Accuracy of DEXA scanning & other methods for determining BMD

¾ In general, densitometry techniques can be performed in either the axial or the appendicular skeleton, depending on the modality employed. ¾ Peripheral measurements, performed in the appendicular skeleton, help to predict the risk of fracture, however, they are less sensitive for the monitoring of therapy than are measurements in the axial skeleton because changes due to age, therapeutic intervention, and estrogen deficiencies occur less rapidly in appendicular bone than they do in the axial skeleton. ¾ For single energy measurements it is often necessary to surround the anatomical site by a constant thickness of tissue equivalent material to correct for overlying soft tissue – this restricts measurement to peripheral sites for these modalities. ¾ The SPA technique uses a single gamma ray source (125 I) and a scintillation detector to measure photons transmitted through a particular anatomical site in the appendicular skeleton. ¾ The gamma source and detector were coupled and scanned in a rectilinear fashion across the region of interest. ¾ To correct for overlying soft tissue the anatomical site has to be surrounded by tissue equivalent material. Water is normally used because its attenuation closely matches that of soft tissue. ¾ This technique was applied to peripheral skeletal sites, most commonly the non-dominant forearm, wrist or heel. ¾ For application to sites in the axial skeleton, dual absorptiometry (DPA) was developed. DPA ¾ DPA allows for simultaneous measurement of the transmission of gamma radiation of two different energies which compensates for the variation in overlying tissue and removes the need for the tissue equivalent material. ¾ 153 Gd is a common radionuclide used which provided dual energy peaks at 44 & 100keV photons which were counted separately by scintillation detectors. ¾ Factors which affect the accuracy of photon absoprtiometry include: ¾ Inhomogeneity of soft tissue ¾ Uncertainty in values of density and attenuation coefficients for both bone mineral and soft tissue components. DPA ¾ The accuracy error of photon absorptiometry has been estimated at 4-8% [Blake et al, 1999]. ¾ Radionuclide decays and has to be replaced regularly ¾ Long scanning times due to low photon flux (~40 mins) ¾ Poor spatial resolution. ¾ The introduction of SXA & DXA systems which employ the use of an X-ray tube as a source of radiation overcame these limitations.