Systems and methods for improving penetration of radiographic scanners

[0062] This specification describes a scanning system with increased penetration and a smaller xenophobic area, resulting in improved performance and ease of deployment in a wide range of environments. The implementation in this specification is very suitable for applications in environments including but not limited to inspections of containers, trucks, and rail cars. Some of the embodiments in this specification are particularly suitable for use when inspecting slow moving vehicles.

[0063] This specification is devoted to systems and methods for reducing the xenophobic area and increasing the penetration capability of radiological imaging systems such as X-ray scanners. In an embodiment, the imaging system described in this specification can deeply scan high-density goods with sufficient penetrating power to detect contraband, resulting in a low probability of a dark alarm (a secondary inspection may be required). This specification also describes an imaging system that has lower influence from scattered radiation (observed in a conventional X-ray scanner) and can be used to inspect high-density cargo. This specification also describes a new method that allows the intensity of radiation applied to the cargo and the environment to be optimized, thereby further increasing the penetration.

[0064] In an embodiment, this specification describes a novel mechanism for reducing scattering by generating a vertically moving fan beam with a smaller angular range than the angular coverage of the scanned object. This manual provides a vertical moving fan beam or "small fan" synchronized with the pulsed X-ray source and data acquisition system. In the embodiment, the “small fan shape” means a part of the entire fan beam and is translated vertically to cover the range of the object.

[0065] In an embodiment, the vertical collimator projects a small sector with a smaller angular range than the angular coverage of the scanned object. In an embodiment, the angular range is achieved by using a collimator that has dimensional characteristics independent of the object, but is customized to ensure that the highest and widest possible object dimensions are taken into account. In an embodiment, the collimator is designed to provide a pre-defined object height and object width (larger than the standard object height and width) collimation, thereby ensuring that any part of the object is scanned.

[0066] The small sector is translated vertically through the collimator mechanism to cover the angular spread of the object. The pulse linear accelerator X-ray source and the data acquisition system are synchronized with the mobile collimator in such a way that images of the object are acquired at intervals, in which, in one cycle, small sectors have no gaps and optionally have minimal overlap. Cover the slice of the object. Then, the images from each small sector are combined to generate sliced ​​images. In one embodiment, in order to minimize the influence of object motion, the source pulse frequency is increased by the number of small sectors. The advantage of this embodiment is that since the irradiated area for each acquisition is reduced, the scattering is reduced.

[0067] This manual is also dedicated to reducing the area of ​​radiation exclusion. In an additional embodiment, the signal from each small sector is used to control the intensity of the small sector in the next cycle to optimize the source intensity. In an embodiment, the beam intensity and/or energy are modulated based on the transmission observed in each small sector to expose the object to the minimum intensity required for penetration, while reducing the dose to the cargo and the environment, thereby causing The xenophobic area becomes smaller. This is similar to the intensity modulation described in PCT Publication No. WO 2011095810A applied to a complete fan beam, the entire content of which is incorporated herein by reference.

[0068] Because the time between pulses at the same vertical position in the standard system is the same, but because the pulse rate is increased accordingly, the embodiment described here can also be used for dual-energy scanning. However, for fast-moving objects, the pulse frequency is high and it may not be possible to increase the pulse frequency by a factor of two or three. In these applications, the preferred embodiment uses a pulsed source, where each pulse contains dual energy separated in a short time.

[0069] In another embodiment, a continuous wave (CW) source is used. In this embodiment, the data acquisition system continuously acquires data at multiple time intervals, and the time is shorter than the time it takes for the collimator to move from the top position to the bottom position to cover the slice.

[0070] This description is devoted to multiple embodiments. The following disclosure is provided in order to enable those of ordinary skill in the art to practice this specification. The language used in this specification should not be construed as an overall denial of any particular implementation or used to limit the claims beyond the meaning of the terms used herein. Without departing from the spirit and scope of this specification, the general principles defined herein can be applied to other embodiments and applications. In addition, the terms and expressions used are for the purpose of describing the exemplary embodiments and should not be considered as limiting. Therefore, this specification is consistent with the widest scope including several alternative embodiments, modifications, and equivalents that conform to the disclosed principles and features. For the purpose of clarity, the details of related technical materials known in the technical fields related to this specification have not been described in detail so as to unnecessarily obscure this specification.

