In vitro evaluation of intravascular stent artifacts in three-dimensional MR angiography.

RATIONALE AND
OBJECTIVES: To evaluate the intraluminal signal characteristics of various stents and stent-grafts in contrast-enhanced three-dimensional MR angiography (3D MRA) in vitro.
METHODS: Fourteen stents made of different materials (steel, nitinol, tantalum, cobalt-based alloy, polyethylene) and six stent-grafts were implanted in plastic tubes simulating the common iliac artery. The tubes were filled with gadopentetate dimeglumine in water at a concentration of 25 mmol/L and positioned in a plastic container filled with water. For imaging, the container was placed in the center of the magnet, parallel, orthogonal, and diagonal to the z axis. A 3D gradient-echo sequence (T1-FFE) was acquired with the following parameters: repetition time 5.3 ms, echo time 1.6 ms, flip angle 50 degrees, slice thickness 1.5 mm, and acquisition matrix 256 with zero filling to 512. To evaluate the influence of the frequency-encoding gradient on the appearance of the artifacts, stents were examined with their axes oriented in all three directions both with the frequency-encoding gradient in the feet-head and right-left directions. The size and pattern of stent-related artifacts were evaluated semiquantitatively for each measurement.
RESULTS: Five different components of artifacts could be distinguished: homogeneous signal reduction inside the stent, narrowing of the stent lumen, structures of various shapes inside the stents, signal reduction or signal increase at the ends of the stents, and shift of the intraluminal signal orthogonal to the longitudinal axis of the vessel. The size of the artifacts depended heavily on the material of the stent. The polyethylene stent showed no artifacts, the tantalum stent only minor artifacts. Nitinol stents were characterized by artifacts at both ends and signal reduction intraluminally. Stents made of steel demonstrated the strongest artifacts, characterized by almost complete signal loss intraluminally. The characteristics of the artifacts of all stents depended on the direction of the stent relative to the frequency-encoding gradient.
CONCLUSIONS: Three-dimensional MRA follow-up after stent placement may be applicable for stent patency evaluation in all instances. However, grading of stenoses seems to be unrealistic in steel stents and in most nitinol stents.