expanding stents? There are two broad categories of self-expanding stents: stents made from woven Elgiloy wires, and stents laser-cut from nitinol...

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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.
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.
8. What is nitinol?
Nitinol was developed by the U.S. Navy and stands for nickel titanium al oy. 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.
EquipmEnt, tErms, and tEchniquEs in intErvEntional radiology
intErvEntional radiology 219
• 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 cal ed 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