[0071] It should be noted here that any feature or component described in combination with a specific embodiment can be used and implemented by any other embodiment, unless otherwise explicitly indicated.

[0072] Figure 1A An X-ray system including an X-ray source 110 and a detector array 120 is shown for scanning a railcar 130 containing cargo 140. The X-ray path 150 represents uninteracted X-rays transmitted through the cargo 140. In an ideal system, these are only X-rays to be detected. The X-ray path 160 represents X-rays scattered by the walls of the container of the rail car 130, and the X-ray path 170 represents X-rays scattered within the cargo 140. The scattered X-rays represented by path 170 constitute the background noise of the X-ray system. In various embodiments, this specification provides systems and methods for reducing background noise.

[0073] Figure 1B The collimator is shown coupled to the detector array to reduce the X-ray scattering signal. in Figure 1B In, the collimator 180 for the detector is coupled with the X-ray detector array 120 for reducing scattered X-rays (such as Figure 1A X-ray shown in 170 etc.). As shown, the path of the main X-ray beam 190 does not interact with the collimator 180 and is detected by the detector array 120, while the subsequent X-ray path 192 is absorbed into the collimator 180 and is not detected. In addition, the subsequent X-ray path 194 passes through the collimator 180 and is detected by the detector array 120, and the subsequent X-ray path 196 is scattered by the collimator 180 into the detector array 120 and is also detected.

[0074] These effects indicate that the collimator reduces scattering, however, a deeper collimator, or a collimator with a wave source longer from the detector, leads to more rejection. The performance of the collimator is affected by the aspect ratio of each collimator opening. The higher the aspect ratio, the more scattering rejection of the collimator; however, this embodiment is more expensive to manufacture.

[0075] Furthermore, since the collimator is made deeper, the scattering in the collimator limits rejection. Thus, since the X-rays scattered in the collimator (used to reduce the scattering from the cargo) can become more in number than the rest of the scattering from the cargo, there is a difference between the use of a depth collimator and the reduction in scattering. The relationship between declining and growing. In the embodiment, the maximum depth of the collimator is 300 mm, and the gain is minimized if the depth is greater than this. It should be noted that since it will reduce the number of unscattered X-rays, the wall thickness of the collimator cannot be made too thick. Therefore, in order to reduce X-ray scattering, a larger number of collimator panes are used.

[0076] In an embodiment, this specification provides a method of reducing the X-ray scattering signal by generating a vertically moving X-ray beam or small fan. figure 2 A system according to an embodiment of the present specification is shown, which includes a pulse source projecting a small sector of vertical movement to scan the cargo with reduced scattering. The system includes a pulsed X-ray source 210 and a detector array 220 for scanning rail cars (or other objects) 230. Examples of suitable X-ray sources include, but are not limited to, electron linear accelerators that strike tungsten and target CW sources, electron linear accelerators such as Rhodotron and superconducting linear accelerators. Those of ordinary skill in the art should recognize that any pulsed X-ray source known in the art can be used. The collimator 240 represents a mechanism that generates vertically moving fan beams or small fans 250, 260, and 270 with an angular range smaller than that of the railcar 230.

[0077] Back to reference figure 2 Compared with the full fan-shaped X-ray beam that is usually used when inspecting goods in conventional systems, the scattering of the signal generated by the small fan 260 is reduced. In an embodiment, in one cycle, when the fan beam is projected to the small fan positions 250, 260, and 270 to cover the vertical extent of the cargo railcar 230, the X-ray pulse is synchronized with the scanning mechanism to collect data. The processing unit combines the data from the small sectors 250, 260, 270 to form a sliced ​​image of the railcar 230 for cargo. Since the collimator defines a small sector and tends to produce beams with fuzzy edges, the small overlap between the small sectors 250, 260, 270 is preferably to allow the small sectors 250, 260, 270 to be better "stitched" into slices Image to eliminate or minimize edge effects. In the embodiment, an overlap of approximately 1 degree is used. It should be noted that in order to stitch the image slices together, any suitable solution known in the art can be adopted.

