Elsevier

Academic Radiology

Volume 18, Issue 3, March 2011, Pages 277-285
Academic Radiology

Original investigation
RF Heating of MRI-Assisted Catheter Steering Coils for Interventional MRI

https://doi.org/10.1016/j.acra.2010.09.012Get rights and content

Rationale and Objectives

The aim of this study was too assess magnetic resonance imaging (MRI) radiofrequency (RF)–related heating of conductive wire coils used in magnetically steerable endovascular catheters.

Materials and Methods

A three-axis microcoil was fabricated onto a 1.8Fr catheter tip. In vitro testing was performed on a 1.5-T MRI system using an agarose gel–filled vessel phantom, a transmit-receive body RF coil, a steady-state free precession pulse sequence, and a fluoroptic thermometry system. Temperature was measured without simulated blood flow at varying distances from the magnet isocenter and at varying flip angles. Additional experiments were performed with laser-lithographed single-axis microcoil-tipped microcatheters in air and in a saline bath with varied grounding of the microcoil wires. Preliminary in vivo evaluation of RF heating was performed in pigs at 1.5 T with coil-tipped catheters in various positions in the common carotid arteries with steady-state free precession pulse sequence on and off and under physiologic-flow and zero-flow conditions.

Results

In tissue-mimicking agarose gel, RF heating resulted in a maximal temperature increase of 0.35°C after 15 minutes of imaging, 15 cm from the magnet isocenter. For a single-axis microcoil, maximal temperature increases were 0.73°C to 1.91°C in air and 0.45°C to 0.55°C in saline. In vivo, delayed contrast-enhanced MRI revealed no evidence of vascular injury, and histopathologic sections from the common carotid arteries confirmed the lack of vascular damage.

Conclusions

Microcatheter tip microcoils for endovascular catheter steering in MRI experience minimal RF heating under the conditions tested. These data provide the basis for further in vivo testing of this promising technology for endovascular interventional MRI.

Section snippets

Device construction

A 1.8Fr Baltacci catheter (BALT, Montmorency, France) was obtained, and the most distal catheter tip containing heavy metal marker was cut to eliminate confounding magnetic forces and MR artifacts. A three-axis coil using 44–American wire gauge magnet wire (California Fine Wire, Grover Beach, CA) was wound on the tip of the modified catheter. The z-axis coil was solenoidal and consisted of 100 turns. The two orthogonal modified Helmholtz coils were wound along the side of catheter over the

RF Heating of Handmade Coils in Agarose Gel Phantom

In the absence of applied direct current, experiments were carried out at various distances from the magnet bore wall to determine RF heating effects induced in the solenoid and Helmholtz catheter coils due to RF pulse sequences during real-time MRI. At the isocenter of the magnet, no heating was detected by any of the probes after 15 minutes of imaging (Fig 2a). As the apparatus was moved 5 cm closer to the magnet bore wall, a slight increase in the temperature (0.2°C) recorded over the

Discussion

This study shows that no clinically significant RF-induced heating occurs during real-time SSFP MRI of MR-assisted catheter tip steering for interventional MRI. The maximum increase in temperature observed after 15 minutes of continuous SSFP imaging was 0.35°C at 15 cm from the magnet isocenter, which is well below the 4°C increase that can cause irreversible tissue damage 17, 18. As expected, temperature increases around the coils were directly proportional to the proximity of the

Conclusions

MRI-assisted steering coils for endovascular catheter steering during vascular intervention do not appear to experience significant RF heating. These results, combined with demonstration that resistive heating due to applied direct current can be minimized by transferring heat to saline coolant flowing through catheter lumen, begin to address safety concerns related to the eventual clinical use of this device in vascular interventions and will serve as the basis for further in vivo testing

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  • Cited by (0)

    This work was supported by grant 5 R01 HL076486-01 to 03 from the National Heart, Lung, and Blood Institute (Bethesda, MD).

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