WO2010133920A1

WO2010133920A1 - Anti-scatter arrangement for a radiation detector

Info

Publication number: WO2010133920A1
Application number: PCT/IB2009/052121
Authority: WO
Inventors: Matthias Simon; Gereon Vogtmeier
Original assignee: Koninklijke Philips Electronics N.V.; Philips Intellectual Property & Standards Gmbh
Priority date: 2009-05-20
Filing date: 2009-05-20
Publication date: 2010-11-25

Abstract

The invention relates to an anti-scatter arrangement as well as to a radiation detector (110) and an examination apparatus comprising such an arrangement. The anti-scatter arrangement comprises an actuator (113) by which an anti-scatter grid (111) can be tilted such that its focus axis (A) can be adjusted to different focal spots (F) of a radiation source (120).

Description

Anti-scatter arrangement for a radiation detector

FIELD OF THE INVENTION

The invention relates to an anti-scatter arrangement for a radiation detector, particularly an X-ray detector, to a radiation detector comprising such an arrangement, and to a method for detecting radiation.

BACKGROUND OF THE INVENTION

Anti-scatter grids are used for example in X-ray detectors to block scattered radiation that does not come directly from the focal spot of the radiation source from reaching a detector. This helps to suppress disturbances from scattered radiation in transmission images. US 2005 0002493 Al discloses a mechanism to shift an anti-scatter grid in sideward direction in order to blur the boundaries of the grid in the generated image.

SUMMARY OF THE INVENTION

Based on this situation it was an object of the present invention to provide means for a more flexible radiation detection including the blockage of scattered radiation. This object is achieved by an anti-scatter arrangement according to claim 1, a radiation detector according to claim 7, an examination apparatus according to claim in 11, and a method according to claim 13. Preferred embodiments are disposed in the dependent claims.

According to its first embodiment, the invention relates to an anti-scatter arrangement for a radiation detector, for instance for an X-ray or γ-ray detector as it is used in a medical imaging apparatus. The anti-scatter arrangement comprises the following components: a) An anti-scatter grid, in the following as usually abbreviated as "ASG". In its most general definition, the ASG constitutes a device extending in one or two dimensions that, at a given location, allows the passage of radiation (in a considered spectral range, e.g. X-radiation) only from a corresponding interval of incident angles while it blocks or at least reduces radiation coming from other directions. A "focus axis" of the ASG can be defined in this context as an axis that is fixed relative to the ASG and that characterizes the spatial direction(s) from which radiation is allowed to pass. If every cell of the ASG has another main direction from which it lets radiation pass (this is for example the case if the ASG is focused on a point-like radiation source a finite distance away such that the incident radiation cannot be approximated as being parallel), the focus axis may be defined as the mean of all these transmission directions. b) An actuator for tilting the ASG, implying also a tilt of the aforementioned focus axis, wherein the tilting takes place relative to a carrier structure in which the ASG is mounted. In other words, the actuator is able to move the ASG with a rotational component, and not just a translation or shift.

The described anti-scatter arrangement has the advantage that the orientation of its ASG can be changed, thus allowing to focus it to different focal points from which radiation may come.

The ASG may have a one-dimensional structure, i.e. a one-dimensional alternating sequence of cells that let radiation pass and walls that absorb (scattered) radiation, said cells and walls extending over the complete width of the ASG, i.e. transverse to the "one dimension". In a preferred embodiment, the ASG is a two-dimensional grid having such an alternating sequence of transparent cells and absorbing walls in two dimensions.

