Multi-angle cargo security screening procedures

Abstract

Method for cargo security screening with multiple viewing angles for screening an object (2) using a cargo security screening system, wherein the cargo security screening system comprises a radiation source (1) for generating a beam of radiation for scanning the object (2) to be screened and a data collection unit (3) for collecting the scanning projection data after the beam of radiation has scanned the object (2) to be screened, wherein the method comprises a scanning step comprising: Rotating the radiation source (1) and / or the object (2) about an axis of rotation to achieve a relative rotation, thereby positioning the radiation source (1) in a plurality of discrete positions with different viewing angles with respect to the object (2) to be inspected, wherein the radiation source (1) moves along a straight line in a direction parallel to the axis of rotation in each viewing angle and simultaneously scans the object (2) to be inspected in each viewing angle to acquire the radiographic projection data; wherein the plurality of discrete positions with different viewing angles is a plurality of positions evenly spaced on a circumference; wherein the method further comprises an imaging step for imaging the object to be controlled (2) on the basis of the X-ray projection data collected by the data collection unit (3), wherein in the imaging step a three-dimensional image of the object to be controlled (2) is reconstructed using the X-ray projection data of the multiple viewing angles in combination, wherein this reconstruction is carried out using a filter back-projection algorithm, expectation maximization algorithm or statistical algorithm with ordered subsets.

Description

Technical field

[0001] The present invention relates to the technical field of radiation detection, in particular a method for cargo security control with multiple viewing angles. State of the art

[0002] Security screening is of paramount importance in areas such as counterterrorism and the fight against illegal drug trafficking and smuggling. Following the terrorist attacks in the United States on September 11, 2001, countries worldwide have increasingly prioritized security screening in civil aviation. Simultaneously, the demands on security screening of various types of cargo are constantly rising with the ongoing efforts to combat illegal drug trafficking and smuggling. A range of security screening measures are implemented to inspect passenger baggage, personal belongings, and cargo containers at public locations such as airports, train stations, customs houses, and port facilities.

[0003] Computed tomography (CT) is widely used in medical diagnostics and industrial non-destructive testing. With societal development, its use in public safety and security is gradually increasing. Among the widely used CT scanning systems, circular path scanning is the most common. This scanning method requires a simpler mechanical setup and is therefore technically easy to implement. Furthermore, the corresponding reconstruction algorithm is mature and reliable. Circular path scanning systems typically employ fan-beam or cone-beam CT. The corresponding detectors are either a linear array or a planar array. The pair of X-ray source and detector is arranged symmetrically around the pivot point of a rotating stage.

[0004] When CT scanning objects with a larger mass using a CT scanning system, the generally feasible method involves scanning a rotation combined with conversion. This means the object being inspected rotates around its central axis, and the radiation source detector simultaneously moves parallel to the axis of rotation, thus creating a spiral scan path around the object. For objects with a larger cross-section, a large number of projections are required if accurate reconstruction is desired. The result is that the speed of the security check is very slow and the efficiency is very low. Furthermore, a large amount of data may not be relevant to the user.Such a CT scanning system is therefore impractical for the airport, which has to perform security checks on a large number of cargo items daily, due to its slower speed.

[0005] If, in addition to perspective distortion, several items are present in a direction parallel to the beam or beams, the items overlap in the image. It is generally very difficult to distinguish the items from one another, which creates many difficulties for the inspection of contraband. From JP 2004 012 407 A, an X-ray CT / DR imaging system is known in which two-dimensional digital X-ray images of an object are created in two different rotational positions in order to locate specific areas of interest (e.g., defects) within the object, which are then measured using a conventional three-dimensional CT imaging procedure. Imaging systems of the aforementioned type are also known from documents US 2004 / 0 109 532 A1, DE 31 50 306 A1, and US 2002 / 0 097 831 A1. Brief description of the invention(I) Technical problem to be solved

[0006] The object of the present invention is to provide a method for cargo security control with multiple viewing angles, in view of the aforementioned deficiency existing in the prior art. (II) Technical solution

