Switching from DXA pencil-beam to fan-beam. II: Studies in vivo

Switching from the Hologic QDR-1000/W to the QDR-2000 DXA densitometer was critically evaluated with regard to cross-calibration and dosimetry. Studies with bone equivalent humanoid spine phantoms and patient studies were done. Fan-beam scanning with the QDR-2000 is problematic because of magnification.

Mean phantom bone mineral content (BMC) and bone mineral density (BMD) were moderately but significantly different. Biological variation disguised differences between the two devices in humans, but significant differences were revealed when individual data were analyzed. Longitudinal assessments of BMC and BMD, initiated with QDR-1000/W and continued with the QDR-2000, should employ singlebeam mode only and not fan-beam mode—but even if that is done, significant errors can be introduced.

The new QDR-2000 should be properly cross-calibrated with the original densitometer, and one should make sure that the same software, phantom, and type of collimator are used. The radiation dose is substantially higher with QDR2000 (fan-beam and high-resolution array mode) than with QDR-1000/W (pencil-beam mode) and QDR-2000 (pencilbeam mode), and higher than claimed by the manufacturer. The typical radiation dose given by the manufacturer was half the actual radiation dose measured (e.g., for fan-beam scan 62 μSv versus 33 μSv). High-resolution array mode does not improve precision, but augments the radiation dose to the patient.     

Cross-calibration of DXA equipment: Upgrading from a hologic QDR 1000/W to a QDR 2000

Summary In this study, the cross-calibration of a fan beam DXA system (Hologic QDR-2000) to a pencil beam scanner from the same manufacturer (Hologic QDR-1000/W) is described. The scanners were calibrated by the manufacturer using the same anthropomorphic spine phantom at installation. To verify consistent machine calibration, a group of 69 female subjects, aged 46–75, had anteroposterior (AP) spine and proximal femur scans on the QDR-1000/W followed by pencil and array scans of the same sites on the QDR-2000 during the same visit. Many of the subjects had bilateral examinations of the proximal femur for a total of 123 hip scans. Pencil and array area, bone mineral content (BMC), and bone mineral density (BMD) from the QDR-2000 were compared with the values obtained on the QDR-1000/W, and linear regression equations were derived for relating the two instruments. At the spine, no differences were found between the QDR-1000/W BMD values and the QDR-2000 array BMD values. A slight difference between pencil beam modes was detected but was not deemed clinically significant. Linear regression models relating the QDR-2000 and QDR-1000/W AP spine BMD measurements showed correlation coefficients greater than 0.99, with slopes of 1.00, intercepts equivalent to zero, and small root mean square errors. Comparisons at the proximal femur showed equivalency at the femoral neck and trochanter regions for the two machines in pencil mode, but slight increases in BMC and BMD at the other femoral sites on the QDR-2000 in both pencil and array mode. Correlation coefficients were 0.97–0.99 for all measurement regions except for Ward's. Regression slopes relating the BMD for the femoral regions were 1.00–1.04, with intercepts not significantly different from zero and small residual errors. As with the spine, the differences were small enough that they were not of clinical significance. However, in longitudinal drug trials requiring highly precise determination of spinal and femoral BMD changes, these differences may be important.     

In vitro comparability of dual energy X-ray absorptiometry (DEXA) bone densitometers

Summary Six Hologic QDR-1000 DEXA bone densitometers at different centers across the USA were compared to determine the intermachine variability. Nine scans in succession were acquired on each machine using a single anthropomorphic lumbar spine phantom (manufactured by Hologic). Values for BMC, area, and BMD were recorded for each measurement. Means, standard deviations (SD), and coefficients of variation (CV) were calculated for each machine. All the CVs (BMC, area, BMD) were less than 1% (range 0.3%–0.6%). The CV of the means at the six sites were 0.4%, 0.6%, and 0.5% for BMC, area, and BMD, respectively. Although several significant differences for BMC, area, and BMD were noted by ANOVA between machines at different sites, the difference between the highest and lowest means of the individual machines was only 1.1%, 1.31%, and 1.07% for BMC, area, and BMD. The small variations between the DEXA systems are encouraging for researchers involved in multicenter trials in which data are pooled.     

