# Time-Domain Near-Infrared Spectroscopy and Imaging: A Review
## 7 Summary  
We reviewed the development of TD-NIRS from the views of its theoretical background, instruments, advanced theories and methods, the literature on future clinical applications, and current clinical applications of commercially available TD-NIRS systems. At the beginning of the summary section, we identified the major/key developments in the instruments as well as the theories, methods, and applications of TD-NIRS and imaging from the previous sections, and they are listed in Tables 1 and 2 in a chronological style. In addition, the major developments in Tables 1 and 2 are schematically and chronologically shown in Figures 19 and 20, where the relationship between the major developments are indicated by vertical arrows to help us view the whole picture of TD-NIRS and imaging technology. From these tables and figures, it can be said that the theories, methods, and applications of TD-NIRS developed steadily in the 1990s and 2000s, while the development of TD-NIRS instruments was rather slow during 1990s and 2000s. However, the development of such instruments began to accelerate during the 2010s, probably owing to the advent of the SiPM (MPPC), as seen from Figure 19. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Year** & **Event** & **Ref.** \\ \hline
1988 & TD-NIRS system using a streak camera or TCSPC & [34,36] \\ \hline \multirow{4}{*}{1999} & 2-wavelength multi-Ch TD-NIRS oximeter using pLDs and multi-anode PMTs & [39] \\ \cline{2-3}  & Commercial 1-Ch TD-NIRS system using TCSPC for research use: “TRS-10” & [40] \\ \cline{2-3}  & 1-wavelength 1-Ch TD optical mammography using a pLD and PMT-TCSPC & [41] \\ \cline{2-3}  & 64-channel TD-DOT system using pLDs and PMT-TCSPCs & [45] \\ \hline
2000 & 32-channel TD-DOT system (MONSTIR) & [49] \\ \hline
2003 & TD-NIRS system using the spread spectrum technique and pseudo-random bit sequences & [63] \\ \hline
2005 & TD-NIRS system using time-gated ICCD & [65] \\ \cline{2-3}  & 16-channel TD-DOT system using pLDs and PMT-TCSPCs & [50] \\ \hline
2009 & Commercial 2-Ch TD-NIRS system using TCSPC for research use: “TRS-20” & [43] \\ \hline
2011 & 48-Ch 3-wavelength TD-NIRS optical mammography & [52] \\ \hline
2013 & TD-DOT system incorporating SPADs into TCSPC & [58] \\ \hline
2014 & Commercial 2-Ch TD-NIRS system using MPPCs for medical use: “tNIRS-1” & [55] \\ \cline{2-3}  & MONSTIR II employing an SC laser with an AOTF for 4 wavelengths & [51] \\ \hline \multirow{4}{*}{2016} & TD-NIRS mammography for imaging the contents of water, lipid, collagen, oxy-Hb and deoxy-Hb & [54] \\ \cline{2-3}  & using 7 wavelengths & \\ \cline{2-3}  & Compact 2-wavelength TD-NIRS system and detector probe using SiPM & [56,57] \\ \cline{2-3}  & TD-NIRS system using an SC laser and SPADs for non-contact measurements & [60] \\ \hline
2017 & 12-Ch TD-NIRS mammography with a hand-held probe & [53] \\ \hline \multirow{4}{*}{2018} & TD-DOT system using an SC laser and SPAD camera & [62] \\ \cline{2-3}  & Compact TD-NIRS system for measuring the contents of water, lipid, oxy-Hb, and deoxy-Hb using & [68] \\ \cline{2-3}  & 6 wavelengths & \\ \cline{2-3}  & Compact 1-Ch TD-NIRS system using telecommunication devices & [3] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chronology of major events in instruments of TD-NIRS (Abbreviations: “Ch” for channel and “pLD” for pulsed laser diode). _Appl. Sci._**2019**, \(9\), 1127