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.

Vertebral morphometry: a comparison of long-term precision of morphometric X-ray absorptiometry and morphometric radiography in normal and osteoporotic subjects.

Vertebral morphometry, the quantification of vertebral body shape, has proved a useful tool in the identification and evaluation of osteoporotic vertebral deformities in both epidemiologic surveys and clinical trials. Although conventionally it has been performed on lateral radiographs of the thoracolumbar spine (morphometric radiography, MRX), it may now be accomplished on morphometric X-ray absorptiometry (MXA) scans, acquired on dual-energy X-ray absorptiometry (DXA) machines. In this study the long-term precision of vertebral height measurement using MXA and MRX was directly compared. Initially 24 postmenopausal women were recruited (mean age 67+/-5.8 years): 12 normal subjects (group 1) and 12 with osteoporosis and known vertebral deformities (group 2). Each subject attended for a baseline visit at which they had a MXA examination and lateral thoracic and lumbar radiographs. Twenty-one subjects then returned 1.7+/-0.4 years later (10 subjects from group 1 and 11 from group 2) for a follow-up visit to repeat both the MXA scans and conventional radiographs. The baseline MXA scans and conventional radiographs were each analyzed quantitatively by two observers in a masked fashion, using a standard six-point method. The follow-up images were then analyzed by the same observers. The MRX observers were masked to the baseline analyses, while the MXA observers utilized the manufacturer's 'compare' facility. On all scans and radiographs anterior (Ha), mid (Hm) and posterior (Hp) vertebral heights were measured and wedge (Ha/Hp) and mid-wedge (Hm/Hp) ratios calculated for each vertebral body, ideally from T4 to L4. MRX analyzed 129 of the 130 available vertebrae in group 1 at both visits and 141 of the 143 available in group 2, while MXA analyzed 124 vertebrae in group 1 at both visits and 127 in group 2. Intra- and inter-observer precision errors, particularly in terms of coefficient of variation (CV%), were larger for MXA than for MRX in both normal subjects and those with vertebral deformities. For example, intra-observer precision errors for vertebral height measurement were 0.62 mm (2.9%) for MXA compared with 0.63 mm (2.2%) for MRX in group 1 (normal) subjects and 0.82 mm (4.2%) for MXA compared with 0.85 mm (3.3%) for MRX for group 2 (osteoporosis and vertebral deformities) subjects. Both MXA and MRX inter-observer precision was clearly poorer than the intra-observer precision, a problem associated with any morphometric technique. This was particularly noticeable for MXA; for example, precision of vertebral height measurement in group 1 subjects was 0.62 mm (2.9%) for intra-observer compared with 0.99 mm (4.6%) for inter-observer analyses. MXA and MRX intra- and inter-observer precision was significantly poorer for subjects with vertebral deformities compared with those without, with the CV% for subjects with vertebral deformity approximately 50% greater than that of normal subjects. For example, MRX intra-observer precision for the midwedge ratio was 2.6% for group 1 subjects compared with 3.8% for group 2 subjects. The precision of vertebral height measurement on deformed vertebrae of group 2 subjects was poorer than that for normal vertebrae in the same subjects using both MXA and MRX, as a result of increased variability in point placement. For example, MXA intra-observer precision (RMS SD) for the wedge ratio precision was 0.037 (3.9%) for normal vertebrae compared with 0.060 (6.6%) for deformed vertebrae. We conclude that MXA precision was generally poorer than MRX, although both techniques were adversely affected by the presence of vertebral deformities and the use of more than one observer. Although precision errors for both techniques were substantially smaller than the 20-25% reduction in vertebral height frequently proposed to identify incident deformities, the poorer precision of MXA may lead to an increased risk of erroneous classification of vertebrae as normal or deformed.

Visual assessment of vertebral deformity by X-ray absorptiometry: a highly predictive method to exclude vertebral deformity.

The accurate identification of prevalent vertebral fractures is important in both the clinical and research setting as they are associated with increased risk of further fracture and irreversible clinical consequences. This study reports a direct comparison of prevalent vertebral deformity identification using X-ray absorptiometry (XA) scans, acquired on a dual-energy X-ray absorptiometry (DXA) machine, and conventional radiographs in a diverse group of 161 postmenopausal women, ranging from healthy subjects with normal bone mineral density (BMD) to osteoporotic subjects with multiple vertebral deformities. Deformities were identified by a trained operator by visual assessment of the XA scans (VXA) and semiquantitatively by an experienced radiologist on the conventional radiographs (XSQ). Subjects were recruited prospectively and were triaged according to their VXA results into normal, equivocal and definite deformity groups. VXA and XSQ demonstrated good agreement (96.3%, K = 0.79) in classifying vertebrae as normal or deformed in the 1978 of 2093 vertebrae deemed analyzable on both the XA scans and conventional radiographs. VXA showed good sensitivity (91.9%) in the identification of moderate/severe XSQ deformities and an excellent negative predictive value (98.0%) was produced when VXA was used to distinguish subjects without vertebral deformities from those with possible or definite deformities on a per subject basis. The majority of disagreement between the two methods resulted from different classification of mild wedge and endplate deformities and the poor visualization of upper thoracic vertebrae on the XA scans. Agreement improved, particularly on a per subject basis, when analysis was restricted to the vertebral levels from L4 to T7. Visual triage of XA scans by a trained operator would seem to be swift, convenient and cost-effective method, with excellent negative predictive value, to distinguish subjects with very low risk of vertebral deformities from those with possible deformities. These 'normal' subjects can then be excluded prior to performing conventional radiographs and further time-consuming and costly methods of vertebral deformity assessment such as XSQ by an experienced radiologist and/or quantitative morphometry. VXA may prove useful in the clinical evaluation of patients at risk of osteoporosis as an adjunct to BMD scans or in the selection of subjects for osteoporosis-related clinical trials.

