Cross calibration of DXA as part of an equipment replacement program.

The purpose of this study was to develop cross calibrations when replacing three dual-energy X-ray absorptiometers (GE Lunar DPXL [Madison, WI], DPXL, GE Lunar Expert [Madison, WI], Expert, Hologic QDR2000 [Waltham, MA]) with two new GE Lunar Prodigy instruments. Subjects previously scanned on the Expert or QDR2000 were transferred to Prodigy 1 and those previously scanned on the DPXL to Prodigy 2. A cohort of subjects was recalled for each old instrument, and approximately 20 subjects had lumbar spine and hip scans on each old instrument and the appropriate new instrument. An in vitro calibration was carried out using a Bona Fide Phantom (Bio-Imaging Technologies, Inc., Newtown, PA). Calibrations were fitted using a standardized principal components method. A Bland and Altman plot was used to calculate the mean difference and limits of agreement between instruments. Standardized bone mineral density (BMD) was also used to calibrate the Hologic to Prodigy 1. There was good agreement between instruments from the same manufacturer. As expected, BMD measured on the Prodigy was about 15% higher than the Hologic. Using standardized BMD to cross calibrate gave a mean difference of 3% at the lumbar spine. The limits of agreement following calibration are clinically significant, so it is not possible to apply a calibration to an individual subject for trending purposes, as the error is similar to the expected annual change in BMD, but can be used for cross calibration in clinical trials. The in vivo calibration gave better agreement than using standardized BMD. The phantom calibration was close to the in vivo calibrations at the spine, but not in some hip regions. When introducing a new instrument, a new baseline BMD has to be obtained for each subject.     

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     

Effect of 99mTc-MDP administration on dual-energy X-ray absorptiometry bone mineral density measurements.

The effect of a diagnostic dose of (99m)Tc-MDP on bone mineral density (BMD) estimations in the lumbar spine and the neck of femur was assessed in 20 patients using a Hologic QDR4500 scanner. Each patient underwent a DXA assessment prior to and 1 h following injection of (99m)Tc-MDP (mean dose-910 MBq). For comparative purposes, the precision of BMD estimation without the presence of a radioisotope was assessed by performing two sequential DXA studies on 30 volunteers and was found to be less than 0.01 g/cm(2). No significant change in BMD reading was detected following (99m)Tc-MDP injection for either measurement site and the precision of the readings was similar to that observed for the 30 volunteers. This study has shown that any effect produced by a typical bone scan dose of (99m)Tc-MDP is small in comparison with the intrasubject variance when estimating BMD using a Hologic QDR4500 scanner.     

A population-based comparison of quantitative dual-energy X-ray absorptiometry with dual-photon absorptiometry of the spine and hip

Summary Dual photon absorptiometry (DPA) is currently the most widely used method for noninvasive bone mineral density (BMD) measurement of the axial skeleton. Dual energy X-ray absorptimetry (DEXA) is a recently developed technique that uses an X-ray tube as a photon source; it has demonstrated several significant advantages over DPA in preliminary studies. We report here a quantitative comparison of the DEXA and DPA technologies using a Hologic DEXA (Hologic QDR model 1000, Waltham, MA) scanner and a Lunar DPA (Lunar Radiation DP3, gandolineum-153 source) scanner at both the proximal femur and lumbar spine sites using bone density measurements from a populationbased sample of older white men and women who had complete DEXA and DPA measurements of the hip (n=217) or the spine (n=176). To examine the relationship of BMD measured by the DPA scanner to BMD measured on the DEXA scanner, normal least squares linear regression was used to regress the DPA BMD on the DEXA BMD for each site. DEXA values were consistently lower than DPA values, with an average difference of 16%. The squared multiple correlation (R²) values were at or above 0.95 for almost all sites, with Ward's triangle having the lowest value (0.89). The slope for all sites was similar, ranging from 0.94 to 1.1. Research and clinical centers that wish to change to DEXA technology because of its shorter examination time and greater precision can therefore compare DEXA with DPA values using representative convesion factors.     

