How short is short? Optimum source-detector distance for short-separation channels in functional near-infrared spectroscopy

Abstract

In recent years, it has been demonstrated that using functional near-infrared spectroscopy (fNIRS) channels with short separations to explicitly sample extra-cerebral tissues can provide a significant improvement in the accuracy and reliability of fNIRS measurements. The aim of these short-separation channels is to measure the same superficial hemodynamics observed by standard fNIRS channels while also being insensitive to the brain. We use Monte Carlo simulations of photon transport in anatomically informed multilayer models to determine the optimum source-detector distance for short-separation channels in adult and newborn populations. We present a look-up plot that provides (for an acceptable value of short-separation channel brain sensitivity relative to standard channel brain sensitivity) the optimum short-separation distance. Though values vary across the scalp, when the acceptable ratio of the short-separation channel brain sensitivity to standard channel brain sensitivity is set at 5%, the optimum short-separation distance is 8.4 mm in the typical adult and 2.15 mm in the term-age infant.

Keywords: Monte Carlo simulations; functional near-infrared spectroscopy; short-separation channel; source–detector distance.

Fig. 1

Circular regions of interest (ROIs) around the chosen 10–5 locations displayed on the adult head model for (a) top view and (b) lateral view. Each ROI is depicted with a different color and the 10–5 location of reference is defined in the color bar.

Fig. 2

(a) and (b) show the spatial distribution of the sum of the scalp, skull, and cerebrospinal fluid (CSF) thickness (i.e., brain depth) in the adult head model, as displayed on the adult scalp in (a) top view and (b) lateral view. (c) and (d) show the spatial distribution of the sum of the extra-cerebral tissues (ECT) and CSF thickness in a representative infant head model (40-week PMA) displayed on the baby’s scalp: (c) top view and (d) lateral view.

Fig. 3

Median values of (a) the ECT and (b) the CSF thickness for each infant’s age. Younger infants tend to have similar values, which have been grouped into two clusters, cluster 1 from 29- to 33-week PMA and cluster 2 from 34- to 38-week PMA.

Fig. 4

Examples of photon measurement density functions at three different source–detector distances: (a) 30 mm channel, (b) 8 mm channel, and (c) 3 mm channel. The log of the sensitivity is displayed. Only the upper 40 mm of the multilayer slab model is displayed for visualization purposes. The white dashed lines indicate the borders of the tissue layers (between scalp and skull, between skull and CSF, and between CSF and GM, from top to bottom).

Fig. 5

The percentage of overlapping voxels (POV), relative brain sensitivity (RBS), and brain sensitivity (BS) results. (a) and (d) The POV as a function of source–detector separation for adults and infants, respectively. (b) and (e) The BS as a function of source–detector separation for adults and infants, respectively. (c) and (f) The RBS as a function of source–detector separation for adults and infants, respectively. In each case, the red shaded area comprises the range of values exhibited across all models (i.e., across all ROIs in the adult and all clusters/ages in the infant). The black lines are an interpolation of the values obtained in the Common model for the adult and the 40-week PMA model for the infant. Note that Figs. 5(b) and 5(e) include a value for each reference channel (30 mm in adult and 25 mm in infant).

Fig. 6

The optimum source–detector distance for a short-separation channel for different RBS thresholds and both (a) adult and (b) infant models. To determine the optimum short-channel separation, first select what RBS threshold is acceptable, and then select the value for most relevant model for your population and probe location. For example, if a 5% RBS value is acceptable and the study in question is of the adult frontal lobe, the optimum short-channel separation would be $\sim 8\mathrm{mm}$.