Scanning systems for security and medical scanners having means to control the radiation source

Field of the Invention

0001The 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 such as security and medical scanners.

Background

0002Conventional cargo inspection systems comprise a radiation source that irradiates with X-rays or gamma-rays through the object under inspection to a sensor array that may be in an arc shape or configured into an "L"-shaped array. Other forms of sensor array will be apparent to the skilled person.

0003Often, such systems use a linear accelerator to produce X-rays or a radioactive source such as ¹³⁷Cs or ⁶⁰Co to produce gamma-rays.

0004Linear accelerators are typically operated with beam energies of 2MV to 9MV where a 2MV beam is one that is generated by firing a 2 MeV electron beam at a target, producing a broad spectrum of X-ray energies up to a maximum of 2 MeV but with a typical 1/3 of peak energy. This high-energy X-ray beam can penetrate large objects such as full containers or complete trucks.

0005Radioactive sources are often used in lower cost installations with a lower radiation penetration requirement. For example, they might be used for screening cars or smaller air-cargo type containers. ¹³⁷Cs provides gamma rays at an energy of 662 keV (more or less equivalent to a 2 MV linear accelerator source) while ⁶⁰Co produces gamma rays at an energy of around 1.2 MeV (more or less equivalent to a 4 MV linear accelerator source).

0006Previously, linear accelerators have often been fabricated using low frequency S-band microwave energy sources. The waveguide used in the linear accelerator defines the overall size of the finished radiation source. Therefore, such sources are typically bulky and heavy since thick radiation shielding must be installed around the whole accelerating structure. <patcit id="pcit0001" dnum="US6713773B1"><text>US6713773 B1</text></patcit> is an example of a prior art scanning system which controls the pulses of radiation based on a level of detected radiation which has passed through an object being scanned.

0007<patcit id="pcit0002" dnum="US5909478A"><text>US5909478</text></patcit> discloses a portable imaging system, wherein a user has the possibility to control the x-ray image acquisition duration, in order to improve the generated image.

Summary of the Invention

0008The present invention provides a scanning system comprising a radiation source arranged to direct pulses of radiation towards an object, detection means arranged to detect the radiation, wherein the detection means includes a plurality of detector banks arranged to produce a detector signal in response to detection of the radiation, acquisition means comprising a plurality of programmable gain integrating amplifiers each connected to a bank of detectors arranged to generate a data signal from the detector signal, image processing means arranged to generate image data which defines images of the object from digital signals from the acquisition means, a decision processing means arranged to receive and review the image data and determine whether the detector signals are outside the dynamic range of the amplifiers, instruct the source to fire more pulses and control the acquisition means so as to vary the number of pulses over which the amplifiers integrate the detector signal until the dynamic range required is achieved thereby to vary the sensitivity of the system.

0009The present invention also provides a method of scanning an object comprising directing pulses of radiation from a radiation source towards an object, detecting the radiation using radiation detection means comprising a plurality of detector banks, producing a detector signal in response to detection of the radiation, generating a data signal from the detector signal using acquisition means which comprises a plurality of programmable gain integrating amplifiers each connected to a bank of detectors, generating image data which defines images of the object using image processing means receiving digital data from the acquisition means, reviewing the image data and determining whether the detector signals are outside the dynamic range of the amplifiers, instructing the source to fire more pulses and controlling the acquisition means so as to vary the number of pulses over which the amplifiers integrate the detector signal until the dynamic range required is achieved thereby to vary the sensitivity of the system.

0010The present invention is directed in part at the use of compact imaging sources for imaging of cargo items from small containers up to large trucks.

0011A radioactive source is naturally compact and therefore of smallest possible weight given that the radiation shield must be located all around the radiation source. However, the radiation source may not be switched on and off, requiring a manual shutter for this purpose. Such a shutter may become stuck so posing a potential safety hazard. Further, the transportation of such sources can be problematic, particularly across national borders. Linear accelerators are convenient but complex radiation sources in that they may be turned on and off simply using electronic means. When the power supply is disconnected, no radiation can be emitted at all. Transportation becomes more straightforward and they are intrinsically safer to operate. In some embodiments of the present invention, a compact imaging source is built around a linear accelerator that operates at high frequency. High frequency results in low wavelength and so a more compact linear accelerator design is the result. Advantageously, an X-band linear accelerator may be used, or alternatively one with similarly high operating frequency.

0012To provide best imaging capability, the compact radiation source may be capable of operating in a number of modes.

