Time- and spectrally-resolved mesoscopic Raman and fluorescence imaging of carious enamel by a CMOS SPAD-based spectrometer

Dental caries is the most widespread non-communicable disease affecting 2.5 billion people worldwide. Recently various optical spectroscopy methods have gained a lot of interest in dental research and caries detection due to their noninvasive and nonionizing nature. We demonstrate here, to the best of our knowledge, for the first time simultaneous Raman, fluorescence intensity and fluorescence lifetime imaging of carious human enamel with a single device based on a time-resolved single-photon avalanche diode (SPAD) sensor fabricated in 110-nm CMOS technology. The sensor has identical 256 spectral channels each of them containing an integrated tunable 7-bit time-to-digital converter (TDC) enabling time- and spectrally-resolved photon counting. We were able to show with the spectrometer built around the CMOS SPAD sensor and by utilizing a simple unsupervised machine learning algorithm (K-means) that a more comprehensive and reliable images of the enamel and caries-affected regions can be obtained by combining the information from the three different optical spectroscopy methods. We believe that the technology presented here could pave the way for novel multimodal optical devices not only in dental research and clinical dentistry but also in other biomedical applications in which molecular and chemical changes are studied.