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1. What are the characteristics of a diagnostic catheter?

Catheters may be selected for a specific application based on many characteristics (

Fig. 29-1

). Although there are

numerous catheters, experience and personal preference play a large role in the selection process.

•

Length: Catheters are available in various lengths, the most common being 65 cm and 100 cm. The appropriate

length is based on the access site and desired application. From a femoral approach, a 100-cm-long catheter may

be used for a cerebral arteriogram, whereas a 65-cm-long catheter would suffice for a renal arteriogram.

•

Tip configuration: Tip configuration describes the curve on the leading edge of the catheter. Various curves are

available that are designed to select branch vessels that originate at different angles. Common catheter curves

include Cobra, Simmons, and Berenstein (see

Fig. 29-1

).

•

Outer diameter : Most diagnostic catheters used today are 4 or 5 Fr, meaning that they are less than 2 mm in diameter.

•

Inner diameter : This describes the inner channel of the catheter.

Most catheters are designed to accommodate guidewires that

are either 0.035 inch or 0.038 inch in diameter. It is important to

match the inner diameter of the catheter with the devices (wire

or coil) that are placed through it.

•

Coating: Some catheters have a hydrophilic coating that

becomes very slippery when wet. This may facilitate crossing a

stenosis.

•

Stiffness: Some catheters may contain braided fibers within

the shaft. Braiding of polymers increases the stiffness of the

catheter. Some clinical applications are better suited to

stiffer catheters, whereas others are better served by floppier

ones.

2. What is the difference between a Cobra 1 and a

Cobra 2 catheter?

A Cobra 1 catheter and a Cobra 2 catheter have the same general

shape except that the radius of the secondary curve of the catheter

is greater for the Cobra 2 (

Fig. 29-2

). A Cobra 3 catheter has the

same general “Cobra” shape, but the secondary curve is even

greater still. The same nomenclature applies to Simmons catheters

and others as well.

3. What is a French? What is a gauge?

A French (Fr) is a unit of measure of diameter that is often applied

to catheters. 1 Fr is equal to one third of a millimeter. A 6-Fr

catheter is therefore 2 mm in diameter. A gauge (G) is also a unit

used to measure diameter and although it can be used to measure

catheters, it is frequently used for needles. An approximation used

for needles is that the diameter (in inches) is equal to the reciprocal

of the gauge. Therefore a 21-gauge needle is about 1/21 of an inch

in diameter.

4. What are the two general categories of stents?

How do they differ?

Improvements in biomedical engineering and metallurgy have

helped expand the clinical applications for noncoronary stents.

Although various commercial stents are available, they can be

Jeffrey A. Solomon, MD, MBA, and
S. William Stavropolous, MD

EquipmEnt, tErms, and tEchniquEs
in intErvEntional radiology

A

B

C

D

E

F

G

H

I

Figure 29-1.

Types of catheters. A = Berenstein, B =

Cobra, C = H1H, D = multipurpose-A, E = pigtail, F =

Simmons, G = straight, H = tennis racket, I = SOS.

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EquipmEnt, tErms, and tEchniquEs in intErvEntional radiology

classified as either balloon-expandable or self-expanding. The

characteristics of each type vary, making one type more suitable for

certain clinical applications than the other. There is significant

overlap for uses of both types of stent, and user preference plays a

role in selection.

5. How is a balloon-expandable stent deployed?

The Palmaz stent is the prototypic balloon-expandable stent. Such

stents come packaged either individually or premounted on a

balloon. When the balloon is inflated, the stent expands to the

diameter of the balloon. As the stent expands, it changes very little

in length. The relatively constant size and method of delivery/

deployment of this type of stent makes for precise and predictable

placement. Balloon-expandable stents are the stent of choice for

treating renal artery stenosis. Because they are made of laser-cut

stainless steel, these stents may cause significant artifact on

magnetic resonance imaging (MRI) examinations. Balloon-

expandable stents may be permanently deformed by extrinsic

compression and should not be used in situations in which they

could be subject to these forces.

