Carotid Artery Disease

In 2006, the UK Health Technology Assessment (HTA) Programme published a meta-analysis of studies evaluating the accuracy of noninvasive imaging (but not including multidetector computed tomographic angiography [MDCTA]) that concluded that although contrast-enhanced MRA (CEMRA) appeared the most accurate imaging modality (Table 98-2), its “clinical effectiveness” was limited by (in)accessibility, (un)availability, and delays.² As a consequence, the researchers, who “constituted a panel of experts in stroke, imaging, vascular surgery, statistics and health economic modelling,” concluded that DUS should remain the first-line imaging modality for identifying patients with 70% to 99% ICA stenoses because of (1) low cost, (2) the much higher number of strokes likely to be prevented in the long term (because patients are scheduled more rapidly for surgery), (3) robustness in sensitivity analyses (see Table 98-2), and (4) the ability of DUS to match the needs of current surgery (i.e., patients were selected for surgery through attendance at “single visit clinics” incorporating DUS with a low probability of the surgeon encountering unexpected findings at operation that might otherwise compromise patient safety).³ The panel did, however, highlight concerns about DUS accuracy for diagnosing 50% to 69% stenoses. Because the benefit conferred by CEA in patients with 50% to 69% stenoses falls significantly with any delay to surgery (see Table 98-1), the panel recommended that if 4 weeks or more have elapsed after the index event, it would be advisable to perform corroborative imaging (CEMRA or MDCTA) in order to confirm stenosis severity. If, however, DUS has been performed within 4 weeks of the index event, the panel judged it reasonable to proceed to CEA on the basis of DUS findings alone because the number of strokes prevented through rapidly performed surgery exceeds the potential risk to patients with less than 50% stenoses undergoing inappropriate surgery.

Vertebrobasilar Disease

Extracranial DUS offers limited information about flow dynamics in the vertebral arteries (VAs), but nothing about the basilar system (which requires color transcranial DUS). It is not always possible to image the vertebral artery origins, and only those sections running between the transverse processes can be imaged more distally. Accordingly, if a patient presents with suspected vertebrobasilar symptoms, DUS assessment can exclude coexistent carotid disease, but diagnostic imaging with MRA/MDCTA is necessary. DUS is, however, useful for demonstrating complete or partial vertebral artery flow reversal, which suggests the possibility of proximal subclavian or innominate artery disease.

Role in Screening

Although the Society for Vascular Surgery advocates carotid screening in patients older than 55 years with cardiovascular risk factors, the 2007 U.S. Preventive Services Task Force recommends against it.⁴ A reluctance to screen also exists in Canada, the UK, and Scandinavia. For those committed to screening, DUS is the first-line modality. Portable DUS machines are available, but there is little data about their accuracy. Accordingly, users of portable machines are advised to undertake a corroborative DUS study in an accredited vascular laboratory (or use MDCTA/CEMRA) if a greater than 50% stenosis is suspected. DUS remains the preferred screening modality prior to coronary artery bypass grafting (CABG) surgery. Screening everyone is not cost-effective (5% yield for 80%-99% stenosis⁵) but the yield is increased in selected patient cohorts (left mainstem disease, carotid bruit, previous stroke/transient ischemic attack [TIA]). Any patient who is being considered for CABG using the internal mammary artery (IMA) and who has weakness or absence of the radial pulses should undergo assessment of the subclavian, innominate, and vertebral arteries. Where inflow disease or vertebral artery flow reversal is suspected, further imaging (MDCTA/CEMRA) is essential, and the cardiac surgeon should be warned that unless the flow reversal is corrected, subsequent use of the ipsilateral internal mammary artery (as a conduit) could cause postoperative coronary steal syndrome (Fig. 98-1).

Role in Trauma

DUS provides a useful screening and surveillance role in patients with zone II carotid artery injuries (between cricoid cartilage and angle of mandible), especially intimal irregularities or small false aneurysms that are being treated conservatively. DUS is, however, less reliable in zone I injuries (clavicle to cricoid cartilage) or zone III injuries (angle of mandible to skull base). MDCTA is now the investigation of choice because vascular imaging can be combined with an evaluation of concurrent bony and soft tissue injuries (see later).

Evaluation of Carotid Body or Glomus Vagale Tumors

Because of their vascularity, DUS is ideally suited for diagnosing carotid body tumor (CBT) and glomus vagale tumor (GVT). Given the rapid availability of DUS, it is unwise to perform biopsy of a “lymph node” in the jugulodigastric region without undertaking DUS. Once such a tumor is suspected, MDCTA or MRA should be performed to exclude bilateral lesions (5% incidence) and to image the upper and lateral extents of the lesion. Large, highly placed lesions require a totally different operative strategy, which must be anticipated preoperatively (temporomandibular subluxation cannot be performed once the operation is under way; see Chapter 104). DUS findings can also alert the surgeon to the possibility that a GVT might be present (Fig. 98-2), thus enabling the patient to be counseled about vagus nerve division (hoarseness, swallowing difficulties). Carotid body tumors cause splaying of the carotid bifurcation. GVTs do not splay the bifurcation, but they do cause displacement of the distal ICA (see Fig. 98-2). In addition, GVTs tend to have vascular serpiginous feeding vessels tracking down the proximal vagus nerve. These usually lie separately from the bifurcation, internal jugular vein, and common carotid artery (CCA).

