is an Associate Professor of Radiology and Section Chief of
Cardiovascular Imaging, and
is an Assistant Professor of Radiology at Stanford University
School of Medicine, Palo Alto, CA.
Computed tomography (CT) is a simple and robust method for
evaluating the peripheral arterial system and for diagnosing
peripheral arterial disease. Advances in technology and variability
in patient physiology make contrast administration challenging,
however. This article will review patient preparation for CT
angiography (CTA) of the lower extremities, as well as optimal
scanner settings, contrast administration, and visualization
A multirow scanner is essential for CTA of the lower
extremities; however, the number of detector rows the scanner is
equipped with (4, 8, or 16) is of less importance. In fact, all of
the clinical studies of lower-extremity CTA reported in the
scientific literature to date have been conducted on 4row scanners,
with impressive results.
Several very practical details are important in ensuring the
success of lower-extremity CTA. It is recommended that tape be used
to keep the patient's knees and feet together. (Usually, at our
institution, we wrap a pillowcase or towel around the knees and
feet first.) Without tape, the patient's natural inclination is to
let the knees fall away from one another. If that happens, the
field-of-view in the reconstruction must be opened substantially in
order to include the proximal anterior tibial arteries, and
in-plane resolution is lost. The patient must not have a pillow
under the knees, however, as the arterial system will move up and
down relative to the table, which creates a similar problem for the
The patient should be positioned near the isocenter of the
scanner. Centering the patient provides the best in-plane and
through-plane resolution for visualizing what are frequently very
We use a 20-gauge antecubital intravenous line for contrast
delivery, and nonionic iodinated contrast material. The scanning
range for most studies is from the celiac artery through the toes,
a total of 105 to 130 cm, depending on the height of the patient.
The entire study takes about 20 minutes to perform.
Three-dimensional (3D) visualization is essential. Volume
renderings, maximum intensity projections (MIPs) and curved planar
reformation (CPRs) are all useful visualization methods.
There are a range of approaches to image acquisition, depending
on the detector configuration and the desired coverage. Full
anatomic coverage, from the celiac arteries through the toes (105
to 130 cm), is generally indicated for the evaluation of
atherosclerotic occlusive disease. More limited distal coverage (40
to 60 cm) is typically indicated prior to reconstructive surgery,
including fibular transfer grafts and pectoral flap mobilization
for revascularization of an area of osteomyelitis or tissue injury
secondary to trauma.
Figure 1 describes available detector configurations for full
anatomic coverage, using a 4-, 8-, or 16-row scanner. In each case,
only 1 configuration emerges as an acceptable choice. For example,
with a 4-channel scanner, a 2.5- to 3.0-mm collimation will
accomplish the study in 30 to 40 seconds, whereas a 1.0- to 1.5-mm
collimation will result in an unacceptably slow image acquisition.
With an 8-channel scanner, by comparison, a detector collimation of
2.5 to 3 mm is seldom used, because a higher-resolution alternative
is available (8 × 1.0 to 1.5 mm). With the 16-row scanner, a 0.5-
to 0.75-mm collimation results in excessive image noise,
particularly in the abdomen and pelvis. A 1.0- to 1.5-mm
collimation accomplishes the study in 15 to 20 seconds, with an
acceptable noise level.
Figure 3 shows a full-coverage acquisition from above the celiac
arteries through the feet, using a 4 × 2.5-mm detector collimation.
The patient had claudication, and femoral pulses were absent. The
scan reveals occlusion of the distal abdominal aorta, proximal
common iliac, and left superficial femoral arteries, with extensive
collateralization reconstituting the lower extremity arterial
The study took approximately 70 seconds to complete. With
2.5-mm-thick sections, the vessels are a little less distinct than
they are with thinner sections, but image quality is more than
adequate for making important observations on vessel patency and
stenosis, and for guiding routine therapy.
