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(Circulation. 1998;98:984-989.)
© 1998 American Heart Association, Inc.

Clinical Investigation and Reports

Spiral Computed Tomography

A Novel Diagnostic Approach for Investigation of the Extracranial Cerebral Arteries and Its Complementary Role in Duplex Ultrasonography

Roberto Corti, MD; Claudio Ferrari, MD; Marzio Roberti, MD; Mario Alerci, MD; Pier Luigi Pedrazzi, MD; ; Augusto Gallino, MD

From the Division of Cardiology and Vascular Medicine (R.C., C.F., M.R., A.G.), Radiology (M.A.), and Neurology (P.L.P.), Ospedale San Giovanni, (E.O.C.) Bellinzona, Switzerland.

Correspondence to Prof Dr Med Gallino Augusto, Chief of the Division of Cardiology and Vascular Medicine, Ospedale San Giovanni, (E.O.C.) 6500 Bellinzona, Switzerland. E-mail osgin3{at}tinet.ch

	Abstract

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Background—For the detection of atherosclerotic lesions of the extracranial cerebral arteries, duplex ultrasonography (US) is an established operator-dependent method, whereas arteriography is associated with the not-insignificant risk of embolic complications. Spiral CT is a promising novel diagnostic tool that allows noninvasive, operator-independent diagnosis of obstruction of extracranial cerebral arteries. The aim of our prospective study was to evaluate in a clinical setting the complementary role of duplex US and spiral CT.

Methods and Results—We compared the results obtained independently by spiral CT and duplex US in 59 consecutive patients with clinical suspicion of an obstructive lesion affecting the carotid arteries. We analyzed a total of 354 segments from the extracranial carotid arteries, including the common, internal, and external carotid arteries. A total of 4 complete occlusions, 38 severe stenoses (70% to 99%), and 32 moderate stenoses (30% to 69%) were concordantly identified by means of duplex US and spiral CT. In 5 cases in which duplex US did not allow sufficient evaluation of the carotid artery because of a poor US window or severe calcification, spiral CT allowed identification and correct measurement of the stenotic lesion. The comparison of the percentage of stenosis with both methods was good (r=0.91, P=0.024).

Conclusions—Our results indicate that spiral CT of the extracranial cerebral arteries is a promising noninvasive complementary and non–operator-dependent examination. Its application is particularly attractive in cases in which duplex US is not reliable (ie, severe kinking, severe calcification, short neck, and high bifurcation) and particularly when an overall view of the vascular field is required.

Key Words: angiography • arteries • imaging • stenosis • tomography

	Introduction

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Cerebrovascular diseases are among the most frequent causes of mortality and morbidity in the elderly and are a frequent concomitant pathology in patients with cardiovascular problems (ie, coronary artery disease). Single or multiple segmental atherosclerotic lesions of the extracranial cerebral arteries account for ischemic stroke syndromes in over half of all cases.^{1 2} The most frequently involved extracranial vessels are the carotid bifurcation, including the origins of the internal (ICA) and external (ECA) carotid arteries (90%), origin of the vertebral arteries, and aortic arch with its proximal branches.³ Stenoses of the carotid arteries are responsible for many transient ischemic attacks and may precede a complete stroke in half of these patients.^{1 2} The risk of stroke associated with symptomatic carotid artery disease is related to the severity of the carotid artery stenosis.^{1 2 4}