Cross calibration of Hologic QDR2000 and GE Lunar Prodigy for forearm bone mineral density measurements.

The purpose of this study was to carry out an in vivo cross calibration for forearm bone mineral density (BMD) between a Hologic QDR2000 (Hologic Inc., Bedford MA, USA) and Lunar Prodigy (GE Healthcare, Madison WI, USA) during equipment upgrade. Nineteen subjects (17 females and 2 males, mean age 57 yr, range: 42-79yr) attending for routine dual energy X-ray absorptiometry scanning were recruited. BMD of the nondominant forearm was measured on both instruments. Cross-calibration equations were derived for total forearm, ultradistal radius and ulna, and 33% radius and ulna. A Bland & Altman plot was used to calculate the mean difference and limits of agreement between instruments. There were significant differences in BMD at all sites. The Prodigy BMD was 15% higher at the total forearm and 20-25% higher in the cortical regions of the 33% ulna and 33% radius. The differences are smaller in the ultradistal regions, as it appears that the Prodigy underestimates BMD at low BMD. The standard error of estimate about the cross calibration was such that it cannot be used to transfer individual patients between instruments, but could be applied to clinical trial data.     

Comparison of narrow-angle fan-beam and pencil-beam densitometers: in vivo and phantom study of the effect of bone density, scan mode, and tissue depth on spine measurements.

This study compared the in vivo and in vitro performances of the Lunar MD and Prodigy dual-energy X-ray absorptiometers (DXAs). Ten volunteers and three different spine phantoms were studied to determine the effect of scan mode, tissue depth, and bone density on measures of spine bone area (BA), bone mineral content (BMC), and areal bone mineral density (BMD). These studies demonstrated that the choice of scan mode was most important for the Prodigy and for subjects who were thin, obese, or had low BMD. Increase in tissue depth caused an increase in measured BMC and BMD for the MD but had a small effect on Prodigy results if the appropriate scan mode was selected. BA was dependent on the BMD for both DXA systems. Results using a hydroxyapatite phantom demonstrated that after correcting for the calibration of Lunar systems, the BMC measured by the MD and Prodigy was similar to the calculated hydroxyapatite content of the phantom. In vivo studies confirmed the in vitro findings and demonstrated that even when the appropriate scan mode was selected, the BMC, BMD, and T-scores were significantly higher on the Prodigy than MD.     

Cross Calibration of Hologic QDR2000 and GE Lunar Prodigy for Whole Body Bone Mineral Density and Body Composition Measurements

The objective of this study was to undertake an in vivo cross calibration of body composition, whole body bone mineral content (BMC) and bone mineral density (BMD) between a Hologic QDR2000 and a GE Healthcare Lunar Prodigy. Twenty-one subjects attending for routine bone densitometry were recruited to the study (19 female and 2 male, aged 30–79 yr). Phantom cross calibrations were carried out using the Bio-Imaging Variable Composition Phantom (VCP) for percentage fat (%fat) and the Bona Fide Phantom (BFP) for BMD. There was no significant difference in whole body lean body mass between the QDR2000 and the Prodigy. Fat mass (FM) and %fat were significantly higher on the QDR2000. BMC and whole body BMD were significantly higher on Prodigy. As the BMC increased, so did the difference between the 2 instruments. The VCP did not provide an adequate cross calibration of %fat compared with in vivo. The BFP provided a good cross calibration of whole body BMD compared with in vivo. The results suggest that the partitioning of the soft tissue component between lean and fat in the 2 instruments is systematically different. The variation between instruments from the same and different manufacturers reported in the literature varies widely, as does the comparison with criterion methods. This makes it difficult to generalize the results of this study to other centers and it is recommended that each center would have to cross calibrate when changing equipment.     

Comparison of imaging methods for localization of parathyroid tumors.

