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Long-term reproducibility of quantitative computed tomography vertebral mineral measurements
CT: THE JOURNAL
OF COMPUTED
TOMOGRAPHY
1985;
9:73-76
73
LONG-TERM REPRODUCIBILITY OF QUANTITATIVE COMPUTED TOMOGRAPHY VERTEBRAL MINERAL MEASUREMENTS VIRGIL B. GRAVES,
MD,
AND ROD WIMMER,
Similar methods for measuring bone density using quantitative computed tomography have been proposed by Cann and Genant and by Firooznia et al. This study was done to evaluate the long-term reproducibility of quantitative computed tomography in the clinical setting. An anthropomorphic phantom was evaluated over an 18-month interval. A variation of -t-2.2% was obtained, substantiating that quantitative computed tomography can be done in a reproducible manner over an extended time interval. KEY WORDS: Bone
density;
Quantitative
computed
tomography
A method for monitoring and accurately quantifying bone density and dynamics has long been sought (1). The history of quantitative bone density measurement is one of progressive technologic refinement, including the photon densitometric methods of Cameron and Sorenson (2) and radiographic microdensitometric methods. More recently, computed tomography (CT) methods have been used to make serial quantitative bone density measurements (3-13). Because of the long-term nature of bone diseases, any system designed to measure quantitative changes must be stable over a long-time interval. Reproducible measurements are paramount for the
From the Department of Radiology, Columbus Hospital, Great Falls, Montana 59405. Address reprint requests to: Virgil B. Graves, MD, Department of Radiology, Columbus Hospital, Great Falls, Montana 59405. Received May 1984. 0 1985 by Elsevier Science Publishing Co., Inc. 52 Vanderbilt Ave., New York, NY 10017 0149-936X/85/$3.30
PhD
assessment of bone density, diagnosis of metabolic bone disease, treatment, and the efficacy of such treatment. Recently, surement of a reference and Genant
a method for precise and accurate meavertebral mineral content using CT and phantom has been developed by Cann
(7, 9-11, 14-16) and by Firooznia et al. (12, 13). In view of the numerous variables that affect the CT data and the need for long-term reproducibility and accuracy in bone density determinations, we wanted to determine the long-term reproducibility and error inherent in this method over an extended period of time. In addition, we wanted to see if this method could be easily and precisely done in a community hospital environment. The short-term reproducibility (1.6%) (16) and accuracy (5 to 8%) (17) of this scanning method and data analysis technique has been determined by Cann and Genant (7). They have reported long-term in vivo reproducibility using off-line software localization. We wanted to examine the long-term reproducibility over an extended period of time, approximately 1.5 years, using commercially available online software with scout-view localization. METHOD A anthropomorphic phantom was used to simulate an actual patient. The anthropomorphic phantom was selected for two reasons: 1) It possesses a nonvariable bone mineral content for the vertebral bodies; and 2) its mass and humanlike construction present the same type of scan problem for the technologist and machine in a repeatable way. In this manner the reproducibility of the composite CT scanning system comprising the technologist, machine, and reference phantom can be measured. The reference phantom contains known standards for si-
74
CT: THE JOURNAL OF COMPUTED
GRAVES & WIMMER
TOMOGRAPHY
VOL. 9 NO. 1
multaneously calibrating the scanner and permiting expression of results in milligrams per cubic centimeters of K2HP04. The anthropomorphic phantom and reference phantom (Figure 1) were scanned on a G.E. CT/T 8800, using a 35cm scan field, single energy - 120 kVp, 80 MA, and a 4.8-set scan time. The method used for localization/selection and data analysis, previously described by Cann and Genant and by Firooznia et al., was used with some variation in slice localization technique, using only the commercially available scout-view for localization (Figure 2) (7, 11, 15). The mean mineral content I was calculated by averaging the measured mineral content of three vertebral bodies of the phantom. The data was collected in a random fashion over an 18-month period. During this time interval, 31 measurements were obtained. RESULTS FIGURE 1.
