Dual-energy X-ray absorptiometry in clinical practice: application and interpretation of scans beyond the numbers

Dual-energy X-ray absorptiometry (DXA) is widely used for measurement of bone mineral density (BMD). In routine clinical practice, a diversity of DXA scan findings that deviate from anticipated anatomical landmarks and require a sophisticated approach to patient positioning and scan analysis may be identified. Recognition of various artifacts and pathologic processes that can falsely increase the measured BMD is essential to accurate DXA scan analysis. Critical evaluation of the DXA scan image, and careful appraisal of numeric data on the computer-generated printout by clinicians and radiology technologists are instrumental to ensure correct DXA scan interpretation. We demonstrate a spectrum of incidental findings that may influence BMD measurements and analyze common pitfalls of DXA scan interpretation. Radiographic correlation can help further evaluate a suspected abnormality displayed on the DXA image.     

Dual-energy scanned projection radiography of osseous metastatic disease

Dual-energy scanned projection radiography was used to evaluate eight patients with both lytic and blastic metastatic disease in the axial skeleton. The ability to selectively cancel obscuring soft-tissue structures from images resulted in improved conspicuity of involved sites, compared with conventional radiographic studies, despite greater quantum noise and lower inherent spatial resolution. Other desirable features of the technique include a projected format, image enhancement by contrast and brightness optimization, rapid data acquisition, convenient image storage and retrieval, and low radiation dose. Since the majority of osseous metastases involve axial sites, dual-energy scanned projection radiography may be a useful adjunctive modality in the management of oncology patients.     

Dual-energy X-ray absorptiometry applied to the assessment of tibial subchondral bone mineral density in osteoarthritis of the knee

Objective Plain X-ray is an imprecise tool for monitoring the subchondral bony changes associated with the development of knee osteoarthritis (OA). Our objective was to develop and validate a technique for assessing tibial subchondral bone density (BMD) in knee OA using dual energy X-ray absorptiometry (DXA).Design Patients with OA of at least one knee underwent DXA scanning of both knees. Regions of interest (ROI) were placed in the lateral and medial compartments of tibial subchondral bone. Weight-bearing plain X-rays and Te ^99m scintiscans of both knees were obtained and scored.Results One hundred and twelve patients (223 knees) underwent DXA and radiography. Intra-observer CV% was 2.4% and 1.0% for the medial and lateral ROI respectively. Definite OA (Kellgren and Lawrence Grade 2, 3 or 4) was correlated with age-related preservation of subchondral BMD compared to radiographically normal knees. Raised BMD was also associated with subchondral sclerosis, and positive scintigraphy.Conclusion DXA may provide a safe, rapid and reliable means of assessing knee OA. Cross-sectional age-related subchondral tibial BMD loss is attenuated by knee OA.Work completed at Bristol Royal Infirmary, Bristol, BS2 8HW, UK     

Dual-energy x-ray absorptiometry in the diagnosis of osteoporosis: a practical guide

OBJECTIVE: The purpose of this essay is a quick and comprehensive review of dual-energy x-ray absorptiometry in the diagnosis of osteoporosis that shows how to achieve the best performance in three steps. CONCLUSION: The three-step procedure for dual-energy x-ray absorptiometry includes image acquisition emphasizing proper patient positioning and regions of interest; analysis, including areas to scan and detection of artifacts that should be excluded from the analysis and noted in the report because they can necessitate additional imaging; and interpretation of results.     

Quantitation in planar renal scintigraphy: which µ value should be used?

The attenuation coefficient value μ used by different authors for quantitation in planar renal scintigraphy varies greatly, from the theoretical value of 0.153 cm^–1 (appropriate for scatter-free data) down to 0.099 cm^–1 (empirical value assumed to compensate for both scatter and attenuation). For a 6-cm-deep kidney, such variations introduce up to 30% differences in absolute measurement of kidney activity. Using technetium-99m phantom studies, we determined the μ values that would yield accurate kidney activity quantitation for different energy windows corresponding to different amounts of scatter, and when using different image analysis approaches similar to those used in renal quantitation. With the 20% energy window, it was found that the μ value was strongly dependent on the size of the region of interest (ROI) and on whether background subtraction was performed: the μ value thus varied from 0.119 cm^–1 (loose ROI, no background subtraction) to 0.150 cm^–1 (kidney ROI and background subtraction). When using data from an energy window that could be considered scatter-free, the μ value became almost independent of the image analysis scheme. It is concluded that: (1) when performing background subtraction, which implicitly reduces the effect of scatter, the μ value to be used for accurate quantitation is close to the theoretical μ value; (2) if the acquired data were initially corrected for scatter, the appropriate μ value would then be the theoretical μ value, whatever the image analysis scheme. Received 15 July and in revised form 25 August 1999     

