Probe-hosted silicon photomultipliers for time-domain functional near-infrared spectroscopy: phantom and in vivo tests

Abstract

We report the development of a compact probe for time-domain (TD) functional near-infrared spectroscopy (fNIRS) based on a fast silicon photomultiplier (SiPM) that can be put directly in contact with the sample without the need of optical fibers for light collection. We directly integrated an avalanche signal amplification stage close to the SiPM, thus reducing the size of the detection channel and optimizing the signal immunity to electromagnetic interferences. The whole detection electronics was placed in a plastic screw holder compatible with the electroencephalography standard cap for measurement on brain or with custom probe holders. The SiPM is inserted into a transparent and insulating resin to avoid the direct contact of the scalp with the 100-V bias voltage. The probe was integrated in an instrument for TD fNIRS spectroscopy. The system was characterized on tissue phantoms in terms of temporal resolution, responsivity, linearity, and capability to detect deep absorption changes. Preliminary in vivo tests on adult volunteers were performed to monitor hemodynamic changes in the arm during a cuff occlusion and in the brain cortex during a motor task.

Keywords: detector; near-infrared spectroscopy; time domain.

Fig. 1

(a) Block schematic of a detection channel, (b) designed SiPM-based detection channel (bottom in the picture) and the same detector, hosted into a plastic screw holder (top), and (c) EEG standard cap.

Fig. 2

Example of IRF at the two wavelengths.

Fig. 3

(a and d) Linearity and (b and c) accuracy for the absorption (${\mu }_{\mathrm{a}}$) and reduced scattering (${\mu }_{\mathrm{s}}^{\prime }$) coefficients for one channel at 690 nm. Points in panel (a) and (d) are the average of different phantoms, the error bars, and the standard deviations.

Fig. 4

Contrasts as function of the position obtained during (a) the $Y$ and (b) scan, for the 5-mm inclusion, in function of the different temporal windows. The first temporal window (0 to 0.35 ns) is not shown.

Fig. 5

Arterial occlusion of the right arm. Variations of the hemodynamic parameters with respect to the baseline for left (light line), not occluded, and the right (thick line), occluded arm. [(a) oxy-hemoglobin, (b) deoxy-hemoglobin, (c) total-hemoglobin, and (d) saturation].

Fig. 6

Right-hand finger tapping. Variations of the hemodynamic parameters with respect to the baseline for (a and b) the left and (c and d) right hemispheres. The error bars are the standard deviations over the four task repetitions.

Fig. 7

Left-hand finger tapping. Variations of the hemodynamic parameters with respect to the baseline for (a and b) the left and (c and d) right hemispheres. The error bars are the standard deviations over the four task repetitions.