Application of near-infrared spectroscopy in clinical neurology

Abstract

Near-infrared spectroscopy (NIRS) monitoring has been used mainly to detect reduced perfusion of the brain during orthostatic stress in order to assess orthostatic intolerance (OI). Many studies have investigated the use of NIRS to reveal the pathophysiology of patients with OI. Research using NIRS in other neurological diseases (e.g., stroke, epilepsy, and migraine) is continuing. NIRS may play an important role in monitoring the regional distribution of the hemodynamic flow in real time and thereby reveal the underlying pathophysiology and facilitate the management of not only patients with OI symptoms but also those with various neurological diseases.Key words: Orthostatic intolerance; Near-infrared spectroscopy; Clinical neurology INTRODUCTIONMany studies have investigated the hemodynamics and functions of the brain in various fields.1 Several noninvasive methods have been introduced in recent decades for measuring neuronal activity in the brain. 2 Neurophysiological techniques such as electroencephalography, magnetoencephalography, and event-related potentials offer the ability to measure overall changes in the electromagnetic field with an excellent time resolution, but these methods have poor spatial resolution. On the other hand, brain imaging techniques such as positron-emission tomography (PET) and functional magnetic resonance imaging (fMRI) have greatly increased our knowledge about neural circuitry. However, PET is highly sensitive to motion artifacts, confines the patient, and involves the injection of radioactive materials. Although fMRI is noninvasive and has excellent spatial resolution, it is expensive, highly sensitive to motion artifacts, and difficult to integrate with other imaging modalities. 3,4 Near-infrared spectroscopy (NIRS) was introduced as a new neuroimaging modality for Principle of NIRSNIRS is a noninvasive optical imaging tool for observing changes in the hemodynamics in the prefrontal area that is based on measuring the concentrations of HbO and HbR in the blood. Since Franz Jöbsis first studied the oxygenation of living tissues, NIRS has been extensively applied in imaging studies of brain activation. 6 The physiological activation of neurons leads to an imbalance between oxygen supply and utilization, increasing the concentration of HbO and decreasing the concentration of HbR. Most tissues are relatively transparent to light in the near-infrared range from 700 to 900 nm, and photons interact with tissues via absorption and scattering. This means that near-infrared light is less absorbed and scattered by tissue than is light at other wavelengths. 7 This range of wavelengths is often called an optical window since the light can easily pass through most tissues; however, it is reflected by HbO and HbR, and hence the absorption and scattering of light used for NIRS can provide information relevant to neural activity. 8 This light penetrates several centimeters through tissue and can still be detected (Fig. 1). The NIRS system consists of a light source and a photodetector. The l...