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Volumetric X-Ray Testing
by Harold Berger* and Robert L. Schulte*
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As radiographic techniques become more sophisticated to solve more sophisticated problems, the basics of the technique become more complicated. Here is a great explanation of how one of the new imaging techniques, tomography, works. Berger and Schulte give a really short and to the point explanation - and without the pages of math usually found in such articles. The photographs of how it works are worth many thousands of words that would never have done the job - at least for me. Frank Iddings Tutorial Projects Editor |
Introduction
Most radiographers have come across an X-ray testing problem that was not solved by a conventional two dimensional X-ray image. The information sought was hidden behind other structures in the object. If the system was a realtime radioscopic system, the information might be obtained (or might not, depending on the geometry) by rotating the object in the beam while the test was being conducted. However, many installed X-ray testing systems, even realtime radioscopic systems, don't have the necessary object rotation capability. In these cases, the desired information may be obtained by a repositioning of the object in the beam, perhaps several times, to see the desired X-ray image detail. An alternative to this approach is a volumetric X-ray imaging system. Volumetric X-ray image techniques have existed for some time (Ellingson and Berger, 1980), but many X-ray people will think first about computed tomography (ASTM International, 1997; Bossi, 1996). However, there are many testing situations where the size, shape or location of the object is not suitable for testing using computed tomography methods.
Images of any region of a testing object can be obtained quickly and easily with volumetric X-ray systems.
There are various terms that have been used to describe the process of creating a useful X-ray image of a selected plane within an object. Planigraphy and laminography are some of the older terms used for this process of bringing a selected plane in the object into good focus. The technique was used for many years in the medical community (Plantes, 1932). In the simplest form, the object remained stationary while the X-ray source was moved linearly in the +X direction and the imaging detector (usually film) was moved at the same time and rate in the -X direction. This resulted in a single image plane in the object (the fulcrum plane) remaining in focus while images from other object planes were blurred along the direction of the source detector movement. The process could be repeated to image other object image planes. The motion for the source detector movement may also be more complicated than the simple linear motion described; the blurring of out of plane detail is more effective when more complex source detector movements are used. For example, circular or hypocycloidal motions provide images of the desired object plane with less blurring from out of plane object details.
The terms used to describe this process for computer methods are "tomography" and "tomosynthesis." Tomography is defined in ASTM E-1316 (1999) as follows: "any radiologic technique that provides an image of a selected plane in an object to the relative exclusion of structures that lie outside the plane of interest." Tomosynthesis is normally used to describe a technique in which the X-rays are directed through the object in a limited number of views, with the objective of reconstructing multiple image planes within the object.
Computed tomography is widely used in the medical community and is receiving increased attention from industrial users. Computed tomography systems are usually configured to take many views of the object, often more than 100; this provides reconstructed images of good quality, with excellent density discrimination. Limited view tomosynthetic techniques are beginning to attract attention from a number of investigators, many of whom are taking advantage of new, semiconductor digital imaging detectors - particularly flat panels (Antonuk, 1995; Jones and Berger, 2000; Jones, 2001). Many reports on advances in computed tomography and tomosynthesis X-ray imaging are described in recent symposium proceedings (Antonuk and Yaffe, 2001; Boone and Dobbins, 1999; Dobbins and Boone, 2000). These advanced techniques have also been described at the recent series of ASNT topical meetings on digital imaging (for example, see American Society for Nondestructive Testing, 2001).
An example of an advanced volumetric X-ray testing technique is an all electronic, digital, multiimage technique that permits an operator to reconstruct any horizontal or vertical X-ray image plane in the object (Schulte et al., 2001; Schulte 2001; Schulte, 2002). This capability allows the viewer to look behind obstructing details that may hide the testing region of interest. The operator also has the capability to scan through selected regions of the object in both the horizontal and vertical directions. Since an integral part of this system is an immediate response (realtime radioscopic capability), the operator can view the usual two dimensional X-ray image and decide if more information is needed to complete the test. If that is the case, additional images can be taken while the testing object is in place to provide the volumetric images needed to complete the test.
Volumetric Imaging System
A simple system for stereo imaging requires only two images taken from angles that permit stereo image viewing. X-ray image stereo viewing can be accomplished with film or filmlike detectors (American Society for Nondestructive Testing, 1985; Evans and Robinson, 2000) or in realtime radioscopy by rapid movement of theX-ray focal spot in a microfocus X-ray tube (Polansky et al., 1990). However, more than two X-ray views are required for reconstruction of multiple X-ray image planes within an object. True tomosynthesis X-ray systems for viewing of multiple image planes require more than two X-ray images and provide much more image information. The system, like all tomosynthesis techniques, makes use of multiple two dimensional X-ray images taken from different perspectives.
