Carotid bifurcation

Review of postcontrast CTA source images might provide a good estimate of whole-brain perfusion." If time allows, MR or CT perfusion maps are obtained to characterize more accurately the ischemic penumbra." Careful but expedited preprocedural analysis of the CTA, done in parallel with transport of the patient to the treatment area, may be extremely helpful in establishing the presence of anatomic variants (e.g., bovine aortic arch) or pathological states (e.g., vessel origin or carotid bifurcation disease) prior to the catheterization procedure. 

CEA involves exposure of the carotid bifurcation in the neck to a point along the internal carotid artery (ICA) beyond which the atherosclerotic plaque terminates. 

Results of in vitro studies suggest an interaction between calcium ions and cyanide in cardiovascular effects (Allen and Smith 1985 Robinson et al. 1985a). It has been demonstrated that exposure to cyanide in metabolically depleted ferret papillary muscle eventually results in elevated intracellular calcium levels, but only after a substantial contracture develops (Allen and Smith 1985). The authors proposed that intracellular calcium may precipitate cell damage and arrhythmias. The mechanism by which calcium levels are raised was not determined. Franchini and Krieger (1993) produced selective denervation of the aortic and carotid bifurcation areas, and confirmed the carotid body chemoreceptor origin of cardiovascular, respiratory and certain behavioral responses to cyanide in rats. Bradycardia and hyperventilation induced by cyanide are typical responses evoked by carotid body chemoreceptor stimulation (Franchini and Krieger 1993). 

Carotid revascularization, initially by CEA, was introduced in early 1950s as a method to prevent stroke due to atherosclerosis of the carotid bifurcation and internal carotid artery (ICA). At least four prospective randomized trials have demonstrated... 

The carotid bifurcation can be seen in 80% of the adult population at the level of cervical vertebrae 3-5. Distal to the physiologic dilatation of the carotid bulb the internal carotid artery (ICA) proceeds dor-sally to the external carotid artery (ECA) into the petrous bone. Entering the bony carotid canal, it describes a sharp bend and thus causes turbulent flow patterns with somewhat typically symmetrical signal loss on TOF-MRA, which should not be misinterpreted as real stenoses (Fig. 5.2). 

Atherosclerosis mainly affects large- and mediumsized arteries. Extracranial manifestations at the carotid bifurcation statistically dominate the intracranial arteries. Besides typical manifestations at the carotid siphon or the vertebrobasilar junction, atherosclerosis is occasionally also found in peripheral intracranial vessel segments. Typical sequelae of atherosclerosis are stenosing plaque formations, ulcerations, dilatations or the evolution of fusiform aneurysms, which can be accompanied by extensive formation of thrombus. 

CE-MRA is a promising technique particularly for the evaluation of the proximal segments from the aortic arch covering the carotid bifurcation (Cloft... 

Fig. 15.9. Combined MRA (left) and plaque imaging (right) in a hypertensive patient with symptomatic left ICA stenosis. Cross-sectional black-blood imaging demonstrates the severely narrowed lumen (A and B) as well as the extension of the plaque to the carotid bifurcation (E), appearing normal on angiography...

Beere et al. (1992) described experimental atherosclerosis at the carotid bifurcation of the cynomolgus monkey by a cholesterol-enriched diet. 

Baxter A, Fitzgerald BJ, Hutson JL et al. (1992) Squalestatin 1, a potent inhibitor of squalene synthase, which lowers cholesterol in vivo. J Biol Chem 267 11705-11708 Beere PA, Glagov S, Zarins ChK (1992) Experimental atherosclerosis at the carotid bifurcation of the cynomolgus monkey. Arterioscl Thrombos 12 1245-1253... 

Fig. 6.6. Digitally subtracted arterial arigiogram showirig an ulcerated plaque at the carotid bifurcation, with contrast seen within the plaque (arrow).

There are a large number of rare arterial disorders or anomalies that can cause ischemic stroke (Tables 3.2 and 6.1). The frequency of many of these disorders has probably been underestimated because of under-investigation and lack of radiological imaging of the cerebral vasculature beyond the carotid bifurcation. 

A localized bruit over the carotid bifurcation (under the jaw) is predictive of some degree of carotid stenosis, but very tight stenosis or occlusion may not cause a bruit at all. Bruits may... 

Schmahmann JD (2003). Vascular syndromes of the thalamus. Stroke 34 2264-2278 Schulz UG, Rothwell PM (2001). Major variation in carotid bifurcation anatomy a possible risk factor for plaque development Stroke 32 2522-2529 Scott BL, Jankovic J (1996). Delayed-onset progressive movement disorders after static brain lesions. Neurology 46 68-74 Wardlaw JM, Merrick MV, Ferrington CM et al. (1996). Comparison of a simple isotope method of predicting likely middle cerebral artery occlusion with transcranial Doppler ultrasound in acute ischaemic stroke. Cerebrovascular Diseases 6 32-39 Wardlaw JM, Lewsi SC, Dennis MS etal. (1999). Is it reasonable to assume a particular embolic source from the type of stroke Cerebrovascular Diseases 9(Supp 1) 14... 

Compared with cut-film selective intra-arterial catheter angiography recorded directly on to X-ray film, intra-arterial digital subtraction angiography (DSA) (Fig. 12.1) is quicker the images are easier to manipulate and store and contrast resolution is better although spatial resolution is less. However, there is no evidence that less contrast is used or that it is much safer (Warnock et al. 1993). Even for imaging only as far as the carotid bifurcation. 

