Near infrared cerebral oxygenation monitoring
نویسنده
چکیده
This technique employs principles of optical spectrophotometry which exploits the fact that biological material including skull, is relatively transparent in the near infrared range. Light transmission depends on a combination of reflectance, scattering and absorption effects. Reflectance is primarily a function of the angle of the light beam to the tissue surface, while scattering decreases with increasing wavelength, favouring transmission of shorter near-infrared (NIR) light (6501100 nm). Absorption occurs at specific wavelengths, determined by the molecular properties of the materials in the light path. Above 1300 nm water absorbs all photons over a pathlength of a few millimetres, while below 700 nm, increasing light scattering and intense absorption bands of hemoglobin (Hb) prevent transmission. In the 700-1300 nm range, however, NIR light penetrates tissue several centimetres [1]. The absorption spectra of oxyhemoglobin (HbO2) ranges from 800-850 nm, deoxyhemoglobin ranges from 650800 nm, and Caa3 has a broad peak at 820-840 nm [2]. In order to compensate for extracerebral tissue, most common techniques employ either spatial resolution or temporal resolution. Spatial resolution commonly involves differentially spaced receiving optodes with the signal from the closer receiver measuring more superficial tissue and distal optode measuring both superficial and deeper tissues and cortical oxygenation being derived from a subtraction algorithm. Temporal resolution involves the principle that photon path length is proportional to tissue transmission time such that by using a pulsed NIR signal, deeper (cortical) tissue will be reflective of receiver-detected photons arriving ‘later’ rather than ‘earlier’ in a pulsed sequence.
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