This paper will review the technologic and physiologic principles fundamental to imaging coronary arteries with CT.
attended The Juilliard School, earning the BM, MM, and DMA in Piano
Performance. He earned his medical degree from the University of
Pittsburgh and is currently a third-year resident in the Department
of Radiology at University of California, San Francisco. Following
residency, he will complete a fellowship in Vascular and
Interventional Radiology at UCSF.
Coronary artery disease is the leading cause of morbidity
and mortality in the United States. Both electron beam computed
tomography (CT) and multidetector row CT can produce diagnostic
quality images of the coronary arteries following intravenous
injection of contrast material. Speed and appropriate timing of
image acquisition are key determinants of successful technique,
and familiarity with the electromechanical relationship of the
electrocardiogram (ECG) tracing to the cardiac cycle is
fundamental to understanding the protocol. Images may be used to
diagnose coronary artery atherosclerotic disease and pathologic
states of coronary anatomy, to document the patency of bypass
grafts and intraluminal stents, and to derive data regarding the
functional status of the heart. This paper will review the
technologic and physiologic principles fundamental to imaging
coronary arteries with CT.
Heart disease is the leading cause of mortality in the United
States, accounting for about 460,000 deaths annually. The most
recent prevalence data from 1998 indicate that 12.4 million
Americans suffer some manifestation of coronary artery disease.
Every day, 600 Americans die from coronary disease outside of the
hospital, and 20% to 25% of first coronary events are sudden
cardiac deaths. Among those who die suddenly, most often, two or
more major coronary arteries are narrowed by atherosclerosis.
Noninvasive imaging studies play an important role in evaluation
of the patient with suspected coronary artery disease. Nuclear
scintigraphy is used to assess for abnormalities of myocardial
perfusion, and gated acquisitions permit quantitative determination
of left ventricular ejection fraction and analysis of wall motion.
Echocardiography evaluates cardiac morphology and function but
cannot reliably demonstrate coronary arterial morphology. Magnetic
resonance (MR) imaging reveals cardiac anatomy in multiple planes
and can be used to assess cardiac function, myocardial perfusion,
and viability. This technique continues to evolve. Scoring of
coronary artery calcium content by computed tomography (CT) is a
relatively new method that possesses the ability to estimate the
total coronary arterial plaque burden, but cannot specifically
assess the severity of obstructive coronary arterial disease.
Cardiac catheterization is the most commonly used method for
direct examination of the coronary arterial lumen. More than 1
million coronary angiograms are performed each year, at an
estimated cost of more than $3 billion. Furthermore, transcatheter
angiography is an invasive examination with a complication rate of
1% to 2% and an associated mortality of 0.15%. Nearly one in five
cardiac catheterizations demonstrates normal coronary arteries or
luminal stenoses of <50%.
Therefore, a less expensive and less invasive method for evaluation
of the coronary arteries has long been sought.
Until recently, electron beam CT (EBCT) was the only CT method
capable of the fast exposure time required to image coronary
arteries effectively. Multidetector row helical CT (MDCT)
technology employs faster tube rotation times than older
single-detector helical CT systems, and has the ability to acquire
multiple thinner sections simultaneously, thus making helical CT
evaluation of the coronary artery lumen now possible.
Electron beam computed tomography
Electron beam CT was developed by Imatron, Inc. (South San
Francisco, CA) in 1983, based on research performed at University
of California at San Francisco by Douglas Boyd and coworkers.
Conceptualized to permit imaging of the cardiovascular system, the
scanner's configuration has fundamental differences from helical CT
design (figure 1). Unlike helical CT scanners, EBCT contains no
X-ray tube. Instead, an electron gun discharges a 130 keV electron
beam into a vacuum funnel. This beam is deflected by
electromagnetic coils such that it strikes one of four semicircular
tungsten targets, which lie in arcs beneath the patient. The
resultant X-rays are collimated into a fan beam prior to
penetrating the patient. After passing through the patient, the
X-ray photons strike a semicircular detector array positioned above
the patient, opposite the tungsten targets. The scanner's use of an
electron gun rather than a moving, mechanical X-ray tube permits
image acquisition in either a 50-msec or 100-msec mode, allowing
excellent temporal resolution for cardiac imaging. Compared
withcurrent conventional CT scanners, EBCT images have increased
noise and may appear somewhat grainy. This and the higher equipment
cost of EBCT may explain why the systems are not in widespread
Multidetector row computed tomography
The introduction of multirow detector arrays to helical scanning
technology in 1998 revolutionized CT imaging. Continuous rotation
of the X-ray tube and the multidetector array during simultaneous
table feed are features of the slip-ring technology characterizing
this generation of scanners. Sub-second gantry rotation time and
simultaneous acquisition of four or more helical slices offers fast
volume coverage with high resolution. Many typical multidetector
scanners have an effective temporal resolution of 300 msec,
achieved by a 500-msec gantry rotation time and utilization of
half-scan interpolation algorithms. Further improvements, not yet
in clinical use, may reduce the effective exposure time to ¾ 125
When considering acquisition times for cardiac CT, note that a
heart rate of 60 beats per minute (bpm) corresponds to cardiac
cycle length of 1000 msec. Electron beam CT acquisitions are
typically accomplished in 50 to 100 msec. The 300-msec temporal
resolution achievable on many MDCT scanners, however, requires
imaging during one quarter of the cardiac cycle at this heart rate.