[0007] To solve the above problem, the technical solution employed in the present invention comprises the following: A method for multi-viewing cargo security screening for inspecting an item using a cargo security screening system, wherein the cargo security screening system comprises a radiation source for generating a beam of light for scanning the item to be screened and a data collection unit for collecting the scanning projection data after the beam of light has scanned the item to be screened, wherein the method comprises a scanning step comprising: rotating the radiation source and / or the item about an axis of rotation to achieve a relative rotation, thereby positioning the radiation source in a plurality of discrete positions with different viewing angles with respect to the item to be screened,wherein the radiation source moves along a straight line in a direction parallel to the axis of rotation at every viewing angle and simultaneously scans the object to be inspected at every viewing angle in order to capture the X-ray projection data.

[0008] In one embodiment, the relative rotation is achieved by keeping the radiation source stationary and rotating the object to be controlled. In another embodiment, the relative rotation is achieved by keeping the object to be controlled stationary and rotating the radiation source around the controlled object.

[0009] Preferably, the radiation source and the data collection unit are arranged on opposite sides of the object to be inspected, and in the scanning step the data collection unit moves synchronously with the movement of the radiation source.

[0010] According to the invention, the plurality of discrete positions with different viewing angles is a plurality of positions evenly spaced on a circumference.

[0011] Preferably, the plurality of discrete positions with different viewing angles comprises 3 to 70 positions. Particularly preferably, the plurality of discrete positions with different viewing angles comprises 4 to 60 viewing angle positions. Further preferably, the plurality of discrete positions with different viewing angles comprises 8 to 50 viewing angle positions. Even more preferably, the plurality of discrete positions with different viewing angles comprises 10 to 40 viewing angle positions. Most preferably, the plurality of discrete positions with different viewing angles comprises 15 to 25 viewing angle positions.

[0012] Preferably, the radiation source moves in opposite directions along straight lines at two adjacent viewing angle positions.

[0013] The method of the present invention further comprises an imaging step for imaging the object to be inspected based on the X-ray projection data collected by the data collection unit.

[0014] Preferably, in the imaging step, a two-dimensional perspective image of the object to be inspected is created for each viewing angle using the X-ray projection data.

[0015] Preferably, the plurality of discrete positions with different viewing angles comprises at least three viewing angle positions. According to the invention, in the imaging step, a three-dimensional image of the object to be inspected is reconstructed using the transmission projection data of the multiple viewing angles in combination.

[0016] According to the invention, the reconstruction is performed using a filter backprojection algorithm, expectation maximization algorithm or statistical algorithm with ordered subsets. (III) Beneficial effects of the invention 1. Compared to CT scanning or spiral CT scanning in the prior art, the scanning path in the method of the present invention is somewhat different. In the method of the present invention, a payload is scanned by the radiation source with several parallel linear paths at different viewing angles relative to the payload in order to acquire fluoroscopy projection data at multiple viewing angles. Such scanning can be performed at a faster speed. Compared to CT scanning or spiral CT scanning, the scanning path in the present invention is significantly faster.In the prior art of spiral CT scanning, the fluoroscopy projection data acquired by the method of the present invention are not complete with respect to the three-dimensional image, but it is possible to acquire a three-dimensional image that meets the accuracy requirement as far as possible, provided that the speed requirement is met by appropriate selection of the number of viewing angles, thereby maintaining a balance between the scanning speed and the imaging accuracy. 2. The method of the present invention is able to implement rapid security checks on cargo (for example, air cargo containers) and thereby significantly increase the efficiency of security checks on cargo, thus fulfilling the airport's requirement for rapid security checks on a large number of cargoes. 3. Since the present invention can reconstruct the three-dimensional image of a cargo, it effectively solves the problem that the objects overlap when reconstructing a perspective image, thereby effectively increasing the accuracy rate of checking objects and thus greatly facilitating the control of contraband. 4. The method of the present invention can be implemented with the current system; therefore, in addition to implementing the method of the present invention, the system can also perform conventional perspective imaging and CT imaging, making it possible to carry out security checks on cargo in a more flexible manner. Description of the enclosed drawings Fig. Figure 1 is a schematic drawing of a conventional cargo security control system; Fig. Figure 2 is a schematic drawing of a cargo control system with multiple viewing angles, which implements the method of the present invention; Fig. Figure 3 is a schematic drawing of a conventional fan beam scan of a circular track; Fig. Figure 4 is a schematic drawing of a scanning method with multiple viewing angles of the multi-viewing-angle cargo control system provided in the present invention; and Fig. 5a and Fig. 5b are simulation results of the Shepp-Logan head model. Implementation of the invention

