Precision Of Dual-energy X-ray Absorptiometry Research Articles

Analytic and quantitative exposition of patient-specific systematic inaccuracies inherent in planar DXA-derived in vivo BMD measurements.

An extensive analytic exposition is developed of systematic inaccuracies inherent in planar dual-energy x-ray absorptiometry (DXA) in vivo bone mineral density (BMD) measurements arising directly from the dual-energy facet of this methodology. Generalized and specific criteria governing these BMD inaccuracies are derived for the presence of relevant absorptiometrically distinguishable extra- and intra-osseous soft tissues (lean muscle tissue, interposed adipose, marrow, and cerebrospinal fluid). These analytic findings are utilized in comprehensive quantitative simulation studies which evaluate the magnitude and stipulate the direction (under- or over-estimate) of such systematic errors for typical, realistic DXA in vivo lumbar vertebral BMD scans over the ranges of soft tissue parameters and trabecular bone volumes (TBV) encountered clinically. It is shown that inherent systematic inaccuracies as high as +/- approximately 20% may be anticipated in typical patient-specific planar DXA vertebral BMD scans, particularly so for older, osteoporotic-prone, and osteoporotic individuals. These inaccuracies exceed considerably the DXA precision error and may mask or exaggerate clinically significant true changes in BMD. Both the scale and trends of patient-specific inaccuracies found to pertain in the present work are in keeping with those observed by recent investigators in actual specimen-specific in situ and in vitro DXA scans of given cadaveric bones. Insofar as the range of BMD inaccuracies found here is comparable to that of variations in the DXA-measured interpatient, age-moderated BMD of normal individuals, the intrinsic specificity of planar DXA for screening/diagnostic purposes is examined critically. It is shown that these inherent inaccuracies may prompt erroneous diagnoses, assessments, and interpretations of DXA-derived BMD measurements undertaken to screen, monitor, and help evaluate patient-specific predictive bone fragility, and to assess the efficacy of drug and other therapeutic regimens.

Instrument performance in bone density testing at five Australian centers

To assess the in vitro precision and accuracy of bone mineral densitometry (BMD) within and between locations at five Australian centres. 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). 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%. 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.

Evaluation of dual-energy X-ray absorptiometry for body-composition assessment in piglets and term human neonates

The reproducibility, accuracy, and precision of dual-energy X-ray absorptiometry (DXA) was assessed by scanning 13 piglets (1471-5507 g) in triplicate. In four piglets, fat content was increased with porcine lard around the abdomen; additional measurements were performed on these animals. Reproducibility in DXA measurements from the animals without added fat was 0.09% for body weight, 1.95% for bone mineral content (BMC), and 5.35% for fat content. DXA estimates of body weight, BMC, and fat content were significantly correlated with scale body weight, ash weight, chemical calcium, and chemical fat. Body weight was measured accurately but fat content was overestimated by DXA. Mean BMC estimated by DXA represented 48% of ash weight and 215% of calcium content. The precision of DXA was 0.23% for body weight, 10.99% for ash weight, and 4.44% for calcium content. The precision of DXA for fat content was poor. However, for measurements performed in piglets with > 250 g fat, the precision was 8.85%. Thirty appropriate-forgestational-age term human neonates (birth weight: 3188 +/- 217 g) were scanned once during the first week of life. BMC and fat content were 54 +/- 6 and 470 +/- 92 g, respectively, which corresponded to 26.4 +/- 2.6 g calcium and 427 +/- 82 g fat. These were close to the reference values previously determined by chemical analysis. This study suggests that DXA is accurate and reliable for measurement of calcium and fat contents in human neonates. Further refinements would be beneficial for determining fat content in preterm human infants.

Reproducibility of DXA absorptiometry: a model for bone loss estimates.

The enhanced precision of dual-energy X-ray absorptiometry (DXA) allows the detection of very small changes in bone mineral density (BMD). True clinical changes in BMD in patients must be evaluated with the appropriate error of variance. We evaluated the responsiveness of our measures to true bone loss using a statistical variance components model that characterizes the variability associated with error introduced by the machine, operator, and subjects. Our techniques were applied to data from a prospective study of BMD measurements on spine phantoms and on pre- and postmenopausal women performed on the same day or up to 4 weeks apart with DXA (QDR 1000W, Hologic). Our model determined that most of the error in measurements was introduced by operators' and subjects' variability rather than machine performance. The false-positive rates for true bone change are significantly reduced when the appropriate CV% is used to estimate the significance of bone loss over time. Our study underscores the need to use the appropriate precision error to evaluate the clinical significance of changes in bone mass in individual subjects over time.

Precision of measurement by dual-energy X-ray absorptiometry of bone mineral density and content in rat hindlimb in vitro.