A comparison of two dual-energy X-ray absorptiometry systems for spinal bone mineral measurement

Summary Two dual-energy X-ray absorptiometry (DEXA) systems—the Hologic QDR-1000 and the Norland XR-26 bone densitometers—were evaluated in terms of precision, accuracy, linearity of response, X-ray exposure, and correlation of in vivo spinal measurements. In vitro precision and accuracy studies were performed using the Hologic anthropomorphic spine phantom; linearity of response was determined with increasing thicknesses of aluminum slabs and concentrations of Tums E-X in a constant-level water bath. Both systems were comparable in precision, achieving coefficients of variation (CVs) of less than 1% in bone mineral content (BMC, g), bone area (cm²), and bone mineral density (BMD, g/cm²). Both were accurate in their determination of BMC, bone area, and BMD with reference to the Hologic spine phantom. Both systems also showed good BMC and BMD linearity of response. Measured X-ray skin surface exposures for the Hologic and the Norland systems were 3.11 and 3.02 mR, respectively. In vivo spinal measurements (n=65) on the systems were highly correlated (BMC: r=0.993, SEE=1.770 g; area: r=0.984, SEE=1.713 cm²; BMD: r=0.990, SEE=0.028 g/cm²). In conclusion, both systems are comparable in terms of precision, accuracy, linearity of response, and exposure efficiency.     

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.     

Validation and application of dual-energy X-ray absorptiometry to measure bone mineral density in rabbit vertebrae.

The rabbit could be a superior animal model to use in bone physiology studies, for the rabbit does attain true skeletal maturity. However, there are neither normative bone mineral density (BMD) data on the rabbit nor are there any validation studies on the use of dual-energy X-ray absorptiometry (DXA) to measure spinal BMD in the rabbit. Therefore, our aim was twofold: first, to investigate whether DXA could be used precisely and accurately to determine the bone mineral content (BMC). bone area (BA). and BMD of the rabbit lumbar spine: Second. to evaluate the new generation fan-beam DXA (Hologic QDR-4500) with small animal software by comparing two DXA methodologies QDR-1000 and QDR-4500 with each other, as well as against volumetric bone density (VBMD) derived from Archimedes principle. As expected. there was a magnification error in the QDR-4500 (BMC, BA. and BMD increased by 52%. 38%. and 10%, respectively, when the vertebrae were positioned flat against the scanning table). With the magnification error kept constant (vertebrae positioned 10 cm above the scanning table to match the height in vivo). there were no differences among the mean BMC. BA. and BMD of the rabbit vertebrae (Ll-L7) in vivo and in vitro using the QDR-4500 (p > 0.05). BMC, BA, and BMD differed between QDR-1000 and QDR-4500 in vitro because of a magnification error when the vertebrae were flat on the table (p <0.0001). and, consequently. the machines did not correlate with one another (p > 0.05). However, the BMC, BA, and BMD of the two DXAs did significantly correlate with each other in vivo and in vitro when the magnification error was compensated for (r = 0.44 and 0.52. i2 = 0.45 and 0.63. and 12 = 0.41 and 0.60. respectively. p < 0.05-0.008). The BMC and BMD (in vivo and in vitro) of the rabbit vertebrae measured by QDR-4500 was significantly correlated with VMBD, ash weight, and mineral content (,2 = 0.67-0.90,j <0.01-0.0001). Therefore, the QDR-4500 can be used to yield precise and accurate measurements of the rabbit spine.     

Patient dose in dual x-ray absorptiometry

Dual x-ray absorptiometry (DXA) provides a convenient, non-invasive method of assessing skeletal bone mineral which is widely used for clinical studies. This report describes a study to estimate the effective dose of radiation (ICRP-60(1990)) to a typical female patient from scans performed on three DXA scanners: the Hologic QDR-1000, QDR-1000/W and QDR-2000. The scans modes studied were: total body; anteroposterior (AP) lumbar spine; lateral lumbar spine; proximal femur; distal forearm. An ionization chamber and tissue-equivalent phantom were used to determine entrance surface dose and percentage depth-dose curves for each scan mode. Anatomical data from ICRP-23 (Reference Man) and a body section atlas were used to estimate the absorbed dose to each organ in the scan fields. Effective dose was estimated using the ICRP-60 tissue weighting factors and the fraction of each organ in the scan field. Results are summarized below. Figures for the effective dose are given both excluding and (in brackets) including the ovaries to cover the cases of postmenopausal and premenopausal women respectively.      

