Dr. Rajiah is a Cardiovascular Imaging Fellow at the Cardiovascular Imaging Laboratory, Imaging Institute, Dr. Halliburton
is a Medical Imaging Scientist, Cardiovascular Imaging Laboratory,
Imaging Institute, and Cardiovascular Medicine, Heart and Vascular
Institute, and Dr. Flamm is the Section Head of the
Cardiovascular Imaging Laboratory, Imaging Institute, and Cardiovascular
Medicine, Heart and Vascular Institute, at Cleveland Clinic, Cleveland,
Over the past decade, computed tomography (CT) has emerged as an
important noninvasive imaging modality for the evaluation of
cardiovascular diseases.1 However, the increasing use of
cardiac CT has raised concern about the long-term health consequences of
medical radiation, particularly stochastic effects, such as cancer, and
the probability of these effects has no clear threshold dose and
increases linearly with the dose.2,3 To reduce radiation dose
without compromising image quality, cardiovascular imagers have
developed and adopted several novel scan acquisition techniques and scan
parameter modifications.4 This review discusses the various
dose-reduction strategies available for cardiovascular CT and how these
strategies can be employed in various clinical protocols.
proper selection of patients is a critical step in reducing overall
radiation burden from CT. Cardiac CT should be performed only when the
benefits of the scan outweigh both the risks of the imparted radiation
and when similar diagnostic information cannot be obtained from a
different imaging modality that does not expose the patient to ionizing
radiation. Appropriateness criteria for cardiovascular CT have been
published by various societies; these criteria take into account pretest
probability of cardiovascular disease, patient characteristics,
procedural risk, and the expected benefit to the patient.1 If
CT is deemed inappropriate when these criteria are applied, tests
without ionizing radiation exposure should be performed instead.
Additionally, CT should not be performed if evidence suggests that
diagnostic images would be difficult to obtain in a particular patient
(eg, CT coronary angiography in a patient with an irregular heart rhythm
or a heavy coronary calcium burden or with an irregular heart rhythm).
the radiation dose can be achieved by manually or automatically
altering several scanning factors. These dose-reduction strategies are
reviewed in this article.
X-ray filters, prepatient collimators, and shields
that are placed beneath the x-ray tube selectively attenuate low-energy
x-rays that do not contribute to image formation, but do add to the
total radiation dose.4 Filtered beams have lower intensity
and higher mean energy, but the maximum energy is not altered. Depending
on patient size, the imager can use small, medium, or large flat and
bow-tie filters; the choice of filter size affects both the acquisition
field-of-view (FOV) and the radiation dose. The smallest filter allowing
the entire region of interest to be within the FOV should be used.
Similarly, cardiovascular imagers can use prepatient z-collimators to reduce radiation reaching the patient.5
Prepatient collimators are positioned close to the x-ray tube, allowing
the width of the collimator to be altered. This limits the x-ray beam
to the section thickness intended at the area of interest in the z-axis
and avoiding unnecessary radiation exposure in adjacent tissues. On a
single-slice CT, a prepatient collimator also determines the slice
Bismuth-impregnated latex shields are another option
for reducing radiation dose; specifically, these shields can reduce
radiation dose to the breast when the shields are located between the
x-ray beam and the breast (ie, in anteroposterior projections of CT, but
not in posteroanterior projections). Although bismuth shielding has
been found to reduce radiation dose to the breast by 29% to 57%,6,7 such
shielding is also associated with degradation of image quality because
of increased pixel attenuation values from beam-hardening effects,
higher levels of image noise, streak artifacts, and internally scattered
radiation. The routine use of bismuth shields is therefore not
recommended for patients undergoing cardiovascular CT.4 When
the shields are used, they should be placed after the topogram is
performed so that increased radiation from anatomic tube current
modulation in response to the shields can be prevented.
scan length is defined as the volume that is irradiated along the
z-axis (cranio-caudal axis) of the patient.
Radiation dose is directly proportional to the scan length; hence, the
scan length should be tailored to the specific clinical indication so
that tissues outside the field of interest are not unnecessarily exposed
to radiation.8 For instance, scan length should involve only
the craniocaudal extent of the heart for calcium scoring and CT
angiography, but must involve the entire thorax for the evaluation of
bypass grafts. Although imagers typically use topogram-based
prespecified scan lengths, the scan should be stopped early if the
anatomy of interest is covered before the planned z end-point is
Several acquisition modes
are available for cardiovascular imaging, each of which can affect the
radiation dose. Here we review the current commercially available
acquisition modes for cardiovascular imaging, including retrospective
and prospective electrocardiogram (ECG) referencing, standard and wide
detector arrays, and low- and high-pitch techniques.
