Download Dual-source cardiac computed tomography

Eur Radiol (2008) 18: 1188–1198
DOI 10.1007/s00330-008-0883-3
Stephan Achenbach
Katharina Anders
Willi A. Kalender
Received: 10 September 2007
Revised: 10 December 2007
Accepted: 11 January 2008
Published online: 26 February 2008
# European Society of Radiology 2008
S. Achenbach
Department of Cardiology,
University Erlangen-Nuernberg,
Ulmenweg 18,
91054 Erlangen, Germany
K. Anders
Institute of Diagnostic Radiology,
University Erlangen-Nuernberg,
Maximiliansplatz 1,
91054 Erlangen, Germany
W. A. Kalender (*)
Institute of Medical Physics,
University Erlangen-Nuernberg,
Henkestr. 91,
91052 Erlangen, Germany
e-mail: [email protected]
Tel.: +49-9131-8522310
Fax: +49-9131-8522824
COMPUTER TOMOG RAPHY
Dual-source cardiac computed tomography:
image quality and dose considerations
Abstract Computed tomography
(CT) imaging of the heart, most
prominently coronary CT angiography, is currently subject to intense
interest and is increasingly incorporated into clinical decision-making. In
spite of tremendous progress in CT
technology over the past decade, the
limited temporal resolution has remained one of the most severe problems, especially for cardiac imaging.
The novel design concept of dualsource CT (DSCT) allows for an
effective scan time of 83 ms independent of heart rate. While large trials are
still missing, initial studies have
shown improved image quality, especially for visualizing the coronary
arteries and detecting coronary artery
stenoses. Further investigations have
shown that routine beta blockade to
lower the heart rate is not necessary to
reliably achieve diagnostic image
Introduction
Rapid motion of the heart, the small dimensions of cardiac
structures—especially the coronary vessels—and the need
to synchronize image acquisition or image reconstruction to
the cardiac cycle have constituted tremendous obstacles
to the use of computed tomography for cardiac imaging. In
the late 1990s, however, the design of multidetector
computed tomography (CT) systems and the development
of ECG-correlated partial scan image reconstruction
algorithms allowed first attempts at cardiac and coronary
imaging [1–4]. From the very beginning, coronary artery
visualization constituted of the most prominent areas of
interest and “coronary CT angiography” has been the major
driving force for further technical development and clinical
evaluation. The first multidetector CT systems, with four
quality. Other applications that may
particularly benefit from increased
temporal resolution are the analysis of
ventricular function and of the cardiac
valves. Dose issues which are of
interest for cardiac CT in general are
discussed in some detail, including a
quantitative analysis of dose values
and three-dimensional dose distributions. Various strategies to lower
radiation exposure are available today,
and DSCT offers specific potential for
this.
Keywords Computed tomography .
Dual-source CT . Heart . Coronary
arteries . Coronary angiography .
Dose
detector rows, provided a gantry rotation time of
approximately 0.5 s, and imaging of the heart could be
completed in about 30–40 s. Initial reports on visualization
of the coronary artery lumen by four-slice CT after
intravenous injection of contrast agent were published in
the year 2000 [4, 5]. However, because of limitations in
temporal and spatial resolution, the obtained data sets were
frequently of insufficient quality for interpretation: in the
initial studies, up to 30% of arteries were classified as
“unevaluable” [6, 7]. In addition, the requirement of a 30-s
breath-hold was an obstacle to broad clinical use.
Rapid improvements in CT technology occurred in the
subsequent years, and they rapidly led to improved
temporal and spatial resolution as well as to reduced
overall acquisition time [8]. Sixteen-slice CT was introduced around 2002 and 64-slice scanners with gantry
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rotation times of 330–420 ms, introduced in 2004, are
currently considered “state-of-the-art” equipment for CT
visualization of the coronary arteries. These systems
provide a slice collimation of 0.5–0.625 mm and acquisition of a data set for coronary visualization can be
accomplished in a breath-hold of 6–12 s duration [9].
