iDXA,Prodigy, and DPXL Dual-Energy X-ray Absorptiometry Whole-Body Scans: A Cross-Calibration Study

PurposeTotal body fat, lean, and bone mineral content (BMC) in addition to regional fat and lean mass values for arms, legs, and trunk were compared across a pencil-beam (Lunar DPXL) and 2 fan-beam (GE Lunar Prodigy and GE Lunar iDXA) dual-energy X-ray absorptiometry (DXA) systems.MethodsSubjects were a multiethnic sample of 99 healthy adult males (47%) and females (mean ± SD: age, 46.3 ± 16.9 yr; weight, 73.4 ± 16.6 kg; height, 167.6 ± 9.7 cm; body mass index, 26.0 ± 5.2 kg/m²) who had whole-body scans performed within a 3-h period on the 3 systems. Repeated measures ANOVA was used to test the null hypothesis that the mean values for the 3 systems were equal. Translation equations between the methods were derived using regression techniques.ResultsBone mineral content (BMC): For both genders, total BMC by iDXA was lower (p ≤ 0.004) than the other systems. Lean: for males, iDXA was lower (p ≤ 0.03) than the other systems for total, trunk, and arms. For females, DPXL estimated higher (p < 0.001) lean mass compared with the other systems for total, trunk, and arms, but iDXA estimated greater legs lean mass. For both genders, all DPXL mean values were greater than Prodigy mean values (p < 0.001).Fat: in females, all the 3 systems were different from each other for total, trunk, and legs (p ≤ 0.04). For arms, DPXL and iDXA were higher than Prodigy (p < 0.0004). For males, DPXL was less (p < 0.001) for total body, trunk, and legs compared with the other 2 systems and greater than Prodigy only for arms (p < 0.0007). These data were used to derive translation equations between systems. For several measurements, the differences between systems were related to gender.ConclusionFor estimation of BMC and body composition, there was high agreement between all DXA systems (R² = 0.85–0.99). Even so, cross-calibration equations should be used to examine data across systems to avoid erroneous conclusions.     

Cross-Calibration of iDXA and Prodigy on Spine and Femur Scans in Korean Adults

In this study, the authors compared bone mineral density (BMD) determined using GE Lunar iDXA and Prodigy and derived cross-calibration equations for the 2 devices in Korean adults. One hundred subjects (66 women and 34 men) participated in this study. Bone mineral density of spine and femur was measured by iDXA and Prodigy dual-energy X-ray absorptiometry (GE Lunar, Madison, WI). Subjects were divided into 3 groups. The first group (30 subjects) was scanned twice using Prodigy for precision testing and then once using iDXA. The second group (30 subjects) was scanned twice using iDXA and then once using Prodigy. Cross-calibration equations were derived using these results. The derived equations were tested in the third group (40 subjects). Predicted values from calculations based on Prodigy findings were compared with measured iDXA data. A significant difference was found between the BMD determined using the 2 devices (p < 0.001). However, linear regression analysis showed a high level of agreement between the two (r² from 0.984 to 0.994, p < 0.001). Bland-Altman analysis revealed no significant correlations between Prodigy and iDXA. Cross-calibration equations decreased systematic errors between Prodigy and iDXA by 0.4% at the spine, 0.8% at the femoral neck, and 0.1% at the total femur. A high level of agreement was found between Prodigy and iDXA in Korean adults. Cross-calibration equations proved reliable based on comparisons of measured and calculated BMD values.     

