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Quantitative Computed Tomography Precision and Accuracy for Long-Term Follow-up of Bone Mineral Density Measurements
Journal of Clinical Densitometry, vol. 5, no. 3, 259–266, Fall 2002 © Copyright 2002 by Humana Press Inc. All rights of any nature whatsoever reserved. 1094-6950/02/5:259–266/$12.00
Original Article
Quantitative Computed Tomography Precision and Accuracy for Long-Term Follow-up of Bone Mineral Density Measurements A Five-Year In Vitro Assessment
Pierre M. Braillon, PHD, MD Department of Radiology (Pav. B), Hospital Edouard Herriot, Place d’Arsonval, 69347 Lyon Cedex 03, France
Abstract The purpose of this work was to test the long-term precision of quantitative computed tomography (QCT) on a CT scanner partly used for the measurement of bone mineral density (BMD). A spine phantom (ESP®), which simulates three lumbar vertebrae (Li, i = 2–4) with given mineral densities of 50, 100, and 200 mg hydroxyapatite equivalents (HAP)/cm3, respectively, was measured periodically over more than 5 yr on a Elscint-Marconi CT-Twin® scanner. A total of 80 measurements were taken. The measured BMDi values were 48.4 ± 1.2, 101.3 ± 1.1, and 212.6 ± 1.7 mg HAP/cm3, respectively (coefficient of variation [CV%] = 2.4, 1.1, and 0.8), and they were linearly correlated with the given density values (r > 0.99). The mean BMD value of the three simulated vertebrae was 120.8 ± 1.1 mg HAP/cm3 (CV% = 0.9), a value that corresponds to the mean lumbar BMD value in normal 65-yr-old women. We concluded that QCT is a precise and accurate method for long-term follow-up of BMD assessment in the population affected by osteoporosis. Key Words: Bone mineral density (BMD); quantitative computed tomography (QCT); long-term precision; osteoporosis.
Introduction
these systems, especially the shorter scanning times and the better image quality, their precision remains a concern for radiologists who require reliable body mineral density (BMD) measurements (11). In fact, for patients between the ages of 45 and 70, covering the most important period of life for BMD checks, the mean annual decrease of trabecular BMD at the spine level, measured by QCT, is approximately 1.8% in women and 0.8% in males, in our experience. Therefore, it is imperative that investigators be aware of how precise the system used in BMD follow-up measurements is. The short-term precision of CT scanners has been widely studied (8–10,12–15). However, other than the study of Laval-Jeantet et al. (16) covering a 2-yr period, to our knowledge there
The use of quantitative computed tomography (QCT) to measure the bone mineral density (BMD) at various skeletal sites, especially at the lumbar vertebral level, was developed some 20 yr ago (1–9). Since that time, an extensive literature has been published on the subject (10) and the technology has evolved. However, despite the improvements in
Received 08/07/01; Revised 11/13/01; Accepted 11/28/01. Address correspondence to Dr. Pierre M. Braillon, Department of Radiology (Pav. B), Hospital Edouard Herriot, Place d’Arsonval, 69347 Lyon Cedex 03, France. E-mail: pierre.braillon@chu_lyon.fr
259
260
Braillon
Fig. 1. (a) Spiral three-dimensional CT reconstruction of the bony part of the ESP phantom used. The bone mineral densities of the three simulated lumbar vertebrae are 50, 100, and 200 mg/cm3 (in HAP). (b) Image of the slice obtained at the mid-level of the second simulated vertebra, with the elliptical region of interest positioned for the BMD measurement. (c) Example of slice obtained in the reference CIRS phantom. Two successive reference measurements were performed in this phantom with the 50- and 150-mg HAP/cm3 inserts, respectively, and with the same scanning parameters as for the ESP measurements. (Figure continues)
are no data available that confirm the long-term precision of the QCT technique over several years. The aim of this study was to estimate this parameter from in vitro measurements performed on a modern CT scanner.
