Download Can abdominal multi-detector CT diagnose spinal osteoporosis?

Eur Radiol (2009) 19: 172–176
DOI 10.1007/s00330-008-1099-2
MUSCULO SKELETAL
Antonios E. Papadakis
Apostolos H. Karantanas
Giorgos Papadokostakis
Effie Petinellis
John Damilakis
Can abdominal multi-detector CT diagnose
spinal osteoporosis?
Received: 7 September 2007
Revised: 9 May 2008
Accepted: 17 May 2008
Published online: 19 July 2008
# European Society of Radiology 2008
E. Petinellis
Department of Radiotherapy,
Faculty of Medicine,
University Hospital of Heraklion,
P.O. Box 2208
Iraklion, 71003, Crete, Greece
A. E. Papadakis (*) . J. Damilakis
Department of Medical Physics,
Faculty of Medicine,
University of Crete,
P.O. Box 2208
Iraklion, 71003, Crete, Greece
e-mail: [email protected]
Tel.: +30-2810-392092
Fax: +30-2810-542095
A. H. Karantanas
Department of Radiology,
Faculty of Medicine,
University of Crete,
P.O. Box 2208
Iraklion, 71003, Crete, Greece
G. Papadokostakis
Department of Orthopedics,
Faculty of Medicine,
University Hospital of Heraklion,
P.O. Box 2208
Iraklion, 71003, Crete, Greece
Abstract The aim of this study was to
(1) generate quantitative CT (QCT)
densitometric data based on routine
abdominal multi-detector (MDCT)
examinations and (2) investigate
whether these data can be used to
differentiate osteoporotic from healthy
females. Twenty-five female patients
(group A) with a history of radiotherapy were examined both with
routine abdominal MDCT and
standard QCT to generate a MDCTto-QCT conversion equation. Twentyone osteoporotic (group B) and 23
healthy female patients (group C)
were also recruited in the study.
Patients of groups B and C underwent
routine abdominal MDCT examina-
Introduction
Osteoporosis is the most common metabolic bone disorder
leading to enhanced bone fragility and, consequently, to lowenergy bone fractures. As the elderly population is expected
to grow in the coming years, the total number of individuals
affected by osteoporosis is also expected to increase
dramatically [1]. Quantitative assessment of osteoporosis
and estimation of fracture risk relies mainly on bone mineral
density (BMD) assessment [2–5]. Dual-energy X-ray
tion for various clinical indications.
Mean bone mineral density (BMD) in
patients of group A was 103.4 mg/ml±
32.8 with routine abdominal MDCT
and 91.0 mg/ml±28.5 with QCT.
Quantitative CT BMDQCT values for
patients in groups B and C were
calculated utilizing the BMDMDCT
values derived from routine abdominal MDCT data sets and
the MDCT to QCT conversion equation: BMDQCT ¼ 0:78 BMDMDCT þ
10:13 . The calculated QCT densitometric data adequately differentiated
osteoporotic from healthy females
(area under ROC curve 0.828,
p=0.05). In conclusion, this study
showed that in a group of female
patients, QCT data derived from
routine abdominal MDCT examinations discriminated osteoporotic from
healthy subjects.
Keywords Osteoporosis . Multidetector CT . Abdominal imaging .
CT densitometry . Image analysis
absorptiometry (DXA) and quantitative computed tomography (QCT) of the lumbar spine are currently considered by
the clinical and research facilities as the methods most
clinically relevant for the evaluation of BMD. The former is
accepted by the World Health Organization (WHO) as the
gold standard for diagnosing osteoporosis, since it is a
method that is inexpensive, rapid, easy to perform, and
involves low-radiation exposure. However, it is widely
accepted that QCT is the most sensitive method available to
detect osteoporosis [6, 7]. Each image slice of a given
173
thickness represents a tissue volume and thus a true
volumetric BMD can be obtained, as opposed to areal
BMD obtained with DXA. In addition, QCT is able to
separately assess the trabecular and cortical BMD.
Abdominal MDCT is a quite common radiologic
examination, which includes densitometric data on the
lumbar vertebrae. The question that arises is whether these
densitometric data may be used for accurate BMD
assessment. Preliminary studies have shown that high
correlation between BMD, as determined with routine
abdominal CT, versus lumbar QCT can be achieved [8–10].
The aim of this study was to (1) generate QCT
densitometric data based on routine abdominal MDCT
examinations and (2) investigate whether these data can be
used to differentiate osteoporotic from healthy females.
