses" on the desired part.
Figure 3 diagrammatically shows another detection system. First slit plate 128 has three first slits 130, while a second slit plate 134 has three second slits 136. By means of the plates 128 and 134, a focal part 142 of an object 120 is imaged onto detector plate 140 as nine focus images 115.
By providing a plurality of slits 130, 136 in at least one of the plates 128, 134, more than one image of the part 142 is made, whereby more angular information becomes available. Based on more angular information, a better reconstruction of the part 142 may be made. Herein, the number of slits 130, 136 is not particularly limited. In principle, it holds that the more slits, the more images, and thus the more angular information. Of course, the images will often each be smaller.
Figure 4 shows a detection system that largely corresponds to Figure 3, now in a side elevational view. In Figure 4 it holds, just as in the other Figures, that similar parts are denoted with the same reference numerals. Furthermore it holds that parts of which the reference numerals only differ in their hundreds have a similar function. For example, the parts 28, 128, 228 and so on are equivalent in their function.
In Figure 4, there is added a framing plate 150 having framing openings 152. The framing plate serves to prevent images of the part to be imaged from overlapping each other (too much) on the detector 140. Thereto, the framing plate 150 comprises a desired number of frame openings 152 having the desired dimensions.
In order to do justice to the desired flexibility of the detection device, it is preferred if at least one of the framing plate 150 and the slit plates 134, 128 is provided with one or more openings that are displaceable. Herein, 'being displaceable' comprises both a displacement within the plane that is substantially perpendicular to the path of the radiation, and a displacement that is substantially directed along the path of the radiation. The latter displacement has already been discussed with Figure 2. The first displacement, i.e. within a plane that is substantially perpendicular to the path of the radiation, will be discussed herebelow, in connection with Figures 5 and 6.
Figure 5a diagrammatically provides a detection system, in which a small area is being imaged. Figure 5b provides an adapted detection system in which a large area is being imaged.
In Figure 5a the focal part 142 of the object 120 is imaged onto the detector screen 140 via second slit plate 134 and framing plate 150. Herein, images 115 are projected.
The three slits 136 are located at mutual distances d₁. The framing plate 150 comprises three framing openings 152, which, in this case, exactly limit the beams, of which there is only one drawn here, in dashed lines. Of course, the framing openings 152 may also be e.g. narrower, such that a desired part may be stopped in the case of a beam that is wider than the framing opening. Thereby, e.g. overlap may be prevented.
In the example shown, a relatively large imaging ratio is achieved. This may e.g. useful when examining small structures, such as organs in for example rodents. A disadvantage may be that information about the surroundings may not be detected.
It is noted here that the first framing plate 128 is not shown here, for the sake of the overview. Furthermore it is noted that here, too, the number of slits 136 may be selected arbitrarily.
Figure 5b shows basically the same device as in Figure 5a, with which now however a much larger object 120' is being imaged. The images 115' on the detector plate 140' are basically about the same size, and together substantially fill up the detector plate 140'. However, the magnification factor has now become much smaller, because the ratio of the distance between the detector plate 140' and the slit plate 134' to the distance between the slit plate 134' and the object 120' has become much smaller. Accordingly, however, the position of the slits 136' should be adjusted. A simple geometric construction now shows that the slits 136' now have to lie at a distance d₂, that is larger than d₁. Furthermore the opening angle of the beam that produces the image 115' is larger than the corresponding opening angle in the case of the beam for the image 115 in Figure 5a. In turn this means that the framing openings 152' should be adjusted, i.c. be made narrower. It is noted that this effect becomes smaller, the closer the framing plate 150, 150' is to the detector plate 140, 140'. For a thin framing plate that is mounted to the detector plate, the phenomenon is essentially negligible. However, because of the desired radiation shielding properties, a framing plate should have a certain minimum thickness, depending on the material.
Figure 6 shows a slit plate with first through fifth slit plate parts 160-164. In principle, all slit plate parts are displaceable in the direction indicated by the arrows. For that purpose, there are provided slit plate part displacement means 165, 166 that are shown only partly.