Even with selective catheter angiography, there can be difficulty in distinguishing occlusion from extreme internal carotid artery stenosis, and then late views are needed to see contrast eventually passing up into the head. Moreover, because of the localized and non-concentric nature of atherosclerotic plaques, biplanar, and preferably triplanar (Jeans et al. 1986 Cuffe and Rothwell 2006), views of the carotid bifurcation are required to measure the degree of carotid stenosis accurately that is, to visualize the residual lumen without overlap of other vessels, to measure at the narrowest point and to compare with a suitable denominator to derive the percentage diameter stenosis. 

A similar trade-off between diagnostic accuracy and risk is necessary when imaging the carotid bifurcation in patients with TIA or ischemic stroke. Performing intra-arterial catheter angiography in everyone is clearly unacceptable because of the risks and cost. Fewer than 20% of patients will have an operable carotid stenosis even if only those with cortical rather than lacunar events are selected (Hankey and Warlow 1991 Hankey et al. 1991 Mead et oL 1999). Coirfining angiography to patients with a carotid bifurcation bruit will miss some patients with severe stenosis and still subject too many with mild or moderate stenosis to the risks. Nor will a combination of a cervical bruit with various clinical features do much better (Mead et al. 1999). 

Despite these limitations, duplex sonography is a remarkably quick and simple investigation in experienced hands, and it is neither unpleasant nor risky. Very rarely, the pressure of the Doppler probe on the carotid bifurcation can dislodge thrombus, or cause enough carotid sinus stimulation to lead to bradycardia or hypotension (Rosario et al. 1987 Friedman 1990). The same conceivably applies to the various arterial compression maneuvres that may be carried out during transcranial Doppler, and any such compression should be avoided in patients who may have carotid bifurcation disease. 

Jeans WD, Mackenzie S, Baird RN (1986). Angiography in transient cerebral ischemia using three views of the carotid bifurcation. British Journal of Radiology 59 135-142... 

Naylor AR, Merrick MV, Sandercock PAG et al. (1993a). Serial imaging of the carotid bifurcation and cerebrovascular reserve after carotid endarterectomy. British Journal of Surgery 80 1278-1282... 

Fig. 27.1. Selective arterial angiograms of both carotid circulations in a patient with a recently symptomatic carotid "near occlusion" (lower left), and a mild stenosis at the contralateral carotid bifurcation (lower right).

Fig. 27.2. A selective catheter aragiogram of the carotid bifurcation showing a 90% stenosis. To calculate the degree of stenosis, the lumen diameter at the point of maximum stenosis (A) was measured as the numerator in both the European Carotid Surgery Trial (ECST) method and the North American Symptomatic Carotid Endarterectomy Trial (NASCET) method. However, the NASCET used the lumen diameter of the distal internal carotid artery (B) as the denominator, whereas the ECST used the estimated normal lumen diameter (dotted lines) at the point of maximum stenosis.

Fig. 12.3. A color-flow Doppler ultrasound of the carotid bifurcation showing a plaque (arrow) at the origin of the internal carotid artery (ICA) and the resulting stenosis. EGA, external carotid artery CCA, common carotid artery. 

Figure 4 Diagrammatic representation of size-dependent particle route differentiation as well as fluid flow in branching network. In (his schematic, larger particles are prevented from entering the lower branch by their size while the main vessel branch accommodates both large and smaller particles. The base diagram is from a simulation of carotid bifurcation and complex flows by L. Fatone and P. Gervasio, Ecole Polytechnique de Lausanne (which is used here simply to illustrate not only geometrical complexities but also flow complexities that will occur to a greater or lesser in smaller vessels also).

Chemoreceptors. Early investigators assumed that the chemically sensitive areas controlling respiration were located in the brain. In 1926 De Castro (15) suggested that the carotid bodies, located near the carotid bifurcation of each common carotid artery, also could be important chemoreceptors. Shortly thereafter, Heymans and Heymans (16) found that ventilation was stimulated when the aortic arch of an animal was perfused with blood from an animal breathing a low oxygen air mixture. This study established the existence and general location of chemosensi-tive bodies in the aortic arch (the aortic bodies). Additional studies by Heymans and co-workers (17) delineated the location and function of the carotid bodies and demonstrated that they were stimulated by hypoxia and hypercapnia. The exact location and function of the aortic bodies was described by Comroe (18). 

This one-pass protocol is designed to allow the optimal visualization of not only an occlusive circle of Willis thrombus, but also the possible origin or source of this thrombus in the carotid bifurcations. 

In the neck, time of flight techniques are preferred over phase contrast techniques due to the longer scan times of phase contrast techniques needed to provide the same coverage and spatial resolution. 2D TOF MRA provides superior flow-related enhancement and allows coverage of the entire neck. Compared to 2D TOF MRA, 3D TOF MRA provides superior spatial resolution and is less susceptible to phase dispersion artifacts, but is more susceptible to saturation effects and cannot cover a large area. 3D TOF flight techniqnes are, therefore, used to delineate the carotid bifurcation only. 2D PC techniques are used to evaluate flow direction in the vertebral arteries when subclavian steal is... 

D time of flight (3D TOF) Noninvasive High spatial resolution Shows complex vascular flow Less susceptible to intravoxel dephasing Can be repeated if suboptimal Can be obtained after contrast Only small volumes due to marked saturation effects Cannot image slow flow because of saturation effects Time-consuming, susceptible to patient motion Artifact from T1 hyperintense lesions Routine to evaluate circle of Wfllis for large vessel stenoses/ occlusions Can estimate carotid bifurcation stenoses... 

Patel, M.R., et al. Preoperative assessment of the carotid bifurcation. Can magnetic resonance angiography and duplex ultrasonography replace contrast arteriography Stroke, 1995. 26(10) p. 1753-8. 

Ku, D.N., et al. Pulsatile flow and atherosclerosis in the human carotid bifurcation. Positive correlation between plaque location and low oscillating shear stress. Arteriosclerosis, Thrombosis, and Vascular Biology 5(3), 293-302 (1985)... 

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