Ultrasound Clinics 2006

Robbin ML, Lockhart ME. Carotid Artery Ultrasound Interpretation Using a Pattern Recognition Approach. Ultrasound Clin 1 (2006) 111131   B-mode (gray-scale) Transverse gray-scale images of at least the proximal and distal CCA, bulb, bifurcation, and proximal ICA and ECA should be obtained with detailed labeling. Longitudinal gray-scale images of at least the proximal and distal CCA, bulb to CCA, proximal ECA extending into the CCA, and proximal ICA extending into the CCA should be obtained. It is not necessary to record gray-scale images of the vertebral artery without spectral Doppler, as only a limited portion of the vertebral artery can be seen between the vertebral segments.   Color/power Doppler One of the first things that should be addressed when imaging a vessel is the color Doppler gain. It should be increased until color speckles are seen outside of the lumen, then turned down until the color is displayed only within the vessel lumen. Spectral Doppler The normal default position should be parallel to the posterior carotid wall segment. The exception to this rule occurs when there is a color jet seen, usually extending distally or downstream from a stenosis. In this instance, the angle should be placed parallel to the direction of the jet and not the posterior carotid wall. A small sample gate width should be used, between 1.5 and 2.5 mm wide.   Disturbed flow patterns indicate that the movement of blood is less orderly than in laminar flow, and can be found in vessel kinks, curves, areas of arterial branching, and areas of mild stenosis. Turbulent flow is defined as areas of flow reversal, and usually occurs at areas of severe stenosis when a jet of blood with high velocity encounters a normal or increased lumen diameter beyond the stenosis [9]. The location at which the distal CCA waveform is measured within the CCA is standardized at 2 cm proximal to the bulb. If any stenoses are found, spectral waveforms should be obtained at and distal to the stenosis, with corresponding PSV and EDV measurements.   Waveform analysis The classic cause of a high-resistance waveform pattern in the CCA is a high-grade ICA stenosis or occlusion, termed externalization of the CCA. If a high-resistance waveform is seen in the CCA, the diagnosis of severe ICA stenosis or occlusion is suggested, and the distal waveforms should be assessed to determine whether they support the diagnosis [Fig. 2]. However, there is an exception to this rule. If the process is bilateral and the peak PSVs are low, this waveform pattern could indicate aortic stenosis or severe cardiac failure, discussed later.   Common carotid artery The first thing that should be assessed when interpreting a carotid ultrasound is the CCA waveform, because the CCA waveform is a combination of the ECA and ICA waveforms, and should have a moderate amount of diastolic flow. If this waveform is abnormal, downstream or upstream stenoses or occlusions are likely. A normal CCA PSV should be in the range of approximately 0.6 to 1.0 m/s [12]. If the CCA PSV is lower than this range, the contralateral CCA waveform should be examined. If the PSVs are symmetric, the cause of the low PSV is likely secondary to a low cardiac output, such as that seen with congestive heart failure. If the contralateral PSV appears more normal, the CCA with the low PSV is usually abnormal and needs to be investigated further. Potential causes for a unilateral low PSV in the CCA include a proximal or distal high-grade stenosis or occlusion. The first step in differentiating between these two causes can be made by determining whether the waveform is high-resistance or low-resistance. A high-grade stenosis can occur in either proximal CCA. On the right side, the stenosis or occlusion can also occur in the innominate artery (brachiocephalic artery), typically at its origin off the aortic arch. A high-grade proximal CCA stenosis will be evidenced by a downstream low-resistance waveform. Poststenotic turbulence may be demonstrated if sampled close to the stenosis, and sometimes a tardus parvus waveform can be seen [Fig. 4]. Occasionally the waveforms look normal, and the only way a proximal CCA stenosis can be suspected is by noting the asymmetry in the PSVs. A high-grade stenosis distal to the point of sampling in the CCA, such as higher in the neck or in the carotid siphon, will be reflected in a high-resistance waveform. The ratio of the highest PSV at the CCA stenosis divided by the PSV 2 cm proximal to the stenosis should be calculated as PSVcca at stenosis/PSVcca prox [Fig. 5]. Fig. 5. CCA stenosis. The PSV ratio is used to characterize the severity of the stenosis in this region. PSVB is at the CCA stenosis and PSV   Internal carotid artery Thus, a ‘‘normal’’ PSV can be either normal or reflect an underlying high-grade stenosis. This potential pitfall occurs when the PSV is the only criterion used for stenosis quantification, and can be avoided by also analyzing the ICAPSV/CCAPSV ratio. In the case of high-grade stenosis, the ICA EDV will also rise, as the blood flow downstream to the stenosis becomes more turbulent and low-resistance. The ICA EDV should also be assessed to avoid this pitfall. The maximum PSV in the ICA is also used to compute the ICAPSV/CCAPSV ratio. The maximum PSV in the ICA is divided by the PSV in the CCA (measured at 2 cm proximal to the carotid bulb). This ratio is also used to assess the degree of ICA stenosis (see Table 1). The ICAPSV/ CCAPSV ratio is an important double check of the PSV measured in the ICA to determine whether there is a high-grade stenosis as described earlier. This table is only used for ICA stenosis, not ECA or CCA stenosis. If there is a high-grade stenosis in one internal carotid artery, there may be a compensatory elevation in the contralateral ICA PSV not caused by a stenosis.   