The inspiration and development of multidetector-row computed tomography (MDCT) has been driven by an interest in developing noninvasive coronary angiography. However, there is, as yet, no single way to perform and interpret coronary artery CT. The author reviews and illustrates one ap-proach to the current techniques of coronary artery MDCT data acquisition, processing, and clinical application.
is an Assistant Professor of Cross Sectional and Interventional
Radiology, Johns Hopkins University, Baltimore, MD.
Coronary artery disease remains one of the leading killers in
the western world. Given that many of those dying of coronary
artery disease have no antecedent history and that a large
percentage of cardiac catheter interventions do not involve
therapy, there is clearly a role for noninvasive imaging. Alhough
it certainly has benefits in other areas, the inspiration and
development of multidetector-row computed tomography (MDCT) has
largely been driven by an interest in developing such noninvasive
Fundamentally, cardiac imaging is a straightforward extension of
the principles underpinning stationary vessel CT angiography (CTA),
though the beating heart introduces some new issues for data
acquisition and processing. Although the application of mechanical
helical CT for coronary imaging as yet lacks definitive outcome and
comparative data, it has nevertheless gained a degree of acceptance
in daily cardiology and radiology practice.
Like many rapidly developing imaging modalities, cardiac CT
implementation appears unlikely to pause for the results of
long-term, evidence-based data.
With MDCT still in its infancy, there is no monopoly on the
correct way to perform and interpret coronary artery CT at this
time. This paper will review and illustrate one approach to the
current techniques of coronary artery MDCT data acquisition,
processing, and clinical application.
With most current scanners, coronary CTA should be reserved for
those patients with heart rates <65 beats per minute. Even with
the temporal resolution of current MDCT scanners, higher heart
rates frequently result in significant motion artifact. One should
note that a patient's resting prescan heart rate may rise with
anxiety on the scanner and also with the administration of
iodinated contrast.Patients should be instructed to avoid high
doses of caffeine and other chronotropes the morning of the study.
A beta-blocker will slow the heart rate and increase the relative
duration of the diastolic phase within the cardiac cycle.
Short-acting metoprolol is the most commonly prescribed
beta-blocking agent. It may be given as a 50-mg oral tablet on the
morning of the procedure or as a 5-mg dose intravenously 30 minutes
before the procedure.
Coronary artery CT is one of the most demanding of CT studies,
as it requires simultaneously high spatial, contrast, and temporal
resolution. Though the in-plane resolution of the 512 matrix has
changed little with new detector arrays, the z-axis resolution and
coverage speed has increased. Across the vendors, most current
detector designs have 0.625- to 0.75-mm detector units. These
provide near isotropic (square) voxels where the viewing
perspective for interpretation is almost independent of the plane
of acquisition. The short scan duration of 12 to 15 seconds permits
a breath-hold imaging duration that can capture homogenous contrast
opacification around the narrow peak of contrast enhancement.
Temporal resolution (TR) may be thought of as the "shutter speed"
of the scanner and is the key to recent advancements in MDCT
technology. Temporal resolution in the region of 100 msec, which
approximates the performance of electron-beam CT (EBCT), is
required to create relatively motionless images of the beating
heart. This is largely achieved through fast gantry rotation (~0.37
to 0.42 msec) and segmental reconstruction.
ECG triggering and gating
Because of the limitations of current helical mechanical CT, one
needs a method to extract the coronary images that are the most
diastolic and motionless from a series of complete cardiac cycles
(Figure 1). Two methods in use are echocardiographic (ECG)
triggering and ECG gating. Using ECG triggering, the scanner
acquires data only for a defined period after the signal from the R
wave of the ECG trace. This is applied to a nonhelical,
translational "step and shoot" scan technique, and an image is
acquired every second heartbeat to allow table translation between
image generation. Using ECG gating, the scanner acquires data in a
nonstop, helical mode while an independent ECG trace is generated
at the same time. Images are thus acquired both during systole and
diastole. Subsequently, the ECG trace is mapped alongside the
acquired image data so that one can deduce the systolic and
To summarize, ECG triggering uses the ECG to acquire data and is
prospective, while ECG gating is used to reconstruct data and is
retrospective. Also, ECG triggering may be used for coronary
calcification scoring and aortic root imaging, whereas ECG gating
is required for coronary artery CTA.