Retrospective ECG-gated low-pitch helical acquisition
retrospective ECG-gated low-pitch helical acquisition technique, the
most commonly used mode for cardiac CT, is associated with the least
susceptibility to cardiac motion artifacts, but also with the highest
dose of radiation. With this method, continuous data and ECG acquisition
occurs simultaneously during gantry rotation and table translation
until the entire scan length is covered; after this, images are
reconstructed, with data retrospectively gated to the ECG signal. If the
full tube current is used throughout the cardiac cycle, images with
similar noise levels can be reconstructed at any phase of the cycle.
Radiation dose associated with this technique can be significantly
reduced with ECG-based tube current modulation.
Prospective ECG-triggered axial acquisition
the prospective ECG-triggered axial mode of acquisition, data are
acquired during gantry rotation while the table remains stationary. Data
acquisition with this technique is limited to a predefined phase of the
cardiac cycle and is prospectively triggered with the ECG signal (QRS
complex), which is simultaneously monitored. After data are acquired
during a single-gantry rotation, the table moves to the next position
and data acquisition is continued until the entire scan length is
covered. A dose reduction of up to 68%, without significant impairment
in image quality, has been associated with this technique versus
low-pitch helical imaging with retrospective ECG gating.9
Functional information cannot be obtained with this imaging technique on
most scanners however, some scanners do, however, allow for data
acquisition of data during 2 or more phases of the cardiac cycle, though
again at the expense of higher radiation dose. With this mode of
acquisition, misalignment artifact may be seen if the position of the
heart varies slightly from cycle to cycle or if there are alterations in
the length of the cardiac cycle; the probability of this artifact
increases with the number of table movements required. On very
wide-array scanners (eg, 320 detectors x 0.5-mm detector row width), z
coverage of up to 16 cm can be achieved; additionally, the heart can be
imaged within 1 heartbeat and without table movement, thus eliminating
the likelihood of misalignment artifact.
A regular and low heart
rate is necessary in patients undergoing CT scanning with this mode, as
this technique depends on an accurate estimation of the upcoming R-R
interval duration.4 To this end, some scanners have an
automatic arrhythmia-rejection technique that postpones image
acquisition until the patient’s heart rate is stable. The data
acquisition window may also be widened in patients with high or
irregular heart rates to enable minor retrospective adjustments of the
reconstruction window; this technique, however, is associated with a
higher radiation dose.
Prospective ECG-triggered high-pitch helical acquisition
highest pitch possible for gapless data acquisition is 1.5 with a
conventional single-source system. Beyond this pitch level, gaps will
occur in image data, which will result in artifacts. Second-generation
dual-source scanners, however, can achieve pitch values up to 3.4 by
interleaving data from 2 detector arrays that are separated by 90°. This
technique reduces the amount of redundant data and allows imagers to
acquire prospective ECG-triggered helical data with lower radiation
doses and shorter acquisition times. In patients with low and stable
heart rates, this mode can acquire images of the entire heart within the
diastolic window of 1 R-R interval and with a radiation dose as low as 1
Non-ECG-gated helical acquisition
Helical data acquisition without ECG referencing is typically used
for indications in which cardiac motion is not an issue (eg, aortic
studies before and after descending thoracic endovascular stent
placement, or evaluation of the left atrium and pulmonary veins). In
such cases, this acquisition mode is associated with decreased radiation
dose and reduced motion artifacts when compared with ECG-gated helical
is defined as the number of electrons accelerated across the x-ray tube
per unit of time; tube current-time product is defined as the product of
tube current and time. Both factors indicate x-ray tube output.
Radiation dose is directly proportional to the tube current, whereas
image noise is inversely proportional to the square root of the tube
current; hence, lowering the tube current proportionally lowers the
radiation dose, but increases image noise. For cardiovascular CT, the
lowest possible tube current producing images with acceptable noise
levels should be used.
