Abstract:
A scanner system comprises: a radiation generator arranged to generate radiation to irradiate an object, the radiation generator comprising a radiation source arranged to produce radiation and a filter arranged to provide variable filtering of the radiation from the source; a detector structure arranged to detect the radiation after it has interacted with the object and generate a sequence of detector data sets as the object is moved relative to the generator, and a processing system arranged to process each of the detector data sets thereby to generate a control output arranged to control the radiation generator so as to vary the filtering, thereby to vary the radiation output by the radiation generator as the object is scanned.

Description:
CROSS-REFERENCE 
       [0001]    The present application is a 371 national stage application of PCT/IB2011/050469, filed on Feb. 3, 2011, which relies on Great Britain Patent Application No. 1001738.2, filed on Feb. 3, 2010. All of the aforementioned applications are herein incorporated by reference. 
     
    
     FIELD 
       [0002]    The present application relates to scanning systems, in particular security scanning systems, and has particular application in the use of high energy X-radiation to inspect packages, cargo, containerised loads and vehicles for the presence of illicit materials and devices. 
       BACKGROUND 
       [0003]    X-ray radiography imaging of cargo containers and trucks for the detection of nuclear material and other contraband requires high-intensity X-ray radiation sources. The higher the source intensity at a specific source energy, the greater the amount of material the X-ray beam can penetrate and the better the contrast resolution. In current practice, the X-ray source intensity of an inspection system is typically set to the highest level allowable (referred to herein as the Output Set Point or OSP) under the particular circumstances of the system and the inspection area, and all cargo is inspected using this fixed intensity, whether the accurate inspection of the cargo requires this intensity or not. This OSP is, however, typically not the highest rated intensity that the source is capable of producing. More often than not, the OSP is set to not exceed a specified radiation dose limit at the boundary of a predefined exclusion zone, or, in case of a portal inspection system (where truck drivers drive their trucks through the inspection system), to stay below a certain dose limit to the driver of the inspected truck. There are, therefore, two deficiencies of the current practice:
       1. The rated maximum intensity of the source is usually higher than the OSP and the inspection system is therefore intrinsically capable of providing higher cargo penetration than it is set to provide;   2. The system uses much higher intensity for certain cargos (or parts thereof) than needed, leading, on average, to unnecessarily high radiation levels around the inspection system.       
 
       SUMMARY 
       [0006]    The present application discloses a scanner system. The scanner system may comprise a radiation generator arranged to generate radiation to irradiate an object. The radiation generator may comprise a radiation source arranged to produce radiation and a filter arranged to provide variable filtering of the radiation from the source. The system may further comprise detection means, for example a detection system, arranged to detect the radiation after it has interacted with the object, and which may generate a sequence of detector data sets as the object is moved relative to the generator. It may further comprise processing means, for example at least one processor, arranged to process each of the detector data sets thereby to generate a control output arranged to control the radiation generator, for example so as to vary the filtering, thereby to vary the radiation output by the radiation generator as the object is scanned. 
         [0007]    The processing means may be arranged to define a parameter of the detector data, to determine a value of the parameter for each data set, and generate a control output arranged to vary the radiation output depending on the value, for example if the value of the parameter does not meet a predetermined condition. 
         [0008]    The detection means may comprise a plurality of detectors. The detector data may comprise a set of intensity values indicative of the intensity of radiation at each of the detectors. 
         [0009]    The filter may comprise a filter element, which may be movable between a plurality of positions so as to provide a plurality of different levels of filtering. 
         [0010]    The filter may comprise a plurality of filter sections and may be arranged to move so that each of the filter sections is aligned with the radiation in turn. 
         [0011]    Indeed the present invention further provides a scanner system comprising a radiation source arranged to produce radiation, detection means arranged to detect the radiation after it has interacted with an object thereby to produce data sets, a filter comprising a plurality of filter sections and arranged to move so that each of the filter sections is aligned with the radiation in turn, and a processing system arranged to control at least one of the filter, the source, and the detection means thereby to control the level of filtering of the detected radiation. 
         [0012]    The radiation generator may be arranged to generate the radiation in pulses, and may, for example under the control of the processing system, be arranged to vary the timing of the pulses so as to vary which of the filter sections is aligned with the radiation when the data sets are collected. 