Dual-energy x-ray absorptiometry: a precise method of measuring bone mineral density in the lumbar spine

We compared two methods of measuring spinal bone mineral content and density (BMC/BMD): conventional dual-photon absorptiometry (DPA) and a more recent method, dual-energy x-ray absorptiometry (DEXA). The clinical usefulness of both methods was compared in the measurement of BMC in the forearm. DEXA had a long-term in vivo precision of 1% which was significantly better than that of DPA. Changes in the distribution of fatty tissue influenced the accuracy of the two spinal methods in different ways. Forearm BMC discriminated between the bone mass of early and late postmenopausal women to the same degree as DPA and DEXA. The variability in the response to estrogen treatment and placebo was much lower with DEXA and forearm BMC than with DPA. We conclude that DEXA provides a fast and precise measurement of spinal BMC/BMD. The accuracy remains to be evaluated for in vivo studies.     

Measures of body composition via Dual-energy X-ray absorptiometry,ultrasound and skinfolds are not impacted by the menstrual cycle in active eumenorrheic females

Objectives(1) Compare changes in body composition estimates over the menstrual cycle in active females using Dual-energy X-ray absorptiometry, standardised brightness-mode ultrasound and skinfolds (2) Compare the predictability of Dual-energy X-ray absorptiometry fat mass estimate via standardised brightness-mode ultrasound versus skinfolds measurements.DesignThirty active females (27 ± 5 y) with regularly occurring menstrual cycles participated in a cross sectional study.MethodsParticipants completed four assessment sessions scheduled according to each individual's menstrual cycle. These sessions took place during their (1) early follicular, (2) mid-to-late follicular, (3) mid-luteal and (4) second early follicular phases. Body composition estimates were acquired using Dual-energy X-ray absorptiometry, subcutaneous adipose tissue thickness was measured at eight sites using standardised brightness-mode ultrasound and skinfolds.ResultsThe sum of eight subcutaneous adipose tissue thickness measured using standardised brightness-mode ultrasound and skinfolds were not different between the cycle phases (p > 0.05). Body mass and Dual-energy X-ray absorptiometry total mass estimate as well as Dual-energy X-ray absorptiometry estimates of total and regional lean and fat mass were also not different between cycle phases (p > 0.05) and any changes were within the 95% confidence intervals of their respective least significant change values.ConclusionsThere were no true and meaningful changes in the sum of eight subcutaneous adipose tissue thickness measured via standardised brightness-mode ultrasound and skinfolds or Dual-energy X-ray absorptiometry total and regional tissue mass estimates across the menstrual cycle in active eumenorrheic females. Body composition may thus be assessed via these methods in this population at any cycle phase with standardised participant presentation.     

Bone mineral measurement of phalanges: comparison of radiographic absorptiometry and area dual X-ray absorptiometry

With a standard, image-intensifier-based, digital radiographic system, high-spatial-resolution images of the hand were acquired for analysis of phalangeal bone mineral density with dual x-ray absorptiometry (DXA). Results with phalangeal DXA had precision of plus or minus 0.67% and accuracy of 4.1% and correlated well with those with radiographic absorptiometry. This phalangeal DXA technique is potentially useful for clinical diagnosis of osteoporosis.     

Measurement of breast density with dual X-ray absorptiometry: feasibility

Dual x-ray absorptiometry (DXA) was used to quantify breast density with a phantom and with cadaveric breasts. With DXA, percentage of fat correlated with percentage of glandular density of the phantom (r > 0.998) and with density at mammography (r(adjusted) = 0.83). DXA precision (SD) was 0.5% without and 1.1% with breast repositioning. DXA devices can be used to accurately and precisely estimate breast tissue density.     

