Transcript [] Optical Tracking

Optical Tracking
How this pertains to our project
• Distributed Instrument Control with TINI
using CORBA
Distributed
Computer
Ethernet
RS-232
TINI
Polaris
Optical
Tracker
Drew and I will be developing an IDL that can interface between
The TINI device and the network and between the Polaris and the
TINI.
What is Tracking?
• The process of pinpointing the location of
instruments, anatomical structures, and/or
landmarks in three dimensional space and in
relationship to each other
• Synonymous with Localization
• Localization is used for registration of the surgical
space to image space, as represented by
preoperative MRI and CT images
Why do we need tracking?
• Intraoperative Guidance
• Without tracking, the surgeon is required to
rely on succesive steps of needle placement,
image verification, needle advancement and
re-imaging and so on, until the target is
reached
• This is slow, which raises the likelihood that
a patient can move and reduce accuracy
General Concept of Tracking
• Almost all tracking, with the exception of
mechanical, is done using some form of
electromagnetic radiation
General Concept of Tracking
• Markers are placed on the body of which
the position is to be determined
• These markers are adapted to emit energy in
response to an activation signal (active) or
reflect energy from an activable source
(passive)
General Concept of Tracking
• A sensor detects the energy emitted or
reflected by the markers
• The energy detection is translated to
positional information using various
techniques: triangulation, time-of-flight
calculation
General Concept of Tracking
• Markers can be placed on a probe with
known fixed length to allow the
measurement of discrete points on the
surfaces of exposed, rigid anatomical
structures
Sensor Modalities in Tracking
Mechanical
• Determines position of a sensor endpoint based
upon measurements of joint angles,
potentiometers
• Example systems: Faro Arm, NeuroNavigator
Sensor Modalities in Tracking
Optical
• Tracks the positions of one or more actively
illuminated or passively reflective markers and
uses geometric triangulation to determine the
locations of these markers.
• Example systems: NDI Optotrak, Polaris
Sensor Modalities in Tracking
Magnetic
• Measures electrical currents induced in
receiver coils when the receiver is moved
within a magnetic field generated by an emitter
• Example systems: Polhemous, Flock of Birds
Sensor Modalities in Tracking
Acoustic
• Sensors receive signals which are emitted by
ultrasonic emitters and determine location via
time-of-flight
• Example system: Sonic Wand
Optical Tracking: How it Works
From the patent papers of Polaris
• Multiple charge couple device (CCD)
sensors are used to detect the energy
emitted (active) or reflected (passive) by the
marker.
• A single point marker is energized per
sensor cycle to emit infrared energy
Optical Tracking: How it Works
From the patent papers of Polaris
• During each sensor cycle, the emitted
energy focused on to the sensor is collected
• It is then shifted to the sensor processing
circuitry
• To determine the 3D position of the marker,
the marker must be detected on at least
three sensor axes (to cover a minimum of
three orthogonal planes)
Optical Tracking: How it Works
From the patent papers of Polaris
• Mathematical processing using the
technique of triangulation determines the
3D coordinates and angular orientation i.e.
6 DOF
What is Triangulation?
• Given three rays that intersect at one point,
if you know the angles of the rays from the
three sources and the 3D coordinates of the
three sources, the distance from the point of
intersection of the three rays and the sources
can be determined.
• Similar process used in GPS receivers
Comparing Effectiveness: Some
Sensor Characteristics
• Accuracy – measure of the difference between
estimated and correct measurement values, where
all sensor measurements are estimates
• Resolution – smallest change which can be
detected by the sensor
• Bandwidth – measure of amount of information
which can be acquired and processed by the sensor
per unit of time (Hz)
Comparing Effectiveness: Some
Sensor Characteristics
• Active/passive – mentioned previously
• Contact/Non-Contact – whether or not the
sensor comes into physical contact with the
object being measured
• Cost
Comparison of Position/ Orientation Sensing Modalites
Accuracy
Mechanical
Optical
Magnetic
Acoustic
0.1 – 2.5 mm
0.1 – 2.5 mm
~5 mm
~1 mm
Resolution
Best ~ 0.01
mm
~0.1 mm
Bandwidth
>3000Hz
100-2500 Hz
20-100Hz
500-1000Hz
Interference
Sources
Physical
occlusion
Heat,
occlusion
Ferrous
objects,
magnetic
fields
Temp,
humidity,
occlusion
Examples
Faro Arm,
OptoTrak
NeuroNavigat 3020, Polaris
or
Polhemus,
Sonic Wand
Flock of Birds
Contact/ NonContact
Direct
Contact
Contact
w/targets
Contact
w/targets
Contact
w/targets
Passive/Active
Passive
Either
Active
Active
Optical Tracking: Advantages
• Minimally invasive compared to fiducial
approach, no need for extra surgery, extra
cost
• Small, light weight, unobtrusive in the OR
• Very high resolution: .01 mm
• High bandwidth, though not as high as
mechanical
• Low Cost (according to manufacturer)
Optical Tracking: Advantages
Most Importantly
• High Degree of Accuracy
– 0.1 to 2.5 mm accuracy
– In a study reported in the paper “Comparison of
Relative Accuracy Between a Mechanical and
an Optical Position Tracker for Image-Guided
Neurosurgery,” Rohling et al, found that optical
was more accurate than mechanical in the case
of NDI’s Optotrak vs. FARO
Optical Tracking: Disadvantages
• “Line of sight” requirement
– According to Cleary et al, in “Technology
Improvements for Image Guided and Minimally
Invasive Spine Procedures,” ‘the major drawback of
optical systems is the requirement that a line-of-sight
between the trackers and the camera remain at all times.
This line of sight requirement can be cumbersome and
difficult to maintain in the delicate surgical
environment…This may reduce the acceptance of
image-guided spine surgery among physicians.’
Conclusion
• Optical Tracking is a highly effective and
accurate technique for localization with the
only disadvantage being the maintenance of
‘line-of-sight’ with the cameras
References
• Howe, Robert and Matsuoka, Yoky, “Robotics for
Surgery,” Draft, Ann. Rev Biomed Eng. 1:211240, 1999.
• Leis; Eldon, Stephen, US Patent 6,061,644
“System for determining the spatial postion and
orientation of a body,” Dec 5, 1997.
• Simon, D.A. “Intra-Operative Position Sensing
and Tracking Devices”
• Cleary, et al. “Technology Improvements for
Image-guided and Minimally Invasive Spine
Procedures,” Draft, Transactions on IT in
Biomedicine, Jan 2001.
References
• Rohling, et al. “Comparison of Relative
Accuracy Between a Mechanical and an
Optical Position Tracker for Image-Guided
Neurosurgery”
• Northern Digital Product Information,
www.ndigital.com