6. What two materials are used to make self-

expanding stents?

There are two broad categories of self-expanding stents: stents

made from woven Elgiloy wires, and stents laser-cut from nitinol tubes. Self-expanding stents exert a continuous

outward force and resist deformation, and are preferable to balloon-expandable stents in regions potentially subject to

external compressive forces. To ensure full expansion, self-expanding stents are dilated with a balloon of appropriate

diameter after deployment.

7. How do woven Elgiloy and nitinol self-expanding stents differ?

•

Woven stents, such as Wallstent (Boston Scientific), have several unique characteristics. They are very radiopaque

and can be easily seen on fluoroscopy, even in obese patients. The stents are reconstrainable, meaning that they

can be almost entirely deployed, recaptured, moved, and then deployed in a different location. The tradeoff is,

however, that the length of the stent depends on its fully expanded diameter. These stents may shorten

significantly as they expand over time, uncovering a region of pathology. Alternatively, if the stent does not expand

to the degree expected, the stent may remain too long. Woven stents are available in large sizes (up to 24 mm in

diameter) and are often used to create transjugular intrahepatic portosystemic shunts or to stent large central

veins.

•

Laser cut self-expanding stents are not reconstrainable. Because the stents are constructed of rings linked

together they are subject to significant foreshortening and remain at a relatively stable length regardless of

diameter. Nitinol is less radiopaque then Elgiloy, and these stents may be difficult to see, especially in obese

patients.

8. What is nitinol?

Nitinol was developed by the U.S. Navy and stands for nickel titanium alloy. This metal is particularly useful for

medical applications because it has thermal memory. This property allows stents to be made at a certain diameter,

cooled, and then compressed onto a delivery system. When the stent is deployed at body temperature, the stent

attempts to regain its original configuration and diameter. A nitinol stent that is slightly oversized (by approximately

20%) with regard to the vessel exerts an outward force to keep the vessel open as the stent attempts to regain its

original diameter.

9. What do the terms hoop strength and radial force mean?

•

Hoop strength is a measure of a stent’s ability to avoid collapse and withstand the radial compressive forces

of a vessel after dilation.

A

B

Figure 29-2.

A and B, The difference between a

Cobra 1 and a Cobra 2 catheter: The radius of the

secondary curve on the Cobra 1 catheter (

A) is smaller

than the Cobra 2 (

B), although the overall configuration

of the catheters is similar.

Key Points: Choice of Stent Type

1. Balloon-expandable stents are the stent of choice for renal arteries because they can be placed with greater

accuracy.

2. Self-expanding stents are indicated when there may be an extrinsic compressive force acting on the stent.

EquipmEnt, tErms, and tEchniquEs in intErvEntional radiology

219

intErvEntional radiology

•

Chronic outward radial force is the force a self-expanding stent exerts on a vessel as it tries to expand to its

original diameter. The radial resistive force is the force a self-expanding stent exerts as it resists squeezing by

a vessel.

10. What is a sheath?

A sheath is a device that may be placed into a vessel at the site of percutaneous access. Sheaths permit rapid

exchanges of guidewires and catheters while maintaining access. Sheaths are sized based on the diameter of the

catheter or device that they allow to pass. A 7-Fr sheath accepts devices up to 7 Fr in outer diameter. A 7-Fr sheath

is in fact closer to 8 or 9 Fr in diameter.

11. What is a guiding catheter?

A guiding catheter is a special type of catheter that does not taper at its tip to the diameter of the guidewire. This

configuration allows the passage of devices of large diameter through the catheter. When used in this manner, a

guiding catheter functions similarly to a long sheath. In contrast to sheaths, guiding catheters lack a side-port and

hemostatic valve. Guiding catheters are sized based on the outer diameter. A 7-Fr guiding catheter fits through a

7-Fr sheath, but a 7-Fr device does not fit through a 7-Fr guiding catheter. Guiding catheters are available with

various tip configurations.

