Rationale and Objectives
The aims of this study were to investigate image noise (standard deviation of computed tomographic value) and to assess variability in repeated coronary artery calcium (CAC) scoring on prospective electrocardiographically triggered 64-detctor computed tomography.
Materials and Methods
Patients ( n = 428) suspected of having coronary artery disease were scanned twice using three protocols: with tube current modified by body mass index (BMI; group A), by BMI and body height (group B), and by attenuation at the maximal heart diameter (group C). Image noise was plotted against BMI. Interscan variability of CAC scores was determined. The effective dose was estimated by computed tomographic dose index.
Results
The mean effective dose and image noise, respectively, were 0.9 ± 0.2 mSv (range, 0.6–1.5 mSv) and 19 ± 4 Hounsfield units (HU) (range, 10–32 HU) for group A; 0.8 ± 0.2 mSv (range, 0.5–1.4 mSv) and 18 ± 4 HU (range, 10–31 HU) for group B; and 0.8 ± 0.4 mSv (range, 0.3–2.2 mSv) and 20 ± 2 HU (range, 16–26 HU) for group C. Group C used a wide dose range and controlled noise within a small range. The positive slopes of image noise versus BMI, 0.81 HU/(kg/m 2 ) in group A and 0.62 HU/(kg/m 2 ) in group B, suggested insufficient control of the tube current. In contrast, the nearly flat slope in group C, 0.091 HU/(kg/m 2 ), indicated optimal control. The interscan variability for Agatston score, volume, and mass in patients with CAC ( n = 300) was 13% (median, 8%), 12% (median, 7%), and 11% (median, 6%), respectively.
Conclusions
CAC scoring on prospective electrocardiographically triggered 64-detector computed tomography using attenuation-based tube current control has the potential to favorably control image noise with low dose and low interscan variability.
The validity of serial coronary calcium measurements as a method to monitor the progression of atherosclerosis requires that the progression of coronary artery calcium (CAC) have biologic relevance to atherosclerosis activity, the progression of CAC can be detected relative to intertest variability, changes in CAC severity have prognostic relevance, and the modification of cardiovascular risk factors modulate the progression of CAC . Therefore, regarding the technical aspects of CAC scoring, low radiation exposure and low interscan variability are key requirements.
To reduce radiation exposure, a fixed and lower tube current–time product of 40 mAs or 55 mAs and body weight–adapted or body mass index (BMI)–adapted protocols have been introduced, but these do not account for the body habitus of patients, such as the size of heart or the presence of pericardial effusion. In a recent report of the International Consortium on Standardization in Cardiac Computed Tomography, a standard deviation (SD) level target of 20 Hounsfield units (HU) for small and medium-sized patients and an SD level target of 23 HU for large patients have been recommended . Mühlenbruch et al reported automated attenuation-based tube current adaptation whereby the tube current was chosen from a proprietary control curve calculated on the basis of the attenuation values derived from the scanogram. Their study showed that automated attenuation-based tube current adaptation can better control tube current than a fixed “standard” dose protocol, but their regression analysis revealed a statistically significant influence of patient BMI on image noise.
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Materials and methods
Patients
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Prospective Electrocardiographically Triggered Step-and-Shoot CT Protocol
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Group A: Tube Current Modified by BMI
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tube current=250×(BMI/25)mA=10×body weight(kg)/[body height(m)]2mA. tube current
=
250
×
(
BMI
/
25
)
mA
=
10
×
body weight
(
kg
)
/
[
body height
(
m
)
]
2
mA
.
This was based on the strategy that patients with standard BMIs of 25 kg/m 2 would receive tube current–time product of 58 mAs, which is almost the same level as the recommendation for CAC scoring using low-dose 4-slice CT imaging .
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tube current(mA)×gantry rotation speed(seconds)×exposure time per rotation time=250mA×0.35seconds×(2/3)=58mAs. tube current
(
mA
)
×
gantry rotation speed
(
seconds
)
×
exposure time per rotation time
=
250
mA
×
0.35
seconds
×
(
2
/
3
)
=
58
mAs
.
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Group B: Tube Current Modified by BMI and Body Height
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tube current=250×(BMI/25)mA×(body height/1.7)mA=5.88×body weight(kg)/body height(m)mA. tube current
=
250
×
(
BMI
/
25
)
mA
×
(
body height
/
1.7
)
mA
=
5.88
×
body weight
(
kg
)
/
body height
(
m
)
mA
.
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Group C: Attenuation-Based Tube Current Adaptation at the Maximal Heart Diameter
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tube current=recommended tube current×(3/2)×(0.4/0.35)mA=recommended tube current×1.71mA. tube current
=
recommended tube current
×
(
3
/
2
)
×
(
0.4
/
0.35
)
mA
=
recommended tube current
×
1.71
mA
.