Coronary artery visualization by 64-slice CT
A number of studies were able to demonstrate relatively
high accuracy for the detection of hemodynamically
relevant coronary artery stenoses by 64-slice CT. Obviously, the accuracy for stenosis detection will depend on
the operators’ expertise, on the scanner technology that is
used, and also on the prevalence of disease in the patient
population that was studied. For 64-slice CT, sensitivity
values ranging from 73% to 99% and specificity values
between 93 and 97% were obtained in patients referred for
a first diagnostic coronary angiogram [9]. A recent metaanalysis showed a clear increase in diagnostic accuracy as
technology improved from four-slice to 16-slice and 64slice CT, with a pooled sensitivity of 95% and specificity of
93% for 64-slice CT on a per-vessel basis. In a per-patient
analysis, pooled sensitivity for 64-slice CT was 99% and
specificity was 93% [10]. Importantly, patients in all
published studies were somewhat preselected (e.g., exclusion of all patients with renal failure or arrhythmias,
exclusion of unstable patients and patients with acute chest
pain). In addition, interpretation was usually limited to
coronary segments with a diameter of at least 1.5–2.0 mm.
The available data demonstrate that with adequate data
acquisition and reconstruction, as well as experience in
interpretation, high sensitivity and specificity values for the
detection of hemodynamically relevant stenoses can be
achieved by 64-slice CT. All the same, up to 12% of
coronary artery segments had to be excluded from these
analyses because they were deemed to be “unevaluable”
[11]. While sometimes a consequence of massive calcification, motion artifacts are another frequent reason for
impaired evaluability in 64-slice CT. The temporal resolution of 64-slice CT is still not sufficient for imaging of
unselected patients. In fact, heart rate is a major predictor of
image quality [12–16] and usually, heart rates of less than
65 bpm or, optimally, less than 60 bpm are suggested in
order to achieve predictably good image quality. This often
requires premedication with beta blockers or other heartrate lowering agents and next to concerns of side effects
and logistic challenges, some patients cannot achieve the
target heart rates in spite of pretreatment. Finally, heart rate
variability has been identified as another parameter that
negatively influences image quality [12] in 64-slice CT.
For this reason, improvements in temporal resolution have
been considered very desirable in cardiac CT imaging and,
especially, coronary CT angiography.
Dual-source computed tomography (DSCT)
DSCT was introduced in late 2005 [17]. The system at
hand, the SOMATOM Definition (Siemens Medical
Solutions, Forchheim, Germany) contains two sets of Xray tube and detector, which are arranged in a single gantry
at 90° offset. Since data acquired over 180° are required to
reconstruct a single cross-sectional image (an assumption
which holds true for the center of rotation, slightly more
data are needed for off-center regions), a one-quarter
rotation of the gantry is sufficient to collect the data
necessary for one image when the two tubes and detectors
of the DSCT system are operated simultaneously. With a
gantry rotation time of 330 ms, DSCT therefore provides
an effective scan time of 83 ms in the center of rotation.
Notably, this can be achieved without multisegment
Fig. 1 Visualization of the coronary arteries by DSCT. a Here, the
left anterior descending coronary artery (large arrow) which
contains a proximal calcification (small arrow) is visualized in a
curved multiplanar reconstruction. b Three-dimensional visualization of the heart and coronary arteries (large arrow left anterior
descending coronary artery, small arrows right coronary artery)
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reconstruction; we use single-segment reconstruction (with
all data originating from the same cardiac cycle) in all
cases. This allows use of larger pitch values CT with less
overlap compared with single-source and reduced dose.
Artifacts that may occur when data collected during more
than one heart beat are “averaged” for image reconstruction
are avoided. This approach also assures that temporal
resolution is entirely independent of heart rate. Accordingly, we observed high image quality of DSCT for
coronary artery visualization from the beginning of our
DSCT work in 2005 [18] (Fig. 1).