Preoperative localization of parathyroid tumors by computed tomography (CT), thallium-201/technetium-99m pertechnetate subtraction scintigraphy (Tl-201/Tc-99m), ultrasonography (US), and magnetic resonance imaging (MRI) was compared in patients with hyperparathyroidism (HPT) to examine the characteristics of each method. A total of 87 patients with HPT were divided into two groups according to the time when they were examined. Patients in group I were examined before MRI had been introduced in our hospital, and a 2.5-MHz transducer probe was used for US. Those in group II were examined by MRI and US using a 7.5-MHz transducer probe. Group I included 45 patients (36 with primary hyperparathyroidism [PHPT] and 9 with secondary hyperparathyroidism [SHPT]), and group II included 42 patients (15 with PHPT and 27 with SHPT). In both PHPT and SHPT and SHPT of group I and PHPT of group II, there was no significant difference in detection rates between all diagnostic methods. In patients with SHPT in group II, the detection rate was significantly higher for CT than for Tl-201/Tc-99m and MRI (both p less than 0.01), and for US than for Tl-201/Tc-99m (p less than 0.01). In both groups I and II, the detection rate of each study method was significantly higher in patients with PHPT than in those with SHPT (all p less than 0.01). Compared with group I, the rate was significantly improved in group II, in both types of patients. Regarding the location of the parathyroid tumor, the detection rate of CT was significantly higher for upper parathyroid glands than for lower glands, whereas that of US and Tl-201/Tc-99m was significantly higher for lower glands. The detection rate sharply increased when the tumor weight reached 250 mg (CT, US) or 1,000 mg (Tl-201/Tc-99m, MRI).     

Dual-energy x-ray absorptiometry measurements of total-body bone mineral during weight change.

Bone mineral measurements were made using dual-energy X-ray absorptiometry during a multicenter diet trial.There were five centers, two using Hologic QDR4500 fan-beam scanners, two using Lunar Prodigy fan-beam scanners,and one using a pencil-beam Lunar DPX. Measurements were made at 0, 2.5, and 6 mo. The mean weight loss was 7.9 kg, but there was a wide range. With the Lunar instruments, the total-body bone mineral density reduced with weight loss, but with the Hologic scanners, it appeared to increase. This anomaly is similar to that observed previously with a Hologic QDR1000 pencil-beam scanner. It was shown that changes of fat distribution can lead to alterations in bone measurement without any real change in the skeleton. With all of the scanners, there was a strong correlation between the change in the bone mineral content and bone area, with some values of the latter being quite implausible. There was an associated worsening of long-term precision compared with that derived from short-term duplicated scans, more marked with the Lunar scanners. It is concluded that measurement artifacts preclude the valid assessment of total-body bone mineral during weight change.     

Follow-up of Individual Patients on Two DXA Scanners of the Same Manufacturer

Measuring and monitoring changes in bone mineral density (BMD) is usually done by dual-energy X-ray absorptiometry (DXA). Replacement of old devices is becoming increasingly frequent. To cross-calibrate two Hologic devices, a QDR 1000 and a QDR 4500A, we measured three phantoms – a Hologic spine phantom, a Hologic block phantom (without and with subregions analysis) and a European Spine Phantom – 20 times each without repositioning on both devices. The mean difference between BMD obtained on the two devices was 0.003, 0.033, 0.051 and −0.045 g/cm² respectively. We also measured the spine and hip of 60 women aged 19–78 years twice on the same day on both devices. Another group of 30 women aged 52–83 years were measured twice on the QDR 4500 A device (15 days apart). We analyzed the data using Pearson’s correlation coefficient, and Bland and Altman’s method, and calculated the smallest detectable difference (SDD). Results on the two devices were highly correlated: r ²= 0.99, 0.95, 0.96 for spine, femoral neck and total hip BMD respectively. SDD was higher for scans done on different devices than for those done twice on the same device: the SDDs were 0.048, 0.046 and 0.047 g/cm² for spine, femoral neck and total hip BMD respectively measured on two different devices, while the equivalent values were 0.034, 0.036 and 0.027 g/cm² using a single device. The difference in BMD results was not dependent on BMD. Our results suggest that, although devices are properly cross-calibrated, differences among them great enough to be clinically relevant can be observed in vivo. Received: 30 June 1999 / Accepted: 7 February 2000     