         [0013]    The processing system may be arranged to vary the timing of collection of the data sets, thereby to vary which of the filter sections is aligned with the radiation when the data sets are collected. 
         [0014]    The filter sections may have different filter characteristics. For example they may be of different thicknesses, and they may be made of the same material, or they may be made of different materials. 
         [0015]    The filter may be rotatable, or it may be movable linearly, or in a reciprocating manner. 
         [0016]    Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a schematic view of a radiation imaging system according to an embodiment of the invention; 
           [0018]      FIG. 2  is a schematic view of a radiation imaging system according to a further embodiment of the invention; 
           [0019]      FIG. 3  is a front view of a filter wheel forming part of a radiation imaging system according to a further embodiment of the invention; and 
           [0020]      FIG. 4  is a section through the filter wheel of  FIG. 2 ; 
           [0021]      FIG. 5  is a further section through the filter wheel of  FIG. 2 ; and 
           [0022]      FIG. 6  is a section similar to  FIG. 5  through a filter wheel according to a further embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    Referring to  FIG. 1 , an X-ray imaging system comprises an X-ray generator  10  arranged to generate a beam  11  of X-rays, and a detector array  12  arranged to detect the X-rays after they have passed through an imaging volume  14 . The X-ray generator is arranged to produce the beam in the form of a vertically extending fan beam, and the detector array  12  comprises a vertical linear detector array  12   a  and a horizontal linear detector array  12   b.  Therefore the imaging volume  14  is in the form of a thin vertical slice. A processing system  16  is arranged to receive detector signals from each of the detectors in the detector array  12  and to process the detector signals so as to generate image data. A variable filter system  18  comprises a filter element  20 , which in this case is formed of sheet metal, and is movably mounted so that it can move between a deployed position in which it is placed in the path of the X-ray beam  11 , and a retracted position in which it is not in the path of the X-ray beam. An actuator  22  is connected to the filter element  20  and is arranged to move the filter element  20  between its deployed and retracted positions. The processing system  16  is arranged to analyse the detectors signals and control the actuator  22  to control the position of the filter element in response to that analysis. 
         [0024]    In operation, the system starts with the filter element in its deployed position, and the source  10  is arranged to generate X-rays in pulses, and for each pulse the processing system  16  is arranged to sample the detector signals to generate an image data set, from which a two-dimensional image can be generated. The image data set in this case comprises a set of intensity values, one for each detector in the array  12 . As the object moves through the imaging volume, in the direction into or out of the page of  FIG. 1 , a series of data sets is collected which can then be built up to form a 2D image of the object. As each data set is collected, the processing system  16  is arranged to analyse it to determine whether it meets a condition, which in this case is that all of the detectors in the detector array  12  receive at least a minimum level of radiation. If this condition is met, then the processing system simply waits for the next radiation pulse so that it can collect the next set of data. However, if the condition is not met, the processing system sends a signal to the actuator  22  which is arranged to respond by moving the filter element to its retracted position. This reduces the amount of radiation that the filter blocks, and therefore increases the intensity or the X-ray beam  11 , which in turn increases the level of radiation detected by the detector array  12 . The filter is maintained in the retracted position until a further condition is met, which in this case is that all of the detectors detect a second level of radiation, which is higher than the minimum level. If this condition is met, then the processing system is arranged to control the actuator to move the filter element back to the deployed position. The gap between the higher and lower set levels provides a degree of hysteresis around the switch point. 
         [0025]    Referring to  FIG. 2 , in a second embodiment of the invention, which is similar to the first embodiment, but here shown in plan view, the filter element  120  is of varying thickness, having a number of sections  120   a,    120   b,    120   c,    120   d  of different thicknesses. In this case the filter element is located to the side of the X-ray fan beam  111  so that the distance that it needs to move is minimized. In this embodiment the filter element has five positions, corresponding to each of the four sections  120   a,    120   b,    120   c,    120   d  being in line with the X-ray beam  111 , and the filter element  120  being completely clear of the X-ray beam a shown in  FIG. 2 . In this case, the processing system (not shown) is arranged to analyse each set of image data, which again relates to a 2D image slice, and select the appropriate position for the filter element  120  for the subsequent slice image. It is then arranged to send a signal to the actuator  122  to move the filter element  120  to the required position. As with the first embodiment, the conditions which have to be met for the filter element to be moved can be defined in a number of ways. 
