INTRODUCTION
Carotid artery stenting has become an accepted treatment for carotid artery disease and is often the preferred treatment in high-risk patients or those who may present a surgical challenge. Carotid artery stents are routinely followed with duplex ultrasound using surveillance protocols similar to those for patients with carotid endarterectomy surgery. While there are common components with carotid stent and endarterectomy ultrasound evaluations, there are additional areas that require close examination.
ULTRASOUND PROTOCOL
Ultrasound of carotid stents should be performed with standardized techniques. Imaging frequency should be optimized to provide the best image resolution possible for the depth of insonation required. Focal zones, gain, power and other imaging presets should also be optimized. Spectral Doppler should be performed with the angle of insonation at 60 degrees and the sample volume within the center of the vessel. Angles greater than 60 degrees should never be used. With most patients a consistent 60 degree angle is possible using variation in Doppler steering, toe-heal maneuvering techniques or alternate scanning approaches to the vessel (medial, lateral, anterior or posterior). Laboratories align the angle to be parallel to the vessel walls although some use the color jet. Studies have shown little variation between velocities determined with Doppler cursor alignment to the wall as compared to the color jet. However to limit technical variability a laboratory should consistently use the same alignment method. Color imaging presets may also need to be optimized to avoid artifacts that can sometimes be encountered with scanning through stents. Color gain and scale should be carefully adjusted to allow for appropriate color filling of the vessels without producing bleeding of the color displays outside the lumen. Lastly, as with routine carotid ultrasound imaging, power Doppler can be employed to achieve clearer edge definition of the stent walls, in vessels with low flow states, in vessels with suspected occlusion or to aid in characterizing flow in tortuous vessels.
The entire cervical portion of the carotid system should be examined. Using only gray-scale imaging the common (CCA), internal (ICA) and external (ECA) carotid arteries should be scanned using both transverse and longitudinal views. At a minimum a representative longitudinal image should be documented for the CCA, the entire portion of the stent and the distal ICA beyond the distal edge of the stent.
Spectral Doppler should be recorded at multiple levels in the native vessels as well as the stented portions. Typically at least two Doppler waveforms may be recorded from the CCA, one from the ECA and one from the vertebral artery. Doppler waveforms from the proximal, mid and distal portions of the stent are obtained. In addition an ICA waveform distal to the stent is also recorded. Figure 1 illustrates an example of a carotid stent extending from the distal CCA through the mid ICA and the levels at which spectral waveforms should be obtained. Peak systolic velocity (PSV), end diastolic velocity (EDV) and ICA/CCA
PSV ratio are recorded.
Figure1: A diagram of a carotid stent (in red) placed from the distal
CCA through themid ICA. The blue asterisks indicate the levels a
which spectral Doppler should be recorded.
Color flow imaging should be used to assess blood flow dynamics. Areas of disturbed flow and stenoses can be rapidly identified with color flow imaging. Color can be used to assist in the placement of the spectral Doppler gate at areas of increased velocities. Power Doppler can also be used to evaluate
flow in tortuous segments or in cases of suspected occlusion.
DIAGNOSTIC CRITERIA
The gray-scale image should demonstrate normal smooth walls of the native vessels adjacent to the stent. Any evidence of residual disease in the CCA or distal ICA should be documented with transverse or longitudinal views. In figure 2, the proximal edge of this carotid stent is placed in the mid-portion of the CCA.
Figure 2: A longitudinal view of the CCA with a stent placed
beginning in the mid portion of the CCA.
Figure 3 illustrates a carotid stent placed entirely within the ICA with the proximal edge of the stent at the beginning of the ICA. The walls of the stent should be apposed to the walls of the vessel and the stent itself should appear relatively uniform in diameter (Figure 4).
Figure 3: A longitudinal view at the level of the carotid bifurcation.
A stent is placed beginning at the origin of the ICA.
Figure 4: A longitudinal view of a carotid stent completely positioned within the ICA.
The stent walls appear fully deployed producing a uniform lumen throughout the vessel.