12. What is an up-and-over sheath?

Sometimes called a Balkin sheath, an up-and-over sheath is a

U-shaped sheath (

Fig. 29-3

). It is designed to facilitate interventions in

which the arterial access is in one femoral artery, and the lesion to be

treated is in the contralateral extremity. A catheter and guidewire are

placed into the aorta, and the contralateral iliac artery is selected. The

sheath is placed over the wire. The preshaped “U” curve assists

passage over the aortic bifurcation. After angioplasty or stenting,

arteriography can be performed by injecting contrast agent through

the side-arm of the sheath.

13. Explain the Trojan horse technique.

The original balloon-expandable stents came from the manufacturer

packaged in a small box. To use the stent, an appropriate balloon was

selected, and the stent was hand-crimped onto the balloon by the

operator. If not mounted properly, stents had the tendency to slip on the

balloon when being advanced across a tight stenosis. The Trojan horse

technique minimizes this risk. Instead of pushing the stent across the

lesion, the lesion is crossed with a sheath or guiding catheter (

Fig. 29-4

).

The balloon-mounted stent is advanced through the catheter or sheath to

the desired location, and then the sheath or catheter is withdrawn to

expose the stent in the proper location. In this way, complications related

to stent slippage are minimized. This is just one example of how the

Trojan horse technique is used. The term applies to the technique in

general and can be used to deliver any device in this manner, not just

a balloon-expandable stent.

14. What are the defining characteristics of

guidewires?

Guidewires are available in numerous configurations. The selection of wire type depends on the intended application.

Defining characteristics include the following:

•

Length: Wires are available in lengths from 70 cm to more than 300 cm. Short wires may be used for obtaining

vascular access or placing drains. Longer, 300-cm wires are used in catheter exchanges or to perform procedures at

a great distance from the access site.

•

Diameter : The most commonly used wires are 0.035 inch, 0.038 inch, or 0.018 inch in diameter. Wires 0.010 inch are

available for special applications, such as cerebral interventions. The size of the wires should be selected based on

the catheter used and the intended application.

•

Stiffness: Wires vary from very floppy to extremely stiff.

•

Coating: Some wires may have a hydrophilic coating. When wet, these wires become very slippery, and the coating

may facilitate the crossing of tight stenoses.

Figure 29-3.

A Balkin sheath is commonly

used when performing interventions in the

contralateral lower extremity. The “up-and-over”

design allows diagnostic angiography and

interventional procedures to be performed via an

access site in the contralateral leg.

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EquipmEnt, tErms, and tEchniquEs in intErvEntional radiology

•

Tip configuration: Wires have specialized tips that have

been engineered for specific applications. Some wires have

preformed angles to assist the selection of vessels. Other

wires have floppy or atraumatic tips to prevent vascular

injury. Some specialized wires have tips that can be shaped

during a procedure to accomplish a specific task.

15. What is an exchange-length wire? How long

does it have to be?

An exchange-length wire is long enough to perform a

catheter exchange without having to withdraw the wire. Most

wires are 150 cm long and are paired with catheters that are

either 65 or 100 cm long. If a catheter needs to be

exchanged over a wire, the wire needs to be at least twice as

long as the catheter to make the exchange without having to

pull the wire back and risk losing access. This is easy to

remember: If a 100-cm-long catheter is advanced all the way

into a vessel, a 200-cm wire would be needed to exchange

the catheter; 100 cm of the wire would be used before the tip

of the wire comes out of the catheter, and another 100 cm of

wire is needed so that the catheter can be pulled out without

moving the wire.

16. What is a Cope loop?

Dr Cope is one of the pioneers of interventional radiology and

is credited with some of the field’s most ingenious inventions.

One of these is the Cope loop, which is a pigtail catheter with a

locking mechanism to prevent accidental displacement. A small

string runs through the center of the catheter and is fixed to the

distal curved end. The other end of the string exits the hub of the

catheter. The curl on the catheter is straightened out as it is

advanced over a guidewire. After the wire is removed, the string is

pulled and tied, which causes the end of the catheter to curl and lock.

