Influence of optical aberrations on depth-specific spatial frequency domain techniques

Abstract

Significance: Spatial frequency domain imaging (SFDI) and spatial frequency domain spectroscopy (SFDS) are emerging tools to non-invasively assess tissues. However, the presence of aberrations can complicate processing and interpretation.

Aim: This study develops a method to characterize optical aberrations when performing SFDI/S measurements. Additionally, we propose a post-processing method to compensate for these aberrations and recover arbitrary subsurface optical properties.

Approach: Using a custom SFDS system, we extract absorption and scattering coefficients from a reference phantom at 0 to 15 mm distances from the ideal focus. In post-processing, we characterize aberrations in terms of errors in absorption and scattering relative to the expected in-focus values. We subsequently evaluate a compensation approach in multi-distance measurements of phantoms with different optical properties and in multi-layer phantom constructs to mimic subsurface targets.

Results: Characterizing depth-specific aberrations revealed a strong power law such as wavelength dependence from ∼40 to ∼10 % error in both scattering and absorption. When applying the compensation method, scattering remained within 1.3% (root-mean-square) of the ideal values, independent of depth or top layer thickness, and absorption remained within 3.8%.

Conclusions: We have developed a protocol that allows for instrument-specific characterization and compensation for the effects of defocus and chromatic aberrations on spatial frequency domain measurements.

Keywords: aberrations; quantitative spectroscopy; spatial frequency domain imaging; subsurface optical properties; tissue simulating phantoms.

Fig. 1

The calculated ratios of each reduced scattering coefficient spectra at all depths relative to when the phantom was in the focused position ($z=0$), dashed lines are the data fitted using Eq. (1).

Fig. 2

Reduced scattering coefficient calculated at each distance and the respective aberration compensated data for the (a) and (d) reference phantom; (b) and (e) test phantom; and (c) and (f) multi-layered phantoms. $z=0$ refers to the ideal, in-focus reduced scattering spectra.

Fig. 3

Absorption coefficient calculated at each distance and the respective recovered aberration compensated data for the (a) and (d) reference phantom; (b) and (e) test phantom; and (c) and (f) multi-layered phantoms. $z=0$ refers to the ideal, in-focus absorption spectra.