In addition, DSCT offers the possibility for simultaneous
data acquisition with different X-ray energies, which may
permit improved tissue differentiation [17, 19, 20]. One
tube is operated at 80 kV tube voltage, while the other tube
is operated at 140 kV. Thereby the increased temporal
resolution compared with single-source CT (SSCT) is
sacrificed for this mode of operation. Except for phantom
studies, which indicated potential for plaque tissue differentiation [20], the potential of dual-energy imaging for
cardiac applications has not yet been thoroughly explored.
Potential limitations include high image noise, especially
for images acquired at 80 kV, and the lack of thoroughly
Table 1 Typical data acquisition parameters for DSCT
coronary angiography
Premedication
Rotation time
Total scan time
Slice width
Collimation
Pitch
“ECG pulsing”
Tube voltage
Tube current
mAs value per rotation
Effective mAs value
Contrast agent
Contrast timing
validated software to achieve complex tasks, such as
removal of certain plaque components.
Typical image acquisition parameters for DSCT coronary angiography are listed in Table 1. The typical scan
duration is 7–10 s and between 50 and 80 ml of contrast
agent are usually injected at a flow rate of 5 ml/s [18, 21,
22]. Images are most frequently reconstructed using
0.75-mm slice thickness, a slice increment of 0.4–0.5 mm,
and the dedicated kernel “B26f,” which uses a threedimensional (3D) noise reduction algorithm [23].
Coronary CT angiography by DSCT
Based on a number of publications, it has become evident
that the image quality of DSCT coronary angiography is
less dependent on heart rate than for 64-slice CT and allows
coronary CT angiography data sets of fully diagnostic
quality to be obtained, even at higher heart rates. We [18]
published an initial series of 14 patients in whom DSCT
coronary angiography data sets were obtained without the
use of beta blockers for lowering the heart rate. With a
mean heart rate of 71 bpm, 98% of all coronary segments
Nitrates s.l.
330 ms
7–10 s
0.6 mm (64 overlapping slices)
19.2 mm
0.20
Heart rate <50 bpm
0.22
Heart rate 50–59 bpm
0.28
Heart rate 60–69 bpm
0.33
Heart rate 70–79 bpm
0.39
Heart rate 80–89 bpm
0.44
Heart rate 90–99 bpm
0.50
Heart rate ≥100 bpm
Tube current modulation should always be used; typically, dose
reduction factors of 0.4–0.8 can be achieved
120 kV
100 kV for patients <85 kg
80 kV for very slim patients
e.g., 400+400 mA
e.g., 267 mAs (= 800×0.33); the resulting value is relevant for
image quality considerations
e.g., 534 mAs (= 267×0.6/0.3); in this example, a pitch of 0.3
and an ECG pulsing efficiency factor of 0.6 were assumed; the resulting
value is relevant for dose considerations
50–80 ml at 5 ml/s (consider 6 ml/s in patients >100 kg)
Test bolus or bolus tracking
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were visualized free of motion artifact. Most frequently, an
image reconstruction window starting at 70% of the R-toR-interval provided optimal image quality. Similarly,
Johnson et al. [21] published early results of 24 patients
with heart rates between 44 and 92 bpm and did not find
significiant impairment of image quality at high heart rates.
They reported that image reconstruction windows around
70% of the cardiac cycle are usually optimal for low heart
rates, and end-systolic time instants are often better for
heart rates above 75 bpm.
This was investigated in more detail by Leschka et al.