Patients with symptomatic carotid artery disease may benefit from carotid endarterectomy when stenosis is between 70% and 99% with a relative risk reduction of stroke of 65% in patients undergoing surgery compared with patients treated medically.^{4 5 6} There is a growing consensus related to the indication for endarterectomy in selected patients with asymptomatic carotid artery stenosis of severity >60%.⁷ Therefore, the detection of stenotic lesions and determination of their severity are essential for the prevention of strokes. The determination of the degree of stenosis has, until recently, been the domain of selective angiography. Although the image quality resulting from this invasive technique is almost always optimal, the complication rate is not insignificant. The overall incidence of complications described in the literature ranges from 0.1% to 10%, the risk of neurological complications (transient ischemic attacks or stroke) is about 4%, and a permanent neurological deficit (disabling stroke) occurs in about 1% of the patients.⁸ Furthermore, angiography does not allow evaluation of the vessel wall and plaque composition,⁹ causes discomfort, and is expensive. Duplex ultrasonography (US) has become the investigation of first choice because it provides detailed information on the localization and degree of stenosis, flow dynamics, and condition of vessel wall. It is, however, a highly operator-dependent investigational method and does not always yield good-quality images (ie, in patients with short neck, high carotid bifurcation, or highly calcified lesions).¹⁰ Duplex US also fails to provide a plastic overview of the surgical field that the vascular surgeon often needs to plan surgical procedures, which explains why preoperative angiography is still required in many cases.⁸

Spiral CT, consisting of the simultaneous radiographic source rotation and movement of the patient during data acquisition, represents, together with magnetic resonance (MR) angiography, 1 of the latest technologically advanced imaging techniques. This technique, introduced in 1989¹¹ and further improved by the technological advances of the last 2 to 3 years, seems to be a promising tool for the investigation of the carotid vessels.

In several studies, good correlation was obtained between spiral CT and conventional angiography (r=0.89 to 1.0, P<0.001),^{12 13 14 15 16 17 18 19} with a discrimination rate of 100% between severe stenosis and occlusion.^{12 13 14 15 16 17 18 19} The correlation seems to be even higher for severely stenotic lesions.¹² Spiral CT seems to be particularly useful and relatively noninvasive for those patients in whom the results of duplex US are hampered by anatomical causes (short neck or high carotid bifurcation) or by the presence of heavily calcified lesions.

The aim of our prospective study was to evaluate in a clinical setting the complementary role of color-coded duplex US and spiral CT, ie, the functional and morphological assessment obtained by duplex US and the overview of the vascular field obtained by spiral CT.

	Methods

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A total of 59 consecutive patients, referred to our clinic because of clinical suspicion of symptomatic carotid artery stenosis, were prospectively and independently evaluated with duplex US and spiral CT. The characteristics of patients and symptoms giving rise to the clinical suspicion of an obstructive lesion of the carotid arteries are described in Table 1.

View this table: [in this window] [in a new window]   Table 1. Patient Characteristics and Clinical Presentation

All patients were studied by 2 independent observers using duplex US following a standardized technique, performed with an Acuson Inc 128 XP 10c computer sonography system with 5- or 7.5-MHz transducers. Transversal and longitudinal scanning of the common carotid artery (CCA), ECA, and ICA was performed, and the spectrum of the Doppler signal pulse frequencies was evaluated, centering on the sample volume in the central zone of the residual carotid lumen. Duplex US determination of the degree of stenosis was based on previously published criteria (ie, peak systolic velocity criteria^{20 21 22 23 24 25 26} and, whenever possible, determination of the stenosis lumen by B-mode ultrasonography¹⁰). B-mode sonograms were used to analyze stenoses and plaque morphology. Color-coded blood flow was superimposed, and the velocity measurements of the Doppler signals were recorded at peak systole. Diagnosis of complete occlusion of the carotid artery was based on the absence of spectral flow and the color Doppler signal. If occlusion of the vessel was suspected, the sonography instrument was calibrated to have maximum flow sensitivity and minimum wall filter, and the Doppler sample volume was increased to the total diameter of the vessel examined.

The mean time interval between duplex examination and spiral CT angiography was 23±23.8 days (mean±SD; median, 17 days).

