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Original Research |
1 Department of Radiology and Center for Imaging Science, Samsung Medical
Center, Sungkyunkwan University School of Medicine, 50 Ilwon-dong, Kangnam-ku,
Seoul 135-710, Korea.
2 Department of Preventive Medicine, School of Medicine, Kyunghee University,
Seoul, Korea.
Received August 6, 2007;
accepted after revision October 19, 2007.
Address correspondence to M. J. Chung.
Abstract
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MATERIALS AND METHODS. One hundred twenty-eight consecutive participants (all men; mean age, 52 ± 7 years) underwent low-dose chest CT and calcium-scoring CT with prospective ECG gating using 40-MDCT. Low-dose chest CT volume data were reconstructed as 25-cm field of view and three slice thicknesses: 1, 2.5, and 5 mm. For each examination, the lesion area, Agatston calcium score, and calcium mass were measured at 90- and 130-H thresholds. All measurements (130-H threshold) from the calcium-scoring CT were used as reference standards. Spearman's correlation test was used to compare the results.
RESULTS. Among the low-dose chest CT examinations, sensitivity was best determined with a 1-mm slice thickness at 130 H and 2.5-mm slice thickness at 90 H. Specificity was best determined with a 5-mm slice thickness at 130 H. Accuracy (90%) was best determined with a 2.5-mm slice thickness at 130 H. Of all protocols, calcium area, score, and mass from a 2.5-mm slice thickness at 130 H correlated best with the reference results (r = 0.89 for all three criteria).
CONCLUSION. Using a low radiation dose and nongated MDCT, we can detect coronary artery calcium and obtain results comparable to those obtained with dedicated calcium-scoring CT that uses a higher dose and ECG gating.
Keywords: calcium-scoring CT • coronary artery calcium • lung cancer screening • MDCT • radiation dose • screening
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However, the high image quality of MDCT is achieved at the expense of a substantial amount of ionizing radiation. Exposure to radiation is a concern, especially for individuals in the screening population because many participants undergo repeated examinations. Although the risk from CT radiation may be relatively low per patient, the added risk to the population may be substantial, with unknown long-term effects. Thus, it is essential to reduce the CT radiation dose to the minimum level that will allow an adequate quantification of coronary artery calcium.
The purpose of this study was to compare prospectively the results of coronary artery calcium measurement obtained from a retrospective reconstruction of low-dose chest CT for lung cancer screening and from high-dose dedicated calcium-scoring CT.
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Participants
Between March and April 2007, 128 consecutive participants (all men; mean
age, 52 ± 7 years; range, 40–68 years) were included. They were
smokers who came to the health care center of our institute to undergo
low-dose chest CT for lung cancer screening and calcium-scoring CT for
coronary risk factors. Participants with a history of cardiac surgery,
coronary stent insertion, or other cardiac intervention were excluded from the
study. We calculated body mass index (BMI) from the individual's height and
weight. The overall mean BMI was 24.2 kg/m2 (range, 17.1–30.9
kg/m2).
Calcium-Scoring CT Protocol
CT of the heart was performed using 40-MDCT (Brilliance 40, Philips Medical
Systems). This exam ination was performed with 120 kVp, 55 mAs, and
prospective ECG gating. The imaging parameters included a 600-millisecond
scanning time and 2.5-mm single slice thickness, with a total of 60 slices
obtained during a single breath-hold. The single-section mode of imaging was
used, with imaging ECG-triggered to 75% of the R-R interval (for a diastolic
blood pressure > 80 mm Hg, 45% of the R-R interval). Imaging was
reconstructed into a 512 x 512 matrix with a 25-cm field of view. All
calcification related to a coronary artery equal to or greater than the
minimum attenuation of 130 H was considered potential coronary calcium.
Low-Dose Chest CT Protocol
CT of the lung was also performed using 40-MDCT (Brilliance 40).
Participants remained stationary on the table during both examinations without
changing position. This examination was performed with 120 kVp, 30 mAs,
0.5-second gantry rotation, and a table pitch of 1.3. The low-dose chest CT
volume data were retrospectively reconstructed into a 512 x 512 matrix
with a 25-cm field of view (carina–heart base), and three slice
thicknesses: 1, 2.5, and 5 mm (Figs.
