Part 01 - fNIRS: a cost-effective and robust
technique for measuring neuroactivation
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NIRx Offices (Year Established)
New York, USA (1999)
Berlin, Germany (2006)
Los Angeles, USA (2013)
São Paulo, Brazil (2014)
Istanbul, Turkey (Pending, October 2014)
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What is NIRS?
NIRS = Near- InfraRed Spectroscopy
A form of Optical Imaging.
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A bit about Spectroscopy
The interaction of EM
radiation with matter…
… can be used to identify
molecular structures.
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The molecule of interest: Hemoglobin (Hb)
The oxygen transporter in the blood of vertebrate animals.
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The Hemodynamic Response
Neural activation
↓
Metabolic demand
↓
Increased blood flow
↓
Increase in oxy-hemoglobin
&
Wash-out of
deoxy-hemoglobin
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EM absorption of oxy-Hb and deoxy-Hb
To discriminate between HbO and HbR, NIRS systems use multiple
wavelengths in measurements. (example: 760nm & 850nm)
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Optimal Light Frequencies of NIRS
Best frequencies for physiologic chromophores are partially within the red & NIR spectrum
Lower absorption of light = Better transmission through tissue and bone
Well suited for modern LED's or lasers (650…900nm)
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Practical Example
Skin and bone are mostly transparent to near-infrared
light and do not significantly impede the signal.
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Principles of Optical Imaging
Relies on properties of absorption and scattering of photons.
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Principles of Optical Imaging
The sensitivity profile light conforms to an elliptical “banana” shape as
it travels from source to detector. This is known as a photon banana.
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Principles of Optical Imaging
Light Source
Detector
Hemoglobin
Detector
Hemoglobin
Ambient Light
Ambient Light
Detector
Detector
A single light source sends light out indiscriminately, with signal detected at
potentially infinite positions.
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NIRS Topography
Collects functional information from
top 1cm of cortex, using “nearest
neighbor” source-detector
measurements.
Source
Detector
The source-to-detector pairing
distance effects the depth of the
photon banana penetration.
One data channel is produced per wavelength
used per source/detector pairing.
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NIRS Topography
2 – 3 cm
‘Nearest-neighbor’
measurements are done with
spacing of 2-3cm between
sources and detectors.
S
D
S
D
S
D
S
D
S
D
S
= light source
= light detector
= data channel
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NIRS Tomography
Like Topography, collects functional
information from top 1cm of cortex, but
uses multi-distance measurements with a
single source sending light to multiple
detectors in 1-dimention.
Source
Detector 1
Detector 2
Multi-distance measurements allow
for depth discrimination of signal
Eliminates systemic interference of superficial origin
(closest channel => mostly non-cortical signal)
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The NIRS Hemodynamic Response
Stimulation period
-3
x10
oxy-Hb (HbO)
D concentration [mM]
2
deoxy-Hb (Hb)
1
0
-1
0
2
4
6
8
10
12
14
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time in [s]
Kohl M ,.. , Dirnagl U. Phys. Med. Bio. 2000
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Physical and Biological Principles
• Spatial resolution of neuro-activation within
the mm-to-cm range (depending on probe density)
• Excellent sensitivity to Hb and HbO
• Excellent sampling rate (several Hz to 60Hz+)
• Measures top 1cm of cortex
• Greatest light transmission is on forehead
(prefrontal) due to lack of hair, supporting vascular
structure, and sinuses
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How does NIRS compare to
more well-known functional
brain-imaging modalities?
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Functional Hemodynamic Modalities
BOLD fMRI
•
Identifies deoxyhemoglobin as well
as detailed tissue structure from
entire brain
•
Hundreds of thousands of peerreviewed publications
•
Large form-factor, noisy operation,
very expensive systems and
operation
•
Confined environment
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Functional Hemodynamic Modalities
fNIRS
•
Identifies deoxyhemoglobin as well as oxyhemoglobin
(total hemoglobin derived) from top 1cm of cortex
(deeper at forehead)
•
Thousands of peer-reviewed publications, but rapidly
increasing in number
•
Relatively small form-factor (portable options
available)
•
Silent system operation (ideal for sensitive subjects)
•
System cost is a fraction of the cost of fMRI,
operational cost is very low
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Comparison - fMRI vs. fNIRS
•
Spatial mapping of activation area in response to right-hand activity
L
L
NIRS,
D HbR
fMRI
BOLD
Jan Mehnert, OHBM Annual Meeting, 2010
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Most Significant Activation Response
seconds
fNIRS
fMRI
Jan Mehnert, OHBM Annual Meeting, 2010
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Small Form-Factor Neuro-Modalities
EEG
•
Measures cortical electrophysiology from scalp
•
Tens of thousands of peer-reviewed publications
•
Excellent temporal resolution (sampling rates 2kHz+)
–
Measures very quick paradigms related to action potentials (P300,
N200, etc.) as well as frequency bands (alpha, beta, etc.)
•
Relatively poor spatial resolution (several cm)
•
Portable/wearable systems available, but highly sensitive to
motion artifacts
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Small Form-Factor Neuro-Modalities
fNIRS
•
Measures hemodynamic cortical activity
- Similar headgear and form-factor to EEG
•
Lower temporal resolution than EEG (fNIRS typically ~5-10Hz)
–
Though lower, the sampling rate for fNIRS is more than sufficient for
measuring hemodynamic responses of interest (.01 - .2Hz)
•
Greater spatial resolution than EEG (fNIRS is ~6mm)
•
Better noise performance than EEG with even very extreme
movements and muscle usage
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Summary of NIRS
NIRS technology offers:
• Compact, low cost sensing hardware
• Noninvasive, continuous measures
• Highly configurable and scalable platforms
• Information-rich sensing data
– BOLD-like response without the magnet
– Source localization >> EEG
– Minimal sensitivity to movement artifacts
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Thanks!
... Questions?
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