You are in: eMedicine Specialties > Neurology > Neuro-imaging Carotid UltrasoundArticle Last Updated: Jan 4, 2007AUTHOR AND EDITOR INFORMATIONSection 1 of 11
  Author: Brian Silver, MD, Consulting Staff, Assistant Professor of Neurology (Clinician-Educator), Department of Neurology, Henry Ford Hospital Brian Silver is a member of the following medical societies: American Academy of Neurology, American Medical Association, American Society of Neuroimaging, American Stroke Association, Massachusetts Medical Society, Michigan State Medical Society, and Royal College of Physicians and Surgeons of Canada Editors: Draga Jichici, HBSc, MD, FRCP(C), FAHA, Assistant Professor, Department of Medicine, Division of Critical Care Medicine, McMaster University Medical School, Canada; Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine; Howard Kirshner, MD, Vice-Chair, Professor, Department of Neurology, Vanderbilt University School of Medicine; Matthew J Baker, MD, Consulting Staff, Collier Neurologic Specialists, Naples Community Hospital; Helmi L Lutsep, MD, Associate Professor, Department of Neurology, Oregon Health and Science University; Associate Director, Oregon Stroke Center Author and Editor Disclosure Synonyms and related keywords: carotid duplex ultrasonography, CUS, cervical carotid artery disease, blood flow velocity, blood velocity, color flow imaging, Doppler, Doppler imaging, brightness-mode imaging, B-mode imaging, duplex, frequency, power Doppler, sonography, spectral analysis, angiography, pulse-echo, hemodynamics, stenosis, Doppler phenomenon, Doppler shift, wave properties, ultrasound, spatial pulse length, SPL, tissue attenuation, hemodynamics, Reynolds number, law of Poiseuille, volume flow, Doppler effect, ultrasonic waves, pulse repetition frequency, PRF, pulser, mechanical beam former, electronic beam former, transducer, plaque, common carotid artery, CCA, internal carotid artery, ICA, external carotid artery, ECA, arterial occlusion, peak systolic velocity, PSV, end diastolic velocity, EDV INTRODUCTIONSection 2 of 11
  Carotid duplex ultrasonography (CUS) is a useful diagnostic tool for assessing cervical carotid artery disease. With proper angiographic correlation, CUS can be highly reliable, and fewer risks are associated with CUS than with angiography. The North American Symptomatic Carotid Endarterectomy Trial, Asymptomatic Carotid Artery Stenosis Trial, and Asymptomatic Carotid Surgery Trial have helped establish guidelines for possible interventions based on CUS results. DEFINITIONSSection 3 of 11
CUS is a 2-step, or duplex, procedure. The 2 steps are as follows: Imaging
Spectral analysis
PHYSICS: BASICSSection 4 of 11
Knowledge of 3 physical properties is helpful in understanding basic CUS: Pulse-echo technique - Used to image the vessel
Hemodynamics - Principles of blood movement within an artery
Doppler phenomenon - Used to assess the velocity of blood as it moves past the probe
PHYSICS: ADVANCEDSection 5 of 11
This section describes, in mathematical language, the concepts presented in Physics: Basics. Properties of waves Like light, radiation, and audible sound, ultrasound is a wave. The properties of any wave are as follows: c = fl (c = propagation speed of a wave [constant in a given medium], f = frequency, l = wavelength). A medium is a conduit through which the wave passes. Examples of media are air, water, and tissue. The value of c for a given wave differs from medium to medium but is always constant in a particular medium. In the context of this discussion, the medium is tissue and the value of c for ultrasound is 1540 m/s (or 1.54 mm/ms). Because c is constant, the wavelength increases as the frequency decreases; the converse is also true. Frequency is measured in cycles per second, called hertz (Hz). A cycle is one revolution of the wavelength. In CUS, the emitted frequency is millions of cycles per second. Therefore, it is measured in megahertz (MHz) (1 MHz = 1 million cycles/s). During spectral analysis, the Doppler shift is 1000-fold less than the emitted frequency. Therefore, it is measured in kilohertz (kHz) (1 kHz = 1000 cycles/s). Sometimes, the term spatial pulse length (SPL) is used. It is equal to the number of cycles (n) in a pulse multiplied by the wavelength (SPL = n X l). SPL is important in determining resolution. Tissue attenuation As ultrasound passes through tissue, some of the signal is lost through scattering, reflection, and absorption (conversion to heat). In tissue, attenuation (in decibels [dB]) = 0.5 X frequency X path length. Therefore, the higher the frequency of the machine, the greater the attenuation, and the less can be imaged at a greater distance from the probe. On the other hand, higher frequencies mean shorter wavelengths and better