1A,
1B,
1C, and
1D). The thresholds for
identifying calcification in the coronary arter ies were 90 H (modified
Agatston score) that has been used previously with nongated helical CT
[3] and 130 H (conventional
Agatston score), along with the requirement for two contiguous pixels.
Therefore, we acquired data with six protocols using low-dose chest CT as
follows (threshold, slice thickness): 90 H, 1 mm; 90 H, 2.5 mm; 90 H, 5 mm;
130 H, 1 mm; 130 H, 2.5 mm; and 130 H, 5 mm.
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Also, on the basis of the score from the best low-dose chest CT protocol, we classified participants using the risk stratification proposed by Rumberger et al. [9]: very low risk (Agatston score = 0), low risk (> 0–10), moderate risk (> 10–100), moderately high risk (> 100–400), and high risk (> 400). Then we compared the risk stratification of the participant with that of calcium-scoring CT.
Statistical Analysis
The lesion area, the calcium score, and the calcium mass for each coronary
vessel obtained in the two examinations were compared using a two-tailed
paired sample test. Correlation of the calcium scores between the two
examinations was measured using the Spearman's correlation test. Multiple
regression analysis was used to show the partial effect of BMI and the calcium
score from the calcium-scoring CT on measured values in each low-dose chest CT
protocol. A p value of 0.05 or less was considered to indicate a
statistically significant difference.
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The correlation coefficients of coronary artery calcium measurements between low-dose chest CT (six protocols) and calcium-scoring CT (as reference standards) are shown in Table 2. Of all protocols, calcium area, score, and mass from a 2.5-mm slice thickness at 130 H correlated best with the reference results (r = 0.89 for all three criteria).
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When the results of each coronary branch were evaluated at the protocol of a 2.5-mm slice thickness at 130 H, the correlation coeffi cients were poor (r = 0.215, p = 0.015) for the posterior descending artery, but the total number of samples was insufficient (the number of vessels with a positive calcium score on calcium-scoring CT was only four). The correlation coefficients were sufficiently high in all other coronary branches (r > 0.654, p < 0.001) (Table 3).
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On the basis of the data from the protocol of a 2.5-mm slice thickness at 130 H, we compared the risk stratification of a patient with that of calcium-scoring CT (Table 4). Among 54 participants who had coronary artery calcium deposits as seen on the calcium-scoring CT examination, five were not detected on the low-dose chest CT protocol. All five cases are low-risk category in the calcium-scoring CT examination and thus they were moved into the very-low-risk category in the low-dose chest CT protocol. In contrast, among 74 participants who had no coronary artery calcium deposits as seen on the calcium-scoring CT examination, eight were detected on the low-dose chest CT protocol. All eight participants were moved into the low-risk category in the low-dose chest CT protocol. Among all participants, no individual differed by more than the adjacent risk category. By the result of multiple regression analysis, BMI has only a very weak negative effect with no significant effect on the measured values of each low-dose chest CT protocol (p > 0.1) (Table 5).
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Electron beam CT and conventional single-detector and MDCT have been used for the detection and quantification of coronary calcification as a sign of coronary atherosclerosis [2]. The standard method for quantification of coronary artery calcium is electron beam CT. After the introduction of MDCT and rapid technical advances, the number of MDCT examinations of the coronary vasculature has been rapidly increasing. Higher radiation doses are delivered with cardiac MDCT as compared with the doses delivered using electron beam CT. The effective radiation doses for calcium scoring with electron beam CT were 1.0 and 1.3 mSv for men and women, respectively. The effective radiation doses for calcium scoring with MDCT were in the range of 1.5–5.2 mSv for men and 1.8–6.2 mSv for women [12]. Because the detection and quantification of coronary artery calcification is considered a screening tool in potentially healthy people, radiation exposure should be kept as low as possible.
Much recent work has focused on the value of low-dose chest CT as a screening tool for detecting early asymptomatic lung cancers. The radiation dose for low-dose chest CT is 0.6–1.1 mSv, compared with 7.0 mSv for conventional CT [13]. In low-dose chest CT screening for lung cancer, Brenner [14] reported an estimated upper limit of a 5.5% increase in the lung cancer risk attributable to annual CT-related radiation exposure. The risk of radiation-induced lung cancer associated with repeated CT screening for lung cancer or coronary artery disease may not be negligible. Therefore, one should choose a CT examination with a lower radiation dose for a screening study and minimize the number of CT examinations.
