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Hyperspectral imaging (HSI) is a state-of-the-art technique that captures and processes information across a given electromagnetic spectrum. Unlike traditional imaging techniques that capture light intensity at specific wavelengths, HSI collects a full spectrum at each pixel in an image. This rich spectral data enables the distinction between different materials and substances based on their unique spectral signatures. Near-infrared hyperspectral imaging (NIR-HSI) has attracted significant attention in the food and industrial fields as a non-destructive technique for analyzing the composition of objects. A notable aspect of NIR-HSI is over-thousand-nanometer (OTN) spectroscopy, which can be used for the identification of organic substances, their concentration estimation, and 2D map creation. Additionally, NIR-HSI can be used to acquire information deep into the body, making it useful for the visualization of lesions hidden in normal tissues.
Various types of HSI devices have been developed to suit different imaging targets and situations, such as for imaging under a microscope or portable imaging and imaging in confined spaces. However, for OTN wavelengths, ordinary visible cameras lose sensitivity and only a few commercially available lenses exist that can correct chromatic aberration. Moreover, it is necessary to construct cameras, optical systems, and illumination systems for portable NRI-HSI devices, but no device that can acquire NIR-HSI with a rigid scope, crucial for portability, has been reported yet.
Now, in a new study, a team of researchers, led by Professor Hiroshi Takemura from Tokyo University of Science (TUS) and including Toshihiro Takamatsu, Ryodai Fukushima, Kounosuke Sato, Masakazu Umezawa, and Kohei Soga, all from TUS, Hideo Yokota from RIKEN, and Abian Hernandez Guedes and Gustavo M. Calico, both from the University of Las Palmas de Gran Canaria, has recently developed the world’s first rigid endoscope system capable of HSI from visible to OTN wavelengths. Their findings were published in Volume 32, Issue 9 of Optics Express on April 17, 2024.
At the core of this innovative system lies a supercontinuum (SC) light source and an acoustic-opto tunable filter (AOTF) that can emit specific wavelengths. Prof. Takemura explains, “An SC light source can output intense coherent white light, whereas an AOTF can extract light containing a specific wavelength. This combination offers easy light transmission to the light guide and the ability to electrically switch between a broad range of wavelengths within a millisecond.”
The team verified the optical performance and classification ability of the system, demonstrating its capability to perform HSI in the range of 490-1600 nm, enabling visible as well as NIR-HSI. Additionally, the results highlighted several advantages, such as the low light power of extracted wavelengths, enabling non-destructive imaging, and downsizing capability. Moreover, a more continuous NIR spectrum can be obtained compared to that of conventional rigid-scope-type devices.
To demonstrate their system’s capability, the researchers used it to acquire the spectra of six types of resins and employed a neural network to classify the spectra pixel-by-pixel in multiple wavelengths. The results revealed that when the OTN wavelength range was extracted from the HSI data for training, the neural network could classify seven different targets, including the six resins and a white reference, with an accuracy of 99.6%, reproducibility of 93.7%, and specificity of 99.1%. This means that the system can successfully extract molecular vibration information of each resin at each pixel.
Prof. Takemura and his team also identified several future research directions for improving this method, including enhancing image quality and recall in the visible region and refining the design of the rigid endoscope to correct chromatic aberrations over a wide area. With these further advancements, in the coming years, the proposed HSI technology is expected to facilitate new applications in industrial inspection and quality control, working as a “superhuman vision” tool that unlocks new ways of perceiving and understanding the world around us.
“This breakthrough, which combines expertise from different fields through a collaborative, cross-disciplinary approach, enables the identification of invaded cancer areas and the visualization of deep tissues such as blood vessels, nerves, and ureters during medical procedures, leading to improved surgical navigation. Additionally, it enables measurement using light previously unseen in industrial applications, potentially creating new areas of non-use and non-destructive testing,” remarks Prof. Takemura. “By visualizing the invisible, we aim to accelerate the development of medicine and improve the quality of life of physicians as well as patients.”