Morphologic imaging of the coronary arteries was ﬁrst performed on electron-beam computed tomography (EBCT) scanners in 1984.1,2 Advantages included a temporal resolution of 50 to 100 msec.3 Poor resolution in the Z-axis, limited volume coverage, and low power limited its use for coronary CT angiography (CTA).
is currently a third-year Radiology Resident at the University of
California-San Francisco, San Francisco, CA. He received his
medical degree from The Northwestern University Feinberg School
of Medicine in Chicago, IL, and completed his Internship at The
Queen's Medical Center, Honolulu, HI. He plans to pursue a
Cardiovascular Imaging Fellowship followed by an Interventional
Noninvasive, accurate morphologic imaging of the coronary
arteries is the Holy Grail of emergency medicine, cardiology, and
radiology. The coronary arteries are small vessels requiring
submillimeter isotropic resolution to resolve distal branches.
High temporal resolution is a priori to freeze cardiac motion.
Dose-reducing algorithms and low-dose protocols have reduced the
coronary CT angiography (CTA) dose to one that is comparable to
that of catheter angiography. This article will discuss coronary
CTA technique, components of image quality, radiation dosimetry,
Morphologic imaging of the coronary arteries was first performed
on electron-beam computed tomography (EBCT) scanners in 1984.
Advantages included a temporal resolution of 50 to 100 msec.
Poor resolution in the Z-axis, limited volume coverage, and low
power limited its use for coronary CT angiography (CTA).
The introduction of helical multidetector CT (MDCT) in the 1990s
revolutionized imaging of the coronary arteries. Improvements in
volume coverage were achieved through simultaneous acquisition of 4
slices per gantry rotation, reducing acquisition time to 40
seconds. An effective temporal resolution of 250 msec limited image
quality. Multisegment reconstruction algorithms were developed to
overcome this limitation.
Despite improvements in axial collimation to 1 mm, partial volume
artifacts remained problematic.
Sixteen-slice MDCT scanners with simultaneous acquisition of 12
to 16 slices further reduced acquisition time to 15 to 20 seconds.
Gantry rotation increased to 375 msec. Spatial resolution improved
with axial collimation to 0.75 mm. Investigators reported fewer
nondiagnostic segments with improvement in accuracy compared with
Poor temporal resolution necessitated pharmacologic heart rate
reduction and the use of multisegment reconstruction algorithms.
Partial volume artifacts remained challenging in patients with
stents and high Agatson scores.
Sixty-four slice MDCT scanners were introduced in 2004. The
latest-generation 64-slice scanner (SOMATOM Sensation 64, Siemens
Medical Solutions, Forchheim, Germany) enables simultaneous
acquisition of 64 overlapping slices through advanced z-sampling
techniques, improving temporal resolution to isotropic voxels of
Greater volume coverage per gantry rotation with an acquisition
time of 5 to 14 seconds is within a comfortable breath-hold for all
but the most dyspneic patients. An effective temporal resolution of
165 msec is achieved with partial scan algorithms. Many
investigators recommend routine pharmacologic heart-rate- reducing
The recent introduction of a dual-source MDCT scanner (SOMATOM
Definition, Siemens Medical Solutions, Forchheim, Germany)
presented a solution to the limited temporal resolution achievable
with single-source systems. Combining multiple X-ray sources to
improve temporal resolution at CT was initially described in 1979
by Robb and Ritman.
The incorporation of a second X-ray source and detector array into
the gantry offset 90˚ enables the acquisition of half-scan data in
a quarter gantry rotation. An effective temporal resolution of 83
msec is achieved without employing multisegment reconstruction
algorithms. Improved temporal resolution enables
heart-rate-independent imaging and clear visualization of
A 256-slice scanner has been introduced by Toshiba Medical
Systems (Ohtawara, Japan). This scanner employs a cone-beam design
with a Z-axis coverage of 12.8 cm per rotation, enabling
whole-heart coverage without table movement.
Stationary volume acquisition eliminates interpolation artifacts.
Spatial resolution is improved with 0.5-mm detector elements.
Gantry rotation speeds on the alpha prototype are as fast as 350
The relatively high radiation dose associated with the cone-beam
X-ray source is a limitation of this scanner design.
Investigators have proposed prospectively gated acquisitions and
improved detector design as solutions to this problem.
Several aspects of coronary CTA technique are pertinent to this
discussion of image quality and radiation dose.
