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Pictorial Essay: Color Duplex Evaluation of Carotid Occlusive Lesions
 

Rajagopal KV, Bhushan N Lakhkar, Shekhar Banavali, Nitin Kumar Singh

Key Words: Color Doppler, carotid occlusive lesions, stenosis
 

Flow limiting stenosis of the carotid artery is a common predisposing cause of stroke. The major role of Doppler examination is the detection of stenotic lesions in the vicinity of the carotid bifurcation. Atherosclerotic plaque with resultant stenosis in the carotid arteries usually involves the internal carotid artery (ICA) within 2 cm of the carotid bifurcation. Accurate diagnosis of significant stenosis is important to identify those patients who would benefit from surgical intervention. Carotid endarterectomy is more beneficial than medical therapy in symptomatic patients with greater than 70% carotid stenosis [1]. Ultrasound can also assess plaque morphology such as hemorrhagic or ulcerative plaque, which increases the risk of thromboembolic events.

Carotid doppler - technique and instrumentation:

Carotid examination is performed with the patient supine, the neck slightly extended and head turned away from the side being examined. Examination can be done facing the patient or sitting behind the patient. Only linear transducers are recommended for the examination of cervical arteries. An acceptable Doppler angle can be achieved with beam steering. A 5-Mhz transducer is essential and will adequately demonstrate cervical arteries even in obese patients.

 

Gray-scale examination begins in the transverse projection. The transducer may be applied more from the anteromedial or lateral side of the sternocleidomastoid muscle. Scans are obtained along the entire course of the cervical carotid artery from the supraclavicular notch cephalad to the angle of the mandible. Inferior angulation of the transducer in the supraclavicular area images the common carotid artery (CCA) origin. The left CCA origin is deeper and more difficult to image consistently than the right. The carotid bulb is identified as mild widening of the CCA at the bifurcation. The examination plane necessary for optimal longitudinal scans of the carotid artery to perform Doppler spectral analysis is determined by the course of the vessels demonstrated on the transverse study. Images are obtained to display the relationship of both branches of the carotid bifurcation to the visualized plaque disease and the extent of the plaque is measured.

 

The normal carotid arteries:

The longitudinal view of the normal carotid wall demonstrates two nearly parallel echogenic lines; the inner line is the lumen-intima interface and the outer line is the media-adventitia interface. The distance between these lines is the combined thickness of intima and media (I-M complex)

 

Thickening of I-M complex more than 0.8 mm represents early changes of atherosclerosis. The intimal reflection should be straight, thin and parallel to the adventitial layer (Fig. 1). Undulations and thickening indicate plaque deposition or more

Fig 1
Fig. 1
Longitudinal and transverse scan of CCA. The innerline (arrowhead) indicates lumen-intima interphase. The outerline (arrow) indicates media-adventitia interphase.

rarely fibromuscular hyperplasia. The CCA lies immediately adjacent to the jugular vein but the two vessels are easily differentiated. The carotid artery exhibits pulsatile flow pattern whereas the jugular vein shows continuous low velocity signal. Several anatomic features differentiate ICA from external carotid artery (ECA). In about 95% of the patients, the ICA is posterior and lateral to the ECA. The ICA is usually larger than the ECA and has no branches in the neck whereas the ECA possess branching vessels.

 

Flow characteristics on color doppler image:

Laminar flow is apparent in normal CCA and ICA as manifested by gradations of shades of color from the periphery to the center of the vessel (Fig. 2). This can be appreciated

Fig 2
Fig. 2
Colour Doppler image of CCA showing laminar flow.

on longitudinal as well as transverse images. A tortuous vessel or bifurcation of the vessel may produce flow disturbances that vary in severity in proportion to the curvature or angular measurements of the vessel. Flow disturbances may be manifested by mixtures of shades of color, all flowing cephalad or mixtures of colors representing forward or reversed flow. Normal flow disturbances occur at the carotid bulb.

 

Aspects of carotid pulsatility that assist with the identification of ECA and ICA are also manifested by the Doppler image. The CCA and ICA exhibit a continuous flow pattern with antegrade flow in diastole indicated by persistence of color throughout the entire cardiac cycle. ECA shows cessation or marked diminution of diastolic flow and this is indicated by the disappearance of color during the diastolic portion of the cardiac cycle.

 

The ICA and ECA have distinct spectral waveforms. The ECA shows a sharp velocity rise during systole and a rapid fall during diastole, approaching zero or transient reverse direction. This flow pattern is due to the high resistance vascular bed of the facial musculature supplied by the ECA (Fig. 3). The ICA supplies the low resistance circulation

Fig 3
Fig. 3
Normal Doppler wave form in ECA

 

Fig 4
Fig. 4
Normal Doppler wave form in ICA

 

Fig 5
Fig. 5
Temporal artery tap causing serrate distortion of the Doppler waveform in ECA

of the brain. Thus it shows large quantity of forward flow in diastole (Fig. 4). Percussion of the superficial temporal artery (temporal artery tap) often results in a serrate distortion of the Doppler waveform in the ECA (Fig. 5).

