Abstract:
A drive-through scanning system comprises a radiation generating means arranged to generate radiation at two different energy levels and direct it towards a scanning volume, detection means arranged to detect the radiation after it has passed through the scanning volume, and control means arranged to identify a part of a vehicle within the scanning volume, to allocate the part of the vehicle to one of a plurality of categories, and to control the radiation generating means and to select one or more of the energy levels depending on the category to which the part of the vehicle is allocated.

Description:
CROSS-REFERENCE 
       [0001]    The present specification is a continuation of U.S. patent application Ser. No. 12/919,482, of the same title, and filed on Aug. 26, 2010, which is a 371 national stage application of PCT/GB2009/000515, filed on Feb. 26, 2009, which relies on Great Britain Patent Application Number 0803642.8, filed on Feb. 28, 2008, for priority. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to scanning systems. It has particular application in scanning systems for cargo, but can also be used in scanners for other applications. 
       BACKGROUND 
       [0003]    There exists a requirement for inspection of cargo for the detection of illicit materials and devices. Currently, a popular way of performing such inspection is through the use of X-ray screening equipment. 
         [0004]    In this apparatus, a high energy X-ray source (typically a 4 MV to 9 MV X-ray linear accelerator) is collimated to a fan beam which irradiates through the cargo item under inspection through to a set of X-ray detectors in a substantially linear configuration. 
         [0005]    To form a two-dimensional image from the set of one-dimensional projection data, it is necessary to scan the cargo item through the X-ray beam. This can be achieved by placing the cargo item on a moving floor which moves the cargo item through the X-ray beam with controlled velocity. Alternatively, the X-ray system can be placed on rails and, with the cargo item stationary, the X-ray system can be scanned along the rails to form the image. Alternatively, the X-ray system can be mounted on a vehicle which can be driven past the stationary cargo item to form the image. Alternatively, the cargo item can be dragged through a stationary X-ray beam using a tow truck. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention provides a drive-through scanning system comprising a radiation generating means arranged to generate radiation and direct it towards a scanning volume, detection means arranged to detect the radiation after it has passed through the scanning volume, and control means arranged to identify a part of a vehicle within the scanning volume, to allocate the part of the vehicle to one of a plurality of categories, and to control the radiation generating means depending on the category to which the part of the vehicle is allocated. 
         [0007]    The categories may include at least one of: a human-occupied category, a human un-occupied category, and a cargo category. They may also include an animal-occupied category or a human-or-animal occupied category. 
         [0008]    The radiation generating means may be arranged to generate radiation at two different energy levels and the control means be arranged to control the generating means to operate at one of the energy levels depending on the category allocated to the part of the vehicle. The two energy levels may correspond to two different energy spectra, for example one having a higher peak energy, or a higher mean energy, than the other. These may be a high level and a low level, to enable, for example, high energy scanning of a cargo-containing part of the vehicle, and low energy scanning of a human or animal occupied part of the vehicle. Alternatively one of the radiation levels may effectively be zero so that, for example, only un-occupied parts of the vehicle are scanned. 
         [0009]    The radiation generating means may include two radiation sources arranged to generate radiation at said two different energy levels, or there may be three or more energy levels, and optionally a corresponding three or more sources. 
         [0010]    The control means may be arranged to receive signals from the detection means and to identify the part of the vehicle at least partly on the basis of those signals. In addition, or alternatively, the control means may be arranged to receive signals from the sensing means and to identify the part of the vehicle at least partly on the basis of those signals. 
         [0011]    The present invention further provides a method of scanning a vehicle comprising moving the vehicle through a scanning system according to any foregoing claim. 