The stent should be evaluated for evidence of incomplete deployment or stent compression. The distal edge of the stent should be closely examined. The transition from the stent to the distal ICA usually appears smooth (Figure 5a). In some carotid vessels there is more of an angle present in the distal ICA which results in a sharper angle between the distal ICA and the stent (Figure 5B). This more abrupt angle can occasionally be the location of hyperplasia and narrowing.
Figure 5a: This image of the distal edge of a carotid stent shows a smooth
transition from the stent to the native distal ICA.
Figure 5b: This image of a carotid stent demonstrates a sharper angle
between the distal portion of the stent and the native distal ICA.
Occasionally, the vessel walls may be injured during the placement of the stent or various cerebral protection devices. The image of the vessels walls should be closely examined for pathology such as intimal flaps or tears, dissections or other trauma to the vessel wall which may result in platelet aggregate formation. Rarely, injury to the wall may result in the serious complication of a pseudoaneurysm. First using gray-scale imaging, an anechoic area may appear adjacent to the stented vessel (Figure 6a). The presence of blood flow outside the vessel lumen is confirmed using both color (Figure 6b) and spectral Doppler (Figure 6c).
Figure 6a: Gray-scale imaging of a carotid stent with a small anechoic
area adjacent to the anterior wall.
Figure 6b: A transverse view with color indicating flow
outside the lumen of the ICA consistent with a pseudoaneurysm.
Figure 6c: Spectral Doppler within the area of the pseudoaneurysm
demonstrates dampened pulsatile flow.
Even though the stents themselves are highly reflective they usually do not produce acoustic artifacts. However, the primary artifact encountered is due to calcific plaque. During the stenting procedure, the plaque is not removed. Angioplasty techniques fracture the plaque followed by the immediate deployment of the stent to dilated vessel. The plaque remains and so does any calcification which may be present and thus giving rise to acoustic shadowing (Figure7 ).
Figure 7: Image of an ICA stent with large calcific plaque at proximal
portion of vessel producing acoustic shadowing as indicated at red arrow.
The calcification and resultant shadowing may be significant enough to block the ultrasound evaluation of some portions of the stent. In these instances, signals distal to the calcification will be important to indirectly determine the presence of disease. Post-stenotic turbulence will help identify areas of significant disease.
Within the first two years following the placement of a carotid stent neointimal hyperplasia may occur. The degree at which this occurs varies between patients and if severe will lead to in-stent restenosis. High-resolution imaging will detect echogenic material within the vessel lumen along the walls of the stent. The magnitude of the restenosis will be determined via the velocity parameters measured.
Of note, in patients where a carotid stent is placed across the origin of the ECA, changes in ECA velocities may be observed. Increased velocities or turbulence are often encountered within the ECA without the presence of significant plaque.
Currently there is no universally accepted velocity parameters used to characterize carotid stents. Figure 8 illustrates normal velocities through a patent stent and normal color filling.
Figure 8: A patent carotid stent with normal velocities and good color filling.
Many publications have reported carotid stent velocities higher than those observed in native vessels. These higher than expected velocities are found in stents which angiography have been shown to be free of disease. An early study by Ringer, et al, describes finding elevated velocities of greater than 125 cm/s in 40 of 114 patients despite no angiographic evidence of significant stenosis. They suggest a variation in compliance, resulting from the placement of the stent, may be the result of the elevated velocities. Researchers have postulated that the higher velocities are related to the degree of calcification or stiffness of a plaque. It is thought that since the plaque is not removed, dense stiff plaques will decrease the compliance of a vessel such that velocities are elevated.
Lal and colleagues have examined carotid stent restenosis and have concluded that <150 cm/s in a carotid stent best correlates with a normal lumen. They have investigated patterns and restenosis and have published their criteria for restenosis. They suggest a PSV of 220-339 cm/s and an ICA/CCA > 2.7 as criteria for a 50-79% stenosis. A PSV > 340 cm/s and an ICA/CCA >4.15 suggests a > 80% stenosis. These criteria are similar to an earlier study by Stanziale et al which used a PSV > 225 cm/s and an ICA/CCA ratio of> 2.5 for a > 50% stenosis while a > 70% stenosis was characterized as having a PSV > 350 cm/s and an ICA/CCA ratio of > 4.75.
