1UniversitaÃtsklinik fuÃr Radiodiagnostik, Osteologie/MR, Waehringer Guertel 18-2, 1090 Vienna, 2Krankenhaus der Barmherzigen Schwestern, Abt. Innere ...
The British Journal of Radiology, 73 (2000), 275±277
E
2000 The British Institute of Radiology
Peripheral quantitative CT of the forearm: scanner cross-calibration using patient data 1
S GRAMPP, MD, 2A NATHER, MD, 3B RINTELEN, MD, 1C HENK, MD, A RESCH-HOLECZKE, MD, 1H IMHOF, MD and 2H RESCH, MD
4 1
UniversitaÈtsklinik fuÈr Radiodiagnostik, Osteologie/MR, Waehringer Guertel 18-2, 1090 Vienna, 2Krankenhaus der Barmherzigen Schwestern, Abt. Innere Medizin, Stumpergasse 13, 1060 Vienna, 3Kaiser Franz-JosefSpital, Abt. Innere Medizin, Kundratstrasse 3, 1100 Vienna and 4UniversitaÈtsklinik fuÈr Strahlentherapie, Waehringer Guertel 18-2, 1090 Vienna, Austria
Abstract. Bone density measurements by peripheral quantitative CT (pQCT) of the distal radius in 87 women (age 38.4¡12.3 years) were obtained on a both a Stratec XCT 900 and an updated version XCT 1400. Axial area, and total and trabecular bone mineral density (BMD) results from both scanners were compared for all patients using linear regression analysis. To achieve scanner calibration we used the intercept and slope of the patients' correlations. The correlations of the patients' values were good for area (r50.83) as well as total BMD (r50.90) and excellent for trabecular BMD (r50.97). The cross-calibration approach in our study provided compatibility of pQCT for trabecular and total ROIs, the clinically mostly utilized volumes of measurement. Peripheral quantitative CT (pQCT) measurements of BMD at the distal radius can be used to distinguish between osteoporotic and non-osteoporotic subjects and to monitor patients during clinical studies [1±3]. The pQCT machine (XCT 900) evaluated in our study is the most widely used design worldwide (about 1000 scanners) [4] and is being replaced by an updated version (XCT 1400). By quantifying the differences in readings between the two pQCT instruments, we aim to determine the cross-calibration relationships that will allow the results to be expressed such that the two instruments give comparable bone mineral density (BMD) values.
Methods 87 healthy, non-pregnant women were examined; the mean age was 38.4 years (age 20±84 years, median 36.0 years, SD512.3 years). Those with medical conditions that could affect forearm measurements (e.g. history of fracture) were excluded. Other exclusion criteria were the presence of a history of generalized disease of bone, trauma, or malignant disease, as well as treatment with drugs which could in¯uence bone metabolism. Permission for the study was given by the local ethics committee. pQCT measurements were made with both a Stratec XCT 900 scanner and a Stratec XCT 1400 (Stratec GmbH, Pforzheim, Germany), which produce a narrow fan beam using a heavily Received 12 August 1999 and in revised form 6 October 1999, accepted 3 November 1999. The British Journal of Radiology, March 2000
®ltered X-ray tube (47 kVp, 0.3 mA). The XCT 900 produces 72 projections per scan with a slice thickness of 2.5 mm and the XCT 1400 produces 90 projections per scan with a slice thickness of 2 mm. The same software for image analysis is used in both machines. There is a signi®cant difference in gantry width: the XCT 900 can image objects with a diameter of up to 85 mm, whereas the XCT 1400 can be used for object diameters of up to 135 mm. Measurements on both machines were performed on the same day using standard procedures speci®ed by the manufacturer [5, 6]. Total area, BMD and bone mineral content (BMC) as well as trabecular BMD and BMC results measured by both scanners were compared for all patients by linear regression analysis. From the regression line the slope, intercept and standard error of the estimate (SEE) were calculated and cross-calibration equations were derived. The differences of the means of all three measures were calculated for the total population and compared by the paired t-test.
