Optical Guidance
Sanford L. Meeks
July 22, 2010
Optical Tracking
• Optical tracking is a means of determining in real-time the
position of a patient relative to the treatment unit. Markerbased systems track the positions of either active or
passive infrared markers attached to the patient as a
surrogate for actual patient position. Markerless systems
track the reflection of light from the surface of the patient.
The position of the point of reflection is determined using a
camera system.
Active markers
Passive
markers
Camera
Calibration
Camera has been correlated with linac isocenter to correlate
“virtual” space with “real” space.
Clinical Applications of Optical Tracking
1. Markers are indexed to patient
Clinical Applications of Optical Tracking
2. Markers are identified in CT scan relative to treatment
isocenter.
Clinical Applications of Optical Tracking
3. Markers are tracked using camera system.
CCD Camera
Control Computer
Fiducial Array
Optical guidance to track patient position using
passive markers and a bite-plate system.
Clinical Applications of Optical Tracking
3. Markers are tracked using camera system.
4. Misalignments are corrected.
Optical Tracking Extracranial Targets
• Tracking surface alone may be inadequate
• Target motion relative to surface anatomy
• Imaging coupled with optical tracking
• Ultrasound
• X-Ray
Varian SonArray
High-resolution infrared camera
Detects passive and active
fiducials
Computer with
Spatial Digitizer linked to Camera
Image Processing and Localizing
Software
Optically tracked ultrasound
system
Correlation of US with Planning CT
BrainLab ExacTrac
infrared
camera
aSi
detectors
Floor mounted
stereoscopic kV X-ray
tubes
X-Ray Registration
process:
Either bony anatomy
(automated) or implanted
fiducial markers
Patient tracked optically
using reflective body
markers
Marker Alignment
Observed
positions
Expected
positions
Manual
alignment
Table Offsets
Expected
positions
Observed
positions
Manual
alignment
Patient aligned and
tracked using
optical guidance
Gating with Marker-Based System
(BrainLab Exactrac)
Because optical tracking gives real-time feedback, it is also useful for
generating respiratory gating signal. Gated x-ray verification is
performed using implanted fiducials. Can verify at any time, including
during treatment
Once the breathing signal is obtained the user selects the level
of the breathing pattern for which x-rays are to be obtained
X-Ray Level
Markerless Tracking
•Use of projection of patterned or structured light onto the scene.
•Use image feature correspondence in stereo images to track surface.
Images from: Siebert et al, “Human body 3D imaging by
speckle texture projection photogrammetry,” Sensor Review
20:3, p 218, 2000.
Waldron ACMP 2009
Markerless Tracking
•RT Vision sensor system arranged
in pods, each is capable of stereo
“vision”. A pod contains 1 stereo
pair of cameras and a speckle
pattern projector. A texture camera,
white flash, and speckle flash
projector are also present.
Stereo camera pair
Speckle flash
Speckle projector
Speckle Pattern
Waldron ACMP 2009
Texture
camera
Commercial Systems
Company
Varian
BrainLab
Accuray
CMS
Nomos
Resonant
Medical
Medical
Intelligence
VisionRT
External tracking,
+ultrasound, gating
External tracking + x-ray,
gating
External tracking + x-ray
Ultrasound
Ultrasound
Ultrasound
External tracking
Surface tracking
TG147: QA for non-radiographic
localization and positioning systems
• Disclaimer: This task group is still under review and has
not been published (a work in progress)
Installation
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Machine isocenter and in-room laser
accuracy (per customer ATP / TG40)
Linac interference with localization system
Localization system interference Linac (and
collision space)
Establish Localization Volume (Volume of
View)
Establish Tracking volume (VOV for Tracking)
System Performance
Installation
• Machine isocenter and in-room laser accuracy
(per customer ATP / TG40)
• Recommend the use of a Winston/Lutz film
process for establishing machine isocenter /
laser QA documentation prior to installation of
add-on equipment
System Performance
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Calibration Procedures
Overall Localization Accuracy (End to End
Test)
System Drift
Reproducibility
Tracking Accuracy
Response Time (latency)
Target Localization Accuracy and Overall
Positional Accuracy (End to End Test)
Localization on Treatment
Planning System
Setup of “Patient” under Optical
Guidance
Resulting Film: Measure of Total
Accuracy of System
Results:
Z: 0.49 mm to T
X: 0.35 mm to A
Y: 0.50 mm High
Overall error = 0.789mm