12.1\ Vascular Surgery

12.1.1\ Direct Extracranial-

Intracranial Revascularization

12.1.1.1\ Discussion

Extracranial-intracranial (EC-IC) bypass is a revascularization option for complex cerebrovascular disease such as moyamoya in adults and flow replacement prior to planned vessel sacrifice for treatment of complex and fusiform aneurysms that are not amenable to coiling or clipping. Usually in EC-IC bypass, the superficial temporal artery is anastomosed to a middle cerebral artery branch (Fig. 12.1). Alternatively, saphenous vein grafts can be used, in which the venous graft normally appears relatively patulous with respect to the artery (Fig. 12.2). If these options fail, the occipital artery can be anastomosed to the middle cerebral artery (Fig. 12.3), or it can be used to

supply the posterior cerebral artery. Regardless of the particular vessels used, EC-IC bypass is performed via a small craniectomy in the temporal region, so as to expose the Sylvian fissure and the right temporal lobe. On follow-up angiography, increase in caliber of the recipient and donor arteries can be observed. On the other hand, basal collateral vessels often regress. Graft patency can be readily assessed via MRA or CTA. Stenosis or occlusion of the bypass typically occurs at or near the anastomosis (Fig. 12.4). Correlation with precontrast images is recommended, since early clot can appear hyperattenuating, mimicking graft patency on CTA. A pitfall with time-of-flight MRA in particular is the loss of signal associated with the presence of adjacent surgical clips, which can obscure the bypass, mimicking a stenosis (Fig. 12.5). However, the presence of flow-related enhancement distally suggests that the vessel is indeed patent.

Fig 12.3  Occipital artery-MCA bypass. The patient has a history of failed left STA-MCA bypass. The 3D reformatted CTA image (a) shows the microcraniotomy (encircled) for the failed STA-MCA bypass and the left occipital

artery (arrowhead) entering an additional craniotomy. The 3D reformatted CTA image (b) shows a patent anastomosis between the left occipital artery (arrowhead) and left middle cerebral artery (arrow)

Fig. 12.4  EC-IC bypass occlusion. The patient has a history of complex left MCA aneurysm requiring left ICA occlusion and EC-IC bypass. Initial axial CT image (a) shows a small amount of encephalomalacia in the left temporal lobe and insula. The corresponding axial CTA

MIP image (b) shows a patent bypass. Follow-up axial CT image (c) obtained 11 months later shows increased encephalomalacia. The corresponding axial CTA MIP image (d) now shows occlusion of the bypass near the anastomosis (encircled)

Fig. 12.5  Metal artifact simulating steno-occlusive disease of the STA-MCA bypass. Axial time-of-flight MRA image (a) shows susceptibility effect from a metallic clip (arrow) adjacent to the left superficial temporal artery branch (arrowhead). The corresponding MRA MIP image

(b) shows focal loss of signal (arrow) along the course of the left superficial temporal artery, but there intact flow-­ related enhancement distally (arrow), indicating patency of the vessel