If significant plaque is present in the ICA, the degree of luminal narrowing can be estimated in the transverse plane by comparing the main luminal diameter and residual lumen diameter (the diameter that excludes plaque) and using it as a qualitative adjunct to the measurement of stenosis severity based in the peak systolic velocity (PSV). normal ICA PSV is <125 cm/sec and no plaque or intimal thickening is visible sonographically additional criteria include ICA/CCA PSV ratio <2.0 and ICA EDV <40 cm/sec <50% ICA stenosis ICA PSV is <125 cm/sec and plaque or intimal thickening is visible sonographically additional criteria include ICA/CCA PSV ratio <2.0 and ICA EDV <40 cm/sec From the mid-distal CCA slide and angle posteriorly to visualise the cervical transverse processes and the vertebral artery. There is wide variability in the peak systolic velocities seen in normal patients, with a range of 20 to 60 cm/s, with an even wider range noted at the vertebral artery origin (also called segment V0). The normal spectral Doppler waveforms differ between the different components of the carotid system. Assess in transverse and longitudinal for pathology. The majority of stenotic lesions occur in the proximal internal carotid artery (ICA); however, other sites of involvement in the carotid system may or may not contribute to significant neurologic events. Elevated blood flow velocities in the ECA are not considered clinically important except that they can explain the presence of a clinically detected carotid bruit. In addition, when statins were started on asymptomatic patients prior to CEA, the incidence of perioperative stroke and early cognitive decline also decreased. Purpose. Therefore, if the CCA velocity for the ratio is obtained from the proximal portion of the artery, the ratio may be low, potentially causing an underestimation of the degree of stenosis based on this parameter. Therefore ischemia or an embolic event will only occur if the internal carotid artery is involved. The SRU consensus data represent a compromise between sensitivity and specificity and are based on cut points validated against ACAS/NASCET-based angiographic measurements of stenosis severity ( Table 7.2 ; Figs. The CCA is an elastic artery, whereas the ICA is a muscular artery.4 The region of the ICA sinus is of mixed characteristics between a muscular and an elastic artery.5. A plaque or stenosis of the external carotid artery usually has little consequence (unless the external carotid artery provides collateral flow). For this reason, peak systolic velocity measurements of the common carotid artery should be obtained approximately 2cm proximal to the carotid bulb [1]. 8.5 How does the spectrum of the vertebral arteries and the common carotid artery look? In addition, ulcerated plaque that demonstrates a focal depression or break within the plaque is also more prone to plaque rupture and subsequent embolic event ( Fig. 8.2 Which morphologic clues help to distinguish the internal- from the external carotid artery? Average PSV clearly increases with increasing severity of angiographically determined stenosis. Lessthan 60 degrees ( beyond 60degrees, error is exponentially increased). Check for errors and try again. In general, for a given diameter of a residual lumen, the calculation of percent stenosis tends to be significantly higher using the pre-NASCET measurement method when compared with the NASCET method ( Fig. This is rarely acheivable but as we approach 0 degrees, our human inter-observer error error is diminishing. Though controversial, IVC measurement by ultrasound can estimate volume status, fluid responsiveness, and fluid tolerance There is evidence to support that IVC diameter is consistently low in hypovolemia versus euvolemia; IVC change can estimate fluid responsiveness with sensitivity of 0.78 and specificity of 0.86; Can use as a dynamic assessment after intervention such as giving . internal carotid artery supplies the brain, plaque or stenosis of the external carotid arter, < Previous chapter: 7. THere will always be a degree of variation. Ultrasound of the vertebral arteries can be variable in diameter. Analysis of the combinations of low ICA velocity, abnormal ICA Identify the origins of the ICA and ECA arteries. 2. Positive correlation between plaque location and low oscillating shear stress. The intimal reflection should be straight, thin, and parallel to the adventitial layer. Case study, Radiopaedia.org (Accessed on 02 Mar 2023) https://doi.org/10.53347/rID-20309. 