Materials and Methods The study was performed on an Elscint-Marconi CT-Twin® scanner (Elscint-Marconi, Haifa, Israel). This CT scanner works in single energy. In our department, it is heavily used for bone diagnosis imaging, with a mean number of 40,300 ± 4850 slices obtained each month. A preventive maintenance of this machine is scheduled every 2 mo, with an entire set of controls of the electronic circuits and of the image quality. Also, density measurements are made on specific phantoms, to calibrate the system in Hounsfield units. In between, a simple control of the X-ray beam and detectors is completed every Journal of Clinical Densitometry
week by the scanner technologists. The X-ray tube was replaced three times during the study, and an entire calibration of the system was performed each time. The study took place between January 1996 and June 2001 (e.g., over more than 5 yr between the first and the last measurements [1978 d, precisely]). To test the precision of the system, we measured a spine phantom developed by Kalender et al. (17,18) and manufactured by QRM, Erlangen, Germany. This phantom, called the “European Spine Phantom®” (ESP) was previously used in a European study for cross-calibration of bone densitometers (19). It simulates three lumbar vertebrae (Fig. 1a) designated hereafter as Li (i = 2–4). The given mineral densities of the simulated vertebrae were 50, 100, and 200 mg/cm3 in hydroxyapatite equivalent (HAP), respectively. A total of 80 measurements were taken. For each measurement, the following scan parameters were used: 120 kV, 220 mA s, scanVolume 5, 2002
Long-Term In Vitro QCT Precision for BMD Assessment
Fig. 1.
field diameter 500 mm. A 10-mm-thick slice, centered on the mid-plane of each vertebra was obtained, and the BMDi (i = 2–4) were measured within an elliptical region of interest of 14 mm × 21 Journal of Clinical Densitometry
261
(continued)
mm (surface area ≈ 225 mm2), centered in the vertebral body (Fig. 1b). The BMD analysis was conducted the same way as in patient investigations; reference values were systematically obtained from Volume 5, 2002
Fig. 2. Shewhart-type plot of BMDi values, and their mean value, BMDm, against time expressed in days since the first measurement. BMDm, and two standard deviations above and below the mean are shown. The dates on which the X-ray tube has been replaced are indicated by arrowheads.
262
Journal of Clinical Densitometry
Braillon
Volume 5, 2002
Long-Term In Vitro QCT Precision for BMD Assessment
263
Table 1 Mean Bone Mineral Density (BMD) Values Measured in Each Simulated Vertebra of the ESP phantom (Li, i = 2–4), with Their Coefficient of Variation (CV%) and Their Differences (∆%) with the Corresponding Specified Values Given BMD values L2:50 L3:100 L4:200 Mean BMD
Measured values ± 1 SD
CV (%)
∆ (%)
48.4 ± 1.2 101.3 ± 1.1 212.6 ± 1.8 120.8 ± 1.1
2.4 1.1 0.8 0.9
–3.2 +1.3 +6.3 +3.5
Note: BMD values are expressed in mg HAP/cm3.
the measurement of a calibration phantom (CT Simulator®, model 4, from CIRS, Norfolk, VA). This second phantom simulates a 30-mm-thick human body section located at the level of the second lumbar vertebra. Inserts of varying density in HAP equivalent can be introduced for tests, in place of the vertebral body, and two inserts with densities of 50 and 150 mg HAP/cm3 were used to obtain two successive slices (Fig. 1c). Finally, BMDi values, and their mean value, BMDm = ΣBMDi/3, were obtained by reference to the pair of data obtained on the CT Simulator ® phantom.
