High-Resolution Computed Tomography and Digital Radiography of Archaeological and Art-Historical Objects
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HIGH-RESOLUTION COMPUTED TOMOGRAPHY AND DIGITAL RADIOGRAPHY OF ARCHAEOLOGICAL AND ART-HISTORICAL OBJECTS FREDRICK H. SIGUIN Massachusetts Institute of Technology, Building NW-16 Cambridge, Massachusetts 02139 INTRODUCTION Recent advances in x-ray imaging and image processing are providing new options for the nondestructive physical evaluation of archaeological artifacts and works of art. Traditional t'lm-based methods have played an important role in this field for many decades 1,but digital modes developed for medical and industrial applications have been gaining ground 2 and can be expected to become more important as they become more accessible and less expensive. Modes such as digital radiography (DR) and computed tomography (CT) have many advantages in terms of information content, quantitative analysis options, and image display options (including some that make image publication easier). CT, in particular, provides structural information which cannot be obtained through any other nondestructive means. From a technical point of view, the only real drawback to digital methods has been spatial resolution that hasn't compared favorably with the best available from film. That situation is changing 3. The objective here is to discuss a few of the important characteristics peculiar to digital images, to show examples of how high-resolution digital images can provide important structural information, and to illustrate useful types of image processing. The use of experimental CT and DR systems, with spatial resolution as fine as 50 microns, is illustrated in studies of three objects: an ancient terracotta sculpture, a Japanese assembledwoodblock sculpture (ca. 17th century), and an oil painting on canvas (ca. 18th century). DIGITAL X-RAY IMAGING MODES DR is a form of projection imaging and is the digital counterpart of film radiography. A projection image represents a 2-dimensional projection of a 3-dimensional object, with image values indicating the x-ray flux that passes through all material along each line of sight from the x-ray source to the x-ray detector. Projection images are particularly useful for objects which are nearly 2-dimensional, such as paintings on canvas, or at least have a nearly constant thickness, such as some paintings on panels. In many such cases the average x-ray attenuation is similar across the entire object, and structures of interest (such as variations in the densities of pigments) can dominate the variations in image values. Objects in the round, such as sculpture, can also be imaged with projection systems. Average x-ray attenuation may vary strongly across an object, with the result that film images can require different exposures for optimum representation of different parts of the object. Good DR systems have a great advantage in such situations, because they have substantially wider dynamic range than film and can more often record structural detail in the thickest and thinnest parts of the object in one image. They also have the advantage of allowing image processing and quanti
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