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Diagnostic Disagreement of Imaging Quantitative Sonography of the Calcaneus with Dual X-Ray Absorptiometry of the Spine and Femur

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

Top Abstract Introduction Materials and Methods Results Discussion References

 
OBJECTIVE. This study evaluates the diagnostic agreement between imaging quantitative sonography of the calcaneus and dual X-ray absorptiometry of the spine and femur for diagnosing osteoporosis.

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.

Introduction

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Osteoporosis, the most common generalized disease of the skeleton, is defined as a decrease in bone mass accompanied by structural changes leading to an increase in fracture propensity (mostly atraumatic fractures of the spine, distal radius, proximal femur, or ribs) [1]. Current methods for evaluating skeletal status, assessing osteoporosis, and determining fracture risk rely mostly on the noninvasive measurement of bone mineral content and bone mineral density [1, 2]. The most commonly used and widely accepted method for examining the skeletal status is projectional measurement using dual X-ray absorptiometry, typically at the lumbar spine and the proximal femur. The only approach for noninvasive assessment of fracture risk that does not require ionizing radiation is quantitative sonography, which measures ultrasound velocity (speed of sound) and broadband ultrasound attenuation, usually applied at the calcaneus, tibia, and phalanges. Quantitative sonography is most often used at the calcaneus and has shown valid results in clinical studies of fracture risk prediction [2, 3]. Quantitative sonography and bone mineral density can provide complementary information to improve estimates of vertebral fracture risk [4]. Quantitative sonography is known to correlate only moderately with dual X-ray absorptiometry but may predict fracture independently from bone mineral density [5].

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.

Materials and Methods

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We retrospectively studied conventional radiographs and bone density measurements from consecutive clinical patients: 498 women who ranged in age from 20 to 83 years (mean age, 56.2 ± 18.2 years). The patients were referred by various departments and out-patient clinics specializing in orthopedic surgery, internal medicine, and general surgery for the assessment of bone mineral density. All patients were examined with a QDR-1000 scanner (Hologic, Waltham, MA) using dual X-ray absorptiometry with the manufacturer's recommended standard procedures for the posteroanterior lumbar spine at L1-L4 and for the proximal femur (femoral neck, trochanter, intertrochanteric region, total region, and Ward's region).

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.

View larger version (147K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1. —Gray-scale sonogram of calcaneus in lateral projection calculated from broadband ultrasound attenuation values shows automatic placement of region of interest (circle and small arrow) in area of lowest attenuation. Large arrow indicates insertion of Achilles tendon.

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.

Results

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Broadband ultrasound attenuation identified 29.5% of patients as less than or equal to -2.5 SDs below a young normal reference, whereas speed of sound classified only 5.6% as below this threshold (Table 1). Dual X-ray absorptiometry at the spine identified 26.5% of all patients as osteoporotic. Regions of interest of dual X-ray absorptiometry femoral measurements showed generally a high variability in identifying subjects with T-scores less than the given threshold. Dual X-ray absorptiometry of the trochanter identified 17.5% of patient as osteoporotic, but dual X-ray absorptiometry of Ward's triangle identified 56.4% as osteoporotic (Table 1).

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).

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.

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).

Discussion

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We determined the relationship between imaging quantitative sonography of the calcaneus with automatic region-of-interest detection and dual X-ray absorptiometry of the spine and femur. Previously, Fournier et al. [14] found that automatic region-of-interest placement reduced precision errors caused by improved intra- and interobserver variation. Laugier et al. [13] showed that imaging quantitative sonography improved a reproducible selection of a region of interest at the calcaneus.

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.

References

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