Time-gated Raman spectroscopy for biomedical application under ambient or strong background light conditions

Many biomedical applications require measurements of Raman spectra of tissue under ambient lighting conditions. However, the background light often swamps the weaker Raman signal. The use of time-gated (TG) Raman spectroscopy based on a single photon avalanche diode (SPAD) operating in time-correlated single photon counting and near-infrared laser excitation was investigated for acquisition of Raman spectra and spectral images of biological tissue. The results obtained using animal tissue samples (adipose tissue and muscle) show that the time gating modality enables measurement of Raman spectra under background light conditions of similar quality as conventional continuous wave Raman spectroscopy in the absence of background light. Optimal suppression of the background light was observed for time gate widths of 300-1000 ps. The results also showed that TG Raman spectroscopy was able to detect subtle spectral differences required for medical diagnostics, such as differences in Raman spectra of cancer and normal tissue. While the current instrument required scanning of the grating in order to obtain full Raman spectra, leading to impractical times for multi-wavenumber Raman mapping, imaging time could be drastically reduced by spectral multiplexing (compressed detection) using digital micromirror devices or by using SPAD arrays.