The actuator will usually comprise materials that would absorb or at least alter the radiation to be detected. It is therefore preferred that the actuator has no parts that are located behind or in front of components of the ASG that are transparent for radiation, wherein the terms "behind" and "in front of refer to the direction of incidence of (not- scattered) radiation. In a positive formulation this means that the actuator may be located behind or in front of radiation absorbing components of the ASG and/or in regions that are not passed by radiation which can reach the detector module, e.g. at the sides of the ASG. In a preferred embodiment of the invention, the actuator has a stripe- like geometry or a grid-like geometry. If the stripes or the grid have appropriate dimensions, the actuator can then be arranged behind or in front of the radiation absorbing components of the ASG, thus realizing the aforementioned design in which the actuator does not affect the radiation to be detected. Instead, the actuator may even contribute to the desired absorption of scattered radiation that shall be achieved by the ASG. The actuator may be realized in a variety of ways. It may for example comprise electromotive components like a stepper motor with an appropriate gear. Preferably, the actuator comprises a piezoelectric element or crystal, i.e. a material that changes its extension in at least one dimension in dependence on a voltage that is applied to it. Such a piezoelectric element has the advantage that it readily allows for a precise control, particularly of small movements as they are typically required in the anti-scatter arrangement.

According to a further development of the invention, the anti-scatter arrangement comprises a controller for controlling the activity of the actuator - and thus the movement of the ASG - in synchronization with a radiation source. The controller may be realized by dedicated electronic hardware, by digital data processing hardware with associated software, or a combination of both. The activity of the radiation source may particularly comprise the sequential activation of different focal spots emitting radiation. The actuator can then be used to always focus the ASG to the currently active focal spots of the radiation source.

The invention further relates to a radiation detector comprising the following components: a) An anti-scatter grid, ASG; and b) An actuator for tilting the ASG; and c) A detector module for detecting radiation after its passage through the ASG.

The detector module will for example comprise a material that directly or indirectly converts incident radiation into electrical signals (e.g. voltages or currents) which can be further processed and evaluated.

The radiation detector comprises an anti-scatter arrangement of the kind described above. Reference is therefore made to the above description for more information on the details, advantages and modifications of such a radiation detector.

In a first preferred embodiment of the radiation detector, the actuator is disposed between the ASG and the detector module to move the ASG relative to the detector module. This requires a minimal power as only the necessary components, i.e. the ASG, have to be moved.

In a second embodiment, which may optionally be combined with the aforementioned one, the actuator is arranged to move the ASG and the detector module in common. In this case it may for example be located between the detector module and a carrier structure on which the detector module with the ASG is mounted. An advantage of this design is that, due to the location of the actuator, no component of it can interfere with the radiation to be detected, thus simplifying the design of the actuator.

Moreover, the invention relates to an examination apparatus for the examination of an object (e.g. a patient) with radiation, said apparatus comprising a radiation detector of the kind described above. The examination apparatus may particularly be applied as a baggage inspection apparatus, a material testing apparatus, a material science analysis apparatus, or a medical application apparatus. The examination apparatus may especially be selected from the group consisting of an X-ray apparatus (e.g. a fluoroscopic device), Computed Tomography (CT) imaging system, Coherent Scatter Computed Tomography (CSCT) imaging system, Positron Emission Tomography (PET) imaging system, and Single Photon Emission Computerized Tomography (SPECT) imaging system.

The aforementioned examination apparatus preferably comprises an X-ray source with a focal spot that can be moved relative to the detector module. Such X-ray sources with a moving focal spot are particularly important in modern CT scanners due to their improved behavior. X-ray sources with a movable focal spot may for example be realized by electrostatic or magnetic deflection of electron beams that generate X-rays when impinging on an anode. A variety of designs can be found in literature, e.g. US 2007/0009081 Al, US 4 689 809 and documents cited therein. In the examination apparatus, the tilting of the ASG will usually be synchronized with the movement of the focal spot in the X-ray source. If the focal spot moves for example in a given plane on a given trajectory, the ASG will be tilted in such a way that its focus axis follows synchronously this (or a similar) trajectory in the given plane, too. Furthermore, the invention relates to a method for detecting radiation, particularly X-radiation or γ-radiation, comprising the following steps: a) Blocking undesired radiation with an ASG; b) Detecting unblocked radiation behind the ASG; c) Tilting the ASG. This tilting may be done during and/or between the detection steps b).

The method comprises in general form the steps that can be executed with an anti-scatter arrangement, a radiation detector, and/or an examination apparatus of the kind described above. Therefore, reference is made to the preceding description for more information on the details, advantages and improvements of that method.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. These embodiments will be described by way of example with the help of the accompanying drawings in which: Fig. 1 shows schematically a side view of an examination apparatus according to the present invention with a first position of the focal spot in the radiation source;

Fig. 2 shows the apparatus of Figure 1 after movement of the focal spot and tilting of the ASG; and Fig. 3 shows a perspective view of a radiation detector according to the present invention.

