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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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… can be used to identify molecular structures.

The interaction of EM radiation with matter…

A bit about Spectroscopy

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The oxygen transporter in the blood of vertebrate animals.

The molecule of interest: Hemoglobin (Hb)

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Neural activation↓

Metabolic demand↓

Increased blood flow↓

Increase in oxy-hemoglobin&

Wash-out of deoxy-hemoglobin

The Hemodynamic Response

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To discriminate between HbO and HbR, NIRS systems use multiple wavelengths in measurements. (example: 760nm & 850nm)

EM absorption of oxy-Hb and deoxy-Hb

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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)

Optimal Light Frequencies of NIRS

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Skin and bone are mostly transparent to near-infrared light and do not significantly impede the signal.

Practical Example

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Relies on properties of absorption and scattering of photons.

Principles of Optical Imaging

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The sensitivity profile light conforms to an elliptical “banana” shape as it travels from source to detector. This is known as a photon banana.

Principles of Optical Imaging

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A single light source sends light out indiscriminately, with signal detected at potentially infinite positions.

Principles of Optical Imaging

Hemoglobin Hemoglobin

DetectorDetector

Detector Detector

Light Source

Ambient Light Ambient Light

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Source

Detector

Collects functional information from top 1cm of cortex, using “nearest neighbor” source-detector measurements.

One data channel is produced per wavelength used per source/detector pairing.

NIRS Topography

The source-to-detector pairing distance effects the depth of the photon banana penetration.

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S

D

S

D

S

D

S

D

S

2 – 3 cm‘Nearest-neighbor’ measurements are done with spacing of 2-3cm between sources and detectors.

S

D

= light source

= light detector

= data channel

NIRS Topography

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Source

Detector 1

Detector 2

Eliminates systemic interference of superficial origin (closest channel => mostly non-cortical signal)

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.

Multi-distance measurements allow for depth discrimination of signal

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Kohl M ,.. , Dirnagl U. Phys. Med. Bio. 2000

oxy-Hb (HbO)

deoxy-Hb (Hb)

x10-3

time in [s]

0 2 4 6 8 10 12 14 16

-1

0

1

2

D c

once

ntra

tion

[mM

]

Stimulation period

The NIRS Hemodynamic Response

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• 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

Physical and Biological Principles

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How does NIRS compare to more well-known functional brain-imaging modalities?

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BOLD fMRI

• Identifies deoxyhemoglobin as well as detailed tissue structure from entire brain

• Hundreds of thousands of peer-reviewed publications

• Large form-factor, noisy operation, very expensive systems and operation

• Confined environment

Functional Hemodynamic Modalities

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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

Functional Hemodynamic Modalities

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Comparison - fMRI vs. fNIRS

• Spatial mapping of activation area in response to right-hand activity

NIRS,D HbR fMRI

BOLD

Jan Mehnert, OHBM Annual Meeting, 2010

LL

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fNIRS fMRI

seconds

Jan Mehnert, OHBM Annual Meeting, 2010

Most Significant Activation Response

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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

Small Form-Factor Neuro-Modalities

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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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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

Summary of NIRS

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Thanks!

... Questions?