Discovery instrument

A consecutive subset of children was invited to visit the Osteoporosis Centre at Southampton General Hospital for assessment of bone mass and body composition at age 4 years. At this visit, written informed consent for a dual‐energy X‐ray absorptiometry (DXA) scan was obtained from the mother or father/guardian. The child's height (Leicester height measurer, Seca Ltd, Birmingham, UK) and weight (in underpants only, using calibrated digital scales, Seca Ltd) were measured. A whole‐body DXA scan was obtained, using a Hologic Discovery instrument (Hologic Inc., Bedford, MA, USA) in pediatric scan mode. Scans with unacceptable movement artefact were excluded. The manufacturer's coefficient of variation (CV) for the instrument was 0.75% for whole‐body bone mineral density, and the experimental CV when a spine phantom was repeatedly scanned in the same position 16 times was 0.68%. A similar bone assessment was undertaken at age 6 years. The SWS was conducted according to the guidelines laid down in the Declaration of Helsinki, and the Southampton and South West Hampshire Research Ethics Committee approved all procedures.

Infants receive DXA scans within the first 2 weeks after delivery. The reliability of DXA measurements in neonates has been demonstrated in the Southampton Women’s Survey [2 (link)]. The baby is pacified, fed, fully undressed and then swaddled in a towel before being placed on the densitometer (Hologic Discovery instrument using paediatric software (Hologic Inc., Bedford, MA, USA)). The scanner is calibrated against a spine phantom every day together with daily quality assurance and step-wedge calibration, performed as per manufacturer’s instructions (Hologic Inc., Bedford, MA, USA). If possible, the baby is scanned while still an inpatient. If this is not possible, the mother and baby return for assessment within 14 days of birth. Whole body and lumbar spine bone area, bone mineral content and bone mineral density are measured. The infant DXA assessments are associated with a low dose of radiation exposure, roughly equivalent to 2 days of background radiation in Cornwall (UK) and 7 days in other parts of the UK.