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).