The source-detector distance in functional near-infrared spectroscopy (fNIRS) is a critical parameter that determines the depth of light penetration and, consequently, the brain region being measured. This distance is carefully selected to optimize the signal-to-noise ratio, ensuring that the measurement primarily reflects brain activity rather than superficial scalp signals. The optimal distance varies significantly based on the population being studied, primarily due to anatomical differences in tissue thickness.
Typical Distances by Population
The recommended source-detector distance in fNIRS is adjusted to account for variations in skull and scalp thickness across different age groups.
Infants
For the infant population, a shorter source-detector distance is typically utilized. Due to their thinner scalp and skull, the commonly used fNIRS source-detector distance is 20 or 25 mm. This allows for effective light penetration to the infant's cerebral cortex while minimizing the path length through superficial tissues.
Adults
In adults, the scalp and skull tissues are considerably thicker compared to infants. The mean total thickness of the human scalp and skull in an adult head is approximately 13 mm. To adequately penetrate beyond these highly vascularized superficial layers and effectively measure oxygenation changes within the brain's cortical regions, a longer source-detector distance is required. While not explicitly detailed in some contexts for its exact value, a common and effective source-detector distance for fNIRS studies in adults is typically around 30 mm. This distance is generally accepted as optimal for capturing signals from the cerebral cortex.
The table below summarizes the typical fNIRS source-detector distances:
| Population | Typical Source-Detector Distance (mm) | Key Anatomical Consideration |
|---|---|---|
| Infants | 20 or 25 | Thinner scalp and skull |
| Adults | ~30 | Thicker scalp and skull |
Importance of Source-Detector Distance
Selecting the appropriate source-detector distance is paramount for obtaining accurate and meaningful fNIRS data:
- Depth of Penetration: The distance directly influences how deep the light travels into the tissue. Shorter distances predominantly capture signals from superficial layers (scalp, skull), while longer distances are necessary to reach the cerebral cortex.
- Signal Quality: An optimally chosen distance helps differentiate between signals originating from the brain and those from extracerebral tissues. Using a distance that is too short for adults, for instance, might result in measurements dominated by changes in scalp blood flow rather than cortical activity.
- Avoiding Superficial Contamination: While the primary source-detector distance targets brain activity, some fNIRS systems also incorporate "short-separation" channels (e.g., <10 mm) to specifically measure and then regress out superficial hemodynamic changes, further enhancing the specificity of brain signals.
The careful selection of source-detector distance ensures that fNIRS can reliably non-invasively monitor cortical hemodynamics, making it a valuable tool in neuroscience research across various age groups.
