Abstract:
The present invention relates to an apparatus and a method for detection of radiation comprising at least a first collimator arranged to transmitted radiation through at least a first slit in a Z-direction and prevent radiation in said Z-direction apart from through said at least first slit. The invention further comprises at least a first array of at least two radiation detecting elements, that each of said radiation detecting elements having α length a in an X-direction, where said X-direction is the direction of said array of radiation detecting elements, that each of said radiation detecting elements having a length β in a Y-direction, that said at least first slit, for letting through radiation in the Z-direction, has a length in said second X-direction which is at least as long as said array of radiation detecting elements, that said at least first slit has a length in said Y-direction which is substantially shorter than said length β of said radiation detecting elements, and displacement means arranged to move said collimator and/or said array of radiation detecting elements.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to detection of radiation. More specifically, the present invention relates to detection of radiation, particularly X-ray radiation, using scanning.  
         BACKGROUND OF THE INVENTION  
         [0002]    Film has been used in medical X-ray imaging for more than 100 years, and is yet the dominating technique for X-ray detection at hospitals all over the world. The largest improvement came in the 1960s when embedding the film in a fluorescenting screen to increase the sensitivity drastically reduced the dose, but at the expense of a reduced position resolution.  
           [0003]    In the middle of the 1990s, the first digital technique appeared on the market, where the X-ray illumination gives digital signals in the detector, for creating digital images. These detectors either convert the X-ray flux to visible photons in a scintillator or to charge in a semiconductor. The light from the scintillator is detected with e.g. a TFT or a CCD (Charged Coupled Device), and the charge from the semiconductor is detected using e.g. a TFT (Thin Film Transistor). These techniques solves some of the problems associated with using film, but still have some drawbacks.  
           [0004]    The basic CCD consists of a series of metal oxide-semiconductor capacitors that are fabricated close together on a semiconductor surface. Today, CCDs are used in a wide variety of indirect-conversion X-ray imaging devices, including large-area radiographic imaging systems. The single most salient characteristic of CCDs with regard to radiography is that they are physically small in size, typically 2-4 cm 2 , which is much smaller than typically projected X-ray areas. Because of this, cost-effective CCD-based radiographic systems must include some means of optical coupling to reduce the size of the projected visible light image and to transfer the image to the face of one or more CCDs. Some systems are based on an array of CCD cameras, each of which is coupled to a scintillator by a lens or fibre optic taper.  
           [0005]    Recent advantages in photolithography and electronic micro fabrication techniques have enabled the development of large area x-ray detectors with integrated readout mechanisms based on arrays of TFT. Unlike CCD-based detectors that require optical coupling and image demagnification, TFT-based, flat panel systems are constructed such that the pixel charge collection and readout electronics for each pixel are immediately adjacent to the position of the X-ray interactions. TFT-arrays are used as active electronic elements in both indirect and direct conversion flat panel detectors. In indirect systems X-rays produces light in scintillators, which is optically coupled to a light sensitive device where the produced light is converted into charges, which are detected. In direct systems X-rays directly produces charges, which are detected.  
           [0006]    TFT-arrays are typically deposited onto a glass substrate in multiple layers, beginning with readout electronics at the lowest level and followed by charged collector arrays at higher levels. Depending on the type of detector, X-ray-elements (direct conversion), light-sensitive elements (indirect conversion) or both, are deposited to form the top layer of the electronic sandwich structure.  
           [0007]    Typically, amorphous selenium is used as conversion material, due to its good X-ray detection possibilities and high intrinsic spatial resolution, in direct conversion TFT systems. Before exposure, an electric field is applied across the amorphous selenium layer through a bias electrode on the top surface of the selenium. As X-rays are absorbed in the detector, electrons and holes are released within the selenium, and due to the electric field within the selenium, electric charges are drawn directly to the charge collecting electrodes. Pixels are separated by means of field shaping within the selenium layer.  
           [0008]    Indirect conversion systems based on TFT-arrays are constructed by adding an amorphous silicon photodiode circuitry and a scintillator optically coupled to the top layers of the TFT sandwich. When X-rays strike the scintillator, visible light is emitted proportional to the incident X-ray energy. Visible light photons are then converted into an electric charge by the photodiode array.  
