Noninvasive echocardiographic-gated multidetector computed tomography (MDCT) allows for visualization and characterization of native coronary arteries, coronary artery anomalies, quantiﬁcation of atherosclerosis and luminal vessel diameter as well as the evaluation of bypass graft patency. The growing acceptance of MDCT as an alternative or adjunct to conventional cardiac evaluations has fostered the rapidly emerging technical advances in the ﬁeld.
is currently a Third-Year Cardiology Fellow at the University of
Iowa Hospitals and Clinics in Iowa City, Iowa. She plans to
pursue an advanced cardiac imaging fellowship in coronary CT
angiography (CTA), echocardiography, cardiac MRI, and cardiac
nuclear medicine at the University of Iowa. Dr. Staffey graduated
from the University of Illinois College of Medicine, Rockford, IL
in 2001. She completed her internship and residency training at
the Mayo Clinic in Rochester, MN in 2004.
Dr. van Beek
is a Professor in the Division of Diagnostic Radiology,
Department of Radiology, University of Iowa, Iowa City, IA.
is an Associate Clinical Professor of Cardiology, Department of
Internal Medicine, University of Iowa, Iowa City, IA.
Multidetector computed tomography (MDCT) allows for
evaluation of native coronary arteries, coronary artery
anomalies, bypass graft patency and quantification of
atherosclerosis. Multidetector CT provides a safe, accurate,
noninvasive alternative to invasive coronary angiograms in a
select patient population. Successful CTA acquisition involves
various components that this article will focus on, including
adequate patient preparation, utilization of beta-blockers for
heart rate control, understanding the principles of scan
acquisition, and optimization of radiation dose. The current
limitations of CTA and the potential role of dual-source CT
(DSCT) to resolve many of the current limitations are also
Noninvasive echocardiographic-gated multidetector computed
tomography (MDCT) allows for visualization and characterization of
native coronary arteries, coronary artery anomalies, quantification
of atherosclerosis and luminal vessel diameter as well as the
evaluation of bypass graft patency. The growing acceptance of MDCT
as an alternative or adjunct to conventional cardiac evaluations
has fostered the rapidly emerging technical advances in the
Proper patient selection and adequate patient preparation are
vital to the success of the procedure. Radiation exposure is a
significant concern with CT angiography (CTA), and new scanning
protocols are being evaluated for radiation reduction while
maintaining diagnostic image quality. Conventional invasive
coronary angiography remains the gold standard for the detection of
coronary artery stenosis, in-stent restenosis, and the evaluation
of bypass graft patency. The risks of adverse events, cost, and
potential complications of the procedure have led to an intensive
search for a noninvasive alternative. The growing acceptance of
MDCT as an alternative or adjunct to conventional cardiac
evaluation has fostered the rapidly emerging technical advances in
The objectives of this article are to provide current
information from the literature on some of the fundamental aspects
of CTA, including the following: adequate patient preparation,
utilization of beta-blockers for heart rate control, principles of
scan acquisition, optimization of radiation dose, current
limitations of CTA, and potential of dual-source CT (DSCT) to
resolve many of the current limitations of CTA.
The technology is rapidly advancing, and with increasing
experience the clinical acceptance of MDCT is growing.
Multidetector CT will provide a safe, accurate, noninvasive imaging
alternative to invasive coronary angiograms.
Patient selection and preparation
Proper patient selection and adequate patient preparation are
vital to the successful performance of coronary MDCT. First,
patients must be screened for potential risk of iodinated contrast,
including allergic reactions and renal insufficiency. Subsequently,
upon arrival in the CT suite, patients require heart rate and blood
pressure monitoring and need to receive a detailed explanation of
the procedure. It is important that patients receive clear
instructions on breath-holding and are made aware of the reasons
for lying absolutely still during the scanning procedure. Relative
contraindications for CTA include patients who are unable to lie
still, who are unable to hold their breath for up to 15 seconds, or
who have arrhythmias, including atrial fibrillation or frequent
premature atrial or ventricular contractions.