The risk group for lung cancer can overlap the risk group for coronary artery disease because smoking is a heavy risk factor for both. For these risk groups, a double CT screening examination is somewhat wasteful in terms of expense and time. A double CT screening examination also increases overall radiation exposure. A previous study has determined the frequency of calcification of the thoracic aorta and its relationship to risk factors and coronary artery disease using a CT examination for lung cancer and tuberculosis [15]. We also used low-dose chest CT examination for lung cancer screening to reduce the radiation dose. Coronary artery calcium detection and measurement from retrospective reconstruction of images produced by low-dose chest CT gave results that were well correlated with those obtained from dedicated calcium-scoring CT, which means that the risk groups for lung cancer and coronary artery disease can undergo just a single CT examination rather than two consecutive CT examinations.
The main finding of our study is that application of a low milliampere-second setting of 30 mAs for coronary artery calcium detection and measurement from a retrospective reconstruction from low-dose chest CT images produces results that are well correlated with those obtained using dedicated calcium-scoring CT at 55 mAs. An additional finding is that the ECG-gating did not significantly affect the correlation of the results between prospective ECG-gated calcium-scoring CT and nongated low-dose chest CT with retrospective reconstruction. Finally, the effect of BMI is negligible except in patients who are extremely overweight. The BMI of our participants ranged from 17.1 to 30.9 kg/m2.
The results of our study strengthen those of previous studies in which a low milliampere-second setting was used [16–22]. Shemesh et al. [22] assessed the coronary artery calcium measurement with two milliampere-second levels (55 and 165 mAs) using prospective ECG-gated MDCT. Those investigators calculated the calcium mass and calcium score in 51 asymptomatic participants by performing two consecutive CT examinations, the first with a setting of 165 mAs and the second with a setting of 55 mAs. The total calcium score between the high- and low-dose scans was well correlated with respect to the Agatston method and calcium mass (r = 0.97, p < 0.001 and r = 0.99, p < 0.001, respectively). A strong correlation was also found for each vessel. The overall correlation coefficients determined in our study were lower than the values reported by Shemesh et al. The main cause of this difference is the use of a different statistical method. We did not use a typical parametric method (Pearson's correlation test) to determine a correlation coefficient but used a non-parametric method (Spearman's correlation test) because the data of our study had too many null cases and thus correlation coefficients were not different between six different reconstruction protocols (r > 0.998, Pearson's correlation test).
In our study, the optimal protocol has a sensitivity of 91%, which means
the low-dose chest CT protocol did not detect calcium in five participants.
But all five participants were in the low-risk category in the calcium-scoring
CT examination. Fallavollita et al.
[23] evaluated electron beam
CT calcium scoring in 98 men and women who had no significant obstructive
disease detected at coronary angiography and found that 87% with
angiographically smooth coronary arteries had a calcium score of 5 or less. In
another study, although a negative or extremely low calcium score (
10)
could not totally exclude the presence of coronary atherosclerosis, it was
consonant with the absence of a fixed (significant) coronary obstructive
lesion, regardless of age and sex
[9]. Thus, no further specific
cardiac workup is recommended for this group. Therefore, the low-dose chest CT
protocol can be used as the basis for the screening study. When we tried to
use the parametric method forcibly, the correlation coefficients for the
protocol of 2.5-mm slice thickness at 130 H were 0.99 each for area, score,
and mass.
Of all the protocols used for the retrospective reconstruction of low-dose chest CT images, the protocol that provided the best correlation was 130 H and 2.5-mm slice thickness (r = 0.89, p < 0.001). In this protocol, results from the posterior descending artery showed a lower correlation coefficient than the other coronary artery branches. This was statistically due to the small number of samples and physically due to the posterior descending artery being smaller than the other branches and its location being a site associated with higher motion artifacts, as seen in an nongated scan. Cardiac motion artifacts are known to artificially raise the calcium score [24].
In conclusion, our study showed that nongated helical CT with 30 mAs can be an alternative setting that produces results for calcium measurement that are comparable to those obtained with a prospectively ECG-gated axial CT examination at 55 mAs. Thus, for one who undergoes screening for lung cancer and coronary heart disease simultaneously, the radiation exposure can be reduced for coronary artery calcium detection and measurement by the retrospective use of low-dose chest CT data instead of dedicated calcium-scoring CT. However, the appropriate use for CT with its accompanying low-dose radiation exposure for calcium scoring, particularly for large patients, should be further studied.
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