The Interference of Gamma Rays With Bone Mineral Density Measurements in 177Lu-PSMA and DOTATATE Therapy

The main purpose was to describe the interference of gamma radiation emitted by ¹⁷⁷Lu with simultaneous bone mineral density BMD measures for patients undergoing ¹⁷⁷Lu-PSMA and ¹⁷⁷Lu DOTATATE therapy. A cohort of 9 patients underwent ¹⁷⁷Lu-PSMA therapy were randomly selected to speculate the activity in the abdominopelvic region. So that, SPECT/CT scan at 24 h was used with attenuation and scatter correction. The activities were derived from the delineated ROIs over the abdominopelvic zone showing a range of 34?274 MBq. Next, a water path was placed under spine phantom mimicking L1-L4 vertebrae and followed by consecutive DEXA scans made by Hologic 4500 W and GE-Lunar DPX-NT systems. Five scans were performed without/and with different Lu-177 activities 37, 185, 370 and 555 MBq under the same geometric conditions. The obtained BMD readings of L1-L4 by the Hologic device were 1.027, 1.024, 1.021, 1.013, and 1.006 g/cm² with presence of 0, 37, 185, 370, and 555 MBq ¹⁷⁷Lu activity, respectively. Whereas, in Lunar device, it was found as higher as 1.163, 1.121, 1.09, 1.072, and 1.043, respectively. There was no statistically significant difference between both devices (p _value ≥ 0.05). The fluctuation ranges in the L1-L4 BMD readings at the presence of 37?555 MBq were 0.3%?2%, and 3.6%?10.3% for Hologic and Lunar systems, respectively. It was emphasized that gamma radiation emitted by ¹⁷⁷Lu relatively influence DEXA scans and the yielded BMD measures. Postponing DEXA scans as early as 8 d after ¹⁷⁷Lu-PMSA and 11 d after ¹⁷⁷Lu-DOTATATE therapies is recommended to avoid the erroneous contribution of gamma radiation and provide precise bone assessment.     

Antecedent 99mTc-MDP and 99mTc-sestamibi administration corrupts bone mineral density measured by DXA.

Previous reports of the effect of antecedent administration of radionuclide on bone mineral density (BMD) measurements have yielded inconsistent results. Ten subjects scheduled for (99m)Tc-methylene diphosphonate ((99m)Tc-MDP) bone scanning and 10 scheduled for (99m)Tc-sestamibi cardiac scanning had BMD measured by dual X-ray absorptiometry (DXA) (GE/Lunar) before and within 5 hours of diagnostic radionuclide injection. Paired t test and Wilcoxon-signed rank tests were used to compare the measured differences in BMD at multiple skeletal sites. Differences were subjected to multivariate analysis of demographic factors. Mean change in measured BMD following (99m)Tc-sestamibi administration (DeltaBMD-(99m)Tc-sestamibi) was -0.216+/-0.113 g/cm(2) at the total body and -0.348+/-0.300 g/cm(2) at the lumbar spine (p<0.005). Mean change in measured BMD following (99m)Tc-MDP administration (DeltaBMD-(99m)Tc-MDP) was -0.058+/-0.037 g/cm(2) at the total body and -0.053+/-0.049 g/cm(2) at the lumbar spine (p<0.05). Mean DeltaBMD-(99m)Tc-sestamibi exceeded least significant change (LSC) in all skeletal sites except the femoral trochanter. Mean DeltaBMD-(99m)Tc-MDP exceeded LSC only at the lumbar spine. The effect was correlated with (99m)Tc dose but not with gender, age, body mass index, baseline BMD, or time interval from injection to scan acquisition. In conclusion, BMD measured by the GE/Lunar Prodigy densitometer is corrupted by antecedent (99m)Tc-sestamibi and to a lesser extent by (99m)Tc-MDP. This effect is greater at the total body and lumbar spine than at the hip. Caution is warranted in scheduling and interpreting DXA studies when (99m)Tc has been recently administered.     

