12.2\ Endovascular Surgery

12.2.1\ General Imaging

Considerations

Following Endovascular

Cerebrovascular Procedures

Endovascular cerebrovascular procedures include endovascular reconstruction or deconstruction for cerebrovascular occlusive disease or active bleeding using stents or embolic material; embolization­ of tumors, aneurysms, or vascular malformations either preoperatively or for treatment; and mechanical or chemical thrombolysis for acute ischemic stroke or vasospasm. Materials that are typically used during neuroendovascular procedures include metal containing devices, such as coils, plugs, and stents, liquid embolic agents, balloons, and particles. Certain metals contained in some of these endovascular treatment modalities create substantial streak artifact on CT, rendering imaging less sensitive

to vascular assessment. Most intracranial endovascular devices create relatively less artifact on MRI compared to CT. For example, embolic coils used in aneurysm are predominantly made of platinum. These have only mild susceptibility effect on MRI/MRA. Indeed, MRA is an effective means to assess small degrees of aneurysm recurrence following coil embolization (Fig. 12.40). Most intracranial stents have relatively low mass, but still produce susceptibility artifacts on MRI, giving the corresponding vessel’s intraluminal diameter a false appearance of being narrowed (Fig. 12.41). Liquid embolic agents, such as Onyx, generally produce a signal void on MRA, T1-, and T2-weighted MRI without significant­ obscuration of adjacent vasculature (Fig. 12.42). However, Onyx HD500 used for treating aneurysms is associated with more susceptibility effect compared to Onyx used for arteriovenous malformation embolization, which can overestimate the degree of stenosis on MRA (Fig. 12.43).

Fig. 12.40  Embolic coil occlusion. MRA before (a) and after (b, c) the anterior communicating artery aneurysm (arrows) demonstrate complete occlusion of the aneurysm, as demonstrated on preand post-embolization digital subtraction arteriograms (d, e). Axial CT image (f)

following aneurysm embolization demonstrates substantial streak artifact which precludes evaluation for early recurrence as opposed to the MRA, which has negligible artifact, allowing for satisfactory evaluation of potential recurrence

Fig. 12.41  Stents. Unsubtracted angiographic image (a) following Y-shaped stent-assisted coiling of a basilar tip aneurysm demonstrates the proximal and distal markers (arrows) of the stents as well as coils within the aneurysm.

MRA following the procedure (b) demonstrates occlusion of the aneurysm with artifact giving a false impression of stenosis along the stent despite lack of evidence for this on digital subtraction angiography (c)

Fig. 12.42  Onyx liquid embolization. Time-of-flight MRA and CT before (a, b) and after (c, d) embolization of a posterior cingulate gyrus arteriovenous malformation using Onyx. Note that the embolic material creates signifi-

cant artifact on CT preventing adequate evaluation, whereas time-of-flight MRA has the ability to detect a residual component of the arteriovenous malformation (arrow)

Fig. 12.43  Onyx HD500. Digital subtraction angiography (a) after embolization of a giant aneurysm of the left internal carotid artery cavernous segment demonstrates

patency of adjacent vessels, while susceptibility artifact on MRA (b) obscures the surrounding vessels (encircled)

12.2.2\ Endovascular Treatment

for Aneurysms

Endovascular occlusion of cerebral aneurysms can be achieved via coil embolization, liquid embolic embolization, or flow-diverting stents (Figs. 12.40, 12.41, 12.43, and 12.44). The number of coils utilized depends on the size of the lesion and the type of coil. For example, fewer hydrogel coils are required than bare metal coils for comparable aneurysm sizes. Stents are sometimes­ used to support the coils, especially for wide-necked and fusiform aneurysms. Flow-­diverting stents, such as the Pipeline and Silk devices, are an option for treating large, wide-­necked, or otherwise

a

untreatable aneurysms. The devices provide 30–35% metal coverage of the inner surface of the target vessel with a pore size of 0.02– 0.05 mm. The tube mesh implants are believed to achieve their results via functional reconstruction of the parent artery with rerouting of blood flow away from the aneurysm while preserving flow to branch vessels. Although aneurysm opacification is often observed on angiography during the early postoperative period, complete occlusion is achieved in the majority of treated aneurysms by 6 months. Protocols for follow-up imaging after aneurysm coil embolization vary among institutions and include either conventional angiography, CTA, MRA, or a combination of these.

b

Fig. 12.44  Flow-diverting stent. Preoperative CTA image (a) shows a large, wide-necked left supraclinoid internal carotid artery aneurysm (*). CTA obtained at 2 months after

Pipeline stent insertion (b) shows residual filling of the aneurysm (arrow). CTA image obtained 12 months after Pipeline stent insertion (c) shows obliteration of the aneurysm