 

The CCA waveform is a composite of the ICA and ECA waveforms but the CCA more often closely resembles the ICA flow pattern and diastole is generally above the base line (Fig. 6)

Fig 6
Fig. 6
Normal Doppler wave form in CCA

Abnormal Carotid Arteries:

The major cause of non-embolic blood flow disturbances is atherosclerotic luminal narrowing in the setting of stenosis or occlusion. Rare cause of arterial luminal narrowing leading to cerebral ischemia include various forms of arteritis, Moya-Moya disease, traumatic or spontaneous dissections, fibromuscular hyperplasia and vascular compression or infiltration by tumors.

 

Plaque Characterisation:

On gray scale imaging plaque is seen as echogenic material that encroaches on the arterial lumen and produces a flow void. Plaque echogenicity depends on its composition.

 

Low echogenicity plaques: These are fibrofatty plaques containing large amount of lipid material. These may be difficult to identify on gray scale imaging due to their faint echogenicity. This problem is lessened with color Doppler imaging since a flow void is visible even if the plaque is not (Fig. 7).

Fig 7
Fig. 7
Fibrofatty plaque causing luminal narrowing, well demonstrated by power Doppler imaging.

 

Moderately echogenic plaques (Fig 8): These are fibrous plaques in which collagen is a prominent component.

Fig 8
Fig. 8
Fibrous plaque (moderately echogenic).

 

Fig 9
Fig. 9
Calcified plaque showing distal acoustic shadowing.

Strongly echogenic plaque (Fig 9): These plaques show posterior acoustic shadowing secondary to calcifications in the areas of hemorrhage and necrosis. Acoustic shadowing from the plaque may obscure the arterial lumen and wall opposite the plaque and thus may prevent acquisition of Doppler information.

 

Plaque texture is classified as being homogeneous or heterogeneous. Homogeneous plaque has a uniform echo pattern and smooth surface. Heterogeneous plaque has a more complex echo pattern and contains at least one or more focal sonolucent areas representing intraplaque hemorrhage. Sonographic findings suggestive of plaque ulceration include a focal depression or break in the plaque surface or anechoic area within the plaque, which extends to the plaque surface without an intervening echo between the vessel lumen and the anechoic region. Color flow Doppler and power Doppler ultrasound may demonstrate slow moving eddies of color within an anechoic region in the plaque, which suggest ulceration; these findings are highly accurate in predicting plaque ulceration (Fig. 10). Pulsed wave Doppler traces from within the ulcer crater show low-velocity dampened waveforms.

Fig 10
Fig. 10
Ulcerated plaque showing reversed low-density eddy flow within an ulcer.

The cephalo-caudad extent of plaque is reliably visualized with longitudinal images. The thickness of plaque, as well as severity of luminal narrowing should be measured from transverse sections. In addition it is useful to describe plaque as circumferential or noncircumferential.

 

Color Duplex Evaluation of Carotid Stenosis:

The detection of carotid stenosis and occlusions with color duplex sonography relies mainly on the combination of B mode and color encoded flow imaging. The B mode image defines the outer boundary of the vessel wall and the lumen reducing material while the color image demonstrates the flow pattern. Doppler frequency analysis serves mainly to confirm the imaging findings and may be necessary for quantification.

 

The severity of the carotid stenosis may be evaluated by measuring the diameter or

Fig 11
Fig. 11
Cross section Color image of ICA demonstrating percentage area of stenosis.

 

Fig 12
Fig. 12
Significant ICA stenosis showing color flow Doppler aliasing and lighter shades of color indicating turbulence with increased velocity of flow.

area of residual lumen and diameter or area of the original lumen (Fig. 11) [2,3]. Thus the percentage of luminal reduction can be calculated. The accurate measurement of the carotid stenosis is dependent on good quality images and on the attainment of the true cross section of the vessel. Cross sectional images of diagnostic quality cannot always be obtained if there are tortuous vessels and calcified plaques. In such cases severity of stenosis must be estimated from Doppler spectral information [4].

 

Color Doppler ultrasound facilitates Doppler spectral analysis by rapidly identifying areas of flow disturbances. The highest velocity shifts can frequently be identified by color flow Doppler aliasing. Color Doppler ultrasound facilitates this by placing the pulsed wave Doppler sample volume in the region of the most striking color abnormalities (Fig. 12) [5]. Power Doppler ultrasound showing better edge definition and relative angle dependent flow imaging offers the potential for better visual assessment of degree of stenosis (Fig. 7) [5].

Fig 13
Fig. 13
Luminal reduction of > 50% causing spectral broadening in Doppler waveform.

 

Fig 14
Fig. 14
High grade stenosis of ICA showing marked velocity increase (>3metres/sec.)

Doppler Spectral Analysis of Carotid Stenosis:

Carotid stenosis usually begins to cause velocity changes when the stenosis exceeds 50% diameter reduction (reduction of 70% cross sectional area) (Fig. 13) [6]. Flow velocity increases as severity of stenosis increases (Fig. 14). Velocity increases are focal and more pronounced immediately distal to a stenosis. The point of maximum velocity can be easily determined on a color Doppler image.