Image resolution and magnification using a cone beam densitometer: optimizing data acquisition for hip morphometric analysis

Bone mineral density (BMD) is a primary determinant of hip fracture risk. However, other factors, notably the femoral geometry, can influence hip fracture risk. The purpose of this study was to evaluate the potential of a new cone beam densitometer, the DMS Lexxos, in order to visualise femoral morphometry. Resolution, magnification and distortion were assessed in vitro using a line pair test pattern and a matrix test object. Results were given in comparison with currently available systems: the Hologic Discovery A and the Lunar Prodigy densitometers. The DMS Lexxos image resolution was the same in the longitudinal and transversal directions evaluated between 1.4 and 0.5 line pairs/mm (lps/mm) for an attenuation varying from 25 to 325 mm of Perplex. The longitudinal resolution was evaluated between 0.9 and 0.5 lps/mm with the Hologic Discovery densitometer, and inferior to 0.5 with the Lunar Prodigy; as for transversal resolution, it varied from 0.63 to 0.5 lps/mm and from 0.6 to inferior 0.5 lps/mm, respectively. The image was isotropic without magnification with the GE-Lunar Prodigy, whereas there was only a transversal magnification with the Hologic Discovery device. The magnification was about 1.17% cm^–1 in the two directions, while increasing the distance of the phantom above the examination table with the Lexxos. This magnification was isotropic without distortion. The magnification could be evaluated from two images taken before and after translation of the C-arm, and a magnification correction could be applied. This method was applied to a phantom and to a human cadaver femoral bone.     

Longitudinal precision of dual-energy x-ray absorptiometry in a multicenter study. The Nafarelin/Bone Study Group

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.     

Discordance of longitudinal changes in bone density between densitometers

This study examined the concordance in BMD measurement and longitudinal change in BMD between the GE Lunar Prodigy and GE Lunar DPX. Even though a high concordance between the densitometers was observed on a single measurement occasion, a significant discordance in longitudinal changes in BMD was observed. INTRODUCTION: Measurement of bone mineral density (BMD) by dual-energy X-ray absorptiometry (DXA) technology plays an important role in the diagnosis and management of osteoporosis. The present study examined the concordance in BMD measurement and longitudinal change in BMD between GE Lunar Prodigy and DPX. METHODS: BMD at the lumbar spine and femoral neck was measured in 135 individuals (47 men and 88 women, mean age 73+/-9 years) using both GE Lunar DPX and Prodigy densitometers at baseline. In this group, 56 individuals (22 men and 34 women) had repeated BMD measurements using the DPX and Prodigy during a subsequent follow-up visit (average duration: 2.2 years). RESULTS: For a single BMD measurement, the coefficient of concordance between the Prodigy and DPX was greater than 0.98 at the lumbar spine and 0.96 at the femoral neck, with the slope of linear regression being approximately 1.0. During the period of follow-up, the lumbar spine BMD decreased by -0.5% (S.D. 1.8%) when measured by DPX, which was significantly different (p=0.002) from the change measured by Prodigy (mean change=0, S.D. 2.0%). However, there was no significant difference (p=0.95) in the rate of change in femoral neck BMD measured by DPX (mean=-1.6%, S.D.=2.9) and Prodigy (mean=-1%, S.D.=1.8%). The correlation in rates of BMD change between Prodigy and DPX was 0.63 at the lumbar spine and 0.52 at the femoral neck. Simulation analysis showed that the theoretical maximum correlation in rates of BMD change between Prodigy and DPX was 0.71. CONCLUSIONS: Despite both densitometers being highly concordant in a single BMD measurement, discordance in the assessment of BMD changes between the Prodigy and DPX densitometers was observed. These findings have implications regarding the assessment of response to therapy in a multi-centre setting when different densitometers are used.     