[0017] To clarify the technical solution provided in the present invention, this invention is below described in detail with embodiments by reference to the accompanying drawings.

[0018] According to the presentation in Fig. 1 is Fig. Figure 1 shows a schematic drawing of a conventional cargo security screening system that can be used to implement the method of the present invention. A radiation source 101 generates a beam of X-rays or a beam of other radiation for scanning cargo 102. This beam scans the cargo 102 (in one example, the cargo 102 is an air cargo container) which is mounted on a rotary table 104. The scan projection data after scanning the cargo with the beam is collected by a data acquisition unit 103 (such as a detector array) and transmitted to a host and data processing computer (not shown). The data acquisition unit 103 is located opposite the radiation source 101. That is, the data acquisition unit and the radiation source are arranged symmetrically around the central axis of the rotary table.The host and data processing computer provides a human-machine interface and reconstructs an image from the received projection data, displaying the reconstructed image. The rotary table 104 drives the cargo 102 to rotate.

[0019] The freight control system usually also includes a delivery device (see Fig. 2) for supplying the container to the turntable and removing the container from the turntable after inspection is complete. Furthermore, the cargo inspection system usually includes a scanning lifting device (not shown) for carrying the radiation source and the detector and causing them to rise and descend synchronously. The scanning lifting device may comprise two sets of lifting platforms on which the radiation source and the data acquisition unit, respectively, are mounted. Additionally, a horizontal collimator may be mounted on the lifting platforms.

[0020] The system may also include a scanning control device for controlling the operation of the radiation source, the data collection unit and the rotary table based on commands received from the host and data processing computer.

[0021] To achieve fast and accurate control, the cargo control system typically includes a device for measuring or calibrating the following system parameters: the distance D from the radiation source to the data collection unit, the distance R from the radiation source to the axis of rotation of the turntable, the imaging position P (u, v) of the radiation source, the pixel size d of the imaging screen, and the rotation angle θ of the turntable. The device for measuring or calibrating these system parameters is well known in the art, and therefore no further details are given here.

[0022] According to the presentation in Fig. 2 is the Fig. Figure 2 shows a schematic drawing of a cargo inspection system implementing the method of the present invention, featuring multiple viewing angles. During operation of the system, the cargo is fed to a rotary table 204 by a transfer conveyor belt 201. A radiation source 202 and a detector group 203 are located on either side of the rotary table 204. The rotary table 204 can rotate continuously or be positioned at a predetermined angle. The radiation source 202 and the detector group 203 can move up and down synchronously (i.e., in a direction perpendicular to the paper surface). Fig. 2) When the rotary table 204 is stationary, it is possible to acquire the X-ray projection data of the air cargo container in the current viewing angle by synchronously raising and lowering the radiation source 202 and the detector group 203.

[0023] Through the in Fig. The conventional circular scan can also be implemented in the system shown in Figure 2. In this system, the radiation source 202 and the detector group 203 are held at a fixed height, and the rotary table 204 drives the load to rotate continuously in order to acquire the CT projection data of the load in its current disk position. Fig. Figure 3 is a schematic drawing of a conventional circular track fan beam scanning system. In the Fig. In section 3, the radiation source 1 and the data collection unit 3 are each arranged on either side of the cargo 2. Relative to the cargo 2, the radiation source 1 and the data collection unit 3 move along a circular track.

