Equipment, Terms, and Techniques in Interventional Radiology

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What are the characteristics of a diagnostic catheter?

Catheters may be selected for a specific application based on many characteristics ( Figure 66-1 ). 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.

• Outer diameter: Most diagnostic catheters used today are 4 or 5 French (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 inches or 0.038 inches 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.

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What is the difference between Cobra 1, Cobra 2, and Cobra 3 catheters?

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 ( Figure 66-2 ). 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.

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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 is also a unit used to measure diameter where higher gauge values are associated with smaller diameters. Although it can be used to measure catheters, it is frequently used for needles. For example, a 21-gauge needle is approximately 0.03 inches in outer diameter, whereas an 18-gauge needle is 0.05 inches in outer diameter.

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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 classified as either balloon-expandable or self-expandable. 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 stents, and user preference plays a role in selection.

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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 allow for precise and predictable placement. Balloon-expandable stents are the stents 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.

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What two materials are used to make self-expandable stents?

There are two broad categories of self-expandable stents: stents made from woven Elgiloy wires and stents laser-cut from nitinol tubes. Self-expandable stents exert a continuous outward force, resist deformation, and are preferable to balloon-expandable stents in regions potentially subject to external compressive forces. To ensure full expansion, self-expandable stents are dilated with a balloon of appropriate diameter after deployment.

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How do woven Elgiloy and nitinol self-expandable stents differ?

Woven stents, such as the Wallstent, 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 redeployed in a different location. The tradeoff, however, is 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 (TIPS) or to stent large central veins.

Laser-cut self-expandable 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.

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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.

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What do the terms hoop strength , chronic outward force , and radial resistive 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.

Chronic outward force is the continuing radial expansion force that a self-expandable stent exerts on a vessel as it tries to expand to its original diameter.

Radial resistive force is the force that a self-expandable stent exerts as it resists radial compression by a vessel.

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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 intravascular 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.

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