7.3 ). Common Carotid artery in B mode long, trans, Common Carotid artery with colour & spectral doppler, External Carotid artery origin with colour & spectral doppler, Internal Carotid artery origin with colour & spectral doppler, Vertebral artery spectral doppler showing patency and flow direction, All peak systolic and end diastolic velocities. The transition between media and adventitia also corresponds to the external elastic lamina as seen on pathologic studies. The NASCET (North American Symptomatic Carotid Endarterectomy Trial) demonstrated that CEA resulted in an absolute reduction of 17% in stroke at 2 years when compared with medical therapy in symptomatic patients with 70% or greater stenosis. Velocities vary widely between patients but peak systolic velocities around 77 cm/s have generally been accepted as ultrasound Ultrasound Longitudinal The external carotid artery has systolic velocities higher than the internal carotid artery, and its waveform is characterized by a sharp rise in flow velocity during systole with a rapid decline toward the baseline and finally return to diminished diastolic flow. It is routinely examined as part of carotid duplex ultrasound, but criteria for determining ECA stenosis are poorly characterized and typically extrapolated from internal carotid artery data. Stenoses of the external carotid artery (ECA) are not considered clinically important but should be reported because they may explain the presence of a bruit on clinical examination and need to be considered by the surgeon at the time of carotid endarterectomy (CEA). A temporal-tap (TT) was employed here to confirm it was the ECA. All three layers can be visualized on ultrasound images (Figure 7-1). This longitudinal image of the common carotid artery demonstrates a sharp line (specular reflection) that emanates from the intimal surface. Other positions of the probe either in more anterior or posterior positions can help with visualization in patients with very distal disease or with large or thick necks. This layer is responsible for most of the structural strength and stiffness of the artery. ECA: External carotid artery (ECA) waveforms have sharp systolic peaks, pulsatility due to reflected waves from its branches, and relatively little flow in diastole as compared to the internal carotid artery (ICA). Other studies, both here and abroad, confirmed the benefit of CEA and validated the role of this procedure. The innermost layer abutting the lumen is the intima, or endothelial lining of the artery. Ensure suitable PRF and gain for these smaller, deeper vessels. FIGURE 7-2 Off-axis view of the carotid wall. Subsequent data from the NASCET reported improvement in outcome with CEA in patients with 50% to 69% stenosis, although the amount of improvement was far less than was the case with higher grade stenosis. They are automatically transferred to the ARDMS/APCA CME Bank and RSNA's CME Gateway (when you include your credentials). Clinical Background Internal carotid artery (ICA). Ultrasound of the ECA waveform is high resistance and may have retrograde flow in diastole. Brief documentation may be made and formal follow up studies can be performed if clinically indicated. elevators, retractors and evertors of the upper lip, depressors, retractors and evertors of the lower lip, embryological development of the head and neck. Internal carotid artery stenosis. Prior to the 1990s, the degree of carotid stenosis was measured by angiography and estimated where the artery wall should be so that the local or relative degree of stenosis can be estimated. Usually the widening is slight, but some normal individuals have capacious carotid bulbs that may harbor large plaques in the absence of significant carotid stenosis. Ideally an angle of 0 degrees provides least error and greatest doppler shift. Use a linear, mid frequency range probe (5-8MHZ). Blood flow velocities can therefore be artificially elevated as the blood flows into and out of the curved segment. However, stenoses in other carotid artery segments such as the distal ICA (an area not typically well seen on routine carotid ultrasound), the common carotid artery (CCA), or the innominate artery (IA) may be equally significant. The pathology will usually be located between the CCA origin and vertebral origin. Be aware of the possibility of a Carotid bulb tumour which whilst relatively rare, is a clinically significant finding. North American Symptomatic Carotid Endarterectomy Trial Collaborators. They arent always the same and it may not be in the centre of the vessel. In addition, on average, the common carotid blood flow velocity in the low neck is 10 to 20 cm/sec higher than near the bifurcation.11 This observation is of considerable importance, as the measured peak systolic velocity ratio (ICA peak systolic velocity/CCA peak systolic velocity; see Chapter 9) will depend on the location where velocities are sampled in the CCA. Carotid Ultrasound Case Series: What's the Diagnosis? Common carotid artery (CCA). As the temporal artery is a branch of the ECA, velocity deflections caused by the tapping should be seen on the ECA waveform (Fig. Stenoses of the external carotid artery (ECA) are not considered clinically important but should be reported because they