The first slit plate part 160 is shown with, in the drawing, an upper bevel over a thickness d3 and a lower bevel over a thickness d4. Together, these bevels form a sharp edge or tip. The second slit plate part 161 also has such a sharply tapering part at its end that is opposite the first slit plate part 160, wherein the tips are located opposite each other. Together, these tips form the slit proper, wherein the opening angle of the transmitted beam is determined by the bevels, and preferably by the bevels at the part that faces away from the radiation source, the lower bevels in the Figure. By means of these bevels, it is also possible to focus, that is to say, the images on the detector may be limited or cropped to a desired portion, in accordance with the desired part of the object that is to be examined.
The slit plate parts with the pointed ends are a preferred embodiment, over the more general form, wherein only one long bevel is provided for each end, and wherein the opening formed, for each slit, is thus more or less funnel-shaped in cross-section. This last embodiment per se has the advantage that the actual slit is closest to the object, and may thus attain thus the maximum magnification factor and resolution, but has the disadvantage that the plate parts have a less quickly increasing thickness in the direction away from the slit, and as seen in a direction perpendicular to the plate (hence in the direction of radiation). In view of the very high penetrating power of the radiation used (often gamma quanta or other high energy particles), a quickly increasing thickness is favourable for a good edge definition. Furthermore, a certain thickness is required in order to well define also the opening angle of the exit beam, which also contributes to the edge definition. In practice, the skilled person will therefore select an optimum ratio between d3 and d4, that depends on the material used for the slit plate and the desired beam and edge definitions. Preferably, that ratio is thus as small as possible, in order to have a magnification as large as possible. This also has the advantage that the next elements in the device (may) come to lie as close to the object as possible, with the associated advantages. Likewise, for the second slit plate (not shown) a corresponding reasoning will hold, in particular that a corresponding similar thickness ratio should be as small as possible for the second plate, too. Furthermore, it is advantageous if the total thickness of the first slit plate is smaller than that of the second slit plate, because then, in turn, the second slit plate may be positioned closer to the object. Of course, as a whole a sufficient radiation shielding should be ensured, in order to obtain a well defined image. Furthermore, it is noted here that, for a small thickness ration, a possible focusing or framing action is primarily determined by the hindmost or thickest part.
The second slit plate part 161 is moveable, either with respect to the part 160, or with respect to the surrounding fixed world, or both. For that purpose, there are provided for example displacement means 165 and 166 connected thereto, that can move e.g. slidingly or telescopically in the direction indicated by the arrows. In that way, the position of the slit between the parts 160 and 161, and/or the width of that slit may be set. The position of the slit is important to image a desired part of the object to be examined onto the correct position on the detector plate. The width of the slit is important to achieve a good balance between sensitivity and resolving power. In principle it holds that: a wider slit gives a shorter measuring time and higher sensitivity but a lower resolving power. The displacement means 165 and 166 shown are not particularly limited, but in case they are positioned at least partly in the path of the radiation through the slit, it is preferable if the means are made of a material that is transparent to radiation, such as plastic.
Together, third slit plate part 162 and fourth slit plate part 163 form a portion of the slit plate between neighbouring slits. The two parts 162 and 163 are mutually displaceable in the direction of the arrow drawn with it, by means of means that are not indicated any further. In that way, it is easily possible to change the distance between the slits, whereby it is for example possible to use a different magnification factor when imaging. A further advantage is that the edge definition is not affected. Because the effective plate thickness decreases in the mutually overlapping sections of the parts 162 and 163, since they no longer overlap after the outward displacement, it is advantageous to select the plate thickness such that even the thinnest plate section already offers sufficient shielding. Alternatively, it is possible to design the thinnest plate section in a material that is less transparent for radiation.
