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1
Universitätsklinik
für Radiodiagnostik, Osteologie;
Währinger Gürtel
18-20, 1090 Vienna, Austria.
2
Universitätsklinik
für Strahlentherapie,
Währinger Gürtel
18-20, 1090 Vienna, Austria.
3
Krankenhaus der Barmherzigen Schwestern, Abt. Innere Medizin, Stumpergasse 13,
1060 Vienna, Austria.
Received June 14, 2000;
accepted after revision December 19, 2000.
Address correspondence to C. R. Krestan.
Abstract
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MATERIALS AND METHODS. In 498 female patients (56 ± 18 years old), bone mineral density measurements by dual X-ray absorptiometry of the lumbar spine (posteroanterior, L1-L4) and the proximal femur and imaging quantitative sonography of the calcaneus were performed. The percentage of patients having T-scores less than or equal to a threshold of -2.5 standard deviations below a young normal reference was used to compare quantitative sonography with dual X-ray absorptiometry. The diagnostic agreement was assessed using kappa scores.
RESULTS. Approximately 30% of the patients had a T-score less than
or equal to -2.5 standard deviations as assessed by imaging quantitative
sonography (broadband ultrasound attenuation), 26.5% as assessed by dual X-ray
absorptiometry of the spine, and 16.7-56.4% as assessed by dual X-ray
absorptiometry of the different regions of interest at the femur. Kappa
analysis showed that severe diagnostic disagreement exists among broadband
ultrasound attenuation and dual X-ray absorptiometry (
=
0.28-0.42).
CONCLUSION. Considerable diagnostic disagreement exists between imaging quantitative sonography and dual X-ray absorptiometry of the spine and femur. The disagreement is in the same range as that reported recently in comparisons of dual X-ray absorptiometry and nonimaging quantitative sonography. In general, no distinct advantage for imaging quantitative sonography could be found when compared with other techniques.
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The combination of both methods can identify individuals with a high or low risk of fracture [6]. The odds ratios for self-reported fractures were reported to be between 1.4 and 1.52 for age-adjusted quantitative sonography measurements at the heel [7].
The diagnosis of osteoporosis, as well as the decision to initiate treatment, is usually done by comparing measurements of an individual patient with a normative database. These reference databases for healthy populations are supplied by the manufacturers and are implemented into the software for practically all available instruments for imaging (dual X-ray absorptiometry, quantitative CT, quantitative sonography). Results of the individual patients are compared with those of healthy individuals of the same sex and ethnic group.
The patient can be categorized according to the T-score (number of SDs difference compared with the mean of healthy young adults) or to the Z-score (number of SDs difference compared with the mean of the age-matched population). The World Health Organization has taken the initiative in defining a T-score of spine (hip, forearm) bone mineral density measurements from dual X-ray absorptiometry of lower than -2.5 SDs as osteoporotic for white women [8]. However, the use of uncorrected databases derived from various studies can have a significant impact on the classification of an individual patient [9].
Additionally, significant discord is often seen among measurements at different skeletal sites and among measurements performed with different techniques [10, 11].
The calcaneus as a weight-bearing, predominantly cancellous bone has been used extensively for quantitative sonography measurements in the past. Yet the inhomogeneity of the calcaneus has led to the development of imaging quantitative sonography to permit optimal selection of regions of interest [12, 13].
The purpose of our study was to evaluate the diagnostic agreement of imaging quantitative sonography of the calcaneus and dual X-ray absorptiometry of the spine and femur for the presence of osteoporosis in white women.
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Fractured vertebral bodies, if present, were excluded from the analysis. A fracture was determined by a semiquantitative assessment of morphologic changes of the lumbar spine in lateral and anteroposterior conventional radiographs. This semiquantitative assessment was based on the qualitative appearance of each vertebra, with a fracture being defined as altered morphology and a decrease in vertebral height of approximately 25% or more at the anterior, medial, or posterior aspect of the vertebral body [2, 10]. Conventional radiographs of the lumbar spine were also evaluated for degenerative changes and overlying calcifications. Patients with degenerative calcifications that might lead to an overestimation of bone mineral density were excluded from the study.
The projectional bone mineral density values were given in grams per cubic centimeter, and the individual patient's results were expressed as a T-score and a Z-score. At the same session, the patients were examined at the calcaneus with an imaging quantitative sonography device (DTU-one; Osteometer, Herlev, Denmark) according to the manufacturer's recommended standard procedures. The results of imaging quantitative sonography are represented as a gray-scale image of the calcaneus in a lateral projection (Fig. 1). In this approach, low quantitative sonography attenuation is depicted by dark gray-scale pixels and high attenuation by bright gray-scale pixels. The two-dimensional sonogram is calculated from a recti-linear scan with a matrix size of 60 x 80 m-3 and a pixel-size of 6 x 10-4 m.
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Broadband ultrasound attenuation and speed of sound as absolute values were automatically calculated from a circular region of interest with the lowest quantitative sonography result. This region of interest was automatically identified by the software in a default process. The region-of-interest placement is independent of the size and position of the heel as well as of the surrounding soft tissue. T-scores and Z-scores for speed of sound and broadband ultrasound attenuation were obtained. The diagnostic threshold for a T-score less than or equal to -2.5 SDs less than a young normal reference was used to compare quantitative sonography and dual X-ray absorptiometry. The percentage of subjects below the threshold of less than or equal to -2.5 SDs T-score were calculated for each technique or variable. Comparisons of all measured absolute variables and T-scores were performed by linear regression analysis for all patients using Pearson's correlation coefficients (r). Additionally, the percentage of standard errors of the estimate (= coefficient of variation) and p values for testing the significance of correlations were obtained. To compare techniques for their diagnostic agreement, percentages of agreement were given for the comparison of all variables. A kappa score analysis was done for T-scores less than or equal to -2.5 SDs below a young normal reference and for a Z-score less than -1.0 SDs below normal.