Role in the Planning of Carotid Endarterectomy and Carotid Artery Stenting

In many centers, up to 95% of CEAs are undertaken on the basis of DUS findings, and there is no evidence that reliance on DUS compromises safety or operability.³ More importantly, patients can be identified and scheduled for surgery faster than with any other imaging modality, thereby optimizing the long-term benefit conferred by surgery (see Table 98-1). There are, however, important caveats. First, the vascular laboratory should be accredited and should have clear and validated criteria for measuring carotid stenosis. Second, European vascular units must be absolutely clear about which measurement method is being used (NASCET/ECST; see Chapter 16); evidence suggests that there may still be confusion on this matter.⁶ Third, the physician must be aware of DUS findings that suggest tandem inflow/outflow problems or subocclusion (see later). For either situation, corroborative imaging is mandated (MDCTA, CEMRA). In practice, one of the most frequent pathologic lesions to be missed with DUS is the coiled distal ICA (see Chapter 104). In most situations the coil does not cause undue problems for the surgeon, but it can be a cause of shunt failure (and intraoperative hemodynamic stroke) if the distal shunt tip becomes occluded against the distal loop and the problem is not recognized. This can be avoided by using intraoperative transcranial Doppler ultrasound (TCD) to assess shunt flow. DUS can also be useful in identifying unusual causes of cerebral vascular symptoms, such as the ICA stump syndrome (Fig. 98-3). In this rare condition, an occluded ICA stump (identifiable on DUS) acts as a reservoir for fresh thrombus that can then embolize up the external carotid artery (ECA) and into the brain via retrograde flow through the supraorbital and infraorbital vessels. Finally, some surgeons utilize DUS in the operating room to localize the carotid bifurcation and thus vary the level and orientation of the incision.

By contrast, CAS cannot be planned on the basis of DUS findings alone. Table 98-3 details anatomic and morphologic features that either are associated with difficulties in performing CAS or might influence the choice of cerebral protection device (CPD) or stent. For example, choice of CPD depends on ECA patency, integrity of the circle of Willis, and whether the distal ICA is straight and nontortuous. Stent selection is determined by varying combinations of vessel tortuosity and plaque friability. DUS can demonstrate (1) the presence of an appropriate ipsilateral stenosis, (2) ECA patency, (3) extent of contralateral disease, (4) tortuosity around the bifurcation, (5) plaque morphology, and (6) the possibility of coexistent inflow and outflow problems. However, DUS cannot provide information regarding (1) type of aortic arch, (2) extent of calcification at the great vessel origins, (3) proximal common carotid artery tortuosity, (4) whether there is an adequate distal landing zone for a filter CPD, or (5) the status of the circle of Willis. If the circle of Willis is not intact (Fig. 98-4), CAS practitioners tend not to deploy the flow reversal type of CPD. This type of information can be provided by only MDCTA or CEMRA (see later). However, it is important to remind CAS practitioners that reliance on additional imaging will introduce delays in planning the management of symptomatic patients. If delays are likely to be excessive (i.e., >2 weeks), patients may be better treated by CEA.

Role in Evaluating Plaque Morphology

DUS enables interpretation of carotid plaque morphology (Table 98-4)^7,8. Most practitioners would agree that better methods are needed to identify vulnerable or “at risk “ plaques in asymptomatic patients (as opposed to stenosis severity alone), and high-resolution DUS will likely prove useful to be one of them. In the Asymptomatic Carotid Surgery Trial (ACST), there was no evidence that the Gray-Weale classification (see Table 98-4) predicted outcome in medically treated asymptomatic patients,⁹ but the Asymptomatic Carotid Stenosis and Risk of Stroke (ACSRS) study showed that if computerized “image normalization” was under­taken, 94% of strokes destined to occur in a cohort of 1000 asymptomatic patients with 50% to 99% stenosis severity had Geroulakis type 1 to 3 plaques^8,10 (see Table 98-4).

The weakness of DUS plaque morphology studies has been a lack of objectivity. This has been partially addressed by gray-scale median (GSM) measurement,¹¹ in which high-resolution B-mode images are combined with a quantita­tive computer-assisted measurement of plaque echogenicity (Fig. 98-5). The GSM is a numerical measure representing the average echogenicity of the plaque (low values are observed in predominantly echolucent plaques, whereas higher values are observed in more fibrous lesions). Evidence from the Imaging in Carotid Angioplasty and Risk of Stroke (ICAROS) study suggested that a GSM value of 25 or less was associated with a 7.1% stroke rate after CAS, in comparison with 1.5% for a GSM value higher than 25.¹² This study was one of the first to suggest that an objective assessment of plaque morphology using DUS could be used to plan management (i.e., when to avoid CAS).

Ultrasonography shows considerable promise in helping identify a cohort of asymptomatic patients with a higher risk for stroke in whom to prioritize CEA or CAS.¹³ In one study, the presence of three or more plaque ulcers on high-resolution three-dimensional (3D) ultrasound was associated with a significantly higher risk of late ipsilateral stroke (3-year risk 20%) than absence or the presence of up to two micro-ulcers (3-year risk 2%).¹⁴ This study also corroborated data from the Asymptomatic Carotid Emboli Study, which showed that plaque morphology predicted late, ipsilateral stroke in asymptomatic patients and that the combination of spontaneous emboli and plaque morphology conferred a greater predictive value than either imaging modality alone.¹⁵

The ACSRS group, undertaking a number of outcome analyses in a cohort of 1000 asymptomatic patients with 50% to 99% stenoses who were treated medically, has proposed a relatively simple clinically and DUS-derived scoring system for predicting late stroke on the basis of (1) GSM, (2) stenosis severity, (3) a history of contralateral TIA/stroke, and 4) plaque area.¹⁶ This scoring system, shown in Figure 98-6, illustrates how predicted annual rates of ipsilateral stroke might vary from less than 1% to more than 10%.¹³ This scoring system still requires independent validation but could emerge as a very useful method for predicting individual patient risk in the outpatient clinic.

Perioperative Roles

DUS should be repeated immediately prior to CEA if more than 4 weeks has elapsed since the initial DUS. This policy ensures that the ICA has not become occluded (i.e., CEA is unnecessary). It is also the final opportunity to detect distal disease extension (last chance for consideration of temporomandibular subluxation or alternative exposure techniques) and fulfills an important quality control/internal validation role. Intraoperative DUS can also offer completion assessment following flow restoration, by showing (1) residual filling defects (luminal thrombus), (2) undue turbulence, and (3) evidence of intimal flaps/residual stenoses (see Chapter 100). No randomized trial has determined whether a completion assessment by DUS reduces procedural risk. One overview suggested that it made no difference,¹⁷ but this assertion should be interpreted with caution because all events (fatal cardiac, intraoperative/postoperative strokes, all stroke etiologies) were combined in the analyses.

