Rationale and Objectives
The purpose of this prospective study was to assess image noise and variability in repeated coronary artery calcium (CAC) scoring on low-dose prospective electrocardiographically-triggered 64-slice multidetector computed tomography.
Materials and Methods
Patients (n = 115) suspected of having coronary artery disease were scanned twice, using a tube current of 10 × body mass index mA. The standard deviation (SD) of the computed tomographic value in the ascending aorta and (mean + 2 × SD) were obtained. Repeated CAC scores (Agatston, volume, and mass) were measured by two observers, and the interscan and interobserver variability were determined.
Results
The mean tube current used was 246 ± 36 mA. The mean tube current–time product and mean estimated effective dose were 57 ± 8 mA and 0.9 ± 0.2 mSv, respectively. The SD and (mean + 2 × SD) computed tomographic values in the ascending aorta were 16 ± 3 and 75 ± 10 Hounsfield units, respectively. Repeated CAC scores were correlated ( r 2 = 0.995–0.998). The interscan variability for observer 1 and observer 2, respectively, were 13% and 13% for Agatston score, 12% and 11% for volume, and 11% and 11% for mass. The interobserver variability for scan 1 and scan 2, respectively, were 3% and 3% for Agatston score, 5% and 3% for volume, and 3% and 3% for mass.
Conclusion
Low-dose prospective electrocardiographically-triggered 64-slice multidetector computed tomography shows low interscan and interobserver variability on CAC scoring while maintaining low image noise.
The validity of serial coronary artery calcium (CAC) measurements as a method to monitor the progression of atherosclerosis requires ( ) that the progression of CAC has biologic relevance to atherosclerotic activity, ( ) that the progression of CAC can be detected relative to intertest variability, ( ) that changes in CAC severity have prognostic relevance, and ( ) that the modification of cardiovascular risk factors modulates the progression of CAC ( ). The normal progression of CAC score per year is reported to be 14% to 27% (average, 24%) ( ) and is accelerated up to 33% to 48% with significant coronary disease ( ). However in previous studies, the interscan variability of the Agatston score ( ) using electron-beam computed tomography (CT) has been 20% to 37% ( ). Retrospective electrocardiographically-gated overlapping scanning using 4-slice CT ( ) and 16-slice CT ( ) have shown low interscan variability (12%–13%) for CAC scoring, but this method entails high radiation exposure. Because the progression of CAC is not clearly modifiable through standard risk-reducing therapies, and CAC measurement involves both cost and radiation exposure, the clinical monitoring of CAC progression using serial fast CT is not recommended by the American College of Cardiology and American Heart Association expert committee ( ). Under these circumstances, low radiation exposure and low interscan variability are key requirements for CAC scoring.
In a cardiac phantom study, prospective electrocardiographically-triggered 64-slice multidetector CT (MDCT), even with low radiation dose comparable with that of electron-beam CT, showed low variability for CAC scoring, comparable with that of retrospective electrocardiographically-gated 16-slice MDCT ( ). In a clinical study with 64-slice MDCT ( ), prospective electrocardiographically-triggered scanning showed low variability (Agatston score, 18%; volume, 12%; mass, 11%). The study used a tube current of 350 mA, thereby resulting in a tube current–time product of 82 mA. In studies using 4-slice CT, however, lower tube current–time products of 40 mA ( ) and 55 mA ( ) have been recommended for low-dose CAC scoring. Thus, the purpose of this prospective study was to assess image noise and variability in repeated CAC scoring on low-dose prospective electrocardiographically-triggered 64-slice MDCT.
Materials and methods
Patients
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Prospective Electrocardiographically-triggered Step-and-Shoot Computed Tomographic Protocol
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Image Noise
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CAC Scoring
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Interscan and Interobserver Variability
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Radiation Dose
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Statistical Analyses
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Results
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Table 1
Patient Demographics
Variable All Patients Coronary Artery Calcium (Agatston Score) Positive Negative No. of patients 115 80 35 Women/men 81/34 62/18 19/16 Age (y) (range) 64 ± 12 (34–87) 67 ± 11 (39–87) 57 ± 13 (34–80) Body mass index (kg/m 2 ) 25 ± 3 (18–35) 25 ± 3 (24–35) 25 ± 4 (18–34) Symptom 82/115 (71%) 56/80 (70%) 26/35 (74%) Risk factor 80/115 (70%) 56/80 (70%) 14/35 (40%)
Quantitative variables are expressed as mean ± SD (range). Categorical variables are expressed as frequency (percentage).
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Image Noise
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![Figure 1, Representative images showing high and low SD values. Two low-noise images ( a , 11 Hounsfield units [HU]; b , 11 HU) and two high-noise images ( c , 27 HU; d , 26 HU) are shown. (a) Image of a man with a body mass index [BMI] of 20 kg/m 2 and a body height of 164 cm. (b) Image of a woman with a BMI of 22 kg/m 2 and a body height of 146 cm. (c) Image of a man with a BMI of 31 kg/m 2 and a body height of 180 cm. (d) Image of a woman with a BMI of 29 kg/m 2 and a body height of 160 cm, presenting with cardiomegaly.](https://storage.googleapis.com/dl.dentistrykey.com/clinical/CoronaryArteryCalciumScoringonLowdoseProspectiveElectrocardiographicallytriggered64SliceCT/0_1s20S1076633208003358.jpg)
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CAC Scores
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Table 2
Agatston, Volume, and Mass Scores on Two Scans Measured by Two Observers
Agatston Score Volume Mass Observer 1 ⁎ Scan 1 688 (223), 1–5,676 538 (186), 2–4,270 143 (40), 0.2–1,158 Scan 2 683 (211), 1–6,130 533 (180), 3–4,658 143 (40), 0.3–1,185 Observer 2 † Scan 1 709 (229), 1–5,849 552 (189), 2–4,392 144 (40), 0.2–1,167 Scan 2 701 (211), 1–6,262 546 (183), 3–4,748 144 (40), 0.3–1,187
Data are expressed as mean (median), range.
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Interscan Variability
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Interobserver Variability
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Radiation Dose
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Discussion
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Image Noise
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CAC Scores: Interscan and Interobserver Variability
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Radiation Dose
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