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Image Noise
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Calcium Scoring
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Interscan and Interobserver Variability
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interscan variability=[absolute(scan1−scan2)/(scan1+scan2)×0.5]×100, interscan variability
=
[
absolute
(
scan
1
−
scan
2
)
/
(
scan
1
+
scan
2
)
×
0.5
]
×
100
,
and
interobserver variability=[absolute(observer1−observer2)/(observer1+observer2)×0.5]×100, interobserver variability
=
[
absolute
(
observer
1
−
observer
2
)
/
(
observer
1
+
observer
2
)
×
0.5
]
×
100
,
where observer 1 and observer 2 are the CAC scores measured by the respective observers. Interscan variability, on a logarithmic scale, was compared among the three groups and CAC-scoring algorithms.
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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 Group A Group B Group C_P_ No. of patients 428 145 145 138 .48 ‡ Women/women 167/261 52/93 60/85 55/83 .43 † Age (y) 65 ± 12 (28–89) 64 ± 12 (34–87) 65 ± 13 (28–89) 66 ± 11 (31–85) .52 ‡ Prevalence of CAC 300/428 (70%) 100/145 (67%) 100/145 (67%) 100/138 (72%) .27 † BMI (kg/m 2 ) 24 ± 13 (16–45) 24 ± 3 (18–34) 24 ± 4 (16–40) 24 ± 3 (16–32) .95 ‡ Symptoms 334/428 (78%) 115/145 (79%) 109/145 (75%) 110/138 (80%) .06 ‡ Risk factors 268/428 (63%) 80/145 (55%) 97/145 (67%) 91/138 (66%) .47 † HR (beats/min) ∗ 61 ± 11 (38–111) 62 ± 10 (40–87) 62 ± 11 (39–107) 60 ± 12 (38–111) .44 † HRV (beats/min) ∗ 5 ± 12 (0–102) 4 ± 10 (0–76) 6 ± 12 (0–75) 4 ± 14 (0–102) .48 ‡
BMI, body mass index; CAC, coronary artery calcium; HR, heart rate; HRV, heart rate variation.
Categorical variables are expressed as frequency (percentage) and quantitative variables as mean ± standard deviation (range).
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Image Noise
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Table 2
Image Noise in the Ascending Aorta and Right Ventricle
Variable All Patients Group A Group B Group C No. of patients 428 145 145 138 SD (HU) Ascending aorta 17 ± 7 (11–30) 16 ± 3 (11–27) 17 ± 3 (11–30) 17 ± 2 (12–24) Right ventricle 19 ± 4 (10–32) 19 ± 4 (10–32) 18 ± 4 (10–31) 20 ± 2 (16–26) Mean + 2 SDs (HU) Ascending aorta 75 ± 9 (53–109) 75 ± 10 (53–99) 75 ± 10 (53–109) 76 ± 7 (57–100)
HU, Hounsfield units; SD, standard deviation.
Data are expressed as mean ± SD (range).
![Figure 3, The highest standard deviation images in the three groups. Images with the highest noise in the three groups are shown. (a) An image of a 57-year-old woman (body mass index [BMI], 34.1 kg/m 2 ; body height, 155 cm) below the diaphragm level (SD = 32 Hounsfield units [HU]). (b) An image of a 57-year-old man (BMI, 27.1 kg/m 2 ; body height, 165 cm) was sacrificed to streaking artifact from spinal spur (SD = 31 HU). (c) An image of an 80-year-old man (BMI, 26.8 kg/m 2 ; body height, 164 cm) below the diaphragm shows pleural and pericardial effusion (SD = 26 HU).](https://storage.googleapis.com/dl.dentistrykey.com/clinical/EvaluationofAttenuationBasedTubeCurrentControlinCoronaryArteryCalciumScoringonProspectiveECGtriggered64detectorCT/2_1s20S1076633209002621.jpg)
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![Figure 4, The relationship between standard deviation (SD) and body mass index (BMI). Scatterplots show the ratio between BMI (kg/m 2 ) and SD (Hounsfield units [HU]). (a) Group A, in the aorta: SD = 0.51 (95% confidence interval [CI], 0.40–0.62) × BMI + 4 ( P < .01). (b) Group A, in the right ventricle: SD = 0.81 (95% CI, 0.64–0.97) × BMI − 1 ( P < .01). (c) Group B, in the aorta: SD = 0.43 (95% CI, 0.33–0.53) × BMI + 6 ( P < .01). (d) Group B, in the right ventricle: SD = 0.62 (95% CI, 0.48–0.76) × BMI + 4 ( P < .01). (e) Group C, in the aorta: SD = 0.041 (95% CI, −0.069 to 