Dual-Energy X-Ray Absorptiometry

Excerpt

Dual-energy x-ray absorptiometry (DEXA) has sustained a niche for measuring bone mineral density since its approval by the Food and Drug Administration (FDA) for clinical use in 1988. The Bone Mass Measurements act in 1998 solidified its validity in light of other diagnostic modalities such as chemical analysis, direct dissection and ashing, quantitative ultrasonography, and later on against CT/MRI images. DEXA is comparatively inexpensive with notably shorter scan times and radiation exposure compared to other imaging options, and there is a long-standing consensus regarding guidelines for interpreting DEXA images.

A C-arm with the x-ray source below the supine patient emits photons at two distinct energy levels specific for soft tissue and cortical bone. A collimator is situated between the patient and x-ray source to minimize scatter. The attenuations from these low and high-energy photon emissions are detected above the patient and are combined to create a planar image to assess bone mass per unit volume (g/cm), for example, bone mineral density (BMD). A T-score is a number of standard deviations between the patient’s mean BMD and the mean of the population compared with reference populations matched in gender and race. The Z-score is the number of standard deviations above or below the mean of age-matched controls.