Comparison of total-body measurements by dual-energy X-ray absorptiometry and dual-photon absorptiometry

Both dual-photon absorptiometry (DPA) using 153Gd and dual-energy x-ray absorptiometry (DEXA) can be used for measurement of bone mineral content (BMC) and bone mineral density (BMD) of the total skeleton and its seven major regions. The short-term precision (coefficient of variation, CV) of DEXA for total-body BMD using the medium (20 minute) and fast (10 minute) speeds was 0.34 and 0.68% in 5 normal subjects; the corresponding CV in 5 osteoporotic females were 0.70 and 1.04%. The CV for BMD using DPA was 0.82% in 8 normal subjects and 0.70% in 12 osteoporotic patients. The CV for regional BMD using DPA was similar to fast-speed DEXA, without significant differences (p NS); precision with medium-speed DEXA was superior to DPA, and the differences were statistically significant (p less than 0.05) for head, spine, trunk, ribs, and pelvis. Total-body measurements using both DPA and DEXA were done on 99 subjects (84 females and 15 males). Significant correlations (r = 0.98; p less than 0.001) were found between DEXA and DPA measurements of both BMC and BMD. There were also significant correlations (r = 0.94-0.98; p less than 0.001) between DEXA and DPA measurements of anatomic regions (head, trunk, spine, pelvis, ribs, arms, and legs). DPA and DEXA results for BMD of total skeleton, ribs, pelvis, and legs were similar (p NS), and statistically significant differences were found in head, spine, and arm measurements (p less than 0.01, p less than 0.01, and p less than 0.05, respectively); regression equations allowed adjustment of DEXA values in patients already measured with the earlier DPA method.     

Influence of patient's weight on dual-photon absorptiometry and dual-energy X-ray absorptiometry measurements of bone mineral density

Lumbar spine bone mineral density (BMD) was measured by dual-energy X-ray absorptiometry (DXA) (Hologic QDR 1000) and by¹⁵³Gd dual-photon absorptiometry (DPA) (Novo Lab 22a) in 120 postmenopausal women. Though a high correlation existed between the two techniques, the ratio between DXA and DPA values was not constant. Using DXA we observed a higher dependence of BMD on weight than in the DPA measurements. To investigate the different behaviour of DXA and DPA machines with weight, we analysed the effects of increasing thickness of soft tissue equivalents on the BMD of the Hologic spine phantom and on the BMD equivalent of an aluminium standard tube. Increasing tissue-equivalent thickness caused the phantom BMD measured by DPA to decrease significantly but had not effect on the DXA measurements. The different behaviour of DPA and DXA equipment with regard to the phantoms could account for the differences observed in the relations between BMD and weight in the patients. Using multiple regression we studied the influence of weight and body mass index on the relation between BMD measured by the two techniques. The introduction of either of these variables into the regression resulted in an improvement of the prediction of the DXA values from the DPA values. However, the residual standard error of the estimate was still higher than the combined precision errors of the two methods, so that no simple relation allows a conversion of BMD_DPA into BMD_DXA. Our results confirm that BMD is positively correlated with weight in postmenopausal women; the influence of weight on BMD is blunted when the Novo Lab 22a DPA machine is used for measuring bone mineral.     

Effects of radioisotopes on the accuracy of dual-energy X-ray absorptiometry for bone densitometry.

Nuclear medicine procedures are often performed in close-time proximity to bone densitometry studies. The purpose of this study was to determine the effects of various radioisotopes on the accuracy of bone mineral density (BMD) measurements performed using dual-energy x-ray absorptiometry (DEXA) systems. We evaluated two DEXA scanners: the Hologic QDR4500 and the Lunar Prodigy. The effects of various activities of Tc-99m, Tl-201, Ga-67 and I-131 on BMD were assessed by placing vials or syringes containing the appropriate isotope above or below a simulated spine (average BMD = 1.1 g/cm2) embedded in a lucite block. We also placed a spine phantom (average BMD = 2.0 g/cm2) in a water bath containing various concentrations of Tc-99m. Maximum activities evaluated were as follows: Tc-99m, 80 mCi; I-131, 50 mCi; Tl-201, 66 mCi; Ga-67, 20 mCi. For the Hologic QDR4500 system, irrespective of the radioisotope or activity, we found no significant effect of adjacent activity on measured BMD on this system. For the Lunar Prodigy system, the effects of adjacent activity on BMD were found to be dependent on source location, strength, and radioisotope. For sources placed beneath the solid lucite phantom, BMD decreased by approx 0.5%/10 mCi of activity for all isotopes. In general, for sources placed above the lucite phantom, the BMD decreased by 1.6-4.0%/10 mCi of activity, depending on location. The exception was Tl-201, where BMD increased by 0.5-2.5%/10 mCi, depending on location. With the high-density spine in the water phantom, the effects of adjacent activity were more pronounced than in the standard density spine in the lucite block. For a distributed Tc-99m source, the BMD decreased by 1.7%/10 mCi. The effect of radioactivity on DEXA measurements is system dependent. In general, adjacent activity results in a reduction in apparent BMD. The magnitude of the effect increases with increasing BMD and is dependent on the location of the activity.     

