Bessel Beam Beating-Based Spontaneous Raman Tomography Enables High-Contrast Deep Tissue Raman Measurements

Abstract

We report on the development of a novel Bessel beam beating-based spontaneous Raman tomography (B 3 -SRT) technique for depth-resolved deep tissue Raman characterization without the need for mechanical z-scanning. The tissue Raman signal is successfully modulated longitudinally for the first time by B 3 -SRT for depth information encoding and retrieval of the tissue Raman signal. To accomplish B 3 -SRT, we conceived a unique method by designing a coaxial Bessel beam dynamic beating excitation associated with the Bessel-shaped collection scheme, such that the depth-resolved Raman information is encoded by the Bessel beam dynamic beating generated, and then the depthencoded Raman spectra along the Bessel beam excitation region are collected simultaneously by a Bessel-shaped collection optical design. Depth-resolved Raman spectra can be rapidly retrieved using inverse fast Fourier transformation. We demonstrated the ability of B 3 -SRT for high contrast deep tissue Raman measurements in a highly scattering two-layer tissue phantom (e.g., fat−bone tissue model). With the use of a nondiffracting Bessel beam in turbid tissue, the B 3 -SRT technique provides an approximately ∼3.7-fold improvement in deep tissue Raman detection compared to confocal Raman spectroscopy. Further, with the benefit of the effective suppression of the randomly scattered photon interference enabled by the Bessel beam dynamic beating excitation and the Bessel-shaped collection, B 3 -SRT gives ∼1.5-fold enhancement in deep tissue Raman spectral contrast in comparison with confocal Raman spectroscopy. We anticipate that the B 3 -SRT technique developed has the potential to facilitate high contrast depth-resolved deep tissue Raman measurements in biomedical systems.