IEEE Robotics & Automation Magazine - September 2018 - 78

outcomes depend greatly on the correct positioning of the
catheter tip. For instance, during catheter ablation, the catheter
is positioned inside the heart and requires consistent contact
between its tip and the cardiac tissue. Achieving accurate positioning is challenging, especially in the presence of cardiac and
respiratory motions. Moreover, the complexity of the vasculature limits the use of preprogrammed trajectories where an
exact knowledge of the path from the insertion point to the
target location is necessary.
Realizing the appropriate control method for both trajectory following and catheter localization requires sufficient
information about the system dynamics, e.g., a force-deflection relationship of an ablation catheter, or a current-field
map in the case of an electromagnetic passive actuation system. Consequently, accurate mechanical models and continuous device localization are specific fundamental requirements
of control strategies for magnetic catheter actuation [74].
Mechanical models used for accurately describing the system
and environment include Euler-Bernoulli beam deflection
[35], rigid link approximation [75], the Cosserat rod theory
[32], and pseudorigid body modeling [76]. The localization
of catheters has been demonstrated by accurately using electromagnetic tracking [27] and by observing the device shape
and orientation using MR, US, and fluoroscopy.
Open-loop control of magnetic catheters has been established and demonstrated in animals [27] and humans [77].
However, effective open-loop control is challenging to
implement, since many of the catheters used in these procedures exhibit nonlinear behavior [78]. Further drawbacks
include measurement and environmental noise, as well as
inaccuracies in terms of result output due to the absence of a
feedback mechanism.
In contrast, closed-loop control involves constant feedback of the position of the catheter to ensure more accurate
catheter positioning. Tunay et al. [79] introduced one of the
first real-time, closed-loop automated MMS in 3-D. In a
study by Degirmenci et al. [54], a robust method for the
closed-loop control of a 4-DoF catheter tip was successfully
demonstrated. O'Donoghue et al. [80] also presented a novel
closed-loop current feedback amplifier for controlling a
magnetic field used for catheter position sensing. Closedloop control in a constrained setting, e.g., a patient vasculature, remains a significant challenge because of vessel wall
friction and physiological disturbances. To address this problem, Edelmann et al. recently demonstrated a model-free
method enabling the direct control of a flexible endoscope
with an electromagnetic steering system [74]. This approach
presented a novel way of implementing catheter control since
there is no need for device configuration or precise model
formulation, making it a critical improvement to a wide
range of endovascular procedures.
Discussion
The intended applications for steerable surgical instruments
impact cardiology and interventional radiology fields the
most. These fields cover interventions such as peripheral and
78

IEEE ROBOTICS & AUTOMATION MAGAZINE

september 2018

cardiac procedures, including angiography, angioplasty, and
ablation [70]. Clinical trials have reported beneficial effects in
the use of steerable catheters in interventional cardiology,
while most studies concern advances in cardiac catheter technology and interventions [24], [30], [70], [73], with a main
focus on passive magnetic actuation systems.
The broad range of applications of catheterization within
minimally invasive surgeries demands additional improvements of surgical instruments. As the underlying steering
technologies' applicability differs per clinical procedure
type, different catheters or flexible devices should be developed accordingly. The chosen method of actuation influences the structure and assembly of the catheter, especially for
magnetically driven systems. For example, during angiography, a catheter will be inserted into the bloodstream to
deliver contrast agents. For angioplasty, a thin catheter with
an expandable balloon will be used to widen narrowed or
obstructed arteries or veins. For targeted drug delivery,
more focus is placed on developing multilumen catheter
bodies, whereas, for catheter ablation therapy, tools to attach
on the catheter tip are more critical. For the remaining
interventions, targeted maneuvering and steering from an
insertion point require a flexible catheter with a fully controllable tip.
The developments found in magnetically actuated catheters are of high scientific value, as their applications are still
unique among minimally invasive surgery. However, specific
challenges were identified in the methods and solutions provided for steerable catheters.
Challenges
From experimental results of the literature reviews, some limitations were introduced with regards to the actuation type,
catheter model, and control methods. Several studies sought
to design catheters that can be remotely controlled in MR systems, though all revealed some limitations.
For example, the acute angles of origin that target vessels
arise from branches influence the orientation of the catheter
tip relative to the bore of the MR scanner. These angles result
in some alignments of the catheter to scrape the walls of the
vessel and cause damage. Furthermore, since the coils generate a heating effect, high temperatures in the near vicinity of
the catheter tip may cause damage to surrounding vessel walls
and sensitive tissue [81], [82].
Problems like the accuracy of closed-loop control strategies [83], 2-D-US tracking algorithm inaccuracies and 2-D
plane visualization limitations [36], small operational workspaces [84], and magnetic field estimation inaccuracies [30]
seem to arise frequently. Studies that take place in stationary
environments tend to simplify complex scenarios; however,
additional tracking uncertainties may occur because of the
reduced visibility of the catheter [85]. These identified limitations result in the pursuit of further navigation and tracking methods, which are either less expensive or require
minimal manual intervention. Several strategies to minimize these challenges can be followed, e.g., implementing

IEEE Robotics & Automation Magazine - September 2018

Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - September 2018