Morphometric X-ray absorptiometry and morphometric radiography of the spine: a comparison of prevalent vertebral deformity identification.

Prevalent vertebral deformities are associated with a substantially increased risk of subsequent vertebral and nonvertebral fractures. Knowledge of vertebral fracture status is an important component in the prediction of further fractures in patients with osteoporosis. This study reports a comparison of the quantitative identification of vertebral deformities on morphometric X-ray absorptiometry (MXA) scans and conventional radiographs (MRX) in 161 postmenopausal women (mean age +/- SD, 64 +/- 7.1 years) recruited from patients referred by their family doctor for bone density measurement (n = 119) and osteoporotic subjects with known vertebral deformities attending an osteoporosis clinic (n = 42). Each subject had MXA scans and MRXs of the thoracolumbar spine, to image the vertebrae from T4-L4, at a single visit. The scans and radiographs were analyzed by two trained observers using six points to quantify the shape of each vertebral body. From these points, three vertebral heights were measured: anterior, middle, and posterior. Vertebral deformities were identified using the algorithms proposed by Eastell and by McCloskey. Generally good to excellent agreement (per vertebra, kappa = 0.87-0.93; per subject, kappa = 0.81-0.91) was observed between the two algorithms used for quantitative vertebral deformity identification using MXA or MRX. More moderate agreement (per vertebra, kappa = 0.70-0.79; per subject, kappa = 0.67-0.75) was seen when comparing the same algorithm between MXA and MRX. Agreement between MXA and MRX for the McCloskey algorithm was better than for the Eastell algorithm, largely because of the lower number of false positives produced by the McCloskey methodology. Deformity identification by MXA was limited because of poor image quality, primarily in the upper thoracic spine. One in six MRX deformities were missed by MXA as they occurred in vertebrae not visualized sufficiently for analysis on the MXA scans. Deformity identification was poorer in the upper thoracic spine in analyzable vertebrae with a sensitivity of 50.0% for MXA in terms of MRX using the Eastell algorithm for the vertebral levels T4-T7, compared with 80.6% for L1-L4A. MXA proved to be more effective at identifying moderate to severe MRX deformities producing a sensitivity of 22.0% for MXA in terms of identifying MRX grade 1 deformities using the Eastell algorithm, compared with 81.6% for grade 2 deformities. Although MXA image quality is inferior to that of conventional radiographs, MXA has distinct advantages such as a substantially reduced effective dose to the patient and acquisition of a single image of the spine. MXA is a potentially useful, relatively fast, low-radiation technique to identify prevalent vertebral deformities, particularly moderate to severe deformities in the middle/lower thoracic and lumbar spine, in conjunction with morphometric radiography in some patients.

Morphometric X-ray absorptiometry and morphometric radiography of the spine: a comparison of analysis precision in normal and osteoporotic subjects.

Morphometric techniques, which use conventional lateral spine radiographs to quantify vertebral body shape (morphometric radiography, MRX), have proved a useful tool in the identification and evaluation of osteoporotic vertebral deformities. Recently a new method of acquiring the images required for vertebral morphometry using dual-energy X-ray absorptiometry scanners (morphometric X-ray absorptiometry, MXA) has been developed. In this study we compare repeat analysis precision of vertebral height measurement using MXA and MRX. Twenty-four postmenopausal women were recruited (mean age 67 +/- 5.8 years): 12 normal subjects and 12 with osteoporosis and vertebral deformities. Each subject had a MXA scan and lateral thoracic and lumbar radiographs at a single appointment, which were each analyzed quantitatively in a masked fashion, using a standard 6-point method, twice by one observer and once by a second observer. Anterior (Ha), mid (Hm) and posterior (Hp) vertebral heights were measured and wedge (Ha/Hp) and mid-wedge (Hm/Hp) ratios calculated for each vertebral body. Intra- and interobserver precision were consistently poorer in MXA compared with MRX in both normal subjects and those with vertebral deformities, with MXA CV% generally at least 50% higher than corresponding values for MRX. For both MXA and MRX interobserver precision was clearly poorer than intraobserver precision, a problem associated with any morphometric technique. MXA intra- and interobserver precision were significantly poorer for subjects with vertebral deformities compared with those without, with a CV% for deformity subjects up to twice that of normal subjects. Conversely, MRX showed little or no obvious worsening of intra- or interobserver precision for deformity subjects. Comparison of MXA precision in the normal and deformed vertebrae of the deformity subjects demonstrated that the poorer precision in these subjects compared with normal subjects was the result of increased variability in point placement on the deformed vertebrae themselves. However, the precision for normal vertebrae in these subjects was also somewhat poorer than the precision in normal subjects. We conclude that MXA precision is generally poorer than that of MRX and that the presence of vertebral deformities has a more pronounced effect on MXA precision than on MRX precision.