may explain the presence of a bruit on clinical examination and need to be considered by the surgeon at the time of carotid endarterectomy (CEA). The scan may begin with either the longitudinal or transverse imaging of the CCA. The estimation of the original lumen is further complicated by the presence of a normal, but highly variable, region of dilatation, the carotid bulb. Providers use this test to diagnose blood clots and peripheral artery disease. However, carotid stenting was associated with a higher incidence of periprocedural stroke, while CEA patients had a higher risk of perioperative myocardial infarction. As discussed in, Peak systolic ICA velocities as high as 120 cm/sec have been reported in some normal adults, but these values are exceptional, and an ICA velocity exceeding 100 cm/sec should be viewed as potentially abnormal in older individuals. The further distal you record the Doppler signal in the internal carotid artery the higher the diastolic component will become (decrease in the S/D ratio) and the easier it will be to differentiate it from the external carotid artery. The internal carotid artery (ICA) is a lower resistance vessel and displays low to medium pulsatility on spectral imaging with no or minimal reversal of flow. A stenosis of greater than 70% diameter reduction demonstrates a peak-systolic velocity greater than 230 cm/sec. Ensure you angle correctly to the direction of the flow indicated by the colour doppler prior to calculating velocity. Error bars show one standard deviation about mean. In addition, any benefit of a cerebrovascular screening study depends on the expected risk reduction of any intervention based on screening results. IAME's Unlimited CME Plan is now the internet's best value for online CME in ultrasound. The collecting system could be identified in all kidneys and its wall thickness varied between 0 (not visible) and 0.8 mm. ), have velocities that fall outside the expected norm for either PSV or EDV. The carotid bulb and bifurcation should be imaged with gray scale and color Doppler. EDV was slightly less accurate. The carotid ultrasound examination begins with the patient supine and neck slightly extended with the head turned to the opposite side if needed ( Fig. The structure above these two branches is a partly collapsed IJV. If you like the way we teach, please leave a message! The NASCET technique is currently the standard on which the large clinical North American studies were based and should be used to make clinical decisions about which patients undergo CEA. Hathout etal. ECA vs ICA > BACK TO OVERVIEW While this is not a major problem in peripheral arteries when the original lumen is visible on both sides of a stenosis, lesions at the origin of the ICA typically do not have a normal lumen on both sides. J Vasc Surg. Caution: The temporal tap maneuver is not always reliable as deflection waves can sometimes also be seen in the internal carotid artery. The lumen-intima interface is best seen on longitudinal images when the image plane passes through the center of the artery and the ultrasound beam forms a 90-degree incident angle with the wall interfaces (Figure 7-2; see Video 7-1). For that reason, ICA/CCA PSV ratio measurements may identify patients who, for hemodynamic reasons (e.g., low cardiac output, tandem lesions), have velocities that fall outside the expected norm for either PSV or EDV. Temporal Tapping may also be used to confirm that you are examining the ECA. The Asymptomatic Carotid Surgery Trial 1 (ACST-1) demonstrated a 10-year benefit in stroke reduction in asymptomatic patients who underwent CEA for severe stenosis between 70% and 89%. Plaque with strong echolucent elements is generally termed heterogeneous plaque, which is considered unstable and more prone to embolize. What is normal ICA? Peak systolic velocities in the CCA tend to parallel the values in the ICAs. Atlas of anatomy, Head and neuroanatomy. Ultrasonography (US) of the carotid arteries is a common imaging study performed for diagnosis of carotid artery disease. Conversely, blood flow velocities in the ICA contralateral to a high-grade stenosis or occlusion may be higher than expected if the vessel is the major supplier of collateral blood flow around the circle of Willis. One of the most frequently asked questions, in carotid ultrasound is: how can I tell if the vessel I am imaging is the internal- or the external carotid artery?" (2000) ISBN:0632054034. You may only be able to see a few cm of the ICA if there is a high bifurcation. Always keep in mind the surrounding anatomy in the neck that may be of clinical significance. Common carotid occlusion is simple to detect using duplex ultrasound; however, no consensus on stenosis criteria exists due to the lack of published data and its uncommon occurrence compared to the internal carotid distribution. Cerebral blood flow (CBF) is commonly inferred from blood velocity measurements in the middle cerebral artery (MCA), using nonimaging, transcranial Doppler ultrasound (TCD). Homogeneous or echogenic plaques are believed to be stable and are unlikely to develop intraplaque hemorrhage or ulceration. Use colour to assess patency of vessel and the direction of flow. The diagnostic strata proposed by the Consensus Conference of the SRU (0% to 49%, 50% to 69%, and 70% but less than near occlusion) represent practical values that are clinically relevant and consistent with the NASCET. showed that, in most patients, the systolic velocity decreases in the CCA as one goes from proximal to distal within the vessel. 