Guidewires

Advances in interventional MR procedures have resulted in the need for guidewires that are acceptable for endovascular therapy, drainage procedures, and other similar applications. Conventional guidewires are made from stainless steel or Nitinol, materials known to be conductive.

Accordingly, radiofrequency fields used for MR procedures may induce substantial currents in guidewires, leading to excessive temperature increases and potential injuries.
Liu et al. studied the theoretical and experimental aspects of the RF heating resonance phenomenon of an endovascular guidewire.

A Nitinol-based guidewire (Terumo, Tokyo, Japan) was inserted into a vessel phantom and imaged using 1.5- and 0.2-Tesla MR systems with continuous temperature monitoring at the guidewire tip. The guidwire was deployed in the phantom in a "straight" manner. The heating effects due to different experimental conditions were examined. A model was developed for the resonant current and the associated electric field produced by the guidewire acting as an antenna.

Temperature increases of up to 17 degrees C were measured while imaging the guidewire at an off-center position in the 1.5-Tesla MR system. Power absorption produced by the resonating wire decreased as the repetition time was increased. No temperature rise was measured during MRI performed using the 0.2-Tesla MR system. Thus, considering the potential utility of low-field, open MR systems for MR-guided endovascular interventions, it is important to be aware of the safety of such applications for metallic guidewires and the potential hazards associated with using a guidewire with MR systems operating at other static magnetic field strengths.

An investigation conducted by Konings et al. examined the radiofrequency (RF) heating of an endovascular guidewire frequently used in interventional MR procedures. A Terumo guidewire was partly immersed in an oblong saline bath to simulate an endovascular intervention. The temperature rise of the guidewire tip during MR imaging was measured using a Luxtron fluoroptic thermometry system. Starting from a baseline level of 26 degrees C, the tip of the guidewire reached temperatures up to 74 degrees C after 30 seconds of scanning using a 1.5-Tesla MR system. Touching the guidewire produced a skin burn.

The excessive heating of a linear conductor, like the guidewire that underwent evaluation, could only be explained by resonating RF waves. According to Konings et al., the capricious dependencies of this resonance phenomenon have severe consequences for safety guidelines of interventional MR procedures involving guidewires.

Recent advances in guidewire designs reporte by Kocaturk et al. and Kramer et al. have yielded guidewires that appear to be more acceptable for interventional MRI procedures with respect to both safety and visualization aspects.

REFERENCES

Buecker A. Safety of MRI-guided vascular interventions. Minim Invasive Ther Allied Technol. 2006;15:65-70.

Konings MK, Bartels LW, Smits HJ, Bakker CJ. Heating around intravascular guidewires by resonating RF waves. J Magn Reson Imaging 2000;12:79-85.

Lin C-Y, Farahani K, Lu DSK, Shellock FG. Safety of MRI-guided endovascular guidewire applications. Proceedings of the International Society of Magnetic Resonance Imaging, 1999;7:1015.

Shellock FG. Magnetic Resonance Procedures: Health Effects and Safety. CRC Press, LLC, Boca Raton, FL, 2001.

Shellock FG. Radiofrequency-induced heating during MR procedures: A review. Journal of Magnetic Resonance Imaging. 2000;12: 30-36.