IEEE Robotics & Automation Magazine - September 2018 - 77

magnet strength is influenced by its materials; the strength of
an electromagnet can be adjusted by the amount of electric
current that flows through the coil. This can result in the
same electromagnet being used for different levels of magnetic field strength, and therefore, various applications when one
permanent magnet can only be demonstrated with one particular field strength. This rapid manipulation of an electromagnet's magnetic field can be used on a wide range,
although the continuous supply of electrical energy makes it
the more costly method.
Visualization and Control
Guidance and tracking of surgical instruments can be
improved significantly using a variety of imaging modality
methods. Using these methods not only allows visualization
of the structure of the blood vessel, the vessel wall, and all
aspects of the anatomy within the field-of-view, but also
shows functional information about the instrument within
the vessel. In the case of the latter, image-based tracking
methods can be used to track the instrument, e.g., a needle or
catheter tip [46], [47].
Imaging Modalities
Previous studies have stated the importance of real-time clinical imaging modalities to provide magnetic motion control
systems with the position of surgical instruments and targeted
drug delivery systems [48], [49]. Some of the techniques used
to observe the movement of surgical devices include: obtaining images by dark-field microscopy [50]; using fluoroscopy
as an imaging modality [51]; and MR [52], although several
health risks, design challenges, and limitations to clinical
studies exist for these methods [53]. Furthermore, existing
work has demonstrated the use of electromagnetic trackers on
catheter bodies or tips [54], [55]. Their application, however,
is not suitable in electromagnetic actuation systems.
Endovascular procedures that are being conducted under
X-ray guidance have proven to be rapid and efficient when
compared to MR guidance [56]. Using X-rays can also produce images from areas such as bone, retroperitoneum, and
lungs, not well seen by ultrasound (US) guidance [57]. Unfortunately, X-ray and fluoroscopy techniques cause several safety hazards due to the exposure of both patients and surgeons
to ionizing radiation [58]. Furthermore, conventional computerized tomography (CT) scanning delivers insufficient soft
tissue visualization [53] and no real-time images during the
procedure [59]. Even when images are produced successfully,
CT-guided procedures can be complicated in uncooperative
patients and in organs that are prone to respiratory motion
(e.g., the aorta, liver, and lung) [60].
Alternatively, MR has been investigated as a novel, passive
imaging modality for visualizing robotic catheters [19], [24],
[53], [61], [62]. Interventional MR offers a high contrast for
soft tissue and 3-D volumetric image reconstruction [63] and
no ionizing radiation [64], a clear advantage over conventional X-ray angiography. The promise of endovascular MR-guided procedures, however, remains unrealized in part because

of the lack of MR-compatible catheters and guidewires that
the user can safely navigate and track efficiently in real time
[65], especially with strong magnetic fields (1.5 T or greater)
[63]. Furthermore, MR has a low image acquisition rate [66],
making it more difficult to navigate conventional catheters
and electrical components safely [67].
US imaging has served as an imaging modality in endovascular studies for both 2-D and 3-D environments [68]-
[71]. As with MR-imaging modalities, one of the prominent
advantages of US imaging is that it causes no ionizing radiation, allowing for longer and safer surgical procedures.
Instrument visibility and tracking accuracy can be enhanced
and automatically guided [72] using robust US-image analysis techniques.
For visualization only, the majority of interventions
involve the use of real-time X-ray fluoroscopy imaging [53].
Due to the previously mentioned drawbacks to its use, MR
and US have again become attractive alternatives for instrument visualization. Cannon et al. [73] investigated 2-D- and
3-D-US as stand-alone imaging modalities during interventional tasks. The objective was to improve the applications to
eventually include intracardiac surgery and fetal surgery,
while also potentially improving the results of solid organ
interventions. Kesner and Howe [70] demonstrated a combination of US guidance and force control to guide and visualize a robotic catheter for its application in cardiac ablation.
The goal was to precisely track and manipulate the intracardiac tissue structures because of the fast tissue motion and the
potential for applying damaging forces. The MR scanner used
by Liu et al. served as both a means of active actuation and
imaging modality to provide sufficient tip and shaft visualization [24]. They aimed to model the deflection motions of the
catheter inside the left atrium. Boskma et al. [36] investigated
the use of US images as a viable alternative to fluoroscopy for
the real-time visualization and control of a robotic catheter. A
2-D-US modality was used to control the catheter in a stationary environment.
The efficacy of surgical procedures clearly depends on successfully detecting the parts of the cardiac tissue that need to
be investigated. As in the "Visualization and Control" section,
using imaging modality methods allow for both visualizing
and detecting surgical instruments. In the case of flexible surgical instruments, image modalities enable either catheter tip
or target-tissue tracking in both 2-D and 3-D images. Among
the medical imaging devices, 2-D-US imaging is the most
commonly used modality, and methods to improve it are continually being researched using additional control techniques
form part of this research area. Additional control enables clinicians to shift their focus from the manipulation task itself to
more sophisticated medical tasks, e.g., ensuring correct target
trajectories and ablation conditions.
Control Methods
Minimizing the invasiveness of surgical procedures requires
instruments to move accurately within the target vessel during
trajectory following. Furthermore, endovascular procedure
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