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METHOD FOR PRODUCING AND
APPLYING AN ANTISCATTER GRID OR
COLLIMATOR TO AN X-RAY OR GAMMA
DETECTOR
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The present application hereby claims priority under 35 U.S.C. §119 on German patent application number DE 10241423.8 filed Sep. 6, 2002, the entire contents of which are hereby incorporated herein by reference.
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FIELD OF THE INVENTION
The present invention generally relates to a method for producing and applying an antiscatter grid or collimator to an x-ray or gamma detector. Preferably, it relates to a method 15 for producing and applying an antiscatter grid or collimator to an x-ray or gamma detector having an array of detector elements which form a detector surface with detection regions sensitive to x-radiation and/or gamma radiation and less sensitive intermediate regions. It further generally 20 relates to an x-ray and gamma detector having an antiscatter grid or collimator which has been produced and applied using this method.
BACKGROUND OF THE INVENTION 25
In radiography, stringent requirements are currently placed on the image quality of the x-ray images. In such images, as are taken especially in medical x-ray diagnosis, an object to be studied is exposed to x-radiation from an 30 approximately point radiation source, and the attenuation distribution of the x-radiation is registered two-dimensionally on the opposite side of the object from the x-ray source. Line-by-line acquisition of the x-radiation attenuated by the object can also be carried out, for example in computer 35 tomography systems.
Besides x-ray films and gas detectors, solid-state detectors are being used increasingly as x-ray detectors, these generally having a matrix shaped arrangement of optoelectronic semiconductor components as photoelectric receivers. Each 40 pixel of the x-ray image should ideally correspond to the attenuation of the x-radiation by the object on a straight axis from the point x-ray source to the position on the detector surface corresponding to the pixel. X-rays which strike the x-ray detector from the point x-ray source in a straight line 45 on this axis are referred to as primary beams.
The x-radiation emitted by the x-ray source, however, is scattered in the object owing to inevitable interactions, so that, in addition to the primary beams, the detector also receives scattered beams, so-called secondary beams. These 50 scattered beams, which, depending on the properties of the object, can cause up to 90% or more of the total signal response of an x-ray detector in diagnostic images, constitute an additional noise source and therefore reduce the identifiability of fine contrast differences. This substantial 55 disadvantage of scattered radiation is due to the fact that, owing to the quantum nature of the scattered radiation, a significant additional noise component is induced in the image recording.
In order to reduce the scattered radiation components 60 striking the detectors, so-called antiscatter grids are therefore interposed between the object and the detector. Antiscatter grids consist of regularly arranged structures that absorb the x-radiation, between which transmission channels or transmission slits for minimally attenuated transmis- 65 sion of the primary radiation are formed. These transmission channels or transmission slits, in the case of focused antis
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catter grids, are aligned with the focus of the x-ray tube according to the distance from the point x-ray source, that is to say the distance from the focus. In the case of unfocused antiscatter grids, the transmission channels or transmission slits are oriented perpendicularly to the surface of the antiscatter grid over its entire area. However, this leads to a significant loss of primary radiation at the edges of the image recording, since a sizeable part of the incident primary radiation strikes the absorbing regions of the antiscatter grid at these points.
In order to achieve a high image quality, very stringent requirements are placed on the properties of x-ray antiscatter grids. The scattered beams should, on the one hand, be absorbed as well as possible, while on the other hand, the highest possible proportion of primary radiation should be transmitted unattenuated through the antiscatter grid. It is possible to achieve a reduction of the scattered beam component striking the detector surface by a large ratio of the height of the antiscatter grid to the thickness or diameter of the transmission channels or transmission slits, that is to say by a high aspect ratio.
The thickness of the absorbing structure elements or wall elements lying between the transmission channels or transmission slits, however, can lead to image perturbations by absorption of part of the primary radiation. Specifically when solid-state detectors are used, inhomogeneities of the grids, that is to say deviations of the absorbing regions from their ideal position, cause image perturbations by projection of the grids in the x-ray image.
In order to minimize image perturbations due to antiscatter grids, it is known to move the grids in a lateral direction during the recording. In the case of very short exposure times of, for example, 1-3 ms, however, stripes may also occur in the image if the movement speed of the grids is insufficient. Even in the event of very long exposure times, perturbing stripes may occur owing to reversal of the grid movement direction during exposure.
