Quantitative computed tomography in children and adolescents

Methods for quantitative assessment of the skeleton are relevant to the understanding of growth and development during normal childhood and studying the effect of disease and therapy in children with chronic diseases. Dual energy x-ray absorptiometry (DXA) is currently the most widely available and utilised technique in adults and children. The strengths of DXA are: • The technology is widely available in most parts of the world (probably in excess of 40,000 central DXA scanners worldwide; cost approximately £80k-£100k) • It is applicable to sites of the skeleton in which osteoporotic fractures occur in adults (lumbar spine, proximal femur, forearm) • It can be extended beyond measurement of bone mineral density (BMD) – whole body and regional body composition, visceral adipose tissue (VAT), vertebral fracture assessment (VFA), hip strength analysis (HSA) and hip axis length (HAL) • It involves very low doses of ionising radiation at 1-6 microSv per scan. There are some limitations also in that DXA provides a 2D image of a 3D structure so there is a depth of bone that cannot be taken into account. DXA provides areal bone mineral density (aBMD) in g/cm and so is size dependent, a particular problem in children in whom bones are growing in length, increasing in density and changing in shape. DXA will therefore under-estimate aBMD in children who are small for age, which may be the case in chronic illness or growth hormone deficiency. Additionally DXA measures integral (cortical and trabecular) bone aBMD. Quantitative computed tomography (QCT) has advantages in that the cross-sectional imaging provides true volumetric vBMD (mg/cm) so is not size dependent and provides separate measures of cortical and trabecular vBMD. The latter is some eight times more metabolically active than cortical bone and so more sensitive to longitudinal change in BMD. The limitation for central QCT, particularly when applied in children, is that it involves higher doses of ionising radiation than DXA and there are limited reference data available. As with other bone densitometry techniques, QCT in children is best performed by a few well trained and experienced technical staff, and good quality scans are difficult to obtain in children under five years due to movement artefact. QCT actually predates DXA as it was introduced in 1976 whereas DXA replaced singleand dual-photon radionuclide absorptiometry in the late 1980s. However, as DXA was subsequently applied widely in epidemiological and therapeutic efficacy studies in adults it superseded QCT. Over the past decade the advantages of QCT have been recognised and consequently it has been utilised increasingly in research and epidemiology studies in adults, with the number of publications using QCT increasing from 20 per annum in 2003 to 60 in 2013, but this is compared to approximately 1,600 for DXA in 2014. This increasing application of QCT includes its application in paediatric patients with skeletal disorders.