Optimizing data acquisition and analysis of morphometric X-ray absorptiometry.

Morphometric X-ray absorptiometry (MXA) uses dual-energy X-ray absorptiometry (DXA) scanners to perform vertebral morphometric measurements of the vertebrae. In this study we evaluated the four available MXA scan modes--single-energy (SE) and dual-energy fast (F), array (A) and high definition (HD)--on a commercial bone densitometer (Hologic QDR-4500A). Sixty postmenopausal women (mean age 59 years, range 40-73 years) were recruited and split into two groups matched for body mass index (BMI, kg/m2). Three MXA scans, covering 13 vertebrae from T4 to L4, were acquired on each subject; all subjects were scanned in SE and A modes, while the third scan was performed in F mode in group 1 and in HD mode in group 2. Subjects were invited to return 6 months after the commencement of the study to repeat their scans. The HD mode produced the most reliable image, with 97% of all scans analyzable to T7 and the fewest vertebrae being lost to analysis (1.5/13 vertebrae lost per scan). A SE + HD combination (using whichever image allows the analysis of more vertebrae) further decreased the number of vertebrae lost to 0.8 of 13 vertebrae, i.e. a typical scan was analyzable up to and including T5. BMI had a noticeable and scan-mode-dependent effect on MXA image quality, an increase in the number of vertebrae lost to analysis occurring once BMI exceeded 30. BMD had a far smaller effect on image quality and no effect at all using the SE + HD combination. Precision (CV%) was similar for all three dual-energy modes at around 3.5% without the scan 'compare' facility and 2.6% with it. The best precision was obtained with SE scan (2.7%/2.2%). BMI and BMD had little or no effect on precision. We conclude that optimal results are obtained by the acquisition of both SE and HD scans. However, for rapid assessment by trained operators SE scans alone offer almost equal utility.

Morphometric X-ray absorptiometry: reference data for vertebral dimensions.

Vertebral fractures are a common and important consequence of osteoporosis and are often identified via morphometric analysis of conventional lateral spine radiographs (morphometric radiography or MRX). A new method of performing vertebral morphometry using images acquired on dual-energy X-ray absorptiometry (DXA) scanners (morphometric X-ray absorptiometry or MXA) has recently been developed. In this study, we derive reference data for vertebral heights and height ratios using MXA scans as the data source and compare the results with previously published MRX studies. One thousand and nineteen Caucasian women (mean age 63 years, range 33-86) were recruited. An MXA scan, covering 13 vertebrae from L4 to T4, was acquired for each subject on one of four DXA systems located at three centers in the U.K. Analysis of variance found statistically significant but relatively small differences among centers, machines, and scan modes, and therefore data were pooled for reference range calculations. Three vertebral heights (anterior, mid, and posterior) were measured and four ratios (wedge, mid-wedge, and two crush) calculated. These data sets were trimmed using an iterative algorithm to remove extreme values assumed to represent deformed vertebrae, then mean and SD values were calculated using the remaining data. When the data were split by age, a small but statistically significant decrease in vertebral height between the sixth and eighth decades was found, but this was not replicated for the vertebral height ratios. Marked differences were observed between MXA data and MRX, but were comparable to those between different MRX studies. These may result from differences in image quality and point placement protocols, population differences, differences in radiographic technique, and differences in the derivation of a group of "normal" vertebrae. This study suggests that reference data of vertebral dimensions should be specific to the technique which uses those data as a reference, i.e., MXA.

Vertebral morphometry studies using dual-energy x-ray absorptiometry.

Vertebral fractures are one of the most common consequences of osteoporosis. They are usually diagnosed by visual interpretation of lateral radiographs of the lumbar and thoracic spine. Vertebral morphometry, based on measurements of the anterior, middle, and posterior heights of the vertebral bodies from T4 to L4, is a useful adjunct to the visual reading of radiographs. A new generation of dual-energy x-ray absorptiometry (DXA) scanners offers software for acquiring lateral images of the spine and performing vertebral morphometry analysis. Advantages of DXA morphometry include straightforward and reproducible patient positioning, absence of geometrical distortion of the image, low radiation dose, digital acquisition, and simplified, semi-automated scan analysis. The widespread availability of such DXA systems should make the investigation of vertebral fractures more widely accessible.