resolutions. Therefore, a trade-off exists in adjusting frequencies. Hemodynamics Blood flow can be laminar, disturbed, turbulent, or plug. In CUS, the first 3 are important. When no stenosis is present, blood flow is laminar. Flow of blood is even, with the fastest flow in the middle and the slowest at the edges of the vessel. When a small degree of stenosis is present, the blood flow becomes disturbed and loses its laminar quality. Even in normal conditions, such flow can be seen around the carotid bulb. With even greater stenosis, the flow can become turbulent. Reynolds number determines the level at which turbulent flow occurs. Its derivation is not necessary for routine CUS. The basic equation for flow is Q = DP/R (Q = flow, DP = pressure difference, R = resistance). This is known as the law of Poiseuille. Resistance is dependent on vessel length and radius and fluid viscosity, so that R = 8Lv/pr4 (L = length, v = viscosity, r = radius). Thus, in normal hemodynamics, as vessel length increases or as fluid viscosity increases, so does resistance. As vessel radius increases, resistance decreases significantly (by a factor of 4). Volume flow In a stenotic vessel, volume flow remains constant. Volume flow is related to average speed and vessel area according to the following: Q = va X A (Q = volume flow, va = average speed, A = vessel area). The continuity rule states that volume flow remains constant regardless of the degree of narrowing. Mathematically, this equation is Q1 = Q2 = Q3 (ie, volume flow 1 = volume flow 2 = volume flow 3) (see Media file 1). Therefore, v1A1 = v2A2 = v3A3, and v1pr21 = v2pr22 = v3pr23. va is proportional to the Doppler shift. Therefore, as vessel diameter (and area) decreases, blood velocity increases to maintain volume flow. The velocity increases by a factor of 2 for every unit decrease in diameter. (The only time volume flow may not be maintained is with severe stenosis [>90%] when the resistance effect dominates.) Doppler effect The Doppler effect is the change in frequency or wavelength due to motion of the wave source, receiver, or reflector of a wave source (see Media file 2). Since the probe is assumed to be still, the source of the change in frequency is the moving blood, which acts as both a receiver and reflector. Because it has both properties, it is known as a scatterer. The equation for the emitted frequency from a scatterer is fe = fo(c+v)/(c-v) (fe = emitted frequency, fo = source frequency, c = propagation speed, v = scatterer speed). Propagation speed of ultrasound in tissue is 1540 m/s. The Doppler shift is equal to the source frequency minus the emitted frequency: fD = fo - fe = fo(2v)/(c-v) (fD = Doppler shift). Doppler shift Doppler shift is angle dependent in CUS. Ordinarily, the computer would use the equation in the previous paragraph to calculate velocity based on Doppler shift. However, 2 adjustments to the equation are made. First, the scatter speed in the denominator is dropped because its speed is negligible compared to that of ultrasound. Second, because the probe cannot be parallel to the arteries in the neck, angle adjustment is required. The correction is based on the cosine of the angle. This is what is meant by angle dependency. Thus, the previous equation becomes fD = 2fv(cosq)/c. If the equation has c replaced and is rearranged, it becomes v = 77 fD/f(cosq) (v in cm/s, fD in kHz, f in MHz). Because the estimation of actual angle increasingly errs with increasing angles (resulting in erroneous velocities) and the number of ultrasound waves that are reflected at the vessel wall increases with lower angles, an angle of 60o is commonly used. Therefore, the factors that affect Doppler shift are blood flow velocity, emitted frequency, and angle of insonation. ARTIFACTSSection 6 of 11
At least 18 artifacts have been identified in carotid ultrasound; most of them occur during imaging. The common artifacts include the following:
INSTRUMENTSSection 7 of 11
The principal components of the ultrasound instrument are pictured in the following schematic drawing. Beam former: The beam former controls the shape and direction of beam, known as focusing and steering. The beam former can be mechanical or electronic. Mechanical beam formers operate via an oscillatory mechanism. Electronic beam formers may be linear-switched arrays or linear-phased arrays. Focusing the beam narrows the pulse, which improves lateral resolution. Lateral resolution is equal to pulse width. Transducer: The transducer contains the damping element, piezoelectric crystal (converts electric energy to ultrasonic waves), and the matching layer. The damping element is behind the crystal. It acts to reduce the pulse length and to improve lateral resolution. The matching layer is in front of the crystal. It