Heart rate is a modifiable variable that has a major impact on
the quality of the scan. It has been shown that lower heart rates
(<65 bpm) produce better-quality scans with improved
visualization of all artery segments with more accurate stenosis
Lower heart rates provide a longer motion-free time during
Mid-to-end diastole is the portion of the cardiac cycle when the
heart has the least amount of motion. Lower heart rates optimize
scan acquisition by creating a longer R-R interval-thus, a longer
Optimal heart rate control can be achieved with oral and/or
intravenous beta-blockers prior to the scan. One study reported
that 20% of patients had heart rates that were above target level
in spite of administration of beta-blockers.
Thus, most studies have routinely excluded patients with heart
rates >65 bpm and patients with irregular heart rhythm.
Most centers administer 50 to 100 mg oral metoprolol to patients
with heart rates >65 bpm approximately 1 hour prior to the scan,
followed by additional intravenous metoprolol if required.
Knowledge of relative and absolute contraindications for
beta-blocker use must be understood and practiced to allow for safe
The basic principles and sequence of scan acquisitions are quite
similar among various scanners. The sequential steps in performing
CTA include obtaining planar scout images, followed by a
noncontrast electrocardiographic (ECG)-gated CT for calcium
scoring. Most centers then obtain a contrast scan with a test bolus
of contrast to assess the circulation time of contrast (Figure 1).
After a detailed explanation of the study, including breath-hold
instructions, the contrast CTA is performed. It must be emphasized
that optimal heart rate control (<65 bpm) and a proper
breath-hold are integral to the success of the scan.
Spatial resolution is defined as the smallest possible
resolution of 2 line pairs that allows sharp and clear delineation
of the coronary arteries. Temporal resolution is defined as the
speed with which images can be obtained to allow for fast coverage
of the cardiac structures within the least number of cardiac
cycles. Temporal resolution is determined by the pitch (table
advancement speed during a single 360˚ X-ray tube rotation), gantry
rotation time, and the patient's heart rate. Multidetector CT is
highly dependent on increased temporal resolution to minimize scan
time and to reduce coronary motion artifacts. There are current
technologic limitations on the pitch and gantry rotation time.
Hence, optimal heart rate control plays a critical role in
obtaining a good-quality scan. The recent advancements in MSCT
technology-including reduced gantry rotation time, improved
temporal resolution, and an increment in detector arrays to the
current 64channel systems- have improved spatial resolution and,
thus, have dramatically improved image quality.
Improved image quality is best explained by the increased
sensitivity and specificity, and high negative predictive value
(NPV) and positive predictive value (PPV) in regard to stenosis
detection. Studies indicate that the 16-slice scanners detect
>50% stenosis in segments of vessels with sensitivity 92% to
95%, specificity 86% to 97%, NPV 92% to 97%, and PPV 80% to 87%.
However, 20% of coronary artery segments, predominantly in small
vessels and side branch vessels, are nonevaluable. The modern
64-slice scanners also detect significant stenosis in small
coronary artery segments and side branches with higher sensitivity
(81% to 94%) and specificity (93% to 97%), and improved NPV (95% to
97%) and PPV (78% to 90%). The significant decrease in the number
of nonevaluable segments (7%) contributes to the increased
diagnostic accuracy of the 64-slice MDCT.
Planar scout films are unenhanced images used by the
technologist to determine scan volumes. During acquisition of the
scout films, the patient performs a breath-hold, and the resulting
anteroposterior image is used to ensure that all pertinent
vasculature of interest is included in the scanning field. The
patient must be consistent with the depth of breath-hold during the
scout image acquisition, the non-contrast CT, and the contrast CT
examination. A shallow breath-hold will move the heart into a
higher position, and a deeper breath-hold will move the heart into
a lower position in the chest.
A noncontrast CT for calcium scoring is obtained to determine
the total calcium burden.
Besides calcium scoring, the noncontrast scan can be utilized for
the identification of prior stent placement, vascular clips after
bypass grafting, foreign bodies, and other objects that may obscure
the contrast agent.