The tomosynthesis process requires a series of projection X-ray images taken from different angles through the test object. A schematic of the technique is shown in Figure 1. (Jones and Berger, 2000; Schulte et al., 2001; Schulte, 2001). A typical testing geometry is shown with the source located at an oblique angle with respect to the vertical direction. The object under test is rotated to eight different positions and an X-ray image is acquired at each position. The process is very flexible and is not constrained to any specific positions, number of images or geometry. The circular image pattern, however, is convenient to demonstrate the technique. The image detector can be any flat detector, film, scintillator or flat panel. The flat panel, as used in this system, provides rapid response, excellent dynamic range and digital images for input to the reconstruction software. In this case, eight two dimensional digital images are collected, showing details in the testing object from the different image perspectives. Any view through the object can be reconstructed with software using the basic set of oblique incidence images acquired during the X-ray test.
After acquiring the limited X-ray image data set, the application software is used to reconstruct views of any horizontal X-ray image plane parallel to the flat panel image detector face. In addition, the software can display any vertical X-ray image plane (perpendicular to the flat panel face). The reconstructed X-ray image planes provide an excellent dimensional measurement capability with the accuracy dependent on the resolution of the image detector. An interesting feature of the application software is a realtime scan through the X-ray image planes. This dynamic display of internal X-ray images is very useful. It allows an operator to look rapidly through the X-ray image volume of the object, stop at any point and reconstruct the entire X-ray image plane at that level. This dynamic response can't be shown in a hard copy article, but several examples of reconstructed X-ray images are shown in the following section.
Applications
A tomosynthetic examination of a test object can be used to illustrate the power of this technique. Two aluminum plates were prepared - each 12.7 mm (0.5 in.) thick plate contains a pair of flat bottom 3.175 mm (0.125 in.) diameter holes at various depths. The plates were assembled into a single 25 mm (1 in.) thick object with the hole openings facing the interface between the two plates. Reconstructed horizontal X-ray images of each plate show only the holes in that plate. Figure 2 shows horizontal views at two different levels within the assembled plate object. Figure 2a shows a horizontal X-ray image view at a level of 10 mm (0.4 in.) above the detector plane, well within the lower aluminum block (two holes are shown). Figure 2b, taken at 15 mm (0.6 in.), shows only the other two holes in the upper block. A cross sectional view of the plates can be obtained through a vertical image reconstruction. Figure 3 shows the horizontal view at the interface of the two plates at which each of the four holes is just visible. Using the application software, a multisegment line is drawn over all the holes, as is shown in Figure 3. A vertical reconstruction is made along the line segments, starting in the upper right corner (Figure 4). The image is a cross section of the stacked plates showing all of the holes as a function of depth; the holes in each plate extend only to the interface between the plates. The diameters and depths of all the holes can be measured with precision on the order of 127 µm (5 x 10-3 in.).The test object described above illustrates what can be done with this tomosynthesis X-ray imaging system. A practical application of X-ray tomosynthetic imaging is weld testing. The horizontal and/or vertical X-ray reconstructed images provide excellent location information about weld discontinuities. This, coupled with the capability for dimensional measurement, gives an inspector the information needed to decide about repair and from which side of the weld to start. Figure 5 shows a horizontal X-ray image reconstruction through a butt weld of two 12.7 mm (0.5 in.) thick aluminum plates with a single V groove. The total thickness of the weld is 18 mm (0.7 in.). This horizontal image, at a level of 16 mm (0.63 in.) is just below the level of the plates, showing discontinuities (a pore and inclusions) in the weld bead. A vertical X-ray image (Figure 6) reconstructed along the direction of the weld and through the center of the pore shows the size and location of the pore. This information makes it easier to assess the quality of the weld.
Conclusion
Full volumetric X-ray images provide additional information for the detection and characterization of discontinuities or components in all kinds of structures, including welds, castings, electronic devices and electromechanical assemblies. Use of a digital flat panel provides high sensitivity, fast response and good resolution. Images of any region of a testing object can be obtained quickly and easily with volumetric X-ray systems. This includes systems such as the electronic X-ray system described in this article. Compared to other volumetric X-ray image systems, the X-ray system offers advantages, including reconstructed viewing of any horizontal or vertical X-ray image plane, scanning through selected horizontal or vertical image regions and a precision measurement capability. Volumetric X-ray imaging, using methods such as tomosynthesis or computed tomography, can provide the additional information often needed for critical nondestructive testing applications.
Acknowledgments
The authors wish to thank Donald Twyman, of the Digitome Corporation, for his assistance with the preparation and review of the paper.
References
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* Industrial Quality, Inc., 640 E. Diamond Ave., Suite C, Gaithersburg, MD 20877; (301) 948-2460; fax (301) 948-9037; e-mail <hberger@indqual.com>.
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