AbuRahma conducted a prospective study in 144 patient undergoing carotid stenting. They used receiver operating characteristic curve (ROC) analysis to determine the optimal velocity criteria for detection of >30, >50, and >80% in-stent stenosis. They concluded that the optimal duplex velocity criteria for in-stent stenosis of >30%, ?50%, and ?80% were the PSVs of 154, 224, and 325 cm/sec, respectively. The ICA PSV of ?224 cm/sec used for the diagnosis of ?50% in-stent stenosis had a sensitivity of 99% and specificity of 90%. The ICA PSV of ?325 cm/sec used for the diagnosis of ?80% in-stent stenosis had a sensitivity and specificity of 100% and 88%, respectively.
Armstrong and Bandyk followed 114 carotid stents for a mean period of 33 months. They stratified their data into stenosis categories of < 50%, 50-75% and >75%. Figure 9 is a summary of their diagnostic criteria for carotid stent restenosis.
Figure 9: University of South Florida Carotid Stent Diagnostic Criteria
Color image can quickly demonstrate areas of disease. As shown in Figure 10a, poor color filling within the stent can be seen as well as color aliasing. Proper documentation of a stent stenosis will include multiple samples through the stent. Figure 10b demonstrates a severe restenosis with a PSV of 503 cm/s and an EDV of 226 cm/s. Just distal to this area but still within the stent, velocities drop slightly and significant turbulence is evident (Figure 10c). Continuing to sample beyond the stent within the distal ICA, velocities diminish further (Figure 10d) and turbulence is still present owing to the severity of the stenosis.
Figure 10a: Color image of a restenosis within a carotid stent.
Figure 10b: Spectral waveforms from the mid-portion of the stent indicating
increased velocities associated with the severe stenosis.
Figure 10c: Spectral waveforms within the distal stent still demonstrate
elevated velocities with turbulence present.
Figure 10d: Diminished velocities and turbulence present within the distal ICA.
FOLLOW-UP INTERVALS
Carotid stents should be followed at a similar interval used for carotid endarterectomy patients. An initial ultrasound can be performed prior to discharge but often is done at about one month after the stent was placed. At this early scan, technical problems such as failure to fully deploy the stent can be observed. Continued ultrasound surveillance is then done at 6, 12, 18 and 24 months and usually once a year thereafter. The development of neointimal hyperplasia progressing to restenosis can be observed to occur within the first two years after stent placement. The goal of surveillance post-stent or surgical procedure is to detect a restenosis prior to occlusion. Patients that develop elevated velocities should be placed into more frequent follow-up intervals.
CONCLUSION
Duplex ultrasound surveillance is an essential component in the long term management of patients with carotid stents. Sonographers and interpreting physicians must be aware of the unique ultrasound elements associated with carotid stents. While carotid stents may display velocities similar to those observed in native carotid arteries, higher than expected velocities are often encountered. No clear consensus has been reached on velocity values useful in identifying restenosis. Various new criteria have been published and further studies will help define restenosis parameters.
REFERENCES
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AbuRahma AF, Abu-Halimah S, Bensenhaver J, et al.
Optimal carotid duplex velocity criteria for defining the severity of carotid
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Armstrong PA, Bandyk DF, Johnson BL, et al.
Duplex scan surveillance after carotid angioplasty and stenting: a rational
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Lal BK, Hobson RW, Goldstein J, Chakhtoura EY, Duran WN. Carotid artery stenting: Is there a need to revise ultrasound velocity criteria? J Vasc Surg 2004;39:58-66.
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Lal BK, Kaperonis EA, Cuadra S, et al. Patterns of in-stent restenosis after carotid artery stenting: Classification and implications for long-term outcome. Journal of Vascular Surgery. 2007; 46: 833-840.
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Lal BK, Hobson RW II, Tofighi B, Kapadia I, Cuadra S, Jamil Z. Duplex ultrasound velocity criteria for the stented carotid artery. J Vasc Surg 2008;47:63-73. Ringer AJ, German JW, Guterman LR, Hopkins LN.
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Stanziale SF, Wholey MH, Boules TN, Selzer F, Makaroun MS. Determining in-stent stenosis of carotid arteries by duplex ultrasound criteria. J Endovasc Ther 2005;12:346-353.