Coronary calcium is a reliable marker of atherosclerotic plaque
burden and angiographic disease and is an independent predictor of
High calcium scores are useful in predicting coronary artery
disease (CAD) and helpful in determining the role of MDCT in
patients with possible CAD. Calcium scores >1000 or heavily
calcified coronary artery seg ments can make accurate
interpretation of plaques difficult, leading to overestimation of
luminal narrowing. Hoffmann et al
determined that calcification was the major underlying cause of
overestimation of coronary lumen stenosis resulting in the majority
of false-positive findings (94%). Several researchers suggest that
CTA should not be performed to diagnose CAD if the patient's
calcium score is >1000.
On the opposite end of the spectrum, Haberl et al
reported a very high NPV of coronary calcium scanning. A coronary
artery calcium score of 0 effectively excludes hemodynamically
significant CAD in 99% of cases. This information is useful in
adjusting the patients' pretest probability for CAD and assists in
interpretation of the CTA. Of note, radiation exposure from calcium
scoring is minimal secondary to prospective gating techniques and
utilization of only a single predefined cardiac phase for image
reconstruction (1.3 to 1.7 mSv for a 16-slice scanner).
Contrast administration for CTA should, ideally, be performed
via an antecubital vein, although other injection sites in the
upper extremities except for veins in the hands can be used. The
use of venous access in the hands is avoided because of the high
flow rates of contrast injection.
The lower extremities and the hands also cannot be used because of
longer delay times. The calculation of transit time from the time
of contrast injection must be accurate to ensure adequate contrast
opacification of the left ventricle and coronary arteries. Some
centers obtain timing scans with a small contrast test bolus (10 to
20 mL) to determine transit time from the injection site to the
ascending aorta. This test bolus method to detect accurate transit
time can be performed by imaging the ascending aorta every 2
seconds after an initial 10-second delay after contrast injection.
The transit time varies with each patient but is typically 15 to 18
seconds. Several factors, such as low ejection fraction, decreased
cardiac output, and pulmonary disease, can influence the transit
Bolus tracking software eliminates the need for a timing test
bolus, since the software determines when the contrast has arrived
in the coronary arteries and initiates the scan after a preset
limit (eg, 100 HU) is reached in the ascending aorta within a
predefined region of interest.
Most centers use contrast injection rates of 4.5 to 5.0 mL/sec.
Allergic reactions and contrast extravasation from the vein are
rare but highly concerning procedural complications. Dual contrast
injectors, with one injector filled with contrast and the other
with saline, are commonly used to reduce the contrast requirements
and improve opacification of the coronary arteries. The total
contrast volume is based on the required scan time.
Typically, 60 to 80 mL of iodinated contrast with high iodine
concentration (300 to 400 mgI/mL) is injected at a flow rate of 4.5
to 5.0 mL/sec, followed by a saline chase bolus of 40 to 70 mL at
the same flow rate.
Multiple variables affect image quality and the elimination of
respiratory-phase cardiac motion with patient com pliance with
breath-holding technique is critical for a high-quality scan. The
breath-hold time continues to decrease with the increase in the
number of slices that can be obtained in one rotation of the gantry
and with increased speed of image acquisition. Although 4-slice
scanners required breath-holds of 20 to 40 seconds, modern-era
scanners require breath-holds of 15 to 30 seconds for 16-slice and
8 to 20 seconds for 64-slice.
Breathing artifacts are problematic and create blurring of the
images from cardiac motion. Patients need clear and repetitive
instructions on appropriate breath-holding prior to scanning.
Retrospective ECG-gated MDCT utilizes simultaneous acquisition
of imaging and ECG data during scanning. This allows for
flexibility in the reconstruction of the data sets during systolic
or diastolic phases of the cardiac cycle. The R-R intervals must be
regular and at a heart rate of approximately 60 to 65 bpm for
optimal application of the retrospective reconstruction technique.
Hence, it is critical that patients referred for CTA are in sinus
rhythm. Atrial arrhythmias (flutter, fibrillation), frequent
premature atrial contractions, and frequent premature ventricular
contrasts are contraindications to MDCT.