The current temporal resolution of MDCT is useful for patients with
heart rates of ¾ 70 bpm. For heart rates significantly higher than
70 bpm, some investigators have administered beta-blockers prior to
scanning to slow the heart rate to an acceptable level for MDCT
Successful depiction of coronary artery anatomy by CT
angiography (CTA) is a demanding application. Near complete
suppression of motion artifact caused by the beating heart is
determined not only by the speed of image acquisition, but also by
the precise synchronization of image acquisition with periods of
minimal cardiac motion. In a process known as ECG gating, the ECG
trace is used to synchronize the acquisition or reconstruction of
image data to the cardiac cycle.
Known electromechanical relationships between the phase of the
cardiac cycle and the ECG trace permit the portion of the cycle
with the least motion to be chosen as the temporal window for
The cardiac cycle and coronary artery motion
The QRS complex heralds ventricular depolarization and the onset
of ventricular systole (figure 2). Subsequent ventricular
relaxation and repolarization occur during the T wave, resulting in
diastole, the period of ventricular filling. Diastole can be
divided into three portions: Periods of rapid ventricular filling,
reduced ventricular filling, and atrial systole (which occurs on or
around the P wave). The period of reduced ventricular filling, also
known as diastasis of diastole or the "rest period," has been
targeted as the optimal period for CT imaging because cardiac
motion is minimal at this point. At higher heart rates, however,
the rest period--the optimal imaging window--is disproportionately
shortened compared with systole.
The distance between successive R waves (called the R-R
interval) defines the length of a cardiac cycle-- the time between
heart beats (figure 2). Because R wave amplitudes are large, they
are convenient ECG signals for triggering acquisitions.
The time delay after the R wave, which serves as the trigger point
for acquisition or reconstruction of coronary artery CTA images,
can be expressed in either absolute or relative terms. An absolute
time period, such as 450 msec, can be specified. Alternatively, the
time delay can be prescribed as a percent length of the cardiac
cycle, expressed as a percent of the R-R interval. For example,
images acquired at 30% to 50% of the R-R interval correspond to
mid-diastole, whereas the commonly used trigger of 70% to 80% of
the R-R interval produces nearly end-diastolic images.
The coronary arteries, coursing along the surface of the beating
heart within the epicardial fat, move with every heart beat. A
recent angiographic study of coronary artery motion found
considerable variation between patients in the range and pattern of
coronary artery motion.
The length of the rest period for coronary arteries was strongly
related to heart rate, as predicted from physiology. For heart
rates >55 bpm, the rest period for the left and right coronary
arteries asymptotically approached 66 msec. For heart rates <55
bpm, the rest period approached 333 msec for the left coronary
artery and 200 msec for the right coronary artery.
The rest period was preceded by gradual slowing of coronary artery
motion, and was followed by abrupt motion. The right coronary
artery demonstrated both a greater excursion and velocity compared
with the left coronary artery. Importantly, coronary arteries
returned to the same location during the rest period of consecutive
cardiac cycles. These findings support the presence of a window for
near motionless coronary artery imaging and emphasize the challenge
posed by faster heart rates.
Several recent studies report concordant results regarding the
value of individual temporal windows for visualization of the right
and left main coronary arteries.
These studies report improved visualization of the right coronary
artery (RCA) at points earlier in diastole than when the common
default trigger of 70% to 80% of the R-R interval typically occurs.
Kopp et al
reported that, although principal RCA segments could often not be
evaluated at triggers of 60% to 70%, even branches of the RCA could
be visualized at 40%. The left circumflex artery showed optimal
quality in a trigger range of 50% to 60%, while the left anterior
descending coronary artery was best imaged slightly later in
diastole; between 60% and 80% of the R-R interval. Improved
visualization of the RCA and left circumflex artery earlier in
diastole is best explained by the effects of atrial systole. On or
near the P wave, late in diastole, atrial systole imparts a
ballistic effect on those vessels traversing the atrioventricular
groove. Each of the coronary vessels may therefore be best
evaluated at a unique portion of the cardiac cycle.