Results The measured BMDi values were very stable throughout the study, as shown in Fig. 2. Their mean values, coefficients of variation (CV%), and differences with the specified densities are given in Table 1. Despite the differences, which ranged from –3.2% to +6.3%, a high linear correlation was found between the measured and given BMD values (Fig. 3). The mean measured BMD values, plotted on the normal value curve made for the lumbar spine BMD in women (unpublished data), are shown in Fig. 4. BMD4 and BMD3 were in good accordance with the values measured at the spine level in normal women, aged 25–35 and 70, respectively. BMD2 corresponds to a BMD value situated two standard deviations below the normal value at age 75, and finally, the BMDm value is very close to the mean spine BMD value in 65-yrold women. Journal of Clinical Densitometry
Discussion Quantitative CT is a valuable method for assessing spine BMD. It has the advantage of analyzing a well-defined zone, usually the central part of the vertebral body. This region is made of purely trabecular bone and has a high turnover rate. Therefore, BMD measurements in this region have a high sensitivity (20,21), an important point when screening for osteoporosis (22). Moreover, bone artifacts such as arthritis or extraosseous calcifications have no or a very low influence on the measurement. However, the follow-up of osteopenic patients continues over a long period of time, usually several years, and it is important that radiologists be sure that the changes observed in successive measurements, for a given patient, are significant. This means that the system used must be stable and have a sufficient precision to detect small BMD changes between two successive measurements. This is the least significant change (LSC) defined by Cummings and Black (23) and Glüer (24). Also, the comparison of measurements performed with different CT systems (e.g., in multicenter studies) should be possible by using simple cross-calibration data (19). From the results obtained in the present study, it appeared that the CT scanner used was stable over more than 5 yr, despite three X-ray tube replacements. Its accuracy was dependent on the BMD values, the differences with the specified values of 50, 100, and 200 mg HAP/cm3, being –3.2%, +1.3%, and +6.3%, respectively. However, these differences yielded a linear correlation between the measured Volume 5, 2002
Fig. 3. Linear correlation obtained between the measured BMDi (±2 SD) and the given BMD values of the simulated vertebrae. The dotted line is the line of identity.
Fig. 4. Positions of the measured BMDi values on the normal lumbar BMD value curve, established against age, for women. Circles: BMDi diamond: BMDm. BMD4 and BMD3 were in good accordance with the normal BMD at ages 25–35 and 70, respectively, whereas BMD2 corresponded to a BMD value two standard-deviations below the normal mean at age 75. Note that the BMD value decrease, as defined on the normal value curve, is quite linear between ages 45 and 70.
Long-Term In Vitro QCT Precision for BMD Assessment BMD values and the specified values, a relationship that makes it simple to adjust measured data in patients, whenever necessary. Precision, as estimated by the CV values of the 80 measurements taken, ranged from 0.8% for the highest density value to 2.4% for the lowest, and a CV of 0.9% was found for the mean BMD value of the three simulated vertebrae in the ESP phantom. This result could be compared with the longitudinal precision of the dual-energy X-ray absorptiometry (DXA) technique for posterior–anterior spine BMD measurements, which can be considered as the reference method (24) for these kinds of measurement. In a multicenter study, Orwoll et al. (25) found a longitudinal variability ranging from 0.11% to 1.19%, in four in vitro measurements, over the course of 1 yr. We obtained a comparable precision value of 0.70% in 2325 DXA measurements of a spine phantom on a Norland XR-36® DXA system, over a period of time of 54 mo (unpublished data). Therefore, the standard precision error, as defined by Glüer (24), can be estimated to about 1.1% for the QCT measurements of the present study. Consequently, for these BMD measurements, LSC should be 2.8 × 1.1 ≈ 3.0% to be detected with 95% confidence (p ≤ 0.05). Although in our experience the decrease of the spine BMD in women aged from 45 to 70 yr is quite linear, as shown on our normal value curve, and the annual BMD decrease rate is –1.8% over this period of time, we concluded that QCT is a suitable technique for assessing the spine BMD with confidence in perimenopausal and menopausal women over longterm periods of time. In these populations, the reliability of two to three successive QCT examinations, performed within an 18- to 24-mo time interval, over approximately 5 yr, is acceptable, even in the cases where the decrease in the patient’s spine BMD is on the same order as the physiological changes.
Acknowledgments We would like to thank Mr. T. Blanchard, engineer in charge of the CT scanner maintenance for his careful work, and assistance. The study was conducted with the ESP phantom #23, which had previously been used in a concerted action of the European Community’s COMAC (1989–1992). The author also thanks B. Kiraly for his help in digitizing images.
