Digitome is capable of accurate measurement down to the order of microns given a small focal spot size and low pixel depth for a digital image plate, typical conditions for high-resolution digital radiographs. This video demonstrates how Digitome measures objects in an exam; the objects used have well-known sizes to test the software’s accuracy.
The first sample object is a wooden block with two screws and a nail. The screws have been drilled into the block on the same face about half an inch apart. The nail has been placed into the block at the center of an adjacent face of the block so that it is orthogonal to and in between the screws. The most interesting aspects of this composite are its screw threads, which have different densities. One of the screws has about 24 threads/inch while the other has about 32 threads/inch (manufacturer’s quantities, McMaster Carr). With Digitome, we are able to discern the thread patterns and measure about 24 threads/inch and 32 threads/inch, respectively.
The second object is made of two ruby spheres used in fiber-optic cables placed in contact with each other on a flat plane. The spheres in fiber-optic cables must be accurately made to precisely transmit information throughout the cable. Thus, the two spheres have been made with an uncertainty in diameter only the level of microns. Digitome effectively measures a diameter that has only a 1.29% error compared to the manufacturer value.
In contrast to Computed Axial Tomography exams, Digitome exams do not have to interpolate between slices and therefore (theoretically) contain all of the object information in one exam. So accurate measurements made in the software can be used to measure any aspect of an object that is detectable through x-ray imaging.
These measurements were carried out with a tungsten x-ray source with a spot size of 0.5 mm and an amorphous silicon digital image plate with a pixel depth of 127 microns.
Davidson College, a four-year liberal arts college located in Davidson, NC, is using a flexible and portable technology from Digitome Corporation for taking three-dimensional x-radiographic examinations of various archaeological objects. The Digitome software constructs a volumetric display of an imaged object using multiple images of the object from different perspectives. Typically, a 3D exam is constructed from around eight images of the object.
This technology has potential applications in art and object conservation, archaeology, and other fields requiring non-destructive testing. Over the summer of 2014, Dr. Dan Boye (Professor of Physics, Davidson College) and I, (Ryan Kozlowski, Davidson College class of 2016) applied the Digitome technology to archaeological artifacts from the Queen Anne’s Revenge (QAR), the flagship of infamous Blackbeard the Pirate. This research was funded by the Davidson Research Initiative. The QAR Conservation Laboratory, located in Greenville, NC, is a part of the North Carolina Department of Cultural Resources (NCDCR).
The original Pythagoras cup was designed around 500 BC to teach its users to drink in moderation. If liquid is filled into the cup above a specific height, it all completely funnels out of a hole in the bottom of the cup and onto the drinker’s lap. A three-dimensional radiographic exam of a modern Chinese Pythagoras cup was taken to reveal the simple structure of the cup that allows the siphoning mechanism to occur. Though design or object material changes across cultures and time, the functionality of the Pythagoras cup remains the same.
The cup has a neck at its center with an opening at its base. Inside of the neck, which is sealed off at the top, is an inner tube that is open at the top (within the neck) and the bottom (visibly seen on cup surface). As a user pours fluid into the cup, the fluid fills in the neck to the same height as the fluid in the open, visible area of the cup. When a portion of the fluid in the neck overflows and begins to pour down the inner tube by gravity, the remainder of the fluid in the entire cup follows the flow up through the neck and down the inner tube because of reduced pressure and an overall decrease in the gravitational potential energy of the system. The Pythagoras cup is really just a self-priming siphon.
The three-dimensional exam reveals that the inner tube is at a height of approximately 1.20 inches from the bottom layer of the cup. It affirms that the dragon’s head is sealed off from the neck and demonstrates the effectiveness of the Digitome software in accurately measuring object parts and picking up on fine details like the horns of the dragon or the opening in the back of its neck. This video shows a scan upward through the object—Digitome can produce a “stack” of images for an object, and the user can export the stack to commercial software that can then produce a 3-dimensional surface view, as displayed at the end of the video.
Two-dimensional x-radiography is often sufficient for looking at simple, relatively flat objects. But what happens when an object has multiple layers of information or overlapping parts? A coin with two faces is an example of such an object—a two-dimensional radiograph allows the viewer to pick up certain details of each face of the coin, but these details inevitably overlap. Similar issues may occur for other objects, such as a painting with multiple layers or a complex circuit board.
A Digitome exam was taken of an Alaska quarter to see if the two separate faces could be isolated, and it was successful. One distinct layer of the coin scan reveals the grizzly bear springing out of a rushing river and biting his catch—a salmon. Moving down through the coin, the face of George Washington is clear, but analysis of the words reveals that the radiographic image face is actually a mirror image of the real face! The coin had been placed with Washington’s face down on the image plate, so its shadow was flipped. (Imagine an ink stamp or the imprint of a boot in the mud.) In the video, after scanning, the coin is manually rotated and flipped by video software for ease of viewing.
The characters (text and numbers) on each face are visible albeit not completely clear. The designs on the coin are made through pressing blank copper-nickel alloy with imprints that outline the images and characters; the result is that any pressed coin has varying thicknesses and therefore different levels of attenuation (absorption or scatter of x-rays) across the coin. Characters are areas of greater thickness and Digitome distinguishes the changes in thickness on both faces from one another. Still, the characters on the quarter are somewhat difficult to resolve because of the limitations of resolution. Resolution is determined by the image plate used, the position of the object relative to the source and detector, and, in this instance, the size of the letters (on the order of millimeters).
Geodes are hollow, closed shells made of igneous or sedimentary rock. They generally form in cooling lava or in sedimentary geological structures over several thousand years. Though the mechanisms of void formation differ, the same process occurs in time for all geodes: water-carrying minerals, usually calcite (CaCO3), seeps into the microscopic pores of the shell and accumulate the inner lining that, in the final product, is a visually pleasing crystalline structure,.
Since these rocks are closed surfaces, the only practical ways to view the inner crystals are to break them or to use a form of imaging that makes use of light outside of the visible range like x-radiography. A two-dimensional x-ray image will be able to detect that the rock is hollow, but it will not necessarily include information about the varying thickness of the shell or the shape of the crystal lining. A Digitome volumetric exam, however, is able to detect the contour of the void within the rock in a non-destructive manner.
The elemental make-up and thinness of the geode shown in the video allow for easy penetration of x-rays that are relatively low in energy. The quartz is mainly silicon dioxide and the crystal lining’s most absorptive element is calcium, with an atomic number of 20. The exam in the video was taken with an x-ray beam of 80 kVp; metallic elements like iron, of atomic number 26, often require more energy for adequate penetration. Since calcium composes much of the geode, this material is a good model for concretions that would be found on the ocean floor in underwater archaeology excavations. In particular, the geode shown in the video was volumetrically examined to prepare for taking three-dimensional x-radiography exams of artifacts in concretions from the Queen Anne’s Revenge.
Scanning upward through the geode, it is evident that much of the rock is indeed hollow. Areas of particularly low or high thicknesses are identifiable through the exam and can be accurately located on the actual geode. Because of this, one could carefully break open the geode or open a small hole by accessing an area of weakness found in the 3D exam. This potentially provides the opportunity for a cleaner cut of the rock for museum display, personal use, or commercial purposes.