Introduction; Nirs Fundamentals - NIRx NIRSport User Manual

2

Introduction

2.1

NIRS Fundamentals

Near-infrared spectroscopy (NIRS) of brain function employs low-energy optical radiation for measuring
absorption changes in sub-surface tissues, in order to infer local concentration changes of oxy- and de-
oxy-hemoglobin as correlates of functional brain activity.
Each measurement channel is formed by an optical emitter ('source')
and a receiver ('detector') placed on the tissue surface. Because of the
scattering (light diffusing) properties of the tissue, a portion of the
received light will travel through deep tissue structures, where it may
interact with tissue chromophores such as hemoglobin. The penetration
depth and shape of the probing volume is a complex function of the
source-detector distance and other aspects of the measurement
geometry, and is also affected by the local tissue optical properties. As a
general rule, measured cortical NIRS signals are estimated to originate
from an area centered between the source and detector, and from a
tissue depth no more than about half the source-detector distance (see
Figure 1). Optical signals are heavily damped by biological tissue; the intensity will decrease several
orders of magnitude over a few cm. The ideal source-detector distance is therefore a trade-off between
achieving the greatest possible sensing depth while maintaining a sufficient signal quality (signal-to-
noise ratio). Generally, a distance of 30-35 mm is considered optimal, and the sensing depth into the
adult human head is on the order of 15-25 mm.
To achieve spatial mapping (or imaging) of brain activity, arrays of multiple source-detector pairs are
placed over the area of interest. The NIRS imaging equipment developed by NIRx employs a unique
measurement strategy in which EACH source channel forms a measurement channel with EACH detector
channel. Therefore, a setup with X sources (S) and Y detectors (D) will always produce X*Y measurement
channels. This is true irrespective of the source-detector arrangement or distances; however, only the
channels that have S-D separation distances within a certain upper limit will produce signals having
usable amplitudes and noise levels.
Because of this scheme, there is no restriction on how to place sources and detectors over the tissue.
This affords a maximum of freedom and flexibility in realizing any desired experiment, but at the same
time it demands that the user pay careful attention to experimental planning and take a few data-
quality assurance steps during setup and also in the subsequent signal analysis.
To allow this degree of flexibility, NIRx imagers require a calibration operation prior to each experiment,
during which the signal for each S-D combination is optimized. Depending on the amount of light
reaching a given detector from a given source, the detector's light sensitivity is electronically adjusted to
provide the optimal amount of signal amplification and therefore the best possible signal quality. This
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NIRStar 14.1 - User Manual
Source
Detector
Figure 1. Principle of simple
nearest - neighbor NIRS
measurement