The patient is supine with the head resting flat, in a foam donut, or with a rolled up towel under the neck for support if needed. The technologist is seated at the head of the bed with the scanner to the left of the bed so that the controls are within easy reach of the operator.
A towel or tissue is tucked into the patient's clothing for protection, or a gown may be provided. The patient's head is slightly hyper extended and turned toward the contra lateral side at about a 45-degree angle to the midline. The head position can be changed as the examination proceeds to optimize and access the vessel window.
7.2 Scan technique:
The sample volume of the pulsed Doppler should be moved continuously throughout the length of the vessels searching for regions of increased velocity or flow disturbance. It is important to note that a stenosis may be very focal, and the flow distal to it may normalize over a short distance. Therefore, the sample volume must not be skipped around but rather moved methodically through the vessels. Spot recording of velocity waveforms may
overlook a significant lesion. In addition, a methodical study will ensure that the internal carotid and the external carotid artery are indeed continuous with the common carotid artery, preventing misidentification of other vessels in the area, especially in the presence of disease or with poor image quality. Color flow and B-Flow imaging facilitates this initial survey of the vessels but should not eliminate sampling with the pulsed Doppler. Valuable information is contained in the spectral waveforms that may not be appreciated in the color or B-Flow display.
Note: Plaque localization and characterization by B-mode imaging is necessary. In addition to sampling the Doppler arterial signals, the vessel is evaluated for the presence of wall plaque, plaque morphology, vessel tortuosity, wall thickness, and any abnormalities in the surrounding tissue.
During videotaping or digital acquisition, the technologist should use the microphone if available to describe pertinent anatomy and areas of interest (including presence of plaque, plaque morphology, vessel tortuosity, wall thickness, and any abnormalities in the surrounding tissue) to aid the reviewer's interpretation. When hard copy
documentation is used, prints of areas of interest are obtained for subsequent
interpretation. If voice recording is not possible, screen captures and image prints should include text annotations to describe pertinent anatomy and areas of interest (including presence of plaque, plaque morphology, vessel tortuosity, wall thickness, and any abnormalities in the surrounding tissue) to aid the reviewer's interpretation.
Note: Color flow and B-Flow imaging facilitates the identification of anatomy and areas of interest. In order for color and B-Flow Doppler imaging to be integrated adequately into the conventional duplex examination the examiner must first clearly understand the principles of Doppler, spectral analysis, hemodynamics of blood flow and gray scale imaging. The use of color flow and B-Flow technology does not obviate the need to learn these fundamental principles.
7.3 The common carotid artery:
The artery is evaluated throughout its length for the presence of visible plaque, tortuosity, and focal changes in the velocity. A spectral waveform should always be generated from the most proximal, straight segment of the vessel that is accessible to the scanhead. The character of the waveform from the common carotid is that of a low-resistance vessel.
The end-diastolic velocity should be above zero and similar to the end-diastolic velocity of the contra lateral common carotid taken at approximately the same level in the neck.
Important hemodynamic information is contained in the common carotid artery waveform that may indicate proximal or distal disease. In the case of a high-grade stenosis or a total occlusion of the internal carotid artery, the blood flow will be shunted through the external carotid. The common carotid waveform will then take on the characteristics of the external carotid artery with flow of zero or very close to zero in end-diastole, i.e., a peripheral vessel. The contra lateral common carotid artery may have increased flow in end-diastole in the body's effort to compensate for reduced flow to the brain. In addition, in the presence of a significant stenosis at the origin of the common carotid or the brachio-cephalic artery, the ipsilateral common carotid artery may be damped with low velocity and a slower slope to peak systole compared with the contra lateral common carotid waveform. The ipsilateral common carotid waveform may also appear post-stenotic in character. Therefore, it is important to compare the waveforms generated from both proximal common carotid arteries directly, with particular attention paid to the shape and end-diastolic component.