Figure 4 demonstrates a study of distal anatomy, acquired with a
4-row scanner and a 1.5-mm detector collimation. This patient had
osteomyelitis and was preparing to undergo surgery to revascularize
the distal calf. The MIPs depict the arterial system with the
intricate detail necessary for surgical planning.
The images in Figure 5 were acquired with 16-row scanner and a
0.625-mm detector collimation. In this patient with a tibial
plateau fracture, CTA was performed to determine whether the
popliteal artery had been injured. There is no occlusion and no
extravasation of contrast material. Such studies have become a
common application of CTA in our practice.
For all their advantages, fast scan acquisitions can complicate
contrast delivery. Table 1 outlines three protocols we have used
for peripheral runoff studies performed on 4-, 8-, and 16-row
scanners. A total of 89 patients are represented. (Unpublished
data.) There is an almost 4-fold reduction in image acquisition
time when comparing a 4-row scanner with a 16-row scanner.
Enhancement is not nec
essarily better with the 16-row scanner, however. Defined as the
average arterial attenuation from the aorta to the feet,
enhancement increases substantially when going from a 4-row to an
8-row scanner, because contrast delivery is much faster. Similarly,
contrast efficiency, defined as average enhancement divided by the
iodine load and normalized to the coverage distance, goes up by
Contrast efficiency improves further with the 16-row scanner.
Average attenuation drops, however, because there is not as much
time for the contrast bolus to fully develop and opacify the
vessels. Whether such differences in attenuation affect the
accuracy of diagnosis or clinical management decisions is yet to be
Certain aspects of contrast utilization are unique to imaging
the peripheral vasculature. A separate study, as yet unpublished,
by Dominik Fleischmann, MD, at our institution, examined peripheral
arterial enhancement in 20 patients with peripheral arterial
After injecting a small test-bolus, Fleischmann observed an
aortic contrast transit time of 14 to 28 seconds, with a mean of 20
seconds-standard findings for aortic CTA. To measure the contrast
transit time between the aorta and the popliteal artery, he
injected a second contrast bolus and documented its arrival in the
popliteal artery. The aortopopliteal transit time averaged 10
seconds, representing an average contrast flow rate of 65 mm/sec.
There was substantial variability among patients, however, with a
minimum of 4 seconds (177 mm/sec) and a maximum of 24 seconds (30
mm/sec)- almost a 4-fold difference. What is intriguing is that it
confirms angiographic experience, as there was no correlation
between contrast flow rate and clinical stage of disease.
Therefore, contrast transit times are not predictable by any means
other than direct measurement.
Table 2 presents the effect of contrast flow rates on scanning
parameters. With a slower scan (detector collimation: 4 × 2.5 mm, 8
× 1.25 mm, or 16 × 0.625 mm), table speed is 30 mm/sec. The time it
takes to scan from aorta to ankle, or a distance of about 1200 mm,
is 40 seconds. In a patient with a low blood flow rate (30 mm/sec),
contrast flows in perfect synchrony with the table speed. In a
patient with an average blood flow rate, however, contrast flow
from aorta to ankle outpaces the table speed. The contrast bolus
must be at least 22 seconds long to ensure that opacification is
still adequate as the foot is being scanned. In a patient with a
fast blood flow rate (177 mm/sec), the contrast bolus must be at
least 33 seconds in length.
With faster scans (detector collimation: 8 × 2.5 mm or 16 × 1.25
mm), the time it takes to scan from aorta to ankle is only 20
seconds. In a patient with a slow blood flow rate, the scanner can
outrun the contrast bolus, rather than lagging behind it. In some
patients, this has created a challenge when attempting to examine
vessels in the feet using a 16 × 1.25-mm detector collimation. In
such cases, it is necessary to wait 20 seconds after contrast
arrival in the aorta to begin the scan, so that the vessels of the
feet are opacified at the time of data acquisition. (The goal is to
image the tail of the bolus in the abdomen and the head of the
bolus in the feet).