Precision of dual-photon absorptiometry

Summary Precision of dual-photon absorptiometry (DPA) measurements was determined in a lumbar spine phantom and in humans. Approximately half of the measurements were made before and half after a¹⁵³gadolinium source change. The phantom was measured with different amounts of acrylic, which simulates human soft tissue, in order to evaluate the influence of body thickness on bone mineral density (BMD). Results of scans analyzed with two software versions from Lunar Radiation Corp., the widely used 08B and a prototype 08C, are compared. DPA with a cold source significantly overestimated BMD in the phantom in the presence of large amounts (more than 25 cm) of soft tissue equivalent with version 08B but not with the newer version 08C. Similiarly, in nine subjects, there was a significant decrease in spine BMD after a source change when scans were analyzed with version 08B (mean difference 0.026 g/cm²,P=0.002) but not with 08C (0.01 g/cm²,P=0.234). No systematic effect of source change on femoral BMD measurements was observed. The SD of the mean difference of two measurements of the nine subjects was 0.019 g/cm² (1.6% of the mean value) for the spine with software version 08B and 0.024 g/cm² (2.0%) with version 08C, 0.03 g/cm² (3.3%) for the femur neck, 0.03 g/cm² (4.0%) for the greater trochanter, and 0.04 g/cm² (4.9%) for Ward's triangle region of the proximal femur. The spine phanton was scanned on two other commercial bone densitometers in order to assess inter-instrument variation. Phantom measurements of L2-4 BMD made on two Lunar Radiation Corp model DP3 scanners which differed by 2% were 10 and 12% higher than those with a Norland Corp. model 2600 scanner.     

Densitometry of the radius using single and dual energy absorptiometry

Though spinal and femoral measurements are typically preferred for evaluating skeletal density, an abundance of forearm data exists, primarily from single photon absorptiometry (SPA) devices. Most dual X-ray absorptiometry (DXA) scanners are capable of scanning the forearm and provide analysis tools to duplicate conventional SPA measurements. In this study, we have compared the radius density measurements from three commonly available densitometers: a Norland 278 SPA, a Lunar DPX-L, and a Hologic 1000/W. Radius bone mineral density (BMD) on the nondominant forearm was measured in 28 volunteers (21 women and 7 men) aged 24–78, with an average age of 51±17 years. Values were compared and regression relationships derived at corresponding measurement sites. SPA and DXA BMD values were found to be highly correlated (r=0.99) with small standard errors (0.014 g/cm²–0.021 g/cm²), though significant absolute differences were observed at most measurement regions. Correlation slopes ranged from 0.85 to 1.04, with intercepts from 0.01 to 0.08 g/cm². Using the resultant regression equations, SPA BMD values can be converted to DXA values with an expected error of roughly 3%. DXA BMD can also be interconverted between Lunar and Hologic with a similar expected error. In situations where this level of imprecision is acceptable, patient forearm measurements obtained on different systems can be interconverted.     

Dual-energy x-ray absorptiometry of the forearm: reproducibility and correlation with single-photon absorptiometry.

Although single-photon absorptiometry (SPA) has been the predominant tool used to assess bone mineral density (BMD) in the forearm, the development of dual-energy x-ray absorptiometry (DEXA) provides the benefits of greater source stability, reduced scanning time, and improved image resolution compared to SPA. In the present study we used the DEXA bone densitometer (Hologic, Inc., Waltham, MA) to (1) measure BMD in the one-third radius and ultradistal radius; (2) examine the reproducibility of these BMD measurements; and (3) compare the BMD at the one-third radius with SPA (SP2, Lunar Corp., Madison, WI). In 65 normal women (ages 22-74 years) we examined changes in the forearm DEXA BMD with age, revealing significant quadratic regression equations. The reproducibility of DEXA BMD (mean +/- SEM) in 7 normal subjects aged 22-50 years is 0.85 +/- 0.16% for the predominantly cortical one-third radius site and 0.97 +/- 0.15% for the more trabecular ultradistal site. The regression relationship between DEXA and SPA of the one-third radius in 26 subjects (ages 22-68 years) is DEXA BMD = 0.105 + 0.826 (SPA BMD); R = 0.97, R2 = 0.94, p less than 0.0001. Bone densitometry of the forearm using DEXA may be performed relatively rapidly, providing reproducibility and image resolution that are generally superior to those observed with SPA.     

