Applied Radiology

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 to C1.

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 stenosis.9

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 criteria.9

Results

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 5).

Discussion

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 level.

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 satisfaction.

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 carotid bifurcation.

Conclusion

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. AR

References

1. Schwartz RB, Jones KM, Chernoff DM, et al: Common carotid artery bifurcation: evaluation with helical CT. Radiology 185:513-519, 1992.

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 163:517-523, 1994.

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 161:395-398, 1993.

6. Leclerc X, Godefroy O, Pruvo JP, et al: Computed tomographic angiography for the evaluation of carotid artery stenosis. Stroke 26(9):1577-1581, 1995.

7. Napel S, Marks MP, Rubin GD, et al: CT angiography with helical CT and maximum intensity projection. Radiology 185:607-610, 1992.

8. Marks MP, Napel S, Jordan JE, et al: Diagnosis of carotid artery disease: Preliminary experience with maximum-intensity-projection helical CT angiography. AJR 160:1267-1271, 1993.

9. Fox AJ: How to measure carotid stenosis. Radiology 186:316-318, 1993.

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.