In recent years, a large volume of research has been conducted involving technology for measuring living organisms using light in the near infrared region, at wavelengths ranging from 700 to 1,200 nm. This is because scientists have found that light in the near infrared region has a comparatively high transmittance rate in living organisms, and can be used to measure the oxygen metabolism in tissues. Another factor leading to the development of this technology is a fuller complement of devices that support this kind of research, such as laser diodes, optical fibers and sensors that detect extremely faint light.

We will be dealing with an aging society in the 21st century, and medical care that focuses on preventing illness has become a priority topic. Developing and introducing diagnostic technology that could enable continuous measurement around the clock, so that irregularities could be detected early on, is an essential part of that effort. The use of near infrared light, in particular, has been garnering interest because of its possibilities in the ultimate devices that could be used continuously, without causing any physical harm to the subject.

Oxygen is indispensable in maintaining activity in living organisms. However, oxygen can't be measured directly in living organisms using light. To solve that, Hamamatsu Photonics focused on the fact that the absorption spectra of chromoproteins of hemoglobin and other components in the blood that are involved in metabolizing oxygen have different characteristics when coupled with oxygen and when in a free state, and we developed technology that would allow these changes in the spectra to be captured, making it possible to directly ascertain the oxygen metabolism taking place in living organisms.

Images of metabolism can currently be captured using PET (Positron Emission Tomography) and fMRI (functional Magnetic Resonance Imaging), but these require huge items of equipment and are extremely expensive. Additionally, in measurements taken with these systems, it is not possible to distinguish between oxyhemoglobin and deoxyhemoglobin.

The isotopes used with PET and the high magnetic fields used with fMRI are said to be safe enough with ordinary measurements, but measurements of living organisms using near infrared light offer significant advantages over these in terms of safety. Also, the subject doesn't have to be restrained in place, so observation can be done without interfering with normal activities. As a result, measurement of living organisms using near infrared light is drawing keen interest as an imaging diagnostic device that enables continuous measurement that is convenient, safe and non-invasive.

Because the brain has a uniquely wrinkled structure, 3-dimensional (3D) images of the blood flow and other elements are needed in order to pinpoint the actual location of a reaction. Up until now, however, there has been no way to display information such as the distribution of hemoglobin concentration in the brain in 3D.

With brain measurement using "optical mapping", a conventional method of imaging, only 2-dimensional (2D) information could be obtained, in which the center point of the sensors between represented the measurement information. In other words, it was possible to see changes in the brain as a data on the surface, but not to know more detailed information about what was taking place in the interior of the brain.

What makes it difficult to create images of the interior of the brain is that the brain is highly turbid, making it hard to identify the optical path length (the distance that the light has traveled).

With the RDOT developed by Hamamatsu Photonics, pulsed light is used as the light source instead of continuous wave light. The pulsed light is scattered inside the body, and the resulting light follows various optical paths in response to the time that has elapsed.

At Hamamatsu Photonics, we developed our own proprietary algorithms for capturing the state of this pulsed light dispersion using TRS (Time-Resolved Spectroscopy) and reconstructing brain activity as 3D images. This technology was then put to work in RDOT.

The relationship between bodily movements and the movement of the blood in the brain is an area that is not yet clearly understood. The connection between brain function and the mind is also still an unknown realm, with many unanswered questions. However, research in the mutual interaction between light and living organisms undoubtedly holds the key to solving the secrets of the human mind and body.

It will not be long before RDOT begins helping us to understand higher brain functions and contributing to early diagnosis of dementia, as well as playing a role in exploring new possibilities involving the brain in the field of Brain-Machine Interface.

Applications drawing on light-based living organism measurement technology are foreseen in areas other than brain research as well, among them researching opto-biopsies used to diagnose the property of tissue in living organisms, detecting breast cancer, understanding how our limbs move, and other fields involving measurement of living organisms.

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