Measurement of bone by dual-photon absorptiometry (DPA) and dual-energy X-ray absorptiometry (DEXA)

Bone densitometry is essential for (a) confirming a diagnosis of osteoporosis, (b) determining the degree of osteopenia and risk of fracture, and (c) monitoring the response of bone to therapeutic agents. Fracture risk at specific axial fracture sites (spine, proximal femur), is associated directly with bone mineral density (BMD) at these sites. ROC analysis demonstrates that the diagnostic sensitivity of spine and femur BMD for spine and/or femur fracture is substantially superior to BMD of appendicular sites in the immediate postmenopausal period. Femoral neck BMD affords high diagnostic sensitivity for proximal femur fracture even in the elderly. Recent prospective studies have shown that bone densitometry can predict future fractures in postmenopausal women. Conventional DPA with 153Gd provides high accuracy for total body, spine, and femur BMD with adequate clinical precision of 1%, 2% and 3%, respectively. Dual-energy x-ray absorptiometry (DEXA), using either switched kVp or by k-edge filtering, offers better precision; typically the precision error is halved. The higher flux available from x-ray sources provides other advantages over DPA, including: improved spatial resolution (2 vs 4 mm), reduced radiation exposure (1 vs 2 mrem), and decreased scan times (3 to 10X). Improved DPA systems, with automatic gain stabilization to minimize drift, could offer clinical precision comparable to DEXA but the scan time and spatial resolution remain as before. Both DPA and DEXA allow detection of therapeutic efficacy in individual patients over the first year or two of therapy.     

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 dual-photon absorptiometry systems for total-body bone and soft tissue measurements: dual-energy X-rays versus gadolinium 153.

A total of 81 subjects (41 males and 40 females) were scanned by dual-photon absorptiometry by 153Gd source (DPA; Lunar DP4) and by dual-energy x-ray absorptiometry (DEXA; Lunar-DPX) within a 24 h period. Total-body bone mineral density (TBMD), calcium content (Ca), and soft tissue mass (ST) were determined with a precision of about 1-1.5% using DPA and 0.5-1.0% using DEXA. Measurements of TBMD, Ca, ST, bone area (area), percentage fat, and regional bone mineral densities (BMD) were compared. Paired t-tests showed small but significant differences between all measurements. Correlations (r) for TBMD, Ca, area, ST, percentage fat, arm BMD, leg BMD, and trunk BMD were 0.99, 0.99, 0.97, 0.99, 0.97, 0.99, 0.99, and 0.98. There were small systematic differences for TBMD (less than 1%), calcium (3%), bone area (3%), soft tissue mass (7%), and percentage fat (9%) between the two approaches. Regression equations are given relating these measurements.     

Sensitivity of dual-photon absorptiometry in spinal osteoporosis

Summary Lumbar spine bone mass and density were measured with Dual photon absorptiometry (DPA) in 60 patients with crush fractures and 60 age-matched normal women. Short-term reproducibility of bone mineral density (BMD) was 1.3% in normal women and 2.5% in osteoporotic women; long-term reproducibility in normal women was 2.2%. The reproducibility of bone mineral content (BMC) seemed to be poorer than that of BMD. In this study, aortic calcifications had no effect on BMD, and one or two crush fractures in the L2–L4 region increased BMD by an average of 3% (0–10%). Lumbar spine DPA provided high sensitivity for these younger crush fracture osteoporotic patients (x=65 years). The sensitivity at 95% specificity was 74% for BMD and 73% for BMC. This sensitivity is substantially better than that reported for DPA instruments giving higher variances or for quantitative computed tomography.     

Dual-energy X-ray absorptiometry versus single photon absorptiometry of the radius

Summary Radial diaphyseal bone mineral density (BMD) was measured at the standard one-third site by dual-energy X-ray absorptiometry (DEXA) and by¹²⁵I single photon absorptiometry (SPA) in 70 consecutive subjects, aged 12–86 years, with metabolic disorders of the skeleton. Each patient was measured once by the DEXA (Hologic QDR-1000) instrument and four times by the SPA (Norland 2780) instrument on the same day by one or the other of 2 technicians. The DEXA and SPA measurements were linearly related and highly correlated (r=0.975,P<0.0001) over a range from severe osteopenia to high normal BMD. Ninety-five percent of the variation in the BMD determined by SPA was accounted for by DEXA, so that the BMD_SPA=1.035±0.027 (SEM)×BMD_DEXA−0.007±0.019 (SEM). This permits continued use of previously accumulated SPA databases. The coefficient of variation for repeat measurements by DEXA was 1.2% and by SPA 1.6%. Examination time by DEXA was 6–7 minutes, about 45% shorter than the corresponding SPA determinations. DEXA is the superior method for evaluation of the radius, as it provides faster and more precise measurements in clinical practice.     

