Sub-diffusive scattering parameter maps recovered using wide-field high-frequency structured light imaging

Abstract

This study investigates the hypothesis that structured light reflectance imaging with high spatial frequency patterns [Formula: see text] can be used to quantitatively map the anisotropic scattering phase function distribution [Formula: see text] in turbid media. Monte Carlo simulations were used in part to establish a semi-empirical model of demodulated reflectance ([Formula: see text]) in terms of dimensionless scattering [Formula: see text] and [Formula: see text], a metric of the first two moments of the [Formula: see text] distribution. Experiments completed in tissue-simulating phantoms showed that simultaneous analysis of [Formula: see text] spectra sampled at multiple [Formula: see text] in the frequency range [0.05-0.5] [Formula: see text] allowed accurate estimation of both [Formula: see text] in the relevant tissue range [0.4-1.8] [Formula: see text], and [Formula: see text] in the range [1.4-1.75]. Pilot measurements of a healthy volunteer exhibited [Formula: see text]-based contrast between scar tissue and surrounding normal skin, which was not as apparent in wide field diffuse imaging. These results represent the first wide-field maps to quantify sub-diffuse scattering parameters, which are sensitive to sub-microscopic tissue structures and composition, and therefore, offer potential for fast diagnostic imaging of ultrastructure on a size scale that is relevant to surgical applications.

Keywords: (170.3660) Light propagation in tissues; (170.3880) Medical and biological imaging; (170.6510) Spectroscopy, tissue diagnostics; (170.7050) Turbid media; (290.0290) Scattering.

Fig. 1

Schematic of measurement setup for structured light imaging.

Fig. 2

Reflectance intensity expressed on various scales: spatial (left), spatial frequency (middle), dimensionless scattering (right). The top and bottom panels show reflectance for different scattering phase functions (as noted by the $\gamma$-values). The right panel presents the $\gamma$-specific relationship between reflectance and dimensionless scattering.

Fig. 3

(Left) Reflectance intensity vs. dimensionless scattering as simulated by Monte Carlo models (markers) and predicted by the semi-empirical model (lines). Here, different colors indicate different $\gamma$-values of the scattering phase function. (Right) Simulated vs. model-estimated reflectance with the line of unity slope included for visualization of the linearity of the relationship.

Fig. 4

Inversion of demodulated reflectance model using simulated data. (a) Demodulated reflectance spectra sampled at multiple spatial frequencies (color markers) from a medium with background scattering properties, $\gamma \left(\lambda \right)$and ${\mu }_s^{\prime }\left(\lambda \right)$, specified with the color markers in (b) and (c), respectively. (d) Reflectance from spectra in panel (a) plotted vs. dimensionless scattering clearly showing a $\gamma$-specific slope. Here, different symbols indicate wavelength, and colors define spatial frequency. The inversion algorithm returns a fitted estimate of reflectance (shown as black lines in (a)) and estimates optical properties (shown by black markers in (b) and (c)).

Fig. 5

Experimental measurements of structured light in Intralipid phantoms. (a) Sampled lipid volume fractions. (b) Diffuse intensity map ($f_x=0.0m{m}^{-1}$). (c) and (d) show spatially-resolved estimates of ${\mu }_s^{\prime }$and $\gamma$at 730 nm. (e) Spectrally resolved ${\mu }_s^{\prime }\left(\lambda \right)$in each phantom. (f) Corresponding estimates vs. known ${\mu }_s^{\prime }$values. (g) $\gamma \left(\lambda \right)$spectra in each phantom. (h) Corresponding estimates vs. known $\gamma$values.

Fig. 6

Measurement of the scar on the hand of a healthy volunteer. (a) Color photograph. (b) and (c) shows a reflectance remission intensity maps for low ($f_x=0.0m{m}^{-1}$) and high ($f_x=0.5m{m}^{-1}$) spatial frequencies, respectively. (d) and (e) show spatial maps of ${\mu }_s^{\prime }$and $\gamma$at 730 nm. (f) and (g) show${\mu }_s^{\prime }\left(\lambda \right)$and $\gamma \left(\lambda \right)$spectra evaluated at point locations within the scar (red markers) and normal skin (blue markers), with the measurement locations shown by the red and blue arrows in (c).