水平骶椎的临床X线分析

目的分析水平骶椎的影像表现。方法12例临床怀疑水平骶椎患者(男2例,女10例;年龄23～25岁)均摄腰骶椎正侧位片,并在患者直立侧位片上,利用CR后处理功能测量第一骶椎上面与水平线所交的角。其中,3例又经CT检查。结果本组12例中,腰骶角在45～55°之间5例,腰骶角在56～65°之间6例,腰骶角在66°以上1例。腰骶正位片示骶骨投影变短,结构重叠,腰骶侧位片示腰骶部极度侧弯。CT片上有椎间盘突出2例,椎间盘膨出3例,以及椎骨狭窄2例。结论侧位片上腰骶部极度侧弯和腰骶角大于45°是水平骶椎的特征性X线表现。CT扫描可发现一些继发病变。     

X-ray dual-photon absorptiometry: a new method for the measurement of bone density

A recently introduced method (dual-photon X-ray absorptiometry, DEXA) capable of measuring skeletal density in man (at present in the spine and hips, but ultimately for the whole body) has been evaluated in terms of its ability to perform long-term assessment of bone density changes. The method, which uses X rays rather than gamma rays as its photon source, represents a significant improvement over present systems both in image quality and precision (reproducibility) of results, which is better than 1% in vivo. Scanning time is approximately halved compared with present techniques and the radiation dose is reduced by 25%. First data on long-term drift of results and effects of changes in patient composition (i.e. thickness and fat content) are given and show the new method to be superior to present radionuclide systems. It is likely that this new method will become the standard for bone density measurements.     

Development of a phantom for morphometric X-ray absorptiometry

Morphometric X-ray absorptiometry (MXA) has recently been developed to assess vertebral deformity status using dual energy X-ray absorptiometry (DXA) machines. In contrast to bone densitometry, a vertebral morphometry phantom is not supplied by any machine manufacturer. The aim of this study was to develop a suitable phantom to quantify the accuracy and precision of the vertebral measurement software on three DXA scanners in vitro and to perform a weekly quality control (QC) scan over a 30-month period to evaluate any drift or changes in measurement accuracy over time. The phantom was constructed from Perspex and aluminium to simulate soft tissue and bone, respectively. 13 aluminium rectangles (each 30 mm wide, 25 mm high and 3 mm thick, with edges ("endplates") 6 mm thick) were set into one side of a solid Perspex block to represent the vertebral bodies from the fourth thoracic (T4) to the fourth lumbar (L4). The phantom was scanned on both the Hologic QDR2000plus and the QDR-4500A as well as the Lunar Expert-XL. Three consecutive lateral MXA scans were acquired on the Hologic machines using each of the scan modes available. On the QDR-2000plus, the lateral scan modes available are fast, array and high definition, which are all dual energy modes. These three scan modes are also available on the QDR-4500A, with the addition of a single energy scan mode. Four lateral scans were acquired on the Expert-XL machine using the single scan mode available. Each MXA scan was analysed twice by a trained operator using the standard software supplied by each manufacturer. A QC scan was performed approximately weekly over a 30-month period on only the QDR-4500A machine, and total phantom height was measured from the inferior edge of L4 to the superior edge of T4. Accuracy of "vertebral" height measurement varied between the three DXA machines and between the scan modes available. All underestimated "true" vertebral height by between 0.4% and 8.6%, with the scan modes using finer collimation producing the most accurate results. Repeat analysis precision of vertebral height measurement was best on the QDR-4500A, followed by the Expert-XL, and was poorest on the QDR-2000plus. The QC scans acquired on the QDR-4500A suggested that it was a highly stable machine, little affected by even major repairs. It must be remembered that these in vitro phantom results may not be representative of the true in vivo situation. The MXA phantom appears to be a useful tool for documenting the stability of the mechanical instruments and for checking the long-term consistency of operator precision.     

The clinical role of dual energy X-ray absorptiometry

Dual energy X-ray absorptiometry (DXA) measurements of hip and spine bone mineral density (BMD) have an important role in the evaluation of individuals at risk of osteoporosis, and in helping clinicians advise patients about the appropriate use of anti-fracture treatment. Compared with alternative bone densitometry techniques, hip and spine DXA examinations have a number of advantages that include a consensus that BMD results can be interpreted using the World Health Organisation (WHO) T-score definition of osteoporosis, a proven ability to predict fracture risk, proven effectiveness at targeting anti-fracture therapies, and the ability to monitor response to treatment. This review discusses the evidence for these and other clinical aspects of DXA scanning, including its role in the new WHO algorithm for treating patients on the basis of their individual fracture risk.