Results The correlation coef®cients for the comparisons of the two scanners for the patients' total area and total BMD (Table 1) values were high (r50.83 and 0.90, respectively). The correlation of the patients' trabecular BMD values was excellent (r50.97) (Table 1). All intercepts for these linear regressions were signi®cantly different from zero (p,0.05). There were signi®cant differences (p,0.05) in the paired t-test for the 275
S Grampp, A Nather, B Rintelen et al Table 1. Scanner cross-calibration for total and trabecular BMD, total and trabecular BMC and total area with correlation coef®cients (r) and standard errors of the estimate (SEE) (all p,0.00001) Parameter
XCT 1400
Total BMD (g cm23) Trabecular BMD (g cm23) Total BMC (g) Trabecular BMC (g) Total area (mm2)
5(0.8956XCT 5(0.9946XCT 5(0.7726XCT 5(0.7876XCT 5(0.6786XCT
900)+58 900)210 900)+47 900)+2 900)+68
mean values of total BMD, total area and trabecular BMC (Table 2). From the linear regressions, cross-calibration equations were derived for each of the values as given in Table 1.
Discussion pQCT is a technique for the measurement of peripheral bone that is unique in that it gives true volumetric information and can distinguish between bone compartments. pQCT has been established as a reliable method providing high precision and acceptable accuracy. The establishment of a normative dataset helps to unify the diagnostic results obtained using the various systems. By quantifying the differences between systems, a set of independent normative data for cross-calibration can be acquired [7, 8]. In our study we demonstrated the relationship between the measurements of the two pQCT scanners. Although the results obtained with both systems are highly correlated, regression analysis shows that the values are not identical, as the regression lines have slopes different from unity and an intercept different from zero. Signi®cant differences between the absolute results of the two scanners were demonstrated for all parameters investigated. Even though they operate according to the same methods, the pQCT systems examined in this study gave distinct results, which may only partially be due to the marginally different scanner design, scanner speci®cations and the difference in measured volume (slice thickness). In the current study the area values acquired using the different scanners differed more from each other than did the BMD results, as re¯ected by
XCT 900
r/SEE (%)
5(0.8956XCT1400)+14 5(0.9536XCT 1400)+18 5(1.1366XCT 1400)219 5(1.1416XCT 1400)+4 5(1.0046XCT 1400)+40
0.90/9.0 0.97/6.5 0.95/11.9 0.93/8.8 0.83/9.4
the lower correlation coef®cients. Differences in area results may, to a certain degree, be attributed to inaccuracy in patient positioning, which will also have a signi®cant impact on the measured BMD [9, 10]. With pQCT, a small difference in the placement of the slice plane can result in signi®cant differences in axial area. Correlation coef®cients for total and trabecular BMD were reasonably high, indicating the robustness of BMD compared with area. Similar high correlations for BMD values were reported by Genant et al in comparing different dual-energy X-ray absorptiometry scanners at the spine (r50.98± 0.99) and hip (r50.88±0.95) [8]. Our calculated cross-calibration equations were acquired on a variety of females, covering the clinically observed range, therefore ensuring validity of the results. Even considering the observed differences, it is legitimate to estimate the total and trabecular BMD values of both machines by applying the calculated regression relationships. Therefore it seems reasonable to apply the normative data base of the XCT 900 to the measurements of the updated version after correction with the appropriate equations and to perform follow-up measurements.
Acknowledgments The manuscript preparation was supported by: Ludwig-Boltzmann Institut fuÈr klinische und experimentelle Radiologie, Vienna, Austria and the Ludwig-Boltzmann-Institut fuÈr fortgeschrittene Altersforschung, Vienna, Austria.
Table 2. Average values and standard deviations for total and trabecular BMD, total and trabecular BMC and total area for the study population and p values for the paired t tests Scanner
XCT 1400
XCT 900
p value
Total BMD (g cm23) Trabecular BMD (g cm23) Total BMC (g) Trabecular BMC (g) Total area (mm2)
356.1¡66.7 150.4¡47.2 264.3¡58.3 50.8¡19.2 299.6¡50.4
332.5¡66.7 160.9¡46.2 281.6¡70.7 62.0¡23.1 341.1¡61.2
0.02 0.13 0.08 0.0006 0.0001
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Peripheral quantitative CT of the forearm: scanner cross-calibration
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