Comparison of QCT-derived and DXA-derived areal bone mineral density and T scores.

UNLABELLED: Two-dimensional areal bone mineral density (aBMD) of the proximal femur measured by three-dimensional quantitative computed tomography (QCT) in 91 elderly women was compared to dual-energy X-ray absorptiometry (DXA) aBMD results measured in the same patients. The measurements were highly correlated, though QCT aBMD values were marginally lower in absolute units. Transformation of the QCT aBMD values to T score values using National Health and Nutrition Examination Survey (NHANES) DXA-derived reference data improved agreement and clinical utility. INTRODUCTION: World Health Organization guidelines promulgate aBMD (g cm(-2)) measurement of the proximal femur for the diagnosis of bone fragility. In recent years, there has been increasing interest in QCT to facilitate understanding of three-dimensional bone structure and strength. OBJECTIVE: To assist in comparison of QCT-derived data with DXA aBMD results, a technique for deriving aBMD from QCT measurements has been developed. METHODS: To test the validity of the QCT method, 91 elderly females were scanned on both DXA and CT scanners. QCT-derived DXA equivalent aBMD (QCT(DXA) aBMD) was calculated using CTXA Hip software (Mindways Software Inc., Austin, TX, USA) and compared to DXA-derived aBMD results. RESULTS: Test retest analysis indicated lower root mean square (RMS) errors for CTXA; F test between CTXA and DXA was significantly different at femoral neck (FN) and trochanter (TR) (p < 0.05). QCT underestimates DXA values by 0.02 +/- 0.05 g cm(-2) (total hip, TH), 0.01 +/- 0.04 g cm(-2) (FN), 0.03 +/- 0.07 g cm(-2) (inter-trochanter, IT), and 0.02 +/- 0.05 g cm(-2) (TR). The RMS errors (standard error of estimate) between QCT and DXA T scores for TH, FN, IT, and TR were 0.36, 0.40, 0.39, and 0.49, respectively. CONCLUSIONS: This study shows that results from QCT aBMD appropriately adjusted can be evaluated against NHANES reference data to diagnose osteoporosis.

Precision error of fan-beam dual X-ray absorptiometry scans at the spine, hip, and forearm.

The current study investigated short-term precision of fan-beam dual X-ray absorptiometry scans using a sample of subjects typically seen in a routine practice. Seventy-two women were scanned twice each in array fan-beam mode at the lumbar spine and 71 at the hip on a Hologic QDR 2000. Sixty-five were scanned at the forearm on a Hologic QDR 4500A in array mode. Each of the two scans was analysed without reference to its companion scan (independent analysis). The second assessment was also reanalyzed using the manufacturer's "Compare" function (compared analysis). Coefficients of variation (CVs) were calculated for the two forms of analysis at all the sites measured. Paired t-tests were performed to see whether the "Compare" function improved precision. The CV at the lumbar spine was 1.1% in both forms of analysis. The "Compare" function did not improve precision significantly at this site. The highest precision at the hip was found at the (Total) site of the hip (1.2-1.0%, independent and compared analyses, respectively) and the lowest at Ward's triangle. The Compare function significantly improved precision at Ward's triangle. The forearm had the best precision of all the sites measured with the Total site of the forearm being highest (0.5-0.7%) and the ultradistal site lowest (1.6-1.8%). The Compare function significantly decreased precision at the mid-distal and Total sites of the forearm. In conclusion, the highest precision was found at the Total site of the hip, the Total site of the forearm, and the spine. The Compare function did not significantly improve precision at the spine or hip (with the exception of Ward's triangle) and decreased precision at the forearm.

Instrument performance in bone density testing at five Australian centres.

AIMS: To assess the in vitro precision and accuracy of bone mineral densitometry (BMD) within and between locations at five Australian centres. METHODS: Using a multicentre reliability study the accuracy and short- and long-term precision of dual-energy X-ray absorptiometry (DXA) in vitro was compared on five instruments. Measures were performed using pencil beam mode on four Hologic QDR-2000 densitometers and one Hologic QDR-1000/W (Hologic Inc, Waltham, MA). RESULTS: Short-term precision of bone mineral density measurement was less than 0.5% for spine phantoms (n = 10 for each centre, mean intrasite coefficient of variation [CV] 0.39 +/- 0.09% [SD]) and for hip phantoms (n = 10 for each centre, mean intrasite coefficient of variation [CV] 0.34 +/- 0.10% [SD]). Between-centre measurement (n = 10 for each phantom) of a single spine phantom and a single hip phantom (specified mineral contents-58.5 g and 38.6 g, respectively) revealed ranges of bone mineral content of 57.7-58.1 g (all-point CV = 0.52%) and 37.1-37.8 g (all-point CV = 0.70%), respectively. When results from pairs of machines were compared there were statistically different mean BMD results for the majority of the ten possible pairings for both spine and hip measurements. Each study centre measured in vitro stability of phantom BMD measurements over a one year period (n = 45-283, median 157 for spine; and n = 0-262, median 38, for hip); CVs ranged from 0.38 to 0.53% for the spine measurements and from 0.38 to 0.54% for the hip measurements. The mean all-point accuracy of the spine phantom measurements was 99.1% and the hip phantom measurements was 96.7%. CONCLUSIONS: Across a number of instruments DXA demonstrates in vitro all-point precision of 0.5% for the spine phantom and 0.7% for the hip phantom. The instrument demonstrates accuracy of greater than 99% at the spine and 96% at the hip. This finding has clinical, research and quality control implications.