Optical Tracking

How this pertains to our project

• Distributed Instrument Control with TINI using CORBA

DistributedComputer

EthernetTINI

RS-232 PolarisOpticalTracker

Drew and I will be developing an IDL that can interface betweenThe TINI device and the network and between the Polaris and theTINI.

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

• Determines position of a sensor endpoint based upon measurements of joint angles, potentiometers

• Example systems: Faro Arm, NeuroNavigator

Mechanical

Sensor Modalities in Tracking

• 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

Optical

Sensor Modalities in Tracking

• 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

Magnetic

Sensor Modalities in Tracking

• Sensors receive signals which are emitted by ultrasonic emitters and determine location via time-of-flight

• Example system: Sonic Wand

Acoustic

Optical Tracking: How it Works

• 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

From the patent papers of Polaris

Optical Tracking: How it Works

• 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)

From the patent papers of Polaris

Optical Tracking: How it Works

• Mathematical processing using the technique of triangulation determines the 3D coordinates and angular orientation i.e. 6 DOF

From the patent papers of Polaris

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

Mechanical Optical Magnetic Acoustic

Accuracy 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, NeuroNavigator

OptoTrak 3020, Polaris

Polhemus, Flock of Birds

Sonic Wand

Contact/ Non-Contact

Direct Contact

Contact w/targets

Contact w/targets

Contact w/targets

Passive/Active Passive Either Active Active

Comparison of Position/ Orientation Sensing Modalites

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

• 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

Most Importantly

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. 1:211-240, 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