Materials and methods
Patients
Twenty-five female patients (mean age 61±10.7 years), 12
with history of uterine-cervix cancer and 13 with history of
breast cancer, were recruited in the study (group A). These
patients are considered to have a high risk of developing
osteoporosis. All patients had had conformal radiotherapy
and chemotherapy treatments and were referred for
abdominal MDCT to rule out tumor recurrence in their
long-term follow-up. These patients were additionally
scanned with the standard QCT protocol to asses BMD
values. The excess exposure of the QCT examination was
considered negligible compared to the dose delivered from
the previous radiation therapy.
Twenty-one osteoporotic female patients (mean age,
69±5.9 years) referred by an orthopedic surgeon for lumbar
CT to analyze a possible vertebra fracture after a low-energy
trauma were also included in the study (group B). Twentythree female patients (mean age, 70±5.3 years) referred to
clinically indicated, non-emergency routine abdominal
MDCT were also recruited in the study (group C). Women
in group C were selected to match the age range of women
in group B. Moreover, they had no premature menopause,
history of bone disease, low-energy bone fractures, and
trauma or malignancies. They were not subjected to any
medical treatment with drugs that might influence their bone
metabolism. Patients in groups B and C underwent only
routine abdominal MDCT examination. The study was
approved by the local ethics committee, and informed
consent was given by all patients who participated in the
study.
CT scan protocols and image analysis
All CT examinations were performed with a 16-slice CT
system (Somatom Sensation 16, Siemens, Erlangen,
Germany). QCT examinations were performed using an
application-specific QCT protocol according to the manufacturer’s instructions and a dedicated calibration phantom [11] (Osteo calibration phantom, Siemens). The QCT
protocol constituted of 2×5=10-mm slice thickness
sequential scans, acquired with 80 kVp tube potential and
125 mAs tube load. The S80 kernel was used for image
reconstruction. Before acquiring the QCT data, a lateral
scout image of the lumbar spine of the subject was
obtained. For correct patient positioning, a gel pack sponge
was placed in between each patient’s spine and the
calibration phantom during acquisition. Four sequential
QCT data sets were acquired per patient, each corresponding to the midvertebral sections of T12, L1, L2, and L3
vertebra. The gantry was tilted appropriately before each
acquisition so that the orientation of the acquired image
slice was always parallel to the upper and lower endplates.
By using the implemented Osteo software (Siemens),
automatic contour tracing of the trabecular and the cortical
bone was performed on each image slice. An easy manual
interaction for contour tracing modification was allowed.
The trabecular and cortical bone regions of each vertebra
are automatically divided into the left and the right subregions. Images with artifact structures within the vertebral
region were excluded from the measurements. Trabecular
and cortical BMD values are calculated based on the
comparison with a standard table, which includes a healthy
bone reference group with age- and sex-specific data from
three European centers (Siemens, QCT Reference). The
BMDs per subject were derived by averaging four vertebral
bodies (T12, L1-L3).
A routine abdominal MDCT protocol was used for the
examination of all patients (groups A, B, and C). CT
parameters were as follows: tube voltage 120 kVp, tube
load 160 mAs, beam collimation 16×1.5 mm, table feed
24 mm, reconstruction image slice 10 mm, and reconstruction kernel B31. The dedicated calibration phantom was
used as in QCT. Before MDCT, all patients had been given
1,000 ml of oral contrast medium (0.3 g/ml, Iopamiro
Gastro, Bracco, Italy). Based on the scout image and by
analyzing the series of the acquired images, the slices
closest to the midvertebral sections of T12, L1, L2, and L3
vertebra were selected for BMD assessment. In these slices,
BMD values of the trabecular and the cortical bone were
calculated following the same automated procedure
described above for the QCT. All data acquisition and
analyses were performed by a radiology technologist who
was supervised by an experienced radiologist. During this
procedure investigators were blinded to each patient’s
history, fracture status, and BMD as determined with QCT.
To give an indication of the error introduced in the
calculation of the BMDQCT, we have examined an
anthropomorphic Rando phantom (Alderson Research
Laboratories, Stanford, CA) with the QCT and abdominal
CT protocols. This phantom consists of human skeleton
embedded in lung- and tissue-equivalent material and
174
corresponds to a young male individual with average
anatomical characteristics. The Rando was examined on
top of the osteo calibration phantom. To study the effect of
table height on BMD, the Rando anthropomorphic
phantom was also scanned in different table heights, i.e.,
125 mm, 135 mm, 145 mm, and 155 mm. At each table
height, the QCT and the abdominal CT were repeated three
times. Mean BMD values averaged over T12, L1, L2, and
L3 were calculated at each table height.
