Interferometric Near-Infrared Spectroscopy (iNIRS) for determination of optical and dynamical properties of turbid media

Abstract

We introduce and implement interferometric near-infrared spectroscopy (iNIRS), which simultaneously extracts optical and dynamical properties of turbid media through analysis of a spectral interference fringe pattern. The spectral interference fringe pattern is measured using a Mach-Zehnder interferometer with a frequency-swept narrow linewidth laser. Fourier analysis of the detected signal is used to determine time-of-flight (TOF)-resolved intensity, which is then analyzed over time to yield TOF-resolved intensity autocorrelations. This approach enables quantification of optical properties, which is not possible in conventional, continuous-wave near-infrared spectroscopy (NIRS). Furthermore, iNIRS quantifies scatterer motion based on TOF-resolved autocorrelations, which is a feature inaccessible by well-established diffuse correlation spectroscopy (DCS) techniques. We prove this by determining TOF-resolved intensity and temporal autocorrelations for light transmitted through diffusive fluid phantoms with optical thicknesses of up to 55 reduced mean free paths (approximately 120 scattering events). The TOF-resolved intensity is used to determine optical properties with time-resolved diffusion theory, while the TOF-resolved intensity autocorrelations are used to determine dynamics with diffusing wave spectroscopy. iNIRS advances the capabilities of diffuse optical methods and is suitable for in vivo tissue characterization. Moreover, iNIRS combines NIRS and DCS capabilities into a single modality.

Interferometric near-infrared spectroscopy directly quantifies optical field dynamics in turbid media

“…This is achieved through the analysis of the spectral interference between light traversing the sample and reference paths. This work extends beyond the intensity autocorrelations [32] demonstrated previously and determines field autocorrelations directly from the phase of the spectral interference.…”

“…This experimental measurement is related to the TOF-resolved field autocorrelation, $$G_1\left({\tau }_s,{\tau }_d\right)={\langle U_s^{\ast }\left({\tau }_s,t_d\right)U_s\left({\tau }_s,t_d+{\tau }_d\right)\rangle }_{t_d},$$by a convolution integral, $$G_1^{\left(\text{iNIRS}\right)}\left({\tau }_s,{\tau }_d\right)=true{\int }_{-\mathrm{normal\infty }}^{\mathrm{\infty }}{G}_1\left({\tau }_s^{\prime },{\tau }_d\right){\mathrm{scriptI}}_0\left({\tau }_s-{\tau }_s^{\prime }\right)\mathrm{d}{\tau }_s^{\prime },$$where ${\mathcal{I}}_0\left({\tau }_s\right)$ denotes the instrument response function (IRF), which depends on the spectral range over which ${\mathcal{W}}_{rs}$ is measured [32]: $\Delta \nu \approx c\Delta \lambda /{\lambda }_0^2$ ( c is the speed of light, Δ λ denotes the wavelength bandwidth, and λ 0 stands for the center wavelength in free space). Hence, iNIRS measures the convolution of the TOF-resolved autocorrelation G 1 with the IRF.…”

“…To accomplish this, we demonstrate interferometric near-infrared spectroscopy (iNIRS) [32] for direct determination of TOF-resolved field and intensity autocorrelations. This is achieved through the analysis of the spectral interference between light traversing the sample and reference paths.…”

Time-domain diffuse correlation spectroscopy

“…The decay rates of path length-dependent correlation functions are independent of the optical absorption coefficient of the media, which has the potential to reduce uncertainty in the estimation of blood flow. Prior demonstrations of path length-resolved DCS relied on the selection of individual path lengths by nonlinear optics [41] or interferometry [48], limiting throughput and applicability for transcranial human measurements. Here, we have taken the novel approach of exploiting the path length-dependent correlation functions to improve the sensitivity of transcranial blood flow measurements to the brain by distinguishing the contributions from short and long paths.…”

Reflectance-mode interferometric near-infrared spectroscopy quantifies brain absorption, scattering, and blood flow index in vivo

“…Recently, interferometric NIRS (iNIRS), based on the TOF-resolved measurement of coherent light reemitted from a turbid medium, was introduced [10]. This approach is based on interferometry using a tunable laser to achieve path length resolution [11,12].…”