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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_605_Библиотеки_им_академика_М_И_Перельмана.pdf
445Tracking and Navigation Systems
11.1 OPTICAL TRACKING SYSTEMS
OTS are currently standard in clinical applications (Table 11.1).
The 3D position of an object is triangulated between two or more
cameras with overlapping projections. The object whose position shall be
tracked has to be equipped with markers. Markers can be passive (light
reflectors) or active (light emitters, commonly by LEDs). If not only position but also orientation is required, several ($ 3) markers have to be
arranged at a known geometry. The 2D information of each single camera are combined to calculate the spatial position and orientation of the
body carrying the markers.
Infrared (IR) tracking systems are the most common optical systems,
but videometric tracking systems are also available. Laser tracking systems
do not play an important role in clinical settings.
There are a variety of OTS image-guided applications, including surg ical procedures in ear, nose, and throat (ENT) medicine,
neurosurgery, motion correction during imaging acquisition, or
image-guided therapy procedures, perfor med manually or using
robotics.
Instead of infrared cameras, videometric tracking systems use one or
more calibrated cameras to detect and track specially marked objects with
a known pattern of features. These markers are identified by patterns on
video image sequences (
the pattern with known features are then used to define a vector going
from the sensor to the pattern. If more than at least three vectors are
Fig. 11.2A). The different 2D projections of
Table 11.1 Key facts of optical tracking systems
Typical
applications
Tracking of rigid
endoscopes
(e.g., for liver
surgery),
Tracking of
ultrasound
probes
Strengths and
weaknesses
High accuracy,
No
interference,
Line-of-sight
restrictions,
Bulky design
Recent
developments
Application-
specific
technologic
solutions
Research
potential and
future trends
Miniaturization,
Combination
with other
tracking
systems
446
Biomedical Engineering in Gastrointestinal Surgery
Figure 11.2 (A) Probes with markers of a videometric tracking system; (B) Passive IR
tracking markers. (B) Courtesy: NDI.
available, the position and orientation of the target is computed. One
common videometric system is the MicronTracker of ClaroNav, Inc,
Toronto, Canada.
Infrared tracking systems use only light of approximately 900 nm
in wavelength, so that IR systems are independent of ambient light. Both
active as well as passive IR tracking systems are used in practice. Active
systems track IR rays emitted by a series of light-emitting diodes (LEDs)
located on the surgical instruments. The LEDs are tracked by either two
planar or three linear CCD units forming the camera module.
Further more, LEDs are fired sequentially and are detected by each CCD
unit. The central unit uses a process of triangulation based on the known
geometric configuration and firing sequence of each LED and the
known, fixed distance between the CCD elements. A minimum of three
noncollinear LEDs are necessary for determining six degrees-of-freedom
position information
[3]. Since the LEDs must be powered, active systems
are traditionally wired systems.
Passive IR tracking systems apply retro-reflective markers, also
called reflecting spheres, that are attached to instruments and reflect
IR light. In contrast to active tracking systems, passive systems provide
wireless tracking (
Fig. 11.2B).Thepatternofthereflectivemarkers,
which has to be unique for each tracking probe, is identified on a
2D image. For this reason, these systems are always equipped with 2D
CCD cameras
[4].
With optical systems, submillimeter accuracy is possible, but a steady
line-of-sight between the markers and the stationary system is needed
(
Fig. 11.3).
The mode of function is depicted in
Fig. 11.4.
Tracking and Navigation Systems
Figure 11.3 (A) Tracking system for active and passive tools; (B) reflecting spheres
for passive tracking. Courtesy: NDI.
447
Figure 11.4 Comparison of active and passive IR tracking systems. From NDI Optical
Measurement Technology, ,
2013 [accessed 01.09.16]
http://www.ndigital.com/medical/technology-optical.php. ;
[3].
