In vivo differences among scan modes in bone mineral density measurement at dual-energy X-ray absorptiometry

Our aim was to estimate the in vivo reproducibility of bone mineral density (BMD) at dual-energy X-ray absorptiometry (DXA) and to compare fast array, array, and high-definition scan modes.

A total of 378 patients (38 males and 340 females; mean age 63 ± 9 years) underwent DXA using a QDR-Discovery A densitometer (Hologic). Considering the three scan modes on lumbar spine and right femur, six independent groups of 30 patients were examined twice (for a total of 180 patients). Least significant change (LSC) and smallest detectable difference (SDD) were calculated. The remaining 198 patients underwent three scans of the lumbar spine (n = 92) or of the right femur (n = 106), one for each scan mode. The student t test and Bland-Altman analysis used were. Scan times were recorded and radiation dose was estimated using the ICRP60 method.

Intra-scan mode reproducibility was 98-99%, corresponding to an LSC of 1.49-2.08%. The SDD was 0.018-0.023 g/cm(2) (lumbar spine) and 0.017-0.019 g/cm(2) (right femur). All comparisons among scan modes were statistically significant (p < 0.001) but lower than SDDs, i.e. not clinically relevant. Considering lumbar spine and the right femur, scan times were 50 and 38 s for fast array, 98 and 74 s for array, and 195 and 148 s for high definition, respectively; radiation doses were 6.7 and 4.7 μSv for fast array, and 13.3 and 9.3 μSv for both array and high definition, respectively.

Since all BMD differences were lower than the SSDs, the three scan modes can be considered interchangeable. As a consequence, although the absolute reduction in time and radiation dose is relatively low, when BMD measurement is the aim of DXA, fast array can be generally preferred.