In recording x-ray images, increasing use has recently been made of solid-state detectors which are formed from a plurality of an array of detector elements. The detector elements are arranged in this case in a generally square or rectangular grating. In the case of such solid-state detectors, as well, there is a need to employ effective suppression measures to reduce as far as possible the striking of scattered beams on the detector surface formed by the detector elements. Because of the regular structuring of the pixels, formed by the detector elements, of the detector, there is here, in addition, the risk of mutual interference between the structures of pixels and antiscatter grids. Disturbing moire phenomena can thereby arise. These can certainly in specific instances be minimized or removed by a downstream image processing measure. However, this is possible only when their projection image on the detector is absolutely immutable.
The same problem occurs in nuclear medicine, especially when using gamma cameras, for example Anger cameras. With this recording technique also, as with x-ray diagnosis, it is necessary to ensure that the fewest possible scattered gamma quanta reach the detector. In contrast to x-ray diagnosis, the radiation source for the gamma quanta lies inside the object in the case of nuclear diagnosis. In this case, the patient is injected with a metabolic preparation labeled with particular unstable nuclides, which then becomes concentrated in a manner specific to the organ.
By detecting the decay quanta correspondingly emitted from the body, a picture of the organ is then obtained. The profile of the activity in the organ as a function of time
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permits conclusions about its function. In order to obtain an image of the body interior, a collimator that sets the projection direction of the image needs to be placed in front of the gamma detector. In terms of functionality and structure, such a collimator corresponds to the antiscatter grid in x-ray 5 diagnosis. Only the gamma quanta dictated by the preferential direction of the collimator can pass through the collimator, and quanta incident obliquely to it are absorbed in the collimator walls. Because of the higher energy of gamma quanta compared with x-ray quanta, collimators 10 need to be made many times higher than antiscatter grids for x-radiation.
For instance, scattered quanta may be deselected during the image recording by taking only quanta with a particular energy into account in the image. However, each detected 15 scattered quantum entails a dead time in the gamma camera of, for example, one microsecond, during which no further events can be registered. Therefore, if a primary quantum arrives shortly after a scattered quantum has been registered, it cannot be registered and it is lost from the image. 20
Even if a scattered quantum coincides temporally— within certain limits—with a primary quantum, a similar effect arises. Since the evaluation electronics can then no longer separate the two events, too high an energy will be determined and the event will not be registered. Both said 25 situations explain how highly effective scattered beam suppression leads to improved quantum efficiency in nuclear diagnosis as well. As the end result, an improved image quality is thereby achieved for equal dosing of the applied radionuclide or, for equal image quality, a lower radionu- 30 elide dose is made possible, so that the patient's beam exposure can be reduced and shorter image recording times can be achieved.
In future, increasing use will also be made for recording gamma images of solid-state detectors which are formed 35 from an array of detector elements. The detector elements are arranged in this case in a generally square or rectangular grating. In the case of such solid-state detectors, as well, there is a need to employ effective suppression measures to reduce as far as possible the striking of scattered beams on 40 the detector surface formed by the detector elements. Because of the regular structuring of the pixels, formed by the detector elements, of the detector, there is here, in addition, the risk of mutual interference between the structures of pixels and collimators. 45
Collimators for gamma cameras are generally produced from mechanically folded lead lamellae. This is a relatively cost-efficient solution. However, it has the disadvantage that, in particular when using solid-state cameras with an array of detector elements, for example in the case of cadmium-zinc 50 telluride detectors, perturbing aliasing effects can arise because the structure of these collimators is then relatively coarse.
The publication by G. A. Kastis et al., "A Small-Animal Gamma-Ray Imager Using a CdZnTe Pixel Array and a 55 High Resolution Parallel Hole Collimator" discloses a method for producing a cellularly constructed collimator for gamma radiation. In this case, the collimator is produced from laminated layers of metal films, here made of tungsten, which are photochemically etched. However, on account of 60 the large number of photolithographic exposure and etching steps, this production method is very elaborate and costintensive.
