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1 Basic Principles of Embolization 5
Identifying the Target
Clearly it is impossible to treat anything unless you know the location, the local
blood supply, and any other tissues that may be affected by vascular occlusion.
• Often common sense will direct you to the appropriate vascular distribution.
In the example above, bleeding following renal biopsy will originate from the
corresponding renal arterial branches.
• When the situation is not so obvious (e.g., gastrointestinal hemorrhage), then
cross-sectional imaging will commonly identify the site of bleeding. This will
often direct angiographic interrogation.
• Most proximal arterial variations can be identified by careful interpretation of CT
angiography.
• CT will not always identify the vessel of origin. For example, bleeding into the
retroperitoneum can come from a large number of adjacent vessels—and some
work will still be required at angiography!
Knowing the Arterial Anatomy and Collateral Pathways
Understanding the arterial and venous anatomy of the target lesion is essential
to permit safe and effective embolization. Embolization requires both meticulous
angiographic technique and patience to carefully unravel the pathway to the target
lesion.
Normal Anatomy and Anatomic Variations
Arterial and venous variants are a common trap for the unwary, and knowledge of
the common patterns can prevent an easy case going far astray.
• In general, even with the advent of CT and MR angiography, it is always bet-
ter to start by establishing the arterial anatomy with overview (non-selective)
angiography.
• In some vascular territories (e.g., the visceral circulation), variant anatomy will be
present in over a quarter of cases. Subtle arterial variations are still best appreciated at selective angiography. Only after the anatomy has been established should
the operator proceed to more selective catheterization of the target vessel.
Collateral Perfusion
• Collateral pathways are a mixed blessing for embolization therapy. While in some
circumstances the existence of a collateral pathway can prevent tissue ischemia
after embolization, the same network of pathways may continue to perfuse a
target lesion beyond embolization coils.
6 I. Robertson
Fig. 1.1 Gastroduodenal
artery pseudoeneurysm;
a lesion here can be supplied
by both celiac axis and
superior mesenteric artery
• The classic collateral pathway of the gastroduodenal artery (Fig. 1.1) is a per-
fect illustration. A lesion in the mid gastroduodenal artery cannot be successfully
treated by proximal embolization of the common hepatic artery or even the gastroduodenal artery alone. The lesion will continue to be supplied via the superior
mesenteric artery/inferior pancreaticoduodenal arcade.
This introduces the concept of closing both the “front and back doors.” In other
words, successful embolization of this lesion requires occlusion of both the inflow
and outflow arteries. This is usually achieved by starting embolization distal to the
lesion and working backward.
• It is important to remember that collateral pathways are not solely limited to
major vessels and exist within the parenchyma of some solid organs (e.g., the
liver).
End Arterial Supply
End arteries are vessels that provide the sole supply to an organ or vascular territory.
• Occlusion of an end artery will lead to ischemia throughout the territory it sup-
plies. This can have vital implications for embolotherapy, as without care it is
possible to cause more harm than good.
1 Basic Principles of Embolization 7
• Obvious examples are the coronary arteries, renal arteries, splenic artery, and the
brachial artery. When dealing with an end artery, the embolization is normally
performed as distally as possible to minimize the ischemic consequences. In some
cases, however, even this might be unacceptable. For example, there would be no
prizes for occluding a left main stem coronary artery to treat a false aneurysm.
Choosing the Correct Embolic Material
The choice of embolic material is dictated by two principal factors: is permanent or
temporary occlusion required and what is the level of vascular occlusion necessary
for effective treatment.
Temporary occlusion is useful when the aim is to stop bleeding which is from a
lesion that will undergo spontaneous healing.
• A good clinical example is the trauma patient with bleeding from a pelvic frac-
ture; temporary embolization of the entire internal iliac artery can be a rapid and
life-saving maneuver.
• Recanalization of the temporary occlusion will not cause rebleeding, as healing
and fracture immobilization will have occurred.
• The choice of temporary embolic agents is fairly limited. Gelfoam and now
increasingly rarely autologous blood clot are the temporary agents in use.