The contrast dynamics within the coronary arteries reflect those
in systemic elastic arteries elsewhere, with a rapid rise to peak,
short duration plateau, and relatively rapid drop in concentration
after the peak. A peak attenuation value of 2 to 300 HU must be
achieved in the left ventricular outflow tract for clinically
useful coronary CTA images to identify patency and noncalcified
plaque (Figure 2). All postprocessing segmentation tools assume a
level of contrast differentiation so that the vessel lumen may be
defined. In our department, 100 mL of nonionic contrast material
(320 mgI/mL) is given by power injection at a unimodal, constant
rate of 3.5 to 4 mL/sec through an antecubital vein. With the
saline chaser technique, the contrast bolus is followed immediately
with a 20-mL bolus of saline by power injection from a second
syringe in a dedicated dual-head injector. The underlying principle
is that the chaser captures redundant contrast in the arm,
innominate vein, and the superior vena cava, washes out the
beam-hardening density within the right atrium, and increases the
density values within the left heart and coronary arteries by
tightening the bolus and raising its concentration.
There is limited scientific data on the role of the saline chaser,
and its use remains under review without universal recommendations.
I have found its performance variable with 16-detector scanning,
and I am currently looking at its use with 64-channel MDCT.
Although in the past we have used empiric timing of a 25-second
delay with 16-detector scanning with good reproducibility and
consistent ventricle en-hancement, the shorter data acquisition
time of a 64-detector CT requires timing or test-bolus techniques
that are the same as those applied to aortic imaging.
The main coronary artery protocols in use are calcification
scoring and coronary CTA. The former is performed using a low-dose,
noncontrast, prospectively triggered, nonhelical technique, with
relatively large slice collimation (~3 mm). Coronary CTA is
performed with small slice collimation, relatively low pitch, and
slow table translation that allow oversampling of data. The
smallest slice widths are defined by the smallest detector width
chosen. A 0.75- to 1-mm slice collimation produces good coronary
3-dimensional (3D) data sets, though the smaller slice widths may
carry a penalty of higher noise. Some institutions advocate
performing coronary calcification scoring in all patients and,
perhaps, not proceeding to coronary CTA when a heavy burden of
calcified plaque will likely make the study uninterpretable.
The ECG-triggered mode is a relatively low-dose technique (~3
mSv), as data is acquired only during the diastolic phase of the
cardiac cycle. The ECG-gated mode is a high-dose technique (~7
mSv), as it acquires data throughout the cardiac cycle in both
systole and diastole with low pitch and data oversampling. This
dose approximates that of conventional digital subtraction coronary
angiography. Larger detector arrays do decrease the relative
contribution of the wasted penumbra effect at the end of the
detector arrays. ECG modulation is a technique that lowers the
radiation exposure during the systolic phase of imaging, and then
increases the radiation dose during the diastolic phase when
clinically useful imaging is being acquired.
It behooves us to apply the lowest reasonable dose, in particular,
when the CT is used as a screening tool.
Coronary CTA data is reconstructed using multidetector-row
interpolation. The entire 360˚ of gantry rotation is not required
for image reformation. Using partial scan reconstruction with 180˚
of the rotation, the temporal resolution may be halved. Due to the
overlapping data sampling, a smaller arc of gantry rotation from
multiple adjacent cardiac cycles may be summed to create a single
axial image (ie, segmental reconstruction) with increased temporal
resolution. In this setting, the temporal resolution is equal to
the gantry rotation time divided by twice the number of segmental
reconstructions employed. One should note than when more than 2
cardiac cycles are used, the image quality will deteriorate as one
samples data beyond the field-of-view of interest.
An entire set of axial images of the heart from the base to the
apex is made for each of a series of time intervals within the each
cardiac cycle: 10%, 20%, 30%, etc., of the R-R interval (Figure 3).
Of course, this assumes a sinus rhythm with heartbeats of equal
duration, where the 60% R-R image of the top of the heart, made
from the first heartbeats, corresponds in time and cardiac cycle to
the 60% R-R image of the bottom of the heart constructed from the
last heartbeats. Thus, one has images in the most systolic and
diastolic phases and those in between. It is worth noting that
there is both interpatient and intercoronary variation. That is,
the most motionless set of images for 1 patient may be 70%, while
this set may be 60% for another patient. Likewise, the left
anterior descending (LAD) coronary artery may be most motionless at
60% but the right coronary artery may be most motionless at 70% in
the same patient. So there is no "one-size-fits-all," but a routine
reconstruction of the 50%, 60%, and 70% reconstructions will
suffice for most patients.