Trabecular Bone Score (TBS) Cross-Calibration for GE Prodigy and IDXA Scanners

Dual-energy X-ray absorptiometry (DXA) is used for osteoporosis diagnosis, fracture prediction and to monitor changes in bone mineral density (BMD). Change in DXA instrumentation requires formal cross-calibration and procedures have been described by the International Society for Clinical Densitometry. Whether procedures used for BMD cross-calibration are sufficient to ensure lumbar spine trabecular bone score (TBS) cross-calibration is currently uncertain. The Manitoba Bone Density Program underwent a program-wide upgrade in DXA instrumentation from GE Prodigy to iDXA in 2012, and a representative a sample of 108 clinic patients were scanned on both instruments. Lumbar spine TBS (L1-L4) measurements were retrospectively derived in 2013. TBS calibration phantoms were not available at our site when this was performed. We found excellent agreement for lumbar spine BMD, without deviation from the line of perfect agreement, and low random error (standard error of the estimate [SEE] 2.54% of the mean). In contrast, spine TBS (L1-L4) showed significant deviation from the line of identity: TBS(iDXA) = 0.730 x TBS(Prodigy) + 0.372 (p<0.001 for slope and intercept); SEE 5.12% of the mean with negative bias (r=-0.550). Results were worse for scans acquired in thick versus standard mode, but similar when the population was stratified as BMI < or > 35 kg/m². In summary, it cannot be assumed that just because BMD cross-calibration is good that this applies to TBS. This supports the need for using TBS phantom calibration to accommodate between-scanner differences as part of the manufacturer's TBS software installation.     

Cross-Calibration and Comparison of Variability in 2 Bone Densitometers in a Research Setting: The Framingham Experience

New technology introduced over time results in changes in densitometers during longitudinal studies of bone mineral density (BMD). This requires that a cross-calibration process be completed to translate measurements from the old densitometer to the new one. Previously described cross-calibration methods for research settings have collected single measures on each densitometer and used linear regression to estimate cross-calibration corrections. Thus, these methods may produce corrections that have limited precision and underestimate the variability in converted BMD values. Furthermore, most of the previous studies have included small samples recruited from specialized populations. Increasing the sample size, obtaining multiple measures on each machine, and using linear mixed models to account for between- and within-subject variability may improve cross-calibration estimates. The purpose of this study was to conduct an in vivo cross-calibration of a Lunar DPX-L (Lunar Corporation, Madison, WI) with a Lunar Prodigy densitometer (GE Medical Systems Lunar, Madison, WI) using a sample of 249 healthy volunteers who were scanned twice on each densitometer, without repositioning, at both the femur and spine. Scans were analyzed using both automated and manual placement of regions of interest. Wilcoxon rank-sum tests and Bland-Altman plots were used to examine possible differences between repeat scans within and across densitometers. We used linear mixed models to determine the cross-calibration equations for the femoral neck, trochanter, total hip, and lumbar spine (L2–L4) regions. Results using automated and manual placement of the regions of interest did not differ significantly. The DPX-L densitometer exhibited larger median absolute differences in the BMD values by repeat scans of femoral neck (0.016 vs 0.012, p = 0.1) and trochanter (0.011 vs. 0.009, p = 0.06) compared with the Prodigy densitometer. The Bland-Altman plots revealed no statistically significant linear relationship between the differences in paired measures between machines and mean BMD. In our large sample of healthy volunteers, we did detect systematic differences between the DPX-L and Prodigy densitometers. Our proposed cross-calibration method, which includes acquiring multiple measures and using linear mixed models, provides researchers with a more realistic estimate of the variance of cross-calibrated BMD measures, potentially reducing the chance of making a type I error in longitudinal studies of changes in BMD.     

Accuracy and Precision of 62 Bone Densitometers Using a European Spine Phantom

Dual-energy absorptiometry (DXA) is widely used for bone mineral density measurements. Different types of devices are available. Differences between devices from either the same manufacturer or different manufacturers can lead to difficulties in clinical practice when patients are followed on different machines. We calculated the accuracy and precision of 62 DXA devices from two manufacturers (51 Hologic, 11 Lunar) using a European Spine Phantom (ESP, semi-anthropomorphic). The ESP was measured 5 times on each device without repositioning. Accuracy was assessed by comparing bone mineral density (BMD, g/cm²) values measured on each device with the actual value of the phantom. Precision was assessed by the coefficient of variation (CVsd), using the root mean square average. The limits of agreement were estimated from the differences between each replicate measurement of BMD and the estimated true value for a particular manufacturer, according to Bland and Altman. The results confirm the difference between devices from different manufacturers (18.5%). Mean CVsd values were 0.57% and 0.64% for Hologic and Lunar respectively. The limits of agreement among devices from the same manufacturer were 0.026 g/cm² and 0.025 g/cm² for Hologic and Lunar respectively. Differences in extreme results between devices from the same manufacturer were on average 5.4% and 3.6% for Hologic and Lunar respectively. Results of different devices from the same manufacturer are highly comparable, although unpredictable differences exist that may be clinically relevant. Received: 12 June 1998 / Accepted: 20 November 1998     