Effect of treadmill exercise on vertebral and tibial bone mineral content and bone mineral density in the aged adult rat: Determined by dual energy X-ray absorptiometry

Summary Dual energy X-ray absorptiometry (DXA; Hologic QDR-1000W) in an ultrahigh-resolution mode, was used to examine the changes in tibial/fibula and vertebral L4 +L5 bone mineral content (BMC) and bone mineral density (BMD) in each 14-month-old female rat at 0, 9, and 16 weeks of study. Twenty rats were randomized by a stratified weight method into two groups, control and exercised. Exercise consisted of running on a flat-bed treadmill, 17 m/minute, 1 hour/day and 5 days/week. As compared with the control group, a significant increase in tibia/fibula BMC and vertebral BMD was apparent at 9 weeks after exercise training (P=0.014 by 2-way analysis of variance). The slope of the gain of the tibia/fibula BMC and BMD by 16 weeks of training was ninefold and fivefold higher than that of the control group (P<0.01 and P<0.05, respectively, by Mann-Whitney test). The correlation coefficient (r) between the final dry weight of excised bone and the final BMC of the intact rat was 0.843 and 0.71 for tibia/fibula and vertebrae, respectively. In summary, we found that in the aged rat, by 9 weeks, exercise increases BMC and BMD in the tibia, whereas in the vertebrae, only increases in the BMD were found. This study demonstrates that this precise and accurate DXA technique is useful in a longitudinal study of in vivo bone mineral changes in the rat over time by taking into account the individual variation between animals as well as changes between groups.     

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     

Reproducibility of Fan-Beam DXA Measurements in Adults and Phantoms

As part of a multicenter study, we examined the intersite reproducibility of bone mineral content (BMC) and areal density (BMD) among three fan-beam dual-energy X-ray absorptiometry (DXA) instruments from one manufacturer, all using the same software version. Spine, femur, and body-composition phantoms were each scanned nine times at each center. Over a 3-wk period, the same 10 adults were scanned once at each of the three centers. For the spine and femur phantoms, the precision errors were 0.3-0.7%. For the body-composition phantom, the precision errors were 0.8-2.8%. The intersite coefficients of variation for the human measurements varied from 1.1 to 6.8%, depending on the bone site. We conclude that even when using the same fan-beam DXA model and software, an intersite cross-comparison using only phantoms may be inadequate. Comparisons based solely on the use of a spine phantom are insufficient to ensure compatibility of human bone mineral data at other bone sites or for the whole body.     

Dual X-ray Absorptiometry

The DMS Lexxos is the first cone-beam dual X-ray absorptiometry (DXA) system capable of performing bone mineral density (BMD) measurements of the spine and hip. By using a two-dimensional (2-D) detector array rather than the linear array used with conventional fan-beam DXA systems, image acquisition time on Lexxos is only 1.5 s. However, the need to correct for the large signal from scattered radiation reaching the detector is a potential source of error in cone-beam DXA. The aim of this clinical evaluation of the Lexxos bone densitometer was to investigate the relative accuracy of cone-beam BMD measurements compared with conventional DXA by performing an in vivo cross-calibration study with an established fan-beam system, the Hologic QDR-4500. Spine (L1–L4) and hip BMD measurements were performed in 135 patients (111 women, 24 men) referred for a bone densitometry examination. Duplicate Lexxos measurements were performed in 27 female patients to evaluate precision. On average, Lexxos spine and femoral neck BMD measurements were 2% lower than those on the QDR-4500, whereas total hip BMD was 5% higher. Larger differences were found for the trochanter and Wards triangle regions. For all sites, Lexxos BMD measurements showed a strong linear relationship with those measured on the QDR-4500 with correlation coefficients in the range r = 0.95 to 0.97 for the clinically important spine, femoral neck, and total hip regions. The root mean standard error (RMSE) between Lexxos and QDR-4500 BMDs ranged from 0.037 g/cm² for the femoral neck to 0.060 g/cm² for the spine, whereas Lexxos precision was 1.3% for total hip, 2.0% for femoral neck, and 2.3% for spine BMD. Although for the hip BMD sites the RMSE and precision of Lexxos measurements were similar to studies of pencil-beam and fan-beam DXA systems, the results for the spine were poorer than expected. The findings of this study suggest that Lexxos corrects accurately for the effects of scattered radiation at the detector, but that the precision of spine BMD measurements may be limited by involuntary patient movement between the high and low energy X-ray exposures.     

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     

Calibration of dual-energy x-ray absorptiometry for bone density.