Postoperative Roles

Because of its versatility, DUS is invaluable in guiding the management of postoperative neurologic deficits. These often occur when other imaging modalities are unavailable and DUS can be brought to the patient’s bedside. Air in the deep tissues can interfere with insonation early after surgery, but it is usually possible to exclude thrombosis. Diagnosing the exact cause of the deficit can be difficult, and the role of DUS is enhanced if combined with TCD (see later).

The role of serial DUS surveillance after CEA and CAS remains enduringly controversial. As with screening, such surveillance is rarely undertaken in the UK and Scandinavia but remains common practice in North America, Australasia, and mainland Europe. It might be argued that an early postoperative DUS to assess the technical result of the operation and a follow-up study at 1 year would identify the majority of patients who will experience significant intimal hyperplastic recurrent lesions. However, the contrary opinion is that the identification of these lesions is of little clinical utility because this pathology is usually a benign phenomenon (see Chapters 100 and 101). On a practical note, those who offer DUS surveillance after CAS must be aware that stents change the physical properties of the carotid artery, this requiring different diagnostic criteria.¹⁸

DUS is, however, useful in warning about the possibility of late prosthetic patch infection (Fig. 98-7). DUS is the first-line investigation in patients returning with pulsatile neck swellings, discharging sinus tracts, or neck pain, and the recognition of patch “corrugation” may precede onset of clinical symptoms by up to a year¹⁹ (i.e., recognition of this phenomenon can alert the surgeon to the possibility of patch infection and antibiotics can be started early). DUS can also identify “leaks” into early false aneurysms (see Fig. 98-7).

Accuracy and Limitations in Clinical Practice

The most important determinants for the diagnostic accuracy of DUS are the experience and skill of the operator. Second is awareness of the wide array of stenosis thresholds being used in individual centers (>50%, >60%, >70%) and the spectrum of peak systolic velocity (PSV) and velocity ratios being used to diagnoses stenosis subgroups. This uncertainty has been lessened by publication of North American DUS consensus criteria.²⁰

Third is establishing the ECST or NASCET measurement criteria used. UK experience suggests that many vascular laboratories remain unsure about this matter.⁶ Interestingly, the North American consensus document provides no advice for grading the large carotid bulb that contains large amounts of atherothrombotic material. This could score 0% with the NASCET measurement method (i.e., no need for CEA) but 70% stenosis with the ECST method (i.e., CEA justified). Future guideline makers must address this anomaly.

Fourth, in some situations, caution should be exercised in interpretation of DUS findings, and corroborative imaging should be considered. These include (1) inability to image high bifurcations, (2) inability to image above the plaque, (3) damped waveforms in the proximal common carotid artery suggesting inflow stenosis, (4) high-resistance flow in the distal ICA suggesting a distal tandem lesion, (5) the presence of a contralateral occlusion or very severe stenosis, because peak systolic velocity values may be increased in the contralateral ICA (due to hyperemic collateralization), leading to the potential for overestimation of degree of ipsilateral stenosis on the basis peak systolic velocity mea­surement, (6) excessive calcification (which prevents accurate velocity measurement because of acoustic shadowing), and (7) suspicion of subocclusion. The last condition has aroused considerable controversy. Although subocclusion (i.e., a severe ICA stenosis that does not open out into a normal-caliber lumen) was previously considered to be a marker of increased stroke risk, the Carotid Endarterectomy Trialists’ Collaboration observed that it is not a high-risk predictor for stroke and that CEA does not confer long-term benefit for patients with the condition.¹

Carotid Disease

MRA has assumed an increasingly important role in the evaluation of patients with carotid artery disease. Table 98-2 summarizes the sensitivity analyses from the HTA systematic review for time-of-flight (TOF) MRA and CEMRA. The sensitivity and specificity for diagnosing a 70% to 99% stenosis with TOF MRA is identical to that for DUS and similarly poor for diagnosing 50% to 69% stenoses. Two-dimensional (2D) TOF MRA provides strong vascular signals, even when flow is low (useful for differentiating occlusion from subocclusion), whereas 3D TOF MRA offers better spatial resolution for measuring severity of stenosis and also enables assessment of flow directionality (useful in evaluating steal phenomena). However, in order to minimize the long acquisition times and artifacts (flow-related, motion and boundary), the field of view has to be restricted. Accordingly, although it is possible to image both carotid bifurcations simultaneously, two further data acquisitions are required to image the aortic arch/great vessels and the circle of Willis with TOF MRA. This limits its overall versatility and accessibility and means that, for the most part, TOF MRA offers little in addition to DUS.

CEMRA uses the paramagnetic agent gadolinium and obtains images more rapidly than TOF MRA. CEMRA incurs fewer flow-related artifacts and provides a much greater field of view that enables high-resolution imaging from the aortic arch (Fig. 98-8) up to the circle of Willis while retaining the ability to also evaluate flow directionality. The HTA systematic review and meta-analysis (see Table 98-2) concluded that CEMRA was the best (noninvasive) imaging modality (although MDCTA was not evaluated) and it has completely superseded TOF MRA. However, some centers still do not have rapid access to CEMRA imaging. Accordingly, if it is going to take several weeks for CEMRA to be performed, the symptomatic patient will gain greater clinical benefit by undergoing expedited CEA on the basis of DUS findings alone (see Table 98-1). Another advantage of CEMRA is that it can be combined with MR functional brain imaging, although (unlike MDCTA) doing so requires an additional period of data acquisition. The use of blood-pool contrast agents now facilitates high-resolution image acquisition by allowing equilibrium-phase imaging, thereby enlarging the temporal window.