Strengths and Weaknesses
Optical systems, especially LED-based systems, benefit from their very
high accuracy
[57]. The technical accuracy of OTS is in the range of
0.1 to 1.4 mm. NDI, Waterloo, Canada, a manufacturer of optical and
electromagnetic tracking systems (EMTS), reports a theoretical accuracy
of 0.25 mm
[8]. In contrast to EMTS, OTS avoid the field distortion pro-
blems associated with EM trackers, have nearly no interference with other
IR devices in the operating room, and do not necessarily require wired
sensors. With optical systems, the field of view is large, but the line-ofsight restrictions are a significant disadvantage, often making them
impractical for laparoscopic procedures. Markers can certainly be placed
at the end of instruments outside the body, but this will degrade the
tracking accuracy for long tools inserted into the body
[9]. Additionally,
the registration procedure of the markers is time-consuming, and can lead
to target reg istration errors of up to 3 mm positional differences.
448
Biomedical Engineering in Gastrointestinal Surgery
Recent Developments and Current Research
The main disadvantage of OTS, the line-of-sight restrictions, cannot
be overcome. Further developments are more application-oriented than
oriented toward general improvements of the technology. In 2011, a
guidance system that simplifies lung interventional procedures with
needles was developed. It uses cheap single-use materials and integrates
CT images into both the preplanning of the surgery and the verification
of the needle target access.
11.2 ELECTROMAGNETIC TRACKING SYSTEMS
EMTS are tracking technologies in which magnetic fields of known
geometry are used to determine the position and orientation of sensors
by measuring magnetic flux. The magnetic reference field is either
produced by permanent magnets or electromagnetics. Permanent magnets
are not relevant for biomedical purposes; artificially induced magnetic
fields prevail. The geometry of the emitting coil assembly and the type of
current determine the shape and geometry properties of the field. For the
measurement inside the field, specific magnetic sensors are required.
Therefore, typically an EM tracking system consists of three components: the sensor(s), a field generator (FG), and a central control unit
(
Fig. 11.5). The FG has to create the position varying magnetic field, or,
more precisely, three different magnetic fields of a well-defined geometry,
which is used to establish the coordinate space. The most important feature of the FG is the tracking volume, i.e., the area around the generator
where sensors can be tracked reliably. The sensors attached to the tracked
Figure 11.5 (A) Schematic illustration of electromagnetic tracking: 1. Field generator;
2. Sensors fixed to landmarks of the body; 3. Sensor integrated into the surgical
instrument; (B) EM tracking system with field generator, system control unit, and sensors. Courtesy: (A) MITI, (B) NDI.
Tracking and Navigation Systems
449
object induce current in the small containing coils. The system control
unit controls the FG and interprets the current induced by the sensors to
determine the position and the orientation. Today, a broad range of
EM sensors adapted to various applications and requirements are on the
market (
Fig. 11.6).
EM tracking systems can be divided in three categories:
• AC tracking systems
• DC tracking systems
• Passive systems.
In AC tracking systems, driven by alternating current (AC), search
coils use inductors to determine the magnetic flux as a function of the
time. Thus, an alternating magnetic field is needed for these sensors to
measure a voltage.
Figure 11.6 (A) Different DC trackers with sizes from 5 to 1.3 mm; (B) Polhemus AC
tracking sensor; (C) miniaturized 6 DoF AC tracking sensor; (D) wireless EM tracking
sensor for target position monitoring during radiation therapy. From Franz AM,
Haidegger T, Birkfellner W, Cleary K, Peter TM, Maier-Hein L. Electromagnetic tracking in
medicine—a review of technology, validation, and applications. IEEE Trans Med Imaging
2014;33(8):170225
[10].
450
Biomedical Engineering in Gastrointestinal Surgery
DC tracking systems use quasistatic direct current (DC) and use fluxgate
sensors to determine the position and orientation. Fluxgate sensors consist of
two inversely arranged inductors to measure the second harmonic Fourier
component of the magnetic field. A fluxgate can vectorially measure magnetic fields which are static or alternating with a low frequency.
AC tracking systems can determine distances by use of Hall Effect sensors, operating as an analog transducer, directly returning a voltage, but
are of less relevance for exact positioning.