Level of vascular occlusion. The effectiveness of an embolization treatment will
depend on controlling the level of vascular occlusion appropriate for the underlying
pathological processes.
• If there is an ongoing angiogenic stimulus, then proximal occlusion will simply
result in the lesion rapidly re-establishing collateral circulation which will be
much more difficult if not impossible to treat.
• In the treatment of chronic inflammatory and most neoplastic processes, more
distal occlusion with particulate, or less often liquid embolic agents, is required
to obtain a more durable result.
• Particulate agents come in a range of sizes; the smaller the particles, the more
distal the level of embolization down to capillary level. The more distal the
embolization, the greater the risk of tissue necrosis.
The over-riding principle is always to use the safest, most readily controlled,
effective agent.
Understanding the Likely Outcomes/Consequences of Embolization
There is no single answer to this, as it depends on the territory, nature of the pathology, and extent of embolization. The following questions should be considered:
8 I. Robertson
• How much non-target tissue is likely to be affected and what will be the
consequence of this?
• Can supply be preserved to vital structures? It might, for example, be possible to
occlude a carotid artery, if there was good collateral supply to the brain around
the Circle of Willis.
• Whether arteries are end arteries which if occluded will lead to important distal
ischemia.
• If tumor embolization is being performed, then think how much target tissue will
need to be infarcted. Sometimes it is better to stage the procedure than to have the
patient sustain a massive physiological assault from post-embolization syndrome.
• Remember that all these issues are relative in the context of a life-threatening
situation.
Performing the Procedure
Following planning comes the technical part of the procedure; much of this should
be thought out in advance. Before starting ask yourself the following questions:
• Do I understand the procedure to be performed?
• Do I have the technical ability to perform the procedure?
If the answer to either of these is no, then seek assistance if there is time. In
an emergency ask yourself for instance, if a temporizing measure such as balloon
occlusion might act as a bridge to more definitive therapy. If this is not possible,
then there may have to be a decision about “the lesser evil” as long as the patient’s
interests are put first, e.g., it is better to have a less selective embolization than to
bleed to death.
Informing Others: The Patient and Clinicians
The scope for discussion will obviously relate to the urgency of the clinical scenario.
Elective embolization procedures should always be discussed in advance with the
patient and clinician. In a life-threatening emergency, there is no time for this and
the patient’s survival comes first.
• Consent should reflect the likely outcomes and risks of this treatment and this
should be clearly recorded in the patient’s records.
• At a practical level, always give the patient a clear idea about the likely duration
of the treatment.
• The patient will want to know if the procedure is palliative or curative, especially
in the context of cancer therapy.
• Most embolization procedures are performed under conscious sedation and the
patient must be co-operative during the procedure.
1 Basic Principles of Embolization 9
• Whether it will be painful? For long or particularly painful procedures, it may
be best to consider general anesthesia. The same applies in children. This should
remind you to ensure that you prescribe appropriate analgesia for the peri- and
post-procedural period.
• Finally, make sure that senior referring clinicians have a very clear picture of the
risks, success rate for the procedure and that a collaborative management plan
has been discussed.
Planning Vascular Access
The shortest straightest route is usually best and is considered on the basis of
the likely target and associated anatomy. Most cases will be approached from the
femoral artery.
• If imaging is available, review the whole access route on the CTA or MRA.
• Elderly patients frequently have diseased iliac vessels that can require treatment
to achieve access.
• Very tortuous iliac vessels can make alternative access sites such as the brachial
arteries more appropriate.
Selective Catheterization
Embolization often requires selective catheterization of a small branch vessel to
deliver effective treatment and to minimize damage to non-target tissue. A high level
of technical catheter and wire skill is essential and is only acquired with practice.
The following outlines some key concepts which will help.
Achieving a stable catheter position. This is a vital component to successful
embolization and is analogous to ensuring a firm footing when working up a high
ladder!
• A stable position will permit the deployment of coils or injection of liquid
embolic materials.
• In general, the deeper the catheter is within the target arterial circulation, the
more stable the catheter position.