[0078] In an embodiment, in order to reduce the effect of cargo movement, the source pulse frequency is increased approximately in proportion to the number of small sectors. For example, in mobile applications, the pulse frequency is about 100 Hz. If the number of small sectors is 3, the frequency will increase to 300 Hz. In the embodiment, the minimum number of small sectors is generated by splitting the corresponding fan beam into two halves; however, this does not make the scattering significantly reduced. By increasing the number of small sectors (achieved by reducing the angular range of each small sector), scattered radiation is reduced. However, the increase in the number of small sectors can only be obtained by proportionally increasing the pulse frequency of the pulse linac source.

[0079] In an embodiment, the typical angular range of the fan beam of the scanner is approximately 60 degrees. In an embodiment, the angle range of the small sector is from 1 degree to 30 degrees. In the embodiment, ten small sectors are used, and each small sector has an angle range of 5 degrees. Those of ordinary skill in the art should recognize that the small sector has a significantly larger angular range than a conventional pencil beam (the angular range is on the order of a few tenths of a degree).

[0080] Since the total number of X-rays is the same as compared with standard X-ray scanning, the X-ray dose to the cargo and the environment does not increase. However, since there are fewer X-rays when inspecting cargo at any acquisition time relative to the main beam incident on the detector, the scattering is reduced.

[0081] For dual-energy scanning, the source can be interleaved (meaning that in the first pulse, the source can be the first energy, in the second pulse, the source can be the second energy, and in the nth pulse, the source can be the nth energy ) Or can contain small time gaps (> ~100ns) the dual energy of the same pulse. Likewise, the frequency is effectively increased by two factors. For example, when a standard system is operating at 250 Hz, the source emission frequency may be increased to 375 Hz with dual energy per pulse, resulting in an effective frequency of 750 Hz, enabling the use of three small sectors with a smaller cargo movement effect.

[0082] In an embodiment, for interleaved dual-energy scanning, an odd number of small sectors are generated, so that the second energy is in the same small sector position in the next cycle, thereby allowing dual-energy scanning for each vertical position. For example, in the case of three small sectors, in the first cycle, the following patterns will be seen: a small top sector with high energy (HE), a small central sector with low energy (LE), and a small sector with high energy ( HE) has a small fan at the bottom. In the next cycle, the following patterns will be seen: a small top sector with low energy (LE), a small central sector with high energy (HE), and a small bottom sector with low energy (LE). Thus, in an embodiment, the first period is HE-LE-HE and the next period is LE-HE-LE, thereby allowing energy to be interleaved at corresponding small sector positions of consecutive periods. It should be noted that if the number of small sectors is an even number, the energy at each position will be LE or HE, and it will not be possible for LE-HE or HE-LE to be arranged at the same vertical position.

[0083] image 3 A system according to another embodiment of the present specification is shown, which includes a CW source for projecting a continuously moving small sector of vertical motion to scan the cargo with reduced scattering. image 3 An X-ray system including a CW X-ray source 310 and a detector array 320 that scans a railcar 330 of cargo is shown. The collimator 340 represents a mechanism that generates a vertically continuously moving fan beam with an angle range smaller than the angle coverage of the railcar 330. The scanning mechanism is synchronized with the data acquisition module to start data acquisition through the detector array 320 at position 350 and end data acquisition at position 360 so as to cover the angular range of the small sector 370. in image 3 Among them, the end position 360 constitutes the start position of the next acquisition cycle. Continue data collection in a similar manner until the full vertical extent of the cargo is covered by "each" small sectors. as figure 2 The illustrated pulse source embodiment uses a CW source 310 to reduce scattering. It should be noted that the system operation remains the same regardless of whether the source is pulse or CW. Although the pulsed high energy X-ray source generates microsecond pulses separated by several milliseconds, the CW source continuously generates X-rays.