Today, very reliable and very small sensors are available. Even if first
wireless devices were introduced in radiation therapy, still in most applications the sensors have to be connected to the control unit with cables.
However, a cable is always required to connect them with the control unit.
EM-based surgical navigation and tracking systems are the most common choice for laparoscopic surgery, flexible endoscopy, and other minimally invasive procedures because a clear line-of-sight is not required
between the base station and the attached sensors.
Fig. 11.7 gives an over-
view of the specially designed antennas (field generator) for clinical use.
The tracked coils are placed near the end of the tip of the instrument.
It is possible to track miniaturized sensors designed for integration into
surgical tools and instruments, such as needles, catheters, probes, and
scopes (
Fig. 11.8).
EMTS reach a technical accuracy in the range of 0.17 to 1.4 mm
[11]
under laboratory conditions, but it is significantly lower in clinical use.
Figure 11.7 A selection of FGs currently in use. From Franz AM, Haidegger T,
Birkfellner W, Cleary K, Peter TM, Maier-Hein L. Electromagnetic tracking in medicine—a
review of technology, validation, and applications. IEEE Trans Med Imaging 2014;33
(8):1702-25
[10].
451Tracking and Navigation Systems
Figure 11.8 (A) An extremely miniaturized EM sensor. (B) Shape sensor, a series of
seven EM tracking sensors integrated into a catheter. All from MITI.
Table 11.2 Key facts of electromagnetic tracking systems
Typical
applications
Strengths and
weaknesses
Recent
developments
Research potential
and future trends
Laparoscopic
surgeries
Flexible
endoscopy
Catheters Less accuracy Electromagnetic
No line-of-sight
restrictions
Integrable into
every tool
Field distortion
electromagnetic
objects
Miniaturization Improved accuracy
Integration Extension of the
tracking volume
immunity
Strengths and Weaknesses
A major advantage of EMTS is the fact that they do not have line-ofsight constraints; hence, there is no danger of interrupting navigation.
The sensors can be very small and integrated into nearly every tool or
device so that the tracking point of the surgical instrument can be closer
to the anatomical structures than with OTS. That leads to the ability to
track flexible endoscopes and catheters, which is the main advantage of
EMTS. Since EMTS navigation is based on tracking the coils of the
instrument, the relationship between the coils must not be changed
during the procedure
[11] (Table 11.2). The most serious drawback is
field distorsion due to external EM sources.
Recent Developments and Current Research
The use of EM tracking is already clinically established in colonoscopy.
The ScopeGuide (Olympus, Tokyo , Japan) shows an accurate 3D
452
Biomedical Engineering in Gastrointestinal Surgery
reconstruction of the endoscope position and configuration within the colon.
The 3D inforamtion generated by the electromagnetic tracking is then displayed in split-screen mode of both the anterior-posterior and lateral vie w.
The focus of current developments in EMTS technology is the
improvement of EMTS accuracy. Currently, they cannot compete with
OTS in terms of tracking accuracy. Additionally, the range of the
magnetic field needs to be extended to minimize the spatial limitations of
surgeries with EMTS.
One new promising technique for biomedical tracking are superconducting
quantum interference devices; however, further research is still necessary
[11].
11.3 FIBER BRAGG GRATING SENSORS
Fiber sensing is a new technology based on the principle that the wavelengths of reflected light differ under distinct circumstances, such as a
change of temperature or strain, that are achieved by an interference
pattern in the optical fiber
FBG is using the effect that a temperature difference, a strain, an
acceleration or a tilt has an impact on a change of the index of diffraction,
which is caused by concave gratings inside the optical fiber. In diffraction
grating, the emitted light is refracted and reflected. The recording of these
diffractions by a special camera (inter rogator) allows the measurement of
differing environmental influences, such as strain
[12].
[13] (Fig. 11.9).