Journal of Clinical Densitometry
265
References 1. Isherwood I, Rutherford RA, Pullan BR, Adams PH. 1976 Bone mineral estimation by computer assisted transverse axial tomography. Lancet 1:712–715. 2. Ruegsegger P, Elsasser U, Anliker M, Gnehm H, Kind H, Prader A. 1976 Quantification of bone mineralization using computed tomography. Radiology 121:93–97. 3. Genant HK, Boyd DP. 1977 Quantitative bone mineral analysis using dual-energy computed tomography. Invest Radiol 12:545–551. 4. Cann CE, Genant HK. 1980 Precise measurement of vertebral mineral content using computed tomography. J Comput Assist Tomogr 4:493–500. 5. Cann CE, Genant HK. 1982 Cross-sectional studies of vertebral mineral using quantitative computed tomography. J Comput Assist Tomogr 6:216–218. 6. Adams JE, Chen SZ, Adams PH, Isherwood I. 1982 Measurement of trabecular bone mineral by dual-energy computed tomography. J Comput Assist Tomogr 6:601–607. 7. Cann CE, Genant HK. 1983 Single versus dual-energy CT for vertebral mineral quantification. J Comput Assist Tomogr 7:551. 8. Cann CE, Genant HK, Kolb FO, Ettinger B. 1985 Quantitative computed tomography for prediction of vertebral fracture risk. Bone 6:1–7. 9. Genant HK, Cann CE, Ettinger B, Gordan GS. 1982 Quantitative computed tomography of vertebral spongiosa: a sensitive method for detecting early bone loss after oophorectomy. Ann Intern Med 97:699–705. 10. Cann CE. 1988 Quantitative CT for determination of bone mineral density: a review. Radiology 166:509–522. 11. Theodorou DJ, Theodorou SJ, Andre MP, Kubota D, Weigert JM, Sartoris DJ. 2001 Quantitative computed tomography of spine: comparison of three-dimensionnal and two-dimensional imaging approaches in clinical practice. J Clin Densitom 4:57–62. 12. Laval-Jeantet AM, Cann CE, Roger BM, Dallant P. 1984 A post-processing dual-energy technique for vertebral CT densitometry. J Comput Assist Tomogr 9:1164–1167. 13. Louis O, Luypaert R, Kalender W, Osteaux M. 1988 Reproducibility of CT bone densitometry: operator versus automated ROI definition. Eur J Radiol 8:92–94. 14. Reinbold WD, Adler CP, Kalender WA, Lente R. 1991 Accuracy of vertebral mineral determination by dual-energy quantitative computed tomography. Skeletal Radiol 20:25–29. 15. Kalender W, Felsenberg D, Genant HK, Fischer M, Dequeker J, Reeve J. 1995 The European Spine Phantom: a tool for standardization and quality control in spinal bone mineral measurements by DXA and QCT. Eur J Radiol 20:83–92. 16. Laval-Jeantet AM, Genant HK, Wu CY, Gluer CC, Faulkner KG, Steiger P. 1993 Factors influencing long-term in vivo reproducibility of QCT vertebral densitometry. J Comput Assist Tomogr 17:915–921. 17. Kalender WA. 1992 A phantom for standardization and quality control in spinal bone mineral measurements by
Volume 5, 2002
266 QCT and DXA: design considerations and specifications. Med Phys 19:583–586. 18. Kalender W, Felsenberg D, Genant HK, Fischer M, Dequeker J, Reeve J. 1995 The European Spine Phantom: a tool for standardization and quality control in spinal bone mineral measurements by DXA and QCT. Eur J Radiol 20:83–92. 19. Pearson J, Dequeker J, Henley M, et al. 1995 European semi-anthropomorphic spine phantom for the calibration of bone densitometers: assessment of precision, stability and accuracy. Osteoporos Int 5:174–184. 20. Grampp S, Jergas M, Lang P, et al. 1996 Quantitative CT assessment of the lumbar spine and radius in patients with osteoporosis. Am J Roentgenol 167:133–140.
Journal of Clinical Densitometry
Braillon 21. Jones CD, Laval-Jeantet AM, Laval-Jeantet MH, Genant HK. 1987 Importance of measurements of spongious vertebral bone mineral density in the assessment of osteoporosis. Bone 8:201–206. 22. Genant HK, Ettinger B, Cann CE, Reiser U, Gordan GS, Kolb FO. 1985 Osteoporosis: assessment by quantitative computed tomography. Orthop Clin North Am 16:557–568. 23. Cummings SR, Black D. 1986 Should perimenopausal women be screened for osteoporosis? Ann Intern Med 104:817–823. 24. Gluer CC. 1999 Monitoring skeletal changes by radiological techniques. J Bone Miner Res 14:1952–1962. 25. Orwoll ES, Oviatt SK, the Nafarelin/Bone Study Group. 1991 Longitudinal precision of dual-energy X-ray absorptiometry in a multicenter study. J Bone Miner Res 6:191–197.