[0084] Figure 4 It is an exemplary illustration in which the imaging system in this specification is used to scan an ANSI 42.46 standard penetrating force phantom object. Such as Figure 4 As shown, the ANSI 42.46 penetrating force phantom object 401 is placed in the rail cargo 405. The ANSI 42.46 standard penetrating power phantom object 401 is used to evaluate the penetrating ability of the high-energy radiography system. The object 401 includes an iron block 406 composed of a straight line having a length and a width of at least 60 cm. And the iron block 404 of approximately rectangular shape is placed behind the iron block 406 formed by a straight line. The thickness of the rectangular iron block 406 is approximately 20% of the thickness of the iron block 406 formed by a straight line. in Figure 4 In the test procedure shown, the phantom object 401 is placed in the center of the railed cargo container 405 and tilted toward the X-ray source 402. The X-ray detector array 403 is arranged to detect X-rays transmitted through the object 401. The successful ANSI test of the penetration power of the X-ray system is based on the assessment of the X-ray system's ability to determine which direction the tip 407 of the rhomboid object 406 in the captured image points.

[0085] Figure 5 An exemplary simulation image of an ANSI 42.46 penetrating force phantom object according to an embodiment of this specification is shown, which is Figure 4 The imaging system described in uses a full fan beam of X-rays and uses multiple small fans to obtain. By illuminating a phantom object (such as Figure 4 The illustrated object 401 etc.) form an image 510, and the phantom object includes an object composed of straight lines coupled with a rhomboid object. It can be seen that the image quality of the image 510 is not good because it is difficult to distinguish the rhomboid object 502 in the object 501 formed by straight lines in the image. By using references such as Figure 4 Multiple small fan-shaped X-ray phantom objects (such as Figure 4 The illustrated object 401 etc.) to obtain an image 520. By using multiple small sectors, the image contrast is improved due to the reduction in measured scattering. It can be seen that, compared with the image 510, since the rectangular object 502 in the object 501 formed by straight lines is easier to see, the image quality of the image 520 is better. Compared to the one used when obtaining the image 520, a larger number of small fan-shaped X-rays are used to illuminate a phantom object (such as Figure 4 The illustrated object 401, etc.) obtain an image 530. By using a larger number of small sectors, even fewer scattered X-rays are detected. It can be seen from the figure that the rhomboid object 502 in the object 501 formed by straight lines in the image 530 is most clearly visible, and the quality of the image 530 is better than that of the image 520.

[0086] The generation of the small sector of the vertical movement of X-rays requires a system for projecting an X-ray beam, wherein the X-ray beam has an angular range smaller than the angular coverage of the object under inspection. In one embodiment, the system includes a radiation source that emits radiation at an emissivity (Re) and a conveyor that moves an object through the system at a conveyor rate (Rc), wherein the time (Tf) for the small sector to traverse the object is preferably Equal to the time of a single radiation pulse. In this case, the total time to emit a group of small sectors (when combined, cover the entire angular range containing the object) is equal to a multiple of the total number of small sectors (Nf): Tf*Nf. When multiplied by the transmitter rate (Rc), the total time should preferably be equal to or less than the detector width (Dw), thereby ensuring that no part of the object is lost. Therefore: Tf*Nf*Rc

[0087] Figure 6A A mechanism including a plurality of actuators connected to a beam attenuator to generate a small sector of vertical movement according to a preferred embodiment of the present specification is shown. The plurality of actuators 610 are connected to the plurality of beam attenuators 630 through steel push/pull drive rods 620. Such as Figure 6B As described in more detail in, the actuator 610 is computer-controlled to move the beam attenuator 630 to attenuate the beam to project a small sector of vertical movement. In an embodiment, the actuator 610 is a rotary actuator for obtaining a fast response time for scanning a fast moving object. In alternative embodiments that include scanning slow moving or deep scanning of fixed objects, other types of actuators such as pneumatic actuators may be used.

