Purpose
The aim of this study was to compare the calcium score and reproducibility of coronary artery calcium scores obtained on the four kinds of 64-detector computed tomography (CT) scanners using standard (120 kVp) and low tube voltage (80 kVp) scan techniques.
Materials and Methods
We scanned 80 and 120 kVp on all scanners. We calculated Agatston, volume, and mass scores for coronary artery calcium scoring on each scanner and compared the coefficients of variation of the calcium scores to evaluate reproducibility of among CT scanners.
Results
The averages of the total mean Agatston score, total mean volume score, and total mean mass score at 80kVp/120kVp were 798.9/683.8, and 627.2/567.3, and 157.1/156.7, respectively. The total mean mass score was almost constant irrespective of the tube voltage. The total mean coefficients of variation for the four CT scanners were lower at 80 than 120 kVp (4.1% vs. 10.2% [total mean Agatston score], 3.2% vs. 9.6% [total mean volume score], and 3.2% vs. 9.4% [total mean mass score]).
Conclusion
Use of the low tube voltage technique can reduce variations in the coronary artery calcium scores obtained on different CT scanners.
Coronary artery calcium scoring by cardiac computed tomography (CT) is a reliable diagnostic tool in patients with suspected coronary artery disease because the degree of coronary calcification is strongly associated with the degree of atherosclerosis . The coronary artery calcium score on screening CT images yields relevant prognostic information on the development of coronary events . Although coronary artery calcium scoring was devised originally for electron beam CT by Agatston , it is now widely performed on multidetector CT (MDCT). For follow-up studies of patients with coronary artery disease, the use of the same CT scanner is desirable from the perspective of reproducibility among different CT scanners. However, this is not always possible and follow-up studies are often performed on different scanners.
The CT number of a scanned object depends on the effective tube voltage. Individual CT scanners have a specific effective tube voltage that corresponds to the “nominal” kilovolts peak (kVp). Consequently, different CT scanners yield different CT numbers even if the same object is scanned at the same kVp. Calcium attenuation increases as the effective tube voltage decreases . We hypothesized that variations in coronary artery calcium scores obtained on different CT scanners may be mitigated by increasing the signal intensity of calcium by using a low tube voltage technique. Although the image noise level is the same at 120 kVp and 80 kVp, because of the increased signal intensity at 80 kVp, coronary artery calcium can be certainly detected on scans acquired at 80 kVp. Therefore, the purpose of this study was to compare the calcium score and reproducibility of coronary artery calcium scores obtained on different CT scanners using standard (120 kVp) and low tube voltage (80 kVp) scan techniques.
Materials and methods
Phantom
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![Figure 1, (a) Frontal view of the anthropomorphic phantom body with the calibration insert. (b) Diagrams of frontal ( left ) and side ( right ) views of the calibration insert with the nine different calcifications and the two large calibration inserts (Hounsfield units [HU] = 0, water and 200 mg/cm 3 calcium hydroxyapatite).](https://storage.googleapis.com/dl.dentistrykey.com/clinical/CoronaryArteryCalciumScoringonDifferent64detectorScannersUsingaLowtubeVoltage80kVp/0_1s20S1076633212003807.jpg)
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CT Scanning
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Table 1
Scan Parameters for Coronary Artery Calcium Scoring
Scanner Aquilion-64 Light Speed VCT Definition AS+ Briliance-64 Rotation time (sec) 0.25 0.35 0.217 0.28 Tube voltage (kV) 120 80 120 80 120 80 120 80 Tube current (mA) 70 240 95 265 70 254 86 350 Calibration factor 0.70 0.51 0.71 0.51 0.83 0.59 0.81 0.56 Noise (mean ± SD) 21.0 ± 0.4 20.4 ± 0.8 19.9 ± 0.9 21.3 ± 0.6 20.6 ± 1.2 20.8 ± 0.5 19.9 ± 0.6 20.9 ± 1.0 CTDIw (mGy) 1.60 1.90 1.58 1.37 0.88 0.83 1.70 2.10 Detector configuration (mm) 64 × 0.5 64 × 0.625 64 × 0.6 64 × 0.625 Section thickness (mm) 3.0 2.5 3.0 2.5 Table feed (mm/rotation) 32.0 40.0 38.4 40.0 Beam pitch 1 1 1 1 Reconstruction kernel FC13 Standard B35f Heart View medium Cardiac Standard
CTDIw, weighted computed tomography dose index.
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Analysis of Coronary Artery Calcium Scoring
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Agatstonscore=sliceincrement/slicethickness×∑(area×weightingfactor), Agatston
score
=
slice
increment
/
slice
thickness
×
∑
(
area
×
weighting
factor
)
,
where the weighting factor = 1 if 130 HU ≤ CT max < 200 HU, = 2 if 200 HU ≤ CT max < 300 HU, = 3 if 300 HU ≤ CT max < 400 HU, and = 4 if 400 HU ≤ CT max , the volume score =Σ (area × slice increment), and the mass score = Σ (area × slice increment × mean CT density) × calibration factor .
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c=200/(HUCaHA−HUwater). c
=
200
/
(
HU
CaHA
−
HU
water
)
.
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coefficientofvariation=100×[standarddeviation]/mean(9) coefficient
of
variation
=
100
×
[
standard
deviation
]
/
mean
(
9
)
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Statistical Analysis
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Results
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Table 2a
Mean Coronary Artery Calcium Scores Obtained at 120 kVp
∗ Steel-Dwass test, P < .05.
Table 2b
Mean Coronary Artery Calcium Scores Obtained at 80 kVp
∗ Steel-Dwass test, P < .05.
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Table 3
Coefficient of Variation (%) Obtained with Different CT Scanners
Score kVp Scanner Aquilion-64 Light Speed VCT Definition AS+ Briliance-64 Agatston 120 8.8 8.1 7.3 10.6 80 5.3 6.3 4.2 6.7 Volume (mm 3 ) 120 5.6 5.1 4.4 9.1 80 6.3 4.5 3.4 6.3 Mass (mg) 120 2.5 1.0 1.9 7.5 80 2.6 1.2 0.9 6.4
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Discussion
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