The slit plate parts 160-164 are made of a material that has a low transmission for the radiation, such as a metal like lead, tungsten, osmium, iridium, (depleted) uranium, or gold or platinum. From the point of view of costs, it is favourable to design only the critical parts in a material that is very dense, in a radiation technological point of view, but precious, such as gold or platinum. Critical parts are in particular the edges of the slit, i.e. the tips, and possibly thinner sections of the slit plate parts.
Figure 7 diagrammatically shows a framing device for use in the detection system, in a plan view.
The framing device comprises, in this case, three x-framing parts 180, 181 and 182, that are positioned side by side in the x-direction and in parallel, as well as three y-framing parts 190, 191 and 192, that are positioned side by side in the y-direction and in parallel. The mutual distances are d1 through d4 as shown in the Figure. Furthermore, the framing device comprises, preferably but not necessarily, non-shown framing part displacement means.
The framing parts define four image windows. Of course, any other number of windows is possible, by arranging a suitable number of framing parts in a suitable way. The framing windows neither have to be all of equal size. Furthermore, the framing parts do not have to be straight, but may also be at least locally curved, i.e. having an at least locally varying width. The framing parts neither have to run in a rectangular or parallel fashion, but it is also possible to have them run in a lozenge-shape or trapezially. The framing openings not even have to be in a regular pattern, but they may also e.g. be staggered, depending on where the desired images should be formed on the detector plate.
Preferably, the framing parts are displaceable, in particular mutually. In that way, it becomes possible to select the position of the windows, and advantageously also the dimensions of the windows. Advantageously, the x-framing parts and the y-framing parts are mutually independently displaceable, so the windows may be adapted to an image that is changed in the x or y direction. It is also possible to couple the displacement in x and y directions, such that a certain imaging ratio is maintained.
Figure 8 diagrammatically shows a framing part with a variable width. The framing part is composed of three subparts 210, 202 and 203. The total width has been indicated by d. Furthermore, at least one of the subparts 201 and 203 is displaceable in the direction indicated with the arrow, allowing to vary the width d. The subpart 202 ensures the required overlap to prevent radiation leaks. Not shown are subpart displacement means, that are optional incidentally, because it is also possible to mutually displace the subparts with external and removable means, such as along a guide.
Figures 9a and 9b diagrammatically show a framing part with a variable edge angle. There is shown a framing part with two subparts 210 and 211, each able to rotate about a respective axis 212 or 213. In Figure 9a, the subparts 210 and 211 have such an orientation that the edge rays of the passing beams, indicated by dashed lines, run substantially parallel. In Figure 9b, the subparts 210 and 211 are rotated in such a way that the corresponding edge rays converge. Such an orientation may for example be favourable when the beams that are associated with the images have a different opening angle. By adjusting the orientation of the (sub)parts of the framing device to that opening angle, the definition of the edges of the beams remains optimum. Of course, the orientations shown are just an example, the changes in orientation often being much smaller in practice. It is furthermore possible provide only one axis 212, 213 for each framing part, although a symmetrical set-up will often be preferred.
A general remark here is that, in principle, each remark about an embodiment of the framing device and its parts also holds for the slit plates and their parts. For example, a slit plate could be made up of a plurality of subparts that are each rotatable about an axis, to thereby be able to set an edge angle.
Figure 10 depicts a diagrammatic cross-section of a detection device
This comprises a housing 300, on which there are three detector plates 340. Inside, there is a second slit plate, or better: slit cylinder, 334, and coupled thereto a second slit plate displacement means 338. Inside that, there is a first slit plate/slit cylinder 328 with a coupled first slit plate displacement means 332. Inside that, there is an object 320 on an object table 322 with a coupled object table displacement means 324.
The device shown has a triangular external configuration, but of course it may be polygonal, such as tetra-to hexagonal, or even cylindrical as a whole. The first and second slit plates 328 and 334 are cylindrical. Incidentally, these may also be built up of a plurality of individual parts, that together form a cylinder or polygon.
A framing device is optional, and will preferably be located between the second slit plate 328 and the detector plates 340.