Multidetector CT scanning is performed in the spiral mode, and
images are acquired continuously throughout the cardiac cycle. A
slice thickness of ≤0.75 mm with 50% overlap is usually used for
image reconstruction with a 512 × 512 pixel matrix. In obese
patients, images can be reconstructed at 1.0 mm thickness to reduce
noise in the image. A small field of view (18 to 20 cm) is used for
a dedicated CTA. A medium-smooth reconstruction filter (kernel) is
usually used for standard CTA. Reconstruction with a sharper kernel
may be used for evaluation of calcified plaques and intracoronary
stents. As described previously, a retrospective ECG-gated image
reconstruction technique is utilized in CTA to minimize radiation
exposure. Although half-scan reconstruction algorithms are used in
most patients, multisector reconstruction from several consecutive
heartbeats may be used to optimize image quality at faster heart
rates (>65 bpm). The half-scan single reconstruction can be done
in any phase of the cardiac cycle.
It is recognized that each coronary artery has a different pattern
of motion during a cardiac cycle. The visualization of the right
coronary artery (RCA) is highly sensitive to motion on MDCT
secondary to its extensive motion radius and short motion-free
period. Motion artifacts account for a portion of noninterpretable
coronary artery segments.
Reconstructed images earlier in the cardiac cycle (30% to 40%
phase, end-systolic) provide diagnostic-quality images for RCA
evaluation and later in the cardiac cycle (60% to 80% phase,
mid-to-end diastolic) provide quality images of the left coronary
CT angiographic literature indicates that the optimal
reconstruction phase is based on the patients' heart rate. Herzog
recently published a prospective study in 70 patients that
indicated that the R-R interval phase choice should be based on the
heart rate at the time of the MDCT. In patients with higher heart
rates (65 to 75 bpm) the best image quality was obtained during
end-systole and early diastole (27% to 39% of R-R interval), and
for patients with low heart rates (<65 bpm) the best image
quality was obtained during mid-diastole (60% to 65% of R-R
Optimization of radiation dose
Three primary factors determine radiation dosimetry: X-ray
energy or tube voltage, tube current, and exposure time.
Effective dose refers to the amount of radiation absorbed in
different organs and tissues and is a standard means of comparing
various imaging modalities. Radiation exposure is a significant
concern for MDCT; a current estimated range of radiation exposure
is 8 to 22 mSv. Radiation exposure with other cardiac imaging
modalities such as cardiac catheterization is 2.5 to 5 mSv, and
nuclear testing with single-photon emission CT is 15 to 20 mSv. The
current radiation exposure of MDCT is equivalent to 100 to 160
anteroposterior and lateral chest X-rays.
Coles et al
recently published a 91-patient comparison of effective radiation
doses with 16-slice MDCT and diagnostic coronary angiography. The
study results revealed a significantly higher mean effective
radiation dose associated with MDCT: 14.7 mSv for MDCT versus 5.6
mSv for a diagnostic invasive angiogram.
The technical factors utilized to enhance image quality,
particularly increased spatial resolution using a high- voltage
current X-ray tube and an increased number of slices per rotation,
have resulted in increased radiation exposure. Compared with
16-slice scanners, 64-slice scanners result in a signi-ficantly
greater amount of radiation exposure, 11.0 ± 4.1 mSv for 64-slice
CT versus 6.4 ± 1.9 mSv for 16-slice CT.
The U.S. Food and Drug Administration (FDA) has issued a
statement regarding the radiation risks associated with CT
screening describing the potential risk of cancer from radiation
The reported cancer risk associated with 14.7 mSv radiation
exposure is 1 in 1400 versus 1 in 3600 with diagnostic angiogram
Due to the concern and risk associated with medical radiation
exposure, new scanning protocols are being evaluated for radiation
exposure reduction and image quality.
A recent retrospective study conducted by Hausleiter et al
that compared 16-slice to 64-slice scanners in 1035 patients
evaluated 2 different radiation dose reduction strategies and the
outcomes on image quality. The increased radiation dose with the
64-slice MDCT is due to increased spatial and temporal resolution.