Spiral CT angiography was performed with a Tomoscan SR 7000 (Philips Medical System) with volumetric acquisition by continuous radiographic tube rotation and simultaneous table movement. The raw data were acquired in 50 seconds. The intravascular injection of 150 mL of contrast medium (Optiray 350, Guerbet) was performed via the antecubital vein, preferably in the right arm, by a power injector at a rate of 3 mL/s with a delay scan of 20 seconds. We used a table speed of 5 mm/s, with a slice thickness of 3 mm and a reconstruction index of 1 mm. The other scan parameters were as follow: 120 kV, 250 mA, 1-second scan time rotation, and 512x512 matrix. After acquisition of the CT scans, 2-dimensional, maximum intensity projection (MIP), multiplanar reformatting (MPR) and 3-dimensional, shaded surface display (SSD) reconstructions of the vascular anatomy were performed on an independent workstation (Easy Vision, Philips Medical System). The entire process required 1 hour: data acquisition on the CT scanner took <1 minute; slice reconstruction took 15 minutes; and the time needed for segmentation, reconstruction, and display of the vessel lumen at the workstation was 45 minutes. In cases requiring only MIP and MPR reconstruction, the time needed at the workstation was <10 minutes. Three-dimensional reconstruction (SSD) was performed when required by the vascular surgeon or medical radiologist.

Percentages of stenosis with spiral CT were determined in each case according to the criteria established by NASCET investigators.^{9 10 11 12 16 17 18 23} For quantification of the degree of stenosis, the ratio of the narrowest intraluminal diameter in the stenotic segment and the next (1 cm distal) normal arterial segment was used to define the degree of stenosis in terms of 1 of the 3 categories also based on NASCET criteria (mild=<30% stenosis, moderate=30% to 69% stenosis, and severe=70% to 99% stenosis). For spiral CT, the degree of stenosis was assessed in most cases by use of the MIP technique. The MPR technique was preferred in cases of heavily calcified lesions.

The relationship between percentage values of stenoses for duplex US and spiral CT was evaluated by correlation analysis and presented as the linear correlation, Pearson coefficient (r), and SD for regression (_y). The statistical significance of the difference between the percentage values of the stenoses, assessed by the 2 methods, was evaluated by means of Student's t test for paired samples.

	Results

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In 59 patients, we analyzed a total of 354 segments of the extracranial carotid arteries, including the CCA, ICA, and ECA by duplex US and spiral CT (Table 2). The arterial segments were concordantly identified in all patients with 1 exception in which an ICA was erroneously identified by duplex US as an ECA in a patient with a poor US window. The localization of the lesions in artery segments was also concordant in all cases.

View this table: [in this window] [in a new window]   Table 2. Degree of Stenosis According to NASCET Criteria by Duplex US and Spiral CT for All Arterial Segments Analyzed

The comparison of the severity of stenosis assessed by the 2 methods (Figure 1) was good, with a linear correlation coefficient r=0.91, P=0.024, standard error _y=12.9, and a regression line y=A+Bx (regression coefficients A=7.4±13.6 and B=0.90±0.20).

View larger version (13K): [in this window] [in a new window]   Figure 1. Agreement of spiral CT and duplex US. Shown is a comparison of degree of stenosis between the 2 methods. Stenoses of <30% are not considered for statistical analysis and graphical representation.

Duplex US image quality was inadequate in 5 patients for different reasons (ie, short neck, calcification, or high bifurcation). Whereas spiral CT image quality was good in all patients, it also made possible the evaluation of patients in whom duplex US image quality was inadequate. In 5 cases, there was total lack of agreement in the assessment of the severity of the lesion. In 1 patient with marked kinking and high calcification of the vessel wall, stenosis of the ICA was incorrectly overestimated by the duplex US as severe (80%) and identified as moderate stenosis (45%) by spiral CT. The second completely discordant result was in a patient with a poor US window with an ICA stenosis evaluated by duplex US as 30% and by spiral CT as 80%. The third case of discordance was in a patient with complete occlusion on duplex US but a patent lumen on spiral CT, probably related to a dissection of the ICA. The fourth case of discordance was in a patient with occlusion of the ICA, erroneously identified by duplex US as an ECA. In the last case of discordance, duplex US showed a high-grade stenosis (95%), and a complete occlusion was identified by the time of spiral CT examination; however, this was in the patient with the largest time interval between the 2 examinations.

