Near Infrared Spectroscopy (NIRS)

Near Infrared Spectroscopy (NIRS) is a noninvasise optical technique that is able to measure changes in hemoglobin oxygenation state in the biological tissues. Several names are known for this technique like as diffuse optical tomography (or topography: DOT), near infrared imaging (NIRI) and functional near infrared spectroscopy (fNIRS). All of the techniques are based on the optical properties of the light absorbing molecules (chromophores) present in tissue and use the absorption spectra of these substances to interpret the detected light levels as changes in chromophore concentrations. In fact, near infrared light is able to easily penetrate up to few centimetres in biological tissue and allows for monitoring changes in light absorption in human in vivo. Most of all, light transmission in biological tissue depends on reflectance, scattering and absorption effects: in the near infrared wavelengths range the predominant effect is due to the intense absorption by hemoglobin. Using multiple wavelengths of the light transmitted simultaneously through the tissues, it is possible to obtain spectroscopic information about different chromophores concentrations like as oxygenated and deoxygenated hemoglobin/myoglobin and cytochrome oxidase that is a good marker of metabolic demands. Diffuse optical techniques have an excellent temporal sensitivity as well as reasonable spatial sensitivity; in contrast functional Magnetic Resonance Imaging (fMRI), Positron Emission Tomography (PET) and Single Proton Emission Computer Tomography (SPECT) have a better spatial sensitivity but weak in terms of temporal resolution Figure 7.1 . Several advantages of near infrared techniques regard the instrumentation: first of all the measurement is robust to motion artefacts so it can be possible to use with infant, small children or psychiatric patients. Then the portability of the instruments permits their bedside use. Moreover, the non ionizing nature of the infrared light permits long term monitoring. Finally, NIRS instruments are low cost, low power and easy to use. Principal medical applications of near infrared techniques are monitoring of tissue oxygenation during muscle exercise and for studying vascular disease in the upper or lower limbs, detecting tumours in the breast with tomography technique, monitoring of inflammatory process in rheumatoid arthritis disease, functional brain imaging for studies of neuroactivation in infants, small children or patient with epilepsy and brain mapping for neurosurgery