         [0026]    Referring to  FIGS. 3 and 4 , a scanner system according to a third embodiment is similar to that of  FIG. 2 , except that the filter  320  comprises a filter wheel comprising a rotatable plate  321  arranged to rotate about an axis parallel to the centre line of the X-ray beam  311 , with a number of filter blocks  330  formed on it, and evenly spaced around its axis of rotation. The plate  321  comprises a central portion  321   a,  with a number of spokes or blades  321   b  extending radially outwards from the central portion, and having gaps  332  between them. Each of the filter blocks  330  is mounted on a respective one of the spokes  321   b.  Referring to  FIG. 5 , which is a section along an arc of the filter wheel  320 , each of the filter blocks  330  is made up of a number of sections  330   a,    330   b,  side by side in the circumferential direction and each extending across the full width of the filter block  330  in the radial direction, which are of different thicknesses. In this embodiment there are 20 spokes of the wheel and 20 filter blocks  320 . The filter  320  is located so that, as the plate  321  rotates about its axis, each of the filter sections  330   a,    330   b  of each of the filter blocks  330 , alternating with the gaps  332  between the filter blocks, is placed in turn in the path of the X-ray beam  311 , between the X-ray source  310  and the detector array  312 , so that it filters the X-ray beam  311 . The processing system  316  is arranged to receive the detector signals from the detector array  312 , and also to receive signals from a rotary position sensor  324  which is arranged to detect the rotary position of the filter wheel  320 . From the position signal, the processing system  316  can determine which of the filter sections  330   a,    330   b  is in the path of the X-ray beam at any time. The processing system  316  is also arranged to control operation of the X-ray source  310  so as to produce X-rays in pulses, and the timing of the pulses can be controlled by the processing system. The pulses are produced at the frequency of the rotation of the filter wheel  320  multiplied by the number of filter blocks  330 , which in this case is twenty. Therefore twenty pulses (one for each filter block) can be transmitted during each rotation of the filter wheel  320 , each coinciding with a respective one of the filter sections  330   a,    330   b,  or gap  332 , being aligned with the X-ray beam  311 . 
         [0027]    The processing system is arranged to add or remove a delay into the pulse train to shift the pulse timings so as to coincide with alignment of a different set of filter block sections  330   a,    330   b,  or the gaps  332 , thereby to vary the degree of filtering of the X-ray pulses. 
         [0028]    Operation of this system is similar to those described above in that the after each data set is collected, typically comprising a single sample from each detector in the linear array  312 , the processing system analyses the detector signals to check for the meeting of one or more conditions, and then determines whether the degree of filtering needs to be increased or decreased for the next or another subsequent data set. If it does, then the processing system is arranged to shift the timings of the X-ray pulses by a timing offset equal to the filter section transition period to the desired filter section, i.e. the time between alignment of subsequent filter sections with the X-ray beam, thereby to select a different set of operative filter sections of the required thickness. Depending on the type of detector used, the processing system  316  may also be arranged to shift the detector signal sampling times so that they remain synchronized with the X-ray pulses. The degree of filtering, and hence the intensity of the X-ray beam reaching the object, can therefore be adjusted after each detector data set is collected. 
         [0029]    Referring to  FIG. 6 , in a modification to the embodiment of  FIG. 5 , each stepped filter block  320  is replaced by a filter block  420  of continuously varying thickness, the thickness varying in the circumferential direction of the filter wheel. The timing of the X-ray pulses is therefore controlled to determine which part of the filter block  320  they pass through, which determines how much filtering they experience. 
         [0030]    In a further modification to the embodiments of  FIGS. 5 and 6 , the processing system  322  is arranged to control the speed of the motor driving the filter wheel, which enables it to select different thickness filter sections to be the operative filter sections by varying the speed of the filter wheel, while the X-ray pulse frequency remains fixed.