     
    
     
         [0012]    Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which: 
           [0013]      FIG. 1  is a schematic view of a scanning system according to an embodiment of the invention; 
           [0014]      FIG. 2  is a diagram of the data acquisition circuit of a detector of the system of  FIG. 1 ; 
           [0015]      FIG. 3  is a timing diagram showing operation of the circuit of  FIG. 2 ; 
           [0016]      FIGS. 4   a  and  4   b  are schematic views of the system of  FIG. 1  in use; 
           [0017]      FIG. 5  shows a number of driver instruction signals used in the system of  FIG. 1 ; 
           [0018]      FIG. 6  is a schematic plan view of the system of  FIG. 1 ; 
           [0019]      FIG. 7  is a schematic view of an infra-red sensor system of a further embodiment of the invention; 
           [0020]      FIG. 8  is a diagram of the detector circuit associated with each of the sensors of the sensor system of  FIG. 7 ; and 
           [0021]      FIG. 9  is a schematic front view of the sensor system of  FIG. 7  in operation. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]    In the present invention it is recognised that it would be advantageous if the cargo item could be driven through a stationary X-ray inspection system by the normal driver of the vehicle. However, when imaging using a high energy X-ray source, the dose that would be accumulated by the driver during this scanning process would be at an unacceptable level in most commercial operating environments. 
         [0023]    A typical dose rate output from a linear accelerator is in the range 10 to 50 Gy/hr at 1 m. For a scan rate of 0.25 m/s, the dose delivered to a driver at 3 m from the X-ray source can be calculated to be in the range 300 to 1500 μSv. This dose per scan is not generally acceptable. 
         [0024]    However, referring to  FIG. 1 , in one embodiment of the present invention, a scanning system comprises a high energy X-ray source  10  in the form of a linear accelerator, and a low energy X-ray source  12 . The low energy X-ray source  12  can be a stationary or rotating anode X-ray tube operating at a high voltage potential of 60 kVp to 450 kVp. Typically, a tube voltage of 160 kVp provides a good balance between radiation dose, image quality, system reliability and system cost. The high energy X-ray source may comprise stationary anode X-ray tubes. The anode is typically operated at or near ground potential and the cathode is typically operated at negative potential. The anode is then cooled with oil, water or other suitable coolant. In low power X-ray tubes of the low energy source  12 , the anode is typically operated at high positive potential and the cathode is typically operate at high negative potential and no direct anode cooling is provided. 
         [0025]    A detector system  14  comprises a plurality of detectors  16  arranged to detect X-rays from both of the sources  10 ,  12 . The detectors  16  are arranged around a scanning volume  18 , in a vertical array  20  which extends down one side of the scanning volume  18 , on the opposite side of it to the sources  10 ,  12 , and horizontal array  22  which extends over the top of the scanning volume. The sources  10 ,  12  are located close to each other and both in the same plane as the detector arrays. Each of the sources  10 ,  12  is arranged to generate X-rays in a fan beam in the common plane. The dose rate at the output of a low voltage X-ray generator  12  is substantially less than that from a linear accelerator  10 . For example, the dose rate from a standard X-ray source operating at 160 kVp with a 1 mA beam current is typically around 0.3 Gy/hr at lm. For a scan rate of 0.25 m/s, the dose delivered to a driver at 3 m from the X-ray source can be calculated to be around 10 μSv per scan. 
         [0026]    In one practical embodiment of this invention, the scan of a vehicle including a driver&#39;s cab and a cargo container is started using the low energy X-ray source  12  only. As the vehicle is driven through the scanning volume, image data is collected as the driver&#39;s cab passes through the X-ray beam. Once the driver&#39;s cab has passed through the beam, the high energy X-ray linear accelerator  10  is switched on and the low energy X-ray source  12  is turned off. The main cargo load would be inspected with the full intensity high voltage X-ray beam from the linear accelerator  10  to provide a high level of inspection. 
         [0027]    In this hybrid imaging system, the driver will normally be sitting within the cab of a vehicle, and this cab will afford the driver some additional protection which will drop the driver dose further still. 
         [0028]    An X-ray beam at 160 kVp beam quality will be able to penetrate through the driver and 10-20 mm of steel so providing inspection capability of many parts of the drivers cab including the tyres, door panels and roof although little inspection capability would be provided in the main engine compartment. 
         [0029]    The detector elements in the detectors  16  in a cargo screening system will typically be tuned such that their full scale matches the peak intensity that can be delivered from the X-ray linear accelerator  10 . This detector elements are further designed to achieve a dynamic range on the order of 100,000 (i.e. a noise level of around 10 parts per million of full scale range). 
