Magnetically-Assisted Remote Controlled Microcatheter Tip Deflection under Magnetic Resonance Imaging

Summary

Current applied to an endovascular microcatheter with microcoil tip made by laser lathe lithography can achieve controllable deflections under magnetic resonance (MR) guidance, which may improve speed and efficacy of navigation of vasculature during various endovascular procedures.

Abstract

X-ray fluoroscopy-guided endovascular procedures have several significant limitations, including difficult catheter navigation and use of ionizing radiation, which can potentially be overcome using a magnetically steerable catheter under MR guidance.

The main goal of this work is to develop a microcatheter whose tip can be remotely controlled using the magnetic field of the MR scanner. This protocol aims to describe the procedures for applying current to the microcoil-tipped microcatheter to produce consistent and controllable deflections.

A microcoil was fabricated using laser lathe lithography onto a polyimide-tipped endovascular catheter. In vitro testing was performed in a waterbath and vessel phantom under the guidance of a 1.5-T MR system using steady-state free precession (SSFP) sequencing. Various amounts of current were applied to the coils of the microcatheter to produce measureable tip deflections and navigate in vascular phantoms.

The development of this device provides a platform for future testing and opportunity to revolutionize the endovascular interventional MRI environment.

Introduction

Endovascular procedures performed in interventional medicine use x-ray guidance as a tool for catheter navigation through vasculature to treat several major illnesses, such as brain aneurysm, ischemic stroke, solid tumors, atherosclerosis and cardiac arrhythmias targeting over one million patients per year worldwide^1-5. With the use of contrast media, navigation through vasculature is achieved through manual rotation of the catheter and mechanical advancement by the interventionist's hand⁶. However, navigation through small tortuous blood vessels around many vascular bends becomes increasingly difficult, elongating the time before reaching the target site. This poses a problem for time-sensitive procedures such as the removal of a clot in an occluded blood vessel. Additionally, prolonged procedures increase the radiation dose and create the potential for adverse events^7-11. However, endovascular procedures performed under magnetic resonance imaging may provide a solution.

The strong homogenous magnetic field of an MRI scanner can be exploited for catheter tip navigation by remote control^12,13. Current applied to a microcoil located at a catheter tip induces a small magnetic moment, which experiences a torque as it aligns with the bore of the MRI scanner¹³ (Figure 1). If electric current is activated in an individual coil, the catheter tip can be deflected in one plane by remote control. If three coils at a catheter tip are energized, catheter tip deflection can be achieved in three-dimension. Thus, magnetically facilitated steering of a catheter has the potential to increase the speed and efficacy of vascular navigation in endovascular procedures, which could reduce procedure times and improve patient outcomes. In this study, we examined if current applied to a microcoil-tipped endovascular catheter can produce reliable and controlled deflections under MR-guidance as preliminary testing of catheter navigation studies.

Protocol

1. Microcoil Fabrication

1. Obtain a commercially available microcatheter (e.g. 2.3F Rapid Transit Cordis Neurovascular Catheter, Raynham, MA) for a substrate.
2. Ensure catheters have no ferrous components, are considered MR-safe, and range in size 2.3-3.0 F.
3. Sputter a titanium adhesion layer followed by a copper seed layer unto a 1 to 2 mm OD insulating tube. Possible materials include polyimide or alumina (Ortech Advanced Ceramics, Sacramento, CA).
4. Electrodeposit a positive photoresist layer using Shipley's PEPR-2400 (currently being sold by DOW Chemical under the name Intervia 3D-P). Electrodeposition results in a uniform coating on the non-planar cylindrical surface.
5. Photoresist is exposed by a unique laser direct-write system (laser lathe, a non-commercial system developed at Lawrence Livermore National Laboratory) in the pattern of the desired coil form (Figure 2A). This is a modification of the technique originally described in Malba et al.¹⁴
6. Develop the exposed photoresist in a 1% solution of potassium carbonate at 35 °C.
7. Copper is electroplated through the remaining resist mask to form the desired coil. The system can fabricate both solenoid and Helmholtz (racetrack) copper patterns (Figures 2C and 2D).
8. After copper electrodeposition, remove the resist with hot developer. Remove the copper seed layer, followed by the titanium adhesion layer.
9. Attach the insulating tube to the catheter tip using shrink wrap to complete the assembly. Ensure that the shrink wrap covers the entire coiled tip. To assemble multi-axis catheters place insulating tube structures inside each other as shown in Figure 2E.
10. Thread copper wires through the lumen of the microcatheter and solder to the coils at the tip.
11. Modify and shorten a 6 ft RJ11 phone cable to 3 ft in length.
12. Connect the copper wires emanating from the back end hub of the microcatheter to the modified 3 ft phone jack transmission line.

