DOI:10.2214/AJR.04.0793
AJR 2006; 186:193-197
© American Roentgen Ray Society
Different Reconstruction Intervals for Exclusion of Coronary Artery Calcifications by Retrospectively Gated MDCT
Joern J. W. Sandstede1,
Julia Stoffels1,
Frank Wendel1,
Christian Ritter1,
Meinrad Beer1 and
Dietbert Hahn1
1 All authors: Department of Radiology, University of Wuerzburg, Wuerzburg,
Germany.
Received May 18, 2004;
accepted after revision January 4, 2005.
Address correspondence to J. J. W. Sandstede, Roentgenzentrum
Schaeferkampsallee/Hamburg, Schaeferkampsallee 5-7, D-20357 Hamburg, Germany
(sandstede{at}web.de).
Abstract
OBJECTIVE. Exclusion of coronary artery calcifications has a high
negative predictive value for the diagnosis of coronary artery disease.
However, it is known that significant differences in calcium scoring can occur
because of the ECG trigger interval. Thus, the aim of the study was to
evaluate the influence of different reconstruction intervals on detection of
any coronary calcium by using MDCT and retrospective cardiac gating.
CONCLUSION. For a true exclusion of coronary artery calcifications,
different reconstruction intervals have to be evaluated.
Keywords: calcium score • coronary artery disease • heart • MDCT • R-R interval
Introduction
Coronary calcium scoring can be used for either cardiovascular risk
stratification of asymptomatic subjects or for diagnosis of coronary artery
disease (CAD) in symptomatic patients, especially in patients with a low
pretest probability of CAD. In asymptomatic individuals, risk stratification
by calcium scoring depends on the presence and extent of calcifications
[1,
2]. In symptomatic patients, if
no coronary calcifications are detected by CT, there is a high negative
predictive value (nearly 100%) for excluding hemodynamically relevant CAD
[3,
4]. Although these numbers were
assessed by electron beam tomography (EBT), which is regarded as the gold
standard for calcium scoring, EBT is known to have a considerable interscan
variability, especially for the exclusion of coronary calcium
[5]. MDCT has a higher
reproducibility because of a higher signal-to-noise ratio and overlapping
image reconstructions [6].
Furthermore, retrospectively ECG-gated MDCT offers the possibility of
reconstructing images at different points of the R-R interval from the same
raw data set. Although significant differences in calcium scoring due to the
reconstruction point in the ECG trigger interval have been shown
[7], the value of different
reconstructions for the exclusion of any calcifications has not yet been shown
in a clinical setting. Thus, the aim of the study was to evaluate the
influence of different reconstruction intervals on detection of any coronary
calcium for exclusion of CAD by using retrospectively ECG-gated MDCT.
Materials and Methods
From a cohort of 43 consecutive patients who had a low pretest probability
for the presence of CAD and had been referred for detection of calcifications,
noncalcified plaques, or hemodynamically relevant stenoses of the coronary
arteries, this retrospective analysis enrolled 26 patients (12 men and 14
women; mean age, 47 ± 10 [SD] years) with an Agatston score of less
than 10 reconstructed at 60% of the R-R wave interval. All the patients
included in this study underwent retrospectively ECG-gated MDCT of the heart
including both unenhanced quantification of coronary calcifications and
contrast-enhanced CT coronary angiography, because of suspected CAD. The low
pretest probability for the presence of CAD was defined by either the presence
of nonanginal chest pain regardless of age or gender or the presence of
atypical chest pain in men younger than 40 years and women younger than 60
years [8]. An oral
ß-blocker (metoprolol, 50 mg) was administered 1 hr before scanning to
all subjects with a baseline heart rate greater than 60 beats per minute.
Patients who had a persistent heart rate above 80 beats per minute or
contraindications to the iodinated contrast agent were not examined by CT
coronary angiography. The study was approved by the ethics committee of our
institution as a retrospective analysis.
All examinations were performed with a 4-MDCT scanner (Somatom Volume Zoom,
Siemens Medical Solutions) with retrospective ECG gating. For calcium scoring,
a standardized examination protocol with a collimation of 4 x 2.5 mm, a
table feed of 3.8 mm per rotation (pitch, 0.38), and a tube rotation time of
500 msec was applied with scan direction from head to feet. Tube voltage was
120 kV with 133 effective mAs. Images were reconstructed with a field of view
of 180 x 180 mm with a 512 x 512 reconstruction matrix and a
medium smooth convolution kernel (B35f). The temporal resolution of the image
reconstruction algorithm was 250 msec. Slice thickness and increment were 3
and 1.5 mm, respectively.
For the CT coronary angiography, 140 mL of a nonionic iodinated contrast
agent (iopromide [Ultravist 300, Schering]) was injected into an antecubital
vein at a flow of 3.0 mL/sec, followed by a 20-mL NaCl chaser bolus using a
dual-head injector. The bolus was timed by injection of a test bolus.
