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 .
Ultrasound can also assess plaque morphology such as hemorrhagic
or ulcerative plaque, which increases the risk of thromboembolic
doppler - technique and instrumentation:
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.
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.
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)
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
transverse scan of CCA. The innerline (arrowhead)
indicates lumen-intima interphase. The outerline (arrow)
indicates media-adventitia interphase.
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
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
|Colour Doppler image
of CCA showing laminar flow.
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.
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
|Normal Doppler wave
form in ECA
|Normal Doppler wave
form in ICA
|Temporal artery tap
causing serrate distortion of the Doppler waveform in
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).
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)
Doppler wave form in CCA
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.
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.
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
causing luminal narrowing, well demonstrated by power
echogenic plaques (Fig 8): These are fibrous plaques in
which collagen is a prominent component.
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.
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
showing reversed low-density eddy flow within an ulcer.
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.
Evaluation of Carotid Stenosis:
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
|Cross section Color
image of ICA demonstrating percentage area of stenosis.
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 .
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) .
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) .
|Luminal reduction of >
50% causing spectral broadening in Doppler waveform.
|High grade stenosis of
ICA showing marked velocity increase (>3metres/sec.)
Doppler Spectral Analysis of
stenosis usually begins to cause velocity changes when the
stenosis exceeds 50% diameter reduction (reduction of 70% cross
sectional area) (Fig. 13) . 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 .
velocity (PSV) is accurate for quantification of high-grade
stenosis . This parameter bears a
well-defined relationship to the magnitude of luminal narrowing
and is easily measured . 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 . 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 . End-diastolic ratio
(EDV in ICA/EDV in CCA) of more than 5.5 is highly accurate for
predicting more than 80% diameter reduction .
Velocity ratios should always be obtained when unusually high or
low CCA velocities or significant asymmetry of CCA velocities is
- Doppler Spectrum analysis
||Peak systolic Velocity
||End diastolic Velocity
Ratio (Velocity in ICA/CCA)
Ratio (Velocity in ICA/CCA)
Abbreviations: NA, not available.
|Longitudinal (Fig. 15)
and transverse (Fig. 16) images showing occlusion of ICA
and absence of color flow. The lumen is filled with
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.
Evaluation of ICA Occlusion 
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)
dampened flow within ipsilateral CCA. No flow or reverse flow
proximal to ICA occlusion. (Fig. 17)
|Reversal of flow
proximal to the site of occlusion (arrow).
vessel size(chronic occlusion)
Internalization of ipsilateral ECA waveforms.
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.