Because there is substantial and unpredictable variability among
patients in lower-extremity flow rates, the best approach appears
to be the use of a long contrast bolus-30 to 35 seconds-in all
cases. In short, bolus duration is driven not by the speed of the
scan but by the need to ensure that the entire arterial system,
from aorta to feet, is opacified at the time of image
Determining the optimal time to begin image acquisition is more
complex than merely fitting the contrast bolus into the scan range,
however. It may not be desirable to image just as the bolus
arrives. The intensity of enhancement builds up over time, so the
distal end of the bolus is far more enhancing than it is at the
How the scan will be triggered is another important
consideration in determining how much contrast to deliver. There
are two fundamental ways to determine the delay from venous
injection of contrast material to enhancement of the abdominal
aorta. The first is to use a preliminary test bolus. With this
approach, the time to peak contrast enhancement directly measures
contrast medium transit time. Direct bolus triggering is the other
approach. Automated or visual detection of contrast arrival,
defined by a predetermined attenuation threshold, triggers
The length of the contrast bolus will depend on whether scan
timing is determined by a test bolus or bolus triggering. Assume
that a 30- to 35second injection is needed to ensure adequate
opacification of the entire periphery in the majority of patients.
That bolus size will be sufficient if scan timing is determined by
a preliminary test injection. A longer bolus will be necessary,
however, if scan timing is determined by bolus triggering, because
of the delay between the time of contrast arrival at the abdominal
aorta and the actual triggering of the scan. This delay varies from
one scanner to another, but it can be up to 8 seconds. If the delay
is 8 seconds, then a 38- to 43-second contrast bolus will be
With fast scanning, scanning should be delayed by 20 seconds,
even if a test bolus or bolus triggering is used. (It is not
necessary to add 8 seconds to the bolus length to account for the
delays associated with bolus triggering, however.) The benefit of
the 20-second delay is that imaging takes place later in the rising
contrast enhancement curve, which potentially results in more
homogeneous and intense opacification throughout the
A further complication of contrast delivery is that blood flow
in the legs may be asymmetric if, for example, there is a tight
stenosis or aneurysm on only one side. In such cases, use of an
even longer contrast bolus may be warranted.
Venous opacification may make it more difficult to evaluate the
arterial system in certain cases. Early venous opacification almost
always occurs in cases of ipsilateral inflammation, resulting from
cellulitis or ischemic ulceration. Both are common in patients with
peripheral vascular disease. Occasionally, venous opacification is
attributable to spontaneous arteriovenous shunting, which in
patients with atherosclerotic occlusive disease is likely due to
plaque rupture in small vessels.
Maximum-intensity projections provide a useful overview of the
peripheral vessels. To effectively use MIPs in the periphery,
however, bone must be edited out. This is a time-intensive
endeavor, although tools are becoming available to make it more
efficient. In addition, calcification can obscure underlying
Curved planar reformations are longitudinal sections along the
length of the vessel. A CPR enables visualization of both soft and
hard plaque and is especially useful in cases of circumferential
calcium, where it permits examination of the lumen adjacent to the
calcium, including regions of stenosis. In addition, no bone
editing is necessary.
A CPR does not provide as good an overview of the peripheral
vasculature as does the MIP, however. In addition, this technique
is insufficient unless window and level settings are optimized.
When evaluating calcification in peripheral arteries, for example,
it is important to use a bone window to avoiding "blooming" and
overestimation of calcification size. In very small vessels, such
as the peroneal artery, even a CPR may not be able to discriminate
arterial lumen from calcification.
CT has become a simple and robust method for assessing the
peripheral arterial system and diagnosing peripheral arterial
disease. Optimization of contrast administration is increasingly
challenging as scanner speeds increase. New ways to evaluate each
patient's circulatory physiology and compensate for it when
administering contrast material are needed.
Advances in scanner speed are unlikely to reduce the volume of
contrast used during examination of the peripheral vascular system
but will improve image resolution. Finally, additional detector
rows create additional data to analyze. We must continue to find
more effective and automated ways to manipulate and process such
large data sets.