[0088] In an embodiment, the depth scan is performed using a single small sector having an angular range that sufficiently covers the area of ​​the object of interest. In the case where most of the scanned cargo is highly attenuated and the cargo can be scanned at a low speed, the cargo is scanned using a small X-ray fan shape such as the aforementioned X-ray. However, keep the scanning speed lower than the speed used when scanning fast moving goods. In an embodiment, the number of small sectors used when scanning goods at a slow speed is greater than the number of small sectors used when scanning fast moving goods.

[0089] For example, and by way of example only, the linear accelerator source generates 1 X-ray pulse every 1 millisecond (1/1000 Hz=1ms) at a pulse frequency of 1KHz. Although scanning objects moving at 3.6km/h (or moving 1mm per 1ms or moving 1mm per pulse), by using a detector with a width of 10mm, the X-ray covers the entire object because the detector is more The pulse travels a wider distance. Therefore, because each small sector takes 1ms, if 1ms is multiplied by 10 small sectors=10ms, that is, the object travels a distance of 10mm (equal to the detector width), so when scanning the object without losing any part of the object The maximum number of small sectors used is 10. However, if the number of small sectors is increased, for example to 20 small sectors, the time it takes for the small sectors to cover the object will be 20ms, that is, the object will also move 20mm. Because the width of the detector is only 10mm, the X-ray will lose part of the object. However, if the speed of the object is reduced to 1.8km/h, the object moves 10mm in 20ms, thereby allowing various parts of the object to be scanned. Accordingly, in one embodiment, the system monitors whether the total small sector time multiplied by the conveyor rate (Rc) is greater than the detector width (Dw). If the system determines that this is the case, the transmitter rate (Rc) is reduced enough to ensure that the total time multiplied by the transmitter rate (Rc) is equal to or less than the rate of the detector width (Dw).

[0090] Figure 6B Is shown to produce Figure 6A A block diagram of the various attenuator configurations in the mechanism that vertically moves the small sector shown. Such as Figure 6B As shown, the vertical collimator 640 is coupled to a plurality of beam attenuators 630a, 630b, ..., 630n, and the plurality of beam attenuators 630a, 630b, ..., 630n are further connected to Figure 6A Multiple actuators shown ( Figure 6B Not shown in). The vertical collimator 640 projects a fan beam covering the complete vertical extent of the scanned object. The rod 620 coupled with the actuator 610 can control a plurality of attenuators 630a, 630b, ..., 630n to move the projection beam in and out, thereby projecting a small X-ray sector that moves vertically relative to the scanned object. In the configuration 650, the attenuators 630b, 630c, and 630d are moved into the beam to attenuate the beam, and the attenuator 630a is located outside the beam to project a small fan on the upper part of the scanned object. In the configuration 660, the attenuators 630a, 630c, and 630d are moved into the beam to attenuate the beam, and the attenuator 630b is located outside the beam to project a small fan on the upper middle part of the scanned object. In the configuration 670, the attenuators 630a, 630b, and 630d are moved into the beam to attenuate the beam, and the attenuator 630c is located outside the beam to project a small fan on the lower middle part of the scanned object. In the configuration 680, the attenuators 630a, 630b, and 630c are moved into the beam to attenuate the beam, and the attenuator 630d is located outside the beam to project a small fan on the lower part of the scanned object. Therefore, the small fan is moved to project X-rays on different parts of the scanned object by moving the attenuator out of the projected X-ray beam. As described, the movement of the attenuator provides a small sector of X-rays of vertical movement. In various embodiments, the beam attenuators 630a, 630b, ..., 630n are made of high-density materials such as, but not limited to, lead or tungsten.

[0091] In another embodiment, the small X-ray sector can be vertically moved relative to the scanned object through the spiral contour hole formed on the rotating cylinder. Figure 7 An exemplary design of a rotary-roll chopper used to vertically move the small X-ray sector relative to the scanned object according to an alternative embodiment of the present specification is shown. A spin-to-roll chopper is described in US Patent No. 9,058,909 B2, the entire content of which is incorporated herein by reference. The rotation of the roll/chopper provides a small sector of vertical movement of constant size and speed.