[22] in a recently published series of 60 patients, with heart
rates ranging from 35 to 117 bpm (mean: 69 bpm). In this
series, diagnostic image quality was obtained in 98% of all
coronary segments. The authors thoroughly analyzed the
relationship between heart rate and the distribution of the
“optimal” time windows for reconstructing the data set
within the cardiac cycle. While 70% of the R-R interval
was the most frequent “best time instant” for reconstruction, that time instant was found between 60% and 70% for
heart rates of less than 60 bpm, between 60 and 80% for
heart rates between 60 and 70 bpm, between 55 and 80%
for heart rates between 70 and 80 bpm, and between 30%
and 80% for heart rates of more than 80 bpm. These results
have implications for tube current modulation, a strategy
used to limit radiation exposure: the time window of full
tube current can be kept rather short for low heart rates and
should be of larger duration for high heart rates. In a further
study which encompassed 80 patients, the same authors
were able to show that in patients with a heart rate below 65
bpm, all coronary segments were visualized with diagnostic image quality, while in patients with a heart rate of 65
bpm or greater, 98% of coronary segments were visualized
with diagnostic image quality [24].
also the analysis of coronary artery stents, even though no
data on the accuracy for detection of in-stent restenosis by
DSCT are currently available (Fig. 3).
Scheffel et al. [27] investigated the accuracy of DSCT
for the detection of coronary artery stenoses in a group of
patients with high pretest likelihood of disease. The often
severe atherosclerosis in these patients makes stenosis
detection by coronary CT angiography more challenging,
and with previous scanner generations the accuracy for
stenosis detection in high-risk patient groups was consistently lower than in patients with lower prevalence of
disease [26, 28]. In their report, Scheffel et al. [27]
analyzed a group of 30 patients with a mean age of 63±
11 years and an average heart rate of 70±14 bpm. No beta
blockers were given in preparation for the scan. They
reported a sensitivity of 96%, specificity of 98%, positive
predictive value of 86% and negative predictive value of
Advantages of DSCT in challenging situations
While image quality and diagnostic accuracy of 64-slice
SSCT coronary angiography was uniformly found to be
high in patients with low heart rates [9], patients with high
heart rates were often considered problematic, which was a
certain limitation to clinical application. The high temporal
resolution of DSCT provides improved image quality in
patients with high heart rates as outlined above (Fig. 2).
This may be especially valuable in patients with acute
coronary syndromes, in whom there may be no time for
heart-rate lowering medication [25]. In addition, other
situations in which image quality has been somewhat
impaired in 16- and 64-slice CT might profit from the
higher temporal resolution of DSCT, since artifacts are
often aggravated by limited temporal resolution. Examples
include the setting of severe coronary calcification—
motion artifacts and subsequent “blurring” of calcium
have been identified as a frequent reason for false-positive
findings in coronary CT angiography [26]—and potentially
Fig. 2 Patient with a heart rate of 125 bpm as a consequence of
pericardial effusion. a Transaxial image as acquired by contrast
enhanced DSCT (arrow cross-section of right coronary artery,
asterisks pericardial effusion). b Maximum Intensity projection
(MIP) or the right coronary artery (arrows)
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Fig. 3 DSCT visualization of a
coronary artery stent (arrows in
b) implanted in the left anterior
descending coronary artery. b A
magnified image segment of a
99% for the detection of coronary artery stenoses on a persegment basis. This indicates that even in patient groups
with challenging anatomy, DSCT delivers high image
quality and diagnostic accuracy.
Irregular heart rates are also challenging for SSCT,
especially if multi-segment reconstruction algorithms are
used in order to improve temporal resolution. DSCT, with a
high temporal resolution independent of heart rate, appears
to be less susceptible to artifacts by irregular heart rates,
and even the successful imaging of patients in atrial
fibrillation has been anecdotally reported (see Fig. 4) [29].
However, no sufficiently large trials about the accuracy of
DSCT coronary angiography in patients with atrial fibrillation or other arrhythmias are currently available and it
remains to be determined whether such promising initial
results will be viable in a clinical context.
Fig. 4 Visualization of the right
coronary artery in a patient with
atrial fibrillation. a Multiplanar
reconstruction of the right coronary artery (arrows). b ECG
trace. The gray rectangles indicate the time windows used
for image reconstruction. They
are positioned 100 ms before the
peak of the R-wave
One of the most challenging applications of coronary CT
angiography is the visualization and analysis of nonstenotic coronary atherosclerotic plaque (Fig. 5). The small
dimensions of coronary atherosclerotic plaques, along with
the lack of strong contrast between noncalcified plaque
components and the perivascular connective tissue, require
optimal image quality in order to allow detection and,
possibly, quantification and characterization of plaques. In
a phantom study that analyzed the visibility of plaque
components at varying heart rates, Reimann et al. were able
to demonstrate a significant advantage of DSCT over 64slice CT for the visualization of coronary atherosclerotic
plaque [30].
