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.     

Dual-photon Gd-153 absorptiometry of bone

Dual-photon absorptiometry with gadolinium 153 was used to measure the mineral content of lumbar vertebrae in cadavers, excised vertebrae with marrow, and dry, marrow-free vertebrae. The error introduced by the surrounding soft tissue of cadavers was 3%, and the error in determining mineral mass or density in excised vertebrae was about 5%. The correlation coefficient between the results of Gd-153 and corrected iodine 125 (single-photon) absorptiometry on 24 femoral necks was 0.99, and the predictive error was 3.7%. Dual-photon absorptiometry accurately indicates bone mass and bone density and is only slightly affected by either surrounding tissue or fat changes in bone marrow.     

Precision of dual photon absorptiometry measurements

One of the important uses of bone absorptiometry is to examine the rate of bone mineral change in order to evaluate therapy and to identify individuals who need therapy. Generally, this involves comparing the difference between two scans obtained months to years apart. This study investigates the precision of dual photon absorptiometry using a human torso phantom, normal subjects, and abnormal patients. These studies showed that bone mineral calculated as g/cm2 was more precise than g/cm. Reanalysis of the same scan by the same individual produced an average error equivalent to that produced by scanning and analyzing the same subject on multiple occasions. Interobserver analysis error was essentially equal to the intraobserver error. In order to obtain maximum precision, care must be taken that the integrated area of a repeat scan is identical to the previous scan. Our findings indicate that to be confident (95%) of a real change between two scans a difference of at least 5.6% must be measured.     

Emerging trends in dual energy X-ray absorptiometry

The risk of osteoporosis and cardiovascular disease increase significantly with age. Because of the long silent latency of these diseases, there is the opportunity for primary prevention before clinical symptoms occur. The newer fan beam dual energy x-ray absorptiometry (DXA) systems can detect vertebral fractures with fast, low dose lateral scans of the vertabrae from T4 to L4 in as little as 10 s. The next generation of DXA imaging technology will image the hip in 3D and is expected to give a more accurate picture of bone density as well as the geometry and underlying strength of a bone. Visualizing abdominal aortic calcifications (AAC) as a part of a standard vertebral fracture assessment exam is a particularly valuable measurement since it is an independent measure of cardiovascular disease risk, including heart attacks and strokes.     

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.     

Accuracy of dual photon absorptiometry in excised femurs

We investigated the accuracy of assessment of bone mineral content (BMC) by dual photon absorptiometry (DPA). Measurements were compared between BMC and ashed weight using two related scanners. The BMC in different locations of the femur was determined. Twelve cadaver femurs were cleaned of all soft tissue, divided into four parts (head, neck, trochanteric region, and shaft), and measured for BMC in an ethanol/water solution. The bones were then ashed and weighed. Volumetric density was also determined. The correlation coefficient between ash weight and BMC was 0.99 with an s.e.e. of 0.51 g and relative error of 4.8%. Similar correlations were seen within each region. The correlation between the machines was 0.99. Differences in volumetric density were found, with the density of the shaft greater than other regions, and the neck greater than the head or trochanteric regions.     

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.