Figure 11.9 Basics of fiber-optic 3D tracking: (A) Normal grating pattern in the
straight position; (B) depending on the direction, the bars are either dilated or compressed; (C) positioning of three FBGs into one catheter in 120° and respective calculations to compute bending in x- and y-direction. Courtesy: T. Schossig, MIOPAS
GmbH, Goslar, Germany.
Table 11.3 Key facts of fiber bragg grating
Typical
applications
Strengths and
weaknesses
Recent
developments
453Tracking and Navigation Systems
Research
potential and
future trends
Pressure,
temperature,
and
configuration
measurement
Lightweight and
small size
Multiplexing
capability
Simultaneously
sensitive to
temperature,
strain, and
pressure
Cost reduction Specially tailored
optical fibers
Optical fibers
with FBG to
become
standard
for medical
applications
If the fiber cladding and core are applied at different points with different influences, such as a temperature difference, a strain, an acceleration, or a tilt, the refractive index is changed and another wavelength
change is seen in the interference reflection. The measurement is thus
based on a change in wavelength. The absolute wavelength of the individual measuring points is used for a defined state to be calibrated.
A cascading set of different sensors is not a problem
[14] (Table 11.3).
One end of the optical fiber is pro vided with an optical connector , such
as those used in the telecommunication and information technology industry. With this connector, the fiber is connected to the polychr omator, which
contains an LED with a special spectrum that is emitted into the fiber.
The optical fiber includes a light-conducting core, which is set with
impurity atoms (doping).
The core is enveloped by the cladding. It reflects stray light back into
the core minimizing the loss of light even over long distances.
Multiple measurement points, precise and different interface patterns,
may be introduced at any position in the fiber during manufacture. This
is done by different high-energy UV exposures of the doped fiber to an
interference pattern, depending on the UV light exposure. Due to this
exposure, there is a periodic arrangement of refractive index differences
in the fiber core. It creates about 10,000 semipermeable mirror surf aces
with uniform distances at a measurement point. In every periodic refraction change a narrow wavelength is reflected. These reflected light signals
superimpose to one large reflection at a particular wavelength when the
grating period is approximately half the input light’s wavelength. This is
454 Biomedical Engineering in Gastrointestinal Surgery
referred to as the Bragg condition (Equation 11.1), and the wavelength at
which this reflection occur s is called the Bragg wavelength. Different
wavelengths will pass the FBGs without attenuation
[15]. As a protection
against external influences and for mechanical reinforcement, the
so-called buffer coating covers the core and the cladding
[12].
Equation 11.1 Bragg condition, where λBis the center wavelength, n
index, and Λ the period of the index modulation
and strain dependence of the parameters n and Λ, the wavelength of the reflected
component will also change as a function of temperature and strain.
[15]. Because of the temperature
the effective
eff
Strengths and Weaknesses
The main advantages of FBG are the lightweight and small size of the
optical fiber, as well as the excellent performance in hazardous environments and its immunity to electromagnetic and radio frequency interferences. In addition, the optical fibers can be very long without losing
information quality. It is possible to use single and multipoint sensors,
since optical fibers have a high multiplexing capability.
However, there are also some limitations that come along with grating. The most fundamental disadvantage is the fact that they are simultaneously sensitive to strain, temperature, and pressure. Hence, adequate
temperature compensation is always essential in the design and commercialization of reliable and repeatable physical sensors
[16].
For technical applications, FBGs are already available as:
• Temperature sensors
• Strain sensors
• Displacement sensors
• Tiltmeters
• Pressure sensors.
Medical Applications, Navigation
Future fields of application for FBG sensors depend g reatly on a cost
reduction and the development of specific application fields with purpose
built fibers.
One of the first commercially available applications is the pressure
sensor in the TactiCath Quartz ablation catheter (St Jude Medical, St.
Paul, Minnesota, United States).
Navigation could become a key application since multiple FBGs integrated into catheters, endoscopes, or introduced into anatomical structures
(bile duct, blood vessels) could help to define precisely the shape and position
oftherespectiveitem.Thetinydiameterisparticularlyfavorable.Asan