LEO P. LAWLER, MD, FRCR:
Geoff, thank you; it was a very good talk. I agree that nothing
exposes our deficiency in terms of our understanding of contrast
dynamics as much as multidetector CT. It seems to me, however, that
as we go forward with peripheral vascular imaging, it will require
some way of knowing what the Hounsfield units are doing both in the
arteries and the veins as you scan, knowing what is happening both
proximally and distally. Then, somehow, you could feed that back to
the table pitch. That seems to me the only way forward to stop
venous contamination and to make sure asymmetric inflow disease is
imminent. What are your thoughts?
GEOFFREY D. RUBIN, MD:
With respect to venous contamination, I think we are in much better
shape with CT than with MR. It seems to me that venous
contamination typically only occurs in a pathologic setting in
which we have abnormal arteriovenous shunting. In many respects, I
like to be able to know that it is there and to see it. So I am not
sure that it is a big problem moving forward.
But what you have said about being able to somehow detect and
compensate for the variations in flow is very, very intriguing. The
question is, can we really do that on the fly? We know that what is
happening at the injector will influence local arterial opacity
sometime, perhaps 20 seconds, in the future at least; in some
cases, even more. I am not absolutely certain that there is a way
that we can monitor the progression of the injection and feedback
to have any effect on the scan. But, I do wonder if there are
routines that we might develop to preliminarily test a person's
circulation, to spot-check the aorta or the popliteal artery. Then
we could use that to come out with the appropriate strategy. But,
at this stage, that is a rather cumbersome thing to do.
Nevertheless, I think it should be our goal to develop an
algorithmic approach to at least understand these contrast
dynamics. If we do know what these aortopopliteal transit times
are, then we should understand the optimum way to deliver contrast
for that individual.
It is a very small area and it is highly selective. So, perhaps, it
is not a major issue for a lot of people. But I do think the
question really arises in cases such as in the posttraumatic leg
that is hyperemic or perhaps if they are going to fix an ulcer with
a free flap. This is reflected by the lack of literature below the
calf: the question is: Can I sacrifice this vessel to make a free
flap? I think you would be very courageous to make that decision
when you have venous contamination, and you cannot confidently say
that all three vessels running into the foot are clearly open.
It has been our experience that, even in the setting of venous
contamination, we always look at the transverse sections and we
believe we can visualize arterial anatomy in spite of venous
contamination. We have a paper submitted specifically addressing
those issues. The surgical plan was always in agreement with what
was demonstrated on imaging, and the outcomes of the operations
were viable at least 3 months down the line, which suggests that CT
does adequately provide the information.
However, in particular, in acutely traumatized patients in whom
there is massive soft-tissue injury, you can see remarkable venous
opacification, particularly in the upper extremities. I think there
we see more venous contamination than in the legs, and it really
just requires some diligence to examine the transverse sections and
mentally filter out the veins. As long as you are paging through,
it works pretty well. But you are right, the volume renderings can
be very confusing.
U. JOSEPH SCHOEPF, MD:
I believe the images that you show are much more impressive than
anything that we have available with MR these days, for example.
But where exactly do you see CT as opposed to MR in its indication
for doing that kind of study?
Right now it is pretty clear that the rate of voxel acquisition for
an equally sized voxel on CT is far outstripping MR. As I have
indicated, we are getting to the point at which CT may be getting
too fast and we have to wait for the bolus anyway. I think MR's
great hope right now is the successful development of parallel
imaging, which is going to require more widespread use of higher
field-strength magnets, like 3T. A high field-strength magnet will
compensate for the signal-to-noise limitations that come about from
parallel imaging while allowing for the improved spatial resolution
that is possible with parallel imaging.
So, who knows? If there is widespread deployment of 3T MR
scanners or maybe higher field-strength scanners and successful
implementation of parallel imaging, then maybe MR will give images
like this. But at this stage, you are right. We try to do
peripheral MRA as frequently as possible, but at this stage, CTA is
almost always a better quality study.