A waveform should also be generated from the distal common carotid artery, just proximal to the bifurcation. In this area, the vessel may be dilated, and the velocity may be
decreased normally and have a slightly different character, i.e., higher end-diastolic velocity, than the proximal common carotid artery.
7.4 The external carotid artery:
The external carotid supplies the face, structures in the neck, and scalp and is not thought of as a common source of emboli to the brain; however, study of it is invaluable in the accurate identification of the internal carotid artery. The external carotid waveform generally has a very sharp upstroke to systole, followed by a prominent dicrotic wave,
which may reverse at end-systole/early diastole, and velocities approaching or actually reaching zero in end-diastole. In comparison, the internal carotid artery will have
continuous flow throughout the cardiac cycle, the velocities remaining well above zero in end-diastole. The peak velocity of the external carotid is normally higher than that of the internal carotid artery. The external carotid may take on the appearance of the internal carotid artery in end-diastole, i.e., velocities above zero, as the resistance in the face and scalp decreases with warming or in the presence of disease. A direct comparison of the end-diastolic velocities of the external carotid and the internal carotid arteries is essential to separate and identify each of these vessels accurately.
Branches of the external carotid artery, notably the superior thyroid arising near the origin, can often be seen and will help to distinguish the external carotid from the internal carotid, which has no branches in the neck.
7.5 The carotid bulb:
The bulb is usually located in the proximal portion of the internal carotid artery; however, the location is variable, which can be appreciated in the B-mode image. The normal flow patterns become complicated as the blood moves through the dilation and the angled branches of the bifurcation. These normal anatomic features cause a boundary layer or flow separation and secondary helical flow.
The sample volume should be scanned across the bulb along one line of sight from the flow divider to the outer wall. A spectral waveform should be taken from at least three sites across the bulb, documenting the representative flow patterns. The flow divider is defined as the point at which the vessel separates into two daughter vessels. The profile of
waveforms taken across the bulb usually demonstrates unidirectional flow along the flow divider of the bifurcation and a transient reversal of flow at peak systole near center stream and at the outer wall opposite the flow divider. The velocities at the outer wall may drop to zero in end-diastole. These are normal flow changes and, used in conjunction with the absence of visible plaque, will document a normal carotid bulb.
7.6 The mid internal carotid artery:
The mid internal carotid artery waveform is generally taken distal to the normally placed bulb in an area where the vessel is no longer dilated. The flow pattern is that of a typical low-resistance vessel. The normal flow disturbances of the carotid bulb may extend into the mid segment and be reflected in the waveform.
7.7 The distal internal carotid artery:
The distal internal carotid artery is that segment of the vessel beginning at least 3 cm above the bifurcation. Atherosclerosis will usually develop in the first 2 cm of the bifurcation and rarely will be seen isolated in the distal internal carotid artery; however, this does not mean that a thorough search of the area should not be done. The vessel typically will dive away from the transducer or will be tortuous and become exceedingly difficult or impossible to visualize as a result of poor imaging angles and increased scattering. This is an area
where extreme caution should be used in order to prevent inappropriate or inaccurate angle measurements, which can lead to overestimation of velocity increases and hence misdiagnosis of stenosis. The Doppler sample volume can be used as the pathfinder in the cases of poorly visualized vessels. Color Doppler and B-Flow imaging is extremely
valuable in identifying the presence of distal internal carotid artery tortuosity.
There are rare cases where true increases in velocity are detected in the distal internal carotid artery. Fibromuscular hyperplasia, which causes the typical string of beads appearance of the vessel on an angiogram, develops in this region.
A high-resistance waveform, i.e., velocities close to zero in end-diastole, may also be identified throughout the internal carotid artery. This finding suggests a change in resistance of the distal vascular bed. Although not validated, this finding may indirectly suggest the presence of a significant siphon lesion, intracranial stenosis, or distal carotid dissection. A direct comparison of the internal carotid waveforms to identify any asymmetry is essential.