Volume 5, 2002
Original Article
Quantitative Computed Tomography Precision and Accuracy for Long-Term Follow-up of Bone Mineral Density Measurements A Five-Year In Vitro Assessment
Pierre M. Braillon, PHD, MD Department of Radiology (Pav. B), Hospital Edouard Herriot, Place d’Arsonval, 69347 Lyon Cedex 03, France
Abstract The purpose of this work was to test the long-term precision of quantitative computed tomography (QCT) on a CT scanner partly used for the measurement of bone mineral density (BMD). A spine phantom (ESP®), which simulates three lumbar vertebrae (Li, i = 2–4) with given mineral densities of 50, 100, and 200 mg hydroxyapatite equivalents (HAP)/cm3, respectively, was measured periodically over more than 5 yr on a Elscint-Marconi CT-Twin® scanner. A total of 80 measurements were taken. The measured BMDi values were 48.4 ± 1.2, 101.3 ± 1.1, and 212.6 ± 1.7 mg HAP/cm3, respectively (coefficient of variation [CV%] = 2.4, 1.1, and 0.8), and they were linearly correlated with the given density values (r > 0.99). The mean BMD value of the three simulated vertebrae was 120.8 ± 1.1 mg HAP/cm3 (CV% = 0.9), a value that corresponds to the mean lumbar BMD value in normal 65-yr-old women. We concluded that QCT is a precise and accurate method for long-term follow-up of BMD assessment in the population affected by osteoporosis. Key Words: Bone mineral density (BMD); quantitative computed tomography (QCT); long-term precision; osteoporosis.
Introduction
these systems, especially the shorter scanning times and the better image quality, their precision remains a concern for radiologists who require reliable body mineral density (BMD) measurements (11). In fact, for patients between the ages of 45 and 70, covering the most important period of life for BMD checks, the mean annual decrease of trabecular BMD at the spine level, measured by QCT, is approximately 1.8% in women and 0.8% in males, in our experience. Therefore, it is imperative that investigators be aware of how precise the system used in BMD follow-up measurements is. The short-term precision of CT scanners has been widely studied (8–10,12–15). However, other than the study of Laval-Jeantet et al. (16) covering a 2-yr period, to our knowledge there
The use of quantitative computed tomography (QCT) to measure the bone mineral density (BMD) at various skeletal sites, especially at the lumbar vertebral level, was developed some 20 yr ago (1–9). Since that time, an extensive literature has been published on the subject (10) and the technology has evolved. However, despite the improvements in
Received 08/07/01; Revised 11/13/01; Accepted 11/28/01. Address correspondence to Dr. Pierre M. Braillon, Department of Radiology (Pav. B), Hospital Edouard Herriot, Place d’Arsonval, 69347 Lyon Cedex 03, France. E-mail: pierre.braillon@chu_lyon.fr
259
260
Braillon
Fig. 1. (a) Spiral three-dimensional CT reconstruction of the bony part of the ESP phantom used. The bone mineral densities of the three simulated lumbar vertebrae are 50, 100, and 200 mg/cm3 (in HAP). (b) Image of the slice obtained at the mid-level of the second simulated vertebra, with the elliptical region of interest positioned for the BMD measurement. (c) Example of slice obtained in the reference CIRS phantom. Two successive reference measurements were performed in this phantom with the 50- and 150-mg HAP/cm3 inserts, respectively, and with the same scanning parameters as for the ESP measurements. (Figure continues)
are no data available that confirm the long-term precision of the QCT technique over several years. The aim of this study was to estimate this parameter from in vitro measurements performed on a modern CT scanner.
