Abstract
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.
Highlights
Raman spectroscopy is a powerful analytical technique for label-free molecular analysis of tissue [1] and has been widely used for medical applications [2]
In this paper we show that TG Raman spectroscopy using a single photon avalanche diode (SPAD) operating in time-correlated single photon counting (TCSPC) mode is an alternative approach that can overcome these limitations
These results show that time gating Raman spectroscopy can be used for measurements of high-quality Raman spectra of biological samples suitable for medical diagnosis in optically noisy environments
Summary
Raman spectroscopy is a powerful analytical technique for label-free molecular analysis of tissue [1] and has been widely used for medical applications [2]. While TG Raman spectra using iCCD have been reported using 720 nm laser excitation, the materials investigated (minerals [26], polymers and explosives [28]) had much higher Raman scattering cross-sections compared to typical biological tissue samples Another potential drawback is that iCCDs require a fixed time gating window of a specific width determined by either software or by fixed experimental parameters. SPADs have become commercially available with high temporal resolution and detection efficiencies almost equivalent to the best research level CCD detectors They have been shown to be effective for fluorescence suppression in Raman spectroscopy [18, 20], molecular depth analysis of optically turbid materials based upon photon time of flight [29], and TG Raman mapping of materials eliciting strong fluorescence backgrounds when combined with spectral multiplexing (or compressed detection) [25]. The resulting histogram is sent to MATLAB via serial at the end of each acquisition period for storage and analysis
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