Introduction

The development of absorptiometric techniques has revolutionised our ability to measure the mineral content of the human skeleton in living individuals. The first technique to be introduced was single-photon absorptiometry (SPA), capable of the precise measurement of bone mineral in the appendicular skeleton (Cameron et al., 1962; Cameron & Sorenson, 1963; Sorenson & Cameron, 1967). Some years later, dual-photon absorptiometry (DPA) was developed for the measurement of bone in the axial skeleton (Mazess, 1971; Roos & Skoldborn, 1974). Versions of these instruments are capable of scanning the entire body and these provide a measure of whole-body fat and lean masses in addition to total-body bone mineral content. The photon beams required for SPA and DPA are produced by gamma-emitting radioisotopes. In recent years, X-ray technology has replaced the use of gamma-rays and the instruments are referred to as single- or dual-energy X-ray absorptiometers (SXA, DXA).

Absorptiometric techniques are known by a variety of names. For example, bone densitometry refers to any absorptiometric method which measures bone mineral content; X-ray absorptiometry (XRA), quantitative digital radiography (QDR) and X-ray spectrophotometry are synonyms for DXA, and there is a continuing debate as to whether the accepted abbreviation for dual-energy X-ray absorptiometry should be DXA or DEXA.

Currently, the principal application of absorptiometry is for the measurement of bone mineral content in clinical practice (Johnston et al., 1991). This includes screening for osteoporosis, either as a primary condition or secondary to a number of disease states, and long-term monitoring of therapies and drugs known to affect bone. Imaging of prostheses and stones is possible with the latest DXA instruments.