Discussion The high correlations of the 2D HSA measurements of CSA, CSMI, and Z with the 3D QCT gold standard measurements provide support for the validity of interpreting these parameters as being highly correlated to these physical parameters. This is an important point as the HSA algorithm and DXA manufacturer equipment used in this study have already been utilized in many published clinical studies. Because the calibration standards for bone mass differ between the PD0332991 cost two modalities measurements and because they handle bone marrow fat and partial volume effects differently, it is not surprising that the slopes for CSA,
essentially a measurement of the BMC in an ROI, differed from P-gp inhibitor unity. This mass measurement difference also affected CSMI and Z. However, as noted in
the Methods section, there is a further difference for CSMI and Z because the DXA HSA measurements are limited to calculating these values in the DXA planar projection (CSMIHSA and ZHSA, which are around the v axis in Fig. 1), whereas the QCT measurements utilize the 3D data and were calculated around the w (polar) axis. These differences limit the comparison to correlations; thus, individual measurements cannot be substituted one for the other without adjustments which may be population or technician dependent. It is important to note that both the width and FNAL results indicated a high degree of agreement in absolute terms between DXA and QCT despite the use of a fan beam DXA device. Geometrical measurements on fan beam DXA devices are impaired by magnification effects if the bone being measured is not at the height above the table estimated by the scanner software. Based on in vitro studies, some have speculated that fan beam DXA may cause significant errors in geometrical measurements [28–30]. These concerns are not supported by the data in this study of elderly women Isotretinoin with BMI 25.9 ± 3.9 kg/m2, where there was
no evidence for magnification in the population as a whole, as demonstrated by slopes that were nearly unity. Nor did fan beam magnification have an appreciable effect on individual subject results, as the SEEs ranged from only 0.7 to 2.2 mm. While this study does not rule out the possibility that there is a measurable magnification effect in vivo in men or severely obese women, it sets limits on the size of the magnification effect in a typical clinical population. Another possible source of error contributing to the standard error of the estimate (SEE) of FNAL was patient positioning. The FNAL results were calculated independently on the DXA image and QCT dataset without co-registration; thus, if the femur neck during the DXA exam was not positioned parallel to the table in some subjects, it would appear shorter by varying amounts and would cause an increase in the SEE of the correlation.