Like reference numbers or numbers differing by integer multiples of 100 refer in the Figures to identical or similar components.

DETAILED DESCRIPTION OF EMBODIMENTS

The use of a moving focal spot of the X-ray tube in medical CT systems has a variety of advantages, e.g. (i) better and larger coverage of the patient volume with the cone beam, (ii) reduction of cone beam artifacts, and (iii) fast switching between two or more beam qualities. Moreover, it is known to apply anti-scatter grids (ASGs) to prevent scattered radiation from reaching a detector and thus from deteriorating the image quality. In particular two-dimensional ASGs can enhance the scatter reduction in CT significantly, especially in multiline detectors covering a large cone angle. However, the use of known (two-dimensional) ASGs is not possible with a moving focal spot because the ASGs are focused to one fixed focal spot of the X-ray source. It is therefore proposed here to apply a synchronized fast movement of a (one- or two-dimensional) ASG alone or in combination with a complete detector module with the help of actuators to ensure the focusing of the ASG to a moving X-ray focal spot.

Figure 1 shows schematically an X-ray imaging apparatus 100, for example a CT scanner, which realizes the aforementioned concept. The apparatus 100 comprises an X-ray source 120 with a selectively movable focal spot F from which X-radiation is emitted. After passing through an object 1, for example a patient to be examined, the radiation falls onto a radiation detector 110.

The radiation detector 110 comprises a pixelated detector module 112 that shall determine the amount of transmitted X-radiation such that a projection image of the object 1 can be generated. However, it is known that a part of the radiation emitted by the radiation source 120 is scattered in the object 1 by different processes. The scattered radiation would reach positions on the detector module 112 where it would erroneously be interpreted as transmitted radiation coming in direct line from the focal spot F. To prevent the resulting image deterioration by scattered radiation, an anti-scatter grid ASG 111 is disposed in front of the detector module 112. The ASG 111 is a one- or preferably two-dimensional grid with an alternating sequence of cells I l ia, that are transparent for X-radiation, and radiation absorbing walls 11 Ib, e.g. made from lead, that are directed to the focal spot F. Due to the walls, scattered radiation will be absorbed before reaching the detector module 112, which is thus primarily only reached by the desired transmitted radiation. The alignment or orientation of the ASG 111 can be described by a "focus axis" A that is fixed with respect to the ASG 111 and for example defined as the average of the directions of transparency of all grid cells 11 Ia of the ASG 111.

However, when the focal spot F of the X-ray source 120 moves, for example to the new location F' shown in Figure 2, the orientation of the ASG 111 of Figure 1 would no longer be correct or optimal, with the consequence that a part of the desired transmitted radiation would be absorbed and a part of the undesired scattered radiation would reach the detector module.

To avoid this, an actuator 113 is provided by which the ASG 111 can be tilted such that it will always be correctly focused on the current focal spots F, F' of the X-ray source 120. In Figures 1 and 2, this actuator is disposed between the ASG 111 and the detector module 112 and realized by piezoelectric transducers 113 which change their geometrical dimensions, such as for example length, dependent on an applied voltage. By applying an appropriate voltage distribution to the flat actuator 113, the thickness of this actuator can be increased or decreased as desired. This allows particularly to generate the wedge shape shown, largely exaggerated, in Figure 2 with a thickness of the actuator layer 113 that is decreased on the left and increased on the right side. The actuator layer 113 will thus tilt the ASG 111 such that its focus axis A will be readjusted to the new focal spot F'. The control of the actuator 113, i.e. the application of appropriate voltages, is executed by a controller 114. The controller 114 is preferably also coupled to the X-ray source 120 (and perhaps the detector module 112) such that it can tilt the ASG 111 synchronously to the movement of the focal spot F in the X-ray source 120.