Of note, the two tubes of the DSCT scanner can be
combined in order to decrease image noise; as a trade-off,
the high temporal resolution is lost. This option may be
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small patient study, Rist et al. [32] reported close correlation for parameters of left ventricular function in DSCT
and magnetic resonance imaging.
Dose considerations
Fig. 5 Visualization of a nonstenotic coronary atherosclerotic
plaque after intravenous injection of contrast agent (arrow). The
plaque is localized at the bifurcation of the left main coronary artery,
it is partly calcified
Patient dose in general is a point of particular concern for
cardiac CT imaging. This is due to the fact that low pitch
values of typically p=0.2–0.3 are commonly used in SSCT.
This means that each section is exposed several times,
exactly 1/p times. The effective mAs value, where mAseff =
mAs×fECG/p with fECG the ECG pulsing efficiency factor,
and the so-called volume CT dose index CTDIvol =CTDIw/p
characterize this situation appropriately. CTDIvol values are
specified by the manufacturer for the given scanner and
scan protocol [33, 34]. For modern scanners like the Dual
useful for obese patients in whom image noise can be
problematic. If the two tubes are fully combined, and 180°
of rotation from each tube is used, temporal resolution falls
back to the 165 ms of single-source 64-slice CT, but image
noise can be reduced substantially (see Fig. 6). It is also
possible to reconstruct at effective scan times of 105 ms,
125 ms, and 145 ms, to achieve optimal balance between
image noise and temporal resolution. This option has not
yet been systematically evaluated as to the ability to
increase diagnostic accuracy in patients with a high body
mass.
Noncoronary cardiac imaging by DSCT
Potentially, the increased temporal resolution of DSCT
may be of advantage when high-resolution cardiac imaging
is desired, even in phases of more rapid cardiac motion.
While not investigated systematically, it appears possible
that “noncoronary” applications, such as assessment of left
ventricular function and the analysis of valvular function,
would profit from imaging with decreased motion artifact
even in systole (Fig. 7). Johnson et al. [21] reported high
image quality ratings for assessment of the aortic and mitral
valve by contrast-enhanced DSCT, but no systematic
comparison to 64-slice CT has so far been performed.
Mahnken et al. [31] have published data of a phantom
study that compared single-source and dual-source reconstruction for the analysis of left ventricular ejection
fraction. They found that dual-source reconstruction
(with a temporal resolution of 83 ms) provided more
accurate measurements of ejection fraction than singlesource reconstruction with a temporal resolution of 165 ms
(deviation 0.7% vs 4.3%). These differences were
especially pronounced for higher heart rates [27]. In a
Fig. 6 Reduction of image noise by combining the tube output of
both DSCT tubes. In this patient with a body weight of 125 kg,
image noise can be reduced. a Reconstruction with 83 ms temporal
resolution. b Reconstruction with 165 ms temporal resolution and
subsequently reduced image noise
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Fig. 7 Visualization of the aortic valve (here, a nondiseased
tricuspid aortic valve is shown) by DSCT in diastole (valve closed,
a) and systole (valve open, b). In this case, image acquisition was
performed without ECG pulsing (leading to higher radiation dose),
which explains the low noise level in the systolic image
Source CT, the CTDIvol is displayed on the CT console
prior to the scan, the effective mAs value is recorded and
available after the scan. It takes the pitch and ECG pulsing
into account.
