# Time-Domain Near-Infrared Spectroscopy and Imaging: A Review
## 1 Introduction  
The time-domain (TD) technique in near-infrared spectroscopy (NIRS) and imaging technology has the greatest potential among the continuous wave (CW), frequency-domain (FD), and time-domain modalities owing to having the richest information contained in the measured TD data. Although the advantages of the TD technique are widely recognized, only two TD-NIRS systems were commercially available for brain and/or muscle NIRS oximeries, while more than ten CW-NIRS systems were commercially available by 2011, according [PERSON] and [PERSON] [1]. These limited usages of the TD-NIRS technique are due to its drawbacks, to which some articles have referred, as follows.  
In 2014, [PERSON] et al. [2] described the situation of the TD-NIRS technique as follows (with modification), \"TD-NIRS and TD techniques in general had the reputations of being cumbersome, bulky, and very expensive when compared with commercially available continuous wave (CW) functional-NIRS systems. These disadvantages cannot be ignored, and a gap between CW and TD-NIRS technology still exists.\"  
In 2018, [PERSON] et al. [3] also described the drawbacks of TD-NIRS systems more in details (with modification), \"TD-NIRS instruments used so far are bulky and expensive, and typically employ sensitive optoelectronics which are susceptible to vibrations. When solid state lasers are used, switching from one wavelength to another is slow about 10 s. When pulsed laser diodes are used, long _warm-up_ time (about 60 min) is required to achieve a stability of pulse timing in the picosecond range. These factors limit the usages of TD-NIRS not only in hospitals but also in laboratories.\"The drawbacks of TD-NIRS systems described above still exist to some extent and have not been resolved completely, but improvements and developments of the TD-NIRS technique have been progressing steadily. [PERSON] et al. [2] continued the above description with expectations, \"Recent advances in photonic technologies might allow to bridge the gap between TD-NIRS and CW-NIRS and potentially to overtake CW-NIRS.\" Also, [PERSON] et al. [3] expressed expectations for advanced TD-NIRS systems, describing that smaller and more robust instruments might lead to wider applications, for example in emergency medicine.  
In this article for TD-NIRS and imaging, we first review the theoretical background of TD-NIRS and provide an overview of TD-NIRS instruments. Then, we proceed to the advanced theories and methods of the TD-NIRS technique and review studies on the clinical applications of the TD-NIRS technique including TD diffuse optical tomography (TD-DOT) and TD fluorescence tomography (TD-FT). Before summarizing this article, some results of the clinical applications of commercially available TD-NIRS systems by Japanese researchers are reviewed. Please note that many studies of CW and FD approaches were intentionally excluded in this article, although they developed the basis of the TD approach. In the summary, the major/key developments in TD-NIRS and imaging technology are listed in chronological tables and figures to help the reader view the whole TD-NIRS and imaging picture. Finally, we summarize this review article with the expectations of wider usages for the TD-NIRS technique in the near future. There must be many important TD studies which were not referred to in this article, and there may be descriptions with misunderstandings of the references. The authors are grateful for any comments provided by readers.