Effect of bone strontium on BMD measurements.

Strontium ranelate is a new treatment for osteoporosis that is of interest for, among other reasons, its unusual effect on measurements of bone mineral density (BMD). When some of the calcium in bone is replaced by strontium, X-ray absorptiometry measurements of BMD are overestimated because strontium attenuates X-rays more strongly than calcium. In this study, we report the first theoretical estimation of this effect for measurements made using axial (spine and hip) dual-energy X-ray absorptiometry (DXA), peripheral DXA (pDXA), and single-energy quantitative computed tomography (SEQCT). Tables of X-ray attenuation coefficients were used to calculate values of the strontium ratio defined as the ratio of the percentage overestimation of BMD to the molar percentage of strontium (%Sr/[Ca+Sr]) in bone. For DXA measurements, the theoretical value of the strontium ratio increased slightly with increasing effective photon energy of the X-ray beam with figures of 9.0 for Osteometer DTX200 and G4 pDXA devices (Osteometer Meditech Inc., Hawthorne, CA), 10.0 for GE-Lunar DPX and Prodigy DXA systems (GE-Lunar, Madison, WI), 10.4 for Hologic QDR1000 and QDR2000, and 10.8 for Hologic QDR4500 and Discovery (Hologic Inc., Bedford, MA). Results for SEQCT also varied with the effective photon energy with strontium ratios of 6.2 at 60 keV and 4.4 at 80 keV. The results of the theoretical study are in good agreement with the experimental value of 10 reported by Pors Nielsen and colleagues for a variety of different axial DXA systems. A reliable figure for the strontium ratio is important for adjusting BMD measurements in strontium ranelate treated patients for the effect of bone strontium content. This latter correction will be required for the interpretation of future DXA scans in patients who have discontinued strontium ranelate treatment.     

Comparison of pencil-beam and fan-beam DXA systems.

Patients attending a routine bone densitometry clinic were scanned on two different densitometers on the same day using a pencil-beam (Hologic QDR1000W) and fan-beam (Hologic QDR4500W) machine. Subjects were scanned at the lumbar spine site and or the proximal femur. The differences in bone mineral density (BMD) between the fan-beam and pencil-beam (QDR4500W-QDR1000W) were determined for all pairs of scans. The mean difference in BMD was also calculated to see if there was a systematic bias between the machines. The mean difference in BMD was -8 mg/cm2 and 25 mg/cm2 at the spine and total hip, respectively. The individual differences in BMD between the two machines were examined to assess if they were significantly greater than measurement error. The percentage of scans classified as significantly different was calculated for the difference in BMD before and after adjusting for the mean difference in BMD. The percentage of individuals classified as significantly different ranged from 17.1-45.0% before adjustment, at the spine and total hip, respectively, and 16.1-22.6% after adjustment. From a clinical perspective, this degree of misclassification is probably unacceptable. These results suggest that scans obtained from a QDR4500W fan-beam system and QDR1000W pencil-beam system should not be compared, with or without adjustment for systematic bias.     

Cross-Calibration of Prodigy and Horizon A Densitometers and Precision of the Horizon A Densitometer

