The depth of functional Near-Infrared Spectroscopy (fNIRS) is not a single fixed value but varies significantly depending on the tissue type and the specific application.
Understanding fNIRS Penetration Depth
fNIRS utilizes near-infrared light to measure changes in blood oxygenation within tissues, which is indicative of neural activity or tissue function. The ability of the light to penetrate tissue is crucial for its effectiveness, and this penetration depth is highly dependent on the optical properties of the tissue it is examining.
Typical Penetration Depths
The achievable transmission limits of NIRS (and thus fNIRS) vary considerably across different parts of the body:
| Tissue Type | Typical Achievable Penetration Depth |
|---|---|
| Breast Tissue | About 12 cm |
| Arm or Leg Tissue | About 6 cm |
| Brain Tissue | About 3 cm |
These figures represent typical depths that can be probed, highlighting that different tissues allow for different levels of light penetration due to variations in their composition, density, and scattering/absorption properties. For brain imaging, the 3 cm depth means fNIRS is primarily sensitive to cortical activity, typically within the outer layers of the brain.
Factors Influencing fNIRS Depth
The primary factor dictating fNIRS penetration depth is the tissue type. Dense tissues with higher light absorption or scattering properties will limit the depth more significantly than more transparent or less dense tissues. Other contributing factors, while not exhaustively detailed, include:
- Wavelength of Light: Different wavelengths of near-infrared light interact differently with tissue chromophores (like hemoglobin), affecting penetration.
- Source-Detector Separation: Increasing the distance between the light source and detector generally allows for probing deeper tissues, though it also reduces the amount of detected light.
- Optode Design and Placement: The design and precise placement of the fNIRS sensors on the skin can optimize light transmission and collection for specific depths.
Understanding these depth limitations is crucial for correctly interpreting fNIRS data and designing appropriate experiments, particularly when targeting specific anatomical regions or physiological processes.
