Figure 33-1. Arrangement of the major arteries on the right side carrying blood from the heart to the brain. Also shown are collateral vessels that may modify the effects of cerebral ischemia. For example, the posterior communicating artery connects the internal carotid and the posterior cerebral arteries and may provide anastomosis between the carotid and basilar systems. Over the convexity, the subarachnoid interarterial anastomoses linking the middle, anterior, and posterior cerebral arteries are shown, with inset A illustrating that these anastomoses are a continuous network of tiny arteries forming a border zone between the major cerebral arterial territories. Occasionally a persistent trigeminal artery connects the internal carotid and basilar arteries proximal to the circle of Willis, as shown in inset B. Anastomoses between the internal and external carotid arteries via the orbit are illustrated in inset C. Wholly extracranial anastomoses from muscular branches of the cervical arteries to vertebral and external carotid arteries are indicated in inset D.
Posterior cerebral A (P1 segment)A1 segment, anterior cerebral arteryM1 segment, middle cerebral arteryBasilar APenetrating branches ofmiddlecerebral AOptic NInternal carotid AAnterior communicating AParamedianarteriesPosteriorcommunicatingAAnterior inferiorcerebellar A5th N6th N7th N8th N9th, 10th, 11th NPosterior inferiorcerebellar AShort circumferential AAnt. spinal AVertebral AA   4 mm below upperborder of ponsC   2 mm above ponto-medullary junctionD   6 mm below ponto-medullary junctionB   Mid-pontineSuperior cerebellar A3rd N
Figure 33-2. Diagram of the base of the brain showing the principal vessels of the vertebrobasilar system (the circle of Willis and its main branches). The term M1 is used to refer to the initial (stem) segment of the middle cerebral artery; A1 to the initial segment of the anterior cerebral artery proximal to the anterior communicating artery; A2 to the postcommunal segment of the anterior cerebral artery; and P1 and P2 to the corresponding preand post-communicating segments of the posterior cerebral artery. The letters and arrows on the right indicate the levels of the four cross-sections following: A = Fig. 33-16; B = Fig. 33-15; C = Fig. 33-14; D = Fig. 33-13. Although vascular syndromes of the pons and medulla have been designated by sharply outlined shaded areas, one must appreciate that because satisfactory clinicopathologic studies are scarce, the diagrams do not always represent established fact. The frequency with which infarcts fail to produce a well-recognized syndrome and the special tendency for syndromes to merge with one another must be emphasized.
Figure 33-3. MRI showing acute infarctions. The upper images show a right middle cerebral artery infarction that appears bright on diffusion-weighted imaging (DWI) (upper left). There is subtle hyperintensity representing early vasogenic edema on T2-FLAIR sequence (upper right). The lower images show an acute cerebellar infarction in the territory of the posterior inferior cerebellar artery (PICA) that is bright on DWI (lower left) and faintly bright on T2-FLAIR (arrow, lower right). There is also a previous infarction just anterior to the acute cerebellar stroke, that is dark on DWI and bright on T2 due to gliosis.