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Cardiopulmonary Imaging • Original Research
Tatusagami et al.
64-MDCT Coronary Angiography
Cardiopulmonary Imaging
Original Research
FOCUS ON:
Evaluation of a Body Mass Index–
Adapted Protocol for Low-Dose
64-MDCT Coronary Angiography
with Prospective ECG Triggering
Fuminari Tatsugami 1
Lars Husmann 1
Bernhard A. Herzog 1
Nina Burkhard 1
Ines Valenta 1
Oliver Gaemperli 1
Philipp A. Kaufmann 1,2
Tatsugami F, Husmann L, Herzog BA, et al.
Keywords: adapted scanning protocol, body mass index,
coronary CT angiography, image noise, vessel
opacification
DOI:10.2214/AJR.08.1390
OBJECTIVE. Because an increase in body mass index (weight in kilograms divided by
height squared in meters) confers higher image noise at coronary CT angiography, we evaluated
a body mass index–adapted scanning protocol for low-dose 64-MDCT coronary angiography
with prospective ECG triggering.
Subjects AND METHODS. One hundred one consecutively registered patients underwent
coronary CTA with prospective ECG triggering with a fixed contrast protocol (80
mL of iodixanol, 50-mL saline chaser, flow rate of 5 mL/s). Tube voltage (range, 100–120
kV) and current (range, 450–700 mA) were adapted to body mass index. Attenuation was
measured, and contrast-to-noise ratio was calculated for the proximal right coronary artery
and left main coronary artery. Image noise was determined for each patient as the SD of attenuation
in the ascending aorta.
RESULTS. Body mass index ranged from 18.2 to 38.8, and mean effective radiation dose
from 1.0 to 3.2 mSv. There was no correlation between body mass index and image noise (r =
0.11, p = 0.284), supporting the validity of the body mass index–adapted scanning protocol.
However, body mass index was inversely correlated with vessel attenuation (right coronary
artery, r = –0.45, p < 0.001; left main coronary artery, r = –0.47, p < 0.001) and contrast-tonoise
ratio (right coronary artery, r = –0.39, p < 0.001; left main coronary artery, r = –0.37,
p < 0.001).
CONCLUSION. Use of the proposed body mass index–adapted scanning parameters
results in similar image noise regardless of body mass index. Increased bolus dilution due to
larger blood volume may account for the decrease in contrast-to-noise ratio and vessel attenuation
in patients with higher body mass index, but the contrast bolus was not adapted to body
mass index in this study.
Received June 13, 2008; accepted after revision
September 27, 2008.
Supported by grants from the Swiss National Science
Foundation (SNSF professorship grant PP00A-114706)
and the Zurich Center of Integrative Human Physiology.
F. Tatsugami and L. Husmann contributed equally to this
work.
1 Departments of Medical Radiology and Nuclear
Cardiology and the Cardiovascular Center, University
Hospital Zurich, Raemistrasse 100, NUK C 32, CH-8091
Zurich, Switzerland. Address correspondence to
P. A. Kaufmann (pak@usz.ch).
2 Zurich Center for Integrative Human Physiology,
University of Zurich, Zurich, Switzerland.
AJR 2009; 192:635–638
0361–803X/09/1923–635
© American Roentgen Ray Society
C
oronary CT angiography (CTA)
has been found to have high diagnostic
accuracy in the detection
of coronary artery disease
[1–4]. However, high effective radiation exposure
of 9.4–21.4 mSv [5, 6] is a concern.
Previous scanning protocols entailed use of
the helical scanning mode with retrospective
ECG gating and fixed tube voltage and current
regardless of the patients’ body mass
index (BMI) (weight in kilograms divided by
height squared in meters). Scanning protocols
involving prospective ECG triggering
have been found feasible. With these protocols
effective radiation dose can be reduced
to 2.1–2.8 mSv [7, 8]. Coronary CTA with
prospective ECG triggering results in a less
effective radiation dose than with the previous
helical mode because radiation is admin-
istered only in the end-diastolic phase rather
than throughout the cardiac cycle. Furthermore,
BMI-adapted tube voltage and tube
current protocols have been introduced to
low-dose scanning protocols [7, 8].
Because an increase in BMI confers higher
image noise at CT, adaptation of tube voltage
and tube current to individual BMI has been
suggested [9–12]. The goals are to avoid decreased
vessel attenuation and increased image
noise in patients with a high BMI and to
avoid unnecessary overexposure of patients
with a low BMI. However, to our knowledge,
no BMI-adapted scanning protocol for coronary
CTA with prospective ECG triggering
has been evaluated. The purpose of this study
was to evaluate a BMI-adapted scanning protocol
for low-dose 64-MDCT coronary angiography
with prospective ECG triggering.
