The spectroscopic imagery near infra-red ( Near Infrared Spectroscopic Imaging , NIRSI or , fNIR) is the application to the cerebral Imagerie of the Spectroscopie near infra-red. This technique consists in measuring oxygenation of a zone of the Cerveau in order to deduce its activity from it.

Principle of the spectroscopic imagery near infra-red

The human tissues are relatively transparent with the Lumière in the range of the close relation Infrarouge (between 700 and 900 Nm) who can thus cross them on several centimetres, one speaks about optical window of the spectrum. However the molecule of Hémoglobine is a Chromophore which reflects the light in this range and its absorption spectrum is different according to whether it transports a molecule of Dioxygène (oxyhaemoglobine) or not (désoxyhémoglobine). However, increase in neuronal activity induced an increase in the local blood flow baptized hemodynamic Answer and thus a modification of the quantity of oxy/désoxyhémoglobine present on the optical Way of the photons which cross the area of the brain considered. While measuring, the changes of the signal related on absorption and the diffusion, it is thus possible to determine if the areas observed are active or not.

Among the advantages of the fNIR one can quote:

• portability: the transmitters and receivers of infra-red light used for the fNIR measure approximately 1 cm, for which it is necessary to add the electronic system of recording of the signal (a small laptop). It is thus possible to use this technique in an ambulatory way (i.e. by leaving the individual on which one takes the free measurement of his movements)
• the limited cost: the price of complete equipment lies between 20  000 and 200  000 euros, which makes the fNIR much more accessible than of other methods of metabolic cerebral imagery (like functional IRM)

Its limitations are:

• the phenomenon of diffusion of the light in various fabrics of the head (with their inhomogeneousness) makes difficult the calculation of the optical Chemin which is at the base of measurement in fNIR
• the fNIR is limited in its temporal resolution by the fact that it depends on the hemodynamic Réponse (which intervenes several seconds after the increase in neuronal activity)
• With this method, it is possible to reach only the roadbases of the cerebral Cortex