is an Assistant Professor, Department of Radiology, Johns Hopkins
Medical Institutions, Baltimore, MD
With its superb scan coverage and image resolution,
multidetector computed tomography (MDCT) has firmly established its
role in noninvasive angiography. Coronary artery imaging pushes
even today's advanced technology to its limits, however, and
continues to drive research and development. The demands that
coronary CT angiography (CTA) places on multidetector-row
technology in terms of contrast resolution, spatial resolution
(approximately 1-mm isotropic voxels), and temporal resolution
(approximately 100 msec) far exceed those of other applications. In
addition, apart from peripheral vascular imaging, coronary CTA is
perhaps the only application for which three-dimensional (3D)
imaging is mandatory at this time.
This article will review acquisition parameters for MDCT of the
coronary arteries, techniques for contrast administration, the role
of dual-head injection, image postprocessing, and current clinical
Table 1 describes a typical multidetector coronary CTA protocol.
Collimation is as narrow as possible (0.675 mm or 0.75 mm,
depending on the scanner). The effective slice width is ≤1 mm, and
the reconstruction interval is 0.6 mm. The table speed is slow, 2.8
mm per rotation, to enable the oversampling that is required for
coronary CTA. The gantry rotation is very fast, 0.42 seconds.
Segmental reconstruction is used to achieve a temporal resolution
of 100 to 200 msec.
With an mA of 500 and a kVp of 120, radiation exposure during
coronary CTA is relatively high, 42 mGy, or 7 to 10 mSv. The
protocol involves retrospective gating, and the scan time is
short-15 to 20 seconds.
In the past, long scan acquisition times were matched to longer
contrast delivery times. Today, scan acquisition times have been
halved and the demands for fast contrast delivery and bolus capture
are much greater. At the same time, the goals of coronary CTA are
changing. Detection of stenosis is only one objective. Other goals
include evaluation of the arterial wall, characterization of
atherosclerotic plaque, and analysis of cardiac function using
cardiac chamber endocardial opacification.
After intravenous injection, contrast material travels to the
right side of the heart, through the lungs, and into the left side
of the heart. It then fiows through the systemic circulation. After
the contrast bolus is injected, it increases in length and splits,
and portions of the bolus remain in the right side of the heart.
Another challenge is the "dead space" of contrast in arm veins, the
upper thorax, and even the contrast injector tubing-perhaps a total
of 20 to 30 mL.
The final intensity of contrast enhancement depends on many
factors. These are related to patient characteristics (body size;
circulation, including cardiac output and circulating volume; and
renal function), contrast (volume, injection rate, iodine
concentration, and whether a saline chaser is used), and technique
(access and route of administration, scan delay, scan speed, and
Only a few of these factors are of practical importance in
infiuencing routine coronary CTA. This is largely a first-pass
technique that does not depend on the fiow of contrast through the
systemic circulation. At the scanner, there is no way to predict
reliably or to quantify the effect of cardiac function and other
patient factors on the contrast peak in the left ventricle.
Similarly, contrast factors (such as concentration, osmolality, and
ionicity) remain largely consistent in routine practice. The
injection technique is individualized to the study, however,
including the timing of the contrast injection, the timing of image
acquisition, and, perhaps, the mode of injection.
In coronary CTA, a large enough volume of contrast material must
be administered at a sufficiently high rate to reach and maintain a
homogenously high concentration throughout a 15-second data
acquisition, while covering z-axis scan distance of 20 cm (the left
ventricle and outfiow tract).
At the time of image acquisition, the greatest contrast
enhancement (200 to 300 HU) should be in the region of the left
ventricle, the ascending aorta, and, therefore, the coronary
arteries (Figure 1). It is important that contrast density values
are sufficient to facilitate the segmentation techniques used in
The contrast material used in coronary CTA generally has an
iodine concentration of 300 to 350 mg/mL. Most institutions use a
low-osmo-lar, nonionic contrast agent, although some use an
isosmolar preparation, particularly in select groups of patients at
high risk for nephrotoxicity.
Given the limitations of mechanical CT, it is preferable to keep
the patient's heart rate as close to 60 bpm as possible. The use of
beta blockers to achieve this goal will have the effect of
increasing peak maximum enhancement by 10% and increasing time to
peak maximum enhancement by 4 seconds. This effect may be offset,
in part, by the positive chronotropic effects of contrast
Timing is critical in coronary CTA, which necessitates
synchronization of data acquisition with both the
electrocardiographic (ECG) tracing and contrast delivery. Images
are acquired throughout systole and diastole, and the ECG is used
to guide retrospective reconstruction.
