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Principles of embolization


Key points

  • Properly identify the indications and goals for the procedure to guide selection of embolic material.

  • Become familiar with available embolic agents and common uses for each.

  • A clear understanding of the relevant anatomy will allow for a safe and efficient embolization.

Introduction

Transcatheter and percutaneous embolization therapies are a large part of the Interventional Radiologists’ armamentarium. Embolotherapy has a wide array of applications such as control of bleeding in a variety of settings, tumor therapy, and treatment of certain vascular abnormalities. In this chapter, we provide a brief overview of the different embolic agents currently available. The choice of agent must be made according to the appropriate clinical scenario, which can be varied. For example, compare the following two patients: the first being one who is involved in a motor vehicle accident sustaining solid organ injury with evidence of active arterial bleeding on imaging; the second, a consultation regarding nonsurgical treatment options for a liver tumor. Both can be treated with transcatheter embolization; however, because of the different clinical scenarios, the approach to each, risks of embolization, and the choice of embolic agent are all radically different. Some key factors that should be considered include the following:

  • 1.

    What is the goal of the procedure? Is temporary or permanent occlusion required?

  • 2.

    What is the desired level of occlusion?

  • 3.

    What is the relevant anatomy? How does this influence the risks of nontarget embolization?

  • 4.

    What is the appropriate delivery system?

In this chapter, we will provide an overview of the most commonly employed embolic agents and their typical uses. We divide embolic agents into two major categories: temporary versus permanent. The temporary agents are commonly used in the setting of trauma or times when a permanent occlusion may result in unacceptable end-organ damage. Gelfoam is the prototypical temporary agent and is versatile in both its uses and methods of deployment.

Among the permanent embolic agents, we first discuss particles such as polyvinyl alcohol (PVA) and acrylic spheres, which are available in a number of different size ranges and commonly used in tumor and solid organ embolization procedures. Coils are another commonly used embolic agent and are available in a variety of sizes to occlude small, medium, and large vessels. Some common uses for coils include treatment of pseudoaneurysms, blockage of collateral vessels during Y-90 treatments, and treatment of pulmonary arteriovenous malformations (AVMs). Finally, we will review some commonly employed sclerosants and glues, frequently used for the treatment of peripheral vascular malformations. Figure 1-1 provides a brief overview of the most commonly employed embolic agents.

Figure 1-1, Agents for embolization.

Temporary agents

Gelfoam

Gelfoam is a water-insoluble hemostatic gelatin sponge that expands on contact with fluid. A gelatin derived from purified porcine skin, gelfoam is a flow-directed embolic agent used for temporary vascular occlusion that induces hemostasis by promoting platelet aggregation, vessel wall inflammation, and thrombus formation. As a temporary agent, it is most useful in the setting of trauma to control bleeding. Gelfoam is available as sterile sheets of varying thicknesses that can be cut into smaller pieces or as a fine powder, approximately 50 μm in size. Care must be taken when used in the powder form as occlusion is often permanent because of the small size of the particles, and can result in tissue necrosis. Gelfoam sheets are versatile and can also be prepared in a slurry by cutting or shaving small strips of the gelfoam and loading them into a syringe mixed with contrast and/or saline.

It has been observed histologically that within 6 days of gelfoam embolization, an acute inflammatory and foreign body reaction occurs as characterized by giant cells followed by an inflammatory cascade resulting in thrombus formation. Recanalization has been estimated to be anywhere from 3 weeks to 4 months in animal studies with vessel recanalization in two patients undergoing embolization for renal cell carcinoma occurring in 5 and 6 months. , It must be reiterated though that permanent gelfoam occlusion has also been demonstrated and is thought to be secondary to aggressive and densely packed gelfoam.

Avitene

Avitene (Davol Inc., Warwick RI) is a microfibrillar collagen preparation that comes in a variety of forms including powder, sheets, and sponges. The uses and preparation are similar to that described for gelfoam although it is not as commonly used. Although considered a temporary agent with vessel recanalization in a few weeks, this agent causes an intense inflammatory reaction with immediate thrombosis in large and small vessels, which may be permanent in some cases.

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