Bone mineral content (BMC, g) using DEXA (Lunar DPX) was measured on known hydroxyapatite samples in a water bath in the presence of uniform and nonuniform covering of fat-equivalent materials. Selective placement of paraffin over bone had a greater effect than lard in reducing apparent BMC, and polycarbonate plastic had a lesser effect. Measured BMC was 100.1 +/- 1.1% of actual hydroxyapatite weight when (1) fat over bone was about twice the mass of hydroxyapatite, and (2) the surrounding soft tissue was 15-30% fat. There was a linear relationship between observed and expected BMC, area (cm2), and bone mineral density (BMD, g/cm2) measured on an aluminum phantom using either the Lunar DPX or the Hologic QDR-1000. The measured area with the two densitometers was identical, but BMC differed. For both an anthropomorphic phantom and human subjects, use of a constant-threshold (0.2 g/cm2) edge-detection algorithm excluded less low-density bone from the transverse processes than the standard DPX edge-detection algorithm. Differences in edge detection could influence the results obtained with phantoms and in vivo and make system intercomparison difficult.     

Axial and Total-Body Bone Densitometry Using a Narrow-Angle Fan-Beam

We assessed a new dual-energy bone densitometer, the PRODIGY, that uses a narrow-angle fan-beam (4.5°) oriented parallel to the longitudinal axis of the body (i.e., perpendicular to the usual orientation). High-resolution scans across the body can be stepped at 17 mm intervals. The energy-sensitive array detector uses cadmium zinc telluride, which allowed rapid photon counting. Spine and femur scans required 30 s, and total-body scans required 4–5 min; the dose was only 3.7 mrem and 0.04 mrem respectively, or about 5 to 10 times lower than conventional fan-beam densitometry. We found only a small influence of soft-tissue thickness on bone mineral density (BMD) results. There was also a small ( ± 1%) influence of height above the tabletop on BMD results. A software correction for object height allowed a first-order correction for the large magnification effects of position on bone mineral content (BMC) and area. Consequently, the results for BMC and area, as well as BMD, with PRODIGY corresponded closely to those obtained using the predecessor DPX densitometer, both in vitro and in vivo; there was a generally high correlation (r= 0.98–0.99) for BMD values. Spine and femur values for BMC, area and BMD averaged within 0.5% in vivo (n= 122), as did total-body BMC and BMD (n= 46). PRODIGY values for total-body lean tissue and fat also corresponded within 1% to DPX values. Regional and total-body BMD were measured with 0.5% precision in vitro and 1% precision in vivo. The new PRODIGY densitometer appears to combine the low dose and high accuracy of pencil-beam densitometry with the speed of fan-beam densitometers. Received: 2 April 1999 / Accepted: 27 July 1999     

The influence of aortic calcification on spinal bone mineral densityin vitro

We examined the influence of aortic calcification on the spine phantom bone mineral density (BMD). Soft X-ray photographs of human aortae were taken to calculate the percent calcification of aortic tissues. Human aorta laid on lumber spine phantom was placed in the bottom of 15 cm of water and BMD and bone mineral content (BMC) were measured in the anteroposterior view. Samples with severe aortic calcification (over 30%) caused a 2.5% increase of BMD. There might be a relatively small influence of aortic calcification on the value of L2-L4 BMD, but changes over time in a patient could falsely elevate values.     

DXA estimates of vertebral volumetric bone mineral density in children: potential advantages of paired posteroanterior and lateral scans.

Dual-energy X-ray absorptiometry (DXA) estimates of areal bone mineral density (BMD) are confounded by bone size in children. Two strategies have been proposed to estimate vertebral volumetric BMD: (1) bone mineral apparent density (BMAD) is based on the posteroanterior (PA) spine scan; (2) width-adjusted bone mineral density (WABMD) is based on paired PA lateral scans. The objective of this study was to compare DXA estimates of vertebral bone mineral content (BMC), volume and volumetric BMD obtained from Hologic PA scans (Hologic, Inc., Bedford, MA) alone, and paired PA lateral scans in 124 healthy children, ages 4 to 20 yr. The PA scans were used to estimate bone volume (PA Volume) as (PA Area)1.5 and BMAD as [(PA BMC)/(PA Volume)]. Paired PA lateral scans were used to estimate width-adjusted bone volume (WA Volume) as [(pi/4)(PA width)(lateral depth)(vertebral height)] and WABMD as [(lateral BMC)/(WA Volume)]. Generalized estimating equations were used to compare the relationship between scan type (PA vs. paired PA lateral) and bone outcomes, and the effects of height and maturation on this relationship. The estimates of BMC and volume derived from PA scans and paired PA lateral scans were highly correlated (r>0.97); WABMD and BMAD were less correlated (r=0.81). The increases in BMC, volume, and volumetric BMD with greater height and maturation were significantly larger (all p<0.001) when estimated from paired PA lateral scans, compared with PA scans alone. The proportion of spine BMC contained within the vertebral body, versus the cortical spinous processes, increased significantly with age (p<0.001) from 28% to 69%. The smaller increases in bone measures on PA scans may have been due to magnification error by the fan beam as posterior tissue thickness increased in taller, more mature subjects, and the distance of the vertebrae from the X-ray source increased. In conclusion, paired Hologic PA lateral scans may increase sensitivity to growth-related increases in trabecular BMC and density in the spine, with less bias due to magnification error.     

