Optical Topography: An overview Nima Kasraie Spring 2007.
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Transcript of Optical Topography: An overview Nima Kasraie Spring 2007.
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Optical Topography: An overview
Nima KasraieSpring 2007
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• Oxymetric Near Infrared Imaging
• Images higher order brain functions
• Developed by Hitachi Advanced Research Laboratory
• Already in clinical use in Japan.
What When Where
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Red Light Transmission Red Light Transmission
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Features
• Small
• Easy to use
• Patient mobility
• Extended recordings
• Not noisy
• Aclaustrophobic
• “Real time” imaging
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Principle of OT system
• 1.5mW NIR laser diode illuminates head from optical fibers attached to headset.
• dmax ≈ 3cm
• Reflection (0.1%)
• Detection
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73cm ≈ dcortex
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SNR
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• 780nm
• 830nm
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• Simultaneous position encoding by frequency modulation produces a better SNR than time-sharing and time-resolved methods.
• Optimal wavelength pair: 830/692 nm.
• Applicable to other molecular species
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Resolution
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Physiological Noise
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• The presence of low frequency modulation of both vascular (Hb, HbO) and metabolic responses to visual stimuli with unknown origin have been observed.
• The cause of these low frequency oscillations are not exactly known
• but may be attributed to extra-cerebral activities such as respiration.
• One method of removing the low frequency artifact is to convolve the response signal with a model of stimulus signal.
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Similarities with fMRI
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• Both measure brain’s hemodynamic response
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Differences with fMRI
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In concept fMRI measures differences between the Fe content in blood based on the metal’s response to the magnetic field. OT relies on different light absorption characteristics of the two forms.
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In operation Lower spatial resolution than fMRI
2mm vs. 20mm
Higher temporal resolution than fMRI
1 Sec vs. 1 mSec
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• fMRI and OT can be complementary
• Can also be combined with structural imaging
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Current applications
• Developmental plasticity– Neonatology (only functional imaging method)
• Neurosurgery– Identifying the focus of epileptic seizures i.e.
check for dominant hemisphere (e.g. Broca’s area) before removal of eliptogenic tissue
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Concluding remark
OT does not measure exact blood volume concentrations; it measures concentration changes of molecular species, including oxyhemoglobin and deoxyhemoglobin.
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References and further reading
• Spinney, Laura. "Optical topography and the color of blood: OT gives neuroscientists a new and faster view of the brain, and an alternative to fMRI." The Scientist 19.2 (Jan 31, 2005): 25(3).
• Hideaki Koizumi, “Optical topography: practical problems and new applications”. Applied Optics, Vol. 42, No. 16, June 2003
• Il-Young Son, “Near Infrared Imaging and Spectroscopy for Brain Activity Monitoring”.
• http://www.medphys.ucl.ac.uk/research/borl/sheddinglight/brain.htm
• http://www.hitachi-medical.co.jp/info/opt-e/index.html