         [0030]    With no object present in the beam, the output from the conventional X-ray generator  12  will be equivalent to approximately 0.05% to 0.3% of full scale depending on how the detectors  16  are tuned. After attenuation by the driver and 10 mm of steel, the signal, i.e. X-ray intensity, at the detector  16  is expected to drop by a further factor of 1000. This gives a signal at the detector of 1/20,000 of full scale which is still within the reasonable dynamic range of the detector  16 . 
         [0031]    Referring to  FIG. 2 , the scanning system further comprises a data acquisition system that is capable of acquiring and merging the two sets of X-ray image data from the detectors  16 , generated by X-rays from the two sources  10 ,  12  respectively. With reference to  FIG. 2 , for each detector  16 , a preamplifier/integrator circuit  30  is provided with two independent integrator circuits; side A and side B, connected in parallel between the sensor  16  and an analogue-to-digital converter (ADC)  32 . Each integrator feeds into the shared ADC  32  through a simple multiplexor. 
         [0032]    Each preamplifier/integrator circuit  30  comprises an amplifier  34  in parallel with a capacitor  36  and a re-set switch  38 . The input to the amplifier is connected to the sensor  16  by an integrate switch  40  and the output from the amplifier is connected to the ADC by a digitize switch  42 . Each of the switches can be closed by a control signal from a controller  44 . Closing the integrate switch starts the circuit integrating the signal from the sensor, increasing the charge on the capacitor  36 , and opening it stops the integration. Closing the digitizing switch connects the capacitor  38  to the ADC which converts the stored voltage to a digital output signal. The capacitor can then be discharged by closing the re-set switch  38  before the next integration. 
         [0033]    As shown in  FIG. 3 , the integration time on side A, when the control signal A int  from the controller  40  is high, is short, while the integration time on side B, when the control signal B int  from the controller  40  is high, is long. In each case the integration time corresponds with the time that the appropriate source  10 ,  12  is turned on, also under control of the controller  40 , the source being turned on at the beginning of the associated integration time and turned off at the end of the associated integration time. The sources  10 ,  12  are therefore turned on alternately. As can be seen from  FIG. 3 , this means that the low energy source  10  is turned on for relatively long periods, and turned off for shorter periods, and the high energy source  10  is only turned on for the short periods while the low energy source is off. The cycle time is typically on the order of  10  ms with an A side integration time typically of 10 μs and a B side integration time of 9.990 ms. In each case, the digitizing switch  42  is closed, by a short pulse in the appropriate control signal A digitize  or B digitize  from the controller  40 , to digitize the integrated signal at the end of the integration time over which integration has taken place. 
         [0034]    When imaging with the low energy X-ray source  12 , the primary signal is read out using the B side digitised data. When imaging with the linear accelerator source  10 , the primary signal is read out using the A side digitised data. It will be appreciated that the timing described above allows the two sources to be used alternately to form alternate two-dimensional image slices, or one of the sources to be turned off so that just one of the sources is used to generate a series of two-dimensional image slices. 
         [0035]    In one mode of operation of this embodiment of this invention, when imaging with the high energy X-ray source  10 , the low energy X-ray generator  12  is turned off. However the B-side digitised data is used to collect pulse-by-pulse dark offset data which is time and position correlated with the image data from A side and subtracted as dark noise from the imaging signal to provide correction of the imaging signal to correct for the dark noise. 
         [0036]    Referring to  FIG. 4 , the X-ray sources  16  and multi-element detector arrays  20 ,  22  are located within a fixed housing  50  which is firmly attached to the ground and forms an arch over the scanning volume. The system further comprises a traffic control system which includes a signalling system  52 , including traffic lights  54 , and a signal display  56 , arranged to provide signals to the driver of the vehicle to regulate the speed and/or timing of driving the vehicle through the scanner. The traffic control system further comprises one or more speed detectors, in this case a radar gun  58 , arranged to measure the speed of the vehicle. Referring to  FIG. 6 , the traffic control system further comprises a first camera  60  on one side of the scanner and a second camera  62  on the other side of the scanner. As shown in  FIG. 4   a , the driver drives the vehicle including the truck  70  and cargo load  72  through the detection system, following speed indications that are provided via the traffic light system. As shown in  FIG. 4   b , the truck  70  and cargo load  72  pass through the X-ray beam between the X-ray sources  10 ,  12  and the detector arrays  20 ,  22 . 