Rapid small-animal dual-energy X-ray absorptiometry using digital radiography.

Although dual-energy X-ray absorptiometry (DEXA) is an established technique for clinical assessment of areal bone mineral density (BMD), the spatial resolution, signal-to-noise ratio, scan time, and availability of clinical DEXA systems may be limiting factors for small-animal investigations using a large number of specimens. To avoid these limitations, we have implemented a clinical digital radiography system to perform rapid area DEXA analysis on in vitro rat bone specimens. A crossed step-wedge (comprised of epoxy-based materials that mimic the radiographic properties of tissue and bone) was used to calibrate the system. Digital radiographs of bone specimens (pelvis, spine, femur, and tibia from sham-ovariectomized [SHAM] and ovariectomized [OVX] rats) were obtained at 40 kilovolt peak (kVp) and 125 kVp, and the resulting areal BMD values were compared with those obtained with a clinical fan-beam DEXA system (Hologics QDR 4500). Our investigation indicates that the cross-wedge calibrated (CWC) DEXA technique provides high-precision measurements of bone mineral content (BMC; CV = 0.6%) and BMD (CV = 0.8%) within a short acquisition time (<30 s). Areal BMD measurements reported by the CWC-DEXA system are within 8.5% of those reported by a clinical fan-beam scanner, and BMC values are within 5% of the known value of test specimens. In an in vivo application, the CWC-DEXA system is capable of reporting significant differences between study groups (SHAM and OVX) that are not reported by a clinical fan-beam DEXA system, because of the reduced variance and improved object segmentation provided by the CWC-DEXA system.     

Precision and stability of dual-energy X-ray absorptiometry measurements

Summary This study was performed to determine the precision and stability of dual-energy X-ray absorptiometry (DEXA) measurements, to compare bone mineral density (BMD) of subjects measured by DEXA and radionuclide dual-photon absorptiometry (DPA), and to evaluate different absorber materials for use with an external standard. Short-term precision (% coefficient of variation, CV) was determined in 6 subjects scanned six times each with repositioning, initially and 9 months later. Mean CV was 1.04% for spine and 2.13% for femoral neck BMD; for whole-body measurements in 5 subjects, mean CV was 0.64% for BMD, 2.2% for fat, and 1.05% for lean body mass. Precision of aluminum phantom measurements made over a 9-month period was 0.89% with the phantom in 15.2 cm, 0.88% in 20.3 cm, and 1.42% in 27.9 cm of water. In 51 subjects, BMD by DEXA and DPA was correlated for the spine (r=0.98,P=0.000) and femoral neck (r=0.91,P=0.000). Spine BMD was 4.5% lower and femoral neck BMD 3.1% higher by DEXA than by DPA. An aluminum phantom was scanned repeatedly, in both water and in an oil/water (30∶70) mixture at thicknesses ranging from 15.2 through 27.9 cm. Phantom BMD was lower at 15.2 cm than at higher thicknesses of both water and oil/water (P=0.05, ANOVA). The phantom was scanned repeatedly in 15.2, 20.3, and 27.9 cm of water over a 9 month period. In 15.2 and 20.3 cm of water, phantom BMD did not vary significantly whereas in 27.9 cm of water (equivalent to a human over 30 cm thick), phantom BMD increased 2.3% (P=0.01) over the 9 months.     

Cross calibrated standard values of vertebral bone mineral density in Japanese

The reference value of L2-4 bone mineral density (BMD) was calculated in Japanese population by using cross-calibration formulae among the different types of dual energy X-ray absorptiometry (DEXA) and dual photon absorptiometry (DPA). The scan data of lumbar spine were collected from 16 institutes equipped 3 types of DEXA and 3 types of DPA. After subtraction of inadequate scan data, a total of 1038 cases (376 males and 662 females) were accepted to be a representative “normal” population of Japanese. The L2-4 BMD values were adjusted to one equipment by using cross-calibration equations. There were significant decrease in L2-4 BMD in both sexes at age of seventies or more (9.5% decrease in males and 27.5% in females from the peak bone density).     

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.     

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.     

Precision and sensitivity of dual-energy x-ray absorptiometry in spinal osteoporosis.