The object table displacement means 324 e.g. serves to displace the object table 322 with the object 320, in the direction perpendicular to the plane of the paper. A displacement within the plane of the paper is also possible with a suitable means.
Displacement means 332 and 338 may ensure displacement, in this case often a rotation, of the first and second slit plates 328 and 334. Thus, the "pinholes,", that formed by the (not separately shown) slits in the slit plates, may be displaced with respect to the object.
The detector plates 340 comprise any detector type known in the art. This may e.g. be arrays of small detectors, or a single large position-dependent detector, in each e.g. of the scintillator type.
In the case that the slit plates 328 and 334 are also composed of three parts, one can also speak of a triple detector, in which each detector plate is associated with one of the three plates 328 and 334. Then, it is advantageous to provide that such a complete detector, i.e. detector plate with slit plates, and if desired a framing device, may be displaced as a whole. This offers the advantage that the complex procedure to calibrate the detector, in which it is determined where and with what probability an incident particle will be incident on the detector, does not have to be repeated. By this displacement of the detector as a whole, it is true that the focusing of the detection system as a whole changes, but this may be calculated rather well with mathematical correction methods. As an alternative or addition, it is also possible to provide one or more of such detectors as detector "heads", that may thus be displaced as a whole with respect to the object, but in addition with the adjustable plates, slits, framings and so on.
Figure 11 gives a diagrammatic cross-section through a focused first slit plate 328. In this case, this comprises some five slits 330, formed in such a way that their passed beams overlap. The main pass directions, drawn in dashed lines, converge in one point in the object 320. This means that the "pinholes" not only look at the object 320 under different angles, but that these are furthermore arranged such that they present a maximum pass surface area. This is favourable for the sensitivity.
Note that, in this case, the slit plate 328 may be composed of either parallel rings with the slits therebetween, or of a single cylinder with slits that not go round completely or that are each built up of a plurality of partial slits in line with each other or that rather stagger. In the case of a cylinder, built up of rings, mutual coherence may be provided by connecting the rings by means of a material that is transparent to the radiation. Of course, the side walls of the slits 330 will not be completely parallel, but they will, in dependence of the desired framing, be e.g. somewhat funnel-shaped inwards, and outwards again, to thereby form a slit according to Figure 6. This has not been indicated any further.
Figure 12 diagrammatically shows another example of slit plates 428, and 434, respectively, with a helical slit 430, and 436, respectively. Here, the slit plates 428 and 434 are concentric. Again, for coherence, it is also possible to build up the helical slit of a number of partial slits, or to use a material that is transparent to radiation, in order to connect the otherwise helically extending parts. It is also possible to provide a plurality of mutually parallel helical slits, e.g. for a larger pitch.
For example, in a configuration according to Figure 10, and a slit formation according to Figure 11, it is possible, with this double helix, to obtain a series of running "pinholes", that are furthermore constantly focused.
Alternatively, it is possible to provide only one slit plate with a helical slit, or of a plurality of partial slits, that are either in line with each other, or are mutually staggered. The other slit plate then has e.g. parallel slits in the direction of the longitudinal axis of the drawn cylindrical slit plates 428 and 434.
Figure 13 diagrammatically shows, in a perspective view, an alternative embodiment of a framing device or slit plate according to the invention. It comprises a perspex plate 500, on which there are leaden framing or plate parts, the latter being displaceable in the direction of arrow A.
With such a framing device, there is provided, in a simple and elegant way, an adjustable framing device or slit plate, wherein the perspex plate serves as a plate that is transparent to radiation, wherein also another material may be included. Furthermore, if there would be selected width-adjustable parts as the parts 502, and possibly also 504, the slits or the openings between the parts could be set in many different ways. Of course, again it also holds that the openings do not have to be regular, nor rectangular, et cetera. Incidentally, in this Figure, as in all Figures, it holds that the slits are not drawn to scale. In particular, the slits are often too narrow for a clear rendering.
Figure 14 shows a variant having a two-layered structure, in which 600 indicates two perspex plates, 602 a number of first slit plate parts, 604 a number of second slit plate parts and 606 diagrammatically the effective "pinholes".