U.S. Pat. No. 6,021,173 A describes an approach which is intended to avoid moire structures during operation of an 65 x-ray detector having an array of detector elements in conjunction with an antiscatter grid arranged in a stationary
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fashion. In this publication, the antiscatter grid is applied directly to the x-ray detector over the detector surface. The absorbing structure elements of the antiscatter grid are designed at a spacing from one another which is smaller than the extent of the smallest resolvable detail in the x-ray image. The regularly arranged absorbing structure elements are consequently formed at so high a spatial frequency
The post-published German patent application DE 101 51 568 discloses a method for applying an antiscatter grid to an x-ray detector in the case of which a basic structure for the antiscatter grid is produced directly on the detector surface by way of a rapid prototyping technique such that absorbing regions of the antiscatter grid are situated in less sensitive intermediate regions of the x-ray detector. However, the risk exists in this method of damaging the x-ray detector when producing the antiscatter grid.
SUMMARY OF THE INVENTION
An object of an embodiment of the present invention to specify a method for producing and applying an antiscatter grid or collimator to an x ray or gamma detector having an array of detector elements and with the aid of which it is possible to realize an arrangement of an antiscatter grid or collimator on an x-ray or gamma detector which permits image recording without moire structures in conjunction with a high detective quantum efficiency.
An object may be achieved by a method of the present application. An embodiment specifies an x ray and gamma detector having an antiscatter grid or collimator produced and applied in accordance with the method. Advantageous refinements of the method can be gathered from the following description and the exemplary embodiments.
In an embodiment of the present method, a basic structure for the antiscatter grid or collimator is firstly produced by means of a rapid prototyping technique, preferably with the aid of the technique of stereolithography, through which transmission channels and intermediate walls of the antiscatter grid or collimator are formed which have at least in a first direction a center-to-center spacing which is equal to or an integral multiple of the center-to-center spacing of the sensitive detection regions of the detector. The intermediate walls are subsequently coated with a material which strongly absorbs x-radiation and/or gamma radiation in order to finish the antiscatter grid or collimator. The basic structure coated in this way, that is to say the antiscatter grid or collimator, is subsequently applied to the detector surface, and connected to the latter, in such a way that at least the intermediate walls running perpendicular to the first directions, or their coating, are situated over the less sensitive intermediate regions of the detector surface. These intermediate regions of the detector surface which are less or not sensitive correspond to the regions in which the individual detector elements abut one another. Since the detector elements are additionally generally not sensitive to radiation over the entire surface, such insensitive intermediate regions arise in the edge regions of the individual detector elements.
The basic structure for the antiscatter grid or collimator is constructed in the case of the present method in such a way that at least in one direction the intermediate walls or the absorbing coating provided on the intermediate walls inside the transmission openings extend on one side of the intermediate walls over the intermediate regions on the detector. If, because of particularly narrow intermediate regions, the intermediate walls cannot be produced with a sufficiently
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small thickness, they can be situated in the contact region with the detector surface at least partially over the intermediate regions.
The intermediate walls extending in the other direction are preferably likewise arranged in such a way that they or 5 their coating are situated on one side over the non-sensitive intermediate regions of the detector surface. In both dimensions, the spacings of the intermediate walls can thereby assume the value of an integral multiple of the center-tocenter spacing of the sensitive detection regions of the 10 detector. However, the center-to-center spacings of the intermediate walls preferably correspond in both mutually perpendicular directions to the corresponding center-to-center spacings of the sensitive detection regions.
By using a rapid prototyping technique when constructing 15 the basic structure, very filigree structures can be produced with very high accuracy. In the rapid prototyping technique, 3D CAD designs, here the geometry of the basic structure, are converted into volume data in the CAD system. The 3D volume model for the rapid prototyping is then divided into 20 cross sections in a computer. The cross sections have a layer thickness of 100 um or less. After the data have been sent to a rapid prototyping system, the original shape is built up layer by layer. The present method in this case uses a rapid prototyping technique in which the layer construction is 25 carried out by action of radiation, in particular laser radiation. Laser radiation, specifically, offers the advantage of producing very filigree structures in this case.