           [0009]    TFT-arrays, however, have a limited position resolution set by the minimum pixel size, which is currently limited to approximately 100 μm×100 μm. Researchers are currently reaching minimum pixel sizes as small as 70 μm×70 μm, whereas for affordable consumer products pixel sizes are still reasonably large, for instance approximately 450 μm×450 μm in computer screens. It would be advantageous if the resolution achieved in X-ray imaging could be made even greater.  
           [0010]    Large TFT-array systems are also costly. An array, with maximum resolution, i.e. 100 μm×100 μm, and a size of 25 cm×25 cm will have 6.25 million individual pixels and would cost more than approximately $100000. The high cost is mainly due to a low yield when trying to minimize the area of the thin-film transistor in amorphous silicon under the requirement that very few dead pixels are allowed.  
           [0011]    One individual pixel comprises for indirect systems, apart from the X-ray detecting means, a photo diode for detecting the light, a capacitor for storing the charge, a switch transistor for reading out the value of the pixel, data and address lines and an isolation distance. This means that a relative small part is available for the X-ray detecting means, for a 100 μm×100 μm pixel only approximately 50%. This is called the fill factor.  
           [0012]    It would be advantageous if the cost could be reduced, while keeping the position resolution high and the size large.  
           [0013]    Another problem with conventional X-ray detecting apparatuses is that the X-rays is scattered in the object to be imaged. Approximately as much as 50% of the incident X-ray radiation is randomly scattered, producing a fog or haze in the X-ray image.  
           [0014]    It would be advantageous if the amount of scattered X-ray radiation could be reduced.  
         SUMMARY OF THE INVENTION  
         [0015]    It is a main object of the present invention to provide such apparatus and method, wherein the obtained resolution in X-ray imaging is increased.  
           [0016]    It is in this respect a particular object of the invention to provide such apparatus and method that allow for X-ray imaging of large objects with improved resolution.  
           [0017]    It is still a further object of the invention to provide such apparatus and method that reduce the administrated radiation dose to an object, such as an examined patient.  
           [0018]    It is still a further object of the invention to provide such apparatus and method that reduces the noise in the image caused by scattered X-ray radiation.  
           [0019]    It is still a further object of the invention to provide such apparatus and method that reduces the cost of X-ray detectors.  
           [0020]    It is still a further object of the invention to provide such apparatus and method that reduces the number of electronic channels in large area X-ray detectors.  
           [0021]    These objects among others are attained, according to the present invention, by apparatus and methods as claimed in the appended patent claims.  
           [0022]    An advantage of the present invention is that the resolution of the X-ray image is increased.  
           [0023]    A further advantage is that the administrated radiation dose to an object, to be imaged, is reduced.  
           [0024]    A further advantage is that the amount of scattered X-ray radiation in the object to be imaged is reduced.  
           [0025]    Further characteristics of the invention and advantages thereof will be evident from the following detailed description of embodiments of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    The present invention will become more fully understood from the detailed description of embodiments of the present invention given herein below and the accompanying FIGS.  1 - 7 , which are given by way of illustration only, and thus are not limitative of the present invention.  
         [0027]    [0027]FIG. 1 a  shows a top view of a preferred embodiment of a radiation detector according to the invention with a collimator having two elongated slits and two arrays of elongated detector elements.  
         [0028]    [0028]FIG. 1 b  shows a side view taken along the line A-A of the embodiment in FIG. 1.  
         [0029]    [0029]FIG. 1 c  shows a side view taken along line B-B of the embodiment in FIG. 1.  
         [0030]    [0030]FIG. 2 shows a top view of a preferred embodiment of a radiation detector according to the invention with a collimator having a single slit and one array of detector elements.  
         [0031]    [0031]FIG. 3 shows a side view of a preferred embodiment of a radiation detector according to the invention with two arrays of detector elements and two collimators each having two slits.  
         [0032]    [0032]FIG. 4 shows a side view of an E-arm type of X-ray radiation detecting apparatus according to the invention.  
         [0033]    [0033]FIG. 5 shows a side view of a preferred embodiment of a radiation detector according to the invention employing a gas detector.  