A better assessment of organ dose and effective dose is
offered by specific dose calculation tools, which have
become available just recently. We use the Monte Carlo
methods-based tool ImpactDose (VAMP, Erlangen, Germany), which provides scan protocol-specific organ dose
estimates for standard man [34], and ImpactMC (VAMP,
Erlangen, Germany), which provides scanner- and protocolspecific 3D dose distributions for individual patients [35], as
shown by examples below. The results of calculations using
these tools and measurements consistently show that maximum dose values of 50–100 mSv are frequently given and
often even exceeded in the directly exposed volume when no
specific measures, such as ECG pulsing, are taken.
Mean organ dose values for the complete body due to
direct exposure and exposure to scattered radiation are
derived from the 3D dose distributions, and the effective
dose is calculated as the weighted mean of the relevant
organ values. The effective dose is sometimes called the
whole-body equivalent dose and is a useful measure, in
particular, for comparison to other exposures. In cardiac
CT it is much lower than the dose to the directly exposed
cardiac region due to the averaging process since most
other organs receive very low doses only.
The effective dose for coronary CT angiography with
64-slice CT has been shown to be approximately 9.4 mSv
with ECG pulsing and 14.8 mSv without pulsing [36]. A
recent report on effective dose in coronary angiography
performed by DSCT listed mean values of 7.8–8.8 mSv
and found that radiation dose decreased with increasing
heart rate, which is an effect of the increased pitch values
[37]. For comparison, conventional catheter-based coronary angiography has been found to be associated with an
effective dose of approximately 5.6–5.8 mSv [38]. However, when comparing radiation risks of CT and invasive
angiography, the risk of arterial access complications with
subsequent risk of morbiditiy and mortality must also be
taken into account.
Dose considerations are principally the same for SSCT
and DSCT. Particular concern has been voiced with the
introduction of DSCT imaging, since higher power levels
are available, and some misconceptions resulted. It is
correct that twice the X-ray power is available; it is not
correct, however, that higher doses must result. In most
applications, the image quality level regarding noise and
spatial resolution and dose are kept constant: the tube
current is effectively doubled, but the scan time is halved,
i.e., the tube current-time product, measured in mAs,
Fig. 8 Prospectively triggered DSCT coronary angiography scan.
The scan is performed in a “step-and shoot” fashion and radiation is
applied only for acquisition only of data that would actually be used
for image reconstruction (dose here: 3.8 mSv). A curved multiplanar
reconstruction of the left circumflex coronary artery is shown
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remains constant. So does the dose, as it is linearly
proportional to the mAs value when all other scan
parameters remain the same. This is common practice in
general CT, e.g., for obese patients.
For cardiac CT, the situation is more complex. Dose may
increase relative to SSCT since not all data are being used
at maximum temporal resolution [23]. A number of
features and modifications which help reduce radiation
dose for coronary and cardiac imaging were provided by
the SOMATOM Definition [8, 17, 22, 23]. First, a
dedicated “bowtie” filter for cardiac applications was
introduced to reduce the X-ray intensity towards the
periphery; this has no influence on image quality for the
cardiac region but reduces total dose. Second, pitch values
are increased with heart rate in order to reduce overlapping
exposure; no large overlap is needed for DSCT because
single-segment reconstruction is always used. Since DSCT
Fig. 9 Example for dose reduction in cardiac CT (74-yearold patient with 63 kg body
weight) scanned with 80 kV
tube current and ECG pulsing.
a Transaxial image showing the
cardiac chambers and coronary
arteries (large arrow right coronary artery; small arrow left
anterior descending coronary
artery) with good contrast and
acceptable image noise.
b Three-dimensional dose distribution revealed a dose reduction of about 80% compared
with a standard 120 kV scan
with the same contrast-to-noise
ratio. The effective dose was
estimated at about 3 mSv
with its higher temporal resolution yields diagnostic image
quality with single-segment reconstruction even for high
heart rates, dose can be reduced very efficiently for these
patients. Third, ECG pulsing, i.e., tube current reduction
during heart motion phases for which no image reconstruction is planned [39], can be used more effectively with
DSCT compared with SSCT. Fourth, a dedicated reconstruction kernel (“B26f”) which uses a 3D adaptive noise
reconstruction algorithm was developed for use with the
DSCT system.