[0024] An embodiment of the scanning process of the method of the invention is described below, comprising the following steps: (1) Starting up the system so that the radiation source and the data acquisition unit perform a relative rotation with respect to the cargo, and positioning the radiation source and the data acquisition unit at a first viewing angle with respect to the cargo, whereby, when implementing the scanning process with the system according to representations in Fig. 1 and Fig. 2, the radiation source and the data collection unit are kept stationary during the process of relative rotation while the cargo spins driven by the turntable; however, it is very easy to understand that during the process of relative rotation it is also possible to keep the cargo stationary while the radiation source and the data collection unit rotate around the cargo and at this moment the radiation source and the data collection unit can be located near the lower end of the cargo; (2) Causing the radiation source to generate a beam of light in the first viewing angle to scan the cargo guided on the turntable; and causing the radiation source and the data acquisition unit to move synchronously upwards on a linear track and scan the cargo, the linear track being perpendicular to the plane of the relative rotation in step (1), for example, if the relative rotation is carried out in a horizontal plane, in other words, if the linear track is in a perpendicular direction, the direction of the linear track being parallel to the axis of rotation of the relative rotation; and at the same time, as the radiation source scans, the data acquisition unit receives scan projection data of the beam of light that has scanned the cargo. (3) Stopping the movement of the radiation source and the data collection unit after their movement towards the tip, wherein the turntable drives the cargo to rotate at such an angle that the radiation source is positioned in a second viewing angle which differs from the first viewing angle with respect to the cargo; (4) Causing the radiation source to generate a beam of light in the second viewing angle to illuminate the cargo guided on the turntable and to move vertically downwards and the data collection unit to move synchronously with the radiation source and to receive the illumination projection data of the beam of light that has illuminated the cargo; (5) Repeat a process similar to the previous one until the turntable carries the cargo to rotate one revolution, so that the data collection unit receives all the projection data of the beam that has scanned the cargo.

[0025] Fig. Figure 4 shows a relative positional relationship between the radiation source and the cargo in one embodiment. In the Fig. 4. The radiation source is located in four different positions of the viewing angles S1, S2, S3, and S4 with respect to the cargo 2. Each position of the viewing angle corresponds to a linear scan, as described above. In the method of the present invention, several positions of viewing angles are several positions evenly spaced around a circumference, as in the Fig. Figure 4 illustrates this. The number of these viewing angle positions can be selected according to the requirements of the desired scanning speed and imaging accuracy. This is easily done by a person skilled in the art. For example, the multiple discrete viewing angle positions can comprise 3 to 70 positions. To further increase the scanning speed, the multiple discrete viewing angle positions can comprise 4 to 60 positions. To increase the scanning speed even further, the multiple discrete viewing angle positions can comprise 8 to 50 positions. To increase the scanning speed yet again, the multiple discrete viewing angle positions can comprise 10 to 40 positions.Preferably, the multiple discrete viewing angle positions can comprise 15 to 25 positions. If the multiple viewing angle positions are evenly spaced around a circumference, it is very easy to determine the angle of each relative rotation according to the predetermined number of viewing angle positions. For example, if the number of viewing angle positions is 24, the load is driven by the turntable to rotate by 15 degrees each time.

[0026] Based on the X-ray projection data acquired during the aforementioned scanning process, the host and data processing computer can reconstruct this projection data into an image and display it. According to the present invention, it is possible, with respect to each viewing angle during imaging, to use the X-ray projection data at that viewing angle to create a two-dimensional image of the cargo at that viewing angle, or to use the X-ray projection data from multiple viewing angles in combination to reconstruct a three-dimensional image of the cargo. Naturally, it is possible to acquire both the two-dimensional perspective image and the three-dimensional perspective image of the cargo at each viewing angle.When the three-dimensional image is reconstructed according to the invention, it preferably requires transmission projection data captured in at least three positions of viewing angles.