To reduce total radiation exposure, imagers
can modulate tube current during various phases of the cardiac cycle
(ECG-based modulation) or according to body thickness (anatomic-based
ECG-based tube current modulation
a retrospective ECG-gated low-pitch helical acquisition, images are
needed throughout the cardiac cycle for functional evaluation of
ventricles or valves; for most remaining indications, images are needed
from only a single phase of the cardiac cycle. For imaging of coronary
arteries and the aorta, the mid- to end-diastolic phase (75% of R-R
interval) typically is used when the heart rate is low, whereas the
iso-volumic phase of relaxation (40% of R-R interval) typically is used
when the heart rate is high. Pulmonary veins are also imaged in systole
as they have maximum caliber during this phase. In ECG-based tube
current modulation, full tube current is typically applied during a
single phase of the cardiac cycle and the tube current is reduced
(typically 20%-25% of full tube current) or completely switched off for
the remaining phases of the cardiac cycle. With this technique,
radiation doses can be reduced by up to 50%, depending on the patient’s
heart rate,13 the minimum tube current value,14 and the duration of the maximum tube current phase.14
Images with lower tube current have higher noise levels, but such
images can still provide information about functional parameters.
Additionally, imagers can improve the quality of these images can be
improved by increasing the reconstruction slice thickness.
this method of modulating tube current is based on averaging previous
R-R interval changes, this technique requires a regular and preferably
low heart rate. An irregular heart rhythm may lead to an unintended
decrease in tube current during a portion of the cardiac cycle that is
needed for diagnostic evaluation. In patients with irregular heart
rhythms, modulation of tube current may be suspended automatically when
beat-to-beat variation exceeds a predefined threshold. Alternatively,
the window of maximal tube current may be widened. However, both of
these methods will increase the overall radiation dose.
Anatomy-based tube current modulation
this method of modulation, the tube current is adjusted according to
patient size or anatomic shape. Because the tube current necessary to
effectively scan patients is directly proportional to patient size,
imagers can use a lower tube current and thus lower radiation dose in
thin patients, whereas they must use a higher tube current in obese
patients. Imagers can assess patient size through visual inspection,
measurement of body weight or body mass index (BMI), measurement, noise
measurement from a cross-sectional prescan, cross-sectional measurement
from a topogram, or noise measurement from a cross-sectional prescan.
Online tube current modulation during acquisition can also be performed
based on the thickness of the body part along the path of the x-ray (as
estimated by the topogram): imagers can reduce tube current can be
reduced for x-ray projections in which the body part is thin. This
online tube current modulation can reduce the radiation dose by 20%
without increasing noise on thoracic CT images.15 For
cardiovascular CT scans, this technique is used to determine the current
that is necessary to achieve a desired noise level based on the tissue
thickness in from scout images. Once the tube current is selected,
online tube current modulation during the scan is based on the ECG
Tube potential, defined
as the electrical potential across the x-ray tube, determines both the
energy and the intensity of the x-ray beam. The radiation dose is
proportional to the square of the tube voltage: reducing the tube
potential from 120 to 100 kVp can lead to a dose reduction of up to 53%,9 as low-potential x-rays have lower energy and thus lower penetration.16 Because noise is inversely proportional to the tube potential, image noise increases (by up to 26%) with lower tube potential.9
In addition, signal, signal-to-noise ratio, and contrast-to-noise ratio
are increased, as there is a higher probability of photoelectric
interaction than Compton interaction at lower energy levels.9
A lower dose of iodinated contrast material can therefore be used to
achieve the same blood signal, and the attenuation threshold values
should be set at higher levels when automatic bolus tracking is used.
Image noise increases with body weight because of scattering and
radiation absorption, so a tube potential of 100 kV should be used for
nonobese patients (weight ≤90 kg; BMI ≤30 kg/m2).16 In obese patients (weight >90 kg; BMI >30 kg/m2),
120 to 140 kV can be used. In children and very thin patients, a tube
potential of 80 kV may be used. In protocols that require a higher
contrast-to-noise ratio (eg, myocardial perfusion imaging), a tube
potential of 80 kV should be used.
