Assessment of the frequency-domain multi-distance method to evaluate the brain optical properties: Monte Carlo simulations from neonate to adult

Abstract

The near infrared spectroscopy (NIRS) frequency-domain multi-distance (FD-MD) method allows for the estimation of optical properties in biological tissue using the phase and intensity of radiofrequency modulated light at different source-detector separations. In this study, we evaluated the accuracy of this method to retrieve the absorption coefficient of the brain at different ages. Synthetic measurements were generated with Monte Carlo simulations in magnetic resonance imaging (MRI)-based heterogeneous head models for four ages: newborn, 6 and 12 month old infants, and adult. For each age, we determined the optimal set of source-detector separations and estimated the corresponding errors. Errors arise from different origins: methodological (FD-MD) and anatomical (curvature, head size and contamination by extra-cerebral tissues). We found that the brain optical absorption could be retrieved with an error between 8-24% in neonates and infants, while the error increased to 19-44% in adults over all source-detector distances. The dominant contribution to the error was found to be the head curvature in neonates and infants, and the extra-cerebral tissues in adults.

Keywords: (110.3080) Infrared imaging; (170.3660) Light propagation in tissues; (170.5280) Photon migration.

Fig. 1.

Segmented head models: 3D and 2D axial views. Segmentations consist of 4 tissue types: superficial layers (superf) of scalp and skull, CSF, gray matter (GM), and white matter (WM). Sources and detectors are represented by red crosses and blue dots respectively. The black bar (3 cm) in the bottom right of the image indicates the scale of all 2D slices.

Fig. 2.

Retrieved absorption coefficients of a homogeneous slab (calibrated with μ_a = 0.1cm⁻¹). Calibrated (dashed) and uncalibrated (solid) are compared to true (dash-dotted) values with respect to SDs for different homogeneous coefficients of absorption [cm⁻¹]: 0.05 (blue); 0.10 (green); 0.15 (red); 0.20 (cyan); 0.25 (magenta); 0.30 (yellow).

Fig. 3.

Estimated μ_a,brain when the head is treated as a homogeneous medium, i.e. when μ_a,superf = μ_a,brain = [0.05,0.06, . . . ,0.29,0.3]cm⁻¹. For each sub-figure, solid lines represent the 7 groups of SDs used in the estimation (legend is shown in top left sub-figure) and dotted lines represent the 1-ratio between estimated and true brain absorption values.

Fig. 4.

Uncertainty and retrieved absorption for three particular cases of the homogeneous head: μ_a,superf = μ_a,brain = [0.05,0.15,0.3]cm⁻¹ for the 7 sets of SDs.

Fig. 5.

Averages and standard deviations of the relative error on retrieved μ_a,brain over 26 combinations of head absorption [0.05,0.06, . . . ,0.29,0.3]cm⁻¹, for the 7 sets of SDs.

Fig. 6.

Brain absorption coefficients μ_a,brain estimated across subject age (row-wise) and for two values of superficial absorption μ_a,superf = [0.1,0.2]cm⁻¹ (column-wise). For each sub-figure, solid lines represent the 7 groups of SDs used in the estimation (legend is shown in bottom left sub-figure), dotted lines represent the 1-ratio between estimated and true brain absorption values and dash-dotted lines represent the superficial absorption.

Fig. 7.

Two-tissue head model: averages and standard deviations of the relative error in retrieved μ_a,brain over 52 combinations of μ_a,superf = [0.1,0.2]cm⁻¹ and μ_a,brain = [0.05,0.06, . . . ,0.29,0.3]cm⁻¹ for the 7 sets of SDs.

Fig. 8.

Brain absorption coefficients μ_a,brain estimated across subject age (row-wise) and for two values of superficial absorption of μ_a = [0.1,0.2]cm⁻¹ (column-wise) when the CSF tissue is taken into account. For each sub-figure, solid lines represent the 7 groups of SDs used in the estimation (legend is shown in bottom left sub-figure), dotted lines represent the 1-ratio between estimated and true brain absorption values and dash-dotted lines represent the superficial absorption. Note the presence of the axial slice of the newborn (NB) left and right hemispheres superimposed with the optical probe.

Fig. 9.

Three-tissue head model: averages and standard deviations of the relative error on retrieved μ_a,brain over 52 combinations of μ_a,superf = [0.1,0.2]cm⁻¹ and μ_a,brain = [0.05,0.06, . . . ,0.29,0.3]cm⁻¹ for the 7 sets of SDs.