The fixed diameter of the locked coil prevents migration. To remove the catheter, the hub and string are cut, releasing

the distal lock.

17. What is the origin of the term stent?

The word stent derives from the surname of 19th century English dentist Stent. He developed gums and resins used

to make models of jaws, and the verb “to stent” came to mean “to hold tissue in place.” The term stent was later

adopted by surgeons to describe a device or material used to prop open a space.

18. What does it mean to “Dotter” a lesion?

Dr Charles Dotter was an early pioneer in interventional radiology. Before the availability of angioplasty balloons, he

described a technique in which stenoses were treated by passing dilators of successively larger diameter through them.

19. What is a micropuncture set?

A micropuncture set is used to obtain vascular access. It contains a 21G, single-wall needle; an 0.018-inch wire, and a

dilator. The dilator is tapered to the 0.018-inch wire to allow easy placement. The inner portion of the dilator can be

removed along with the wire to allow the dilator to accept a standard 0.035-inch wire. This set can be used as an

alternative to double-wall needles. Some prefer this set for gaining access for thrombolysis because it uses the

single-wall technique.

20. What is the difference between the single-wall and the double-wall technique?

To perform a single-wall technique, only the ventral wall of the vessel is punctured to gain entry to the vessel. A double-

wall puncture (Seldinger technique) is performed by puncturing the dorsal and ventral walls of the vessel

and subsequently withdrawing the needle. When pulsatile blood is encountered, the tip of the needle is in an

intraluminal position, and a guidewire can be placed safely.

21. What are the advantages and disadvantages of a single-wall puncture?

The single-wall technique is preferred for bypass grafts and for patients at risk for puncture site hemorrhage, such as

patients undergoing thrombolysis. The disadvantage is an increased risk for access site dissection.

A

C

B

D

Figure 29-4.

Trojan horse technique.

A, The lesion is

crossed with a wire.

B, A sheath or guiding catheter is

used to cross the lesion.

C, A stent is advanced through

the sheath or guiding catheter and centered over the

lesion.

D, The guiding catheter or sheath is withdrawn

exposing the stent.

EquipmEnt, tErms, and tEchniquEs in intErvEntional radiology

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intErvEntional radiology

22. What is a snare?

A snare is a device that may be used to remove

intravascular foreign bodies, such as wires or coils (

Fig.

29-5

). A snare consists of a wire with a nitinol loop at the

end. The plane of the loop is oriented perpendicular to

the long axis of the wire. The snare is advanced through

a catheter. Under fluoroscopic guidance, the loop is used

to engage a free edge of the foreign body. The wire and

loop are retracted into the catheter. This locks the foreign

body between the snare loop and catheter. The snare,

loop, and foreign body are removed in unison.

23. What is a reverse curve catheter?

A reverse curve catheter is one in which the tip of the

catheter doubles back on itself to form a partial loop

(

Fig. 29-6

). Because of the loop at the end, special

maneuvers must often be done to form the loop when

the catheter is inside the patient. After the loop is

formed, the catheter is pulled down to select a branch

vessel. When the tip of the catheter engages the orifice

of the vessel, the use of the catheter may be slightly

counterintuitive. The presence of the loop at the end of

the catheter causes the tip of the catheter to advance

distally in the selected vessel if the catheter is pulled

out. This is exactly opposite of what happens with a

conventional catheter. One must push a reverse curve

catheter inward to deselect a branch vessel. Reverse

curve catheters may often provide stable access to

vessels because of the way they behave. The Simmons

catheter is the prototypic reverse curve catheter.

24. What are the different ways to form a

Simmons catheter?