No evidence of fibromuscular dysplasias was found in our patients. We examined the vessel according to the MPR technique in cases of severe vessel calcification in which the degree of stenosis was uncertain on spiral CT on the MIP image. In all cases, evaluation of the degree of stenosis could be carried out without removing the calcification from the image before reconstruction of the segmented data¹⁷; the carotid lumen was clearly identified in all cases by spiral CT, and there were no instances of motion artifact that precluded the examination.

Evaluation of the origin of the left CCA, left subclavian artery, and brachiocephalic trunk by duplex US was almost never possible because of a poor US window, although spiral CT evaluation of the aortic branch origins was feasible in most patients.

Spiral CT permitted plastic overview of the vascular field in all patients, including the CCA up to the distal portions of the extracranial ICA and ECA. Improvement in the spiral CT technique also allowed visualization of the intracranial part of the ICA (except the carotid siphon), the circle of Willis, and the M1 and partially the M2 segments of the middle cerebral artery in the last 20 patients examined.

	Discussion

Top Abstract Introduction Methods Results Discussion References

 
Our study, performed in a clinical setting, demonstrates the complementarity and usefulness of spiral CT as an adjunctive method for the detection and assessment of the severity of stenosis affecting the extracranial cerebral arteries.

The advantages of spiral CT angiography for carotid artery investigation are the rapidity of examination, minimum patient discomfort, lower radiation doses, much less invasiveness, and lower costs compared with angiography.¹¹ Furthermore, it provides in-depth information on the vessel lumen, vessel wall, and surrounding structures¹⁴ and, in contrast to angiography, may detect carotid dissection hematoma. It also is a reproducible and less operator-dependent diagnostic tool than duplex US.^{11 18}

Because of its cost-effectiveness, the importance of duplex US in the preliminary diagnosis of patients with cerebrovascular disease is indisputable (1/10^th the cost of angiography^9,27) because of the precise information it provides on flow characteristics within the stenosis and in its proximal and distal portions. Moreover, duplex US is valued for its ability to identify plaque complications (ulcers and hemorrhage) with greater accuracy than other techniques, as shown by correlation studies between duplex and intraoperative specimens,^{9 28 29} and for the precise detection of the degree of stenosis of the carotid artery when >70%.^{10 21 22 23 24 25 26 27 30 31 32}

Duplex US offers the advantage of noninvasiveness, but it is an operator-dependent diagnostic method. In certain circumstances—ie, in cases of extensive calcification, for particular anatomical reasons (severe kinking), or because of high carotid bifurcation—it does not provide sufficient and complete information, as confirmed by the results of our study. Furthermore, in selected cases, it may not be able to distinguish high-degree (subtotal) stenosis from complete occlusion.^{15 25}

MR angiography has also been used to study the pathology of extracranial carotid arteries disease. However, its accuracy is poor in cases of turbulent flow, which causes a loss of signal.^{9 15 18 33} MR angiography shows a high negative but low positive predictive value and currently does not provide results that are sufficiently reliable for preparation of surgical intervention.^{18 33} In addition, the examination times required for standard MR angiography are longer than with spiral CT, and a higher degree of cooperation from the patient is needed; MR cannot be performed in patients with pacemakers or with claustrophobia.¹⁸

The disadvantages of spiral CT angiography lie in the administration of the contrast medium, which is impossible in patients with known sensitivity to the dye, in patients with recent deterioration of renal function or preterminal renal failure, and in the volume overload (150 mL) not suitable for patients with severe heart failure. Nephrotoxicity after intravenous injection of contrast media is a rare and well-known complication described for almost all different agents (ionic, nonionic, and high- or low-osmolality contrast medium). Ionic low-osmolarity contrast media (as we used in our study) show less nephrotoxicity and are used in patients with renal transplant. Patients at high risk for contrast media nephropathy usually have preexisting renal failure or diabetes with renal disease. In our setting, there were no patients with recent important impairment of the renal function, which seems the most important risk factor. Good hydration before the procedure is essential, and mannitol and loop diuretics may be used to diminish the induced renal insufficiency. In our collection of patients, no acute deterioration of renal function requiring a specific therapy was registered.