Prospective triggering initiates CT scanning at a specified
point in the cardiac cycle (Figure 1A). This is based on the length
of a recent RR interval on the electrocardiogram (ECG) (ie, 60% of
the R-R interval or 500 msec after the R wave). Data acquisition is
continued for a chosen length of time (ie, time corresponding to
20% of the R-R interval). Lower radiation dose is an advantage of
prospective triggering. Disadvantages include susceptibility to
heart rate variability, inability to edit data to compensate for
arrhythmias, and limited cardiac phases for data reconstruction.
Functional parameters can be determined by including end systole
and end diastole in the triggered acquisitions.
Retrospectively gated acquisitions record the ECG simultaneously
with CT scanning. Imaging data are acquired throughout the entire
cardiac cycle (Figure 1B). Volume data can be reconstructed at any
point in systole or diastole. An advantage of this technique is the
ability to edit the ECG tracing to exclude data acquired following
ectopic beats. Cardiac functional parameters such as ejection
fraction, wall motion, and myocardial thickening can also be
Multisegmentation refers to acquiring cardiac cycle data at a
point along the Z-axis from ≥2 adjacent heartbeats (Figure 2).
Although an effective solution to limited temporal resolution, this
technique is particularly susceptible to beat-to-beat variability
and ectopic beats. Pitch is necessarily reduced, increasing scan
time and radiation dose.
CT kernels are filters designed to enhance particular spatial
frequencies. Sharp filters are applied when high-density objects
such as stents, bones, and dense vascular calcifications are
imaged. Sharp filters enhance edge definition at high-contrast
interfaces at the expense of increased noise and reduced
signal-to-noise ratio (SNR). Coronary CTA is usually performed with
a medium-sharp filter, which balances edge definition with
acceptable noise (Figure 3). In patients with high Agatson scores
and stents, sharp filters result in subjective improvements in
Spatial resolution is the most fundamental aspect of image
quality and is described in voxel dimensions. CT data are
reconstructed on a 512 × 512 pixel matrix. Spatial resolution is
dependent on the field of view, slice collimation, and detector
element size. Utilizing clinical protocols and a sharp convolution
kernel, a maximum in-plane resolution of 0.5 mm × 0.5 mm and
through-plane resolution of 0.4 mm can be achieved.
The SNR is a practical measure of image quality associated with
spatial resolution. Reducing the reconstructed field of view or
utilizing thinner collimation while keeping kVp and mAs constant
results in fewer X-rays traversing each voxel relative to a
constant noise background. Consequently, SNR decreases, and the
image appears grainy. The tube current must be increased in order
to maintain an acceptable SNR. Similarly, tube current must be
in-creased to maintain an adequate SNR with smaller detector
elements. Gains in SNR are not directly proportional to tube
current, however. A portion of X-rays scattered within the patient
are deflected out of a straight path and are absorbed by peripheral
detector elements, thus increasing noise (Figure 4). Doubling the
tube current results in an increase in SNR of 1.41.
These considerations have practical implications regarding
imaging the coronary arteries. Most investigators administer
vasodilator medications to increase vessel size.
Current scanner spatial resolution is adequate for the evaluation
of the epicardial coronary arteries and major branches. High
spatial resolution reduces beam-hardening artifacts from stents and
calcified vessels and improves accurate assessment of stenoses
Techniques to reduce beam-hardening artifacts are currently under
investigation (Sven Prevrhal, University of California, San
Francisco, CA, personal communication). Further improvement in
spatial resolution to 0.2 to 0.25 mm is necessary for accurate
imaging of in-stent restenosis and plaque characterization.
Future gains in spatial resolution can be achieved with smaller
detector elements. Flat-panel detectors have a high spatial
resolution of up to 0.1 mm. A prototype CT scanner utilizing
flat-panel detectors has been developed.
This scanner is capable of a spatial resolution of 0.25 mm.
Although this is an exciting development, to achieve acceptable
radiation doses and temporal resolution, further improvements in
flat-panel detector efficiency and flat-panel data read-out are
Slice acquisition time must be less than an isovolumetric
portion of the cardiac cycle to effectively freeze cardiac motion.
Using partial scan algorithms, nominal slice acquisition time can
be reduced to 165 msec in scanners with 330-msec gantry rotation
time. At a heart rate of 72 beats per minute (bpm), this nominal
acquisition time corresponds to approximately 20% of the R-R
Pharmacologic heart rate control is routine at single X-ray
source coronary CTA. Target heart rates of <70 and <65 bpm
are reported in the literature.
Reduction of temporal resolution to <100 msec is necessary for
heart-rate-independent imaging across most resting heart rates.
