Bone Densitometry using CT Imaging

Abstract

Porosity of bone directly affects the mechanical characteristics of the tissue. Dual-energy x-ray absorptiometry (DXA) is commonly used to measure bone mineral density (BMD), reflecting the bone mineral content averaged across a specific region of interest. However, this technique only produces a 2D projected measurement of the bone mineral density. In this study, we used 3D computed tomography (CT) imaging to provide a measurement of the 3D bone mineral density of human spine, which could facilitate more accurate analysis of the bone mechanical characteristics. The accurate measurement of bone mineral density from CT imaging requires a consideration of beam hardening artifacts. This effect, if not corrected, can cause severe errors in the measurements which result in inaccurate calculations. Dual energy correction as described by Alvarez (1976) is capable of eliminating beam hardening artifacts by decomposing the linear attenuation coefficient into two components. However, dual energy imaging requires a sophisticated hardware setup or extra radiation dosage in a CT examination and may introduce motion artifacts into the images and cause misregistration. A post-reconstruction single energy correction method that could efficiently remove beam hardening artifacts could be potentially useful. The process requires the precise measurement of the beam spectral intensity for 80kVp and 120kVp settings. Phantoms from different a??purea?? materials were constructed and scanned using the clinical CT machine from NUH. With the scan images and the material characteristics, an effective x-ray beam spectrum was estimated. Both a single energy and a dual energy correction have been implemented on a hydroxyapatite (HA) bone density phantom and human cadaver spine sample. By incorporating the polychromatic characteristics of the x-ray beam into the reconstruction process, both single and dual energy correction algorithms are capable of eliminating beam hardening artifacts. Bone mineral measurements of single energy correction and dual energy correction were compared with the commonly used DXA. Experimental results show that, compared to dual energy processing, the single energy correction has an equivalent capability with the dual energy CT correction in eliminating beam hardening artifacts and producing an accurate measurement of bone mineral density. Though DXA is highly reproducible and uses very low dose, it lacks absolute accuracy due to its inability to account for the large variability in skeletal size and body composition, and the influence of soft tissue and/or the posterior elements of the scanning sample.This work has potential applications in bone and osteoporosis related research.