Assessment of the Stratos, a New Pencil-Beam Bone Densitometer: Dosimetry, Precision, and Cross Calibration

The goal of this study was to assess a new pencil-beam densitometer, the Stratos (Diagnostic Medical Systems, Pérols, France). Evaluation of the dosimetry and precision were done together with an in vivo cross-calibration study performed with the fan beam densitometer Discovery A (Hologic, Bedford, MA). The results indicated that the Stratos performed bone mineral density (BMD) measurements with a good precision, low radiation dose, and good agreement with the Discovery A. The air dose, measured by an ionization chamber, was 40 μGy. The effective dose was assessed using an anthropomorphic phantom and thermoluminescent detectors resulting in 1.96 and 0.31 μSv for a lumbar spine and proximal femur scan, respectively. The average scattered dose rate at a distance of 1 m from the device was 1.06 and 1.21 μSv.h⁻¹ in the lumbar spine and left proximal femur scan mode, respectively. For the precision evaluation, 30 patients underwent 2 lumbar spine and 2 proximal femur scans with repositioning after each scan. The percentage root-mean-square coefficient of variation was 1.22%, 1.38%, 2.11%, and 0.86% for the lumbar spine (L1–L4), lumbar spine (L2–L4), femoral neck, and total hip, respectively. The cross-calibration studies were done on 57 patients (60 ± 9 yr). Lumbar spine, left neck, and left total hip mean BMD were 3.10% lower and 11.94% and 8.83% higher, respectively, with the Stratos compared with the Discovery A. Cross-calibration equations were calculated with a correlation coefficient of 98% (p < 0.01) for the lumbar spine (L2–L4), 94% (p < 0.01) for the left neck, and 92% (p < 0.01) for the left total hip. After standardizing the Stratos measures using the cross-calibration equations, LIN’s concordance correlation coefficient was 0.98, 0.93, and 0.92 for the lumbar spine (L2–L4), left neck, and total hip, respectively.     

A Comparison of Phantoms for Cross-Calibration of Lumbar Spine DXA

The aim of this project was to compare three phantoms used for cross-calibration of dual-energy X-ray absorptiometers with an in vivo cross-calibration. The phantoms used were the Bona Fide Phantom (BFP), the European Spine Phantom (ESP) and the GE Lunar Aluminum Spine Phantom (ASP). The cross calibration was for L2–L4 lumbar spine bone mineral density (BMD) on a GE Lunar DPX-L and Hologic QDR 2000. The in vivo cross-calibration was obtained using 72 subjects (61 female, 11 male; mean age 49 years, range 14–84 years). The phantoms were measured 10 times without repositioning on both instruments. A further, long-term cross-calibration was obtained with the BFP over a 9 month period. The true linear relationship between the two instruments was calculated used a standardized principal components method. The mean residuals were calculated between each phantom cross-calibration line and the in vivo data to obtain a measure of the goodness of fit between the phantom cross-calibration and the in vivo data. There was no significant difference between the in vitro and in vivo cross-calibrations. The long-term BFP cross-calibration gave an in vitro cross-calibration that is closest to the in vivo cross-calibration in this group of subjects. When calculating Hologic QDR BMD from results on the GE Lunar DPX-L, the ASP underestimates Hologic QDR 2000 BMD by 4% at high BMD and overestimates by 4% at low BMD. The ESP cross-calibration overestimates Hologic QDR2000 BMD by 1% at high BMD and 4% at low BMD. The BFP performs best, overestimating Hologic QDR2000 BMD by between 1.2% and 1.8%, whilst the difference between the long-term BFP cross-calibration and the in vivo data is less than 1% over the range of BMD covered. Received: 19 October 2001 / Accepted: 9 July 2002     

Evaluation of Differences Between Fan-Beam and Pencil-Beam Densitometers

We compared the bone and body composition results in vivo on two bone densitometers using fan-beam geometry (EXPERT and PRODIGY) with those using pencil-beam geometry (DPX). Measurements were made on large groups of adults ranging in weight from about 50 to 120 kg. Both spine and femur neck BMD on the fan-beam densitometers averaged within 1% of the pencil-beam results, and there was no magnitude dependence of the results by Bland-Altman analysis. Total body BMC and BMD on the PRODIGY and DPX were congruent, but on the EXPERT, BMC was about 2% lower and BMD 2% higher than corresponding values on the DPX. Soft-tissue composition was closely congruent for the PRODIGY and DPX; the comparable EXPERT-DPX differences showed greater scatter but no significant magnitude dependence. The smaller fan-angle of the PRODIGY (4°) probably contributed to its better congruence to pencil-beam results compared with the EXPERT (12°). Received: 23 February 2000 / Accepted: 14 April 2000 / Online publication: 27 July 2000     