One way is first to place a wire over the aortic

bifurcation and then form the catheter by placing the

tip over the bifurcation and pushing up. The catheter

may also be formed in the thoracic aorta where the

diameter is sufficiently large. A technique commonly

used at the Hospital of the University of Pennsylvania

involves anchoring a suture with a wire in the tip of the

catheter. The loop is formed by gently pulling on the

string while advancing the catheter.

25. What is a Waltman loop?

The distal tip of any catheter can be looped back to

form a reverse curve catheter. A Waltman loop is the

configuration of a standard catheter when the distal

end has been formed into a reverse curve loop. The simplest technique to do this involves selection of an aortic

branch vessel. The tip is maintained in a constant position as wire and catheter are advanced in unison. This creates

a large reverse curve that can stabilize access and facilitate difficult catheterizations.

26. What is a “road map”?

“Road mapping” is an imaging technique present on many modern fluoroscopy units. An arteriogram is performed first.

When the road map function is selected, the fluoroscopy monitor displays a static subtracted angiographic image from

the prior run. When fluoroscopy is performed, a live subtracted fluoroscopic image is superimposed on the static

angiogram. In effect, the vessels are highlighted with everything but the motion of the catheter and guidewire subtracted

out. This technique facilitates selection of small or tortuous vessels.

27. How do you select the proper injection rate for an arteriogram?

The easiest way to select the proper rate is to give a test injection of contrast agent by hand to estimate the rate of flow.

The injection rate should approximate the intrinsic rate of flow of the vessel. Giving a test injection is always wise to

A

B

C

D

Figure 29-5.

Steps used to retrieve a catheter fragment.

A, A gooseneck snare is deployed in the vessel adjacent to the free

end of the fragment.

B, The free end of the catheter is engaged by

the loop of the snare.

C, The snare is pulled back into its outer

catheter to grasp the catheter tightly.

D, The snare, outer catheter,

and catheter fragment are withdrawn in unison.

A

B

C

Figure 29-6.

Steps in using a reverse curve catheter.

A, The

catheter is formed in the aorta.

B, The vessel is selected. C, As the

catheter is pulled out, its tip moves further into the vessel.

222

EquipmEnt, tErms, and tEchniquEs in intErvEntional radiology

ensure that the catheter is in the appropriate location. Accidental power injection into the subintimal space or a tiny

branch vessel may occur if this recommendation is not followed.

28. What is meant by an injection of “20 for 40”?

One must dictate the rate and volume for a power injected run. The term “20 for 40” means that the injection rate is

20 mL/s for a total volume of 40 mL. This would be a 2-second injection.

29. What is a rate rise?

A rate rise is another way that a power injection can be modified and describes the time in seconds that it takes from

the beginning of the injection to reach the desired injection rate. For an injection of “20 for 40” with no rate rise, the

velocity of contrast agent in the catheter immediately jumps from 0 to 20 mL/s. No rate rise implies a step function.

If a rate rise of 0.6 second is used, it takes 0.6 second for the injection to reach maximum velocity. Rate rises are used

to minimize the recoil of the catheter during rapid injections.

30. What is a Hickman catheter?

A Hickman catheter is a device used for long-term intravenous access, most commonly for chemotherapy or total

parenteral nutrition. The line is available in single-lumen, double-lumen, or triple-lumen models. It is ideally placed in

the internal jugular vein, with the exit site tunneled several centimeters away. The catheter has an antimicrobial cuff on

its surface.

31. What is a PermaCath?

PermaCath is a brand name and should not be used to describe the device in general terms. It is a type of tunneled

dialysis catheter.

32. What is a Medcomp?

Medcomp is also a brand name and should not be used as a generic reference. It is a brand of nontunneled dialysis

catheter.

B

iBliography

[1] S.H. Duda, J. Wiskirchen, G. Tepe, et al., Physical properties of endovascular stents: an experimental comparison, J. Vasc. Interv. Radiol.

11 (2000) 645–654.

Key Point: Injection Rate

1. The best way to determine the appropriate injection rate for a given vessel is to give a test injection by hand

and then to approximate the native rate of flow.