There is emerging evidence that the presence of infarction on CT/MRI can identify patients at increased risk of suffering an early recurrent stroke after presenting with a TIA or minor stroke (Table 98-5). The ABCD2 score is a clinically based scoring system that predicts the 48-hour and 7-day risk of stroke on the basis of age, blood pressure, clinical presentation, duration of symptoms, and diabetes.²¹ The maximum score is 7, and the risk of recurrent stroke increases as the score increases (see Table 98-5). Accordingly, the presence of infarction on CT/MRI in a patient with a recent TIA or minor stroke should alert the surgeon about an increased risk of early recurrent stroke, prompting expedited CEA. Interestingly, one study found that where no infarction was present, a high ABCD2 score no longer predicted an increased risk of recurrent stroke.²²

Role in Vertebrobasilar Disease

One of the advantages of TOF MRA and CEMRA is that they can provide information regarding flow directionality. This information is useful in evaluating steal phenomena (subclavian, coronary). Both modalities, however, are limited by an increased likelihood of artifact at the vertebral artery origins due to vessel tortuosity, respiratory motion, and cardiac pulsation. In practice, MDCTA is probably the preferred (noninvasive) investigation for evaluating patients with suspected vertebrobasilar symptoms.

Role in Screening

MRA has no role as a screening modality. It is simply not cost-effective.

Role in the Evaluation of Trauma

MRI/MRA studies are seldom undertaken in trauma patients. The reason is logistical issues, including scanner location (in relation to the emergency department or operating room), prolonged acquisition times, and poor access to an unstable patient for monitoring. In practice, MDCTA is the preferred modality because of its rapid acquisition times (5 seconds for aortic arch to circle of Willis) and its ability to image adjacent bony and soft tissue structures (including the brain) with a single scan.

Role in the Evaluation of Carotid Body or Glomus Vagale Tumors

MRA and CEMRA provide basic information, which is useful for planning surgical management and for excluding contralateral lesions (Fig. 98-9). However, MRI is limited by the fact that no “bony landmarks’ ” are retained on the image. Accordingly, MDCTA is probably the best imaging modality in this situation, largely because of the high-resolution axial images that detail the full extent of the lesion and its relationship to adjacent bony structures.

Role in the Planning of Carotid Endarterectomy and Carotid Artery Stenting

Surgeons who regularly perform CEA on the basis of DUS alone are unlikely to find any additional role for CEMRA and little or no use for TOF MRA. In the event that DUS findings are nondiagnostic, MDCTA is probably preferable to CEMRA (see later). However, CAS requires much more imaging information during the planning process than CEA (see Table 98-3). Accordingly, the choice among CEMRA, MDCTA, or digital subtraction angiography (DSA) for planning CAS will reflect resources, clinician preferences, and local expertise. CEMRA is preferable to conventional spiral CT, whereas MDCTA is generally accepted to be superior because of its versatility and ability to combine anatomic imaging with the identification of calcification, vessel tortuosity, and vascular anomalies in addition to the ability to perform functional brain imaging during one period of data acquisition.

CEMRA can identify vessel anomalies, but if arteries are occluded, they can sometimes be missed because of the way images are acquired. As is apparent from Table 98-3, the main limitation of CEMRA is its inability to display calcification. Documenting extent of calcification in the arch and origins of the great vessels is not a major problem for CEA, but with increasing age, the arch and great vessels become more tortuous and calcified. These features make CAS more challenging (especially for less experienced practitioners²³) and are associated with greater procedural risks. A further problem with CEMRA is the tendency for multiple vessels to be included in the final image (some venous). This can lead to difficulties in overall image interpretation (see Fig. 98-8).

Role in Evaluating Plaque Morphology

There is growing interest in the role of MRI for evaluating subtle changes in carotid plaque morphology, plaque type, and plaque volume that may help identify a subgroup of patients who may be at higher risk of suffering a stroke if left untreated (see Chapter 99). In this respect, MRA has shown considerable research potential, although it still requires relatively long periods of data acquisition (i.e., plaque morphology data cannot be acquired during vessel or brain imaging studies). MRI is probably the best modality for visualization of the fibrous cap²⁴ and lipid core,²⁵ whereas inflammatory molecules within the plaque can be imaged using MR spectroscopy (still very much a research tool). Newer contrast agents that facilitate prolonged imaging now permit higher imaging resolutions to be achieved without compromising the signal-to-noise ratio. They allow greater accuracy in the depiction of ulcers and of subtle plaque surface irregularities.²⁶

Considerable interest has focused on the ability of MRI to diagnose intraplaque hemorrhage (IPH), thought by many to be one of the most important predictors of increased stroke risk. In a multivariate analysis, the High-Resolution Magnetic Resonance Imaging in Atherosclerotic Stenosis of the Carotid Artery (HIRISC) study showed that an MRI diagnosis of IPH was associated with increasing stenosis severity (measured using both the European and North American measurement methods), hemispheric (as opposed to retinal) events, and very recent symptoms.²⁷ In a series of themed projects, Altaf et al^28–31 showed that an MRA diagnosis of IPH was associated with (1) increased spontaneous embolization in recently symptomatic patients, (2) increased embolization during the operative dissection portion of CEA in symptomatic patients, (3) an increased prevalence of white matter hyperintense lesions (leukoaraiosis), and (4) a much higher risk of ipsilateral recurrent stroke/TIA in symptomatic patients with 70% to 99% stenoses awaiting CEA and in recently symptomatic patients with 30% to 69% stenoses managed medically.

One of the key imaging goals is to identify the asymptomatic patient a who is at high risk of stroke, and the HIRISC study showed (in a multivariate analysis) that MRI evidence of IPH in asymptomatic patients was associated with increasing stenosis severity (using the European but not the North American measurement method²⁷). A few studies have now evaluated MRI plaque features in asymptomatic patients, although most have been restricted to patients with less severe carotid stenosis (i.e., 50%-79%). Takaya et al³² observed that asymptomatic patients with MRI evidence of a thinned or ruptured fibrous cap, large lipid core, or IPH faced a higher risk of late ipsilateral stroke than those without these features. Similarly, Singh et al³³ showed that 6 of 36 asymptomatic patients (17%) with MRI evidence of IPH suffered an ipsilateral stroke during follow-up, compared with 0 of 62 who did not exhibit this feature on MRI.