Also, I was really intrigued by the data that you showed on
hemodynamics in the lower extremity runoff studies. One of the
arguments that is brought against the use of CT, over digital
subtraction angiography, for example, is exactly that point, that
you are not able to visualize or assess a phenomenon, such as
arterial inflow, to determine whether the lesion within the lower
runoff is really hemodynamically significant. Do you believe that
some of those developments might come in handy to overcome those
In general, what our vascular surgeons like to repeatedly remind us
is that they treat patients, not scans; or they treat patients, not
angiograms. Therefore, they are going to perform bypass on a
patient based on that patient's symptoms. They know that if there
appears to be a high-grade stenosis downstream and there is a
high-grade stenosis upstream, they need to fix them both.
Otherwise, their graft is ultimately going to be less viable and
So these issues of using the contrast dynamics in the setting of
occlusive disease to lead to treatment have been less relevant for
us. There are situations, such as in arterial venous fistulae,
which obviously are very specialized. In those cases, the contrast
dynamics may be more useful. As we move ahead to 64-row CT
scanners, chances are, we will be able to park ourselves at the
nidus of an arteriovenous malformation and really ob-serve these
sorts of phenomena. But I do not think that comes up very
frequently at this stage.
ELLIOT K. FISHMAN, MD:
At least in my experience with peripheral imaging of both upper and
lower extremities, saline chasers have worked really well. I think
it really helps in terms of getting good homogeneous opacification,
especially of the lower extremities. Do you have any experience
Yes. We are using saline chasers on the dual-chamber injector.
Although, I must say that I do not have a really good intuition
that our opacification is any more homogeneous. Typically, what it
does is prolong the bolus, which gives us a little more usable
I am not sure that homogeneous opacification is necessarily what
we are after here in the periphery. In a sense, the proximal
vessels are big and the distal vessels are small and are more
susceptible to volume averaging. So, in some respects, I think it
is a good thing that the vessels get brighter as you go along,
because you have a harder time detecting small vessels than you do
big ones. I am sure that using a saline chaser is an advantage. It
is definitely an advantage to return pooled contrast in the veins
into the central circulation. I think we need more investigation as
to what ultimate impact it has on the quality of the CTA data
: Right, but I think it has made a very big difference in terms of
getting smaller vessels in, say, the legs. We have been using a 50
mL saline chaser. That seems to have made a major difference, at
least for us.
W. DENNIS FOLEY, MD:
Geoff, I was interested in your different transit times. You said
that it was unrelated to the patient's clinical symptomatology.
Yes, unrelated even to the severity of their disease.
BRIAN R. HERTS, MD:
Was it correlated with heart rate?
It was not correlated with anything that we measured.
You are filling the tube of the aorta with so much contrast, and it
is a big elastic artery that is so highly variable. So, before the
contrast even gets to the lower extremities, there is going to be a
massive variation. It will vary even if you use a constant because
of the rate that ends up feeding into the inflow vessel of the
iliac. Again, it is going around the whole lungs, as well, before
it comes to the left side. There are so many variables before you
even get there.
There are a tremendous number of variables in terms of the relative
vascular resistance of different major beds and what percentage of
the cardiac output is being directed to the legs or not. That is
: You probably have to determine the transit time from the aorta to
But, how are we going to do that? You notice that I made a big
assumption here, which is that it is a constant rate. Dominik
measured from the aorta to the popliteal and I just assumed that it
is the same rate down to the ankle. I think it is going to be very
difficult because if you just give a 16-mL bolus, you are not going
to be able to see that in those tiny vessels down there.
So I think it is probably not unreasonable to consider that the
aortopopliteal transit time is very closely linked to the
aortoanterior tibial or dorsalis pedis transit time. I think we are
going to have to begin operating on the assumption that measuring
aortopopliteal and not aorto-ankle arterial transit times probably
would get us pretty close and just see what limitations are
associated with those measurements.