0.15) × BMI + 16 (P = .46). (f) Group C, in the right ventricle: SD = 0.091 (95% CI, −0.013 to 0.20) × BMI + 18 ( P = .09). The positive slopes of image noise versus BMI, 0.51 and 0.81 HU/(kg/m 2 ) in group A and 0.43 and 0.62 HU/(kg/m 2 ) in group B, suggest insufficient control of the tube current. In contrast, the nearly flat slopes of 0.041 and 0.091 HU/(kg/m 2 ) in group C indicate optimal control of tube current across patients.](https://storage.googleapis.com/dl.dentistrykey.com/clinical/EvaluationofAttenuationBasedTubeCurrentControlinCoronaryArteryCalciumScoringonProspectiveECGtriggered64detectorCT/3_1s20S1076633209002621.jpg)
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CAC Scores and Interscan and Interobserver Variability
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Table 3
Coronary Artery Calcium Scores and Interscan and Interobserver Variability
Variable All Patients Group A Group B Group C No. of patients 300 100 100 100 Agatston score Scan 1 Observer 1 175 (48, 648) 206 (46, 664) 167 (42, 464) 152 (48, 775) Observer 2 185 (48, 648) 206 (48, 668) 174 (42, 488) 157 (48, 789) Scan 2 Observer 1 179 (43, 610) 201 (46, 638) 171 (42, 444) 148 (39, 797) Observer 2 176 (45, 611) 204 (46, 638) 176 (42, 462) 148 (39, 803) Volume score Scan 1 Observer 1 150 (43, 515) 165 (38, 548) 138 (39, 367) 125 (44, 639) Observer 2 149 (43, 528) 166 (40, 552) 146 (37, 392) 129 (44, 622) Scan 2 Observer 1 144 (41, 502) 176 (39, 502) 144 (35, 356) 122 (41, 665) Observer 2 146 (41, 502) 176 (40, 502) 146 (36, 366) 121 (41, 668) Calcium mass Scan 1 Observer 1 33 (8, 127) 37 (8, 130) 30 (7, 93) 28 (8, 139) Observer 2 33 (8, 130) 37 (8, 132) 30 (7, 93) 28 (8, 139) Scan 2 Observer 1 32 (8, 123) 37 (8, 131) 30 (7, 89) 25 (7, 153) Observer 2 32 (8, 123) 37 (8, 131) 30 (6, 91) 26 (7, 149) Interscan variability (%) Agatston Observer 1 13, 8 (3, 17) 13, 7 (2, 15) 12, 6 (2, 15) 14, 10 (4, 18) Observer 2 13, 8 (3, 17) 13, 7 (3, 16) 13, 6 (2, 16) 14, 10 (3, 20) Volume Observer 1 12, 7 (3, 16) 12, 6 (3, 16) 11, 6 (2, 15) 11, 8 (3, 15) Observer 2 11, 6 (3, 16) 11, 6 (2, 15) 10, 6 (2, 15) 12, 8 (3, 17) Mass Observer 1 11, 6 (2, 14) 10, 4 (2, 14) 10, 5 (2, 11) 12, 8 (2, 14) Observer 2 11, 6 (2, 14) 10, 4 (2, 14) 12, 5 (2, 11) 11, 8 (2, 14) Interobserver variability (%) Agatston Scan 1 4, 1 (0, 3) 3, 1 (0, 4) 5, 0 (0, 3) 3, 0 (0, 2) Scan 2 3, 0 (0, 3) 5, 0 (0, 3) 4, 0 (0, 2) 2, 1 (0, 1) Volume Scan 1 2, 0 (0, 2) 3, 0 (0, 3) 3, 0 (0, 1) 1, 0 (0, 0) Scan 2 2, 0 (0, 2) 3, 0 (0, 2) 4, 0 (0, 2) 1, 0 (0, 3) Mass Scan 1 3, 1 (0, 1) 3, 0 (0, 2) 4, 0 (0, 2) 1, 0 (0, 1) Scan 2 3, 0 (0, 1) 3, 0 (0, 2) 4, 0 (0, 1) 2, 0 (0, 1)
Coronary artery calcium is expressed as median (25th, 75th percentiles). Variability is expressed as mean, median (25th, 75th percentiles).
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Radiation Dose
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Table 4
Tube Current, Tube Current–Time Product, and Radiation Dose
Variable All Patients Group A Group B Group C_P_ Tube current (mA) 227 ± 65 (75–610) 245 ± 36 (180–350) 227 ± 43 (150–370) 209 ± 94 (75–610) <.01 ∗ Tube current–time product (mAs) 53 ± 15 (18–142) 57 ± 8 (42–56) 53 ± 10 (35–86) 49 ± 25 (18–142) <.01 ∗ Dose0length product (mGy · cm) 49 ± 15 (16–131) 53 ± 8 (37–87) 49 ± 10 (30–84) 44 ± 21 (16–131) <.01 ∗ Estimate effective dose (mSv) 0.8 ± 0.3 (0.3–2.2) 0.9 ± 0.2 (0.6–1.5) 0.8 ± 0.2 (0.5–1.4) 0.8 ± 0.4 (0.3–2.2) <.01 ∗
Data are expressed as mean ± standard deviation (range).
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Discussion
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Image Noise
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Interscan and Interobserver Variability in CAC Scoring
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Radiation Dose
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