This study evaluated the performance of dual-energy x-ray absorptiometry (DEXA) with regard to (1) the correlation with dual-photon absorptiometry (DPA), (2) the ability to discriminate between normal and osteoporotic patients, and (3) long-term reproducibility. The bone mineral density (BMD) of the spine in 112 subjects, both normal and osteoporotic, was measured with DPA and DEXA (Lunar Corporation, Madison, Wisconsin) of the spine. The femur BMD of 22 cases was also measured with both machines. The results for the two techniques were highly correlated (r greater than 0.9, SEM = 0.02 to 0.04 g/cm2). BMD was measured using DEXA in 80 women (mean age = 61 years) with established spinal osteoporosis and 110 normal age-matched controls. The osteoporotic patients had significantly reduced spine and femur BMDs compared to the controls: -23% for L2-4 BMD (Z score = 2.6) and -13 to -20% for femur BMD (Z score = 1.1-1.3). L2-4 BMD had the best discriminative value, with an area under the ROC curve of 94%; the Ward's triangle BMD had an area of 84%. The precision error in vitro in a phantom over a 1-year period was 0.7%. The measured precision in vivo with young adults was approximately 1% (SD = 0.012 g/cm2) for L2-4 BMD and 1.7-2.3% (SD = 0.015-0.022 g/cm2) for femur over the 1 year period. The reproducibility was not as good for osteoporotic patients (SD = 0.017 g/cm2).     

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.     

Reproducibility of lateral spine scans using dual energy X-ray absorptiometry

Summary Reproducibility of lateral spine dual energy X-ray absorptiometry (LAT DEXA) scans using a Lunar DPX-L scanner was assessed in a cadaveric phantom and in patients. One hundred phantom measurements over 7 months demonstrated a longitudinal stability of 1.7% (coefficient of variation, CV). Additional scans were performed with the phantom rotated by up to 20° in each of the three orthogonal planes to assess the effects of variable patient positioning. Horizontal and vertical rotation of the spine had little effect on the estimated bone mineral density (BMD), however, axial rotation of greater than 8° led to errors in the BMD measurement. One hundred consecutive patients had two lateral scans performed within 1 month. BMD (range 0.10–1.6 g/cm²) was determined for each scan by one operator. Significant overlap from ribs and pelvis was often seen with L2 and L4 vertebrae but one vertebra (L3) could be measured in every case. Intraoperator and interoperator variability was assessed by three experienced operators, each analyzing 10 patients' scans on five separate occasions, and was found to be less than 1.1% for a single vertebra. BMD estimation of vertebral bodies and midslices by lateral DEXA scans (CV% of 3.8% and 4.6%) have a 95% confidence interval of 0.074 g/cm² and 0.096 g/cm², respectively for two vertebrae. This variability is due mainly to axial rotation, with operator variability, horizontal rotation, and vertical rotation having little effect on BMD estimation.     

An evaluation of dual-energy X-ray absorptiometry and comparison with dual-photon absorptiometry

Dual-photon absorptiometry (DPA) is a well-established procedure for measuring bone mineral density (BMD). Recently, dual-energy X-ray absorptiomery (DXA) has become available, which has the ability to measure BMD both regionally and in the total body (TB). We have evaluated the in vivo and in vitro precision of a DXA instrument and compared it with a DPA instrument with similar software characteristics.The short-term precision of BMD measurements using DXA was assessed in 65 postmenopausal women who had duplicate scans performed, with repositioning between scans. Precision was 0.9% in the lumbar spine and 1.4% in the femoral neck.The midterm precision of DXA was compared with DPA by scanning 10 volunteers a mean of four times over 24 weeks, on both instruments. The precision of the bone mineral content (BMC) and area measurements was significantly better (P<0.05) with DXA than with DPA. Long-term in vitro precision was assessed by scanning an aluminium spine phantom over 42 weeks, and a cadaveric sample over 52 weeks, on both instruments. Precision was similar using the aluminium phantom, but was significantly improved (P<0.001) when using DXA for scanning the cadaveric sample.Highly significant correlations (allP<0.001) of BMD, BMC and area measurements were observed when 70 volunteers were scanned on both instruments. However, there was a systematic difference in BMD values between the instruments. The precision of TB composition measurements assessed in 16 volunteers, over a 16-week period, were TB BMD 0.65%, TB lean tissue 1.47%, and TB fat tissue 2.73%. The correlation between weight measured by electronic scales and TB mass as measured by DXA, which was assessed in 70 volunteers, was excellent (r=0.99,p<0.001).We conclude that DXA offers improvements in measuring BMD over DPA in terms of faster scanning times and improved resolution, resulting in better precision, with the additional advantage of the ability to measure TB composition with high precision.