Spatially Offset and Transmission Raman Spectroscopy for Determination of Depth of Inclusion in Turbid Matrix

Abstract

We propose an approach for the prediction of the depth of a single buried object within a turbid medium combining spatially offset Raman spectroscopy (SORS) and transmission Raman spectroscopy (TRS) and relying on differential attenuation of individual Raman bands brought about by the spectral variation of matrix absorption (and scattering). The relative degree of the Raman band changes is directly related to the path length of Raman photons traveling through the medium, thereby encoding the information on the depth of the object within the matrix. Through a calibration procedure with root mean square error of calibration (RMSEC) = 3.4%, it was possible to predict the depth of a paracetamol (acetaminophen) inclusion within a turbid matrix consisting of polyethylene (PE) by monitoring the relative intensity of two Raman bands of paracetamol exhibiting differential absorption by the matrix. The approach was shown to be largely insensitive to variations of the amount of the inclusion (paracetamol) and to the overall thickness of the turbid matrix (PE) with a root mean square error of prediction (RMSEP) maintained below 10% for the tested cases. This represents a major advantage over previously demonstrated comparable depth determination Raman approaches (with the exception of full Raman tomography requiring complex mathematical reconstruction algorithms). The obtained experimental data validate the proposed approach as an effective tool for the noninvasive determination of the depth of buried objects in turbid media with potential applications including determining noninvasively the depth of a lesion in cancer diagnosis in vivo.

Determination of inclusion depth in ex vivo animal tissues using surface enhanced deep Raman spectroscopy

“…The scheme of the deep Raman setup used in these measurements is presented in Figure A. It consists of a custom‐built Raman system described in detail elsewhere . Briefly, the setup is based on an 830 nm continuous wave diode laser capable of performing Raman measurements both in point‐like spatial offset configuration (SORS) and in transmission mode (TRS).…”

“…For each offset and depth of the target, Raman spectra were recorded with a laser power at sample of 300 mW for 20 s × 10 accumulations in the spectral range 112 to 1938 cm −1 with a spectral resolution of ∼8 cm −1 . Different calibration models for depth prediction were created following an approach proposed in our previous work . The spectra were analysed using a Gaussian‐shape curve fit to the 930 and 1580 cm −1 Raman bands of the target for evaluating the effect of differential absorption due to ex vivo tissue components (eg, water, Figure C) on their relative Raman intensities (evaluated as the area of the Gaussian‐shape curve).…”

“…The spectra were analysed using a Gaussian‐shape curve fit to the 930 and 1580 cm −1 Raman bands of the target for evaluating the effect of differential absorption due to ex vivo tissue components (eg, water, Figure C) on their relative Raman intensities (evaluated as the area of the Gaussian‐shape curve). For each spatial offset and transmission measurement, a linear fit (see Figure S2 in Supporting Information) was performed to obtain the trend of the natural logarithm of the intensity ratios of the two bands (1580‐930 cm −1 ) vs the depth (z) of the target .…”

“…In a clinical environment, the in vivo localisation of a Raman reporter related to a specific disease (eg, cancer lesion) within the body could potentially improve the effectiveness of diagnosis and subsequent treatments. Within this context, previous research has demonstrated the use of TRS alone or in combination with SORS to retrieve the depth information of a single inclusion buried within a diffusive synthetic phantom. The approach exploits the differential attenuation of Raman photons at different wavelengths due to the optical properties of the diffusive media (absorption coefficient, μa; reduced scattering coefficient, μs′).…”

“…Within this context, previous research has demonstrated the use of TRS alone or in combination with SORS to retrieve the depth information of a single inclusion buried within a diffusive synthetic phantom. The approach exploits the differential attenuation of Raman photons at different wavelengths due to the optical properties of the diffusive media (absorption coefficient, μa; reduced scattering coefficient, μs′). The resulting change in the relative Raman intensity of the different Raman bands of an inclusion detected at the surface of a sample is related to the mean distance propagated by the Raman photons.…”