Perioperative Roles

There is no role for MRI in the perioperative setting.

Postoperative Roles

MRI/MRA does not have a role in the investigation of patients suffering a neurologic deficit after CEA or CAS. The exception might be the rare patient suffering a suspected hyperperfusion syndrome stroke in whom CT suggests there is an evolving ischemic infarct (usually in the posterior circulation). Perfusion MRI usually demonstrates that this area of “infarcted” brain tissue remains normally perfused and that the area of low attenuation on CT represents vasogenic as opposed to cytotoxic edema.³⁴ New hyperintense white matter lesions (attributed to microembolization) have been well documented in the early period after CEA (and particularly after CAS³⁵), but there is no evidence (as yet) that theses lesions predispose to declining cognitive function in the long term. Finally, CEMRA has no role in surveillance after CEA/CAS, partly because it will never be cost-effective, but also because of the metallic stents used in CAS.

Carotid Artery Disease

PET-CT has no role in the routine evaluation of patients being considered for CEA or CAS. However, because of its ability to measure cerebral blood flow (CBF) and oxygen extraction fraction (OEF), PET remains the gold standard for evaluating the hemodynamic effect of extracranial disease,³⁷ particularly autoregulatory failure and impaired cerebral vascular reserve. To date, a measurement of perfu­sion reserve has never found a role in the investigation of patients with classic thromboembolic carotid disease, but it could evolve into an important role in identifying patients with carotid occlusion, hemodynamic failure, and recur­rent carotid territory symptoms who might benefit from extracranial-intracranial bypass. This possibility is currently being evaluated in ongoing research studies. There is also evidence from a systematic review and meta-analysis that the presence of impaired cerebral vascular reserve in patients with asymptomatic carotid stenoses may serve as a predictor of increased late stroke risk if treated medically.³⁸ Although this could become a novel role for PET in the future, it is likely that less expensive, more accessible, and less technologically complicated imaging strategies will assume this function.

Role in Evaluating Plaque Morphology

PET scanners offer (very) good spatial resolution (2 mm) and many are now combined with CT scanners (PET-CT devices) to allow tomographic radionuclide imaging (with anatomic correlation). This raises the possibility that a number of inflammatory processes could be targeted within the carotid plaque (inflammation, macrophages, apoptosis, angiogenesis, protease enzyme function³⁹). The ability to link metabolic activity with plasma biomarkers and plaque imaging could have important implications for the future (patient selection, monitoring of drug therapy). Some studies have utilized novel PET tracers to differentiate plaque types, but (currently), PET-CT cannot compete with DUS or MRI.

Other Roles

PET-CT has no role in the evaluation of vertebrobasilar disease, screening, the evaluation of trauma, perioperative management, or the evaluation of carotid body/glomus tumors.

Carotid Artery Disease

CTA is less likely to overestimate stenosis severity, than MRA. In the 2006 HTA Programme meta-analysis of noninvasive imaging modalities (summarized in Table 98-2²), only CTA with spiral CT was evaluated. The published data suggested little overall advantage (compared with DUS and TOF MRA) with regard to diagnosing surgically important (and irrelevant) carotid disease. Accordingly, those increasingly rare centers with this older type of CT technology will preferably use CEMRA if DUS provides discordant findings.

Since the HTA panel reported their deliberations in 2006, 64-, 256-, and 320-section MDCTA have been developed and are widely available. MDCTA has revolutionized the role of CTA because (1) it is extremely fast (5 seconds to perform a scan from aortic arch to circle of Willis; Fig. 98-10), (2) it is cheaper than CEMRA, (3) it offers submillimeter spatial resolution⁴⁰ (0.3-mm spatial resolution compared with 0.8-mm for CEMRA and 0.2-mm for DSA), (4) sophisticated imaging workstations enable the production of high-quality 2D and 3D reformatted images from the aortic arch to circle of Willis that are easy to interpret (see Fig. 98-8), (5) it offers faster processing times, (6) it can visualize soft tissue, bone, and vessel at the same time, (7) it can rapidly demonstrate vascular anomalies even if vessels are occluded (Fig. 98-11), and (8) it is able to provide data regarding the extent of vessel calcification, especially in the aortic arch (Fig. 98-12). These characteristics make it possible to define extracranial and intracranial vascular anatomy in combination with functional brain imaging during one period of data acquisition. Stenoses (NASCET or ECST) can be measured directly with electronic microcalipers.

Vertebrobasilar Disease

Imaging the vertebral arteries has always presented a challenge (see Fig. 98-10). The vertebral artery has a tortuous course, a thick bony covering, adjacent veins, and a large variation in normal vessel caliber, and it is not unusual to find that one is hypoplastic. MDCTA provides better images of the extracranial and intracranial vertebrobasilar system than CEMRA, although the latter is preferable if only spiral CT is available. One important advantage of MDCTA is that it retains bony landmarks (see Fig. 98-10) and vessel coverings (which can then be “virtually dissected off” if required). Vessel tortuosity is easily demonstrated, and there is no problem with artifact at the vertebral artery origins. MDCTA cannot, however, provide information regarding flow directionality. If this is required (subclavian, coronary steal phenomena), CEMRA (or DUS) should be undertaken.

Role in Screening

There is no role for CTA as a screening tool. It is not cost-effective, and exposing patients to the radiation involved would be unethical.

Role in the Evaluation of Trauma

MDCTA has an important role in the evaluation of trauma, largely because it can rapidly provide anatomic vascular images from the arch to the circle of Willis, together with bony, soft tissue, and functional brain imaging during a single scan, even in a patient undergoing mechanical ventilation.

Role in the Evaluation of Carotid Body or Glomus Vagale Tumors

Owing to its better spatial resolution and easy interpretation, MDCTA is probably the optimum imaging modality for evaluating tumor circulation (see Fig. 98-9), the extent of the tumor, whether it is bilateral, and the relationship between tumor and adjacent soft tissue structures. This information is usually more than enough to plan a safe surgical resection and can be relied upon to show whether a high or difficult dissection might be encountered (i.e., enabling the surgeon to plan in advance for temporomandibular subluxation as required).