Prospective ECG gating
Electron beam CT coronary angiography and MDCT for calcium
scoring utilize prospective gating, also known as ECG-triggered
sequential scanning or the "step and shoot" method. Prior to
scanning, using the ECG tracing, the operator specifies a delay
interval after the R wave as the trigger for scan acquisition
(figure 3). The selection of delay time is intended to image the
coronary arteries during their period of least motion. Once
selected, the delay interval is fixed for the examination,
regardless of variation in the patient's heart rate or rhythm.
Scanner software predicts the length of the R-R interval (and thus,
the heart rate) based on the length of a variable number of
preceding R-R intervals.
Advantages of prospective ECG triggering methods include a
decreased radiation dose compared to retrospective methods, because
scanning is confined strictly to those periods of the cardiac cycle
that were prospectively chosen for image reconstruction. However,
omitting portions of the cardiac cycle may result in suboptimal
images because individual coronary arteries may be best evaluated
in unique portions of the cardiac cycle. Current EBCT coronary
angiography software addresses this issue by offering the
acquisition of three prospectively determined scan intervals within
each cardiac cycle. Other disadvantages of prospective ECG
triggering include its sensitivity to arrhythmias and heart rate
variability during the breath-hold period. Finally, the sequential
scan technique does not provide continuous volumetric coverage of
the heart. While this is acceptable for coronary artery calcium
determinations, degradation in z-axis resolution is an impediment
to high-quality CTA reconstructions.
Retrospective ECG gating
Coronary CTA performed with MDCT is gated retrospectively. A
digitized ECG trace is continuously recorded during helical
scanning following contrast injection. Slow table motion (pitch 1.5
to 3.0, depending on heart rate) is employed during multidetector
helical scanning. Because the ECG is obtained simultaneously,
postprocessing steps can match portions of the helical scan data
set that coincide with any portion of the cardiac cycle. This
permits retrospective reconstruction of data that is selective for
any cardiac phase.
As in prospective gating, the portion of the cardiac cycle used
for retrospective reconstruction is defined with reference to the R
wave on the ECG tracing. Several different approaches to
retrospective gating are technically possible (figure 4). The
relative delay method designates the trigger time for image
reconstruction as a fraction of the R-R interval after the previous
R wave. Using this method, the delay time is calculated and applied
for each individual cardiac cycle so that variations in heart rate
have little or no effect on the quality of image reconstruction.
The absolute delay method (also called the absolute forward method)
defines the beginning of the reconstruction interval using a fixed
time after the onset of the previous R wave. The absolute reverse
method defines the start point for the reconstruction interval as a
fixed time before the onset of the next R wave. The absolute
reverse technique appears to be superior in cases of arrhythmia.
Advantages of retrospective gating include a decreased
sensitivity to variable heart rate and rhythm. Continuous
three-dimensional (3D) volumetric coverage of the heart permits
reconstruction of data from any phase of the cardiac cycle. The
principal disadvantage is a higher radiation dose compared with
prospective techniques. The helical acquisition required for
retrospective gating results in radiation throughout the entire
cardiac cycle, even during those portions that are not useful for
reconstructing images of the coronary arteries. Because each
coronary artery may have an individual best phase for image
reconstruction, it is estimated that only approximately 20% of the
cardiac cycle could be potentially excluded in dose minimizing
Investigations are under way to synchronize reductions in tube
output, with portions of the cardiac cycle not likely to be
utilized for image reconstruction.
Multidetector CT coronary artery angiograms require at least
four detector channels operating at 1-mm or 1.25-mm collimation.
Narrow collimation reduces partial volume effects. Pitches between
1.5 and 3 are selected by scanner software or by reference to a
standard table based upon the patient's heart rate in order to
assure continuous volume coverage. Using these parameters, the
cardiac volume can be covered in about 30 seconds, well within the
limits of a breath hold.
Contrast bolus technique
Optimal visualization of coronary arteries by CTA demands
opacification of the arterial lumen with minimal venous
opacification (figures 5 and 6). Circulation time is estimated in
most protocols using a test bolus. Ten to 20 mL of contrast
material is infused into a cubital venous line at 4 mL/sec and
serial transaxial images are obtained through the ascending aorta
every 1 to 2 seconds in order to determine the time to peak
enhancement. Most investigators add 2 to 3 seconds to the measured
circulation time to ensure adequate opacification of the coronary
artery lumen at the time of scanning. For the diagnostic
examination, 120 to 160 mL of contrast is injected at 3 to 5 mL/sec
and imaging commences following the circulation interval with
simultaneous recording of a digitized ECG.