Early studies with dual-source technology demonstrated excellent
image quality across a wide range of heart rates, obviating the
need for pharmacologic heart rate reduction.
Multisegmentation reduces the effective temporal resolution by
acquiring slice-specific, isophasic data from 2 to 4 consecutive
heart beats. Effective temporal resolution can be reduced to 43
Improvements in temporal resolution with this technique are related
to heart rate; 83 msec temporal resolution is achievable only at
heart rates of 68 and 82 bpm
(Figure 6). These "sweet spots" occur when there is optimal
desynchronization between the gantry rotation and heart rate.
Effective temporal resolution with dual-segment reconstruction at
other heart rates averages 124 msec. A recent study investigated
the accuracy of single- versus dual-segment reconstruction at
64slice coronary CTA.
Although dual-segment reconstruction resulted in improved image
quality, there was no difference in diagnostic accuracy.
Increases in gantry rotation speeds have occurred with each
generation of MDCT scanners, improving temporal resolution. Higher
gantry rotation rates would generate significant centrifugal forces
and necessitate increases in processing power to accommodate the
high rate of data transmission.
The dual-source scanner design provides an efficient solution to
improve temporal resolution. A temporal resolution of 83 msec is
achieved with single-segment reconstruction; an average temporal
resolution of 60 msec is achieved with dual-segment reconstruction
(Figure 6). Several authors reported motion-free coronary CTA
without pharmacologic agents using single-segment reconstruction.
Flohr and colleagues
discourage the routine use of dual-segment algorithms, citing
Retrospectively gated acquisitions enable the user to specify
the cardiac phase for reconstruction. Although a single-phasic
reconstruction may not be motion-free, an analysis of multiple
reconstructions may yield a complete evaluation of the coronary
tree. Reconstructions at end diastole and end systole are
preferable for low and high heart rates, respectively.
Contrast resolution refers to the ability of an imaging system
to resolve 2 objects of similar size but different attenuation.
This is simply demonstrated by concentric solid squares. As the
shading pattern of the center square closely approximates that of
the outer square, they blend together and cannot be distinguished,
although slight differences in shading density do exist (Figure 7).
Contrast resolution is important in differentiating fatty,
fibrofatty, and calcific plaque as well as calcifications from
luminal contrast. An important limitation of coronary CTA contrast
resolution is the small size of luminal plaque components. Small
structures require greater differences in attenuation for
differentiation at coronary CT. Ap-propriate selection of window
and level settings is necessary to observe the minimum contrast
during image review.
Dual-source CT scanners may be able to utilize differential
energy spectra subtraction to improve contrast resolution.
Applications at coronary CTA are the focus of ongoing
Patient-specific factors impact image quality. Image noise is
directly related to the thickness of tissue that the X-ray photon
traverses. In patients with high body mass indices, increasing the
tube current and slice collimation is necessary to maintain SNR.
Similarly, in women with large breasts, positioning the breasts out
of the field of the X-ray beam reduces beam attenuation and
maintains SNR. Scan times with state-of-the-art 64-slice scanners
average 5 to 9 seconds for coronary CTA; whole thoracic gated
acquisitions can take 20 seconds.
Patient preparation for the length of the breath-hold as well as
the sensation of warmth from intravenous (IV) contrast injection
improves cooperation. Respiratory motion causes interpolation and
misregistration artifacts, which can mimic stenoses.
If patients cannot complete the breath-hold, they should be
instructed to slowly exhale. Shortening the scan time by increasing
the pitch or reducing the scan volume reduces respiratory motion
artifacts. Finally, a regular heart rhythm is necessary. The ECG
tracing can be edited in retrospectively gated acquisitions to
reduce artifacts from ectopic beats. Frequent ectopy or irregular
atrial fibrillation rhythms may result in nondiagnostic studies
(Figure 8). Tachycardic patients who are not candidates for
pharmacologic heart rate reduction can be imaged with dual-source
Other technical factors
Optimal opacification of the coronary arteries is critical to
CTA. A large-bore antecubital IV line is the preferred access site.
Review of prior imaging is prudent to exclude central venous
stenosis or occlusion. A test bolus is preferred by most authors to
determine transit time to the aortic root.
Contrast media with an iodine concentration >350 mg/mL is
preferred. The volume of contrast is equal to the scan time
multiplied by the injection rate. Contrast is injected, followed by
a saline flush at 5 to 6 mL/sec. This dual-injection technique
clears contrast from the central veins and right heart, preventing
beam-hardening artifacts from obscuring the mid portion of the
right coronary artery.