Coronary artery enhancement is characterized by a rapid rise to
a peak enhancement of fairly short duration, followed by a rapid
fall-off (Figure 2). The optimal scan delay to coincide with peak
enhancement can be determined through use of a test contrast bolus,
a tracking bolus, or empiric observation.
The first two techniques have their advocates, but studies
examining their success in achieving a more consistent and
reproducible level of contrast enhancement have reported variable
They do, however, add another step in the protocol, and there are,
as yet, no studies of their particular application in coronary
At our institution, we administer approximately 100 mL of
nonionic, isosmolar contrast material with an iodine concentration
of 320 mg I/mL. We inject at a constant unimodal rate of 3.5 to 4
mL/sec. Recently, we have been experimenting with the use of a
25-mL saline chaser delivered through a dual-head injector. We use
an empiric scan delay of 25 to 28 seconds.
Figure 3 illustrates the consequences of poor bolus geometry and
a less than optimal scan delay. In this case, contrast material has
not yet reached the aorta; instead, it is primarily in the
pulmonary artery. The ability to visualize coronary vessels is
limited, as is the ability to visualize the lumen and arterial
Beam hardening artifact in the superior vena cava (SVC) can also
reduce visualization (Figure 4). The need to overcome this problem
has spurred interest in dual-head injection, whereby contrast
injection is followed by a saline chaser.
Two methods of administering the saline chaser have been
The layering technique involves partially filling the injector
syringe with contrast material, followed by a layer of water. Upon
inversion of the syringe, differences in specific gravity cause the
water to move to the top of the syringe and the contrast material
to the bottom. Therefore, a single injection will deliver both
contrast material and a saline chaser.
An alternative method is to invest in a dedicated dual-head
injector with two separate syringes, one for contrast material and
one for saline. After delivering contrast, the injector
automatically releases the saline chaser.
There is no method to incorporate the saline chaser into pretest
timing techniques; however, optimal scan timing is probably
dictated more by the delivery of contrast itself.
The theoretical advantage of using a saline chaser is the
opportunity it presents to recapture a part of the contrast bolus
that otherwise would be wasted in the venous dead space. By
increasing the circulating volume of contrast, the saline chaser
may increase peak enhancement in the left heart, but there are as
yet no convincing data supporting this possibility.
In addition, use of a saline chaser "tightens" the contrast
bolus, preventing contrast material from lingering in the SVC and
the right side of the heart. The end result should be improvement
in left ventricular contrast enhancement and a reduction in
Some data suggest that use of a saline chaser enables a reduction
in contrast volume while producing an equivalent bolus geometry in
the aorta and the coronaries and reduced contrast enhancement in
the right side of the heart.
To date, experience with the saline chaser has been mixed. At
our institution, the saline chaser has not consistently improved
left-heart opacification. It is not clear that applying a constant
20-mL saline chaser to a technique that contains many
unquantifiable variables (including the heart function and
pulmonary circulation) will produce a more consistent outcome in
In addition, not everyone finds beam hardening of the SVC to be
a substantial problem. Although the right coronary artery courses
along the atri-al-ventricular groove, beam hardening most often
occurs in the dependent portion of the right atrium, away from the
Even with excellent contrast delivery and image acquisition
techniques, certain challenges must be overcome during
postprocessing. Layering artifact, for example, is often observed
in images created through multiplanar postprocessing and, to some
extent, volume rendering. This artifact, which appears as lines
running through the image of the heart, refiects the cobbling
together of retrospectively selected images from different cardiac
cycles and from different points in a single cardiac cycle. By
definition, each block of data represents a different time point in
the geometry of the contrast bolus. Layering artifact can be
differentiated from motion artifact by looking closely at the edges
of the vessels and observing no stepoff.
Opacification of the coronary arteries creates another
challenge. The better the opacification, the more difficult it is
to differentiate the vessel lumen from calcification or stents
during postprocessing (Figure 5). This problem can be overcome
through the use of a maximum-intensity projection (MIP) and bone
The primary clinical application of coronary CTA is the
examination of native coronary vessels (Figure 6). This includes
evaluation of an aberrant coronary artery origin or course,
detection and measurement of stenosis, plaque characterization, and
evaluation of stent patency. Coronary CTA readily detects stenosis
and obstruction and, therefore, has a high negative predictive
value (>90%) for significant coronary artery disease. It is
often used in patients with a low pretest probability of disease,
although more complex patients are now referred for coronary CTA as
well. It is also useful for the evaluation of bypass grafts.
