SWITCHING FROM DXA PENCIL-BEAM TO FAN-BEAM .1. STUDIES IN-VITRO AT 4 CENTERS

The performance of the Hologic QDR-2000 DXA osteoden-sitometer was cri tically evaluated at four centers, using at all four centers one bone equivalent humanoid spine phantom supplied by the manufacturer. Result s were compared with results from Hologic QDR-1000/W using that phanto m tested at the same centers. It appears that the concept of fan-beam scanning-as used in the QDR-2000: a fan-beam, a linear array detector above the phantom, and an x-ray tube located rather close to the spine below the phantom-creates problems due to the magnification effect of the fan beam. The effect of decreasing the distance between the ''ver tebrae''' of the phantom and the couch are: bone mineral content (BMC) increases by 2.8% per cm, projected area (Area) by 2.8% per cm, and b one mineral density (BMD) is unchanged. When QDR-1000/W is upgraded to QDR-2000, BMD is relatively constant, but there are shifts of BMC and Area which are partly due to the magnification effect of the fan-beam . Replacement of a QDR-1000/W with a QDR-2000 can invalidate longitudi nal measurements, even for BMD, unless the proportionality factors of the QDR-2000 are checked and, if necessary, changed. This is true for switching from QDR-1000/W to pencil-beam mode of QDR-2000 or to fan-be am mode of QDR-2000. Even with pencil-beam mode, the long-term precisi on error with phantoms is higher for QDR-2000 than for QDR-1000/W (for BMD, 0.47% versus 0.35%).