Circle of Willis

For the examinations of intracranial vessels the same coil systems are used as for conventional MRI of the skull. For a combined examination of intracranial and cervical vessels specially integrated coil systems are needed which can receive signals from the circle of Willis down to the aortic arch. Improvements of coil design and high frequency (HF) capabilities allow remarkable reduction in acquisition times by the use of parallel acquisition techniques (Tintera et al. 2004). 

Anomalies of the ICA are infrequent, and aplasia of the ICA is a rarity (Van de Perre et al. 2004). In hypoplasia, the vessel tapers off behind a normal proximal segment and can sometimes be followed up to the circle of Willis. Occasionally differential diagnosis is difficult against an arterial dissection or a very high grade stenosis with a collapsing distal lumen (pseudoocclusion). In cases of doubt, the CT shows the hypoplastic osseous carotid canal (Chen et al. 1998). An aberrant lateral course of ICA... 

Fig. 5.16. Typical pseudoocclusion on CE-MRA. Note subtotal left proximal ACI stenosis (arrow). The distal vessel is collapsed and very thin, but relatively visible up to the circle of Willis...

In many instances TOF-MRA provides indirect indicators on collateral circulation at the level of the circle of Willis. While the anatomic conditions for collateral circulation can be studied, the recognition... 

While rMTT can depict changes in cerebral hemodynamics, which are due to vessel occlusion of cerebral arteries and the respective compensation mechanisms, the time to arrival of contrast (TTA) is sensitive to vessel diseases, which are more upstream of the arterial flow, most commonly high grade stenoses or occlusions of the carotid arteries. In such patients, blood flow in the hemisphere ipsilateral to the stenosis is mainly supplied by the contralateral carotid artery via the circle of Willis. Due to this detour, TTA is prolonged in the ipsilateral hemisphere (Reith et al. 1997). At the same time, rMTT may be prolonged in the ipsilateral hemisphere resulting from decreased blood flow (Dorfler et al. 2001). If TTP is calculated instead of MTT, the effect of the bolus delay cannot be separated from that of the perfusion decrease, because TTP is influenced by both rMTT and TTA. 

In the past, two different basic mechanisms have been proposed to account for ischemic events in ICA occlusive disease (1) intracranial embolism and (2) a low-perfusion state also referred to as hemodynamic insufficiency. In the post-mortem arteriographic and pathologic study by Rodda and Path (1986), massive infarcts involving two major cerebral artery territories were associated with distal ICA occlusion, middle cerebral artery (MCA) territory infarcts were seen when the ICA was occluded or stenosed, and borderzone infarcts were characterized by ICA disease and limited circle of Willis anastomosis. 

MRA has been shown to be well suited to investigate the circle of Willis (CW) since abnormalities can be detected by MRI blood flow techniques (Anzola et al. 1995). 3D TOF sequences, although more time consuming than CE-MRA, are still the... 

Hendrikse and coworkers (2001) investigated whether the presence of borderzone infarcts is related to the collateral ability of the CW in symptomatic and asymptomatic patients with unilateral occlusion of the ICA. They found that in patients with unilateral ICA occlusion, the presence of collateral flow via the posterior communicating artery in the circle of Willis is associated with a low prevalence of borderzone infarcts and that asymptomatic patients with an ICA occlusion do not have an increased collateral function of the CW. Figure 15.10 shows the four patterns of collateral flow via the CW to the hemisphere ipsilateral to the ICA occlusion. 

Fig. 15.10. The four patterns of collateral flow via the circle of Willis to the hemisphere ipsilateral to the ICA occlusion. A1 segment indicates A1 segment of ipsilateral ACA, PcomA indicates posterior communicating artery...

The following two examples show different configurations of the circle of Willis in two patients with occlusive ICA disease. While the 54-year-old patient in Fig. 15.12 suffering from left ICA occlusion shows good collateralization via the posterior communi-... 

Baumgartner RW, Baumgartner I, Mattie HP et al (1997) Transcranial color-coded duplex sonography in the evaluation of collateral flow through the circle of Willis. Am J Neuro-radiol 18 127-133... 

Q2 The brain forms about 2% of body weight but receives approximately 20% of the cardiac output and approximately 20% of the body s oxygen supply. Blood reaches the brain via two internal carotids and two vertebral arteries the latter fuse inside the cranium to form the basilar artery. The carotid and basilar arteries are interconnected via the Circle of Willis, which forms a ring of blood vessels in the brain. This arrangement ensures that brain tissues can be supplied with blood from either the carotid or vertebral arteries and reduces the chances of an interrupted blood supply. 

The brain makes up only 2% of the total body weight, but when the body is at rest, it receives 20% of the cardiac output and consumes about 20% of the total inspired oxygen. The anterior two-thirds of the brain is supplied by the two internal carotid arteries, and the posterior third of the brain by the two vertebral arteries (Fig. 4.1). These four arteries anastomose at the base of the brain to form the circle of Willis (Fig. 4.2). 

Fig. 4.2. The circle of Willis at the base of the brain as seen from below. There is considerable anatomica variation and this figure represents one of the more common arrangements,...

The circle of Willis. This is formed by the proximal part of the two anterior cerebral arteries connected by the anterior communicating artery, and the proximal part of the two posterior cerebral arteries, which are connected to the distal internal carotid arteries by the posterior communicating arteries. However, approximately 50% of circles have one or more hypoplastic or absent segments, usually one of the communicating arteries, and atheroma may limit the potential for collateral flow (Fig. 4.2). 

In Japanese, moyamoya means puff of smoke and describes the characteristic radiological appearance of the fine anastomotic collaterals that develop from the perforating and pial arteries at the base of the brain, the orbital and ethmoidal branches of the external carotid artery and the leptomeningeal and transdural vessels in response to severe stenosis or occlusion of one, or both, distal internal carotid arteries (Yonekawa and Khan 2003). The circle of Willis and the proximal cerebral and basilar arteries may also be involved. 

Borggreve F, de Deyn PP, Marien P etal. (1994). Bilateral infarction in the anterior cerebral artery vascular territory due to an unusual anomaly of the circle of Willis. Stroke 25 1279-1281... 

Fig. 12.5. A "time of flight" MR angiogram of the circle of Willis in a patient with no posterior communicating arteries.

Endarterectomy of severe carotid stenosis to improve collateral blood flow, via the circle of Willis, to the basilar artery distal to vertebral or basilar artery stenosis or occlusion Resection and anastomosis Resection and reimplantation... 

Intracranial aneurysms are not congenital but develop over the course of life. Approximately 10% of aneurysms are familial, and candidate genes identified thus far include those coding for the extracellular matrix. Saccular aneurysms tend to occur at branching points on the circle of Willis and proximal cerebral arteries approximately 40% on the anterior communicating artery complex, 30% on the posterior communicating artery or distal internal carotid artery, 20% on the middle cerebral artery and 10% in the posterior... 

Figure 3.5. Gross external apijearance of die circle of Willis and subarachanoid vessels.

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