Near infrared spectroscopy

The Near infrared spectroscopy is an analysis method based on the absorption of electromagnetic radiation in the range of short-wave infrared light. It has a wide range of uses in chemistry, food technology and medicine. In medicine, it is, among other things, an imaging method for displaying brain activity.

What is near infrared spectroscopy?

Near-infrared spectroscopy, also called NIRS abbreviated, is a sub-area of ​​infrared spectroscopy (IR spectroscopy). Physically, IR spectroscopy is based on the absorption of electromagnetic radiation through the excitation of oscillation states in molecules and groups of atoms.

The NIRS examines materials that absorb in the frequency range from 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 and CH groups are mainly excited. If electromagnetic radiation in this frequency range hits the corresponding substances, the vibrations are excited with absorption of photons with a characteristic frequency. The absorption spectrum is recorded after the radiation has passed through the sample or is reflected.

This spectrum then shows the absorptions in the form of lines at certain wavelengths. In combination with other analysis methods, IR spectroscopy and, in particular, near-infrared spectroscopy can make statements about the molecular structure of the substances examined and thus opens up a wide range of applications, from chemical analyzes to industrial and food technology applications to medicine.

Function, effect & goals

Near-infrared spectroscopy has been used in medicine for 30 years. Here it is used, among other things, as an imaging method for determining brain activity. In addition, it can be used to measure the oxygen content of the blood, the blood volume and the blood flow in various tissues.

The procedure is non-invasive and painless. The advantage of the short-wave infrared light is its good tissue permeability, so that it is predestined for medical use. Using near-infrared spectroscopy through the skullcap, brain activity is determined through the measured dynamic changes in the oxygen content in the blood. This procedure is based on the principle of neurovascular coupling. The neurovascular coupling is based on the fact that changes in brain activity also mean changes in the energy requirement and thus also the oxygen requirement.

Any increase in brain activity also requires a higher concentration of oxygen in the blood, which is determined by near-infrared spectroscopy. The oxygen-binding substrate in the blood is hemoglobin. Hemoglobin is a protein-bound dye that occurs in two different forms. There are oxygenated and deoxygenated hemoglobin. That means it is either oxygenated or oxygen free. When moving from one shape to another, its color changes. This also affects the transmission of light. Oxygenated blood is more permeable to infrared light than oxygen-deficient blood.

When the infrared light passes through, the differences in oxygen load can be determined. The changes in the absorption spectra are computed and provide information about the current brain activity. On this basis, NIRS is now increasingly being used as an imaging method to display brain activity. Thus, near-infrared spectroscopy also allows the investigation of cognitive processes, because every thought also generates a higher level of brain activity. It is also possible to locate the areas of increased activity. This method is also suitable for realizing 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.

They can use the computer to trigger certain actions, such as the movement of prostheses, with pure power of thought. Other areas of application of NIRS in medicine relate, among other things, to emergency medicine. The devices monitor the oxygen supply in intensive care units or after operations. This ensures a quick reaction in the event of an acute lack of oxygen. Near-infrared spectroscopy is also useful for monitoring circulatory disorders or for optimizing the oxygen supply to the muscles during training.

Risks, side effects & dangers

The use of near-infrared spectroscopy is problem-free and does not cause any side effects. Infrared radiation is low-energy radiation that does not damage biologically important substances. The genetic makeup is also not attacked. The radiation only stimulates 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 tomography), PET (positron emission tomography) or SPECT (single-photon emission computed tomography), near-infrared spectroscopy can depict brain activities well. Furthermore, near-infrared spectroscopy has great potential for monitoring the oxygen concentration in intensive care medicine. A study at the Clinic for Cardiac Surgery in Lübeck shows that operational risks in cardiac surgery can be predicted more reliably by determining the cerebral oxygen saturation with the help of NIRS than with previous methods.

Near-infrared spectroscopy also provides good results for other intensive care applications. For example, it is also used to monitor seriously ill patients in intensive care units in order to avert an oxygen deficiency. In various studies, NIRS is compared with conventional monitoring methods. The studies show the potential, but also the limits of near-infrared spectroscopy.

However, more and more complex measurements can be carried out due to technical developments of the process in recent years. This enables the metabolic processes taking place in biological tissue to be recorded better and better and to represent them graphically. Near-infrared spectroscopy will play an even greater role in medicine in the future.