Role in the Planning of Carotid Endarterectomy and Carotid Artery Stenting

Like CEMRA, MDCTA is not needed before CEA in most patients. However, if there is evidence of inflow/outflow disease or excessive calcification on DUS, MDCTA can reliably provide the additional information required to plan management. MDCTA is, however, currently the optimal noninvasive imaging modality for planning CAS (see Table 98-3). The reason is partly the production of easily interpretable 3D images from the aortic arch to the circle of Willis (see Fig. 98-10) but also the ease of demonstrating vessel tortuosity (see Fig. 98-10), abnormalities of the circle of Willis (see Fig. 98-4), patterns of calcification in the aortic arch (see Fig. 98-12), and status of the origins of the great vessels (see Fig. 98-8). Unlike CEMRA, MDCTA is unaffected by occluded anomalous vessels arising from the arch (see Fig. 98-11). Procedural risks after CAS increase with age, probably in relation to progressive calcification/tortuosity of the arch and great vessels, which can lead to difficulties in accessing the carotid arteries with guide wires, sheaths, and so on (see Fig. 98-12). Although many of the adverse features detailed in Table 98-4 may not “deter” the experienced interventionist, they should help the less experienced operator (on his/her learning curve) to identify patients in whom CAS would be unsuitable or suboptimal.

Role in Evaluating Plaque Morphology

At present, the role of CT in evaluating plaque morphology is limited (compared with DUS and MRI). MDCTA has been used to divide carotid plaques into three subgroups on the basis of plaque density measurement (measured in Hounsfield units [HU]). Other visualization or postprocessing tools, such as MIP (maximum intensity projection), MPR (multiplanar reformat), and VR (volume rendition), are less effective (especially MIP) at evaluating plaque components. A “soft” plaque (<50 HU) usually has a soft lipid-rich core and is more likely to be associated with symptoms and perhaps a higher procedural risk after CAS. At the other extreme is the “calcified” plaque (>120 HU), which seems to be associated with a lower risk of symptoms. In between is the “inter­mediate” plaque (50-119 HU).^41,42 These findings have been corroborated in a systematic review on the association between carotid plaque calcification and stroke risk, in which Kwee⁴³ observed that plaques with the lowest volume and proportion of calcification were more likely to be clinically symptomatic, possibly because of the effect on shear stress.

The preceding section on MRI (see Table 98-5) observed that the presence of infarction on CT/MRI is highly predictive of an increased risk of stroke in the early time after onset of TIA or minor stroke symptoms²² irrespective of the ABCD2 score. This finding should alert the surgeon of the need to consider expedited CEA.

With regard to plaque morphology, the available evidence suggests that MRI is better than CT for visualizing thrombus, the fibrous cap, and its rupture (largely because MRI is better than CT at differentiating soft tissues structures), but there is evidence that MDCTA may be better at identifying plaque ulcers.^44,45 IPH (provided it is >1 mm in diameter) can be visualized indirectly on MDCTA but is not as reliably predicted as with MRI. MDCTA can readily measure carotid artery wall thickness (CAWT), with a thicker (>1 mm) wall being associated with an increased risk of stroke.⁴⁶

Perioperative Roles

There is no role for CTA in the perioperative management of the patient with cerebral vascular disease.

Postoperative Roles

MDCTA has a potentially important role in the evaluation of any patient suffering a stroke or other neurologic problem in the postoperative period after CEA and CAS. In the first 12 hours, the likelihood of intracranial hemorrhage (ICH) is remote. Accordingly, the patient should generally be considered for re-exploration, which should not be delayed in order to perform a CT scan (in most circumstances). However, because MDCTA can often be performed very rapidly, it may be useful in selected cases of early postoperative neurologic deficit, especially if completion assessment DUS or DSA findings were normal and intracranial embolism is suspected. After 12 hours has elapsed, there is an increasing likelihood that the stroke is due to intracranial hemorrhage or the hyperperfusion syndrome. In this situation, it is important to avoid an unnecessary (and potentially dangerous) return of the patient to the operating room. An emergency CT scan excludes hemorrhage but may demonstrate features associated with the hyperperfusion syndrome (Fig. 98-13). In the latter condition, middle cerebral artery velocities are generally elevated (frequently in association with hypertension) and it is not unusual for the CT scan to display areas of low attenuation (usually in the posterior fossa) that can sometimes be misinterpreted as representing an evolving infarct.³⁴ Diffusion-weighted MRI studies have shown that the white matter edema seen in the hyperperfusion syndrome on CT is vasogenic (if it was an evolving infarct, it would be cytotoxic), whereas perfusion studies show that there is no loss of blood supply.⁴⁷ Quite why the hyperperfusion syndrome causes vasogenic edema in the posterior circulation has never been adequately explained.

If intracranial hemorrhage and hyperperfusion syndrome have been excluded, there remains the possibility that the neurologic deficit was thromboembolic in origin (see later section on TCD). DUS of the carotid bifurcation can be difficult to interpret in the early postoperative period (air in the deeper tissues, edema, overlying hemostatic gauze), and it may not be possible to exclude an intraluminal filling defect. In this situation, MDCTA quickly provides high-quality images of the extracranial circulation, which will not unduly delay return of the patient to surgery if necessary. Finally, MDCTA does not have a routine role in serial surveillance after CEA and CAS; this is better achieved with DUS.