• A more difficult position occurs when the target vessel or lesion lies close to the
origin of a major vessel. In this position, it is all too easy for the delivery catheter
to “back off” during delivery of the embolic agent with loss of embolic material
into a major non-target vessel.
• After selective catheterization has been achieved, it is essential to test if the posi-
tion of the tip of the catheter will remain in position. For “solid materials” like
embolization coils, this is usually done by advancing the guidewire that will be
used for coil deployment to the tip of the catheter.
• Liquid materials, e.g., PVA are assessed by initially using a test injection of
contrast of a similar volume and rate to that anticipated for the embolic material.
10 I. Robertson
(a) (b)
Fig. 1.2 (a) Vertebral catheter at vessel origin; the catheter is easily backed out as it does not reach
the contralateral aortic wall. (b) Cobra catheter at the same site; the catheter curve is supported by
the contralateral aortic wall and achieves a more stable position
• If the catheter backs out during the test, then more stable positions can be
achieved by modifying the shape of the delivery catheter, e.g., exchanging a vertebral catheter for a cobra catheter as the wider cobra shape is better supported
by the aortic wall (Fig. 1.2); when this is not possible consider using a co-axial
system either guidecatheter/sheath around a standard catheter or a microcatheter
through a standard catheter. Sometimes it is necessary to use a tri-axial system
comprising guide catheter, standard catheter, and microcatheter.
Single End Hole Catheters
Regardless of the shape of catheter used, it is essential to use an end hole-only
catheter during embolization procedures.
• Catheters with side holes can result in non-target embolization via the side holes
if used with particulate or liquid materials.
• Side hole catheters are equally hazardous with coil embolization as the end of the
coil may get caught in the side hole trapping the coil within the catheter either
before extrusion or as the coil tail exits the catheter lumen.
• Either way it is a sure fire way to make a controlled procedure more difficult,
time consuming, and hazardous.
Avoiding Wedged Catheter Positions
Finally, in the search to achieve the perfect catheter position, it is possible to impact
or wedge the catheter into the vessel.
1 Basic Principles of Embolization 11
• In this position, the catheter occupies the entire lumen and no blood can get past.
• Flow-directed embolization is then very hazardous, as pressure can build up in
the distal vascular bed leading to the following:
◦ rupture the target vessel
◦ non-target embolization by back-pressure reflux of embolic agent when the
catheter is removed or by opening collateral channels
• If the target vessel is so small that a conventional catheter is wedging, then the
simplest solution is to use a microcatheter.
Microcatheters
Microcatheters have extended the range of embolization into small and tortuous vascular beds and permit targeted embolization in even the most challenging
circulations.
• In general, it is far better to accept using a microcatheter early than provoke
vasospasm or catheter wedging by persisting with a conventional catheter in a
difficult circulation.
• A large range of microcatheter designs are available; generic microcatheters are
small caliber, usually less then 3F, unshaped catheters.
• Microcatheters are generally straight and therefore not steerable. They are usu-
ally used in conjunction with steerable 0.014–0.018 wires to allow selective
catheterization of the distal circulation.
• Microcatheters have to be delivered within a conventional catheter to a branch in
the proximal circulation. It is best to use a Tuohy Borst adapter to allow circulation of saline between the conventional “guiding catheter” and the microcatheter,
as this both helps prevent thrombus formation and provides lubrication to allow
the microcatheter to slide easily within the conventional catheter lumen.
Delivery of Embolic Materials
Embolization requires careful control of the materials used and this is usually
achieved by direct visualization of the delivery under continuous fluoroscopy. Some
embolic agents are already opaque, e.g., coils, but most other materials are not
opaque in themselves. Particulate embolic materials such as gelfoam, PVA, and
microspheres are all suspended in contrast to allow controlled delivery.
Sclerosant agents pose a different problem; dilution with contrast will reduce
their effectiveness and there are several approaches to permit safe embolization.
• Mixing with a contrast agent. Opacification with tantalum powder, which is very
radioopaque, is sometimes used.