Statistical analysis
Mean BMD values and standard deviations (SDs) were
calculated for all patients. The relationship between
measurements obtained with QCT and abdominal MDCT
in group A was assessed based on a linear regression
analysis. The correlation parameters between QCT and
abdominal MDCT calculated for group A were used to
predict the BMDQCT values for groups B and C. A twotailed Student’s t-test was applied to compare mean BMD
values among groups. The capability of QCT and abdominal MDCT to discriminate between fractured and
nonfractured subjects was assessed by calculating ROC
curves. Statistical significance was taken at P<0.05. All
statistical computations were processed using the MedCalc
software package (MedCalc software, Belgium).
Results
Figure 1 illustrates the BMD values obtained from standard
QCT as a function of the corresponding values obtained
from abdominal MDCT in patients with history of
radiotherapy (group A). Each point in the scatter plot is
an average of the trabecular BMD value over the T12 and
L1 through L3 vertebrae for each patient. The linear
BMDQCT ¼ 0:78 BMDMDCT þ 10:13
The correlation coefficient r was 0.90 (p<0.0001). Table 1
lists the mean BMDMDCT values derived from the abdominal
MDCT of the patients in the three groups, the mean BMDQCT
values derived from the QCT of the patients in group A, and
the mean BMDQCT values of the patients in groups B and C.
The latter values were calculated utilizing the measured
BMDMDCT values and the above equation. The difference
between the mean BMDQCT values of the osteoporotic
(group B) and healthy patients (group C) was statistically
significant (p=0.0001; 95% CI: 9.41 to 25.94). Moreover,
the receiving-operator characteristic curve (ROC), shown in
Fig. 2, indicated that abdominal MDCT may adequately
discriminate healthy and osteoporotic women. Area under the
curve is 0.828 with 0.063 standard error at the 95% confidence
interval. Mean BMD values averaged over the T12, L1, L2,
and L3 vertebra of the Rando anthropomorphic phantom for
QCT and abdominal CT were 209.5 mg/ml and 231.5 mg/ml,
respectively. If the above equation is applied to calculate the
anticipated BMDQCT value,it would be: BMDQCT ¼ 0:78 231:5 þ 10:13 ¼ 190:7 mg ml . Given that the measured by
the QCT scan BMDQCT value is 209.5 mg/ml, the error
introduced by the conversion equation in estimating BMDQCT
value is 8.9%. Although this error is not negligible, we consider
that it is partly due to the fact that the above phantom represents
a young male individual, while the conversion equation was
generated on female patients aged 61±10.7 years. The
coefficient of variation in the mean BMD among scans
performed at different table heights was less than 1.5%.
Discussion
The advent of multi-detector CT has significantly increased
the number of CT examinations. Surveys performed in the
US reveal that the annual number of CT examinations has
increased almost ten-fold in less than 2 decades [12].
Quantitative CT is a well-established technique used to
160
BMDQCT (mg Ca-HA/ml)
regression analysis relating the two BMD measurement
methods provided the following equation:
140
120
100
Table 1 Mean densitometric measurements in different group
populations obtained from standard abdominal MDCT and QCT
80
60
40
Abdominal MDCT
mg/ml
a
QCT
mg/ml
103.4±32.8
84.7±15.3
109.9±15.6
91.0±28.5
76.3±11.9
96.08±12.2
20
40
60
80
100
120
140
160
180
BMDMDCT (mg Ca-HA/ml)
Fig. 1 Correlation between bone mineral densities determined with
QCT (BMDQCT) and abdominal MDCT (BMDMDCT); r=0.90,
P<0.0001
Group A
Group B
Group C
a
QCT values for groups B and C were calculated based on the QCTabdominal CT calibration equation derived from patients in group A
175
100
Sensitivity
80
60
40
20
0
0
20
40
60
80
100
100-Specificity
Fig. 2 ROC curve of abdominal MDCT in differentiating
osteoporotic (group B) from healthy women (group C)
measure BMD. With optimized acquisition parameters and
dedicated software, the method has been widely used to
detect patients with osteoporosis. The results of this study
show that the densitometric data generated from routine
abdominal MDCT can be additionally used to differentiate
osteoporotic from healthy females.
