Patent Publication Number: US-8976926-B2

Title: Portable 3-dimensional X-ray imaging system

Description:
GOVERNMENTAL RIGHTS 
     This invention was made with United States Government support under Contract No. N00014-11-C-0142 awarded by the Office of Naval Research. The Government has certain rights in this invention. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     This invention relates to x-ray imaging systems and methods, and more particularly to using an x-ray imaging system to provide three-dimensional information about the contents of an opaque object. 
     BACKGROUND OF THE INVENTION 
     Explosive Ordnance Disposal (EOD) personnel, often called “bomb squads,” are charged with the task of investigating and disposing of unexploded ordnance (UXO) in both domestic and military settings. Most UXO is detonated in-place, using a “disrupter” projectile fired at the ignition source of the ordnance. However, in some cases, such as when the UXO is an improvised nuclear device or when it endangers a high-value asset, it cannot be detonated in-place. In these situations, EOD personnel are tasked with diffusing the UXO without detonation, which can be challenging, especially for IEDs equipped with booby-trap triggers and decoy detonators. 
     One of the tools used by EOD personnel to aid in both detonation-in-place and diffusing is an x-ray imaging system, such as the SAIC RTR-4 system. For detonation-in-place, a single X-ray image can provide sufficient information for aiming the disruptor projectile. However, for diffusing, EOD personnel often find it necessary to view multiple x-ray images from numerous perspectives. Taking images from multiple perspectives enables EOD personnel to estimate the relative locations of components within the IED to ensure entry into the IED at the correct locations. 
     Unfortunately, collecting multiple images for several perspectives requires physically moving the source, and sometimes the imaging screen. This can significantly increase the time-on-target for EOD personnel, which is undesirable. Furthermore, even when a series of perspectives is acquired, determining the z-position of components within the IED is difficult. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present embodiments and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings, in which like reference numbers indicate like features, and wherein: 
         FIG. 1  illustrates an x-ray imaging system for providing three-dimensional data representing the contents of an opaque object. 
         FIG. 2  illustrates a motorized embodiment of the x-ray source and pivot arm of the system of  FIG. 1 . 
         FIGS. 3A ,  3 B and  3 C illustrate three x-ray projection images of a simulated IED, each obtained from a different perspective of the x-ray source. 
         FIGS. 4A ,  4 B and  4 C illustrate an example of three convergence slices at various planes of the z dimension. 
         FIGS. 5 and 6  illustrate the convergence concept of the method of providing three-dimensional data representing the location of contents of an opaque object. 
         FIG. 7  illustrates the basic steps of a method of providing three-dimensional data representing the location of contents of an opaque object. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As indicated in the Background, x-ray images are notoriously “flat”. They are not images, per se, but rather projections, similar to shadows. An x-ray image provides a two-dimensional view of x-axis and y-axis positions, but provides very little z-axis (depth) information. 
     The following description is directed to using x-ray images to determine three-dimensional location information about components contained within an object having an opaque enclosure, such as an improvised explosive device (IED). As explained below, an x-ray source is mounted to a moveable arm, which moves through a number of positions to collect multiple images from multiple perspectives. Each of these multiple images is a conventional two-dimensional x-ray projection. However, the collection of images is processed to provide depth information about the contents of the object. 
     In this manner, the system and method described herein determine the three-dimensional positions of components within an object. The components&#39; x-y as well as z-axis (depth) positions are determined. The information may be presented in various ways, such as in the form of a three-dimensional computer model of the object. In the case of an IED, this information will enable safe access into the IED so that it can be defused. 
       FIG. 1  illustrates an x-ray imaging system for providing three dimensional data about the contents of an opaque object. For purposes of example, the object is assumed to be an IED. An x-ray source  11  in front of the object is mounted on a pivot arm  12 . The pivot arm  12  can be moved in known increments to step the x-ray source  11  through multiple positions. 
     In the embodiment of  FIG. 1 , the pivot arm  12  moves the x-ray source  11  in a vertical arc, that is, an arc in the same plane as the x-ray sensor screen  13 . The x-ray source  11  is pointed down a cone angle towards the center of screen  13 , which is placed behind the object under investigation. 
     The motion of the pivot arm  12 , as well as the tilt of the x-ray source  11  toward the object, can be performed manually or automated. In more sophisticated embodiments, the pivot arm  12  may be attached to a robot, which performs all or some of the re-positioning tasks described herein. 