Radiation dose considerations
All imaging studies utilizing ionizing radiation should adhere
to the "as low as reasonably achievable" (ALARA) principle.
Reported doses at coronary CTA vary significantly because of
inconsistent application of dose-reducing methodology and varied
coronary CTA protocols (Table 1). Multiple measures of radiation
dose have been reported in the literature, adding to the confusion
Radiation dose at coronary CTA is dependent on multiple
technical factors. Dose is directly proportional to the square of
the kVp and varies linearly with mAs.
All cardiac CTA protocols are acquired helically, where pitch
setting determines the degree of overscanning. Pitch is defined by
the length of table translation per gantry rotation divided by the
total detector width. Pitch should be maximized to reduce the
The scanning volume should be confined to the cardiac anatomy with
the minimal necessary z-overscanning for slice interpolation at the
edges of the volume. Exposure time should be minimized for all
Reducing dose at coronary CTA
It is estimated that 20% of the radiation dose is used to
reconstruct a single phase. The remainder of the dose enables
functional evaluation, unless additional phasic reconstructions are
required for analysis.
Multiple strategies have been developed to reduce radiation dose.
As discussed under spatial resolution above, with smaller detector
elements, increased tube current is necessary to maintain SNR.
Without the advent of dose-saving strategies, a significant
increase in exposure would have been realized with higher slice
However, a steady decline in radiation exposure has occurred from
4-slice to 64-slice MDCT because of the implementation of dose
reduction strategies (Table 1).
Electrocardiographic tube current modulation was initially
described in 1999.
This strategy is used with retrospectively gated acquisitions,
where an 80% to 90% reduction in the nominal tube current is
applied during systole and early diastole (Figure 9). The authors
recommend a 400-msec interval of nominal tube current applied
during diastole or during late systole and diastole. Phases
reconstructed within the pulsing interval have low signal-to-noise
ratio, which is adequate for measurements of functional parameters
but may be insufficient for the evaluation of coronary stenoses
(Figure 10). Hence, appropriate selection of the pulsing interval
is mandatory. Dose savings from 37% to 56% are reported with this
Anatomic tube current modulation
Anatomic tube current modulation is a recently developed
dose-saving technique. Several approaches to estimate or measure
patient X-ray attenuation have been described in the literature.
Studies that compared angular tube current modulation based on
anthropomorphic phantoms with modulation from angular
projection-specific attenuation reported greater dose savings with
in-line modulation (Figure 11). This results in an average dose
savings of 44% in children and 53% in adults.
The addition of automated exposure control to 16-slice coronary CTA
performed with ECG modulation resulted in a further 43% reduction
in radiation dose.
Prospectively triggered acquisitions impart less radiation dose
than retrospective acquisitions (see Figure 1). Radiation in a
prospectively triggered acquisition occurs only during a specified
portion of the cardiac cycle. This reduces exposure time and the
radiation dose compared with retrospectively gated acquisitions.
Dose reductions of 70% to 80% have been reported with this
The use of a prospectively triggered acquisition has been proposed
as a means of reducing dose at 256-slice MDCT, reducing the dose
from 14.2 to 2.4 mSv.
Low kVp and mAs protocols have been promulgated in smaller
patients and children for coronary CTA and extracoronary CT.
An approach pioneered by Paul and Abada
measures image noise on a low-dose CT scan with 120 kVp and 20 mAs
at the inferior margin of the heart. Scan parameters are
subsequently adjusted based on control tables. A recent study by
Hausleiter and colleagues
compared doses at 16- and 64-slice coronary CTA using 100 kVp and
120 kVp protocols with ECG modulation. They reported 53% and 64%
dose reduction at 16- and 64-slice coronary MDCT, respectively,
without a difference in evaluable coronary segments. A recent study
by Delhaye et al
evaluated the feasibility of coronary segment analysis on 64-slice
MDCT gated thoracic imaging with a low-dose protocol (120 kVp and
200 mAs). Although the study was not tailored for coronary imaging,
75% of coronary segments were evaluable. The average dose for
low-dose gated thoracic evaluation was 5 mSv. The accuracy of
low-dose protocols is the subject of ongoing research.
X-rays are classified as carcinogens by the National Institute
of Environmental Health Sciences, the World Health Organization,
and the Centers for Disease Control.