[0092] In one embodiment, the chopper 702 is manufactured in a cylindrical shape made of a material that is highly attenuating X-rays. The chopper 702 includes a spiral slit 704 for the chopper. The cylindrical shape enables the chopper 702 to rotate around the Z axis 703 and along the spiral slit (hole) 704 to establish a rolling motion to provide an effective vertical projection that can project a small sector of vertical movement of X-rays on the scanned object. Move the slit 704 straight. In one embodiment, as required by the system in this specification, the slit 704 is wide enough to allow projection of a small fan beam. It should be noted that narrow slits will produce pencil beams instead of fan or small fan beams.

[0093] Figure 8A An exemplary mechanism for generating a small moving sector shape according to another alternative embodiment of the present specification is shown. reference Figure 8A The rotating mechanism 800 includes a wheel 801 with three slits 802, 803, and 804, and the three slits 802, 803, and 804 are arc-shaped or partially circular in shape. In one embodiment, the wheels are made of highly X-ray attenuating materials such as lead or tungsten. The wheel 801 further includes a vertical collimator 805. In operation, as the wheel rotates, the interaction between the slit 802 and the vertical collimator 805 results in obstructing radiation from the slit, except for the small fan-shaped portion 806a. In one embodiment, the width of the slit is configured to produce the desired small fan-angle width. In one embodiment, the rotation frequency of the wheel is determined based on the small sector width and the linear accelerator pulse frequency. The wheel rotation is synchronized with the pulse frequency of the linear accelerator to generate a small sector with small overlap and covering the breadth of the cargo in one cycle.

[0094] Figure 8B , Figure 8C ,as well as Figure 8D Is a series of diagrams showing the various positions of the wheel, which indicate how to create a small fan shape and how to move to cover the breadth of the scanned object. Reference 8B, Figure 8C ,as well as Figure 8D and Figure 8A , Position 810 shows the small sector 806a at the uppermost position. When the wheel 801 rotates in the counterclockwise direction, the small sector 806b moves downward, as Figure 8B Shown in position 820. Those of ordinary skill in the art should recognize that the wheel can also rotate in a counterclockwise direction. Thus, after the position 820 rotates further, the small sector 806c moves further downward, as Figure 8C In the position 830 shown. When the small fan shape exists in the lowest position, the next slit 803 in the wheel projects the upper small fan shape 807. Figure 8D Position 840 in shows this situation. Repeat the rotation cycle until the complete object is scanned.

[0095] It should be noted that although the use of a small fan for scanning reduces the scattering, there is still some scattering within the small fan due to the interaction of the cargo and the X-ray beam. Therefore, in one embodiment, the system in this specification uses a detector located outside the small sector to measure the scattering and uses this measurement to eliminate the scattering in the small sector. Then, the estimated scatter is subtracted from the transmitted image data to increase the contrast of the composite image.

[0096] Those of ordinary skill in the art should recognize that even if the penetration power provided by the embodiments in this specification is increased, however, there is a dark alarm that requires labor-intensive manual inspection. Therefore, in another embodiment, this specification describes a method of scanning objects using a two-step process to further reduce dark alarms. by Picture 9 The flowchart in shows this process.

[0097] reference Picture 9 In the first scan 901, a standard fan beam or a small fan of single-energy or multi-energy high-energy radiation is used to scan the truck or cargo container, and the transmitted radiation is measured by the detector array. In an embodiment, the truck or cargo container is scanned through a complete cycle, where, as described above, the complete cycle is using a standard fan beam with an angular range or multiple small fan pairs with a total angular range of the standard fan beam. The vertical extent of the object is scanned. Thus, in an embodiment, in a complete cycle, the small sector is vertically translated via the collimator mechanism to cover the angular spread of the object. The pulse linear accelerator X-ray source and the data acquisition system are synchronized with the mobile collimator in such a way that images of the object are acquired at intervals, in which, in one cycle, small sectors have no gaps (optionally with minimal overlap) Ground) to cover the slice of the object. Then, the images from each small sector are combined to produce sliced ​​images.