         [0034]    [0034]FIG. 6 shows a top view of a preferred embodiment of a radiation detector according to the invention where the detector elements are arranged with a distance between adjacent detector elements. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0035]    In the following description, for purposes of explanation and not limitation, specific details are set forth, such as particular techniques and applications in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known methods and apparatuses are omitted so as not to obscure the description of the present invention with unnecessary details.  
         [0036]    The present invention may be applied using a variety of X-ray detectors, such as detectors employing direct TFTs, indirect TFTs, CCDs equipped with scintillator, Cmos detectors, PIN-diodes and gas detectors.  
         [0037]    TFT detector elements have a minimum size of e.g. 100×100 micrometer. However, if a TFT detector element is made longer it may also be made narrower still maintaining a high fill factor. Thus, it is possible to achieve a TFT detector element being narrower than a conventional TFT and for instance 0.1-100 mm long. Thus, a detector element is achieved having a better spatial resolution than the spatial resolution normally achieved in one direction at the expense of the resolution in the other direction.  
         [0038]    [0038]FIG. 1 a  shows a top view of a preferred embodiment according to the invention where such elongated detector elements are used. A first array of detector elements  101  and a second array of detector elements  102  are covered with a collimator  103 . Each of the arrays  101  and  102  comprise several elongated detector elements arranged, with their respective longer side, side by side, to make up an e.g. 25 cm long array of detector elements. Each of the individual detector elements can be made between 0.01 mm and 5 mm wide, e.g. 50 μm. This is the distance α in FIG. 1 b . Each element can then be made between 0.05 and 100 mm long, e.g. 1 mm, distance β in FIG. 1 a . Even though FIGS. 1 a - 1   c  only show two arrays, more arrays may of course be incorporated. Thus, as an example, to cover a 25 cm×25 cm area using 50 μm wide and 1 mm long detector elements, 250 arrays of 5000 detector elements would be needed.  
         [0039]    In the figures each detector element is schematically shown separated from its neighbours by some distance. However, this separation can be made very short, e.g. 3-5 μm, so that the radiation detecting elements can be regarded, for all practical purposes, to be arranged side-by-side. The figures are thus not to scale.  
         [0040]    The collimator  103  is arranged to prevent radiation from reaching the detector elements, apart from at selected areas. Thus, the radiation source (not shown) is placed above the collimator  103 , as taken in a Z-direction. The collimator  103  has a first elongated slit  104  and a second elongated slit  105 . Each of the elongated slits  104  and  105  is e.g. 50 μm wide and 25 cm long, thus covering the complete length of each respective array  101  and  102  in a X-direction but only a relative short distance of the length of each individual detector element in a Y-direction. The radiation reaching each detector element is thus limited in the X-direction by the width of the detector element to 50 μm and limited in the Y-direction to the width of the elongated slits  104  and  105 , respectively, to 50 μm. Thus a spatial resolution is obtained where each pixel is 50×50 μm. The X-direction, Y-direction and Z-direction are substantially orthogonal.  
         [0041]    [0041]FIG. 1 b  shows the arrangement in FIG. 1 a  taken along the line A-A. FIG. 1 c  shows the arrangement in FIG. 1 a  taken along the line B-B. To achieve an image of the radiation the collimator  103  is moved at constant speed, or alternatively step-wise with e.g. 50 μm increment in the Y-direction. The slits  104  and  105  will move over the detector element arrays  101  and  102  and let through radiation from different parts of the scanned area to the detector elements during the scanning. The radiation reaching the detector elements is continuously monitored and recorded in a computer (not shown) and an image of the radiation is assembled.  
         [0042]    As an alternative the slit  104  may pass over both the detector element arrays  101  and  102 . An over sampling, or a double scanning would thus be achieved of the part of object to be imaged that are located above the original position of detecting element array  102 . If, as sometimes occur in these kind of detectors, a detector element in array  102  is broken an improved imaged is obtained, since it is very unlikely that the corresponding element in detector element array  101  would also be broken. It should be clear to the man skilled in the art that in practical applications where not only two arrays are used but a multitude of arrays, substantially the complete object would be subject to over sampling. Only the part over the first array would not be subject to over sampling.  
         [0043]    It would of course also be possible to move the object to be imaged and the detecting element arrays in relation to a fixed collimator. It is also preferable to move the radiation source with the collimator, as will be discussed later.  