Helical technique with 1-mm collimation and sub-millimeter
reconstruction increments produces a near isotropic voxel, with
unparalleled resolution in the z-axis, through which the coronary
arteries run. Using dedicated software and workstations, the
resulting 200 to 300 axial images can be reformatted for basic
interpretation in approximately 5 minutes. Curved multiplanar
formats, maximum intensity projections, volume rendering, and
surface shaded displays can be quickly generated to optimally
display coronary arterial morphology and spatial relationships
(figures 7 to 13).
Estimates for radiation dose for CT examination of the heart
vary. Electron beam CT coronary calcium examinations deliver 0.8 to
0.9 mSv, while EBCT coronary angiography results in 1.7 mSv.
Prospectively gated MDCT examinations for quantification of
coronary artery calcium provide doses of 0.6 to 3.2 mSv. Reported
doses from helical MDCT coronary angiography range from 2.4 to 8
mSv; Achenbach et al
report the effective radiation dose as 3.9 mSv in men and 5.8 mSv
in women. Effective radiation dose reflects tissue-specific
susceptibilities, and it is higher in women because they have
breast tissue within the field. By comparison, a routine thoracic
MDCT CT examination may result in a dose of 4 to 6 mSv, similar to
the mean 5.6 mSv dose reported for diagnostic coronary angiography.
Natural background radiation levels range 2 to 5 mSv per year.
The American Heart Association has adopted numeric segmental
nomenclature for the coronary arteries.
Using this nomenclature in the description of stenotic lesions aids
in communication with cardiologists and cardiovascular
Abnormalities of the coronary artery wall deserve particular
attention. A normal coronary artery wall is 0.1 mm thick and is not
readily apparent on CTA.
Atherosclerosis may include both calcified and noncalcified
components (figure 5). In fact, the most vulnerable plaques may
have little or no associated calcium, being composed of a thin
fibrous cap and a lipid-rich core. Such soft-tissue plaques may
produce only modest lumen stenosis, but their rupture can lead to
vessel thrombosis and occlusion, precipitating myocardial
infarction. Detection of soft-tissue plaque, which can otherwise be
evaluated only by intracoronary ultrasound, may prove valuable in
risk stratification and monitoring of drug therapies.
Finally, caution must be exercised when evaluating coronary
artery segments distal to sites of angioplasty or stent placement
(figure 14). Opacification of the arterial lumen distal to a site
of intervention may occur as the result of contrast passage through
patent lumen or contrast passage to the distal lumen via collateral
circulation. Time-attenuation curves, that compare peak enhancement
and contrast transit times within a distal coronary arterial
segment to controls, are routinely used in EBCT studies and should
be equally useful for exams obtained on multidetector scanners.
Patients suspected of stroke are often evaluated using a CT
stroke protocol consisting of noncontrast brain CT, CT brain
perfusion imaging, and CTA from the aortic arch to the circle of
Willis. Similarly, a CT chest pain protocol may soon become a
clinical reality. A noncontrast CT scan tailored for coronary
calcium quantification could be followed by ECG-gated coronary
angiography. Postprocessing steps enabling functional analysis have
been described, permitting assessment of regional wall motion and
left ventricular ejection fraction.
Multidetector CT scanners in current clinical use are routinely
capable of retrospectively gated angiography of the coronary
arteries for heart rates <70 bpm. Further studies are needed to
clarify the appropriate use of beta-blockers in patients with
faster heart rates. Bolus infusion of intravenous contrast material
is followed by multidetector scanning of the heart at 1-mm or
1.25-mm collimation. Helical data is collected in a single breath
hold with simultaneous digital recording of the ECG waveform. Data
reconstruction is performed retrospectively in sub-millimeter
increments with user-specified portions of the cardiac cycle chosen
for optimal visualization of coronary arteries.
CTA of the coronary arteries is emerging from a boutique
application restricted to academic referral centers to a powerful
clinical tool for the evaluation of ischemic heart disease. The
promise of sixteen or more detectors and further improvements in
temporal resolution of scanners will make the technique even more
robust. Attention to the requirements of the coronary artery CTA
protocols will produce high-quality noninvasive coronary artery
imaging and thereby crucial information to patients and referring
The author would like to acknowledge and thank Michael B.
Gotway, MD, Department of Radiology, University of California, San
Francisco, and Harold I. Litt, MD, PhD, Department of Radiology,
University of California, San Francisco, for their assistance in
the preparation of this manuscript.