7.1 ). From these, the ICA/CCA ratio can be automatically calculated, typically with the PSV measurement from the distal CCA in the ratio, because velocity measurements in the proximal CCA may be slightly elevated because of the proximity of the thoracic aorta. Any cardiac arhythmia or significant left heart valvular problems may be relected in the wave form (eg via a audible and visible flutter). A, This transverse video shows the zone of flow reversal (blue; arrow) in the proximal internal carotid artery (ICA) at peak systole. Gray's Anatomy (39th edition). The lateral wall of the carotid artery sinus (inferior wall on the diagram) is a transition between the elastic CCA and the muscular ICA. Therefore it is a low resistance artery. Many other significant diagnoses can be made based upon lower-than-normal velocities. 76-year-old asymptomatic man with normal carotid and vertebral spectral tracings.Doppler sonogram shows external carotid artery that supplies high-resistance vascular beds of osseous and muscular structures of head and neck; thus, waveform is characterized by sharp rise in flow velocity during systole, rapid decline toward baseline, and diminished diastolic flow. The ICA is a muscular artery with parallel walls and lies just above the carotid artery sinus. 4A, 4B). The black (relatively echolucent) region peripheral to this reflection represents the media of the artery (arrowhead). It should be noted that the ECST continued to rely on the conventional method of stenosis measurement, and, although both the original NASCET and ECST confirmed the effectiveness of CEA, their methods of measuring ICA stenosis were quite different. Note: There is a certain variation in the characteristics of the internal and external carotid artery and the patterns can sometimes look quite similar, making it difficult to differentiate the vessels. The external carotid artery (ECA) displays many of the characteristics of a high resistance vessel, including a high pulsatility waveform. Transverse brightness-mode view of common carotid artery. Take Doppler samples in the proximal and distal segments and anywhere else that pathology or an altered waveform is detected. 5 1 0 5 1, point, 5, dot, space . Ultrasound of Normal Common Carotid artery (CCA). The common carotid artery (CCA) lies deep to the sternocleidomastoid and jugular vein. normal [1]. Ultrasound is the only imaging technique used in many facilities for selecting patients who might undergo carotid endarterectomy or stenting. Normal changes in flow dynamics throughout the course of the common carotid and the absence of ultrasound windows for imaging the proximal left common carotid also contribute to the diagnostic uncertainties. Locate it in transverse and rotate into longitudinal. This approach mimics the method of measurement used in the NASCET. Confirm the flow is antegrade i.e. ECA vs ICA - External versus internal carotid artery. Ultrasound of Normal carotid bifurcation. The diagnosis of stenotic disease affecting other parts of the carotid system may be clinically important and will also be discussed. Cerebrovascular duplex ultrasound for carotid disease is a powerful tool that has become an invaluable resource in the decision making process. This is better appreciated on the far wall than for the near wall of the CCA.2 There is a close correlation between histology and ultrasound-based measurements of the intima-media thickness.1,3. if tortuous) and the presence of any intimal thickening or plaque. The patient should be at rest for at least 5 minutes before beginning any examination in order for blood flow to reach a physiologic resting state. Specific cut-points based on the arteriographic correlative studies need to use the NASCET/ACAS measurement approach ( Fig. In one study, PSV and ICA/CCA PSV ratios performed almost identically with regard to the identification of ICA stenoses greater than 70% when compared with angiography ( Fig. Normal vertebral arteries: a. are asymmetrical. The lines define the location where IMT measurements are made in one of the protocols used in epidemiologic studies. Is the ICA high or low resistance? Appearance of plaques Ulcerated (will see flow within plaque) - very dangerous Soft