High spatial resolution is necessary for optimal evaluation of
artery segments. Temporal resolution is improved due to increase
gantry rotation speed. To maintain image quality with the reduced
slice thickness and faster gantry rotation, the tube current is
increased which results in more radiation exposure.
The 2 radiation dose reduction algorithms evaluated were
ECG-dependent modulation of tube current and reduction in tube
ECG-dependent modulation of tube current (dose modulation)
reduces radiation by decreasing the tube current 80% during
ventricular systole. This algorithm allows for high tube current
with optimal image quality during mid-diastole, the portion of the
cardiac cycle with the least amount of motion and the phases
typically utilized for coronary interpretation. Hausleiter et al
utilized this algorithm in 82.2% of the patients with a radiation
dose reduction of 40% with 16-slice and 32% with 64-slice scanners.
The dose reduction did not result in a reduction in image quality.
ECG-dependent dose modulation is limited to patients with low heart
rates and regular sinus rhythms without premature ventricular
The second radiation reduction method involved a reduction of
tube voltage (X-ray energy). Tube voltage reduction results in
greater attenuation by contrast media, creating a higher contrast
between arteries and the surrounding tissue. This method provides
improved signal and decreased radiation exposure but creates
increased image noise. A combination of the 2 methods of radiation
exposure reduction reduced exposure 53% with the 16-slice scanner
and 64% with the 64-slice scanner.
Current limitations of CTA
Multidetector CT has limitations that must be recognized and
well understood by clinicians and technologists. Not all patients
are candidates for MDCT, and proper patient selection is critical.
Artifacts during MDCT acquisition create significant limitations in
imaging quality and diagnosis.
Cardiac motion artifacts occur when heart rates exceed
approximately 70 to 75 bpm or with increased heart rate
variability. This is particularly relevant in the evaluation of the
Cardiac motion typically creates blurring or stepladder artifacts.
Blurring occurs because the cardiac movement has exceeded the
temporal resolution of the scanner. Stepladder artifacts are seen
at the end of the breath-hold secondary to increased heart rate and
increased heart rate variability (Figure 2).
Pulmonary/respiratory artifacts produce stair-step artifacts
through the entire dataset and cannot by corrected with image data
reconstruction. These artifacts involve the heart border and the
anterior chest wall, creating blurring, image gaps, and overlapping
of image sections.
Pulmonary artifacts can be avoided with careful patient instruction
and education on breath-hold technique (Figure 3).
Blooming artifacts result from high-attenuation structures, such
as calcified plaques or stents. They create an enlarged "bloomed"
appearance secondary to partial volume averaging effects that
obscure the surrounding coronary lumen. Coronary calcium is very
problematic in image interpretation and may result in
overestimation of the degree of stenosis, creating false-positive
results. Secondary to the significant interpretation limitations of
heavily calcified coronary arteries, most centers will not proceed
with contrast MDCT if the Agatston score is >1000.
Hoffman et al
reported that 94% of the false-positive findings in their
prospective, blinded assessment of 33 patients were secondary to
overestimation of coronary lumen obstruction from calcification.
Despite the increased temporal and spatial resolution of 64-slice
scanners, which allow for more optimal assessment of calcified
segments of the coronary arteries, severely calcified coronary
plaques remain a significant limitation to interpretation (Figure
Beam-hardening artifacts result when the X-ray beams cross a
high-density structure (calcified plaque, stent, surgical clips),
creating artifacts that resemble noncalcified plaques in adjacent
coronary lumen (Figure 5).
Structure-related artifacts are the result of overlapping
contrast-filled normal anatomic structures, such as left atrial
appendage and cardiac veins. Cardiac veins may obscure evaluation
of the coronary arteries, leading to pseudostenosis.
Obesity creates image noise, and utilization of soft kernels may
improve image quality.
A current significant limitation of MDCT is the assessment of
stent patency and/or in-stent restenosis. Currently, stent
diameters >3 mm can accurately be evaluated with MDCT.