[0027] According to the invention, the reconstruction of the three-dimensional image is carried out by a filter back-projection algorithm (abbreviated as "FBP"), an expectation maximization algorithm (abbreviated as "EM") or EM with an ordered subset (abbreviated as "OSEM").

[0028] When reconstructing received projection data from multiple viewing angles into an image using FBP, the reconstruction process includes the following steps: (1) Filtering the received projection data from multiple viewing angles, the specific process is as follows: assuming that F ρ(ρ,θ) is a one-dimensional Fourier transform of the received projection data of the multiple viewing angles in a direction parallel to the receiving plane of the data collection unit, filter processing of the received projection data of the multiple viewing angles according to the formula Mθ(t)=∫−∞+∞Fρ(ρ,θ)|ρ|e2πjρtdρ to capture the filter-processed result of the projection data from the multiple viewing angles, where ρ and θ are the radial coordinate and the angular coordinate, respectively; and (2) Back-projecting the filter-processed projection data of the multiple viewing angles, the specific process is as follows: Assuming that f̂(x,y) is the reconstructed image, backprojection processing of the filter-processed result according to the formula Mθ(t)=∫−∞+∞Fρ(ρ,θ)|ρ|e2πjρt dρ to capture the reconstructed image of the projection data of the multiple viewing angles, where x and y are the horizontal coordinate and the longitudinal coordinate, respectively.

[0029] When reconstructing the projection data of the multiple viewing angles into an image using EM, the reconstruction process comprises the following two steps: E-step: calculating the expected value of the conditional probability function; and M-step: calculating the maximum of the anticipation function.

[0030] The EM process is then described in detail using a specific example: (1) suppose that X is the reconstructed image, α ij a projection matrix coefficient and the initialization m = 0, x̂ m positive; (2) Carry out the following steps until convergence: a) x 1 = x̂ m , m = m + 1; b) Calculating the projection value, where μti=∑j=1Jatjxji,t∈si; c) Back-projecting the projection value, xji+1=xji∑t∈Siytαtjμtj|∑t∈Siαtj,j=1,2,…,J; and d) Capturing x̂ m = x'.

[0031] The OSEM is similar to the EM, but its convergence rate is higher and its image quality is close to that of the EM. The present invention can be used on the OSEM. When the received projection data from multiple viewing angles are reconstructed into an image by the OSEM, the reconstruction process comprises the following steps: (1) suppose that X is the reconstructed image, α ij a projection matrix coefficient and the initialization m = 0, x̂ m positive; (2) Carry out the following steps until convergence: a) x1 = x̂ m , m = m + 1; b) Calculating the projection value with respect to each subset i = 1,2,...,n, where μti=∑j=1Jαtjxji,t∈Si, and performing the backprojection of the projection value, where xji+1=xji∑t∈Siytαtjμtj|∑t∈Siαtj,j=1,2,…,J; and c) Capturing x̂ m = x'.

[0032] After the image reconstruction of the received projection value from the multiple viewing angles is complete, the reconstructed image is displayed by the host and data processing computer. The detailed result of the reconstructed image is available from Fig. 5 is visible. Fig. 5a and Fig. 5b are results of the simulation of the Shepp-Logan head model.

[0033] In addition, the system implementing the present invention can also implement the dual viewing angle scanning mode and CT disk scanning mode.

[0034] In dual-viewing-angle scanning mode, the system captures only a two-dimensional perspective image from two orthogonal viewing angles, and two perspective views are simultaneously displayed on the computer screen for the operator to determine. This mode features a short scanning time and a high throughput rate. However, the operator's decision-making process and understanding require a higher level of experience and responsibility, similar to the current control system.

[0035] In CT disk scanning mode, the system first acquires the CT projection data of the prescribed disk position of an air cargo container, then generates a corresponding CT image of this position through data reconstruction, and an alarm is triggered. Since this disk image can reflect the size and distribution of the density information of the cargo in the corresponding section, the accuracy of the alarm is significantly increased, but the scanning time is longer.