reduces reflection of ultrasound at the transducer surface, improving ultrasonic transmission. Gel is applied to improve ultrasonic transmission. Even a very thin layer of air can reflect virtually all ultrasound. Receiver: The receiver amplifies (ie, increases small signals), compensates (ie, equalizes signals that are at different distances from the transducer), compresses (ie, reduces the brightness scale to that which is visible to the eye), demodulates (ie, changes bidirectional signals and smoothens), and rejects (ie, gets rid of ambient noise). Memory: Memory often is coded in binary. Display: The display comprises a series of lines representing adjacent scan lines. The varieties of scanning formats are unlimited. The most commonly used methods are linear (ie, rectangular) and sector (ie, pie shaped). Deeper scanning theoretically requires that fewer scan lines be made available in order to maintain a real-time display. Alternatively, the number of scan lines can be preserved at the cost of a slower real-time display. TECHNIQUESection 8 of 11
An ultrasound examination may be performed in many ways. Recommended techniques include the following: Enter the patient's name and examiner's name into the computer, take a focused history, measure bilateral brachial artery blood pressure, ask the patient to lie supine, and choose a conventional starting site (right or left - most use right). Have the patient's head turned contralateral to the side being tested, place a towel on clothing for protection, apply gel liberally to the transducer or neck, and start the scan transversely from the proximal common carotid artery (CCA) moving distally. Note the carotid bifurcation, look for plaques, attempt to characterize the nature of the plaque, and switch to the sagittal view; by convention, the patient's head is at the left on the screen, and images at the top of the screen are closest to the transducer; color may be used at this point to identify flow within the artery and potential areas of high velocity. The on-screen probe is placed in the artery parallel to vessel walls; make sure to correct for excessive angles. The "gate" is the width of the listening window; the larger the gate, the more likely that signal will be detected. However, the trade-off is increased noise. Perform spectral analysis and find the highest velocity or frequency. The procedure is done in the CCA, internal carotid artery (ICA), and external carotid artery (ECA); at least 2 or 3 spectral analyses of each vessel should be obtained. Color imaging and power Doppler may be used but may not necessarily provide additional information. After assessment of the anterior circulation, assess the vertebral circulation. Usually, the C4-C6 segment is accessible; to find the vertebral artery, angle the transducer laterally and inferiorly and identify the presence and direction of flow; velocity measurements may also be obtained. Vessel identification Identify common carotid artery
Distinguishing internal and external carotid arteries (see Media files 4-6)
Identifying the vertebral arteries
Use of color flow Doppler
Power Doppler (see Media file 7)
DETERMINING DEGREE OF STENOSISSection 9 of 11
While some ultrasonographers attempt to characterize the degree of stenosis based on visual characteristics alone (similar to the North American Symptomatic Carotid Endarterectomy Trial method of angiographic estimation), estimation of stenosis solely based on this criterion is not reliable. Commonly used methods of acoustic estimation of the degree of stenosis include the following:
Some laboratories characterize degree of stenosis in terms of exact percentages. A range (eg, 50-69% stenosis) is probably more accurate. The ranges and measurements vary from laboratory to laboratory. Factors that affect measurements include the equipment used, the person performing the ultrasound, and the sites sampled for measurement (eg, the distal ICA often has higher velocities than the proximal ICA). When possible, laboratories should perform their own correlations with angiographic measurements for quality control. A consensus conference in 2003 of the Society of Radiologists in Ultrasound recommended the following criteria for estimating stenosis:
With stenosis over 90% (near occlusion), velocities may actually drop as mechanisms that maintain flow fail. Ratios may be particularly helpful in situations in which cardiovascular factors (eg, poor ejection fraction) limit the increase in velocity. In such cases, ICA/CCA ratios above 3 may signify significant stenosis. With normal cardiovascular function and normal velocities, changes in ratios should be interpreted with caution. MULTIMEDIASection 10 of 11
REFERENCESSection 11 of 11
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