In a preferred embodiment of the present method, the technique of stereolithography is used for constructing the 30 basic structure. In this method, a computer-controlled UV laser beam forms the respective contours of the individual layers of the 3D volume model of the basic structure on a liquid polymer resin. The resin is cured under the action of the laser at the exposed points or areas. The component 35 platform of the system is then lowered, and a new thin layer of photopolymer resin is applied. By repeating these steps, the complete geometry of the basic structure is successively constructed from the bottom upward. In one embodiment of the present method, it is also possible to use the technique 40 of microstereolithography to produce the basic structure.
By comparison with the post-published document specified in the introduction to the description, the production and application of an antiscatter grid or collimator directly onto the detector is simplified by the present method and can be 45 realized more cost-effectively. Owing to the configuration and proposed application of the antiscatter grid or collimator in the specified way such that one or two of the total of four intermediate walls or their coating correspond sufficiently on one side with a boundary line between the detector pixels, 50 the required dose when using this antiscatter grid or collimator is reduced. On the basis of this arrangement in regions which have a lesser sensitivity to the x ray or gamma radiation, the intermediate walls or the absorbing coating provided thereon act to attenuate primary beams less or not 55 at all. The proposed configuration and the application of the antiscatter grid or collimator principally in the said intermediate regions additionally excludes interference with the pixel structure of the detector.
The detectors virtually always have a filling factor which 60 is smaller than 1. This holds, in particular, for a Si detector surfaces coated with phosphor. Even in the case of detector surfaces coated with selenium, the filling factor differs from 1, particularly for small detection regions or pixels. Consequently, the quantum efficiency is primarily reduced in the 65 regions between the pixel surfaces. If the primary radiation is now attenuated only between the pixels by structures
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absorbing the scattered beams, this is more advantageous for achieving a high quantum efficiency than if these structures are arranged arbitrarily. Moire interference between the pixels and the absorbing structure is thereby impossible. The present arrangement of the antiscatter grid or collimator thus permits the primary radiation to be more effectively rendered useful, since the unavoidable primary absorption of the antiscatter grid or collimator occurs chiefly in geometrical regions of the detector which make a reduced contribution to the image signal.
In a preferred embodiment of the present method, the end faces of the intermediate walls are kept free of the absorbing coating, or the absorbing coating possibly applied is removed from these end faces. Here, the end faces are understood to be the sides of the intermediate walls which face the detector, and the sides which are averted from the detector, that is to say the sides which are not situated inside the transmission channels. These end faces can be kept free, for example, by appropriate masks when the coating is being applied. However, the coating is preferably applied to the entire basic structure and subsequently removed by an appropriate chemical or mechanical method.
If, moreover, use is made as material of the basic structure of a material which is substantially transparent to x-radiation and/or gamma radiation, the primary beam transmission of the antiscatter grid or collimator is substantially increased by this measure, since it is possible even in the material regions between the coated inner surfaces of the intermediate walls for appropriate primary radiation to pass through without attenuation or only slight attenuation and to contribute to the construction of the image. When use is made of the technique of stereolithography for constructing the basic structure, such a refinement can be realized without difficulty by selecting a suitable polymer. Applying the absorbing layer can be performed in this case by various known methods, for example vapor deposition, by sputtering or by an electrolytic process. One possibility for applying the layer also consists in using sputtering to apply a thin metal layer which then serves as starting layer for subsequent electrolytic deposition of the layer.
In a further advantageous refinement of the present method, the antiscatter grid or collimator is applied to the detector surface and connected to the latter in such a way that in each case a corner region of the coating inside a transmission channel comes to be situated over a switching element of a detector element. Switching elements of this type, such as a diode or TFT, have no photosensitivity at all, and therefore make no contribution to the detection of radiation. Consequently, the attenuation of the primary radiation in this region has no great influence owing to the positioning of the corner regions of the coating over these switching elements.
Various techniques can be used for the adjusted application of the antiscatter grid or collimator to the detector surface. One technique consists in marking the precise desired position of the basic structure with reference to the pixels, situated therebelow, of the detector as fiducial markers or reference lines on the surface of the detector or on a protective layer which is applied to the scintillator. If appropriate, the desired position can also be projected on optically.
The marking of the desired position can be controlled by means way of infrared microscopy. Subsequently, the antiscatter grid or collimator is connected to the detector surface, for example by bonding. In this case, the bonding is performed step by step such that a small subarea is firstly bonded after the position of the basic structure in relation to the pixels has been precisely set in this region. Thereafter,
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