The Brain & Neuroimaging: fNIRS

Question 1

Describe fNIRS? and what does it stand for?

Answer 1

Functional Near-IR Spectroscopy
* Method of measuring brain activity
* Only looks at top cortical brain matter, not deeper structures like hippocampus, thalamus or amygdala
* Non-invasive
* Resolution ~1 cm spatially
* Not affected by electrical or magnetic fields
* Relatively cheap and portable

Question 2

Describe the theories & principle of operation in fNIRS?

Answer 2

• Near-IR light penetrates up to 2-3 cm below surface of scalp
• Detectors pick up changes in cortical activity associated w/ blood flow in an optical window of 680-900 nm (near-IR light)
• Theory of light propagation in highly scattering media (tissues) & derivation of an approximate solution for absorption & scattering based on multi-distance measurements in frequency domain
• NIRS is typically used to estimate changes in oxygenated & deoxygenated haemoglobin conc. ([HbO] & [Hb]) requiring minimum of 2 λs

Question 3

Describe the modified Beer-Lambert Law in fNIRS?

Answer 3

• Concs of [HbR] & [HbO] are determined from Absorbance (A) or Absorption Coefficient (µa) at 2 (or more) λs
• The Beer Lambert Law allows us to do this
• But it is modified to consider light scatter through a non-uniform medium like brain
• In NIRS of brain tissue measure [HbR] & [HbO] using this
  Looking at path of light through brain tissue – detector detects any photons coming back to reach it

Question 4

How does the fNIRS record from the brain & other tissues?

Answer 4

• [HbR] & [HbO] have different absorption spectra
• The attenuation changes at the 2 λs are measured to tell difference between [HbR] & [HbO]
• Then know which part of brain/tissue is active

Question 5

Describe NIRS absorption in the brain?

Answer 5

• Photon propagation in adult head
  o Light penetrates skull, skin & CSF to reach grey matter
• Absorbers of light are water, lipid, melanin, Haemoglobin (Hb), enzymes
  o Most of these are constant absorbers
• Except Hb chromophores: Their concentration is affected by brain activity

Question 6

Describe the 3 types of instruments used for NIRS measurements in tissue?

Answer 6

• Continuous Wave instruments:
  o Intensity only
• Time Domain Instruments:
  o Intensity & transit time through tissue (pulsed on for less than 100 ps)
  o Use pulse light sources & time sensitive detection system
• Frequency Domain Instruments: (ISS)
  o Intensity & “average transit time” through tissue
  o Use intensity modulated light sources (typically between 50 & 500 MHz – usually 750-810MHz) & phase sensitive detection

Question 7

Why is fNIRS used? (what are the +ves?)

Answer 7

• Non-invasive
• Non-ionising
• Real-time monitoring
• Portable systems
• Can be used on breast tissue & babies v safely
• If know distance between detectors & emitters of light, then get absolute measures
• It can give absolute (or just relative, Δ) measures of
  o [HbO] in µM
  o [HbR] in µM
  o Total haemoglobin [HbT] in µM
• Get blood oxygen saturation in % (exactly same principle as a routinely used pulse oximeter)