Helical CTA offers a fast, accurate, noninvasive means to the complete diagnosis and preoperative evaluation of carotid artery stenosis without the need for catheter angiography. In this study, the authors describe its potential as a factor in the work-up of carotid artery disease.
The use of conventional angiography to diagnose and evaluate
carotid disease in patients with ischemic cerebral events has been
the gold standard for many years. However, the risks and costs of
this procedure have encouraged the development of faster, less
expensive, noninvasive modalities.1-6 Helical computed tomographic
angiography (CTA) is a relatively new modality which has shown
promise in providing an accurate and safe method of displaying
carotid artery stenosis. Advantages such as the elimination of
respiratory artifacts by volumetric data acquisition in a single
breath-hold, as well as the reconstructions of overlapping images
into high quality multiplanar and three-dimensional images have
made CTA an important modality in defining carotid artery
stenosis.7,8 CTA provides useful information on the degree and
extent of carotid artery stenosis, the presence of atherosclerotic
plaques, and the different plaque compositions.
Helical CTA has proved to match or surpass the utility of
conventional angiography in the diagnosis and preoperative
evaluation of carotid artery stenosis.
Materials and methods
Starting in early 1995, we evaluated 131 patients (262 carotid
bifurcations) with suspected carotid stenosis by helical CTA with
2D and 3D reconstructions using maximum intensity projection (MIP)
and multiplanar reconstruction (MPR) techniques. The patients'
medical records, helical CTA scans with reconstructions, and
angiograms (when available) were reviewed. Sixty-one of these
patients received selective conventional angiography after helical
CTA, but 104 carotid systems were examined because some patients
only had a unilateral conventional angiogram. Patient ages ranged
from 55 to 80 years (mean=70.2 years). There were 129 men and 2
women participating in the study. As part of the pre-operative
evaluation, patients were referred for carotid CTA because of
either clinical suspicion of carotid artery stenosis or a previous
carotid ultrasound suggesting carotid artery stenosis.
Helical CT was performed using a Picker PQ 2000 or Picker PQ
5000 (Picker Medical Systems, Cleveland, OH). The former machine
was used since 1995 and the latter since 1997. The data collection
protocols were the same in both systems. A 2-mm collimation
thickness with a 1-mm index and a pitch of 1.25 to 1.5 were used
for all patients. A 20-G or 22-G catheter system was placed
intravenously in the antecubital fossa. A mechanical power injector
(Angiomat CT Digital Injection System, Liebel-Flarshem Company,
Cincinnati, OH) was then hooked up to the intravenous catheter, and
a total of 120 to 150 cc of Isovue 370 nonionic contrast (Bracco
Diagnostics, Princeton, NJ) was injected at 2.5 to 3 cc per second.
After a delay of 10 to 12 seconds, the patient was scanned from C6
Measurement of carotid artery stenosis on CTA was performed
directly from the monitor after the scan. With the axial images,
the narrowest lumen in the stenotic segment was used to determine
the residual lumen. Then, using the plaques to determine the
boundary of the vessel walls, the original lumen size was
ascertained at the same location as the residual lumen. For
comparison with conventional angiogram measurements, the normal
distal lumen also was measured. Because axial images were being
used, the computer could measure the narrowed lumen either as areas
in square millimeters or as diameters in millimeters. With the
capability of measuring the vessel boundary at the narrowest point
and at a distal location, both the European Carotid Surgery Trial
(ECST) and the North American Symptomatic Carotid Endarterectomy
Trial (NASCET) criteria could be used to define the degree of
The axial raw data was then processed using a Picker Voxel Q
computer (Picker Medical Systems). First, the vessel of study was
marked in the 2D mode. Next, using the Angiomask mode, the unmarked
sites were eliminated. The reconstruction formats were then
performed with the MIP and MPR techniques.
The different plaque morphology was recorded while each carotid
system was being examined on CTA. Plaque morphology was sorted as
calcified, soft (including clot versus fat versus fibrosis), or
ulcerated. Furthermore, the length and shape of the plaque were
determined on CTA images.
Measurement of carotid artery stenosis on conventional
angiograms for those patients also who had undergone catheter
angiography was performed using a magnifying glass and a
submillimeter scale. The diameter of the residual lumen was
determined with two orthogonal planes. The original lumen size was
estimated in the same location as the residual lumen in the
catheter angiogram, as in ECST criteria. The normal distal lumen
was also used to determine the degree of stenosis, as in the NASCET
Each carotid system (table 1) was placed into a category based
upon the measured percentage of stenosis as follows: complete
occlusion (100% stenosis), critical stenosis (90-99%), severe
stenosis (70-89%), moderate stenosis (50-69%), minimal stenosis
(<50%), or normal.
In the preoperative evaluation of carotid artery stenosis,
helical CTA was demonstrated to resolve any question of stenosis
which may have been raised by ultrasound and, thus, could save a
patient from having to undergo catheter angiography (figure 1).
Helical CTA can display the site of carotid artery stenosis in
either area or diameter, using either the ECST or NASCET guidelines
(figure 2). In contrast, catheter angiography can only display the
diameter of the site of stenosis, and it requires two orthogonal
planes for a complete evaluation (figure 3).
The utility of helical CTA is further expanded with MIP and MPR.
MIP allows a 360° rotation of the carotid arteries in anatomic
projection after computerized reconstruction of the axial images.
MPR can provide even more spatial information (figure 4).
The important end result of preoperative evaluation of carotid
artery stenosis is confidence in the decision to manage the patient
medically versus surgically. Because carotid endarterectomy carries
inherent high risks, only patients in the preoperative evaluation
who have severe to critical stenosis will be candidates.
