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. 2011 Jul;38(7):4215-24.
doi: 10.1118/1.3600693.

Control of intravascular catheters using an array of active steering coils

N Gudino  1 , J A Heilman, J J Derakhshan, J L Sunshine, J L Duerk, M A Griswold
Affiliations

Control of intravascular catheters using an array of active steering coils

N Gudino et al. Med Phys. 2011 Jul.

Abstract

Purpose: To extend the concept of deflecting the tip of a catheter with the magnetic force created in an MRI system through the use of an array of independently controllable steering coils located in the catheter tip, and to present methods for visualization of the catheter and/or surrounding areas while the catheter is deflected.

Methods: An array of steering coils made of 42-gauge wire was built over a 2.5 Fr (0.83 mm) fiber braided microcatheter. Two of the coils were 70 turn axial coils separated by 1 cm, and the third was a 15-turn square side coil that was 2 x 4 mm2. Each coil was driven independently by a pulse width modulation (PWM) current source controlled by a microprocessor that received commands from a MATLAB routine that dynamically set current amplitude and direction for each coil. The catheter was immersed in a water phantom containing 1% Gd-DTPA that was placed at the isocenter of a 1.5 T MRI scanner. Deflections of the catheter tip were measured from image-based data obtained with a real-time radio frequency (RF) spoiled gradient echo sequence (GRE). The small local magnetic fields generated by the steering coils were exploited to generate a hyperintense signal at the catheter tip by using a modified GRE sequence that did not include slice-select rewinding gradients. Imaging and excitation modes were implemented by synchronizing the excitation of the steering coil array with the scanner by ensuring that no current was driven through the coils during the data acquisition window; this allowed visualization of the surrounding tissue while not affecting the desired catheter position.

Results: Deflections as large as 2.5 cm were measured when exciting the steering coils sequentially with a 100 mA maximum current per coil. When exciting a single axial coil, the deflection was half this value with 30% higher current. A hyperintense catheter tip useful for catheter tracking was obtained by imaging with the modified GRE sequence. Clear visualization of the areas surrounding the catheter was obtained by using the excitation and imaging mode even with a repetition time (TR) as small as 10 ms.

Conclusions: A new system for catheter steering is presented that allows large deflections through the use of an integrated array of steering coils. Additionally, two imaging techniques for tracking the catheter tip and visualization of surrounding areas, without interference from the active catheter, were shown. Together the demonstrated steerable catheter, control system and the imaging techniques will ultimately contribute to the development of a steerable system for interventional MRI procedures.

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Figures

Figure 1
Figure 1
(a) Diagram of a small deflection (δ) that results from the force (F) applied in the catheter tip due to the interaction of the magnetic moment (M) generated by an axial current loop and the static magnetic field of the scanner (B 0) (b) In a 90° deflection condition there is not net magnetization perpendicular to the static field and an additional force source (F′) is necessary to generate further deflection.
Figure 2
Figure 2
Simplified diagram of driving steering coils sequentially (a) Only coil one is active and generates a magnetization vector (M) that interact with the static magnetic field (B 0) and produce deflection. (b) At 45° deflection coil two reached a position to add to the deflection and it is turn on, now the total magnetic moment is the combinations of the magnetization produced by each coil. (c) At 90° deflection coil one axis is now parallel with the static magnetic field and should be turn off.
Figure 3
Figure 3
(a) System schematic diagram. (b) Current value, current polarity and coil/s are set from the serial data sequence generated by matlab routine and transmitted from the main PC to a microprocessor. (c) Current bridge controlled by the PWM generated by microprocessor, and (d) Coil array made of 42‐gauge wire and set on a 2.5 Fr braided microcatheter.
Figure 4
Figure 4
Current excitation diagram for tip deflection experiment. The coil excitation time T i is set in matlab and ideally at this time the resulting magnetization vector is perpendicular to the main magnetic field to generate maximum torque.
Figure 5
Figure 5
(a) Modified GRE sequence with suppressed rewinding gradients and (b) Sequence diagram of imaging mode—excitation mode method. Current excitation sequence is synchronized with scanner imaging sequence.
Figure 6
Figure 6
Increasing deflection of the catheter: (a) no coil excited, (b) first axial coil excited, (c) both axial coils excited, and (d) three coils are excited. The white arrow indicates the direction of the magnetic field (B 0). B 0 = 1.5 T. Current per coil I = 110 mA. The image was acquired using a real‐time GRE sequence, 25º flip angle, 200 mm FOV, 10 mm slice thickness, and 7.8 ms TR. Artifact indicated by white arrow in Fig 6 (b) is due to interference of the current control system with the scanner.
Figure 7
Figure 7
Total deflection of the catheter in the magnet while exciting one axial coil with increasing values of currents (solid diamond), and exciting sequentially the steering coil array with a 100 mA current per coil (open triangle).
Figure 8
Figure 8
Temperature curve of steering catheter tip measured on the benchtop. Temperature was sensed on a single axial coil (70 turns of 42 AWG wire) driven by 100 mA.
Figure 9
Figure 9
(a) Imaging catheter with GRE 5.5 ms TE, 400 ms TR, 300 Hz BW/pixel while switching off the current through the coils during 8.2 ms T OFF. (b) Imaging catheter with GRE 1.8 ms TE, 400 ms TR, 980 Hz BW/pixel while switching off the current through the coils during 3.3 ms T OFF
Figure 10
Figure 10
Deflection angle that result from changing TR when exciting only one axial coil with 100 mA.
Figure 11
Figure 11
Catheter images using different imaging approaches: (a) no coil excited using a GRE sequence. After coil excitation (b) GRE sequence (c) GRE sequence with excitation and imaging mode (d) GRE sequence with unbalanced slice‐select gradient. Artifact indicated by white arrow in Fig 10 (d) is due to interference of the current control system with the scanner.
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