ReviewA brief review on the history of human functional near-infrared spectroscopy (fNIRS) development and fields of application
Highlights
► The paper celebrates the 20th anniversary of functional fNIRS. ► Events that have shaped the present status of fNIRS are reported. ► fNIRS methodology has undergone consistent improvements over the years. ► The application of fNIRS in Medicine and basic research have been increasing. ► Technological development will augment fNIRS application in adults and infants.
Introduction
This review is aimed at reporting the main steps of the progresses, over the last two decades, of the human functional near-infrared spectroscopy (fNIRS) instrumental development, and at highlighting the explosion of its application in several medical fields. fNIRS reveals noninvasively and in a natural environment human infant and adult cortical activation in response to diverse stimuli. The chronology of the major events leading up to human functional cortical imaging by fNIRS is reported in Table 1. Although, in the meanwhile fNIRS methodological development and data analysis made wonderful progress, these important topics are beyond the goal of this article.
Considering that fNIRS exploits the principles of near-infrared spectroscopy (NIRS), the principles of NIRS, the three different NIRS techniques (each based on a specific type of illumination) and the main feature of the NIRS instrumentation are briefly reviewed. The principles of fNIRS are reported. The development of fNIRS instrumentation from 1992 (single channel system with a low temporal resolution and poor sensitivity) up to the multi-channel systems (the first 10-channel system was introduced in 1995) is reported in detail and sketched in Fig. 1. The present high temporal resolution multi-channel systems, using the three different NIRS techniques and complex data analysis systems, provide simultaneous multiple measurements and display the results in the form of a map or image over a specific cortical area. The potential that exists for fNIRS more than for any other neuroimaging modality is represented by the realization of multi-channel wearable and/or wireless systems that allow fNIRS measurements even in normal daily activities.
Section snippets
The discovery and basics of near-infrared spectroscopy for monitoring brain oxygenation
fNIRS, a neuroimaging technology for mapping the functioning human cortex, exploits the principles of near-infrared (NIR) spectroscopy (NIRS). Therefore, it is appropriate to start with the discovery and principles of NIRS.
The basics principles of fNIRS
Although the image intensity that varies with HHb content, termed “blood oxygenation level dependent” (BOLD), was suggested for potential use in functional studies of the brain in 1990 (Ogawa et al., 1990), the era of BOLD-based functional MRI (fMRI) initiated two years later (for a review: Raichle, 2000) with the publication of three exciting papers (Bandettini et al., 1992, Kwong et al., 1992, Ogawa et al., 1992). Interestingly, the discovery of human fNIRS goes back to 1992. fNIRS or optical
The first near-infrared images of human tissues
The first transillumination (diaphanoscopy) using white light of the larynx, the paranasal sinuses and testis goes back to the eighteen-century. Later, transillumination was adopted as a means of diagnosing lesions of the female breast, as described by Cutler (1929). The first one-dimension transillumination of the human hand oxygenation was realized by Jarry et al. (1989) using two pulsed NIR lasers and concluded that “these results could be applied to imaging”.
Brain optical imaging was the
The introduction of time resolved instrumentation
In vivo NIR TD and FD measurements were introduced in 1988 and 1990, respectively. To generate images that can discriminate between the effects of absorption and scattering in tissue, it is necessary to acquire more than just measurements of intensity. In fact, to evaluate changes in chromophore concentration in non-arbitrary units, it is necessary to determine the average pathlength of transmitted (or reflected) photons. The pathlength of individual photons can be recorded directly using a
Wearable/wireless fNIRT systems
The commercial fNIRS systems listed in Table 3 utilize fiber optic bundles. The disadvantage of using fiber optic bundles is that the fibers are often heavy and of limited flexibility, perhaps provoking discomfort (especially in patients). In addition, these fNIRS systems require that the subject's head position not move beyond the length of the fiber optic bundles. Multi-channel wearable and/or wireless systems could make fNIRS measurements more comfortable. These advanced fNIRS systems would
The first 19 years of fNIRS research and perspectives
The summary of the results of almost two decades of fNIRS research goes beyond the goal of this article. From a search on the databases PubMed, Scopus and Web of Science, performed using the keywords: “NIRS”, “functional NIRS”, “functional near-infrared topography”, and “optical imaging”, about 400 articles have been published over the last 3 years. The main fields where fNIRS has been applied since its birth are listed in Table 5. Several recently published review articles summarize the fNIRS
Acknowledgments
Part of the content of this article has been included in the Keynote Talk “NIRS: a historical perspective” presented at the Functional Near-Infrared Spectroscopy: 2010 Conference (Harvard University, Cambridge, MA, October 15–17, 2010).
The authors would like to thank David Boas for his helpful suggestions. The authors wish to thank the European (Hebden, Obrig, Torricelli, Wabnitz), Japanese (Eda, Hoshi, Kato, Kohno, Maki, Okada, Shiga, Tamura, Yamada, Yamashita), and American colleagues
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