Quantitative coherent-scatter-computed tomography

Abstract

Conventional means of diagnosiing and assessing the progression of osteoporosis, including radiographic absorptiometry and quantitative CT, are directly or indirectly dependent upon bone density. This is, how ever, not always a reliable indicator of fracture risk. Changes in the trabecular structure and bone mineral content (BMC) are thought to provide a better indication of the change of spontaneous fractures occurring. Coherent-scatter CT (CSCT) is a technique which produces images based on the low angle (0 - 10 degrees) x-ray diffraction properties of tissue. Diffraction patterns from an object are acquired using first-generation CT geometry with a diagnostic x-ray image intensifier based system. These patterns are used to reconstruct a series of maps of the angle dependent coherent scatter cross section in a tomographic slice which are dependent upon the molecular structure of the scatterer. Hydroxyapatite has a very different cross section to that of soft tissue, and the CSCT method may, therefore, form the basis for a more direct measure of BMC. Our original CSCT images suffered from a 'cupping' artifact, resulting in increased intensities for pixels at the periphery of the object. This artifact, which is due to self-attenuation of scattered x rays, caused a systematic error of up to 20% in cross-sections measured from a CT image. This effect has been removed by monitoring the transmitted intensity using a photodiode mounted on the primary beam stop, and normalizing the scatter intensity to that of the transmitted beam for each projection. Images reconstructed from data normalized in this way do not exhibit observable attenuation artifacts. Elimination of this artifact enables the determination of accurate quantitative measures of BMC at each pixel in a tomograph.