         [0037]    To maintain a high quality image, it is preferable that the velocity of the object, in this case the vehicle, under inspection should remain substantially constant throughout the whole of the scanning of the object. The traffic control system is provided for this purpose. The radar speed gun  58  is arranged to continuously monitor the speed of the vehicle, including the load  72  and to feed back to a control unit which controls the visual display  56 , mounted by the roadside, which advantageously can be arranged to provide a number of display signals as shown in  FIG. 5 . At the left hand side of  FIG. 5 , a horizontal arrow  80  is lit in a green colour when the driver is at the optimal speed, i.e. within a predetermined speed range. When the truck is travelling too fast, a downwards pointing orange coloured arrow  82  will be displayed. Conversely, when the load is travelling too slowly, an upwards pointing arrow  84  will be displayed. If the velocity of the load becomes too low for the scan to continue, or if the load stops, a red T sign  86  will be displayed and the scan will be terminated (see middle graphic of  FIG. 5 ). When the load is going much too fast, a red “hand” sign  88  will be displayed and the scan will be terminated (see right hand graphic in  FIG. 5 ). Other traffic control systems can be used, for example giving numerical displays of desired vehicle speeds, 
         [0038]    The traffic lights  54  (with Red, Amber and Green indicators) are arranged to control the movement of each vehicle to be inspected through the scanner. The use of such traffic control measures substantially reduces the human effort required to co-ordinate scanning of cargo loads. This is advantageous in reducing cost of operation as well as in reducing employee radiation dose exposure. 
         [0039]    In a further aspect of this invention, it is necessary to control the imaging system in order to control which one of the two X-ray sources  10 ,  12  should be switched on at all times during a scan of a vehicle and between scans of different vehicles. To facilitate this process, a small number of video cameras  60 ,  62  is installed around the X-ray installation, typically as shown in  FIG. 6 . One camera  60  views the front of the vehicle as it approaches the scanner. Another camera  62  views the rear of the vehicle as it exits from the scanner. A third camera  64  views down between the vertical detector array  20  and the side of the load furthest from the X-ray sources  10 ,  12 . A fourth camera  66  views down between the side of the load closest to the X-ray sources  10 ,  12  and the vertical supporting structure  50 . 
         [0040]    Prior to the vehicle entering the image inspection area, all X-ray sources  10 ,  12  are normally be switched off. As the vehicle enters the image inspection area, the vertical viewing cameras  64 ,  66  are used to monitor the exact position of the vehicle and to control turn on of the low energy X-ray beam when the front of the vehicle is around 10 cm from the vertical imaging plane. It is prudent to utilise one or more secondary sensors, such as an infra-red light beam to validate the position of the vehicle with respect to the imaging plane. The vertical viewing cameras  64 ,  66  continue to monitor the position of the vehicle as it moves through the scanning plane, seeking to determine when the trailing edge of the driver&#39;s cab  70  has passed through the X-ray beam. Once this feature has been detected, the X-ray linear accelerator source  10  is prepared for operation, but no pulses will be allowed to be generated by that source until such time as the video cameras  60 ,  62 ,  64 ,  66 , have detected that the leading edge of the cargo load  72  has entered the imaging plane. At this point, the X-ray linear accelerator is activated to generate a high energy X-ray beam and the low energy X-ray source  12  is turned off. The scan can now proceed until cameras  62 ,  64 , and  66  all verify that the cargo load  72  has exited the imaging plane. At this point both X-ray sources  10 ,  12  are turned off. 