By simply moving the parts 602 and/or 604, the position, orientation and of course the width of the slits, and thus of the "pinholes", may be set. A non-shown framing device may serve well to stop radiation that has not passed through slits between slit parts.
Figure 15 shows an extremely variable and yet relatively easily produceable slit plate. It comprises a base plate 700, having a number of base holes 702 and a number of subunits 704 with slit parts 706, between which slits 708 are defined.
The base plate 700 is made of a material that is opaque for radiation, such as lead, tungsten, and so on. The base holes 702, of which there are only shown three here, but any number of which may of course be provided, have been cut out in the base plate at positions that indicate certain borders, for example the borders of where a detector is located. The base plate then serves to frame the outermost borders.
The subunits 704 are capable of being moved in the directions indicated by the arrows A and B. Thereby, the position of slit 708 may be adjusted in those directions. Such units are relatively easy to produce, not only because of the relatively short slit, that is more easy to make with the strict tolerances than a much longer slit. In accordance with the invention, the subunits 704 have slit parts 706 that ensure an adjustable slit 708. This may be done in any way described above or known in any way. Advantageously, the number of subunits is at the most equal to the number of base holes, but that may also be selected arbitrarily large. In this way, there is obtained a slit part with a very large freedom in position and width of the (partial) slits, wherein the components are easily produceable and replaceable. It is also possible to attach two such subunits to each other or to the two sides of the plate, e.g. by gluing, soldering or the like. The two subunits may then fulfill the functions of the first and second slit plate. Preferably, these subunits are then located at the object side.
As an alternative, Figure 15 may also be valid for a framing device, wherein of course the diverse openings should be adapted in size.
For the control and displacement of the subunits, use may be made of various possibilities. For example, each moveable component may be provided with a handle or the like. Preferably, that handle is made of a radiation transmitting material, such as plastics, for example perspex. However, it is preferred to make it possible to operate the subunits from the outside. Herein, 'from the outside' means outside the radiation used, the user thus being shielded. To that end, one can work with a connection between the subunits and an operating device located outside the area with a radiation exposure. Herein, it is possible to provide cable or wire connections between the handles or the like, and that operating device.
Figure 16 shows a very flexible detector system in a diagrammatic perspective view.
Herein, 801 is a mouse that is oriented in the z-direction. *10 is a detector housing with a first side wall 812 and a second side wall 814.
828 is a first slit plate having first slits 830, and 834 is a second slit plate having second slits 836.
First framing strips 820 are connected to first framing strip shafts 824 that pass through openings 822 in the second side wall 812 and the opposite side wall. Second framing strips 840 pass, with second framings strip shafts 844, through elongated openings 842.
Furthermore, a detector plate with detection fields 815 is located at the bottom.
It is noted that the shown detection system is not rendered to scale, and that, in principle, it is often a component of a larger whole, e.g. having three or more detection systems positioned around the mouse 801, and then preferably parallel to the z axis.
The detection system shown is very useful and flexible for examination of e.g. the shown mouse 801. Nine images are formed onto as many detection fields 815, by means of the slits 830 and 836. To this is added that, in each case, the first slits 830 are provided staggeredly over 1/3 of the mutual slit distance, whereby the information about the mouse 801 is obtained under in each case a different angle. This makes it possible to obtain more angular information. All this carries with it that then the corresponding detection fields 815 and framing openings, and thus in this case also the framing strips 840, are provided in a correspondingly staggered fashion. Incidentally, detection fields 815 may also be "provided" in a larger one-piece detector by reading out only certain parts of it.
A different part of the mouse 801 may be examined by displacing it, or by displacing one or more of the first and second slit plates 828, 834, the first framing strips 820, 840, and possibly by adapting the detection fields 815 correspondingly. In the case of the second framing strips 840, this is e.g. possible by displacing the shafts 844 in the slots 842, through which they protrude. This may be brought about when the device is active, because both ends of the shafts 844 are located outside the area with radiation exposure. This embodiment may also be combined with other features indicated to be advantageous or special, such as bevels that show a focusing action, end so on.