We performed this study to enable a reliable transition for clinical study participants and patients from a GE Lunar Prodigy to a Hologic Horizon A dual-energy X-ray absorptiometry (DXA) scanner and to assess the reproducibility of measurements made on the new DXA scanner. Forty-five older adults had one spine, hip, and total body scan on a Prodigy dual-energy X-ray absorptiometry (DXA) scanner and 2 spine, hip, and total body scans, with repositioning, on a new Hologic Horizon A DXA scanner. Linear regression models were used to derive cross calibration equations for each measure on the 2 scanners. Precision (group root-mean-square average coefficient of variation) of bone mineral density (BMD) of the total hip, femoral neck, and lumbar spine (L1-L4), and total body fat, bone, and lean mass, appendicular lean mass, and trabecular bone score (TBS) was assessed using the International Society of Clinical Densitometry's (ISCD's) Advanced Precision Calculation Tool. Correlation coefficients for the BMD and body composition measures on the 2 scanners ranged from 0.94 to 0.99 (p<0.001). When compared with values on the Prodigy, mean BMD on the Horizon A was lower at each skeletal site (0.136 g/cm² lower at the femoral neck and 0.169 g/cm² lower at the lumbar spine (L1-4)), fat mass was 0.47 kg lower, and lean mass was 4.50 kg higher. Precision of the Horizon A scans was 1.60% for total hip, 1.94% for femoral neck, and 1.25% for spine (L1-4) BMD. Precision of TBS was 1.67%. Precision of total body fat mass was 2.16%, total body lean mass was 1.26%, appendicular lean mass was 1.97%, and total body bone mass was 1.12%. The differences in BMD and body composition values on the 2 scanners illustrate the importance of cross-calibration to account for these differences when transitioning clinical study participants and patients from one scanner to another.     

Anomalies in the Measurement of Changes in Bone Mineral Density of the Spine by Dual-Energy X-ray Absorptiometry

Dual-energy X-ray absorptiometry (DXA) is frequently used for longitudinal studies of bone mineral status because of the high precision obtained, but evidence is emerging that the accuracy of measurements of changes may be a limitation because of artefacts of the analysis procedure, in particular, a dependence of the measured bone area (BA) on the bone mineral content (BMC). Results of spine bone mineral measurements taken at intervals with two DXA scanners, a Hologic QDR 1000W, and a Norland XR 26 HS, were examined. There was a consistent correlation between changes in BA and in BMC, with a slope of approximately 0.25 when expressed as percentages. A real change of BA of the magnitude observed is not feasible. There were no differences among the correlations for different instruments, genders, ages, or weight changes. There would appear to be an underestimation of changes in bone mineral density (BMD), but there is a possibility that some of the anomaly is manifested as an overestimation of a change in BMC. Phantom measurements were undertaken with the DXA scanners mentioned above and with a Lunar DPX. The phantoms consisted of simulations of the spine cut from aluminium sheet, so that the effective BMD could be varied. The dependence of the measured BA on BMC varied with the phantom outline, particularly the thickness of the transverse processes. Evidence was obtained of both an underestimate of BMD changes and an overestimate of BMC changes. There are errors in measuring spine changes, but these do not seem to be as serious as a previous report suggests for the Hologic scanner and are not likely to lead to misinterpretation of results. Received: 17 June 1997 / Accepted: 23 January 1998     

The Effect of 99mTc on Dual-Energy X-Ray Absorptiometry Measurement of Body Composition and Bone Mineral Density

Whether the γ-emission by radioisotopes influences the outcome of dual-energy X-ray absorptiometry (DXA) measurements is not fully elucidated. The aim of this study was to evaluate the effect of antecedent administration of ^99mTc on DXA measurements regarding body composition and bone mineral density (BMD) using a K-edge filter scanner. The phantom measurements were performed by placing a urinary bladder phantom containing 40 mL of radioisotope solution on the pelvic region of a whole-body phantom. Twenty-seven patients attending our department for a routine examination involving the administration of a tracer marked with ^99mTc were included. The patients underwent a whole-body DXA scan before and within 2 h after tracer injection using a GE/Lunar Prodigy scanner. Control scans were performed on 40 volunteers, who had not received any radioactive tracer. In both phantom and patient measurements, we found a significant dose-related decrease in fat mass and BMD and a corresponding increase in fat-free mass (p < 0.001). Based on the linear regression analysis, we suggest upper dose limits for the measurement of BMD at 0.77 μSv/h and body composition at 0.21 μSv/h (dose rate measured at a distance of 1 m from the patient). Caution should be taken when interpreting the results of DXA scans performed in close temporal proximity to procedures involving the administration of ^99mTc.