In Vivo and In Vitro Comparison of Densitometers in the NOREPOS Study

The purpose of this study was to assess the agreement of in vivo hip scans on 3 densitometers (1 GE Lunar DPX-IQ and 2 GE Lunar Prodigy scanners) and to evaluate whether the European Spine Phantom (ESP) was able to reproduce the in vivo variability. Sixteen subjects had 3 repeated scans (with repositioning) on each densitometer, and the ESP was measured on each densitometer at least 40 times. Mean differences between hip scans on the Prodigy scanners were small and insignificant, and the in vivo results were not significantly different from the in vitro results. Bland and Altman plots showed no systematic differences between the Prodigy scanners over the range of bone mineral density (BMD). On the other hand, differences between Prodigy and DPX-IQ changed systematically over the range of BMD. The ESP did not fully reproduce the in vivo difference between Prodigy and DPX-IQ. In conclusion, the ESP is a valid substitute when assessing agreement between Prodigy scanners. However, when assessing agreement between different types of scanners, substitution of in vivo with in vitro measurements should be made with caution.     

Direct Comparison of the Precision of the New Hologic Horizon Model With the Old Discovery Model

Previous publications suggested that the precision of the new Hologic Horizon densitometer might be better than that of the previous Discovery model, but these observations were confounded by not using the same participants and technologists on both densitometers. We sought to study this issue methodically by measuring in vivo precision in both densitometers using the same patients and technologists. Precision studies for the Horizon and Discovery models were done by acquiring spine, hip, and forearm bone mineral density twice on 30 participants. The set of 4 scans on each participant (2 on the Discovery, 2 on the Horizon) was acquired by the same technologist using the same scanning mode. The pairs of data were used to calculate the least significant change according to the International Society for Clinical Densitometry guidelines. The significance of the difference between least significant changes was assessed using a Wilcoxon signed-rank test of the difference between the mean square error of the absolute value of the differences between paired measurements on the Discovery (Δ-Discovery) and the mean square error of the absolute value of the differences between paired measurements on the Horizon (Δ-Horizon). At virtually all anatomic sites, there was a nonsignificant trend for the precision to be better for the Horizon than for the Discovery. As more vertebrae were excluded from analysis, the precision deteriorated on both densitometers. The precision between densitometers was almost identical when reporting only 1 vertebral body. (1) There was a nonsignificant trend for greater precision on the new Hologic Horizon compared with the older Discovery model. (2) The difference in precision of the spine bone mineral density between the Horizon and the Discovery models decreases as fewer vertebrae are included. (3) These findings are substantially similar to previously published results which had not controlled as well for confounding from using different subjects and technologists.     

In vivo Precision of the GE Lunar iDXA Densitometer for the Measurement of Total-Body, Lumbar Spine, and Femoral Bone Mineral Density in Adults

Knowledge of precision is integral to the monitoring of bone mineral density (BMD) changes using dual-energy X-ray absorptiometry (DXA). We evaluated the precision for bone measurements acquired using a GE Lunar iDXA (GE Healthcare, Waukesha, WI) in self-selected men and women, with mean age of 34.8 yr (standard deviation [SD]: 8.4; range: 20.1–50.5), heterogeneous in terms of body mass index (mean: 25.8 kg/m²; SD: 5.1; range: 16.7–42.7 kg/m²). Two consecutive iDXA scans (with repositioning) of the total body, lumbar spine, and femur were conducted within 1 h, for each subject. The coefficient of variation (CV), the root-mean-square (RMS) averages of SDs of repeated measurements, and the corresponding 95% least significant change were calculated. Linear regression analyses were also undertaken. We found a high level of precision for BMD measurements, particularly for scans of the total body, lumbar spine, and total hip (RMS: 0.007, 0.004, and 0.007 g/cm²; CV: 0.63%, 0.41%, and 0.53%, respectively). Precision error for the femoral neck was higher but still represented good reproducibility (RMS: 0.014 g/cm²; CV: 1.36%). There were associations between body size and total-body BMD and total-hip BMD SD precisions (r = 0.534–0.806, p < 0.05) in male subjects. Regression parameters showed good association between consecutive measurements for all body sites (r² = 0.98–0.99). The Lunar iDXA provided excellent precision for BMD measurements of the total body, lumbar spine, femoral neck, and total hip.     