Carotid Artery Disease

There is no role for routine catheter diagnostic arteriography in modern cerebrovascular practice. However, there are still situations in which intra-arterial digital subtraction angiography (IADSA) can provide valuable diagnostic information (see Fig. 98-14). Improvements in CEMRA and (especially) MDCTA mean that it is rarely necessary to undertake IADSA in order to look at the origins of the great vessels (see Figs. 98-8 and 98-11). Similarly, MDCTA can reliably image patients with heavily calcified plaques in the arch or great vessel origins (see Fig. 98-12). However, CEMRA and MDCTA can provide misleading information in the patient with severe distal ICA disease (Fig. 98-15) and the patient with marked coiling of the ICA in the upper reaches of the neck (see Fig. 98-14). In the former, slow flow through a tiny lumen can be missed. In the latter, unless extreme care is taken with image processing (i.e., ensuring that the axial measurements are in a true perpendicular cross-sectional plane), it is possible that an artifact introduced during image processing may persuade the surgeon or interventionist that significant carotid disease exists.

Vertebrobasilar Disease

There is no role for routine IADSA in the investigation of patients with suspected vertebrobasilar disease. CEMRA and MDCTA are now the investigations of choice, and it is rare that a diagnostic IADSA is necessary.

Role in Screening

There is no role for IADSA as a screening tool in carotid or vertebrobasilar disease.

Role in Trauma

Angiography was previously the first-line imaging modality in patients with a suspected extracranial arterial injury. This role has, once again, been superseded by MDCTA, which can perform a diagnostic study from arch to circle of Willis in 5 seconds. However, IADSA remains an important secondary option in those situations in which MDCTA image quality is compromised or its findings difficult to interpret. Furthermore, angiography retains the unique distinction of allowing one to proceed with endovascular therapeutic interventions (coil/balloon occlusion, insertion of a covered stent).

Role in the Evaluation of Carotid Body or Glomus Vagale Tumors

There is no role for IADSA in the routine evaluation of patients being considered for resection of carotid body tumor or GVT. It provides no information on the overall extent or operability of the lesion. However, preoperative embolization of ECA feeding vessels is considered by some to reduce perioperative bleeding (Fig. 98-16), whereas insertion of a covered stent into the ECA can significantly reduce bleeding in patients with very large tumors.⁵⁰

Role in the Planning of CEA/CAS

Most centers now perform the majority of CEAs without preoperative IADSA. Large studies suggest that reliance on such a strategy does not compromise patient safety and the surgeon rarely encounters operative scenarios that cannot be dealt with safely.³ In reality, the safe performance of CEA does not depend on preoperative imaging of inflow or distal vessels (unless DUS suggests an abnormal flow pattern), and there is no evidence that imaging the intracranial circulation alters outcome. The preceding section does, however, summarize those situations in which a diagnostic IADSA would be advisable.

By contrast, some CAS practitioners have been reluctant to dispense with diagnostic IADSA (usually as an arch angiogram) as part of the routine evaluation of their patients. Advocates of this strategy must, however, remember that the need for a diagnostic angiogram (in place of or in addition to CEMRA/MDCTA) will inevitably delay treatment and so potentially reduce the overall effectiveness of any procedure (see Table 98-1) while exposing the patient to two catheterization procedures. The advent of CEMRA and (especially) MDCTA, together with the drive toward expedited treatment, has probably rendered this practice obsolete. In practical terms, this reliance on an additional diagnostic arch angiogram could mean that on very rare occasions, a patient may be evaluated for CAS solely on the basis of CEMRA/MDCTA findings only to have the procedure abandoned following the diagnostic IADSA that immediately precedes the planned CAS procedure. In reality, this is unlikely to happen because of excellent pre-interventional imaging and planning.

Role in Evaluating Plaque Morphology

DSA has no role in evaluating plaque morphology other than demonstrating surface irregularity, and angiography is not particularly good at identifying intraluminal thrombus.⁵¹ Although NASCET and ECST showed that plaque irregularity (using angiography) was associated with an increased risk of stroke in medically treated patients, as well as an increased risk of major cardiac events during follow-up,⁵¹ there is no evidence to justify IADSA for plaque morphology evaluation.

Perioperative Roles

Intraoperative completion angiography can identify technical errors (intimal flaps, luminal thrombus, residual stenoses) following flow restoration but has been superseded by color DUS (facilitated by dedicated L-shaped probes) or angioscopy. Completion angiography can be cumbersome; it requires the presence of a C-arm and radiology technician in the operating room and (for most patients), represents an unnecessary exposure to radiation.

Angiography does, however, retain a useful diagnostic role in the rare patient who suffers a stroke due to carotid thrombosis or acute distal dissection in the early postoperative period after CEA or CAS (Fig. 98-17). After CEA, the key to ensuring the best outcome is early re-exploration (preferably within 1 hour). At re-exploration, it is not uncommon to find a “pulse” in the ICA (suggesting to the unwary surgeon that the vessel is still patent). In this situation, it is important to avoid manipulation of the bifurcation, which could precipitate major embolization. Despite the apparent pulsation, it is not unusual for the angiogram to demonstrate complete occlusion (see Fig. 98-17). Angiography can also identify the very rare case of acute distal dissection that may follow shunt trauma (see Fig. 98-17). In the latter situation, the diagnostic angiogram can be followed by insertion of a stent/covered stent into the true lumen, whereas embolic occlusion of intracranial vessels can be treated by intra-arterial, low-dose thrombolysis or catheter-directed embolus retrieval.

Thromboembolic stroke in the first few hours after CAS raises the possibility of in-stent thrombosis (Fig. 98-18). For a detailed discussion of CAS and the prevention and management of its complications, see Chapter 101.

Postoperative Roles

There is no role for IADSA in the serial surveillance of patients after CEA or CAS.

Carotid Artery Disease

TCD has no role in imaging carotid disease, but it can provide a simple evaluation of the hemodynamic effect of a steno­sis (compared with PET) by measuring the proportional increase in middle cerebral artery (MCA) velocity in response to intravenous administration of acetazolamide (a cerebral vasodilator). TCD is, however, the only modality capable of diagnosing in vivo embolization. This is because the larger embolus reflects more back-scattered signal than red cells, and despite initial skepticism, there are now well-accepted and validated criteria for diagnosing embolization and for excluding artifact.⁵² Embolization is more commonly encountered in the MCA ipsilateral to a symptomatic carotid stenosis and is indicative of an unstable plaque. Recogni­tion of this phenomenon should prompt the surgeon to undertake an expedited CEA, and although it seems intuitively obvious that patients with these findings might be suboptimal candidates for CAS, this assumption has never been properly evaluated.