Axial scan of anthropomorphic reference phantom.
FIGURE 2.
Lateral scout localization.
phantom and
Statistical analysis of the data showed a mean X of 70.5 mg/cm3, a standard deviation of -+1.6 mg/cm3, and a variation of -+2.2% of the measured mineral content over the 18-month time interval (Figure 3). This analysis represents one of the extremes in variation as well as the clinically significant “worst case” because the mineral content of 70.5 mg/cm3 of the phantom vertebral bodies approximates that of an osteoporotic individual or a value reduced by over 50% of a normal healthy subject. The same error of +-1.6 mg/cm3 extrapolated to a normal mineral content of 150 mg/cm3 would yield a precision of 1%. CONCLUSIONS In extrapolating from a phantom to a patient one has to deal with potential patient motion; but, if the patient has a scout-view before and after scanning and the slices represented on each scout-view are nearly identical, then significant motion has not occurred. In this case the precision shown for the phantom should be representative of in vivo patient reproducibility. The study suggests vertebral body mineral content can be measured in a reproducible manner and is comparable over an extended period of time using the similar methods and reference phantom described by Cann and Genant and by Firooznia et al. These measurements can be made in a community hospital with reasonable care and attention to technique and positioning.
JANUARY 1985
QUANTITATIVE
MEAN
ii-70
CT
75
.6
S.D.11.6
:
FEB
:
t,.
APR
?? o***0.
JUNE
.
.
.
. .
AUG
?? ? ? ? ???? ? ?? ?
.
??
.
OCT
DEC
bone mineral
content
JULY
of trabecular bone density and other bone mineral parameters Normal values in children and adults. Br J Radio1 1979;52:14-23.
1. Sandler
RB, Herbert DL: Quantitative bone assessments: Applications and expectations. J Am Geriatr Sot 1981;24:97-
9.
Genant HK, Boyd DP: Quantitative bone mineral analysis using dual energy computed tomography. Invest Radio1 1977;12:545-51.
103.
10. Cann CE, Genant HK, Ehinger
B, et al.: Spinal mineral loss in oophorectomized women, determination by quantitative computed tomography. JAMA 1980;294:2056-9.
2.
Cameron SR, Sorenson JA: A reliable in vivo measurement of bone mineral content. J Bone Joint Surg 1967;494:481-97.
3.
Pullan BR, Roberts TE: Bone mineral measurement EM1 scanner and standard methods: A comparative J Radio1 1978;51:24-8.
4.
Jensen PS, Orphanoudakis SC, Rauschkollo EN, et al.: Assessment of bone mass in the radius by computed tomography. AJR 1980;134:285-92.
5.
Orphanoudakis SC, Jensen PS, Rauschkolls EN, et al.: Bone mineral analysis using single-energy computed tomography. Invest Radio1 1979;14:122-30.
12. Firooznia
6
Revak CS: Mineral content of cortical bone measurement by computed tomography. J Comput Assist Tomogr 1980;4:34250. Cann CE, Genant HK: Precise measurement of vertebral mineral content using computed tomography. J Comput Assist Tomogr 1980;4:493-500. Exner GU, Prader A, Elsasser U, et al.: Bone censitometry using computed tomography, part I: Selective determination
13.
8.
JUNE
over an
REFERENCES
7
APA
‘63
‘62
FIGURE 3. Plot of vertebral 18-month time interval.
FE8
using an study. Br
11. Cann CE, Genant
HK, Ehinger B, et al.: Determination of bone mineral loss in the axial skeleton of oophorectomized women using quantitative computed tomography: Proceedings from the second international workshop on bone and soft tissue densitometry, Zuoz, Switzerland. J Comput Assist Tomogr 1982;6:217-8.
14.