12.2.3\ Endovascular Embolization

of Arteriovenous

Malformations and Fistulas

Liquid embolization agents, such as n-butyl cyanoacrylate, Onyx, and particles, such as polyvinyl alcohol (PVA), are commonly used to treat arteriovenous malformations and fistulas, sometimes in conjunction with coils (Fig. 12.45). Liquid embolic agents that are not inherently radiopaque are often mixed with tantalum powder in order to improve visibility during fluoroscopy. The embolic agent forms casts of the embolized vessel, which is visible on CT due to the tantalum powder and creates a signal void on MRI. The presence of tantalum powder within the liquid agents is responsible for the streak

a

artifact on CT and may require catheter angiography for more definitive assessment. On the other hand, particles, such as PVA, used for embolization are not directly apparent on imaging. In the past, arteriovenous malformations were sometimes treated with Silastic beads, which appear as tiny spherical hyperattenuating structures measuring 1–5 mm in diameter (Fig. 12.46). Clinical improvement could be achieved even without occlusion of symptomatic arteriovenous malformation due to reduction of cerebral steal phenomenon. Furthermore, remaining portions of the malformation can sometimes spontaneously thrombose after treatment and not require further intervention. Otherwise, surgical resection is often performed after partial embolization.

b

Fig. 12.45  Arteriovenous malformation embolization. Digital subtraction AP arteriograms of a right frontal lobe arteriovenous malformation before (a) and after (b) embolization using a mixture of n-butyl cyanoacrylate, Lipiodol, and tantalum powder, as well as coils. Axial CT

images following the embolization display streak artifact related to the tantalum powder and coils used (c) and thrombosis of a large intranidal venous structure (d). The AVM did not recur following embolization

662

c d

Fig.12.45  (continued)

Fig. 12.46  Silastic bead embolization. Axial MIP image shows spherical hyperattenuating foci within an arteriovenous malformation treated many years before

D.T. Ginat et al.

12.2.4\ Endovascular Deconstructive

Treatment for Vessel Sacrifice

Vessel sacrifice is an accepted method for treatment of cerebrovascular lesions including carotid blowout, aneurysms, dissections, epistaxis, dissection, or preoperatively to facilitate tumor resection. Occlusive materials may include but are not limited to detachable balloons, coils, particles, plugs, or liquid embolic material. When these involve the carotid or vertebral artery, a test occlusion often precedes the vessel sacrifice (balloon test occlusion). Post-procedural findings include identification of the embolic material within the sacrificed vessel. In particular, intra-

a

c

vascular detachable balloons have been used to treat intracranial aneurysms, often in conjunction with coil embolization. Detachable balloons are generally used to achieve permanent occlusion. Balloons are usually composed of silicone or latex and can be filled with contrast material in order to increase conspicuity on imaging (Fig. 12.47). Vascular plugs can be used successfully for permanent occlusion of head and neck vessels. Amplatzer vascular plugs, for example, are composed of self-expandable nitinol mesh with one or more lobes and radiopaque platinum markers at each end (Fig. 12.48). Major complications are uncommon and include cerebral infarction, blindness, and cranial nerve palsies.

b

Fig. 12.48  Vascular plugs. Lateral radiograph (a) and curved planar reformatted CTA image (b) show an Amplatzer plugs (arrows) within the right common carotid artery

12.2.5\ Preoperative Embolization

of Neoplasms

Preoperative embolization of intracranial vascular neoplasms typically uses particles and occasionally­ liquid embolic material or coils with the aim to occlude vessels within the tumor or immediately proximal to the vascular neoplasm. Since such patients often go to surgery shortly after the embolization, they do not undergo imaging unless symptomatic or for presurgical planning. Possible post-procedural complications include thromboembolic events and intratumoral hemorrhage as well as parent vessel dissection. However, the effects of particle embolization can be apparent. For example, absence of a contrast blush or enhancement due to tumor necrosis indicates successful treatment (Fig. 12.49). Furthermore, restricted diffusion can appear in the embolized tumors due to infarction.

Fig. 12.49  Tumor embolization. The left frontal meningioma underwent PVA particle embolization prior to surgical resection. Pre-embolization DSA image (a) shows a strong tumor blush. The corresponding CT with contrast (b) shows a large, early homogeneously enhancing left frontal extraaxial mass. Following microparticle embolization of the

feeding vessels, there is no longer a tumor blush (c). Postembolization contrast-enhanced T1-weighted MRI (d) and ADC map (e) images obtained within 24 h of the procedure show a large area of nonenhancement with corresponding restricted diffusion within the meningioma (*), which represents embolization-induced tumor infarction

12.2.6\ Endovascular Sclerotherapy

for Head and Neck Lymphatic

Malformations

Percutaneous sclerotherapy is a minimally invasive means to treat low-flow vascular malformations in the head and neck in which sclerosing agents such as bleomycin, sodium tetradecyl sulfate, and alcohol, among other agents are infused directly into the

lesion. Imaging following sclerotherapy for a lowflow vascular malformation of the head and neck is used to identify lesional shrinkage, decreased enhancement, and intralesional fibrosis (Fig. 12.50). MRI is a suitable modality for evaluating treatment response following sclerotherapy in the deep soft tissues, due to the lack of ionizing radiation, excellent soft tissue contrast resolution, and multiplanar