It is also possible to turn the first and/or second framing strips, so that they obtain a different effective width for the radiation, because of their elliptical cross-section. If desired, the mutual distances to the object 801 may also be changed, by means of displacement means not indicated any further.
The embodiments shown and described are only non-limiting examples. The scope of protection is defined by the attached claims.

Claims

A radiation detection system, comprising:
- a detector plate (40) that is sensitive to radiation, and

- an imaging system that is arranged to form an image of an object to be examined on the detector plate and that comprises:
- a first slit plate (28; 128) with at least a first slit (30; 130), and

- a second slit plate (34; 134) that is positioned between the first slit plate and the detector plate and with a second slit (36; 136) that makes a non-zero angle with the first slit, wherein the first and second slits together define a pinhole passage,

characterized in that
at least one of the first and second slit plates (28,34) comprises a slit plate displacement means (32,38) for adjusting the distance of that slit plate with respect to the other slit plate and/or the detector plate and/or the object (20) to be examined, wherein at least one of the slit plates, preferably the first slit plate, comprises at least two slit plate parts (160 - 164), that are moveable with respect to each other by slit part displacement means (165,166), such that the width of at least one of the slits in the slit plate is adjustable.
The radiation detection system according to claim 1, further comprising a base plate (700) that is substantially opaque for the radiation, and wherein the first and/or second slit plate at least comprises a subunit (704) having a slit therein that is displaceable with respect to the base plate.
The radiation detection system according to claim 2, wherein the first slit plate comprises a first subunit and the second slit plate comprises a second subunit, wherein the first and second subunit are arranged behind each other in radiation direction.
The radiation detection system according to one or more of the preceding claims, further comprising a framing device, at least comprising a plate part (150) and having at least one framing opening (152) therein that is formed such that the radiation in the direction of the detector plate (140) can only reach a predetermined section of the detector plate, and wherein preferably the framing device is located between the second slit plate and the detector plate, and wherein preferably the framing device comprises a plurality of mutually parallel framing strips (820) in a first direction, as well as a plurality of mutually parallel second framing strips (840), wherein at least one of the first and second framings strips is displaceable with respect to another of the first and second framing strips, wherein the framing device preferably further comprises framing strip displacement means (844) for displacing the at least one displaceable framing strip.
The radiation detection system according to claim 4, wherein at least one framing strip has an adjustable width.
The radiation detection system according to claim 4 or 5, wherein at least one framing strip comprises at least two framing strip parts that are displaceable with respect to each other, and wherein possibly the at least two framing strip parts are rotatable with respect to each other.
The radiation detection system according to one or more one the preceding claims, wherein at least the first slit plate forms a part of a cylinder.
The radiation detection system according to one or more of the preceding claims, wherein the first and second slit plates form concentric cylinders.
The radiation detection system according to one or more of the preceding claims, wherein the slits in the first and second slit plates have a waisted cross-section, that is limited by bevels that run from the front and the back of the slit plate, and that limit a beam passage.
The radiation detection system according to one or more of the preceding claims, wherein at least one of the slit plates (128,134) comprises a plurality of slits (130,136), and wherein preferably both slit plates comprise a plurality of slits.
The radiation detection system according to claim 10, wherein the slits in the first and second slit plates together define a lattice of pinhole passages, each with a beam passage having a main direction of passage, wherein the bevels are shaped such that the beam passages at least partially overlap each other on the longitudinal axis, and wherein preferably the main directions of passage of at least two beam passages, more preferably of each beam passage, pass substantially through one point.
The radiation detection system according to one or more of the preceding claims,. wherein the detector plate is concentric to the slit plates.
The radiation detection system according to one or more of the preceding claims, wherein at least one of the slit plates comprises a plurality of partial slits having a length that is smaller than the length of the slit plate, as measured in the direction of the partial slit.