IEEE Robotics & Automation Magazine - September 2018 - 75

determined that the upper boundary of electrical currents
were safely usable in a 1.5 T MR scanner. Their findings indicated minimal injury to vessel walls using applied currents of
less than 300 mA, but also revealed problems related to steering the catheter in branches with various orientations. To specifically address such shortcomings, Liu et al. [24] presented
the three-dimensional (3-D) kinematic modeling of a new,
steerable robotic ablation catheter system using the actuation
of a 3 T MR scanner.
Similarly, Liu et al. [25] presented a catheter embedded
with a set of current-carrying microcoils that was actuated by
MR. In their latest study, they presented a prototype with a
single axial coil and two orthogonal coils to allow for control
of the 3-D deflection. A challenge of using microcoils is the
final tip diameter after construction, which may cause the
catheter to become obsolete for use in smaller cerebral and
cardiac vessels; although, it has been suggested that future
prototypes could be constructed from laser-lithographed coils
with very thin heat-shrink tubing [17]. This would then allow
for smaller coils.
The use of MR scanners for the active actuation of magnetic instruments is promising and provides a unique opportunity for the development of new endovascular catheter
steering mechanisms. With orthogonal coils, catheters can be
steered in multiple directions inside an MR scanner. However,
it remains a challenge to generate torques parallel to the direction of the MR magnetic field because of the steering principle of alignment between the fields. Furthermore, surgical
procedures using magnetic actuation are commonly not performed in an MR scanner because of either hospital workflow
or cost considerations. A number of studies addressed these
drawbacks by utilizing passive actuation.
Passive Magnetic Actuation
Passive actuation involves catheters being externally actuated
by electromagnets or external permanent magnets (Figure 4).
While adhering to Hopkinson's law [26], electromagnets are
capable of producing steerable magnetic fields, dependent
upon the current that flows through the windings of the coil.
Multiple electromagnets can thus provide passive actuation in
different directions. In contrast, a field generated using external permanent magnets is constant, and therefore steered by
changing the position of the permanent magnet itself [27],
[28]. Passive actuation also relies on the wrench produced by
a magnetic field. This external field can be relatively smaller
since the constant dipoles usually take much higher values.
Passively actuated catheters typically experience both the
magnetic force F and torque T from (1) when exposed to a
magnetic field B (p), as the gradients of the field are rarely
zero. The torque specifically aligns the magnetic dipole direction of the magnetic element with the applied field, and can,
therefore, be used to specify alignment direction for basic
steering. The force then pulls the magnet in the direction of
the field gradient.
Permanent magnets attached to endovascular catheters
[Figure 2(c)] have delivered significant results regarding

accuracy and predictable passive magnetic navigation. The
majority of magnetically tipped catheters used in studies are
constructed using an alloy of neodymium-iron-boron (NdFe-B), also called NeodymiumN. This ferromagnetic material
is widely utilized in catheter designs because it has strong
magnetization, making it an excellent candidate for steering
catheter tips [29]. Studies using both electromagnets [30],
[31] and permanent magnets [32] for passive actuation have
utilized Nd-Fe-B magnets in their catheter models, although
the majority has implemented single magnetic components.
Recently, Edelmann et al. [33] demonstrated the modeling of
different catheter geometries with multiple magnetic components and various boundary constraints. However, the exact
magnitudes of forces acting upon such catheter tips are, in
some cases, limited by the relative stiffness of catheters [34].
Chautems et al. [18] presented a solution (called the tethered
magnet) by replacing a flexible catheter tip with a string-like
tether [Figure 2(d)].
Commercial and noncommercial systems that use the
principle of passive actuation have been developed. Tunay
[35] managed to determine the position and orientation of a
catheter tip in real time by external electromagnetic means
and illustrated an improved predictable navigation in twodimensional (2-D) planes when compared to manual actuation. Deflection was accomplished with the assumption that
the spatial variation of the field from (1) is small enough
that the force is negligible. Boskma et al. [36] also demonstrated a magnetically actuated catheter in 2-D using the
interaction between permanent magnets embedded inside
the tip, and external electromagnetic fields. A homogeneous
magnetic field was used to steer the catheter in 2-D space
using two Helmholtz coils while also assuming the magnetic
force to be zero.
Tunay [37] again demonstrated the static deflection of a
magnet-tipped catheter, but this time extended the navigation of catheters to operate and work within a 3-D space. The
focus on 3-D was also demonstrated using an electromagnetic system developed by Gang et al. [38], which exerts both
force and torque from (1) by controlling the field magnitude,

Electromagnet

z
x

Permanent T
Magnet

B(p)
Catheter

Axis of Symmetry
Figure 4. The torque that acts on a magnetically tipped catheter,
with the permanent magnet center point positioned at a point p
from an external electromagnet. The magnetic field B (p) induces
a torque T to cause an alignment of the catheter tip.

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