Hopper et al. [9] analyzed contrast-enhanced and
unenhanced abdominal CT and QCT to assess BMD in
the L1, L2, and L3 vertebral bodies. That study aimed to
establish a baseline that could be used as a reference for
subsequent bone density studies. The findings of Hopper et
al. [9] showed that BMD values obtained with QCT
(90.6 mg/ml) are lower than those obtained with unenhanced abdominal CT (98.5 mg/cm3). These findings are in
agreement with those of our study. However, that study was
performed with a single-slice CT system. Lang et al. [7]
showed that BMD values derived from the standard helical
scan of L1 and L2 vertebral bodies may adequately
discriminate patients with vertebral factures from healthy
individuals. However, Lang et al. [7] did not recruit any
population for the generation of a helical vs. QCT
correlation coefficient. Link et al. [8] evaluated if
nondedicated standard spiral CT can be used to obtain
reliable bone mineral density data. That study showed that
there is a significant correlation in the densitometric
measurements between routine spiral CT and QCT. Quantitative CT BMD values can be derived using BMD values
from routine spiral CT multiplied by a conversion factor.
That factor was generated based on the correlation between
standard spiral CT and QCT. The results presented in the
current study are in agreement with those presented by Link
et al. [8]. The current study is differentiated from that of Link
et al. [8]. First, the generation of the abdominal MDCT to
QCT conversion equation was based on non-contrastenhanced abdominal MDCT scans. On the contrary, the
patients examined by Link et al. [8] underwent both
intravenous and oral administration of contrast medium
before abdominal CT, which alters the BMD of the bone
marrow in the vertebrae. The alteration in BMD depends on
the amount of intravenously administered contrast media,
which may vary from patient to patient according to the
specifications. Therefore, we believe that an abdominal
MDCT to QCT conversion based on densitometric data not
affected by contrast interference, similar to those presented in
this study, is more appropriate. Second, the CT system used
in our study is a modern 16-slice scanner as opposed to the 4slice CT machine used by Link et al. [8]. CT systems with
different configuration technologies (beam geometry, tube
potential, tube filtration, etc.) may be associated with diverse
MDCT-to-QCT conversion equations. Hence, the conversion
equation presented in the current study may be used from all
institutions equipped with a similar 16-slice system to generate
QCT densitometric data from routine abdominal MDCT.
Several studies on the comparison of bone densitometry
methods and measurement techniques have shown that
QCT exhibits the highest sensitivity [13, 14]. One of the
fundamental advantages of QCT is that selective measurement of the trabecular region excludes sources of error such
as osteophytes and hypertrophic posterior elements that
artificially elevate BMD values, thus reducing their
diagnostic efficacy. In spite of QCT’s high sensitivity and
valuable power in diagnosing osteoporosis, DXA has
dominated in the routine clinical practice. This is attributed
to the (1) lack of availability of CT systems that are
equipped with the appropriate hardware and software
required for image acquisition and densitometric analysis,
(2) high cost of a MDCT examination, and (3) the
inconvenience of use compared to DXA, which is a more
automated procedure and requires less operator intervention for image analysis and result derivation. Radiation
exposure may be considered as another reason why DXA
has dominated over QCT. The effective dose delivered by
DXA is lower than background radiation and is considered
to be negligible. The findings presented in the current study
suggest that routine abdominal MDCT may detect
osteoporotic women without an excess of radiation burden
due to QCT or DXA. Thus, patients undergoing routine
abdominal MDCT could, at the same time, be potentially
evaluated for bone densitometry if clinically indicated.
The present study has its limitations. The patients
participated in the current study did not undergo DXA.
Further studies are required to validate that patients are
correctly classified as osteoporotic. These studies need to
incorporate parameters such as the patient’s medical
history, physical examination, as well as DXA findings.
The correlation between QCT and abdominal MDCT could
176
be ideally validated using anthropomorphic phantoms.
However, commercially available anthropomorphic phantoms represent only young healthy individuals. Female
anthropomorphic phantoms in a wide range of ages should
be constructed to verify patient data. Moreover, this study
was performed with 16-slice CT. This is a widespread
modern system that is installed in many clinical institutions
around the world. However, it would be interesting to
evaluate the results on 64-slice CT systems, which are now
quite widely available.
In conclusion, this study showed that densitometric data
derived from routine abdominal MDCT examinations
differentiated osteoporotic from healthy females. Further
studies are needed before routine abdominal MDCT can be
considered as a useful method to diagnose spinal osteoporosis in everyday clinical practice.
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