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In comparing absolute variables, linear regressions showed comparable low to modest correlation coefficients in the population. These correlations, the standard error of estimate, and p values based on linear regressions for the selected variables are given in Table 2. Correlations ranged from 0.36 to 0.57 for the comparison of broadband ultrasound attenuation with sites and techniques and were less for speed of sound (between 0.22 and 0.28) (Table 3). The intratechnique comparison for various regions of interest of dual X-ray absorptiometry at the proximal femur showed correlations of T-scores ranging between 0.61 and 0.98 (Table 3). Comparing all techniques, we found that the strongest correlation was within one technique between dual X-ray absorptiometry of the intertrochanteric region and of the total region of interest (r = 0.98; p < 0.0001).
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By kappa analysis of the T-scores, the diagnostic agreement among the dual
X-ray absorptiometry measurement techniques was moderate (range, 0.27-0.86) in
classifying patients as osteoporotic (Table
4). The intratechnique agreement for various regions at the
proximal femur was greater using age-adjusted scores (Z-scores) than using
T-scores ( = 0.55-0.59 vs. 0.26-0.27)
(Table 5). Frequently, the
estimates of risk for different patients were made using different measurement
approaches.
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Between quantitative sonography and dual X-ray absorptiometry, the
agreement of T-scores was poor for broadband ultrasound attenuation (the
greatest agreement between broadband ultrasound attenuation and the femoral
neck was = 0.42; p < 0.0001) and not significant between
speed of sound and the other techniques (the greatest kappa value between
speed of sound and the femoral neck: = 0.12; p < 0.0001).
A similar disagreement was found between broadband ultrasound attenuation and
dual X-ray absorptiometry for Z-scores of less than -1 SD ( =
0.24-0.37) (Table 5).
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In general, the correlation coefficients between dual X-ray absorptiometry of the hip, dual X-ray absorptiometry of the spine, and quantitative sonography were low for absolute values as well as for T-scores and were lower for speed of sound than for broadband ultrasound attenuation. They were in the same range as reported recently in comparisons of dual X-ray absorptiometry and nonimaging quantitative sonography [11]. Our results were generally similar to those reported in prior studies in which correlation coefficients of dual X-ray absorptiometry at the spine, hip, and forearm were found to be between 0.26 and 0.46 compared with calcaneal broadband ultrasound attenuation and between 0.33 and 0.54 compared with speed of sound in women [15, 16]. Higher results were shown in healthy volunteers in whom correlations of speed of sound and broadband ultrasound attenuation with spinal dual X-ray absorptiometry were 0.67 and 0.65, respectively [17].
Correlations between the various regions of interest at the femur using dual X-ray absorptiometry were higher, as expected, but were still modest considering the close proximity of the regions of interest within one technique. T-scores correlated with practically the same levels as absolute values, which might be an indicator that the reference databases supplied by the systems warrant reliable results. In our study, the comparison between radiography-based methods and sonographic measurements yielded lower correlation values than those between radiography-based measurements of the spine and hip alone.
A similar relation in women was shown by Young et al. [18], who showed correlation coefficients of bone mineral density of the spine and broadband ultrasound attenuation of 0.40, but had higher results for bone mineral density of the spine and hip (r = 0.67).
Consistent with the low correlation coefficients was the poor diagnostic agreement (by kappa analysis) among these measures in classifying patients as osteoporotic. In most instances in which a patient was classified differently by two methods, the differences in T-scores were indeed substantial. Considerable differences were also noted in the percentage of the population with scores less than or greater than the given threshold of -2.5 SDs.
One of the drawbacks of our study is that we did not directly compare imaging quantitative sonography with nonimaging quantitative sonography. However, statistical analysis did not reveal any benefit from imaging quantitative sonography compared with nonimaging techniques, as shown in the recent literature [13, 19]. Brooke-Wavell et al. [20] did find that different regions of interest had a significant influence on the results, which may well lead to differences in diagnosis by choosing different locations in the calcaneal bone.
A distinct advantage for imaging quantitative sonography could be the greater short- and long-term precision, but correlation coefficients and prevalence figures among different techniques and sites as done in our study would not necessarily reflect this. Questions regarding the value of sonography in conjunction with dual X-ray absorptiometry for improving the estimation of fracture risk cannot be answered by our study design. As with the study of Grampp et al. [11], we found no improvement in the correlation between absolute values and T-scores. The kappa values for Z-scores did not show any difference from the kappa values for T-scores of quantitative sonography in contrast to dual X-ray absorptiometry, in which kappa statistics for Z-scores showed higher agreement at the femoral neck (Table 5). Our study confirms the poor correlation of quantitative sonography with dual X-ray absorptiometry at different sites. The correlation is in the same range as reported recently [11] in comparisons of dual X-ray absorptiometry and nonimaging quantitative sonography. In general, no distinct advantage for imaging quantitative sonography could be found when compared with other techniques.
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