Cardiac functional analysis is of growing importance, and useful
information is contained within the acquired CTA data set. This
application requires good opacification of the ventricle, rather
than the coronary arteries. The goal is to define the endocardial
surface, which enables calculation of wall thickness and chamber
volumes in systole and diastole and, therefore, functional
indicators, such as ejection fraction (Figure 7). We supply this
information to the referring physician, along with digital or print
images and, occasionally, video demonstrating functional changes
throughout the cardiac cycle.
Optimal coronary CTA requires synchronization of contrast
administration and data acquisition. In the future, much greater
rigor will be required for this task to improve test
reproducibility and intra- and inter-patient consistency.
Improvements will likely include the development of an index of
normal and abnormal contrast circulation times, the ability during
bolus tracking to sample at multiple sites (rather than a single
point), and calcification subtraction techniques. Advances in
scanner technology, including an increase in the number of
detectors, will increase scan speed and reduce contrast volumes,
but may also increase the chance of contrast timing error.
U. JOSEPH SCHOEPF, MD:
Can you give us an idea of the scope of a daily clinical routine to
use cardiac imaging these days in your institution? What tests are
being requested by the referring physician? What do you do? Where
do you feel that you are on the safe side in making a definitive
diagnosis? What are the tests that you are actually getting
LEO P. LAWLER, MD, FRCR:
In terms of what we are doing, I think I would divide it into three
categories. The first category is those areas in which coronary CTA
is clearly superior to coronary angiography. We are finding that
the aberrant origin question is clearly an area in which we find a
great demand, application utility, and great acceptance. Clearly,
that is not a very common condition; however, it is a common
question. When they do not have confidence they are seeing the
vessels very well, that has become a question for us.
The next area is the established areas of coronary angiography,
and we are making inroads in that. We are finding that physicians
are referring those patients in whom they have a low pretest
probability and they are really working on our high negative
predictive value. We are also starting to get the more difficult
cases, and I illustrated some of those, such as the vessels that
are not particularly normal, and the patients who had a high enough
pretest probability. We are starting to see that now and I think
that the word on the street is that there is clear interest among
the cardiology community to find earlier disease, to find
nonstenotic plaque, and to start applying the stents that elute
The other area is new techniques where we are not just trying to
reinvent the wheel and do coronary angiography. We are trying to
sell a package to people in which we will give them the angiography
information, but we will give them an awful lot more. Of course, we
have bias just because of our institution, but the whole area of
mapping of the wall of the heart and its functional changes
throughout systole and diastole is very important to a lot of
people. This refiects a new perspective of cardiac function in
which people are saying that the geometry of the heart is the issue
and restoring geometry with a particular reconstruction is
better--that is the big issue for people. So those are the three
biggest areas for us.
In terms of reimbursement issues, it does not have a code at
this point in time. Basically, we do what others are doing; either
we do them under a research protocol, together with our cardiology
and surgical colleagues. Or, others have been attempting to apply
other thoracic angiography codes to a cardiac CT study as well. I
think these are the ways forward. We do not yet have the weight of
data yet to be able to get it reimbursed as a separate study.
W. DENNIS FOLEY, MD:
Most people who are illustrating cardiac CT and coronary artery CT
do not put much emphasis on the diagonal branches or the other
branch vessels in the coronary arteries. Do you have experience
with that in terms of your ability to visualize those vessels and
That is a very good question and I actually just started looking at
that. I started looking at it more because there is an American
Heart Association publication for angiographers that addresses what
they demand of a study and so forth. There is no question; we have
to not only do what they do, but perhaps even do it better to some
extent. Simply saying that the main vessels are opacified is not
enough. You need to see the diagonals, and so forth.
Looking at individual cases, I have found in patients with
moderate stenosis or fairly normal vessels that you can
consistently see diagonals, you can see marginals, and you can even
see small septal branches. You can say whether they are open or
not, but you cannot give greater detail than that. I think 2-mm
vessel size is our limit right now. Maybe the higher resolution
will change that. I am not sure, but maybe it will.