 

It is suggested that four velocity measurements be routinely obtained in stenotic ICA lumens: peak systolic velocity (PSV), peak end-diastolic velocity (EDV), systolic velocity ratio, and diastolic velocity ratio. Velocity ranges that typically occur with specific degrees of ICA obstruction are listed in Table I [7].

 

Peak systolic velocity (PSV) is accurate for quantification of high-grade stenosis [8]. This parameter bears a well-defined relationship to the magnitude of luminal narrowing and is easily measured [9]. Maximum velocities in the carotid system occur with a lumen diameter of 1 to 1.5 mm. As the lumen diameter narrows beyond that point, the velocity decreases [10]. Peak end-diastolic velocity tends to increase in proportion to the severity of luminal narrowing beyond 50% diameter reduction.

The systolic ratio is derived by dividing the peak systolic velocity at the stenotic zone of the ICA by peak-systolic velocity obtained at the normal portion of CCA (PSV in ICA / PSV in CCA). Systolic ratio exceeding 1.5 predicts ICA stenosis of more than 50% decrease in diameter [11]. End-diastolic ratio (EDV in ICA/EDV in CCA) of more than 5.5 is highly accurate for predicting more than 80% diameter reduction [7]. Velocity ratios should always be obtained when unusually high or low CCA velocities or significant asymmetry of CCA velocities is detected.

Table I - Doppler Spectrum analysis
Diameter Stenosis Peak systolic Velocity End diastolic Velocity Systolic velocity Ratio (Velocity in ICA/CCA) Diastolic velocity Ratio (Velocity in ICA/CCA)
0% < 110 < 40 < 1.8 < 2.6
1-39% < 110 < 40 < 1.8 < 2.6
40-59% < 130 < 40 < 1.8 < 2.6
60-79% > 130 > 40 > 1.8 > 2.6
80-99% > 250 > 100 > 3.7 > 5.5
100% NA NA NA NA
         
Abbreviations: NA, not available.

 

Fig 15
Fig. 15
 
Fig 16
Fig. 16
Longitudinal (Fig. 15) and transverse (Fig. 16) images showing occlusion of ICA and absence of color flow. The lumen is filled with echogenic thrombus.

 

Situations where this is likely to occur are low cardiac output status, aortic stenotic valve disease or high-grade stenosis at the origin of CCA.

 

Color Duplex Evaluation of ICA Occlusion [12]

Occlusion of ICA is characterized by

1) Absence of color flow and Doppler flow signal within ICA (Fig. 15)

2) Lumen filled with echogenic material (Fig. 16)

3) Unilateral dampened flow within ipsilateral CCA. No flow or reverse flow proximal to ICA occlusion. (Fig. 17)

Fig 17
Fig. 17
Reversal of flow proximal to the site of occlusion (arrow).

4) Reduced vessel size(chronic occlusion)

5) Internalization of ipsilateral ECA waveforms.

 

False positive occlusion occurs if the Doppler angle is 90 degree or when the artery is obscured by acoustic shadowing, poor image quality or a weak Doppler signal.

 

REFERENCES

  1. North American Symtomatic Carotid Endarterectomy Trial Collaborators. Beneficial effect of carotid endarterectomy in symtomatic patients with high-grade carotid stenosis. N Engl J Med 1991; 325: 445-453.
  2. Erricson SJ, Mewissen MW, Foley WB, et al. Stenosis of the internal carotid artery: assessment using color Doppler imaging compared with angiography. AJR 1989; 152: 1299-1305.
  3. Middleton WD, Foley WD,Lawson TL. Color flow Doppler imaging of carotid artery abnormalities. AJR 1988; 150: 419-425
  4. Zwiebel WJ, Knighton R. Duplex examination of carotid arteries. Seminars in Ultrasound, CT and MR 1990; 11: 97-135.
  5. Griwing B, Driesner F, Kallwellis G et al. Cerebrovascular disease assessed by color flow and power Doppler ultrasonography. Stroke 1996; 27: 95-100.
  6. Carrol BA. Carotid sonography. Radiology 1991; 178: 303-313.
  7. Bluth EI, Wetzner SM, Stavros AT et al. Carotid duplex sonography: A multicenter reccommendations for standardised imaging and Doppler criteria. Radiographics 1988; 8: 487-506.
  8. Zwiebel WJ, Austin CW, Sackett JF et al. Correlation of high resolution B mode and continuous wave Doppler sonography with arteriography in the diagnosis of carotid stenosis. Radiology 1983; 149: 523-532.
  9. Robinson ML, Sacks D, Perlmutter GS et al. Diagnostic criteria for color Doppler sonography. AJR1988; 151: 1045-1049.
  10. Douville Y, Johnsten KW, Kassam M. Determination of haemodynamic factors which influence carotid Doppler spectral broadening. Ultrasound Med Biol 1985; 11: 417-423.
  11. Garth KE, Carrol BA, Summer EG. Duplex ultrasound scanning of the carotid arteries with velocity spectral analysis. Radiology 1983; 147: 823-827.
  12. Gortter M, Niethamma R, Widder B. Differentiating subtotal carotid artery stenosis from occlusions by color coded duplex sonography. J Neurol 1994; 241: 301-305.

From ijri.org