SWITCHING FROM DXA PENCIL-BEAM TO FAN-BEAM .2. STUDIES IN-VIVO

Switching from the Hologic QDR-1000/W to the QDR-2000 DXA densitometer was critically evaluated with regard to cross-calibration and dosimet ry. Studies with bone equivalent humanoid spine phantoms and patient s tudies were done. Fan-beam scanning with the QDR-2000 is problematic b ecause of magnification. Mean phantom bone mineral content (BMC) and b one mineral density (BMD) were moderately but significantly different. Biological variation disguised differences between the two devices in humans, but significant differences were revealed when individual dat a were analyzed. Longitudinal assessments of BMC and BMD, initiated wi th QDR-1000/W and continued with the QDR-2000, should employ single-be am mode only and not fan-beam mode-but even if that is done, significa nt errors can be introduced. The new QDR-2000 should be properly cross -calibrated with the original densitometer, and one should make sure t hat the same software, phantom, and type of collimator are used. The r adiation dose is substantially higher with QDR-2000 (fan-beam and high -resolution array mode) than with QDR-1000/W (pencil-beam mode) and QD R-2000 (pencil beam mode), and higher than claimed by the manufacturer . The typical radiation dose given by the manufacturer was half the ac tual radiation dose measured (e.g., for fan beam scan 62 mu Sv versus 33 mu Sv). High-resolution array mode does not improve precision, but augments the radiation dose to the patient.