The other thing one must bear in mind is that we often assume
that the gold standard of coronary angiography, like a lot of gold
standards, is not really a gold standard. I mean, it certainly was
the standard of care. But if you look at it, the reports and the
way they quantify disease are fairly binary. I think most of the
AHA guidelines on what they consider a good report include that you
say it is >50% or >70% stenosis. It does not come down to
saying it is exactly a 63% stenosis. I do think we are able to say
that it is >50%, perhaps, but I do not think we can give those
specific numbers yet. But I also think that we will probably get
better absolute numbers ultimately than coronary angiography
probably will be able to, because we give the wall information as
Some authorities have stated that if a coronary calcium score is
high, they will not do intravenous coronary angiog-raphy. Do you
have any comments?
That is another very good point because it is a very practical
point. What we have done, and what a number of people have said, is
that maybe these patients should all have a calcium score as part
of their workup, though obviously it has radiation implications.
But it may obviate the need for CTA, which is a very high-dose
technique. It is much higher than conventional angiography.
In my opinion, if the person has a very high calcification
burden, I am very reluctant to perform coronary angiography because
I do not believe I can tell them the type of information they want
in a comprehensive study. However, if they say that they do not
care about the smaller vessels and they simply want to know does he
have a "widow-maker" lesion, a left main lesion, or a right
coronary lesion, etc.--we are superb at that. We can do that even
if there is a lot of calcification because it is still possible to
image those rather large origins and we can still give the
information, such as opacification of the vessels, the dominance of
the systems, etc. But as a simple answer, I think if there is a
heavy burden of calcification, it is possibly not prudent or in the
patient's best interest to do that test; they will most likely need
a test of greater specificity and sensitivity.
I am curious about your opinion on the role of 3D postprocessing in
the evaluation of CT coronary angiography. There are reports that
with use of 3D there is a greater sensitivity for the detection of
lesions that can be achieved with the individual sections plus some
dedicated MIP of the individual coronary arteries alone. I believe
that one of the reasons why this has not taken off in the community
so far is simply because the evaluation of those data sets is so
cumbersome. That deters some of the cardiologists or radiologists
in private practice who are trying to do that. Is 3D visualization
or 3D postprocessing going to make it a lot easier? If so, to what
I do think it is very important; in fact, I think it is simply
mandatory to do 3D. You simply cannot do coronary angiography just
looking at axial images, and you cannot do it looking at
multiplanar images. So you have got to do 3D. That is just a
There is no doubt, in my opinion at least, that in doing the 3D
processing that the MIPs are the best in terms of stenosis
evaluation. The volume-rendered image is better for the
characterization of disease looking at plaque, and so forth. It is
better for the relative changes along the course of the vessels,
and they certainly seem to be the images that the referring doctors
like for the record. But for quantification and really giving the
kind of detail a lot of people want, it really needs to be MIP.
Together with that, there has to be greater automated
Most radiologists in practice, particularly in busy private or
academic practices, want to do coronary angiography. Therefore, by
definition, they want to do more work. This is a case in which you
have to be careful what you wish for, because if you do it well and
it takes off, you will be doing a lot of cases. It is a very
prevalent disease, of course. You have to accept at the outset that
if you are going to do that, that it will require more time and to
do it you are going to have to invest in it. You cannot simply do
it as a thoracic aortic examination and just add it on. It is going
to require more time. But, as was shown at RSNA this year, all the
vendors seem to be moving toward having systems where you really
have minimal segmentation. You just go straight to the vessels you
want to look at. You are getting rid of the whole chest wall, even
getting rid of the heart in some of the software programs. That
clearly has to happen, because it is very labor-intensive.
BRIAN R. HERTS, MD:
How long does it take to read one of these studies?
I think to do it well, just to the coronaries, it takes at least 30
minutes. For this, I am just talking about reviewing the vessels,
and it takes a good 30 minutes. You have to go between the axials,
then go to the MIPs. You have to check the volume rendering
sometimes. You have to have an interface where you can go do that
fairly quickly, double check, triple check, and go back and forth,
going from the most faithful to the original data, which is
obviously the cleaner data. Then maybe you have to go back to the
MIPs to get an overall sense of stenosis and then prove you are
right. You must be sure you are not getting some false-positive
readings because of thresholding or something by checking some of
the other techniques. I would say it takes a good 30 minutes to do
that and the report. A diligent report takes at least 10 minutes to
do. If you really want to do it well and go through left main and
so on, it takes a lot of time.
GEOFFREY D. RUBIN, MD:
Do you use beta-blockers in any of your patients?