The effectiveness of the above measures has been
proven already in a number of investigations. In a phantom
study at a fixed pitch of 0.2, it was shown that in order to
achieve the same image noise, the dose values of 64-slice
SSCT and DSCT for ECG-gated imaging were comparable
[23]. However, pitch values for cardiac imaging by DSCT
can be increased substantially with heart rate (see Table 1).
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Therefore, at higher heart rates, the dose for DSCT is
significantly lower than for SSCT with a fixed pitch, even if
no ECG pulsing is used [23].
In addition, the high temporal resolution makes it
possible to use ECG-related tube current modulation
more effectively. Narrower time windows of full X-ray
tube current are possible since, for most heart rates, reliable
imaging can be achieved within a very short end-diastolic
window [22]. For the patient study shown in Fig. 4, dose
was effectively reduced by about 60% of the value which
would have been obtained without ECG pulsing.
In addition, the DSCT scanner offers a mode for
prospectively triggered image acquisition in a “step and
shoot” fashion, where again the high temporal resolution is
a unique advantage. With prospectively triggered scans, Xray exposure is only applied during a very short time
interval, which substantially lowers dose (Fig. 8). Downsides of the prospectively triggered scan mode include the
lack of any ability to retrospectively adapt the time instant
of imaging during the cardiac cycle, and thus a high
susceptibility to artifacts caused by variations in cardiac
cycle length or by the occurrence of arrhythmias during
data acquisition. The optimal solution would be a DSCT
system with wide enough Z-coverage to encompass the
entire volume of the heart in one single sweep (e.g., 256
slices). Such a system would allow prospective triggering
with high temporal resolution and, since the entire heart
would be covered during one single heart beat, would not
be vulnerable to arrhythmias.
Independent of the above measures for dose reduction,
the use of lower tube voltages may also lead to substantial
dose reduction. It has been recommended by some groups
[36] but has not yet been fully investigated with respect to
dose implications. Based on Monte Carlo calculations
using the tool ImpactMC and on measurements in
phantoms and patients, the potential for dose reduction
can be quantified. It strongly depends on the patient crosssection [40]; for slim patients, 100 kV or potentially even
80 kVare to be recommended. For the case shown in Fig. 9,
scanning with a tube voltage of 80 kV with retrospective
gating and the use of ECG pulsing resulted in an estimated
effective dose of 3.0 mSv at fully diagnostic image quality
(Fig. 9, heart rate 58 bpm).
Not all of the measures for dose reduction described
above are standard clinical practice today, but there is a
clear trend in that direction. In the authors’ opinion, it will
certainly be possible to limit typical effective dose values
for DSCT and for CT coronary angiography in general to
below 10 mSv and often to below 5 mSv.
Summary
In summary, DSCT provides a number of unique
advantages for cardiac and especially coronary artery
imaging. Most prominently, the high temporal resolution
reduces motion artifacts and it has been shown that, even if
patients are scanned without beta blocker premedication,
the vast majority of coronary artery segments are visualized
with diagnostic image quality. This will be helpful not only
to improve the diagnostic accuracy of coronary CT
angiography but also in order to broaden clinical applicability of the method. This applies, for example, to
patients and clinical situations in which administration of
beta blockers or other heart-rate lowering agents is not
possible for medical (e.g., asthma) or logistic reasons (e.g.,
urgent scan in the setting of acute chest pain). Other
advantages relate to patient dose. It is important to note that
DSCT is not associated with an increase of patient dose;
quite to the contrary, a number of options for dose
reduction are given. The possibilities to reduce radiation
exposure include heart-rate-dependent pitch values and
“aggressive” ECG pulsing with narrow windows of full Xray tube output. The possibility to use “dual energy”
acquisition modes to improve tissue classification may
provide further advantages, even though this has not yet
been evaluated in the context of cardiac imaging.
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