0013Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: <ul id="ul0001" list-style="none"><li><figref idref="f0001"><b>Figure 1</b></figref> schematically shows a scanning system setup;</li><li><figref idref="f0001"><b>Figure 2</b></figref> schematically shows a scanning system according to another example;</li><li><figref idref="f0002"><b>Figure 3</b></figref> is a flowchart showing a method of scanning an object not within the scope of this invention;</li><li><figref idref="f0002"><b>Figure 4</b></figref> is a flowchart showing a method of scanning an object not comprising all steps of the invention;</li><li><figref idref="f0003"><b>Figure 5</b></figref> schematically shows two typical scanning system geometries;</li><li><figref idref="f0003"><b>Figure 6</b></figref> graphically represents the timing of pulses of radiation from a radiation source;</li><li><figref idref="f0003"><b>Figure 7</b></figref> graphically represents variation of pulse frequency with object velocity;</li><li><figref idref="f0003"><b>Figure 8</b></figref> graphically represents variable energy pulsed radiation;</li><li><figref idref="f0003"><b>Figure 9</b></figref> graphically represents different variable energy pulsed radiation;</li><li><figref idref="f0004"><b>Figure 10</b></figref> schematically shows a switching gain integrator circuit for processing detector signals;</li><li><figref idref="f0004"><b>Figure 11</b></figref> graphically represents collection and integration of detector data over several radiation pulses;</li><li><figref idref="f0005"><b>Figure 12</b></figref> schematically shows part of a scanning system;</li><li><figref idref="f0005"><b>Figure 13</b></figref> schematically shows part of a scanning system according to an embodiment of the invention; and</li><li><figref idref="f0005"><b>Figure 14</b></figref> graphically represents operation of a threshold check level within said embodiment of the invention.</li></ul>

0014Referring to <figref idref="f0001">Figure 1</figref>, a scanning system 10 comprises a radiation source 12 arranged to direct radiation towards an object 14 to be scanned. The system 10 also includes a detection means 16 arranged to detect the radiation. The radiation source comprises a linear accelerator of the type previously described or may be any other suitable source (for example, a different type of previously described radiation source). The detector means 16 comprises an L-shaped sensor array, which comprises a plurality of banks of detectors. In this example the object 14 being scanned is a lorry. The system 10 also comprises a controller 18 which is arranged to control the radiation source 12 such that the source 12 produces radiation in pulses. The controller 18 is also arranged to define a check condition and to process signals from the detection means 16 in order to determine whether they meet the check condition. The controller 18 varies the pulses of the radiation source 12 dependent upon whether or not the signals from the detection means meet the check condition.

0015Referring to <figref idref="f0001">Figure 2</figref>, a scanning system 20 comprises a radiation source 22 arranged to direct radiation towards an object 24. The system 20 also includes detection means 26 in the form of a detector which produces a detector signal in response to detection of radiation in a similar manner to the embodiment of <figref idref="f0001">Figure 1</figref>. The system 20 also includes acquisition means 30 comprising an integrator which is arranged to generate a data signal from the signal which is received from the detector (the detector signal). The system 20 also includes control means 28 in communication with both the source 22 and the acquisition means 30. The control means 28 is arranged to control the source 20 and the acquisition means 30 to vary the number of pulses over which the integrator integrates the detector signal - thus allowing variation of the sensitivity of the system 20. Advantageously, if it is deemed necessary by the controller 28, the integrator can be used to generate a data signal which comprises a detector signal relating to more than one pulse of radiation. This may be particularly useful in situations where enough radiation is not detected from a single pulse. The control means 28 controls the source 22 to vary the frequency of the pulses in order to vary the number of pulses in which the integrator integrates the detector signal, or the number of pulses over which the integrator integrates the detector signal is varied by varying the time over which the integrator integrates the detector signal. The control means 18 is arranged to generate image data which defines images of the object 14 from some signals from the detection means.

0016Two typical imaging geometries are shown in <figref idref="f0003">figure 5. Figure 5 (a)</figref> shows a system 51 configured with an "L"-shaped sensor array 52 with a compact radiation source 53 to the left while <figref idref="f0003">figure 5 (b)</figref> shows an arc-shaped sensor array 54 with the compact source 53 to the left.