Materials and Methods The study was performed on an Elscint-Marconi CT-Twin® scanner (Elscint-Marconi, Haifa, Israel). This CT scanner works in single energy. In our department, it is heavily used for bone diagnosis imaging, with a mean number of 40,300 ± 4850 slices obtained each month. A preventive maintenance of this machine is scheduled every 2 mo, with an entire set of controls of the electronic circuits and of the image quality. Also, density measurements are made on specific phantoms, to calibrate the system in Hounsfield units. In between, a simple control of the X-ray beam and detectors is completed every Journal of Clinical Densitometry
week by the scanner technologists. The X-ray tube was replaced three times during the study, and an entire calibration of the system was performed each time. The study took place between January 1996 and June 2001 (e.g., over more than 5 yr between the first and the last measurements [1978 d, precisely]). To test the precision of the system, we measured a spine phantom developed by Kalender et al. (17,18) and manufactured by QRM, Erlangen, Germany. This phantom, called the “European Spine Phantom®” (ESP) was previously used in a European study for cross-calibration of bone densitometers (19). It simulates three lumbar vertebrae (Fig. 1a) designated hereafter as Li (i = 2–4). The given mineral densities of the simulated vertebrae were 50, 100, and 200 mg/cm3 in hydroxyapatite equivalent (HAP), respectively. A total of 80 measurements were taken. For each measurement, the following scan parameters were used: 120 kV, 220 mA s, scanVolume 5, 2002
Long-Term In Vitro QCT Precision for BMD Assessment
Fig. 1.
field diameter 500 mm. A 10-mm-thick slice, centered on the mid-plane of each vertebra was obtained, and the BMDi (i = 2–4) were measured within an elliptical region of interest of 14 mm × 21 Journal of Clinical Densitometry
261
(continued)
mm (surface area ≈ 225 mm2), centered in the vertebral body (Fig. 1b). The BMD analysis was conducted the same way as in patient investigations; reference values were systematically obtained from Volume 5, 2002
Fig. 2. Shewhart-type plot of BMDi values, and their mean value, BMDm, against time expressed in days since the first measurement. BMDm, and two standard deviations above and below the mean are shown. The dates on which the X-ray tube has been replaced are indicated by arrowheads.
262
Journal of Clinical Densitometry
Braillon
Volume 5, 2002
Long-Term In Vitro QCT Precision for BMD Assessment
263
Table 1 Mean Bone Mineral Density (BMD) Values Measured in Each Simulated Vertebra of the ESP phantom (Li, i = 2–4), with Their Coefficient of Variation (CV%) and Their Differences (∆%) with the Corresponding Specified Values Given BMD values L2:50 L3:100 L4:200 Mean BMD
Measured values ± 1 SD
CV (%)
∆ (%)
48.4 ± 1.2 101.3 ± 1.1 212.6 ± 1.8 120.8 ± 1.1
2.4 1.1 0.8 0.9
–3.2 +1.3 +6.3 +3.5
Note: BMD values are expressed in mg HAP/cm3.
the measurement of a calibration phantom (CT Simulator®, model 4, from CIRS, Norfolk, VA). This second phantom simulates a 30-mm-thick human body section located at the level of the second lumbar vertebra. Inserts of varying density in HAP equivalent can be introduced for tests, in place of the vertebral body, and two inserts with densities of 50 and 150 mg HAP/cm3 were used to obtain two successive slices (Fig. 1c). Finally, BMDi values, and their mean value, BMDm = ΣBMDi/3, were obtained by reference to the pair of data obtained on the CT Simulator ® phantom.
Results The measured BMDi values were very stable throughout the study, as shown in Fig. 2. Their mean values, coefficients of variation (CV%), and differences with the specified densities are given in Table 1. Despite the differences, which ranged from –3.2% to +6.3%, a high linear correlation was found between the measured and given BMD values (Fig. 3). The mean measured BMD values, plotted on the normal value curve made for the lumbar spine BMD in women (unpublished data), are shown in Fig. 4. BMD4 and BMD3 were in good accordance with the values measured at the spine level in normal women, aged 25–35 and 70, respectively. BMD2 corresponds to a BMD value situated two standard deviations below the normal value at age 75, and finally, the BMDm value is very close to the mean spine BMD value in 65-yrold women. Journal of Clinical Densitometry
Discussion Quantitative CT is a valuable method for assessing spine BMD. It has the advantage of analyzing a well-defined zone, usually the central part of the vertebral body. This region is made of purely trabecular bone and has a high turnover rate. Therefore, BMD measurements in this region have a high sensitivity (20,21), an important point when screening for osteoporosis (22). Moreover, bone artifacts such as arthritis or extraosseous calcifications have no or a very low influence on the measurement. However, the follow-up of osteopenic patients continues over a long period of time, usually several years, and it is important that radiologists be sure that the changes observed in successive measurements, for a given patient, are significant. This means that the system used must be stable and have a sufficient precision to detect small BMD changes between two successive measurements. This is the least significant change (LSC) defined by Cummings and Black (23) and Glüer (24). Also, the comparison of measurements performed with different CT systems (e.g., in multicenter studies) should be possible by using simple cross-calibration data (19). From the results obtained in the present study, it appeared that the CT scanner used was stable over more than 5 yr, despite three X-ray tube replacements. Its accuracy was dependent on the BMD values, the differences with the specified values of 50, 100, and 200 mg HAP/cm3, being –3.2%, +1.3%, and +6.3%, respectively. However, these differences yielded a linear correlation between the measured Volume 5, 2002
Fig. 3. Linear correlation obtained between the measured BMDi (±2 SD) and the given BMD values of the simulated vertebrae. The dotted line is the line of identity.