H, Golimbu C, Mahvash R, et al.: Quantitative computed tomography assessment of spinal trabecular bone, I. CT 1984;8:91-7. Firooznia H, Golimbu C, Mahvash R, et al.: Quantitative computed tomography assessment of spinal trabecular bone, II. CT 1984;8:99-103. Cann CE, Genant HK: Single versus dual energy CT for vertebral mineral quantification: Proceedings from the third international workshop on bone and soft tissue densitometry, Banff, Alberta, Canada. J Comput Assist Tomogr 1983; 7551-2.
76
CT: THE JOURNAL OF COMPUTED TOMOGRAPHY VOL. 9 NO. 1
GRAVES & WIMMER
15. Genant HK, Cann CE, Ettinger B, et al.: Quantitative computed tomography of vertebral spongiosa: A sensitive method for detecting early bone loss after oophorectomy. Ann Intern Med 1982;97:699-705. 16. Genant HK, Cann CE, Pozzi-Mucelli RS, et al.: Vertebral minera1 determined by quantitative CT: Clinical feasibility and normative data. J Comput Assist Tomogr 1983;7:554.
c. a and b.
d. None of the above. 2. Bone density a. Diagnosis
measurements of metabolic
b. Efficacy of treatment c. Both of the above.
are useful in: bone disease.
of metabolic
bone disease.
17. Genant HK, Cann CE, Boyd DP, et al.: Quantitative computed tomography for vertebral mineral determination: In Proceedings of Henry Ford hospital symposium on clinical disorders of bone and mineral metabolism. Detroit, Michigan, 1983.
3. A vertebral
CONTINUING MEDICAL EDUCATION QUESTIONS
4. Bone mineralization quantitation can be done in a reproducible and comparable manner using commer-
1. Bone density measurement techniques a. Long-term reproducibility. b. Short-term reproducibility.
must have:
mineral
content
equivalent
to 70.5 mgkm”
K,HPO, approximates: a. That of a normal, healthy individual. b. That of an osteoportic individual. c. Neither a or b.
cially
available
a. True. b. False.
on-line
software.
OF COMPUTED
TOMOGRAPHY
1985;
9:73-76
73
LONG-TERM REPRODUCIBILITY OF QUANTITATIVE COMPUTED TOMOGRAPHY VERTEBRAL MINERAL MEASUREMENTS VIRGIL B. GRAVES,
MD,
AND ROD WIMMER,
Similar methods for measuring bone density using quantitative computed tomography have been proposed by Cann and Genant and by Firooznia et al. This study was done to evaluate the long-term reproducibility of quantitative computed tomography in the clinical setting. An anthropomorphic phantom was evaluated over an 18-month interval. A variation of -t-2.2% was obtained, substantiating that quantitative computed tomography can be done in a reproducible manner over an extended time interval. KEY WORDS: Bone
density;
Quantitative
computed
tomography
A method for monitoring and accurately quantifying bone density and dynamics has long been sought (1). The history of quantitative bone density measurement is one of progressive technologic refinement, including the photon densitometric methods of Cameron and Sorenson (2) and radiographic microdensitometric methods. More recently, computed tomography (CT) methods have been used to make serial quantitative bone density measurements (3-13). Because of the long-term nature of bone diseases, any system designed to measure quantitative changes must be stable over a long-time interval. Reproducible measurements are paramount for the
From the Department of Radiology, Columbus Hospital, Great Falls, Montana 59405. Address reprint requests to: Virgil B. Graves, MD, Department of Radiology, Columbus Hospital, Great Falls, Montana 59405. Received May 1984. 0 1985 by Elsevier Science Publishing Co., Inc. 52 Vanderbilt Ave., New York, NY 10017 0149-936X/85/$3.30
PhD
assessment of bone density, diagnosis of metabolic bone disease, treatment, and the efficacy of such treatment. Recently, surement of a reference and Genant
a method for precise and accurate meavertebral mineral content using CT and phantom has been developed by Cann