TCD-diagnosed spontaneous embolization may, however, assume an increasingly important role in identifying a cohort of patients with asymptomatic carotid disease who are at high risk for stroke. A systematic review and meta-analysis of six prospective observational studies involving 1144 patients with asymptomatic 70% to 99% stenosis observed that the presence of a single embolic signal during TCD monitoring was associated with a near eightfold increase in stroke risk during follow-up (odds ratio 7.6, 95% CI 2.3-24.7).⁵³ A further systematic review and meta-analysis of the relationship among spontaneous embolization, carotid stenosis severity, and silent cerebral infarction has shown that (1) spontaneous embolization was significantly more common in symptomatic than asymptomatic patients, (2) the prevalence of spontaneous embolization was significantly higher in patients with a 70% to 99% asymptomatic stenosis (as opposed to a stenosis <70%), (3) that the rate of ipsilateral stroke in patients with asymptomatic embolization was significantly higher than in those with no embolization (10% vs. 1%), and (4) patients with evidence of type A silent cerebral infarction (defined as cortical or subcortical, in or adjacent to the anterior or MCA territories or basal ganglia/thalamic infarctions) were nearly five times more likely to suffer a late stroke (but only if they had a 60%-99% stenosis).⁵⁴

Vertebrobasilar Disease

One useful practical role for TCD in the vertebrobasilar circulation involves patients who report isolated dizziness or vertigo following head movements. In the past, many were labeled as having suffered vertebrobasilar TIAs due to compression of the vertebral arteries within the transverse processes of the upper cervical vertebrae during neck movement. Subsequent research has shown this to be rarely the case. In a series of 46 symptomatic patients undergoing extracranial DUS and intracranial TCD with the head rotated into provocative positions, not one patient exhibited flow reversal or flow reduction,⁵⁵ suggesting that most historical cases were probably misdiagnosed. In reality, most patients with such symptoms have inner ear pathologies.

Role in the Evaluation of Trauma

TCD has no role in the management of trauma other than being used to monitor the effect of carotid ligation/endovascular balloon occlusion in an unconscious patient if reconstruction proves impossible. During CEA, provided that mean MCA velocity is higher than 15 cm/sec, most patients tolerate carotid ligation (balloon occlusion) without suffering a stroke.⁵⁶ Whether this velocity threshold also applies to patients with head trauma has never been evaluated.

Intraoperative Monitoring

The main reason that intraoperative monitoring and/or completion quality control assessment have failed to gain wider acceptance is a simple failure to ask the appropriate questions.⁴⁴ TCD warns of embolization (enabling the surgeon to modify technique), but it cannot prevent a stroke due to embolization of luminal thrombus after restoration of flow. Some other form of quality control completion assessment is required for that. Accordingly, TCD is used to answer only four questions during CEA⁵⁷: (1) is there spontaneous embolization during operative carotid dissection (i.e., warning the surgeon of an unstable plaque, thereby allowing modification of dissection technique); (2) is the shunt working (3% of shunts malfunction following insertion);⁵⁸ (3) is MCA velocity higher than 15 cm/sec (if not flow can be increased by pharmacologic elevation of blood pressure); and (4) is this patient one of the very rare ones destined to have ICA thrombosis before the operation is completed (Fig. 98-19). At present, the role of TCD monitoring during CAS has not been clarified, but the potential to answer some of the questions posed previously during CEA could also be relevant for CAS.

Role in Evaluating Postoperative Strokes

TCD has a useful (but unexploited) role in determining the cause of intraoperative and postoperative stroke. Intraoperative stroke follows inadvertent technical error. Hemodynamic failure accounts for only 20% of intraoperative events, with the remainder being thromboembolic.⁵⁷ In a large, prospective audit involving more than 2000 patients undergoing CEA, intraoperative stroke was virtually abolished with the use of TCD and completion angioscopy.⁵⁹ Although intimal flaps were occasionally diagnosed on angioscopy, 4% to 5% of patients had retained luminal thrombus (despite venting and irrigation), which were derived from transected vasa vasorum.⁵⁹

For practitioners without recourse to angioscopy or other quality control methods, the patient wakening with a new neurologic deficit poses a considerable management dilemma. However, if TCD-derived MCA velocity is now the same as was observed during carotid clamping, it is highly likely that the carotid artery has thrombosed (see Fig. 98-19). If MCA velocity is unchanged but embolization is ongoing, it is likely that there is an evolving thrombus within the endarterectomy zone. Both of these scenarios warrant immediate reoperation.

However, the etiology of postoperative stroke is multifactorial. Most strokes occurring more than 24 hours after surgery are due to intracranial hemorrhage or the hyperperfusion syndrome. Unfortunately, no TCD protocol has been developed to reliably anticipate and prevent either of these situations. In the first 24 hours after CEA, postoperative carotid thrombosis remains the most frequent etiology of stroke.⁶⁰ Although it was previously considered unpredictable, there is now growing evidence that postoperative carotid thrombosis is probably related to patient-mediated factors (e.g., increased platelet sensitivity to adenosine diphosphate⁶¹) and may be preventable.

In the past, the Leicester Vascular Unit performed TCD monitoring for 3 hours after CEA.⁵⁷ The aim was to identify the 5% cohort of patients with high rates of early postoperative embolization (who faced a 50% risk of progression to thrombotic stroke⁶²), in whom to target incremental-dose dextran therapy.⁵⁷ However, a randomized trial subsequently showed that the addition of a single 75-mg dose of clopidogrel the night before surgery (in addition to routine aspirin therapy) virtually abolished postoperative embolization.⁶³ A subsequent prospective audit observed that periopera­tive dual-antiplatelet strategy was associated with virtual abolition of significant postoperative embolization with no progression to thromboembolic stroke.⁶⁴ Consequently, all postoperative TCD monitoring in the Leicester Vascular Unit ceased in 2009.