Bone Mineral Assessment of the Axial Skeleton: Technical Aspects

The goal of this chapter is to describe the underlying principles and error sources associated with X-ray based bone densitometry methods used to assess the central skeleton. The first portion of the chapter focuses on projectional densitometry measurements which are most widely used in the clinical setting. The concepts of single and dual photon absorptiometry are used to provide a simple and clear explanation of the physical principles underlying dual X-ray absorptiometry (DXA). The section then describes the error sources associated with DXA, including precision errors, bone size dependence and the effect of adipose tissue distribution. The second portion of the chapter describes quantitative X-ray computed tomography (QCT), an adaptation of clinical computed tomography imaging for assessment of skeletal integrity. This section describes how CT images are acquired and the physical meaning of the image units as they relate to bone mineral content and density. The section then describes the use of QCT of the hip and spine to assess cortical and trabecular bone mineral density, the physical errors associated with those assessments, and the application of QCT to assess measures of bone quality such as bone geometry and whole bone strength.