The assessment of in-stent restenosis in smaller diameter stents
(<3 mm) is limited secondary to the high-density of the stent
obscuring the visualization of the lumen or beam-hardening
artifacts from the stent struts.
Sharper filters or hard kernels and thinner slices improve in-stent
Multidetector CT cannot detect sluggish, retrograde, or
antegrade flow in the coronary arteries, thus the filling patterns
of the arteries can be difficult or impossible to determine. A
final important limitation of MDCT is its inability to provide
functional information about the physiologic significance of an
intermediate-grade coronary stenosis.
Trends and developments in cardiac CT
Dual-source CT is a new imaging technique that improves temporal
resolution of cardiac scans and eliminates the need for heart rate
control. Temporal resolution of <100 msec is required to enable
adequate cardiac imaging without heart rate control. Current
scanners cannot mechanically achieve this rotation speed. The DSCT
utilizes 2 arrays of X-ray tubes and 2 corresponding detectors that
are mounted on the rotating gantry and angled 90º apart.
One detector covers the entire scan field of view, while the other
detector is limited to the central field of view. The DSCT provides
improved temporal resolution at one-fourth the gantry rotation
time; thus, ECG-gated single-segment (one cardiac cycle only)
reconstruction with temporal resolution of 83 msec can be obtained.
The patient's heart rate is no longer an obstacle to scan
acquisition because only 1 cardiac cycle is needed for image
reconstruction. Initial experiences in the literature on DSCT from
Achenbach et al
and Flohr et al
indicate that DSCT provides improved image quality in patients with
high heart rates.
Another technological advancement in CT is the 256-slice MDCT.
The 256-slice scanners will allow for wide anatomic volume coverage
(128 mm of anatomy with 0.5 mm slices) with a single gantry
rotation. The primary benefit of this wide volume coverage is the
ability to perform dynamic examinations of large organs, such as
the heart, including the whole organ and perfusion in the single
gantry rotation. As described before, conventional MDCT systems
acquire phasic information of the entire volume by performing
multiple helical runs or limited dynamic information for a limited
anatomic volume. The 256-slice MDCT will eliminate this current
imaging limitation and allow for better imaging quality in a more
diverse patient population. The major current concerns about
256-slice scanners are increased radiation exposure and increased
cone angle, which could create cone-beam artifacts that limit
Mori et al
recently compared the radiation dose in 256-slice and 16-slice
scanners. Interestingly, the radiation dose for the 256-slice
scanners was smaller than the dose with 16-slice scanners for all
examinations performed (head, chest, abdomen, and pelvis). The
decreased dose with the 256-slice scanners is the result of the
large beam width resulting in greater anatomic coverage without the
need for overlap margin coverage. The 256-slice scanners provide
sufficient image quality for diagnosis with 1 rotation of the
gantry and, thus, less radiation exposure. The 256-slice scanners
are in evolution and the utility of the application to coronary
artery evaluation remains to be determined. However, the current
literature supports the future application of the 256-slice
scanners to dynamic whole- organ evaluation.
This article reviewed the current literature and addressed 5 key
principles of coronary CTA: appropriate patient selection and
preparation, utilization of beta-blockers for heart rate control,
the principles of scan acquisition and optimization of radiation
dose, recognition of the current limitations of CTA, and potentials
of DSCT to address the current limitations of CTA. Multidector CT
is rapidly gaining acceptance as an imaging modality for coronary
artery disease detection and quantification, stent assessment, and
bypass graft evaluation. However, clinicians must understand the
limitations of MDCT. Severe coronary calcification is an important
limitation that may preclude visualization of coronary artery lumen
and may be a contraindication to CTA acquisition. The current
amount of radiation exposure from MDCT is of concern and must be
addressed. Novel radiation dose reduction algorithms are currently
being evaluated. MDCT technology has made major strides in the last
5 years. MDCT provides a safe noninvasive imaging alternative to
diagnostic invasive coronary angiography. With the progressive
advances in MDCT technology, it is expected to continue playing a
major role in the assessment of the cardiovascular system.