Once the data sets have been generated, postprocessing must be
applied to segment the helical volume and harness the information
contained therein. All postprocessing tools for coronary
calcification scoring detect calcification by choosing those sites
within the field-of-view that have density values above that of
nonenhanced blood. Region growing seeds then may select adjacent
pixels with calcification until all calcification is detected. All
platforms allow one to enumerate the number of lesions in each
named vessel. Though all produce the Agatston score, this has been
largely superseded by the mass and volume of calcification present,
which are thought to be more objective and reproducible.
Routine axial planar images still play a vital role in overall
review. Multiplanar reconstruction (MPR) and curved multiplanar
reconstruction (CMPR) techniques have little role to play in
coronary imaging. The coronary arteries do not conform to any
single plane, and curved plane construction is too subjective for
defining the endoluminal center point for such small vessels.
However, combined with tools that automatically find the center of
the vessel (center-line tools) and with vessel-straightening tools
that undo the tortuous curves, CMPR can have value.
We have generally found these tools to be useful only in relatively
larger and well-opacified vessels with minimum calcification.
Maximum-intensity projection (MIP) is a simple projectional-type
technique that depicts all density values above a certain
threshold. It requires interactive real-time motion to generate
perspective but does not require intensive computer power. Slab
MIPs are a quick and effective means to sequentially review the
coronary arteries and depict stenoses. This tool performs well even
in the presence of calcification, which can be separated using
adjusted window settings (Figures 4 through 6).
Volume rendering (VR) represents the most advanced form of 3D
postprocessing. It is of high fidelity to the acquired data set,
preserving all the density values within the voxels. Using
histogram trap-ezoids with various user-defined settings of
opacity, brightness and color presets may be applied to create
attractive coronary images (Figures 7 through 9). However, one must
interpret volume rendering with caution, as many of the
user-adjusted variables may dramatically alter the size of the
vessel lumen. Though VR can be used to illustrate vessel
opacification, stenoses, and course, MIP is the better choice to
quantify lumen change.
Clinical application and interpretation
Although coronary calcification scoring as a surrogate marker
for cardiac risk factors has some advocates, it also has many
equally loud detractors.
Some researchers do not believe in the role of coronary
calcification, and some do not believe in the use of mechanical CT
for calcification scoring. In the early development of MDCT, there
was an argument that calcification scoring should be performed only
on EBCT scanners where the original coronary work was done. Such
arguments have been largely laid to rest by comparison studies with
MDCT and the proven reproducibility of MDCT.
The reality is that scoring use continues to grow rapidly in the
cardiology and radiology community as we await the outcome of some
large, long-term trials here and in Europe. We know that
calcification in the coronary arteries is pathognomonic for the
presence of atherosclerotic disease, though its absence does not
equate with the absence of plaque. A negative calcification score
does suggest that an acute coronary event is unlikely in the near
future, and it has been noted that patients presenting with
atypical chest pain may be evaluated for noncardiac causes in the
setting of a negative coronary calcification scan. When present,
the risk of a coronary event is related to the burden of
However, the prevalence of calcification is far greater than the
prevalence of coronary events and, at this time, there is no
threshold amount of calcification above which coronary events can
be accurately predicted. The coronary calcification tells us
nothing about plaque stability and the acute event may occur at a
site remote from the calcification.
A few isolated reports have suggested that coronary calcification
may be used to monitor the response of plaque to
cholesterol-lowering agents, though the results have never been
replicated. Some of the greatest interest has centered on coronary
calcification as an independent risk-factor modifier.
There are those who have suggested that it may have more
discriminatory value than age in the Framingham formula when
applied for those >55 years of age. I subjectively describe
larger burdens of plaque and, when possible, describe when deposits
are eccentric or more central. The final score analysis is sent to
the referring clinician, and I do not attempt to deduce the
clinical or management implications; at this time, these are best
handled by those with a comprehensive grasp of the patient's
First, it is important to appreciate which patients should not
have coronary artery CTA. Conventional coronary angiography remains
the standard of care for coronary artery evaluation. Time is
critical for those presenting with a threat to myocardial
viability. Patients presenting with a high pretest probability of
unstable disease should still go on to conventional imaging in
cases in which catheter-directed therapeutic intervention is highly
likely or if emergent surgical intervention is anticipated.
Patients who have a coronary calcification study that reveals a
large burden of disease should be reconsidered for CTA, as the
ability to accurately and comprehensively detect and quantify all
stenoses may be limited.
Coronary MDCT is a reasonable test to evaluate stenoses in those
with a low pretest probability of disease where the
negative-predictive value of the study is very high. It has also
been shown to be of value in depicting aberrant coronary artery
anatomy by accurately defining their site of origin, size, and
Such vessels often defy definition by conventional projectional
technique. It is of proven value in evaluating the patency of
bypass grafts (Figure 10), which are relatively motionless vessels
and can be imaged with ease, although one should note that we are
limited in evaluating the anastamosis and small-vessel disease
beyond the anastamosis.