External carotid artery The two major reasons the ECA needs to be eval‎uated are to differentiate the ECA from the ICA and to identify the source of an audible bruit other than the ICA.   Vertebral artery Reversal of flow in the left vertebral artery is more common than the right and indicates a proximal subclavian artery stenosis or occlusion, usually at the origin from the aortic arch. Flow reversal in the right vertebral artery indicates either a proximal subclavian artery stenosis or occlusion, or a significant brachiocephalic artery stenosis or occlusion. If the flow reversal occurs in the right vertebral artery, a small footprint transducer should be used to attempt to visualize the proximal right subclavian artery and a portion of the brachiocephalic artery, which occasionally can be well visualized. Lesser degrees of subclavian or brachiocephalic stenosis result in transient systolic flow reversal in the vertebral artery [17]. There are progressive degrees of midsystolic deceleration patterns that ultimately completely reverse and correlate to the degree of proximal stenosis. Kliewer and colleagues [17] developed an excellent grading scheme for abnormal vertebral artery waveforms that are not completely reversed, which is also covered in the subsequent article. [17] Kliewer MA, Hertzberg BS, Kim DH, et al. Vertebral artery Doppler waveform changes indicating subclavian steal physiology. AJR Am J Roentgenol 2000;174(3):815–9.   A high-resistance waveform in the vertebral artery is consistent with a distal stenosis, occlusion, or a hypoplastic artery [Fig. 16]. A low-resistance, low PSV waveform in the vertebral artery may indicate a more proximal stenosis[Fig. 17]. Fig. 17. Vertebral artery stenosis. (A) Longitudinal spectral Doppler of right vertebral artery demonstrates lowresistance “parvus-tardus” waveforms. RT, right. (B) Further investigation discovered a focal stenosis of the vertebral artery origin with elevated PSV of 272 cm/s. RT, right; VA, vertebral artery.   Waveform Analysis of the Carotid Arteries Leslie M. Scoutta,*, Fig. 3. Aortic stenosis. Tardus parvus waveforms Fig. 14. Knocking waveform. This observation should raise suspicion for a distal occlusion or high-grade stenosis. Fig. 15. A high-resistance waveform pattern   Pitfalls in Carotid Ultrasound Diagnosis Beverly E. Hashimoto, MD When the stenosis is proximal to the site of insonation, the carotid waveform exhibits a low systolic velocity and a delayed, blunted systolic peak velocity (Fig. 8). If the stenosis is distal, then the carotid waveform shows a large systolic slope (sharp upstroke), and low or no diastolic flow (Fig. 9). If a distal carotid occlusion is suspected, the radiologist should eval‎uate the study to identify whether the flow in the CCA goes to zero at end diastole and whether there is a thump or a short waveform with a low systolic velocity and no diastolic flow. These findings are commonly associated with occlusion. Ultrasound Eval‎uation of Carotid Intima Media Thickness: The Hows and Whys Igor Latich, MD*, Leslie M. Scoutt, MD However, intraplaque hemorrhage, whether from rupture of the capsule or disruption of the vasa vasorum and vessels within the plaque, can sometimes be visualized on high-resolution ultrasound (US) or magnetic resonance (MR) images. On US, hemorrhagic plaque is typically hypoechoic or sometimes even anechoic as well as heterogeneous (Fig. 3). Homogeneous or echogenic plaque (Fig. 4) is more likely to be stable.39–41 Fig. 3. Hypoechoic plaque. Hypoechoic plaque, particularly if focal within heterogeneous plaque, suggests intraplaque hemorrhage. However, this finding is nonspecific and is seen with fibrous fatty plaque as well. Fig. 4. Echogenic plaque. Echogenic plaque has been reported to have no prognostic significance in most studies and does not necessarily indicate calcification, although calcification is more likely if shadowing is observed.   CURRENT RECOMMENDATIONS REGARDING MEASUREMENT OF CIMT In 2000, the Prevention Conference V of the American Heart Association concluded that, in asymptomatic persons more than 45 years old, carefully performed carotid ultrasound examination with CIMT measurement can add incremental information to traditional risk factor assessment. Fig. 10. (A, B) Irregularity of the carotid intima media layer. Whether it is more accurate or has more clinical significance to report the mean measurement for a 1-cm length of the vessel wall versus the maximum thickness, which would be different in these 2 patients, is not known. Reporting methodology is variable in most large trials. the CIMT should only be measured if the double echogenic lines representing the intima-media interface and advential interface can be seen in the near wall as well as the posterior wall, even though the near wall is not routinely measured; his will ensure that the center of the vessel is being scanned and a true CIMT measurement is obtained that is not exaggerated by inclusion of the curvature of the lateral wall. If the CCA is too deep (>4 cm), gentle compression of the overlying soft tissues or a different angle of interrogation can be attempted. If the vessel is too shallow (<4 cm), image degradation caused by slice thickness artifact or near-field artifact may occur and a standoff pad or thick application of gel can be tried.  If the vessel is tortuous, extending the neck or turning to the contralateral side may be helpful to straighten the CCA.