Fig. 4. Positions of the measured BMDi values on the normal lumbar BMD value curve, established against age, for women. Circles: BMDi diamond: BMDm. BMD4 and BMD3 were in good accordance with the normal BMD at ages 25–35 and 70, respectively, whereas BMD2 corresponded to a BMD value two standard-deviations below the normal mean at age 75. Note that the BMD value decrease, as defined on the normal value curve, is quite linear between ages 45 and 70.
Long-Term In Vitro QCT Precision for BMD Assessment BMD values and the specified values, a relationship that makes it simple to adjust measured data in patients, whenever necessary. Precision, as estimated by the CV values of the 80 measurements taken, ranged from 0.8% for the highest density value to 2.4% for the lowest, and a CV of 0.9% was found for the mean BMD value of the three simulated vertebrae in the ESP phantom. This result could be compared with the longitudinal precision of the dual-energy X-ray absorptiometry (DXA) technique for posterior–anterior spine BMD measurements, which can be considered as the reference method (24) for these kinds of measurement. In a multicenter study, Orwoll et al. (25) found a longitudinal variability ranging from 0.11% to 1.19%, in four in vitro measurements, over the course of 1 yr. We obtained a comparable precision value of 0.70% in 2325 DXA measurements of a spine phantom on a Norland XR-36® DXA system, over a period of time of 54 mo (unpublished data). Therefore, the standard precision error, as defined by Glüer (24), can be estimated to about 1.1% for the QCT measurements of the present study. Consequently, for these BMD measurements, LSC should be 2.8 × 1.1 ≈ 3.0% to be detected with 95% confidence (p ≤ 0.05). Although in our experience the decrease of the spine BMD in women aged from 45 to 70 yr is quite linear, as shown on our normal value curve, and the annual BMD decrease rate is –1.8% over this period of time, we concluded that QCT is a suitable technique for assessing the spine BMD with confidence in perimenopausal and menopausal women over longterm periods of time. In these populations, the reliability of two to three successive QCT examinations, performed within an 18- to 24-mo time interval, over approximately 5 yr, is acceptable, even in the cases where the decrease in the patient’s spine BMD is on the same order as the physiological changes.
Acknowledgments We would like to thank Mr. T. Blanchard, engineer in charge of the CT scanner maintenance for his careful work, and assistance. The study was conducted with the ESP phantom #23, which had previously been used in a concerted action of the European Community’s COMAC (1989–1992). The author also thanks B. Kiraly for his help in digitizing images.