AJR:192, March 2009 635

Tatusagami et al.
Subjects and Methods
Patients
The study protocol was approved by the
institutional review board, and written informed
consent was obtained. One hundred one consecutively
registered patients (36 women, 65 men;
mean age, 57.2 ± 12.2 years; range, 20–77 years)
were prospectively enrolled. Exclusion criteria for
CT were hypersensitivity to iodinated contrast
agent, renal insufficiency (creatinine level > 150
µmol/L [> 1.7 mg/dL]), nonsinus rhythm, and
hemo dynamic instability. Patients were referred
because of suspicion of coronary artery disease (n =
92) based on at least one of the following symptoms:
dyspnea (n = 11), typical angina pectoris (n = 12),
atypical chest pain (n = 51), pathologic exercise
test or ECG result (n = 36), high cardiovascular
risk (n = 4). Nine patients with known coronary
artery disease were referred for stent (n = 5) or
bypass (n = 1) or for follow-up after myocardial
infarction (n = 3).
CT Data Acquisition and Postprocessing
All patients received a single dose of 2.5 mg
sublingual isosorbide dinitrate (Isoket, Schwarz
Pharma) 2 minutes before acquisition. In addition,
5–20 mg IV metoprolol (Beloc, AstraZeneca) was
administered before the coronary CTA examination
if necessary to achieve a target heart rate
less than 65 beats/min. A fixed contrast protocol
was used for all patients. A bolus of 80 mL iodixanol
(Visipaque 320, 320 mg/mL, GE Healthcare)
was continuously injected into an antecubital vein
through an 18-gauge catheter at a flow rate of 5
mL/s and followed by 50 mL saline solution. Bolus
tracking was performed with a region of interest
(ROI) placed into the ascending aorta.
All coronary CTA examinations (LightSpeed
VCT XT scanner, GE Healthcare) were performed
with prospective ECG triggering [13] according
to a commercially available protocol (SnapShot
Pulse, GE Healthcare). The scanning parameters
were as follows: slice acquisition, 64 × 0.625 mm;
smallest x-ray window (75% of the R-R cycle);
z-coverage value of 40 mm with an increment of 35
mm; gantry rotation time 350 milliseconds. Tube
voltage and tube current were adapted to individual
BMI according to the protocol shown in Table 1.
Scanning was performed from below the tracheal
bifurcation to the diaphragm in three- to five-scan
blocks. The effective radiation dose of coronary
CTA was calculated as the product of the dose–
length product and a conversion coefficient for
the chest (k = 0.017 mSv/mGy·cm) [14]. Axial
images for contrast-to-noise ratio (CNR) calculations
were reconstructed with a slice thickness
of 0.6 mm and a medium soft-tissue convolution
kernel (stand ard). All images were transferred to
an external workstation (AW 4.4, GE Healthcare).
CT Data Analysis
All parameters were determined by one reader,
who had 4 years of experience in cardiovascular
radiology. Calculation of CNR in the proximal right
(RCA) and left main (LMA) coronary arteries was
performed as previously described [15, 16] and
comprised the following steps. First, attenuation
was measured in an ROI in the proximal RCA and
the LMA. ROIs were drawn to be as large as
possible; calcifications, plaques, and stenoses were
carefully avoided. Vessel contrast was calculated
as the difference in mean attenu ation between the
contrast-enhanced vessel lumen and the adjacent
perivascular tissue. Second, image noise was
determined as the SD of the attenuation value in an
ROI placed in the ascending aorta. Third, CNR was
calculated as the ratio of vessel contrast to noise.
Overall image quality was assessed on a 5-point
scale as previously reported [7] (1, excellent image
quality; 2, blurring of the vessel wall; 3, mild
artifacts; 4, severe artifacts; 5, nonevaluative).
Statistical Analysis
Quantitative variables were expressed as mean ±
SD and categoric variables as frequencies, or percentages.
SPSS software (version 15.0, SPSS) was
used for statistical testing. Pearson's cor relation
analysis was performed to compare BMI with
image noise, contrast enhancement, and CNR of
the LMA and RCA. The relation between BMI and
TABLE 1: Protocol for Body Mass Index–Adapted Scanning Protocol
for Low-Dose 64-MDCT Coronary Angiography with Prospective
ECG Triggering
Body Mass Index Voltage (kV) Current (mA)
< 22.5 100 450
22.5–24.9 100 500
25–27.4 120 550
27.5–29.9 120 600
30–40 120 650
> 40 120 700
overall image quality was analyzed with Spear man’s
rank correlation coefficient. A value of p < 0.05
was considered to indicate statistical significance.