The internal carotid waveform may also be found to have markedly elevated end-diastolic velocities in comparison with the contra lateral internal carotid artery. The elevation in overall flow rate will be noted throughout the common carotid artery as well. This pattern of flow may indirectly suggest a decrease in the resistance of the distal vascular bed as in the presence of a distal arterio-venous malformation.
7.8 The vertebral artery:
The vertebral arteries, although not as important as the carotid arteries, should also be carefully examined. The origins of these vessels, the most common site of disease, lie deep under the clavicle and hence are difficult to scan. They can be small, tortuous, and asymmetric in size and often are inaccessible.
The scanhead is initially placed low in the neck, and the common carotid artery is identified as a landmark. The scanhead is then moved out laterally and tipped inferiorly. The
vertebral artery may then be seen lying deep to the common carotid. From this position, the scanhead is tipped caudad to follow the vessel back to its origin under the clavicle.
Some manipulation of the scanhead is necessary at this location. The origin of the right vertebral artery may be easier to locate than the left because of its more superficial location, originating from the subclavian artery. Color flow and B-Flow study provides a much more rapid and efficient method for locating the vertebral arteries, followed by confirmation with the spectral waveform.
The typical spectral waveform from a vertebral artery is that of a low-resistance vessel similar to the internal carotid artery. The vertebral vessel is assessed for flow direction (i.e., antegrade, retrograde, or to-and-fro) and for any gross abnormalities in the flow. Because a subclavian stenosis or occlusion at its origin may cause the vertebral flow to reverse direction to supply the arm (subclavian steal), proper identification of the direction of flow is important. To-and-fro flow occurs when there is an equal decrease in pressure in both the
distal bed supplied by the vertebral and the subclavian artery, causing the flow in the vertebral artery to appear to slosh back and forth during the cardiac cycle at rest. Reactive hyperemia testing with a blood pressure cuff inflated to suprasystolic pressure at the brachial level for 2 minutes can then be performed. While continuing to record the spectral waveform from the vertebral artery or observing the color display, the cuff is released. A reversal of flow in the direction of the arm on release of the cuff indicates a steal. Exercise of the arm can be substituted for reactive hyperemia; however, the endpoint is not as precise and is more difficult to document.
The vertebral artery may also be interrogated further distally in the neck as it appears to thread its way through the transverse processes of the cervical spine. A spectral
waveform at this site will yield information about the direction of flow but atherosclerotic disease almost always occurs at or near the origin.
7.9 The subclavian artery:
The subclavian artery is located under the clavicle and is difficult to access with some scanheads. The probe is turned so that the proximal common carotid is in a transverse view and then is tipped caudad to interrogate under the clavicle. It often is necessary to move the probe laterally to identify the mid region in a longitudinal view and then to move the probe medially following the vessel to its origin. The vessel drops away from the transducer at this point and may be seen in a transverse view.
The flow velocity in the subclavian artery is classically peripheral in its character. It should have the typical triphasic character similar to that of a femoral artery, i.e., a forward
systolic component, a reversal component in late systole/early diastole, and a second forward component in late diastole. A velocity waveform should be taken as close to the origin of the subclavian artery as possible. If study near the origin is not possible, a signal should be taken in the mid region where the vessel appears to straighten out under the clavicle.
The identification of a subclavian stenosis, usually at the origin of the vessel, is verified by taking the brachial blood pressures bilaterally with a hand held continuous-wave Doppler.
A difference of >15 mmHg between the arm pressures indicates a pressure-reducing subclavian stenosis on the side of the lower pressure. The side with the lower pressure will often have a biphasic or monophasic audible Doppler signal at the brachial level. If a stenosis is suspected with Doppler spectral waveforms and no decrease in pressure is noted, either the stenosis is not pressure reducing or there are bilateral subclavian stenoses causing an equal decrease in blood pressures.
8 DOCUMENTATION