[0036] In the multi-viewing-angle scanning mode of the present invention, the system continuously acquires multiple two-dimensional perspective images from different viewing angles, reconstructs the triaxiality data of the entire object approximately using incomplete data, and then displays it on the computer screen for the operator to determine. In addition, interactive operation can be conducted on the corresponding projection data via a human-machine interface, and at the same time, suspicious key areas are prominently displayed. In this mode, the system can proactively trigger an alarm regarding dangerous items such as explosives. Furthermore, the scanning time is short. This mode is therefore the preferred scanning method of the present invention.

[0037] The system can automatically switch between these three scanning modes without any switching time. Different scanning modes can therefore be used flexibly during the actual application process, depending on the risk assessment requirements of the air cargo container or the airport security level. When searching for throughput, the dual-view angle scanning mode is selected; and under normal circumstances, the multi-view angle scanning mode is used first, followed, depending on the circumstances, by CT slice scanning at the specific location of the suspect air cargo container, which cannot be released during the determination of triaxiality data.

Claims

[1] Method for cargo security screening with multiple viewing angles for screening an object (2) using a cargo security screening system, wherein the cargo security screening system comprises a radiation source (1) for generating a beam of radiation for scanning the object (2) to be screened and a data collection unit (3) for collecting the scanning projection data after the beam of radiation has scanned the object (2) to be screened, wherein the method comprises a scanning step comprising: Rotating the radiation source (1) and / or the object (2) about an axis of rotation to achieve a relative rotation, thereby positioning the radiation source (1) in a plurality of discrete positions with different viewing angles with respect to the object (2) to be inspected, wherein the radiation source (1) moves along a straight line in a direction parallel to the axis of rotation in each viewing angle and simultaneously scans the object (2) to be inspected in each viewing angle to acquire the radiographic projection data; wherein the plurality of discrete positions with different viewing angles is a plurality of positions evenly spaced on a circumference; wherein the method further comprises an imaging step for imaging the object to be controlled (2) on the basis of the X-ray projection data collected by the data collection unit (3), wherein in the imaging step a three-dimensional image of the object to be controlled (2) is reconstructed using the X-ray projection data of the multiple viewing angles in combination, wherein this reconstruction is carried out using a filter back-projection algorithm, expectation maximization algorithm or statistical algorithm with ordered subsets. [2] Method according to claim 1, wherein the relative rotation is achieved by keeping the radiation source (1) stationary and rotating the object (2) to be controlled. [3] Method according to claim 1, wherein the relative rotation is achieved by keeping the object (2) to be controlled stationary and rotating the radiation source (1) around the object (2) to be controlled. [4] Method according to claim 1, wherein the radiation source (1) and the data collection unit (3) are arranged on opposite sides of the object (2) to be inspected, wherein in the scanning step the data collection unit (3) moves synchronously with the movement of the radiation source (1). [5] Method according to claim 1, wherein the plurality of discrete positions with different viewing angles comprises 3 to 70 positions of viewing angles. [6] Method according to claim 5, wherein the plurality of discrete positions with different viewing angles comprises 4 to 60 positions of viewing angles. [7] Method according to claim 6, wherein the plurality of discrete positions with different viewing angles comprises 8 to 50 positions of viewing angles. [8] Method according to claim 7, wherein the plurality of discrete positions with different viewing angles comprises 10 to 40 positions of viewing angles. [9] Method according to claim 8, wherein the plurality of discrete positions with different viewing angles comprises 15-25 positions of viewing angles. [10] Method according to claim 1, wherein at two adjacent positions of viewing angles the radiation source (1) moves along straight lines in opposite directions. [11] Method according to claim 1, comprising in the imaging step imaging a two-dimensional perspective image of the object (2) to be inspected with respect to each viewing angle using the X-ray projection data of that viewing angle. [12] Method according to claim 1, wherein the plurality of discrete positions with different viewing angles comprises at least three positions of viewing angles.

Images