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     

Comparison of body composition measurements obtained by two fan-beam DXA instruments.

Although dual-energy X-ray absorptiometry (DXA) has been widely used for measuring body composition, discrepancies have been reported to exist among results obtained from different instruments. In the course of longitudinal studies lasting for many years, old instruments may be required to be replaced with new ones, necessitating comparison and validation between the values obtained by the old and new instruments. We compared the data obtained by the two fan-beam DXA instruments, QDR-2000 (Hologic, Waltham, MA) and Delphi (Hologic). Body composition was first measured by the Hologic QDR-2000 and next by the Delphi W within 30 days in 99 healthy subjects. Whole-body fat mass (FM), percentage of FM, arm FM, and leg FM measured by the Hologic QDR-2000 were significantly larger than those measured by the Delphi W. Lean tissue mass (LTM), bone mineral content, and bone mineral density of the whole body, trunk FM, arm LTM, and leg LTM measured by the QDR-2000 were significantly smaller than those measured by the Delphi W. After converting the QDR-2000 values by equations developed by multiple regression analysis, they were not significantly different from the corresponding Delphi values. Measurements by the QDR-2000 and the Delphi W were not interchangeable and the conversion equations reduced the discrepancy to a level that enabled direct comparison of the data obtained by the two instruments. However, cautious interpretation is necessary when the conversion equations are applied to other instruments even of the same type or when evaluating data of individual subjects.     

Fundamental evaluation of dual x-ray absorptiometry (DXA) for measurement of bone mineral density in rats

This study was designed to assess the precision and accuracy of newly developed ultra high resolution mode (rat mode) in DXA (Hologic, QDR-1000), determine how body thickness affects measured BMD values and to derive a formula by which BMDs in animals with varying degrees of body thickness can be compared. The long term reproducibility on two phantoms (BMD: 170 and 300 mg/cm²) were under CV 1.0%. The repeated precision in vivo and in vitro lumbar spines and phantoms were within CV 1.5%. Accuracy was evaluated by determining the correlation coefficient between ash weight and tibial BMC. The correlation was excellent (r=0.999, p<0.001) over the ash range of 250–600 mg. Using single regression equations, QDR-1000 BMC values were compared with those obtained by conventional methods. There was a close linear correlation with both SPA (Norland Co., r=0.997) and DCS-600 (Aloka Co., r=0.996) measurements. The effects of body thickness were assessed by immersing phantoms at various water depths. There was a significant linear decrease in BMD, as measured by QDR-1000, with increasing water depth. BMDs in vivo with varying body thickness can be compared with each other by using the following correcting formula: BMD₁=(BMD₂+5.5w+7.6) × 10³/(977-4.8w), where BMD₁=expected BMD of extracted bone (mg/cm²), BMD₂=BMD in vivo (mg/cm²), w=body thickness (cm). There was a significant positive correlation (r=0.996, p<0.001) between calculated BMDs from this equation and the values actually obtained. These results confirm that QDR-1000 rat mode yields data useful for assessing BMD and BMC in small animal bones.     

Lumbar vertebral bone mineral density in Japanese infants and children: Measurement with dual x-ray absorptiometry

Dual x-ray absorptiometry (DXA) (Hologic QDR-1000/W; Hologic, Inc.) was used to measure lumbar vertebral bone mineral density (BMD) in 83 healthy Japanese infants and children (55 boys and 28 girls) aged 0–17 years, and the values obtained were correlated with age, body weight and body height. The lumbar BMD (average of L1–L4 values) increased with age, with a nearly twofold increase found from preschool age to adolescence. It also increased with body weight and body height. Our results on normal Japanese infants and children appear almost similar to those reported in French and American studies. Because of its great precision and accuracy, low radiation exposure and rapid scanning, DXA may be the most suitable for use in infants and children. With normal Japanese data now available with this technique, pediatricians can better detect metabolic bone diseases in infants and children and follow the bone response to medical intervention in patients with these conditions.