SUB-MICRON X-RAY TOMOGRAPHY OF CALCIFIED TISSUE

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DESCRIPTION (Adapted from the applicant's abstract): Calcium precipitates in human tissue as a part of the formation of bone, teeth, or in certain abnormal processes (i.e., such as benign or malignant calcifications of the breast, vascular plaque, stones formed in the kidney or gallbladder, etc.). In bone, tissue is formed with a complex hierarchical structure which produces oriented material properties. Conventional microscopy methods are not suitable for studying this three dimensional (3-D) structure since both light and energetic electrons have minimal penetration into mineralized tissue. X-rays in the energy range from 7 to 11 keV penetrate small calcified tissue specimens of 50 to 250 micrometer dimension with characteristics that may permit measurement of the 3-D distribution of mineral concentration with a resolution of 0.1 to 0.5 micrometer. 3D X-ray microtomography is a method wherein many radiographic views of a specimen are acquired from different direction and the data reconstructed with a computer to produce an array (512 x 512 x 256) of values proportional to the X-ray attenuation coefficient of the specimen at many locations. Methods using microfocus X-ray tubes have been frequently used to study bone specimens with a size ranging from 4 to 20 mm with a resolution of 25 to 125 microns. Microtomography using synchrotron X-ray sources is capable of better resolution but detector limitations have limited present methods to a resolution of 2 to 25 micrometers. The applicant proposes to investigate a method using X-rays from a synchrotron source (the Advanced Photon Source, APS, at Argonne National Laboratory) which might achieve a resolution of 0.1 to 0.5 micrometer. An innovative approach using a zone plate to focus an X-ray beam to a sub-micron spot is proposed. The approach combines methods developed for scanning X-ray microscopy using lower X-ray energies and methods for tomographic reconstruction developed for cone beam tomography using electron beam X-ray sources. The methods developed will be evaluated using wet bone specimens. The ability to determine the 3-D ultrastructure with the collagen matrix, the structure and orientation of mineral within bone lamella, and the porosity of bone tissue will be specifically examined. If successful, this method will permit investigations of 3-D bone structure at a resolution that has previously not been possible.