Collimation was 4 x 1 mm, table feed was 1.5 mm per rotation (pitch,
0.38), and tube rotation time was 500 msec. Tube voltage was 120 kV with 400
effective mAs. Scanning was with scan direction from head to feet. Image
reconstruction also was performed with a field of view of 180 x 180 mm
with a 512 x 512 reconstruction matrix and a temporal resolution of the
image reconstruction algorithm of 250 msec. A smooth convolution kernel (B20)
was applied. Slice thickness and increment were 1.25 and 0.6 mm,
respectively.
In every patient, the axial images for both unenhanced quantification of
coronary calcifications and contrast-enhanced CT coronary angiography were
reconstructed at 50%, 60%, 70%, and 80% of the R-R wave interval. For
quantification of calcifications, Agatston score, calcium volume, and calcium
mass were assessed for each reconstruction interval. For differentiation from
artifacts, only calcifications that were also visible on CT coronary
angiography were diagnosed as true calcified plaque. Calcium scoring on images
reconstructed at 60% of the R-R wave interval was chosen as the standard of
reference according to Mahnken et al.
[7,
9]. CT coronary angiography was
also evaluated at all four reconstruction intervals for the presence of
noncalcified plaques or hemodynamically significant stenoses by two
experienced radiologists in consensus.
Results
The Agatston score assessed from the images reconstructed at 60% of the R-R
wave interval ranged from 0 to 3.3, with a mean of 0.33 ± 0.8. Calcium
volume and calcium mass were 0.44 ± 1.22 mm3 and 0.08
± 0.18 mg of calcium hydroxyapatite, respectively. The results of all
four reconstruction points of the R-R interval are listed in
Table 1.
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TABLE 1: Results for Four Reconstruction Points in 26 Patients with Low Pretest
Probability of Coronary Artery Disease
|
|
Regarding exclusion of coronary calcifications, 19 of the 26 patients had a
calcium score of 0 reconstructed at 60% of the R-R interval. At 50%, 70%, and
80%, a calcium score of 0 was found for 20, 19, and 18 of the 26 patients,
respectively. Only 11 of the 26 patients had an overall calcium score of 0 at
all four reconstruction points. Of the 15 (of 26) patients who had coronary
calcifications at any reconstruction, no patient had positive calcium results
at all four reconstructions. Detection of coronary calcification was positive
at one, two, and three reconstruction intervals for six, five, and four of the
15 patients, respectively (Figs.
1A,
1B,
1C,
1D,
1E, and
1F). With regard to the
reconstruction points, coronary calcifications were detected at 50%, 60%, 70%,
and 80% in six, seven, seven, and eight of the 15 patients, respectively.
Calcium was found in only one coronary artery in each of the 15 patients.
Table 2 shows the Agatston
scores and the locations of the calcifications at all four reconstruction
points for these 15 patients.
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Fig. 1E —56-year-old man with calcification of left anterior
descending artery. Maximum-intensity-projection reconstructions of enhanced CT
coronary angiography confirm calcified plaque (arrow, E and
F) and reveal additional noncalcified plaque of left anterior
descending artery (arrowheads, F).
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View larger version (121K):
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[in a new window]
[as a PowerPoint slide]
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Fig. 1F —56-year-old man with calcification of left anterior
descending artery. Maximum-intensity-projection reconstructions of enhanced CT
coronary angiography confirm calcified plaque (arrow, E and
F) and reveal additional noncalcified plaque of left anterior
descending artery (arrowheads, F).
|
|
At CT coronary angiography, no hemodynamically significant stenoses were
found. The proximal and middle parts of the main coronary arteries were imaged
in all patients. Of the 26 patients, one each had noncalcified plaques in the
left main, left anterior descending, and right coronary arteries, and one
patient had noncalcified plaques in three coronary arteries (left anterior
descending, left circumflex, and right) (Figs.
1A,
1B,
1C,
1D,
1E, and
1F). Agatston scores at all
four reconstruction points for these patients are also listed in
Table 2. During the consensus
reading, the reviewers did not significantly differ in their evaluations.
Discussion
Detection or exclusion of small calcifications in the coronary arteries is
improved by the evaluation of reconstructions at different points of the R-R
interval. Whereas in the present study 69–77% of patients with a low
pretest probability for the presence of CAD had a calcium score of 0 at one
reconstruction interval at least, in only 42% of the patients could coronary
calcifications be excluded after evaluation of all reconstructions at 50%,
60%, 70%, and 80% of the R-R interval.
Limited reproducibility is one of the major drawbacks for the clinical use
of coronary calcium scoring. This is true not only for risk stratification of
asymptomatic patients with follow-up under therapy but also, especially, for
the exclusion of CAD by exclusion of any coronary calcifications. EBT is known
to have a mean interscan variability of between 20% and 24% as assessed in
smaller patient groups [10,
11]. In a larger group of 951
asymptomatic individuals who were examined twice, Yoon et al.
[5] reported an interscan
variability of 28% and 43% for women and men, respectively. In this group, 415
individuals had a calcium score of 0 on one of the two scans but only 314 of
these individuals had a calcium score of 0 on both scans. Thus, in this study
only 76% of the patients with a calcium score of 0 at one examination indeed
had no calcifications.