Additionally, plaque morphology plays an important role in whether
embolic events are a concern. Accurate demonstrations of the degree
of stenosis, site of stenosis, and composition of atherosclerotic
plaques are important to treatment decisions, and helical CTA has
the abilities to play a key role in these determinations (figure
During helical CTA scanning, volumetric data is acquired with
continuous tube rotation and table movement. This allows for
coverage of a large volume of the patient with thin sections in a
short amount of time. The collected three-dimensional data set
allows reconstruction of imaged vessels in any desired projection.
Because data acquisition is continuous, the carotid artery can be
imaged during optimal contrast enhancement. Data is also acquired
during a single breath-hold, making this technique less sensitive
to patient motion and breathing artifacts. Furthermore,
retrospective slice reconstructions can be performed at any
Helical CTA offers many advantages in the diagnosis and
preoperative planning of carotid artery stenosis. The range of
anatomic detail provided by helical CTA is broader than that of
catheter angiography. Helical CTA can detail the location, size,
and extent of the stenotic segment, as well as the atherosclerotic
plaque characteristics. With the axial cuts, helical CTA measures
stenosis using area and/or diameter measurements. Thus, it provides
a more accurate estimate of stenosis than that measured by catheter
angiography, which uses only a diameter measurement. We believe
that measurement of carotid artery stenosis using area (two
dimensional, but three dimensions are seen with reconstructions)
offers a better representation of the correct degree of stenosis
than using diameter only (single dimension), which may
underestimate the actual degree of stenosis. Helical CTA correlates
well with carotid endarterectomy results.
Furthermore, helical CTA has demonstrated utility for the
differentiation of atherosclerotic plaque morphology. Helical CTA
characterizes calcifications, soft plaques, and ulcers better than
a conventional angiogram can. Helical CTA easily identifies plaque
composition and shows a close to 100% correlation with tissue
samples; however, hemorrhagic plaque was an exception which did not
correlate well with the operative findings. Distinguishing between
the different plaque morphologies is helpful in planning medical
versus surgical management.10,11
The practical and theoretical advantages of helical CTA over
conventional angiography are multiple. Helical CTA is much less
invasive than angiography. It only needs a peripheral intravenous
line, as opposed to cannulation of the femoral artery with a large
bore catheter. This ameliorates the risk of groin hematoma,
retroperitoneal bleed, and traumatic injury to the femoral artery.
Patient safety and comfort issues are better satisfied with the use
of helical CTA over catheter angiography. In our experience,
examination of the carotid arteries using helical CTA usually takes
less than one minute to complete. Patients spend less time in the
radiology department and do not need to be preadmitted or to stay
in a recovery room after the helical CTA study. Not only does this
increase the patient's safety, but also his comfort and
The only requirement helical CTA imposes on the patient is a
single breath-hold. The radiologist's time requirement is reduced
to less than 5 minutes-the time it takes to read the study.
Additionally, the technician's time is reduced to approximately 20
minutes, and only one technician is required.
By using the combination of axial images, multiplanar
reconstruction, and 3-D reconstruction, helical CTA has proved to
be the study of choice in the evaluation of carotid stenosis.
However, it does have a few minor limitations and pitfalls: 1) the
need for good concentration of contrast in the vessels; 2) a short
field of view; 3) tortuous carotid arteries; 4) short stenotic
segments; and 5) satisfaction of various technical factors
including motion, cardiac output, and anatomic position of the
The results of this study suggest that helical CTA offers
complete diagnosis and preoperative evaluation of carotid artery
stenosis without the need for catheter angiography in most
patients. Taken with the results of ultrasound and/or MRA studies,
helical CTA is a necessary factor in the work-up of carotid artery
disease because it can provide an accurate measurement of the area
of stenosis and can identify the different plaque morphology before
carotid endarterectomy. Catheter angiography should be reserved for
patients who need further evaluation and possible percutaneous
transluminal angioplasty and/or stenting of the stenotic region.
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artery bifurcation: evaluation with helical CT. Radiology
2. Dillon EH, van Leeuwen MS, Fernandez MA, et al: CT
angiography: Application to the evaluation of carotid artery
stenosis. Radiology 189:211-219, 1993.
3. Cumming MJ, Morrow IM: Carotid artery stenosis: A prospective
comparison of CT angiography and conventional angiography. AJR
4. Castillo M, Wilson JD: CT angiography of the common carotid
artery bifurcation: Comparison between two techniques and
conventional angiography. Neuroradiology 36:602-604, 1994.
5. Castillo M: Diagnosis of disease of the common carotid artery
bifurcation: CT angiography vs catheter angiography. AJR
6. Leclerc X, Godefroy O, Pruvo JP, et al: Computed tomographic
angiography for the evaluation of carotid artery stenosis. Stroke
7. Napel S, Marks MP, Rubin GD, et al: CT angiography with
helical CT and maximum intensity projection. Radiology 185:607-610,
8. Marks MP, Napel S, Jordan JE, et al: Diagnosis of carotid
artery disease: Preliminary experience with
maximum-intensity-projection helical CT angiography. AJR
9. Fox AJ: How to measure carotid stenosis. Radiology
10. Seeger JM, Barratt E, Lawson GA, et al: The relationship
between carotid plaque composition, plaque morphology, and
neurologic symptoms. J Surg Res 58:330-336, 1995.
11. Hayward JK, Davies AH, Lamont PM: Carotid plaque morphology:
a review. Eur J Vasc Endovasc Surg 9:368-374, 1995.
Dr. Lee is in the Department of Radiological Sciences at the
University of California, Irvine, Medical Center in Orange, CA. Mr.
Boos, Dr. Nguyen, and Dr. Yu are in the Department of Radiology at
the Long Beach Veterans Affairs Medical Center in Long Beach, CA,
where Dr. Pham is Chief of the Ultrasound section.