defined as the ratio of table movement to the detector-width per gantry
rotation of a helical acquisition; pitch values <1 indicate
acquisitions, whereas pitch values
>1 are associated with gaps between
successive acquisitions. The radiation dose is inversely proportional to
the pitch value. Multidetector row cardiac CT scans are typically
performed with a pitch value of 0.2 to ensure adequate data sampling and
to avoid gaps in image volume. For single-source scans, data from 2
consecutive cardiac cycles are typically necessary to reconstruct each
image, and a lower pitch value insures adequate data sampling. Higher
pitch values (up to 3.4) and thus lower radiation doses can be achieved
with the newest dual-source high-pitch scanners. Pitch does not affect
noise levels on multidetector row CT scans, but does affect the z-axis
spatial resolution, depending on the type of interpolation
reconstruction algorithm that is used.4
back-projection is the most commonly used algorithm in the
reconstruction of CT images from projection data. Recently, a
statistical reconstruction method, iterative reconstruction, also became
available for CT imaging. When iterative reconstruction algorithms are
used, projection data are predicted based on an assumption about the
initial attenuation coefficients of all voxels. These predicted data are
compared to measured projection data, and the voxel attenuation values
are modified until an acceptable level of error between the predicted
and measured data is achieved. Iterative reconstruction can be used for
image data, projection data, or both. This technique is associated with
lower noise levels than filtered back-projection with similar radiation
doses; as a result, lower tube potential and lower tube current levels
can be used with iterative reconstruction, reducing the radiation dose
needed for this technique (Figure 1).4,17
a scan is performed, several types of postprocessing filters can be
applied to the image to further reduce the dose. Most of these filters
reduce the noise in the postprocessed image, which is beneficial in
images acquired with low-dose protocols. Adaptive nonlinear filters are
based on mathematical algorithms that resemble the human vision system.
These algorithms analyze the entire image and remove random noise,
preserving contrast and edge sharpness based on the local orientation in
each pixel. The filtering process is not homogeneous throughout the
image, but varies with each pixel. The reduction of noise with this
technique enables acquisition of images with a lower dose (reduction of
up to 80%) without loss of image quality.18
Reconstructed slice thickness
slice thickness does not directly affect x-ray exposure; however, this
factor does determine how many photons contribute to the final image in
the filtered back-projection technique, with noise being inversely
proportional to the square root of reconstructed slice thickness. For
high special resolution, thinner slices must be used for high spatial
resolution, but these images will also have higher noise levels, which
may require the use of higher tube potential or current. Although
thicker slices have lower spatial resolution, they also have lower noise
levels, allowing for the use of a lower tube
potential or current (Figure 2). Imagers should use the maximum
reconstructed slice thickness providing the required information for a
clinical indication should be used, as the overall tube output can then
be reduced and thus radiation dose minimized.
FOV is generally smaller than the gantry size and the area of
interest should be located within the
acquisition FOV. Reconstructed FOV, which is is the area over which the
image is reconstructed and is the same as or less than the acquisition
Reconstructed FOV does not affect
Practical application of dose-reduction strategies
goal of cardiovascular imagers should be to use a combination of the
above-mentioned dose-reduction techniques to limit radiation dose to the
lowest possible amount without compromising image quality. The CT
protocol should always be customized to the specific clinical question
(Table 1). For certain indications, it is possible to use protocols with
lower spatial or temporal resolution or with higher noise without
significantly compromising the diagnostic quality of the images. For
example, evaluating coronary arteries or cardiac valves requires high
spatial and temporal resolution; however, calcium scoring and evaluation
of pulmonary veins, coronary artery origin, and myocardium require
neither high spatial nor temporal resolution. If fine detail is not
absolutely essential, a lower tube current can be applied and thicker
slices can be reconstructed. However, imagers must exercise caution in
applying these dose reductions, as reducing the dose too much could
result in poor-quality, nondiagnostic results, which will necessitate a
repeat study and a net increase in radiation dose.
protocol should also be tailored to patient characteristics,
particularly heart rate, heart rhythm, and body habitus. As described
earlier, ECG-based techniques, such as tube current modulation and
prospective ECG triggering, rely on a wide diastolic window, which is
possible only in patients with low and regular heart rates (<60-65
beats/min). Beta-blockers are therefore sometimes used to reduce and
stabilize heart rate in patients undergoing CT. Heavier patients tend to
require higher x-ray exposure because of higher attenuation and scatter
of x-rays in these patients; even smaller patients with more upper body
weight distribution require higher x-ray exposure.4
dose from cardiovascular CT should be limited to a level as low as
reasonably achievable without compromising diagnostic image quality.
Additionally, CT should be performed only when the procedure is
clinically indicated and when the benefit to the patient outweighs the
risks of stochastic effects of low-dose radiation and that of forgoing a
necessary examination. The CT protocol should be customized to the
individual clinical scenario and to the patient characteristics. In
patients with low heart rate, regular heart rhythm, and low body weight,
low-dose CT protocols should be the default in patients with a low
heart rate, regular heart rhythm, and low body weight.
authors wish to thank Ms. Megan Griffiths, a scientific writer for the
Imaging Institute, Cleveland Clinic, for her editorial assistance.
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