[0098] In step 902, the transmission information is analyzed to determine the dark alert area. As shown in 909, if no dark warning area is found, the transmitted image is analyzed to determine the existence of contraband and other items of interest.

[0099] As shown in step 903, if the beam does not penetrate one or more areas of the image (dark alarm), the area is subjected to a second scan. In the second scan, adjust the horizontal collimator to cover only the vertical extent of the dark area and the suspicious area (if any).

[0100] This operation is shown in 904. Then, reposition the container to allow rescanning of the location of the suspicious area. In one embodiment, as shown at 905, the radiation source is tilted to align with the center of the dark area. In one embodiment, it is preferable to perform the rescan at a lower speed than the initial scan, such as, for example, at 1/40 th Perform a rescan at the standard scan speed. This operation is shown in 906.

[0101] In one embodiment of the system, the source and detector are mounted on a hanger to allow the system to be repositioned and scan any part of the object at a wide range of speeds. Optionally, because the Bremsstrahlung X-rays are more intense, the source is tilted in such a way that the beam centerline is aligned with the center of the dark area to increase the beam intensity.

[0102] By appropriately using a collimator to reduce the vertical extent, scattering from other areas of the container is prevented and penetration is increased. It should be noted that because the single-energy image is cleaner due to scattering that distorts the X-ray spectrum, the reduction in scattering also helps to improve material separation by the dual-energy beam. The decrease in scanning speed further allows for improved statistical accuracy and also increased penetration.

[0103] After that, as shown in 907, the scanning system checks the transmitted image again to check if there are more dark alarms. If more dark alarms are found in the scanned image, the rescan is performed again by repeating steps 904, 905, and 906. This process continues until all dark alarms are lifted.

[0104] As indicated by 908, when there are no more dark alerts, the rescanned portion of the image is integrated into the original image of the object. In one embodiment, this is done by replacing the initial part of the image with the corresponding rescanned part. Then, as shown in 909, the transmission image is analyzed to determine the existence of contraband and other items of interest.

[0105] In addition to clearing the dark alarm, another motivation for the second scan is to clear the automated high-Z alarm. It should be noted that when detecting high-Z materials, the system in this manual uses an automated procedure to generate an alarm. U.S. Patent Application No. 14/104,625 entitled "Systems and Methods for Automated, Rapid Detection of High Atomic Number Materials" and filed by the applicant of this specification describes the system and method for automatically generating an alarm when detecting high-Z materials, by reference Combine its entire contents here.

[0106] It should be noted that the method of automatically detecting high-Z materials uses attenuation information from the segmented object and the surrounding background. Therefore, the need for active interrogation is reduced due to the improved single-energy and dual-energy contrast, and rescanning suspicious objects with less scattering can clear the alarm. Therefore, in one embodiment, the system in this specification uses the above reference Picture 9 The described rescanning scheme clears the automatic high-Z alarm in a similar way to clearing the dark alarm. In one embodiment, an additional improvement is obtained by another scan performed at an angle of 10°-20° to allow different fields of view of cargo with different sets of superimposed objects. Those of ordinary skill in the art should recognize that requiring the confirmation of alarms in all scanning stages will even result in a lower false alarm rate. Those skilled in the art should also realize that the secondary inspection can be applied not only to high-Z materials, but also to other objects of interest, such as suspected contraband such as explosives, fire alarms, and drugs.

[0107] In one embodiment, the X-ray source can be replaced by a neutron source. It should be noted that when the X-ray source is replaced by a neutron source, the detector is replaced by a neutron detector and the collimator is replaced by a neutron attenuating material instead of lead. However, the operation of the system remains the same.

[0108] In the description and claims of the application, the various words "comprise", "include", and "have" and their forms are not necessarily limited to the elements in the list associated with the word.

[0109] The above embodiments only show many applications of the system and method in this specification. Although only a few implementations of this specification are described here, it should be understood that this specification can cover many other specific forms without departing from the spirit or scope of this specification. Therefore, the present examples and implementations are regarded as illustrative and non-restrictive, and this specification can be modified within the scope of the appended claims.