Medical near Infrared Spectroscopy 35 Years after the Discovery

Abstract

Optical methods using visible and near infrared (NIR) light are a powerful tool for understanding human health and improving disease detection and treatment. The medical applications of optical methods are very old but as optical technologies advance, old medical applications evolve and new ones emerge. The main advantages of the optical methods are: non-invasive, no side-effects, good temporal and spatial resolution, real-time functional information, cost effective and portable. Optical methods rely on different types of interactions between visible or NIR light and biological tissue to provide structural, biochemical, physiological and morphological information. There are two main tissue optical properties which characterise light–tissue interactions and determine therapeutic or diagnostic outcome: absorption coefficient and reduced scattering coefficient. The penetration depth of visible and NIR light in tissue ranges from microns to centimetres depending on wavelength, source/detector separation, light delivery/collection geometry etc. The major tissue absorbers include: haemoglobin, lipids, melanin, water and proteins. Oxygenated and de-oxygenated haemoglobin (O2Hb; HHb) preferentially absorb visible and NIR light of different wavelengths, as evidenced by the more reddish colour of oxygenated blood and the more bluish hue of deoxygenated blood. O2Hb and HHb have distinct spectra, therefore optical measurements can provide information on tissue oxygenation, oxygen consumption and blood haemodynamics. Absorption and/or fluorescence NIR spectroscopy is utilised in different techniques with different resolution and penetration depth, such as conventional microscopy, confocal/multi-photon microscopy, optical coherence tomography (OCT), photodynamic therapy, “in vivo” molecular imaging and medical diffuse NIR optical spectroscopy/imaging. The main applications of medical NIR diffuse optical spectroscopy/imaging are: pulse oximetry, brain/muscle oximetry, functional brain cortex mapping and optical mammography. NIR light was utilised for the first time in pulse oximetry by Takuo Aoyagi in 1972. Pulse oximetry, an extremely simplistic version of reflectance spectroscopy, measures continuously arterial O2Hb saturation in the index finger or in other vascular districts and is considered the most significant technological advance ever made in monitoring the safety of patients during anaesthesia and critical care. Another step in expanding the usefulness of using NIR light in the medical field is represented by the discovery of medical NIR diffuse spectroscopy. Frans Jobsis is considered the founder of “in vivo” medical NIR spectroscopy. In 1977, he reported that the relatively high degree of transparency of brain tissue in part of the NIR range (700–1000 nm; the “optical window”) enables real-time non-invasive detection of regional haemoglobin oxygenation using trans-illumination spectroscopy.1 Starting in 1992 the opportunity of measuring quantitatively, by different NIR spectroscopy techniques, regional O2Hb saturation by NIR oximetry made it possible to monitor brain/muscle reserve capacity following tissue oxygen M. Ferrari, K.H. Norris and M.G. Sowa, J. Near Infrared Spectrosc. 20, vii–ix (2012)