       FIG. 2  illustrates a motorized embodiment of the x-ray source  11  and pivot arm  12 . A base  21  provides secure support for the pivot arm  12  and x-ay source  11 , and is typically placed on the ground or floor. A hinged attachment mechanism  22  allows the pivot arm  12  to travel through an arc. Typically, the range of motion will be 180 degrees. A servo motor  23  controls the position of the pivot arm  12  to step it through multiple positions on its arc. This configuration allows the x-ray source  11  to be placed in any position along the arc. 
     The x-ray source  11  is mounted on the pivot arm  12  so that its tilt may be re-positioned to point toward the center of the x-ray sensor screen  13 . Its aperture is aligned to the pivot arm axis, which helps to provide accurate arc positions. Lasers  24  mounted on the base  21  are used to align the pivot axis with the center of the sensor screen  13  and to ensure that the plane of the pivot arm sweep is approximately parallel to the plane of the screen  13 . A motor  25  may be used in automated versions of the system, to control the tilt of the x-ray source. 
     If the motion of pivot arm  12  is automated, such as with motor  23 , processing unit  18  controls the motion of pivot arm  12  by generating control signals to the motor. Likewise, adjustment of the field of view of x-ray source  11  may also be automated and controlled by processing unit  18  with control signals to motor  25 . 
     Also, processing unit  18  may be programmed to perform all or part of the image processing described below. To this end, it is assumed that processing unit  18  is in data communication with sensor screen  13  to receive x-ray images, and has appropriate hardware and software for performing the image processing tasks described herein. All or part of processing unit  18  may be remote from the rest of the x-ray imaging system. 
     A display and user interface unit  19  is in data communication with processing unit  18 . Unit  19  allows an operator to control the motion of pivot arm  12  and x-ray source  11 , and to view the results of the processing. 
     Optionally, pivot arm  12  may be telescoping. The ability to shorten the length of pivot arm  12  enables the x-ray system to break down for compact stowage. Also, a longer arm length may be desirable for larger objects or for a longer distance between the object and the x-ray source  11 . 
     In operation, the x-ray sensor screen  13  is placed behind the object. The pivot arm  12  is placed in front of the object, with the x-ray source  11  pointed toward the center of the screen  13 . The pivot arm  12  is stepped through a series of discrete positions and a set of x-ray projection images is collected. 
       FIGS. 3A ,  3 B and  3 C illustrate three x-ray projection images of a simulated IED, each obtained at a different perspective of the x-ray source  11 . The simulated IED has various components, including three booby trap triggers  31 - 33 . 
     In an actual test, these images were a subset of thirteen images taken at 15-degree increments from 0 to 180 degrees.  FIGS. 3A ,  3 B and  3 C are of projections at 0, 90 and 180 degrees, respectively. 
     The projection image collected at each position of the x-ray source  11  provides a different perspective on the contents of the object. The set of images from the different perspectives is referred to herein as the “perspective images”. 
     Although not readily apparent from the images, because of the different perspectives, the components within the object are shifted in each image. This shifted feature of the perspective images is used to extract z-dimensional information from a set of processed images. 
     Specifically, each perspective image is processed to achieve back-projections in the direction of the source along a series of discrete depth positions. This is achieved by mathematically shifting and shrinking each projection image at known increments along the z-axis in the direction of the source x-ray. The result is a set of back-projection images for each perspective image. 
     Next, the resized-and-shifted back-projections from all images are superimposed (converged) at each position in z. The effect is that components of the object that exist at a particular z-location will coincide in x and y and thus achieve an additive effect. The result is a set of 2-D convergence images of the 3-D object in the z-dimension. 
     The 2-D convergence images can be presented as a sequence of convergence frames, where each frame represents a 2-D slice in the third (z) dimension. A threshold operation may be used to select the elements that are present in each convergence frame. 
       FIGS. 4A ,  4 B and  4 C illustrate an example of three convergence slices at various planes of the z dimension (depth). The components in the images that are more sharply resolved are located in the z-plane associated with each image. Thus, the z-axis location of components inside the object can be determined. 
     Of particular interest in the IED example of this description is the fact that there are three booby-trap triggers  31 - 33  in this simulated IED. Each booby-trap trigger can be located at its respective depth in the IED, allowing the EOD technician to enter the device with confidence while avoiding inadvertent detonation. 
     The pivot arm length and angle and the pivot-point location relative to the center of the sensor screen are important for determining the position of the source relative to the screen, which is used by the processing software to extract the z-dimension information. 