         [0044]    [0044]FIG. 2 shows a top view of a radiation detector according to a preferred embodiment of the invention where only one array of detector elements  201  is used and a collimator  202  comprises a corresponding elongated slit  203 . If an object  204 , larger than the detector, as defined by the length of the array of detecting elements in one dimension and the length of the individual detector elements in the other dimension, should be imaged, the detector can be made to move between successive scannings. FIG. 2 shows the detector in a first starting position  200   a , indicated by dotted lines and in a second end position  200   b , where the detector has been moved in the Y-direction in successive steps of length L, where L is the length of the detector elements in the y-direction, so as to scan the object  204 . Thus the slit  203  is brought over the complete area of the object  204  and an image is recorded. That is, both the detector, comprising the array of detector elements  201 , and the collimator is moved during the scan.  
         [0045]    [0045]FIG. 3 shows a side view of a preferred embodiment according to the invention comprising a first collimator  301  and a second collimator  302 . The first collimator  301  has two elongated slits  303  and  304  and the second collimator has two elongated slits  305  and  306  aligned with the first collimators  301  slits  303  and  304 . The elongated slits  303  and  304  are preferably arranged at the projection of incident X-ray radiation, emitted by a radiation source  310 , through the slits  305  and  306 .  
         [0046]    A first array of detector elements  307 , of which only the first detector element is visible in this view, and a second array of detector elements  308 , of which only the first detector element is visible in this view, are arranged below said first collimator  301 . The first  301  and second  302  collimator are arranged at a distance to allow for an object  309 , to be imaged, to be positioned there between. The primary purpose of the first collimator  301  is to prevent radiation from reaching the detector elements  307  and  308  in other positions than governed by the slits  303  and  304 . The primary purpose of the second collimator  302  is to reduce the radiation dose to the object  309 . This is specifically important where a living object, such as a human, is to be imaged.  
         [0047]    The radiation source  310  radiates X-rays towards the second collimator  302 . The second collimator transmits photons only through the slits  305  and  306  and photons thus radiate the object  309  substantially only directly under the slits  305  and  306 .  
         [0048]    The slits  303  and  304  transmit radiation towards the detector elements  307  and  308  so that a reading is obtained relating to the radiation passing through the object  309 .  
         [0049]    The collimators  301  and  302  and the radiation source  310  are moved in fixed relation to each other in the Y-direction, while radiation data is continuously read by the detector elements  307  and  308 , scanning the object  309 .  
         [0050]    [0050]FIG. 4 shows a schematic drawing in side view of an E-arm detector apparatus according to a preferred embodiment of the invention. The E-arm comprises a first arm  401  carrying the radiation source, a second arm  402  carrying a first collimator and a third arm  403  carrying a second collimator. The first, second and third arms are fixed in relation to each other. The radiation source is arranged to radiate X-rays towards the first collimator. The E-arm is attached to a stand  404  so as to allow the E-arm to expose a pendulum movement around an axis  405 , and/or a horizontal transverse movement along the x- or y-direction. An object  406  to be imaged is positioned between the first and second collimators and is fixed in relation to the stand  404 . By moving the E-arm a scanning of the object  406  is achieved. A fourth arm  407  is carrying the radiation detecting elements. The fourth arm  407  may be independently movable in relation to the E-arm as required by some embodiments described herein.  
         [0051]    [0051]FIG. 5 shows a side view of a preferred embodiment according to the invention where a gas detector  501  is employed. Gas detectors are per se known and thus no detail will be given as to the specific operation of the gas detector  501 . In this respect we refer to our co-pending patent applications by Francke et al. having U.S. Pat. Nos. 09/443,294, 09/698,173, 09/709,305, 09/752,722 and the Swedish patent application by Francke et al. having Swedish patent application number SE 0102097-3 all incorporated herein by reference.  