0017A compact linear-accelerator source will typically run with a pulse configuration as shown in <figref idref="f0003">figure 6</figref>. Here, each pulse will typically have a pulse width of a few microseconds (1 - 10 µs is typical) with a pulse repetition frequency that is often in the range 50 to 200 Hz. The compact radiation source is capable of pulse frequency modulation such that the timing of each pulse from the linear accelerator is timed to match the velocity of the object under inspection as it passes through the slice-like radiation beam. A suitable pulse frequency modulation example is shown in <figref idref="f0003">figure 7. Figure 7(a)</figref> represents the object velocity, whilst <figref idref="f0003">Figure 7(b)</figref> shows the pulse timing. The pulse frequency increases as the object velocity increases. An object of the imaging system is to identify threat materials within the object under inspection. This may be achieved using an operator (such as a human operator) to look for objects within this image. However, it is also possible to provide some materials discrimination by using a variable energy radiation beam. As shown in <figref idref="f0003">figure 8</figref>, it is possible to pulse the energy of the linear accelerator source to alternate energies, for example a high energy of 6MV and a low energy of 2 MV. In this case, the radiation energy transmitted by the object will be different for the high and low energy pulses (since each material in the beam has a characteristic and energy-varying radiation absorbance). Such variation in energy of the pulses is incorporated with pulse frequency modulation to provide high quality scanning of the object under inspection. An objective of the compact cargo inspection system is to minimise the size and weight of the radiation source. The higher the energy of operation of the radiation source, the larger and heavier the source becomes. It can therefore be advantageous to operate the radiation source at the lowest possible energy. However, in some situations, the energy of the radiation source is too low to provide effective penetration through the object under inspection. In this case, the cargo item can be treated as not being properly inspected. Therefore, to address this problem, a pulse sequence as shown in <figref idref="f0003">figure 9</figref> is adopted. Here, the majority of pulses are emitted at low energy (say 2 MV). The detector signals are recorded and checked to see if sufficient object penetration has been measured. If not, the radiation source is instructed to emit a high energy pulse at say 6MV (marked (a) in <figref idref="f0003">figure 9</figref>) in order to provide a set of data with the required penetration. Now, it is possible to resolve the majority of dark alarms (a dark alarm is defined as occurring where insufficient penetration was recorded by the low energy pulse) without providing excessive dose at the external surface of the unit. In some situations (marked (b) in <figref idref="f0003">figure 9</figref>), it may be necessary to provide several consecutive high energy pulses to get sufficient signal for the transmission measurement.

0018Typically, the high energy pulse will contain more X-ray photons than a low energy pulse. This can place severe demands on the dynamic range and noise of the front-end readout electronics. As shown in <figref idref="f0004">figure 10</figref>, a switching gain front end integrator circuit is provided whose operation is linked to the pulse generator for the linear accelerator. In normal operation with low energy pulses from the radiation source, the Gain switch is open and the value of C1 alone determines the gain. For example, if C1 = 3pF, then the output of the integrator with be 1V for a 3pC input charge from the front end photodiode. In high-energy mode the Gain switch is closed and now the total capacitance is equal to C1 + C2. For example, if C2 = 9pF, then the output of the integrator will be 0.25V for a 3pC input charge from the front-end photodiode. <figref idref="f0004">Figure 11</figref> shows how the integrator may be held in integrating mode over several radiation pulses. Now, it is possible to acquire a burst containing multiple radiation pulses so that regions in which previously there was a dark alarm can now be resolved with measurable signal. Ideally, the base pulse frequency of the imaging system will be maintained (at say 50 Hz) with the additional pulses being fired within the normal beam off time (say at 400 Hz pulse repetition frequency). In this way, the normal imaging quality is not compromised by the additional measurement. <figref idref="f0004">Figure 11</figref> shows the Gain switch being turned to low gain mode during the extra pulse measurement period although it would also be possible to operate in the standard high gain mode for extra sensitivity.

0019<figref idref="f0005">Figure 12</figref> s hows that the gain switch may be operated differently on different detector banks or groups (or different detector portions provided on the same detector) in order to optimise the dynamic range of the X-ray detector system. In this case six detector banks are shown labelled 1-6.

0020<figref idref="f0005">Figure 13</figref> shows an embodiment of the invention, in which there is provided a system for generating an optimal pulse sequence and gain selection by analysis of the image data itself. Here, programmable gain integrating amplifiers 130 are connected to each bank of detectors. Data from these detectors pass to a data acquisition unit 131 which outputs digital signals to an Image Processing Unit 132 which appends the new data to the image data to be displayed as an image on a monitor 133. A Decision Processing Unit reviews the new image and determines whether the new data is outside of the dynamic range of the integrating amplifier. If it is, the Decision Processing Unit 134 will cause the amplifier to continue to integrate and will instruct the radiation source to fire more pulses until the dynamic range required of the measurement has been achieved.

0021The Decision Processing Unit may use a range of algorithms, but a simple threshold algorithm such as that shown in <figref idref="f0005">figure 14</figref> is typically sufficient. If a signal is received below threshold 140, the Decision Processor will instruct the source to provide further pulses. More sophisticated algorithms will use methods such as image segmentation and connected component labelling to identify from one or more 2D image scans the extent of a volume projected into one or more subsequent 2D detected images and to automatically predict when the multiple pulse stream will be required.

0022It is noted that the radiation shielding can be designed around the low energy pulse requirements since for a very large fraction of time it is this radiation beam that is generated. For the small fraction of time that a high energy beam is required, the instantaneous dose will be somewhat higher. However, dose is calculated in a time averaged fashion and hence if the Decision Processor is restricted in the number of high energy pulses that it can emit (for example, no more than 5% of the time), then radiation dose can be kept to an acceptable level with very moderate radiation shielding requirements.

0023Various modifications may be made to the present invention without departing from its scope.