Results
CT was successfully performed without
complications on all 101 patients. Five patients
were excluded from analysis because of
hypoplasia of the RCA (n = 3) or severe motion
artifacts in the proximal RCA (n = 2).
The mean dose–length product at coronary
CTA was 120.8 ± 40.0 mGy·cm (range, 58.3–
189.4 mGy·cm), resulting in an estimated
mean applied radiation dose of 2.1 ± 0.7 mSv
(range, 1.0–3.2 mSv). BMI was significantly
related to effective radiation dose (p < 0.001).
The mean BMI of the study population
was 25.7 ± 4.3 (range, 18.2–38.8). Two patients
(2%) were underweight (BMI < 18.5),
47 patients (47%) were of normal weight
(BMI, 18.5–24.9), 38 patients (38%) were
overweight (BMI, 25–29.9), and 14 patients
(14%) were obese (BMI > 30).
The mean attenuation of the RCA was
404.1 ± 112.1 HU (range, 119–747 HU), and
the mean attenuation of the LMA was 406.6 ±
115.7 HU (range, 140–666 HU). The mean
attenuation of the perivascular tissue adjacent
to the RCA was –68.3 ± 20.8 HU (range, –140
to –19 HU), and of that adjacent to the LMA
was –60.1 ± 23.8 HU (range, –104 to –7 HU).
The SD of attenuation in the ascending aorta
was 34.3 ± 6.4 HU (range, 20.8–55.7 HU).
The calculated CNR in the RCA was 14.1 ±
4.0 (range, 5.4–28.2), and that in the LMA
was 13.9 ± 3.9 (range, 6.5–29.8).
There was no significant correlation between
BMI and image noise (r = 0.11, p =
0.284) (Fig. 1), supporting the validity of the
Image Noise (HU)
60 r = 0.11
p = 0.28
50
40
30
20
20
25 30 35
Body Mass Index
Fig. 1—Linear regression plot of image noise against
body mass index shows no correlation. Solid line =
mean; dashed lines = 95% individual prediction
interval.
40
636 AJR:192, March 2009

64-MDCT Coronary Angiography
BMI-adapted scanning protocol. However,
BMI correlated inversely with vessel attenuation
(RCA, r = –0.45, p < 0.001; LMA, r =
–0.47, p < 0.001) (Fig. 2) and CNR (RCA, r =
–0.39, p < 0.001; LMA, r = –0.37, p < 0.001).
Figures 3 and 4 show that as a result of the
BMI-adapted protocol, noise in patients with
a low BMI (Fig. 3) was similar to that in patients
with a high BMI (Fig. 4). Vessel attenuation
in the LMA and aorta, however, was
lower in patients with a high BMI.
In 101 patients, a total of 1,376 coronary
artery segments with a diameter of 1.5 mm
or greater were evaluated for image quality.
Among these segments, 1,312 (95.4%) were
of diagnostic image quality (score 1–3), and
64 segments (4.6%) were nondiagnostic
(score 4–5). The mean image quality per patient
was 1.9 ± 0.6 (range, 1.0–3.9) and did
not correlate with BMI.
Vessel Attenuation in Right
Coronary Artery (HU)
700
600
500
400
300
200
100
0
20
25 30 35
Body Mass Index
r = −0.45
p < 0.001
40
A
Vessel Attenuation in Left
Main Coronary Artery (HU)
700
600
500
400
25 30 35
Body Mass Index
r = −0.47
p < 0.001
Fig. 2—Linear regression plots.
A and B, Plots of vessel attenuation in right (A) and left main (B) coronary arteries against body mass index show
significant negative dependency. Solid line = mean; dashed lines = 95% individual prediction interval.
300
200
100
0
20
40
B
Discussion
This study is the first, to our knowledge,
evaluation of a BMI-adapted scanning protocol
for coronary CTA with prospective ECG
triggering. Similar image noise regardless of
BMI was observed in all patients, supporting
the validity of adaptation of the protocol to
BMI. However, despite successful compensation
for the effect of higher BMI on image
noise, CNR and vessel attenuation decreased
in patients with a higher BMI.