(7, 9-11, 14-16) and by Firooznia et al. (12, 13). In view of the numerous variables that affect the CT data and the need for long-term reproducibility and accuracy in bone density determinations, we wanted to determine the long-term reproducibility and error inherent in this method over an extended period of time. In addition, we wanted to see if this method could be easily and precisely done in a community hospital environment. The short-term reproducibility (1.6%) (16) and accuracy (5 to 8%) (17) of this scanning method and data analysis technique has been determined by Cann and Genant (7). They have reported long-term in vivo reproducibility using off-line software localization. We wanted to examine the long-term reproducibility over an extended period of time, approximately 1.5 years, using commercially available online software with scout-view localization. METHOD A anthropomorphic phantom was used to simulate an actual patient. The anthropomorphic phantom was selected for two reasons: 1) It possesses a nonvariable bone mineral content for the vertebral bodies; and 2) its mass and humanlike construction present the same type of scan problem for the technologist and machine in a repeatable way. In this manner the reproducibility of the composite CT scanning system comprising the technologist, machine, and reference phantom can be measured. The reference phantom contains known standards for si-
74
CT: THE JOURNAL OF COMPUTED
GRAVES & WIMMER
TOMOGRAPHY
VOL. 9 NO. 1
multaneously calibrating the scanner and permiting expression of results in milligrams per cubic centimeters of K2HP04. The anthropomorphic phantom and reference phantom (Figure 1) were scanned on a G.E. CT/T 8800, using a 35cm scan field, single energy - 120 kVp, 80 MA, and a 4.8-set scan time. The method used for localization/selection and data analysis, previously described by Cann and Genant and by Firooznia et al., was used with some variation in slice localization technique, using only the commercially available scout-view for localization (Figure 2) (7, 11, 15). The mean mineral content I was calculated by averaging the measured mineral content of three vertebral bodies of the phantom. The data was collected in a random fashion over an 18-month period. During this time interval, 31 measurements were obtained. RESULTS FIGURE 1.
Axial scan of anthropomorphic reference phantom.
FIGURE 2.
Lateral scout localization.
phantom and
Statistical analysis of the data showed a mean X of 70.5 mg/cm3, a standard deviation of -+1.6 mg/cm3, and a variation of -+2.2% of the measured mineral content over the 18-month time interval (Figure 3). This analysis represents one of the extremes in variation as well as the clinically significant “worst case” because the mineral content of 70.5 mg/cm3 of the phantom vertebral bodies approximates that of an osteoporotic individual or a value reduced by over 50% of a normal healthy subject. The same error of +-1.6 mg/cm3 extrapolated to a normal mineral content of 150 mg/cm3 would yield a precision of 1%. CONCLUSIONS In extrapolating from a phantom to a patient one has to deal with potential patient motion; but, if the patient has a scout-view before and after scanning and the slices represented on each scout-view are nearly identical, then significant motion has not occurred. In this case the precision shown for the phantom should be representative of in vivo patient reproducibility. The study suggests vertebral body mineral content can be measured in a reproducible manner and is comparable over an extended period of time using the similar methods and reference phantom described by Cann and Genant and by Firooznia et al. These measurements can be made in a community hospital with reasonable care and attention to technique and positioning.
JANUARY 1985
QUANTITATIVE
MEAN
ii-70
CT
75
.6
S.D.11.6
:
FEB
:
t,.
APR
?? o***0.
JUNE
.
.
.
. .
AUG
?? ? ? ? ???? ? ?? ?
.
??
.
OCT
DEC
bone mineral
content
JULY
of trabecular bone density and other bone mineral parameters Normal values in children and adults. Br J Radio1 1979;52:14-23.
1. Sandler
RB, Herbert DL: Quantitative bone assessments: Applications and expectations. J Am Geriatr Sot 1981;24:97-
9.
Genant HK, Boyd DP: Quantitative bone mineral analysis using dual energy computed tomography. Invest Radio1 1977;12:545-51.