Dual-energy x-ray absorptiometry (DXA) is a precise and effective method for measuring bone mineral density (BMD) and content (BMC) at various skeletal sites in the human, but its precision for measuring low BMD and BMC in tiny rat bones has scarcely been evaluated. The short-term precision of DXA (Norland XR-26) for eight regions of interest (ROI) was determined by measuring dissected, not totally defleshed hindlimbs from four rats of differently weights 10 times consecutively. The accuracy of the BMD (area) measurement was also evaluated. The following average precision values (coefficients of variation) with negligible reanalysis error were obtained for BMD and BMC measurements: 0.7 and 1.2% (total); 0.9 and 1.2% (femur); 1.3 and 2.8% (tibia); 1.3 and 2.1% (foot); 2.1 and 4.7% (proximal femur); 1.4 and 2.9% (femoral shaft); 1.8 and 5.6% (distal femur); and 2.7 and 7.6% (proximal tibia). As for the accuracy, BMD measured in tiny bones is likely to be markedly underestimated. We conclude that the standard DXA technique without additional modification for small animal measurements is a precise method for BMD and BMC measurements at multiple sites in rat hindlimb in vitro.

Precision of dual-energy x-ray absorptiometry: Development of quality control rules and their application in longitudinal studies

In research settings, longitudinal measurements of bone mineral density have become an integral part of the assessment of patients with metabolic skeletal disorders. To adequately utilize longitudinal measures, confidence in the long-term precision of the measurement technique must be very high. Dual-energy x-ray absorptiometry (DXA) has become commonly utilized in this context, and to better understand its long-term precision and to develop quality assurance protocols for its use, we examined the performance of eight DXA machines over a 3 year period. Anthropomorphic spine phantoms were measured frequently on each machine during the period of observation, and precision was estimated from the consistency of these determinations. Overall precision was excellent (mean longitudinal coefficient of variation, 0.4%). Nevertheless, by using a series of objective quality control criteria, small alterations in the performance of each machine were identified (mean number of changes, 4.6 in 3 years; mean magnitude, 0.0039 g/cm2, or 0.4%). The cumulative effects of those changes were sufficient to cause a significant (albeit minor) change in the regression slopes (phantom mineral density versus time) of most machines. The same quality control rules were also used to quantitate the magnitude of change and to adjust retrospectively machine performance during the period of observation, such that alterations were minimal and regression slopes were not significantly different from zero. Although the precision of DXA is excellent, alterations in machine function must be anticipated during longitudinal use. The development of quality control protocols provides the means to detect change objectively and to adjust for alterations in performance during the course of longitudinal evaluations.

Precision of dual energy x-ray absorptiometry in the upper extremities

Dual energy x-ray absorptiometry (DEXA) has been shown to be a precise method for measuring bone mineral density (BMD) and content (BMC) in lumbar spine and proximal femur, but it has not been widely used in other skeletal sites. The in vivo day-to-day precision of DEXA (Norland XR-26) for seven anatomic sites in the upper extremities was evaluated by twice measuring both the right and left sides in ten subjects. For consistently defined regions-of-interest, the following precision values (coefficient of variation) were obtained for BMD and BMC: 0.8% and 1.0% (proximal humerus); 0.5% and 0.5% (humeral shaft); 0.7% and 0.5% (radial shaft); 1.3% and 1.1% (ulnar shaft); 0.7% and 1.0% (distal radius); 0.7% and 1.2% (distal ulna); 0.4% and 0.6% (hand). The initially observed relative side-to-side differences did not change significantly in the repeated measurements. Our results indicate that DEXA is a precise method for assessment of BMD and BMC also in the upper extremities.

Longitudinal precision of dual-energy X-ray absorptiometry in a multicenter study

Reproducibility is a key issue in both clinical and research applications of bone mineral density (BMD) measurements. To examine the longitudinal precision of dual-energy x-ray absorptiometry (DEXA) for the measurement of mineral density in vivo and in vitro, the performance of a group of instruments in the course of a multicenter longitudinal clinical trial was monitored. Measures were performed on eight identical machines (Hologic QDR1000) and analyzed using the same automated software program. Short-term precision was good in vitro, [anthropomorphic spine phantoms; mean intrasite coefficient of variation (CV) 0.42 +/- 0.1% (SD)] and in vivo (lumbar spine; CV 1.1 +/- 0.5%). Intersite measures of a single spine phantom (specified mineral content -57.8 g) revealed a range of 57.3-58.4 g (CV 0.7%). In two subjects intersite CV in vivo were 3.7 and 2.1% (spine) and 1.8 and 3.2% (femoral neck). At five sites frequent phantom measures were performed over a 1 year period (mean number of measures 196) and revealed a mean all-point CV of 0.43% (range 0.35-0.53%). Longitudinal precision in vivo was somewhat less (mean CV of spinal measures 1.1%, femoral neck 1.2%, trochanter 1.3%, and Ward's area 2.4%). At one additional site large variations in phantom measures heralded repeated mechanical failures that eventually required machine replacement. In summary, DEXA demonstrates good in vitro and in vivo longitudinal precision, providing the basis for expanded clinical and research usefulness. Nevertheless, stringent quality assurance measures are required to detect and respond to system malfunctions.