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DPX-IQ Operator’s Manual J-5 Precision and Accuracy
Conventional DPA scans have been shown to be well correlated to total body calcium
from neutron activation analysis.
4
Factors that affect accuracy include tissue thickness
and distance of the object from the table top. Measurements were made on two spine
phantoms at different distances from the table top. There was a change of about
0.3%/cm (figure J-4). Subjects should be repositioned carefully to ensure good long-
term precision. Measurements were made on spine phantoms in a water bath to assess
the effect of tissue thickness on results. There was little effect (<1%) of thickness,
prior to correction for scatter and hardening, in up to 20 cm of water (equivalent to 25-
cm patient thickness). Corrections for scatter and hardening reduce this small
thickness dependence at higher thicknesses (figure J-5).
Figure J-5. Effect of tissue thickness on results
J.3 References
1. Mazess R, Collick B, Trempe J, Barden H, Hanson J (1989) Performance evaluation
of a dual-energy x-ray bone densitometer.
Calcified Tissue International 44:228-232.
2. Krolner B (1985) Lumbar spine bone mineral content by photon beam
absorptiometry.
Danish Medical Bulletin 32:152-170.
3. Wahner HW, Dunn WL, Mazess RB, et al. (1985) Dual-photon (Gd
-153
) absorptiometry
of bone. Radiology 156:203-206.
4. Mazess RB, Peppler WW, Chesnut CH, Nelp WB, Cohn SH (1981) Total body bone
mineral and lean body mass by dual-photon absorptiometry. II. Comparison with total
body calcium by neutron activation analysis.
Calcified Tissue International
33:361-363.
110
105
100
95
90
Percent BMD
0
10 20
30
40
Spine
Femur
Thickness (cm water)