       FIGS. 5 and 6  illustrate the convergence concept of the above-described method. As shown in  FIG. 5 , when the x-ray source is stepped through five different angular positions in an arc, Point O 1  is projected to five different locations in the image plane (x-ray screen  13 ), namely P 1 ′, P 2 ′, P 3 ′, P 4 ′, P 5 ′. It is possible to simulate the effect of moving the image plane in the direction of the source by back-projecting and overlaying the five images. The back projection is done by shifting and shrinking each image according to the geometry that follows from the relative distances and angles between the source and screen, which are different for each of the five projection images. 
       FIG. 6  illustrates projection of the image plane closer to O 1 . The five imaging points become closer together. When the projected image plane intersects O 1 , the 5 imaging points become superimposed. This point of intersection, which occurs at the convergence plane, leads to higher signal levels when the projections are added together. If the image plane is further projected, the five superimposed imaging points diverge again. 
     Thus, an object in the summed projected images comes into convergence and then diverges again as a consequence of the simulated movement of the image plane in the direction of the source. When convergence is at its peak, the object has its highest intensity value. 
     The output of the process is a three-dimensional data volume that contains location information about the components in the object. The data volume can be presented as a sequence of data slices in the z-dimension. These data slices can be further presented as frames in a video, or exported as a point cloud to a solid-model rendering software program. Regardless of the specific form of the output, in general terms, the output presents three-dimensional data representing the contents of the object. 
     Additional processing can be performed to eliminate false results and for edge sharpening. False positives can occur when different objects happen to project to the same location; false negatives can occur when objects have weak signal levels due to illumination or absorbance variations. 
     In other embodiments, the x-ray imaging system of  FIG. 1  could be modified such that the x-ray source  11  is moveable from side to side, or up or down, or in any direction, as long as it provides multiple perspectives from positions that vary in height (y-dimension) and from side-to-side (x-dimension), and those positions are known within a reasonable accuracy. It is important for the processing that the relative position of the x-ray source and x-ray sensor screen are known at every projection. An advantage of moving the x-ray source in an arc is that it provides a simple way to span a number of positions in x and y and provide widely varying perspectives on the object. Assuming a well-designed x-ray system apparatus, these positions can be well known by aligning the base of the system to the x-ray sensor screen and moving the pivot arm at known intervals, say 15 degrees each. 
       FIG. 7  illustrates the basic steps of a method of providing three-dimensional data representing the contents of an object. The system of  FIG. 1  may be used to implement the method, with the positioning of the x-ray source  11  and the x-screen  12  being performed either manually or mechanically. Specifically, the initial position of the system, the movement of the pivot arm, and the tilt of the x-ray are all operations that may be performed manually or by robot or motor. 
     Step  71  is the initial positioning of the x-ray source  11  and the x-ray screen  13 . As explained above, the object under inspection is placed in front of the screen  13 , and the pivot arm  12  and x-ray source  11  are positioned so that suitable perspective views can be acquired. 
     Step  72  is acquiring perspective images from a number of different positions of the pivot arm  12 . As explained above, this step can be performed by moving the pivot arm  12  in a vertical arc. The direction of movement of the x-ray source  11  is not important so long as good perspectives are obtained, and each position of the x-ray source relative to the object is known. Step  72  is repeated until a desired number of perspective images is obtained. 
     Step  73  is processing the perspective images to obtain back-projection images. As explained above, this is accomplished by shifting and re-sizing each perspective image at known locations along the z-axis. Step  73  is repeated for each perspective image. 
     Step  74  is superimposing the back-projection images at each location on the z-axis. As explained above, at each z-axis location, the back-projection images are superimposed. The result is an image slice at each z-axis location. Components within the object that are in the same plane in the object will be more intensely illuminated. Further processing can be performed to estimate the location of the component within the object in three-dimensional space. As further explained above, the results of the processing can be presented to the operator in various forms. 
     In an enhanced version of the above method, the x-ray system is used to acquire projection images by re-positioning the system to acquire images from a second orientation of the x-ray system. Typically, the second orientation is in a different plane. For example, as described above, the x-ray imaging system is positioned to collect a series of images in a first arc, such as a vertical arc. Then, the system is re-positioned to collect a series of images in a second vertical arc, such as one 90 degrees from the first arc. Thus, the two arcs of the x-ray source are offset, typically orthogonally. This “multiple orientation” method provides improved three-dimensional data about front and backs of the contents of the object. Once acquired, the projection images from all x-ray source positions are processed (back-projected and superimposed) as described above. 
     The above-described method allows an EOD technician to spend little or no time-on-target while assessing the IED. In practice, a robot could be used to place the screen and pivot arm. A collection of x-ray images at multiple angles can be quickly acquired. The processing to create the depth data of the object is also fast. It is expected that the images can be acquired in less than five minutes and that the processing can be performed in less than two minutes.