Although the technique used in this study should allow the investigation of the whole extracranial and intracranial segments of the carotid circulation, it remains to be demonstrated that spiral CT is sufficiently reliable for the detection of tandem lesions.^{11 12 18} The real importance of these tandem lesions with respect to treatment strategies, however, has not yet been unanimously accepted.^{12 25} Lesions at the origin of the CCA may often be insufficiently visualized because of the undiluted contrast medium present in the brachycephalic venous trunk at the time of scanning. In our experience, this obstacle could easily be overcome by injecting the contrast medium through a narrow catheter in the superior vena cava placed by the antecubital approach.

The MPR technique was often used in this study for correct calculation of the degree of stenosis, particularly when calcified plaques were present because they rendered the MIP image unreliable. The SSD technique was never used to measure the degree of stenosis because it did not provide a correct representation of the stenosis owing to difficulties in selecting the threshold values or the presence of calcification, which may lead to underestimation of the stenosis.³⁴ The SSD technique was used only to give a complete spatial representation of the extracranial carotid artery, especially in complex anatomical situations such as severe kinking (Figure 2) or when required by the surgeon.

View larger version (122K): [in this window] [in a new window]   Figure 2. Kinking of right ICA by spiral CT in MIP and SSD techniques. Shown is severe kinking of right ICA describing a loop of 360° by MIP (left) and SSD reconstruction techniques with 90° clockwise rotation on its long axis (right). MIP reconstruction also shows a severe calcified stenosis (70%) at ICA origin. Arrows indicate knee of kinking loop.

In our experience, the MIP and MPR techniques are performed much more rapidly, are always capable of quantifying stenosis according to the NASCET criteria, and at the same time supply a detailed representation of the plaque or detect the presence of ulcers.

The MPR technique appears to be particularly useful in cases of extensive calcifications. By means of the curvilinear reconstructions, it is possible to follow the entire extracranial route of the vessel, even in cases of particularly tortuous routes passing between calcifications, and to obtain different sections of the vessel and exact visualization of the lumen and vessel wall even within the most stenotic segments.

Differentiation between ICAs and ECAs could always be carried out with spiral CT. Even in cases of occlusion of the former and differentiation of subocclusion from complete occlusion, spiral CT seems to be better because it allows observation and representation of the lesion from various directions. It is likely that our favorable results obtained with spiral CT are related to progress in image manipulation made possible by the latest workstations.

Our study confirms that in most cases, duplex US provides screening and preoperative evaluation of patients with clinical suspicion of an obstructive lesion of the carotid arteries. Spiral CT angiography seems to be a promising complementary diagnostic tool in cases in which duplex US examination is not reliable, ie, in patients with severe kinking of a carotid artery (Figure 2); for the differentiation of critical stenosis from occlusion in cases of severe calcification (Figure 3), short neck, and high bifurcation; and in cases of discrepancy between duplex findings and clinical results. The primary complementary role of this relatively noninvasive technique probably relates to its ability to give an overall view of the vascular field, which the vascular surgeon often still wants to plan the surgical intervention.

View larger version (128K): [in this window] [in a new window]   Figure 3. Severe calcification of the ICA with MIP and MPR techniques. Shown is severe calcification of ICA (arrow) with MIP technique (left). Measurement of lumen was possible only by means of MPR reconstruction (right). Duplex evaluation was difficult because of poor US window, and stenosis was underestimated.

Further studies are needed to confirm our promising preliminary data for the visualization of the intracranial circulation and the usefulness of 3-dimensional reconstruction.

	Acknowledgments

 
We thank Dr Ralf Baumgartner for helpful discussions in preparing this manuscript and Dr Paolo Tutta and Carmen Mondada for technical assistance.

Received January 26, 1998; revision received April 21, 1998; accepted May 3, 1998.

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