         [0052]    A first collimator  502  comprises slits  503  and  504 , respectively. The first collimator  502  is primarily used for preventing excess radiation to reach an object  505  to be imaged. Slits  503  and  504  admit radiation towards to object  505  along defined openings. A second collimator  506 , having two slits  507  and  508  respectively, is arranged over the gas detector  501  for admitting X-ray photons into the gas detector  501 . The photons ionize the gas, resulting in that electrons, schematically indicated  509  in FIG. 6 a , being released. The electrons, accelerated by a voltage  510  applied over a cathode  511  and an anode  512 , is registered by two different detector elements  513  and  514 , respectively. Alternatively, the voltage  510  may be strong enough to cause electron avalanche amplification during the acceleration towards the detector elements  513  and  514 , respectively. The collimators  502  and  506  are moved in the Y direction in relation to the gas detector  501  to perform a scan of the object  505  and thus a 2-dimensional image is recorded.  
         [0053]    It should be clear that, even though two collimators have been shown in FIG. 5, one of the collimators  502  and  506  might be disposed with. If the amount of radiation reaching the object  505  is of no importance, the collimator  502  may be omitted. The arrangement in FIG. 6 a  may instead do without collimator  506  but, due to scattering of radiation, a lower resolution might be achieved.  
         [0054]    It should also be clear that, even though the detector elements  513  and  514  have been depicted as being located below the anode  512 , they could equally well be positioned above the anode  512 , arranged to function as the anode, in which case the anode  512  would be replaced with the detector elements, or the detector elements could even be positioned adjacent the cathode, in which case they would detect positive ions.  
         [0055]    [0055]FIG. 6 shows a preferred embodiment according to the invention having a first  601  and a second  602  array of detector elements. A collimator  603  has two slits  604  and  605  for admitting radiation towards selected parts of the detector element arrays  601  and  602 . The detector elements in the two arrays of detector elements  601  and  602  are arranged with a distance α from each other, e.g. 2 mm, in X-direction. In this embodiment α is substantially larger than a distance ε, being the width of the detecting area in the X-direction of the detector element. In other aspect the present embodiment is similar to the embodiment described in connection with for instance FIG. 1.  
         [0056]    To obtain a complete 2-dimensional image of an object it is thus necessary to scan not only in the Y-direction by moving the collimator  603 , but also to scan in the X-direction by movement of the detector arrangement, as defined by all individual detector elements, and the collimator  603  (and also possibly the radiation source) but not the object. First the collimator  603  moves in the Y-direction from its start position to its end position scanning over substantially the complete length of the detector elements. Thus, a set of image lines is obtained, however the lines are separated by the distance α in the X-direction. To obtain a complete 2-dimensional image the detector arrangement and the collimator is moved in the X-direction a distance ε′ substantially equal to the line width distance ε, e.g. 50 μm. The collimator is again moved in the Y-direction, however this time in the opposite direction from what was the end position during the first scan, to what was the start position during the first scan, to obtain a second set of image lines in the Y-direction. The process is then repeated until the detector elements  601 ,  602  and the collimator  603  has moved substantially the complete distance α.  
         [0057]    Thus, in summary, the collimator  603  can be said to oscillate back and fourth in the X-direction, while the detector elements  601  and  602 , together with the collimator, is moved a line width distance in the X-direction, at each turning of the collimator.  
         [0058]    Even though, the FIG. 6 only shows two arrays of detector elements with  14  detector elements each, a complete detector arrangement would typically include many more arrays having many more detector elements. Typically, in practical applications, a detector arrangement, which would cover 25×25 cm, could employ detector elements having a length in Y-direction of 5 mm, and in X-direction of 50 μm, being separated with 2 mm. Such an arrangement would thus have 250/5=50 arrays of detector elements, each array having 125 detector elements. Of course the collimator would have 50 slits, each slit corresponding to a respective detector array. The detector would need approximately 40 scans to complete a 2-dimensional image. If the dose administrated to the object to be imaged need to be minimal, a second collimator might be employed.  
         [0059]    It should be noted that in the description given above no specific detail has been given to the electronics involved in reading the individual radiation values by the radiation detecting elements or regarding how the assemble the images from the obtained pixel data. This is, however, well known to the man skilled in the art and the present description is therefore not burdened with these details.  
         [0060]    It will be obvious that the invention may be varied in a plurality of ways. For instance, the number of radiation detecting elements, arrays and slits may be varied without limitations. Such variations are not to be regarded as a departure from the scope of the invention. All such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the appended claims.