BMI is a known factor influencing image
quality in CT examinations [9, 10, 12]. In particular,
in CTA, a higher BMI unfavorably affects
CNR, supposedly by decreasing arterial
attenuation while increasing image noise [9,
11]. To compensate for the latter, adaptation of
scanning parameters such as tube voltage and
tube current to BMI has been suggested [11,
12]. Because the resulting image noise was
similar in all patients regardless of BMI, the
results of this study document that our proposed
BMI-adapted parameters proved successful
in compensating for BMI. However,
an increase in BMI was paralleled by a decrease
in coronary artery attenuation and
therefore by a decrease in CNR. Because the
contrast bolus was not adapted to BMI, we
have to assume that the fixed contrast material
protocol might have contributed to this effect.
This finding is in line with the finding by Bae
et al. [17] that image attenuation is affected
by BMI. The lowest attenuation values are
found in obese patients and the highest values
in patients of normal weight.
Several patient- and injection-related factors,
such as BMI; cardiac output; the volume,
Fig. 3—60-year-old man with body mass index of
21.3. Axial CT images obtained at 100 kV and 450 mA
show ascending aorta and proximal left main artery
with image noise of 33.1 HU and vessel attenuation
of 492 HU.
con centration, and type of contrast medium;
and the saline flush, have been identified as
having an influence on contrast enhancement
[17]. As in our study, scanning parameters
have been successfully adapted to individual
BMI. The result is an equal noise level
throughout the range of BMIs. Because all of
the aforementioned injection-related parameters
were kept constant for our whole study
population, the observed variation in attenuation
is probably related to individual differences
in bolus dilution in blood. Higher circulating
blood volumes have been associated
with lower coronary artery attenuation [18].
The proposed BMI-adapted scanning protocol
not only achieves similar image noise
for all sizes of patients but also results in a
reduced radiation dose in patients with a low
Fig. 4—53-year-old man with body mass index of
32.0. Axial CT images obtained at 120 kV and 650 mA
show ascending aorta and proximal left main artery
with image noise of 34.0 HU and vessel attenuation
of 286 HU. Patient has different body mass index from
patient in Figure 3, but image noise is similar to that
in Figure 3.
BMI. Several approaches have been focused
on reducing the effective radiation dose of
coronary CTA examinations by modulating
the tube voltage or tube current settings. The
adaptation of tube current settings to body
weight proved useful, resulting in constant
image noise and reducing radiation dose
11.6–26.3% [9, 19]. Adaptation of tube voltage,
however, also is important for radiation
dose reduction because the dose alters with
the square of tube voltage [12]. Leschka et
al. [6] investigated the quality of images of
the coronary arteries using scanning protocols
with 100 kV and 120 kV. They found
that the protocol with 100 kV was feasible in
patients of normal weight, yielding diagnostic
image quality and a significant reduction
of radiation dose.
AJR:192, March 2009 637

Tatusagami et al.
In coronary CTA with prospective ECG
triggering, radiation is administered only
during the end-diastolic phase of the R-R
cycle rather than throughout the entire cardiac
cycle [13]. Combining prospective ECG
triggering for coronary CTA with BMIadapted
scanning parameters allows a substantial
reduction in radiation dose [7, 8].
The mean effective radiation dose in the
present study (2.1 ± 0.7 mSv) was much lower
than in previous reports [5, 6] of retrospective
ECG gating without a BMI-adapted
scanning protocol (9.4–21.4 mSv).
We acknowledge the following limitations.
First, the dose of contrast material was
not adapted to BMI, as suggested in previous
reports [11, 17]. However, use of only a fixed
amount of contrast material enabled us to
evaluate the influence of BMI on image noise
and coronary vessel attenuation. Second,
coronary attenuation and CNR were selectively
evaluated in the proximal RCA and
LMA. Distal segments were not evaluated
because the small diameters of distal segments
do not allow placement of an ROI
without including parts of the vessel wall and
adjacent tissue, thus causing partial volume
effects. Finally, BMI may not be an exact estimation
of body mass at the level of the
heart. The physique differs, for example, in
men and women in the upper and lower parts
of the thorax, which might have been an additional
reason for the lack of correlation between
BMI and image noise in this study.
We conclude that at coronary CTA with
prospective ECG triggering, a BMI-adapted
scanning protocol yields images with similar
noise regardless of BMI. Increased bolus dilution
due to larger blood volume might have
accounted for a decrease in CNR and vessel
attenuation in patients with a higher BMI in
this study because the contrast material bolus
was not adapted.
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