103.
10. Cann CE, Genant HK, Ehinger
B, et al.: Spinal mineral loss in oophorectomized women, determination by quantitative computed tomography. JAMA 1980;294:2056-9.
2.
Cameron SR, Sorenson JA: A reliable in vivo measurement of bone mineral content. J Bone Joint Surg 1967;494:481-97.
3.
Pullan BR, Roberts TE: Bone mineral measurement EM1 scanner and standard methods: A comparative J Radio1 1978;51:24-8.
4.
Jensen PS, Orphanoudakis SC, Rauschkollo EN, et al.: Assessment of bone mass in the radius by computed tomography. AJR 1980;134:285-92.
5.
Orphanoudakis SC, Jensen PS, Rauschkolls EN, et al.: Bone mineral analysis using single-energy computed tomography. Invest Radio1 1979;14:122-30.
12. Firooznia
6
Revak CS: Mineral content of cortical bone measurement by computed tomography. J Comput Assist Tomogr 1980;4:34250. Cann CE, Genant HK: Precise measurement of vertebral mineral content using computed tomography. J Comput Assist Tomogr 1980;4:493-500. Exner GU, Prader A, Elsasser U, et al.: Bone censitometry using computed tomography, part I: Selective determination
13.
8.
JUNE
over an
REFERENCES
7
APA
‘63
‘62
FIGURE 3. Plot of vertebral 18-month time interval.
FE8
using an study. Br
11. Cann CE, Genant
HK, Ehinger B, et al.: Determination of bone mineral loss in the axial skeleton of oophorectomized women using quantitative computed tomography: Proceedings from the second international workshop on bone and soft tissue densitometry, Zuoz, Switzerland. J Comput Assist Tomogr 1982;6:217-8.
14.
H, Golimbu C, Mahvash R, et al.: Quantitative computed tomography assessment of spinal trabecular bone, I. CT 1984;8:91-7. Firooznia H, Golimbu C, Mahvash R, et al.: Quantitative computed tomography assessment of spinal trabecular bone, II. CT 1984;8:99-103. Cann CE, Genant HK: Single versus dual energy CT for vertebral mineral quantification: Proceedings from the third international workshop on bone and soft tissue densitometry, Banff, Alberta, Canada. J Comput Assist Tomogr 1983; 7551-2.
76
CT: THE JOURNAL OF COMPUTED TOMOGRAPHY VOL. 9 NO. 1
GRAVES & WIMMER
15. Genant HK, Cann CE, Ettinger B, et al.: Quantitative computed tomography of vertebral spongiosa: A sensitive method for detecting early bone loss after oophorectomy. Ann Intern Med 1982;97:699-705. 16. Genant HK, Cann CE, Pozzi-Mucelli RS, et al.: Vertebral minera1 determined by quantitative CT: Clinical feasibility and normative data. J Comput Assist Tomogr 1983;7:554.
c. a and b.
d. None of the above. 2. Bone density a. Diagnosis
measurements of metabolic
b. Efficacy of treatment c. Both of the above.
are useful in: bone disease.
of metabolic
bone disease.
17. Genant HK, Cann CE, Boyd DP, et al.: Quantitative computed tomography for vertebral mineral determination: In Proceedings of Henry Ford hospital symposium on clinical disorders of bone and mineral metabolism. Detroit, Michigan, 1983.
3. A vertebral
CONTINUING MEDICAL EDUCATION QUESTIONS
4. Bone mineralization quantitation can be done in a reproducible and comparable manner using commer-
1. Bone density measurement techniques a. Long-term reproducibility. b. Short-term reproducibility.
must have:
mineral
content
equivalent
to 70.5 mgkm”
K,HPO, approximates: a. That of a normal, healthy individual. b. That of an osteoportic individual. c. Neither a or b.
cially
available
a. True. b. False.
on-line
software.