Pediatric in vivo cross-calibration between the GE Lunar Prodigy and DPX-L bone densitometers

Dual energy x-ray absorptiometry (DXA) machine cross-calibration is an important consideration when upgrading from old to new technology. In a recent cross-calibration study using adult subjects, close agreement between GE Lunar DPX-L and GE Lunar Prodigy scanners was reported. The aim of this work was to cross-calibrate the two machines for bone and body composition parameters for pediatrics from age 5 years onwards. One-hundred ten healthy volunteers aged 5–20 years had total body and lumbar spine densitometry performed on DPX-L and Prodigy densitometers. Cross-calibration was achieved using linear regression and Bland–Altman analysis. There was close agreement between the machines, with r² ranging from 0.85 to 0.99 for bone and body composition parameters. Paired t-tests demonstrated significant differences between machines that were dependent on scan acquisition mode, with the greatest differences reported for the smallest children. At the lumbar spine, Prodigy bone mineral density (BMD) values were on average 1.6% higher compared with DPX-L. For the total body, there were no significant differences in BMD; however, there were significant differences in bone mineral content (BMC) and bone area. For small children, the Prodigy measured lower BMC (9.4%) and bone area (5.8%), whereas for larger children the Prodigy measured both higher BMC (3.1%) and bone area (3.0%). A similar contrasting pattern was also observed for the body composition parameters. Prodigy values for lean body mass were higher (3.0%) for small children and lower (0.5%) for larger children, while fat body mass was lower (16.4%) for small children and higher (2.0%) for large children. Cross-calibration coefficients ranged from 0.84 to 1.12 and were significantly different from 1 (p<0.0001) for BMC and bone area. Bland–Altman plots showed that within the same scan acquisition modes, the magnitude of the difference increased with body weight. The results from this study suggest that the differences between machines are mainly due to differences in bone detection algorithms and that they vary with body weight and scan mode. In general, for population studies the differences are not clinically significant. However, for individual children being measured longitudinally, cross-over scanning may be required.     

Comparison of BMD precision for Prodigy and Delphi spine and femur scans

Introduction Precision error in bone mineral density (BMD) measurement can be affected by patient positioning, variations in scan analysis, automation of software, and both short- and long-term fluctuations of the densitometry equipment. Minimization and characterization of these errors is essential for reliable assessment of BMD change over time.Methods We compared the short-term precision error of two dual-energy X-ray absorptiometry (DXA) devices: the Lunar Prodigy (GE Healthcare) and the Delphi (Hologic). Both are fan-beam DXA devices predominantly used to measure BMD of the spine and proximal femur. In this study, 87 women (mean age 61.6±8.9 years) were measured in duplicate, with repositioning, on both systems, at one of three clinical centers. The technologists were International Society for Clinical Densitometry (ISCD) certified and followed manufacturer-recommended procedures. All scans were acquired using 30-s scan modes. Precision error was calculated as the root-mean-square standard deviation (RMS-SD) and coefficient of variation (RMS-%CV) for the repeated measurements. Right and left femora were evaluated individually and as a combined dual femur precision. Precision error of Prodigy and Delphi measurements at each measurement region was compared using an F test to determine significance of any observed differences.Results While precision errors for both systems were low, Prodigy precision errors were significantly lower than Delphi at L1–L4 spine (1.0% vs 1.2%), total femur (0.9% vs 1.3%), femoral neck (1.5% vs 1.9%), and dual total femur (0.6% vs 0.9%). Dual femur modes decreased precision errors by approximately 25% compared with single femur results.Conclusions This study suggests that short-term BMD precision errors are skeletal-site and manufacturer specific. In clinical practice, precision should be considered when determining: (a) the minimum time interval between baseline and follow-up scans and (b) whether a statistically significant change in the patient’s BMD has occurred.