The effects of ionizing radiation are classified as deterministic
or stochastic (Table 3). Patient doses at coronary CTA are well
below the thresholds for deterministic effects. Stochastic effects
are theoretically possible with the dose from a single coronary CTA
study. Limited data is available to determine the risk of
carcinogenesis from effective doses <50 mSv. Above this level,
population data from the Hiroshima nuclear fallout found a
statistically significant increased cancer incidence.
Wide variation in individual radiation exposure in this population
limits extrapolation to patients exposed to doses common in
Additional studied populations include
U.S. X-ray technologists, Nordic airline pilots, populations
living in high background exposure regions of China, nuclear
radiation workers, patients exposed to Thorotrast contrast, radium
dial painters, and postpartum mastitis and ankylosing spondylitis
Radiation exposure in these cohorts is similar to doses at coronary
CTA. Study results are inconsistent, although a statistically
significant incidence or positive trend for thyroid cancer,
leukemia, melanoma, and nonmelanoma skin cancer, and colorectal
cancer was noted in multiple studies. Data on breast carcinoma
induction is limited, as available study cohorts were predominantly
male. Based on available data, the lifetime risk of fatal cancer is
estimated at 5% to 7.9% per sievert of exposure.
This corresponds 1/1300 to 1/2000 risk of fatal cancer following an
exposure of 10 mSv.
The lifetime attributable risk (LAR) of cancer incidence from
coronary CTA was recently analyzed by Einstein and colleagues.
Monte Carlo simulations and data from the National Academies'
Biological Effects of Ionizing Radiation 7th report formed the
basis for their work. The authors highlighted stochastic risk
associated with coronary CTA in younger patients, particularly
women. The LAR for a 20-year-old woman undergoing 35% ECG modulated
coronary CTA was estimated at 0.46% or 1 in 219. This compared to
an LAR of 0.1% or 1 in 1000 in a 20-year-old man. The LAR declined
progressively with age. A 20-year-old man had 5 times the relative
risk (RR) of an 80-year-old man. However, a 20-year-old woman had
23 times the RR of an 80-year-old man. Gender-specific differences
in LAR were attributed to increased lung parenchyma and breast
doses in women.
Ionizing radiation is a relatively weak carcinogen.
Differentiating ionizing-radiation-induced cancers from sporadic
and other carcinogen-induced cancers is challenging, as
approximately 40% of the U.S. population will develop cancer.
Investigators have turned to population-based data to determine the
scope of medical imaging exposure. In 1980, the collective dose
from medical imaging was 0.55 mSv per person. This quantity has
increased approximately 6-fold to 3.3 mSv per person in 2006.
The U.S. population dose from medical imaging in 2006 totaled
450,000 person-Sv. This compares with 600,000 person-Sv total
exposure from the Chernobyl nuclear accident. The total annual U.S.
population exposure is comparable to that in high background
radiation regions of China.
Gonzalez and colleagues
estimate a 0.6% to 1.8% lifetime cancer risk from radiation
exposure at medical imaging in developed countries.
The European Commission has developed guidelines for exposure
limits for CT.
Currently, no federal guidelines exist for CT dosimetry. The
American College of Radiology (ACR) recently published a white
paper on radiation dose,
which makes recommendations on imaging utilization but does not
specify CT dosimetry guidelines.
Relative effective dose at coronary CTA
Effective doses at coronary CTA, positron emission tomography,
diagnostic angiography, and nuclear imaging are listed in Table 4.
Effective doses at coronary CTA vary significantly based on imaging
protocol and patient factors, but range from 4.95 to 40 mSv. Using
dose reduction techniques, radiation doses at coronary CTA are
similar to diagnostic catheter angiography and technetium-99m
nuclear perfusion studies.
Radiation considerations in special populations
When imaging children, women, and patients with low body mass
indices, special care should be taken to implement dose-reducing
measures at coronary CTA. Pediatric patients are more sensitive to
radiation than older patients.
Manufacturers have developed pediatric protocols which utilize
low-dose algorithms. Investigators have validated low-dose
protocols in whole thoracic and focused coronary CT imag-
Paul and Abada
noted excellent image quality at coronary CTA utilizing 80 kVp
protocols as long as mAs was adapted to patient weight. In women
undergoing coronary CTA, the breasts receive the highest effective
dose. When possible, breasts should be positioned out of the beam.
An alternative approach is the use of bismuth breast shields.
Innovations in scanner design have yielded consistent
improvements in image quality since coronary CTA was first
described with EBCT. Novel dose-reduction strategies realized
reduced effective doses with later-generation CT scanners. Rapid,
accurate, noninvasive coronary imaging is now possible at CTA with
radiation doses comparable to a diagnostic catheter angiogram.