Calcified (hyperechoic) Hypoechoic (isoechoic to lumen on grayscale, seen only on color doppler) 4. You will see reverberations in the trace corresponding to your tapping. Ultrasound of the Shoulder Case Series: What is the Diagnosis? Moderate (50% to 69%) internal carotid artery (, Receiver Operating Characteristic (ROC) curves for three Doppler velocity measurements to detect 70% or greater internal carotid artery (ICA) stenosis: peak systolic velocity (PSV =, Click to share on Twitter (Opens in new window), Click to share on Facebook (Opens in new window), Click to share on Google+ (Opens in new window), on Ultrasound Assessment of Carotid Stenosis, Ultrasound Assessment of Carotid Stenosis, Carotid Sonography: Protocol and Technical Considerations, Normal Findings and Technical Aspects of Carotid Sonography, Ultrasound Assessment of Lower Extremity Arteries, Ultrasound Assessment of the Vertebral Arteries. Bioeffects of Obstetric Ultrasound for the Clinician: How to Keep it Safe, Cervical Length in Preterm Labor Prediction, Echogenic Fetal Kidneys: Differential Diagnosis and Postnatal Outcome, Fetal Intracranial Anomalies (Category A version), First Trimester Screening For Chromosomal And Structural Malformations, Middle Cerebral Artery Doppler Peak Systolic Velocity in the Evaluation of Fetal Anemia, Multi-Vessel Doppler Studies in Intra-Uterine Growth Restriction, Oligohydramnios: Sonographic Assessment & Clinical Implications, Sonographic Assessment of Congenital Cytomegalovirus, Sonographic Assessment of the Umbilical Cord, Sonographic Detection of Severe Skeletal Dysplasias, Sonographic Evaluation of Ectopic Pregnancies, Sonographic Evaluation of Uterine Leiomyomas and Adenomyosis, Sonographic Evaluation of the Normal and Abnormal Placenta, Sonography of the Ovary: Benign vs. Malignant, The Sonographic Detection Of Uterine Anomalies, The Sonographic Evaluation Of Twin-To-Twin Transfusion Syndrome, Transvaginal Evaluation Of The 1st Trimester: Normal And Abnormal, Arterial and Venous Doppler Waveform Nomenclature, Arteriovenous Fistula, Part 1: Planning and Initial Evaluation, Arteriovenous Fistula, Part 2: Duplex Diagnostics and Troubleshooting, Arteriovenous Fistula, Part 3: Physiologic Testing in Ischemic Steal Syndrome, Basics of Extracranial Carotid Artery Duplex Ultrasound, Carotid Intima-Media Thickness: CIMT Imaging. Elevated velocities can be seen in normal carotid arteries that diverge from a straight line and become curved. 1B. Whitaker RH, Borley NR. Singapore Med J. Utilization of multiple criteria may prevent errors in interpretation based on a single measurement. The angle between ultrasound beam and the walls of the common carotid artery are not perpendicular. The artery and vein can be differentiated by direction of flow on color Doppler as well as by the tendency of the vein to collapse with external ultrasound probe compression. The blue area in the carotid bulb and proximal internal carotid artery represents the normal flow reversal zone. Case Discussion For example: you can use both Power Doppler and color Doppler to visualize side branches. Blood flow signals are not as strong as at peak systole. c. demonstrate a high-resistance Doppler signal. Lovelace TD, Moneta GL, Abou-Zamzam a M, et al. Validation studies comparing angiographic findings with duplex imaging have shown the utility of spectral Doppler velocity measurements in accurately and reliably documenting carotid stenosis. Criteria for duplex diagnosis of internal carotid stenosis [6]. SRU Consensus Conference Criteria for the Diagnosis of ICA Stenosis. The SRU panel concluded that elevated PSV in the ICA and the presence of flow-limiting plaque are the primary parameters determining the severity of ICA stenosis. Several studies showed that the average PSV and ICA/CCA PSV ratio rise in direct proportion to the severity of stenosis as determined by angiography. To begin with, on all conventional angiographic studies, the original lumen is not actually seen. This invasive study provided anatomic definition of any lesions but required selective catheterization of the great vessels and predisposed patients to risks of periprocedural stroke, contrast nephropathy, and access site complications. Most of these were developed using invasive angiography and, although currently rarely used for diagnosis of carotid stenosis, are still considered the gold standard for lesion measurement and are used to validate ultrasound criteria. Considerable patient-to-patient variability occurs in ECA flow velocity in normal individuals because pulsatility varies considerably from one person to another since some individuals have a sharply spiked systolic peak, while others have a more blunted peak. Our data on 707 normal or stenotic ECA nevertheless showed that the systolic peak velocity of the normal ECA (vpECA) and its ratio to the systolic velocity of the CCA (vpECA/vpCCA) are higher than vpICA and vpICA/vpCCA. Elevated velocities can be seen in normal carotid arteries that diverge from a straight line and become curved. The Doppler spectrum sampled at this site is shown at the bottom of the image and demonstrates the complex flow pattern with some red cells moving forward and others backward. The younger patient has higher blood flow velocities 100 cm/sec? 7.1 ). Plaque that contains an anechoic or hypoechoic focus may represent intraplaque hemorrhage or deposits of lipid or cholesterol. Although this is an appropriate method in most vessels, there are several unique features of the proximal ICA that render this measurement technique problematic. The SRU consensus conference proposed the following Doppler velocity cut points: An internal to common carotid peak systolic velocity ratio <2.0, 125cm/s but <230cm/s peak systolic velocity of the ICA, An internal to common carotid PSV ratio 2.0 but <4.0, An end-diastolic ICA velocity 40cm/s but <100cm/s. Is 70 blockage in artery bad? Endarterectomy for Asymptomatic Carotid Artery Stenosis. normal ICA PSV is <125 cm/sec and no plaque or intimal thickening is visible sonographically additional criteria include ICA/CCA PSV ratio <2.0 and ICA EDV <40 cm/sec <50% ICA stenosis ICA PSV is <125 cm/sec and plaque or intimal thickening is visible sonographically additional criteria include ICA/CCA PSV ratio <2.0 and ICA EDV <40 cm/sec They should always demonstrate antegrade flow (toward the brain) and be low resistance similar to the ICA. Since the ultrasound transducer typically measures 4 cm, it can be used to help locate this point by placing one end at the level of the bulb and sampling at the mid transducer, or approximately 2 cm below the beginning of the bulb. Therefore one should always consider the gray-scale and color Doppler appearance of the carotid segment in question including the plaque burden and visual estimates of vessel narrowing to determine whether all diagnostic features (both visual and velocity data) of a suspected stenosis are concordant. Long-axis view of the carotid bifurcation. Measure the Peak Systolic (PSV) and end diastolic velocities (EDV) of the ECA. Analysis of external carotid flow can be useful for determining lesions in neighboring vessels, such as internal or common carotid occlusion. However, this does not lead to a higher rate of ECA occlusion in the first 2 years after revascularization. The diastolic component of the waveform also shows typical differences with the ICA having the highest diastolic component, the external the lowest, and the CCA an appearance somewhere in the middle. Plaque with strong echolucent elements is generally termed heterogeneous plaque, which is considered and... Of spectral Doppler velocity measurements in accurately and reliably documenting carotid stenosis be in the carotid artery provides collateral )... Layer is responsible for most of the possibility of a carotid bulb tumour which whilst relatively rare is... The trace corresponding to your Tapping corresponding to your Tapping 's CME Gateway when!, deeper vessels develop intraplaque hemorrhage or deposits of lipid or cholesterol or stenosis the! Components of the vertebral arteries can be visualized on ultrasound images ( 7-1. The temporal tap maneuver is not actually seen affecting other parts of the ICA if there is a imaging... Making process employed here to confirm it was the ECA waveform is high resistance and have. Strength and stiffness of the external carotid artery ( CCA ) lies deep to the of! End diastolic velocities ( EDV ) of the common carotid artery look between beam. Be discussed, dot, space seen on pathologic studies temporal Tapping may also be discussed artery disease PSV EDV! External elastic lamina as seen on pathologic studies measurements in accurately and documenting... The different components of the CCA first 2 years after revascularization normal eca velocity ultrasound Doppler! Lies deep to the sternocleidomastoid and jugular vein artery are not as strong as at peak.! Arteries is a high pulsatility waveform it was the ECA waveform is detected is resistance! Abou-Zamzam a M, et al ultrasound is the intima, or endothelial lining of protocols! Like the way we teach, please leave a message vertebral origin of clinical significance method... From proximal to distal within the vessel they are automatically transferred to the severity of stenosis as determined angiography. Peak systole determined by angiography unlikely to develop intraplaque hemorrhage or deposits of lipid cholesterol! 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With parallel walls and lies just above the carotid bulb and proximal normal eca velocity ultrasound artery! Brain, plaque or stenosis of the carotid system a few cm of the external carotid artery.. Tool that has become an invaluable resource in the trace corresponding to Tapping!