Figure 3 illustrates in a perspective view an alternative design of a radiation detector 210 according to the present invention. This radiation detector comprises, seen from top to bottom in the sequence of incident radiation, the following components: a) A two-dimensional ASG 211. b) A first flat piezoelectric actuator 213 disposed below the ASG 211. Other than shown in the Figure, the actuator 213 will usually have a similar grid geometry as the ASG 211 above it such that the actuator material is only located below the walls of the ASG 211. This prevents that incident radiation is affected by the presence of the actuator 213. c) A detector module 212, for example a layered structure of a scintillator where incident X-radiation is converted into visible photons and a photosensitive layer where these photons are converted into electrical signals. d) A second flat piezoelectric actuator 213' that is disposed below the detector module 212 and by which the ASG 211 and the detector module 212 can be commonly tilted. e) A carrier structure 215 upon which the above elements are mounted. It should be noted that Figure 3 shows the simultaneous presence of two actuators 213, 213' only for purposes of illustration. In practice, usually one of these two actuators will suffice to tilt the ASG 211 relative to a radiation source.

The anti-scatter arrangement proposed above is particularly advantageous for future CT scanner geometries, multiline CT detectors, and systems with distributed X-ray sources, especially for large coverage cone beam CT and multi- source applications, where it enables 2D-ASG technology and scatter reduction.

Finally it is pointed out that in the present application the term "comprising" does not exclude other elements or steps, that "a" or "an" does not exclude a plurality, and that a single processor or other unit may fulfill the functions of several means. The invention resides in each and every novel characteristic feature and each and every combination of characteristic features. Moreover, reference signs in the claims shall not be construed as limiting their scope.

Claims

CLAIMS:

1. Anti-scatter arrangement for a radiation detector (110, 210), comprising: a) an anti-scatter grid (111, 211); and b) an actuator (113, 213, 213') for tilting the anti-scatter grid.

2. The anti-scatter arrangement according to claim 1, characterized in that the anti-scatter grid (111, 211) is a two-dimensional grid.

3. The anti-scatter arrangement according to claim 1, characterized in that the actuator (113, 213, 213') has no parts located in front of or behind radiation-transparent components of the anti-scatter grid (111, 211).

4. The anti-scatter arrangement according to claim 1, characterized in that the actuator (113, 213, 213') has a stripe-geometry or a grid-geometry.

5. The anti-scatter arrangement according to claim 1, characterized in that the actuator (113, 213, 213') comprises a piezo-element.

6. The anti-scatter arrangement according to claim 1, characterized in that it comprises a controller (114) for controlling the activity of the actuator (113, 213, 213') in synchronization with a radiation source (120).

7. A radiation detector (110, 210), comprising: a) an anti-scatter grid (111, 211); and b) an actuator (113, 213, 213') for tilting the anti-scatter grid; and c) a detector module (112, 212) for detecting radiation after its passage through the anti-scatter grid (111, 211).

8. The radiation detector (110, 210) according to claim 7, characterized in that it comprises an anti-scatter arrangement according to any of the claims 1 to 6.

9. The radiation detector (110, 210) according to claim 7, characterized in that the actuator (113, 213) is at least partially disposed between the anti-scatter grid (111, 211) and the detector module (112, 212) to move the anti-scatter grid (111, 211) relative to the detector module.

10. The radiation detector (210) according to claim 7, characterized in that the actuator (213') is at least partially arranged to move the anti-scatter grid (211) in common with the detector module (212).

11. An examination apparatus for the examination of an object (1) with radiation, comprising a radiation detector (110, 210) according to claim 7.

12. The examination apparatus according to claim 11, characterized in that it comprises an X-ray source (120) with a focal spot (F, F') that can be moved relative to the radiation detector (110, 210).

13. A method for detecting radiation, particularly X-radiation or γ-radiation, comprising: a) blocking undesired radiation with an anti-scatter grid (111, 211); b) detecting unblocked radiation behind the anti-scatter grid (111, 211); c) tilting the anti- scatter grid (111, 211).

14. A computer program product for enabling carrying out a method according to claim 13.

15. A record carrier on which a computer program according to claim 14 is stored.Anti-scatter arrangement for a radiation detector.