Download Magnetic Resonance Imaging of Human High

Optical Topography:
An overview
Nima Kasraie
Spring 2007
What When Where
• Oxymetric Near Infrared
Imaging
• Images higher order
brain functions
• Developed by Hitachi
Advanced Research
Laboratory
• Already in clinical use in
Japan.
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Red Light Transmission
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Features
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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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3cm ≈ 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.
In operation
 Lower spatial resolution than fMRI
 2mm vs. 20mm
 Higher temporal resolution than
fMRI
 1 Sec vs. 1 mSec
• 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/br
ain.htm
• http://www.hitachi-medical.co.jp/info/opt-e/index.html
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