Correcting fan-beam magnification in clinical densitometry scans of growing subjects.

As children grow, body and limb girths increase. For serial densitometric measurements, growth increases the distance between the bone region of interest and X-ray source over time, thereby increasing fan-beam magnification. To isolate bone accrual from magnification error in growing subjects, we developed a correction method based on waist girth, a common anthropometric measure. This correction was applied to dual-energy X-ray absorptiometry output obtained in a cohort of premenarcheal gymnasts and nongymnasts. After correcting for magnification, results for projected area and bone mineral content (BMC) increased by 0.4-1.1% at the lumbar spine and 8-16% at the femoral neck, decreasing areal bone mineral density (aBMD) by 0.4-2.3% at both sites. The effects of magnification correction were similar in magnitude to BMC and aBMD gains previously reported in longitudinal studies of normoactive children. Because of body size differences, the effect of correction for BMC and aBMD was 10-20% greater in nongymnasts than in gymnasts, which increased the observed aBMD differential between gymnasts and nongymnasts. Fan-beam magnification distorts true changes in bone mineral measures in growing premenarcheal girls and, therefore, may obscure additional activity-related changes during growth. Our correction technique may enhance detection of skeletal adaptation, particularly in pediatric populations.