         [0041]    As a secondary safety feature, an infra-red light curtain is provided to illuminate a plane close to, and parallel to, the imaging plane to establish the presence of the vehicle, and determine the vertical profile of the part of the vehicle that is within the imaging plane so as to help determine which part of the vehicle is in the imaging plane. Referring to  FIG. 7 , in this embodiment, a series of light sources in the form of infra-red light emitting diodes  80  are arranged in a vertical linear array. A control circuit  82  is connected to each LED  80  and comprises a set of addressable switches each connected to a respective one of the LEDs  80 . The control circuit  82  is arranged to address each light source  80  in turn to turn it on, and the activated light source is pulsed by a clock pulse at a frequency of typically 10 kHz. Each light source is turned on for typically 1 ms at a time. In an array with 20 light sources, it is then possible to scan the system every 20 ms, or equivalently at a 50 Hz repetition rate. 
         [0042]    A series of infra-red sensitive photodiodes  84  are arranged into a vertical linear array on the opposite side of the path of the vehicle to the LEDs, each with their own high speed amplifier. As shown in  FIG. 8 , the output of each amplifier  86  is passed through a band-pass filter  88  that is tuned to the excitation frequency of the associated light emitting diodes  80 , for example 10 kHz. The output from this filter  88  is a switching potential which can be passed into a low pass filter  90  (with a bandwidth of around 1 kHz) which acts to integrate the high frequency switching signal. The output of the low pass filter  90  is then input into a comparator  92  to compare it with a fixed threshold to give a simple binary decision as to whether the receiver  84  is illuminated or not. This binary value for all of the detectors  84  is multiplexed out to a single data line  94  for onwards processing. 
         [0043]    The use of a high frequency switching signal with subsequent a.c. coupling is designed to provide good noise rejection independent of ambient temperature for this safety critical signal. 
         [0044]    Each emitting light emitting diode  80  is arranged to generate a fan beam of infra-red radiation in a vertical plane so that it will illuminate multiple receivers  84 . It is possible to determine the height, and to some extent the profile, of any object in the plane of the beam as shown in  FIG. 9  by determining the lowest illuminated light receiver  84  during activation of each of the light sources  80  in turn. 
         [0045]    The data on the output  94  from the light curtain is input to the processor  44  by means of which it is processed and coupled with that from the video data in order to establish when the trailing edge of the cab  70  has passed through the inspection plane and the leading edge of the load  72  has arrived. 
         [0046]    It will be appreciated that, as well as IR radiation, other wavelengths of electromagnetic radiation, for example visible light, could be used in the light curtain. 
         [0047]    In a further modification to this embodiment of the invention, the X-ray data itself is analysed by the controller  44  and interpreted as it is collected on a pulse by pulse basis to determine when the trailing edge of the drivers cab  70  has passed through the scanner and when the leading edge of the cargo load  72  enters the imaging plane of the scanner. In this modification there are now three types of information that indicate independently, and should all correlate to confirm, the passing of the trailing end of the driver&#39;s cab  70  and the start of the cargo load  72 : (1) video data, (2) infra-red light curtain data, and (3) X-ray image data. These redundant signals are sufficient to build a safety case for the operation of a driver controlled cargo inspection system. 
         [0048]    In a practical embodiment of this system, it is likely that non-cargo loads may be inadvertently passed through the inspection system. For example, a bus or coach carrying passengers may be selected for screening. In this case, no high energy X-ray screening should be performed to minimise dose to the passengers. It can be seen that in this case the three-way redundant data analysis system should not pick up the trailing edge of the drivers cab (since there is not one present), and neither should it pick up the start of the cargo load (since there is not one of these either). This means that the high energy X-ray system will not be turned on, but the load will still have been inspected to a reasonable degree using the low energy source. 
         [0049]    It is understood that the features noted in our related patent applications filed on even date herewith are equally applicable in this case, specifically patent application numbers GB0803646.9 (Agent&#39;s ref ASW42823.GBA), GB0803640.2 (Agent&#39;s ref ASW42822.GBA), GB0803641.0 (Agent&#39;s ref ASW42820.GBA), and GB0803644.4 (Agent&#39;s ref ASW42818.GBA).