Part 01 - fNIRS : a cost-effective and robust technique for measuring neuroactivation

1. Part 01 - fNIRS: a cost-effective and robust technique for measuring neuroactivation

2. NIRx Offices (Year Established) • New York, USA (1999) • Berlin, Germany (2006) • Los Angeles, USA (2013) • São Paulo, Brazil (2014) • Istanbul, Turkey (Pending, October 2014)

3. What is NIRS? NIRS = Near- InfraRedSpectroscopy A form of Optical Imaging.

4. A bit about Spectroscopy The interaction of EM radiation with matter… … can be used to identify molecular structures.

5. The molecule of interest: Hemoglobin (Hb) The oxygen transporter in the blood of vertebrate animals.

6. The Hemodynamic Response Neural activation ↓ Metabolic demand ↓ Increased blood flow ↓ Increase in oxy-hemoglobin& Wash-out of deoxy-hemoglobin

7. EM absorption of oxy-Hb and deoxy-Hb To discriminate between HbO and HbR, NIRS systems use multiple wavelengths in measurements. (example: 760nm & 850nm)

8. 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)

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

10. Principles of Optical Imaging Relies on properties of absorption and scattering of photons.

11. 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.

12. Principles of Optical Imaging Light Source Detector Detector Hemoglobin Hemoglobin Ambient Light Ambient Light Detector Detector A single light source sends light out indiscriminately, with signal detected at potentially infinite positions.

13. NIRS Topography Collects functional information from top 1cm of cortex, using “nearest neighbor” source-detector measurements. Source The source-to-detector pairing distance effects the depth of the photon banana penetration. Detector One data channel is produced per wavelength used per source/detector pairing.

14. NIRS Topography ‘Nearest-neighbor’ measurements are done with spacing of 2-3cm between sources and detectors. 2 – 3 cm S D S S = light source D S D D = light detector S D S = data channel

15. 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)

16. The NIRS Hemodynamic Response -3 x10 oxy-Hb (HbO) 2 deoxy-Hb (Hb) 1 D concentration [mM] 0 -1 0 2 4 6 8 10 12 14 16 time in [s] Stimulation period Kohl M ,.. , Dirnagl U. Phys. Med. Bio. 2000

17. 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

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

19. Functional Hemodynamic Modalities BOLD fMRI • Identifies deoxyhemoglobinas 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

20. 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

21. 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

22. Most Significant Activation Response seconds fNIRS fMRI Jan Mehnert, OHBM Annual Meeting, 2010

23. 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

24. 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

25. 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

26. Thanks! ... Questions?