Techniques of bone mass measurement
In the absence of fractures, making the diagnosis of osteoporosis from radiographs can be imprecise. As there are now effective bone preserving and bone enhancing therapies which reduce future fracture risk, identifying patients at risk before fractures occur is relevant, hence the importance of bone densitometry.The method that is most widely used currently is dual energy X-ray absorptiometry (DXA). This can be applied to axial and appendicular skeletal sites, uses very low radiation doses (1-6 microsieverts) and with modern fan-beam equipment scanning is rapid (1-2 minutes), with improved spatial resolution and image quality from the original rectilinear DXA scanners of the 1980s. DXA at hip & spine has become the ‘gold standard’ for the diagnosis of osteoporosis as defined by bone densitometry (World Health Organisation definition T score below –2.5 standard deviations from sex- and race-matched mean peak bone mineral density [BMD] ). DXA has some limitations: measures integral [cortical and trabecular] bone; in the spine of the elderly degenerative changes, vertebral fractures and other artefacts cause false elevation of BMD; size dependency as DXA is a 2D image (g/cm2) of a 3D structure, a particular issue in children. It is suggested that in those over 65 years hip BMD only should be performed. Another method, quantitative computed tomography (QCT), is applied to spine and periphery (forearm and tibia) and has some unique advantages (separate measures of cortical and trabecular bone; true volumetric density [mg/cm3]). Quantitative ultrasound (QUS) of the heel shows potential in fracture prediction in elderly females, but there are some limitations (temperature dependency; many and varied devices and sites of application).In research studies the different methods provide complementary information.To achieve reproducible results it is essential that technical staff are properly trained and highly motivated and quality assurance (QA) programmes are in place.