We try to, yes. It depends on the patient population again, but
generally, we want the heart rate at approximately 60 bpm.
So you are using oral beta-blockers? How effective are they?
Yes, oral beta-blockers. They are reasonably effective. Often, we
do not know how fast the heart rate was before we saw the patient,
so I am not sure how much it really diminished the heart rate.
You were routinely giving beta-blockers before patients came in,
regardless of their heart rate?
Right. Everyone will have to figure out their own procedures
locally, of course. We set up our plans with one of our referring
doctors who actually is a cardiologist. When he orders the study,
he writes the prescription for the beta-blocker and gives an oral
beta-blocker ahead of time. He is also willing to come on site and
give an intramuscular injection, if necessary.
But I think that beta-blockers are still required because most
of us do not have a heart rate that is suitable for mechanical
What is your dose?
The standard dose they give is 20 mg of labetalol.
If a patient comes in with a heart rate of 80 bpm, do you cancel
I would personally cancel it because I think it is worse to go
ahead with a heart rate like that. Because again, if all they want,
for example, is to know about the left main artery, you can just do
a simple CT for that. You do not even need a coronary CTA; you can
just do an ordinary aorta protocol. If they truly want a coronary
CTA and the patient comes in with a heart rate of 80 bpm, you are
not going to get an interpretable study. The radiation dose ≥7 mSV
is just not justified.
Do you think the heart rate restrictions are the same for bypass
Probably not, because they are relatively motionless. In fact, in
certain cases, I find that the left coronary arteries are fairly
forgiving, even though the left circumfiex is in the AV groove. It
is the right coronary that will hurt you every time and, obviously,
a lot of people are right dominant. Let me just add to that, that
the patient population is very important. A lot of the literature
is saying right now that we should be using this in people who have
relatively lower pretest probability. If they come in with acute
chest pain, you probably should not be doing CTA in the first place
because they may need intervention at the same time.
Do you scan patients with atrial fibrillation?
We have, and we have gotten away with it on a few cases. Again,
arrhythmias are a big problem with any kind of retrospective
gating, as you know. The editing tool of the EKG can save the study
to some extent. But if it is a case in which your data acquisition
for the two R waves got beat all over the place, when you try to do
too much editing you will actually lose so much data the images are
So to summarize, cases of arrhythmia, fast heart rate, and a
heavy burden of calcification are those in which we are not ready
for CTA. These areas show that we are not there yet and we have to
be more selective and have close attention to protocols.
One question concerns MR. You specifically referred to your third
tier of CT applications as being functional. Sometimes I wonder if
we may push what we try to squeeze out of the data a little too far
because of the recognition that our temporal resolution for aspects
such as wall motion and relative wall motion does not even come
close to the capabilities of what we can do with MR.
So I wonder if you think that it is wise to engage upon a clinical
cardiac imaging program that does not somehow integrate the
benefits of MR and CT. Do you think that we can use CT to fully
characterize ventricular function, for example, or even looking at
myocardial ischemia and the issues of hibernating myocardium versus
truly infarcted myocardium? Can you comment upon the relative roles
of MR and CT?
I can give you my own intuition and what I think of the literature.
I think you are absolutely right. Most people are saying right now
to use CT for the coronary angiography and use MR for the perfusion
data and certainly perhaps some of the other functional data. I
generally agree with that.
Although you may have also seen in the February 2004 issue of
that they published a study of a functional analysis looking at CT
functional imaging compared with MR.* The numbers for CT are very
good in terms of ejection fractions and so forth. I think that if
that is the case, and you are exposing these people to a fair dose
of radiation, if it turns out that the ejection fraction
measurements and the wall-thinning and the wall-thickness changes
are reasonably good, it does strengthen the role of CT. If people
still want to know perfusion imaging, that is clearly another
question, and I cannot see how CT is ever going to do that.
But I think the best way forward for a private practice or an
academic institution to do this and to do it well is to form a
single cardiovascular unit that uses all the MR data, CT data,
SPECT data, and, perhaps, PET/CT data. That is the only way to
harness all of our individual potentials and fully realize the
anatomic and functional imaging that will, no doubt, be the
standard of care for the future. There is no doubt about that.
*Juergens KU, Grude M, Maintz D, et al. Multi-detector row CT of
left ventricular function with dedicated analysis software versus
MR imaging: Initial experience. Radiology. 2004;230:403-410.