• Performing test injections with contrast. Test injections can be used to see the
distribution of a known volume and rate of injection. The same volume of
12 I. Robertson
unopacified sclerosant is then injected at the same rate. Remember that the injection rate and volume may change during the procedure due to vasospasm and
occlusion. Repeat test injections should be performed frequently.
• Reducing the flow. Venous outflow can be occluded with a simple tourniquet
in the periphery. Occlusion balloons can be used in less accessible sites. This
combination can permit safe delivery of sclerosant without diluting effectiveness.
Preserving Vessels
If there is a macroscopic vessel lesion such as a pseudoaneursym in a vital vessel,
then preservation of the vessel may be possible. The technique used will depend on
the size of the vessel and the tortuousity of the access vessel.
• A stent graft offers the simplest solution to exclude the lesion from the circulation
and preserve flow. Currently stent grafts are limited to fairly large vessels, certainly greater than 4 mm, and struggle to get round more than one or two gentle
curves.
• The alternative approach is to very gently fill up the space of the psuedoaneurym
with either coils or specialized liquid embolic agents such as Onyx. The vessel lumen can be preserved by either balloon modeling during coil placement
or initially placing a stent across the neck of the pseudoaneurysm which helps
constrain the coils. This is certainly a far more complex technique; however,
this technique is being used very successfully for intra-cerebral aneurysms, and
undoubtedly the interventional neuroradiologists have the greatest experience and
the best equipment for this type of situation.
Non-target Embolization
• Non-target embolization may be unavoidable, but should be minimized by careful
planning and execution.
• Occasionally it is not possible to limit non-target embolization simply by using
selective catheterization. This typically occurs when there are multiple small
vessels arising proximally that are too small and often too many to individually
catheterize (Fig. 1.3).
• It is very rarely possible to exclude these vessels with a stent graft.
• It can still be possible to achieve target-only delivery of embolic material by
intentionally occluding the circulation distal to the lesion, which will direct blood
flow and the embolic agent into the target circulation.
• Depending on vessel size and collateral circulation, this occlusion can either
be temporary via balloon occlusion, which is often more difficult and requires
further vascular access, or by using gelfoam plug occlusion which will recanalize
in time.
1 Basic Principles of Embolization 13
Fig. 1.3 The small vessels at
the vessel origin cannot be
individually catheterized;
temporary occlusion at
point A will permit delivery
into the tumor circulation
while minimizing collateral
damage
A
The chapters which follow will expand on these principles; they are not
complicated and if kept in mind will keep you and your patient safe.
Keypoints
Embolization creates some of the greatest technical and intellectual challenges
for the angiographer.
Careful adherence to the core principles will ensure simpler, safer, and
effective treatments.
Planning is the key to success and requires a thorough knowledge of the
condition and the treatment options. Use all the available information from
clinicians, diagnostic tests, imaging, and operative notes.
Use high-quality imaging at key points in the procedure, e.g., injecting
particles.
Preserve as much tissue and many collaterals as possible by taking care
and paying attention to scrupulous technique.
Chapter 2
Coils, Occluders, and Plugs
David Tuite and Kevin O’Regan
Introduction
Coils, plugs, and vascular occluders are permanent mechanical embolic agents. This
chapter considers the following:
• Mechanism of action
• Physical properties
• Deployment
• Associated complications and strategies for minimizing these
The ideal vascular occlusion device should have the following attributes:
• Simple deployment mechanisms
• Require a low profile delivery system
• Allow precise placement before final deployment
• Resist migration after deployment
• Have a low rate of recanalization
• Should not cause significant vessel damage or inflammatory response
• Have sufficient expansile force to securely attach to the vessel wall in high
pressure and high volume flow studies
• Cost competitive
The choice of embolic material depends on the clinical scenario and the desired
end-point of the procedure.
Mechanism of Action
There are three mechanisms by which these devices occlude vessels:
• Intimal injury
• Promotion of thrombus formation
• Mechanical occlusion
Transcatheter Embolization and Therapy,
Techniques in Interventional Radiology, DOI 10.1007/978-1-84800-897-7_2,
C
Springer-Verlag London Limited 2010
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