Dual X-ray absorptiometry: a comparison between fan beam and pencil beam scans

The recent introduction of dual X-ray absorptiometry (DXA) systems with fan beam instead of conventional pencil beam scanning geometry represents a significant technical advance in bone densitometry. This report describes phantom and in-vivo studies of the effect of the change in beam configuration on DXA measurements. Fan beam and pencil beam measurements acquired on one of the new generation scanners, the Hologic QDR-2000, were compared with scans performed on an earlier pencil beam model, the Hologic QDR-1000. The variation with height above the scanning table of fan beam measurements of an anthropomorphic spine phantom were: bone mineral content (BMC): -3.1% cm-1; projected area (AREA): -2.8% cm-1; bone mineral density (BMD): -0.2% cm-1. For pencil beam scans the magnitude of height variation was less than 0.1% cm-1. QDR-2000 fan and pencil beam scan results for 20 volunteers correlated closely with QDR-1000 pencil beam data (r = 0.966-0.998). For BMD measurements on the spine and hip, differences between fan and pencil beam data were consistent with the errors expected from measurement precision. For AREA and BMC data, however, larger differences were observed with individual deviations which correlated with body habitus of the subjects. Although the change from pencil to fan beam geometry significantly affected AREA and BMC data, the effect on the clinically more important BMD measurements was negligibly small.