Journal of Clinical Densitometry
265
References 1. Isherwood I, Rutherford RA, Pullan BR, Adams PH. 1976 Bone mineral estimation by computer assisted transverse axial tomography. Lancet 1:712–715. 2. Ruegsegger P, Elsasser U, Anliker M, Gnehm H, Kind H, Prader A. 1976 Quantification of bone mineralization using computed tomography. Radiology 121:93–97. 3. Genant HK, Boyd DP. 1977 Quantitative bone mineral analysis using dual-energy computed tomography. Invest Radiol 12:545–551. 4. Cann CE, Genant HK. 1980 Precise measurement of vertebral mineral content using computed tomography. J Comput Assist Tomogr 4:493–500. 5. Cann CE, Genant HK. 1982 Cross-sectional studies of vertebral mineral using quantitative computed tomography. J Comput Assist Tomogr 6:216–218. 6. Adams JE, Chen SZ, Adams PH, Isherwood I. 1982 Measurement of trabecular bone mineral by dual-energy computed tomography. J Comput Assist Tomogr 6:601–607. 7. Cann CE, Genant HK. 1983 Single versus dual-energy CT for vertebral mineral quantification. J Comput Assist Tomogr 7:551. 8. Cann CE, Genant HK, Kolb FO, Ettinger B. 1985 Quantitative computed tomography for prediction of vertebral fracture risk. Bone 6:1–7. 9. Genant HK, Cann CE, Ettinger B, Gordan GS. 1982 Quantitative computed tomography of vertebral spongiosa: a sensitive method for detecting early bone loss after oophorectomy. Ann Intern Med 97:699–705. 10. Cann CE. 1988 Quantitative CT for determination of bone mineral density: a review. Radiology 166:509–522. 11. Theodorou DJ, Theodorou SJ, Andre MP, Kubota D, Weigert JM, Sartoris DJ. 2001 Quantitative computed tomography of spine: comparison of three-dimensionnal and two-dimensional imaging approaches in clinical practice. J Clin Densitom 4:57–62. 12. Laval-Jeantet AM, Cann CE, Roger BM, Dallant P. 1984 A post-processing dual-energy technique for vertebral CT densitometry. J Comput Assist Tomogr 9:1164–1167. 13. Louis O, Luypaert R, Kalender W, Osteaux M. 1988 Reproducibility of CT bone densitometry: operator versus automated ROI definition. Eur J Radiol 8:92–94. 14. Reinbold WD, Adler CP, Kalender WA, Lente R. 1991 Accuracy of vertebral mineral determination by dual-energy quantitative computed tomography. Skeletal Radiol 20:25–29. 15. Kalender W, Felsenberg D, Genant HK, Fischer M, Dequeker J, Reeve J. 1995 The European Spine Phantom: a tool for standardization and quality control in spinal bone mineral measurements by DXA and QCT. Eur J Radiol 20:83–92. 16. Laval-Jeantet AM, Genant HK, Wu CY, Gluer CC, Faulkner KG, Steiger P. 1993 Factors influencing long-term in vivo reproducibility of QCT vertebral densitometry. J Comput Assist Tomogr 17:915–921. 17. Kalender WA. 1992 A phantom for standardization and quality control in spinal bone mineral measurements by
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266 QCT and DXA: design considerations and specifications. Med Phys 19:583–586. 18. Kalender W, Felsenberg D, Genant HK, Fischer M, Dequeker J, Reeve J. 1995 The European Spine Phantom: a tool for standardization and quality control in spinal bone mineral measurements by DXA and QCT. Eur J Radiol 20:83–92. 19. Pearson J, Dequeker J, Henley M, et al. 1995 European semi-anthropomorphic spine phantom for the calibration of bone densitometers: assessment of precision, stability and accuracy. Osteoporos Int 5:174–184. 20. Grampp S, Jergas M, Lang P, et al. 1996 Quantitative CT assessment of the lumbar spine and radius in patients with osteoporosis. Am J Roentgenol 167:133–140.
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Braillon 21. Jones CD, Laval-Jeantet AM, Laval-Jeantet MH, Genant HK. 1987 Importance of measurements of spongious vertebral bone mineral density in the assessment of osteoporosis. Bone 8:201–206. 22. Genant HK, Ettinger B, Cann CE, Reiser U, Gordan GS, Kolb FO. 1985 Osteoporosis: assessment by quantitative computed tomography. Orthop Clin North Am 16:557–568. 23. Cummings SR, Black D. 1986 Should perimenopausal women be screened for osteoporosis? Ann Intern Med 104:817–823. 24. Gluer CC. 1999 Monitoring skeletal changes by radiological techniques. J Bone Miner Res 14:1952–1962. 25. Orwoll ES, Oviatt SK, the Nafarelin/Bone Study Group. 1991 Longitudinal precision of dual-energy X-ray absorptiometry in a multicenter study. J Bone Miner Res 6:191–197.
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