2. Waterbath Setup

1. Make a small hole in the center of the side of a plastic basin about 5 cm from the bottom.
2. Insert a 9F Avanti Cordis vascular sheath (Cordis Endovascular, Miami Lakes, FL) through the hole.
3. Cut the distal tip of the vascular sheath leaving a 4 cm-long piece extending in to the basin.
4. At the end of the sheath, attach a rotating hemostatic or Thuoy-Borst valve to stabilize the location of the microcatheter.
5. Fill the basin with distilled water ensuring complete submersion of the apparatus.
6. Insert the catheter with coiled tip through the vascular sheath and valve.
7. Measure and record the unrestrained length of the microcatheter extending from the valve in to the waterbath.
8. Place the waterbath with microcatheter system within the magnet of the MR scanner and orient with respect to the bore of the magnet.
9. Connect the modified 3 ft phone cable attached to the catheter to a 25 ft RJ11 phone cable transmission line using a 2-way phone jack.
10. Connect the other end of the 25 ft phone cable to a Lambda LPD-422A-FM dual regulated power supply to deliver up to 1 A of current to the device.
11. Place the transmission lines through a waveguide and the power source outside of the MR scanner room outside of the 5 Gauss line.

3. Vessel Phantom Setup

1. Construct a hollow vessel phantom with a Y-shaped intersection from rubber tubing prior to experimentation.
2. Fill the vessel phantom with a 0.0102 M solution of gadopentetate dimeglumine (GdDTPA) (Magnevist, Bayer Healthcare Pharmaceuticals, Montville, New Jersey) in distilled water to create contrast between the phantom vessels and background.
3. Assemble the microcatheter system as outlined in steps 1.1 through 1.9. Connect the catheter to the power supply and position as described in steps 2.9 to 2.11.
4. Position the tip of the microcatheter at the base of the vessel opening.
5. Place the phantom within the magnet of the MR scanner and orient with respect to the bore of the magnet.

4. Magnetic Resonance Imaging

1. Perform imaging with a 1.5T clinical MR system (Siemens Avanto, SW: Syngo B13, Erlangen, Germany; Philips Achieva, SW release 2.1, Cleveland, OH).
2. Apply <50 mA of current to visualize the catheter tip position. Under MRI, a small magnetic moment will be produced at the catheter tip to visualize a distinct artifact of varying shape depending on which coils are energized.
3. Apply variable amounts of current in the range of ±100 mA from the Lambda dual power source to the coils and observe tip deflection (Figures 3A-3C) in the water bath setup. Because tip deflection is almost instantaneous, current need only be applied for ~1-2 sec to visualize maximum deflection.
4. Repeat and record consecutive applications of set amounts of current.
5. Repeat step 4.2 while simultaneously pushing the catheter by hand allowing mechanical advancement through the vessel phantom (Figures 4A and 4B). Apply current at the branch point to deflect the catheter tip into the desired vessel. Advance the catheter into the branch vessel by manually pushing the catheter end (Figure 4C). Retract the catheter to the vessel bifurcation and repeat in the opposite branch (Figure 4D).
6. Acquire MR images using a 2D snapshot-FLASH sequence (TR=30 msec, TE=1.4 msec, a matrix of 256 x128 and flip angle ~30 °).

5. Deflection Measurements

Analyze and measure angle deflections of images captured during water bath experiments with various computer applications (any Digital Imaging and Communications in Medicine (DICOM) Viewer).

Results

From the protocol described above, an angle of deflection between 0 and 90 degrees should be observed from application of 50-300 mA of current delivered simultaneously to both coils of a combined solenoid and Helmholtz coil microcatheter system (Figure 2E). An increase in applied current should result in an increase in microcatheter deflection angle, while a reversal in current polarity should result in deflection in the exact opposite direction as observed with positive current (Figures 5A-5C

Discussion

Here we describe the protocol for deflection of a microcatheter in a MR scanner. The key parameters for success are accurate application of current and measurement of deflection angle. Inaccurate measurement of deflection angle is the most probable error encountered in this protocol. The angles captured in MR images during the waterbath experiment may differ from actual values due to slight differences in the orientation by which the medium is positioned with respect to the bore of the magnet. To address this issue in th...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
GdDTPA Contrast Media (Magnevist)	Bayer HealthCare Pharmaceuticals Inc.	1240340	McKesson Material Number
Positive Photoresist	Shipley	N/A	PEPR-2400, Replacement: Dow Chemicals Intervia 3D-P
Copper Sulfate	ScienceLab	SLC3778	Crystal form
Sulfuric Acid	ScienceLab	SLS1573	50% w/w solution
Parrafin Wax	Carolina	879190
Potassium Carbonate	Acros Organics	424081000