Sources of error in calcium scoring with EBT are signal-to-noise ratio,
partial-volume effects, and motion artifacts
[12]. These errors can be
addressed by the use of MDCT. The signal-to-noise ratio can be improved by a
higher tube current, and partial-volume effects are diminished by an
overlapping image reconstruction that can be applied with retrospectively
gated MDCT [6]. Furthermore,
the point of image acquisition within the R-R interval can be optimized,
reducing motion artifacts. For EBT, Mao et al.
[10] showed that
reproducibility is better at a trigger of 40% than of 80% of the R-R interval.
With MDCT, Mahnken et al. [7]
showed that diastolic image reconstruction at 50% or 60% of the R-R interval
can be recommended for retrospectively ECG-gated MDCT. This finding is in
concordance with the results of Achenbach et al.
[13], who measured coronary
artery motion by EBT and showed that motility is lowest at 48% of the cardiac
cycle. However, also shown was considerable interindividual variation in the
distribution of different velocities of coronary arterial motion during the
cardiac cycle. Because of these interindividual variations, motion artifacts
that are due to the point of image acquisition will still have a major
influence if images reconstructed at only one point of the R-R interval are
used for calcium scoring. Thus, as was shown with the present study, detection
of small calcifications especially will be improved by the evaluation of
images reconstructed at more than one point of the R-R interval for
determination of the individual optimal reconstruction interval. With this
technique, MDCT can reduce motion artifacts, although the temporal resolution
of EBT remains superior. However, regarding the specificity of calcium
detection, MDCT also suffers from streak, edge, and motion artifacts that can
cause pixels in the region of the coronary artery course to exceed the 130-H
threshold. Motion artifacts might also explain the different calcium scores at
the different reconstruction intervals
[14].
CT coronary angiography is performed in addition to calcium scoring in our
clinical routine because of the high negative predictive value of the
former—97% to 98%
[15–18].
However, with the use of a 4-MDCT scanner, sensitivities for the detection of
hemodynamically relevant stenoses are between only 81% and 91% and
specificities are between only 84% and 93%. Image quality is diagnostic in
only 71% of the coronary arteries
[15]. Thus, combined
assessment of coronary calcifications and CT coronary angiography can give
further information to exclude CAD. Certainly, CT coronary angiography with a
4-MDCT scanner cannot now replace conventional coronary angiography but might
be able to in the future with further technical improvements. Despite
hemodynamically relevant stenoses, CT coronary angiography also allows for the
detection of noncalcified plaques. Nikolaou et al.
[19] showed in a larger series
of 179 patients that seven of 48 patients without coronary calcifications had
detectable noncalcified plaques that may provide additional information for
the diagnosis of CAD. However, in our smaller group of patients, only four of
26 patients with a low pretest probability for the presence of CAD had
noncalcified plaques. Although all these patients presented with a calcium
score other than 0 in either reconstruction interval, if images at only one
reconstruction interval were evaluated—for example, 60%—two of
these four patients would have been missed. Thus, it remains unclear whether
the number of patients reported by Nikolaou et al. presenting with
noncalcified plaques but without any calcifications represents the true number
of patients with only noncalcified plaques or whether these patients would
show calcifications if images reconstructed at more than one point in the R-R
interval had been evaluated. This possibility that CT coronary angiography
might add value to calcium scoring in patients with a low pretest probability
of CAD has to be determined by the examination of larger patient groups.
The radiation dose of calcium scoring with the technique used in the
present study as reported by Hunold et al.
[20] was 3 and 3.6 mSv for men
and women, respectively. The radiation dose for CT coronary angiography was
reported to be 10.9 and 13 mSv for men and women, respectively. Today, the
dose can be reduced by about 50% by the use of ECG pulsing. Thus, the current
radiation dose in a clinical setting for the protocol used in the present
study is about 7 and 8.3 mSv for men and women, respectively
[21,
22]. This dose is higher than
doses for a conventional protocol consisting of either only calcium scoring or
only CT coronary angiography. However, this combined protocol brings two major
advances: On the one hand, if calcium scoring reveals a high calcium score, CT
coronary angiography can be omitted because high amounts of calcium hinder
reliable diagnosis of coronary artery stenoses
[23]. On the other hand, an
examination protocol consisting only of calcium scoring is more suitable for
screening purposes but cannot rule out hemodynamically significant CAD in
symptomatic patients.
A major limitation of the present study was the lack of correlation with
invasive coronary angiography findings. Thus, it is not known whether the
patients without any calcifications truly did not have CAD or whether the
patients with calcifications truly did have CAD. However, obtaining this
information was not the focus of our study. Furthermore, it is unclear whether
changes in calcium score from 0 to less than 1 will change patient management.
A larger series of subjects with minimal coronary artery calcifications
detected by any CT reconstruction interval should be followed to determine
whether these minimal calcifications are associated with increased acute
coronary events. However, even one small calcification proves the presence of
any CAD, and this is an important issue in patients with atypical chest pain
in whom CAD must be excluded. Thus, even an Agatston score of less than 1 can
be considered a significant finding in this patient group. As a clinical
implication, one can state that for a true exclusion of coronary artery
calcified plaques by retrospectively ECG-gated MDCT, different reconstruction
intervals have to be evaluated.
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Abstract
Introduction
