Near-infrared spectroscopy is an analytical method based on the absorption of electromagnetic radiation in the range of short-wave infrared light. It has many applications in chemistry, food technology and medicine. In medicine, it is, among other things, an imaging method for showing brain activity.
What is near-infrared spectroscopy?
In medicine, near-infrared spectroscopy is, among other things, an imaging technique for showing brain activity. Near-infrared spectroscopy, also abbreviated as NIRS, is a branch of infrared spectroscopy (IR spectroscopy). Physically, IR spectroscopy is based on the absorption of electromagnetic radiation by excitation of vibrational states in molecules and groups of atoms. NIRS examines materials that absorb in the frequency range of 4,000 to 13,000 vibrations per cm. This corresponds to the wavelength range from 2500 to 760 nm. In this range, vibrations of water molecules and functional groups, such as the hydroxyl, amino, carboxyl as well as the CH group, are mainly excited. When electromagnetic radiation of this frequency range hits the corresponding substances, excitation of the vibrations occurs with absorption of photons with a characteristic frequency. After the radiation has passed through the sample or been reflected, the absorption spectrum is recorded. This spectrum then shows the absorptions in the form of lines at specific wavelengths. In combination with other analytical techniques, IR spectroscopy and, in particular, near-infrared spectroscopy can provide information about the molecular structure of the substances under investigation, opening up a wide range of applications from chemical analysis to industrial and food applications to medicine.
Function, effect and goals
Near-infrared spectroscopy has already been used in medicine for 30 years. Here it serves, among other things, as an imaging method in the determination of brain activity. Furthermore, it can be used to measure the oxygen content of the blood, blood volume, and blood flow in various tissues. The procedure is non-invasive and painless. The advantage of short-wave infrared light is its good tissue permeability, making it virtually predestined for medical application. Using near-infrared spectroscopy through the skullcap, brain activity is determined by the measured dynamic changes of the oxygen content in the blood. This method is based on the principle of neurovascular coupling. Neurovascular coupling is based on the fact that changes in brain activity also mean changes in energy demand and therefore oxygen demand. Any increase in brain activity also requires a higher concentration of oxygen in the blood, as determined by near-infrared spectroscopy. The oxygen binding substrate in the blood is hemoglobin. Hemoglobin is a protein-bound pigment that occurs in two different state forms. There is oxygenated and deoxygenated hemoglobin. This means it is either oxygenated or deoxygenated. When it changes from one form to the other, its color changes. This also affects the transmittance of light. Oxygenated blood is more transparent to infrared light than oxygen-depleted blood. Thus, when the infrared light passes through, the differences in oxygen loading can be determined. The changes in the absorption spectra are computed and offer conclusions about instantaneous brain activity. On this basis, NIRS is now increasingly used as an imaging technique to visualize brain activity. Thus, near-infrared spectroscopy also allows the study of cognitive processes, because every thought also generates higher brain activity. It is also possible to localize the areas of increased activity. This method is also suitable for the realization of an optical brain-computer interface. The brain-computer interface represents an interface between humans and computers. Physically handicapped people in particular benefit from these systems. For example, they can trigger certain actions via the computer with pure thought power, such as the movement of prostheses. Other applications of NIRS in medicine include emergency medicine. For example, the devices monitor the oxygen supply in intensive care units or after operations.This ensures rapid response in the event of acute oxygen deficiency. Near-infrared spectroscopy also performs well in monitoring circulatory disorders or optimizing oxygen supply to the muscle during exercise.
Risks, side effects, and hazards
The use of near-infrared spectroscopy is trouble-free and causes no side effects. Infrared radiation is a low-energy radiation that does not lead to any damage to biologically important substances. Even the genetic material is not attacked. The radiation only causes excitation of the various vibrational states of biological molecules. The procedure is also non-invasive and painless. In combination with other functional methods, such as MEG (magnetoencephalography), fMRI (functional magnetic resonance imaging), PET (positron emission tomography), or SPECT (single photon emission computed tomography), near-infrared spectroscopy can image brain activity well. Furthermore, near-infrared spectroscopy has great potential in monitoring oxygen concentration in critical care. For example, a study at the Clinic for Cardiac Surgery in Lübeck shows that surgical risks in cardiac surgery can be predicted more reliably than with previous methods by determining cerebral oxygen saturation using NIRS. Near-infrared spectroscopy also delivers good results in other intensive care applications. For example, it is also used to monitor critically ill patients in intensive care units to avert hypoxia. In various studies, NIRS is compared with conventional methods for monitoring. The studies show the potential, but also the limitations of near-infrared spectroscopy. However, technical advancements in the technique in recent years have made it possible to perform increasingly complex measurements. This enables the metabolic processes taking place in a biological tissue to be recorded and imaged ever more accurately. Near-infrared spectroscopy will play an even greater role in medicine in the future.