A CTA luminogram does not provide information on flow dynamics and
usually cannot reveal collateral formation. Its use in establishing
coronary stent patency is limited and has been limited by beam
hardening in its sensitivity to early restenosis.
Based on our experience, 70% to 80% of cases are satisfactory
studies with acceptable visualization of 70% of the native vessel
lengths using 16-detector CT. Most vessels ≤2 mm can not be
routinely visualized. It would appear that though the line pair
resolution of 64-detector CT has not significantly changed, the
improved temporal resolution and bolus capture may increase
reproducibility, accuracy, and vessel visualization.
As with all CTA, there is not as yet any standardization of
coronary artery CTA technique or interpretation. The following
discussion reflects only one approach. There are, however, American
Heart Association and American College of Cardiology guidelines on
conventional angiography interpretation that may serve as a guide,
since conventional angiography is as yet the gold standard, albeit
an imperfect one.
It is important that the basic anatomy is understood and the
vernacular of cardiology interpretation is followed.
Initial image review is directed at noncardiac pathology with
axial planar images. Though the heart images are reconstructed with
a smaller field-of-view, it is important to study the entire
field-of-view images in the axial plane to exclude incidental lung
or mediastinal neoplasms. Initial cardiac review is directed at
noncoronary pathology, including pericardiac disease, cardiac
masses, and valvular changes. The shape and size of ventricles is
commented upon as well as any attenuation changes. Starting with 3D
VR, coronary artery interpretation begins with documenting the
vessel origins, course, and dominance. With knowledge of the vessel
course, slab MIP is applied to each vessel, in turn, to assess for
stenotic disease. As with barium studies, readers will quickly
develop their own systematic way of reviewing all of the vessels in
turn. The printed images include the standard cardiologist views of
the right coronary, LAD, and circumflex arteries. These are
supplemented with unique 3D views that depict the individual
patient's disease to best effect. There are no proven tools that
accurately measure coronary artery stenosis, and as with
conventional angiography, subjective percentages may be applied.
One approach is to subjectively describe lesions <50%, 50% to
75%, and >75%. With larger and well-opacified vessels, one can
apply diameter measurement tools. The site, length, and
multiplicity of the lesions may be estimated, along with the
proximity to major branch points. With high-grade disease, the
presence of distal vessel opacification is noted. A full cardiac
evaluation takes an average of 20 to 30 minutes to interpret.
The final report includes a description of the data acquisition
and postprocessing tools that were employed. There is a sequential
description of noncardiac, noncoronary cardiac, and coronary artery
findings. Referring clinicians receive a photographic printout or
digital image of calcium scores and coronary artery CTA.
Recently, CT has been moving into some traditional areas for
MRI, including plaque imaging and functional cardiac imaging. Some
early work has suggested that coronary artery plaque may be
characterized by CT attenuation values with potential to
discriminate sites of stable and unstable disease. Such findings
have been correlated with intravascular ultrasound, but the work
remains in its early stages.
Information on the entire cardiac cycle is contained within the
retrospective data set. From this data, one may obtain dynamic
chamber volumes, wall thickness, and wall thickening information.
Using cine-loops and so-called 4-dimensional images, one may depict
the chamber and valve changes throughout the cardiac cycle.
One may qualitatively assess regional wall motion and dyskinesis
and objectively quantify ejection fraction and stroke volume. Such
findings have found good correlation with MRI and provide useful
supplemental information in patients presenting for cardiac
coronary artery evaluation (Figures 11 through 13).
The advent of MDCT has introduced an exciting dimension to
existing radiology practice. With the latest hardware and software
refinements, cardiac CT imaging is relatively straightforward and
simple to perform and interpret. Perhaps this is best seen by the
rapid progress of nonradiologists in this area. Though it remains
an emerging technology, it is being applied and developed in
clinical practice at both academic and private centers. As sites
implement MDCT technology, coronary CTA should be a part of routine
body imaging for all radiologists. One cannot, of course, assume
the new role of coronary artery imaging without challenges. It
requires a special investment of time and interest as we break new
ground, and it has not reached the fluidity and workflow efficiency
of routine body imaging. It does stretch existing manpower
shortages, and any future application in the emergency room on-call
will present further challenges. However, scanner manufacturers are
working hard to provide more user-friendly tools for scanning and
interpretation and are slowly moving from qualitative, subjective
assessment to objective, quantitative assessment.