Functional Brain Imaging by TCSPC-fNIRS Technology

NIRS (Near-Infrared Spectroscopy) techniques are able to record absorption and scattering changes in biological tissue down to a depth of several centimetres. Applied to the human head, the techniques can record dynamic changes in the time-of-flight distributions caused by the heart beat, by oxy- and deoxyhemoglobin changes during brain activity. The haemodynamic response to brain stimulation is on the time scale of a 100 ms to few seconds. Time-resolved detection by TCSPC provides improved separation of scattering and absorption, and better depth resolution than CW techniques. In particular, moment analysis and time-window analysis of the distributions of time of flight (DTOFs) provide a way to distinguish between intracerebral and extracerebral oxygenation changes.
Instruments for functional brain imaging use similar setups as for static brain imaging. However, because fast sequential recording is required, the signals of the detectors are normally recorded by fully parallel TCSPC channels. The architecture of an instrument for dynamic brain imaging is shown in the figure below. The instrument uses two SPC-134 packages to obtain 8 parallel recording channels. The DTOFs at both hemispheres of the head are recorded simultaneously. Each hemisphere has 9 source fibres and 4 detection fibres attached. Two lasers of different wavelengths are multiplexed in time. In addition, a fibre switch periodically switches through the 9 source positions of each hemisphere. For every source position the DTOFs are acquired for 95 ms. One switching cycle through all 9 source positions is completed within 0.9 seconds. The sequences are recorded in the ‘Continuous Flow’ mode of SPC-130, SPC-130EM, or SPC-150N modules.

Architecture of an 8-channel parallel fNIRS system

To give an impression of the of the recorded data, the figure below shows 20 steps of two different sequences of DTOFs, both recorded at a speed of 100 ms per curve.

20 steps of a TOF sequence recorded in the Continuous Flow mode of an SPC-134. Acquisition time 100 ms per curve, ADC resolution 1024 channels. Left: Source-detector distance 5 cm, count rate 4.5 MHz. Right: source-detector distance 8 cm count rate 1.8 MHz

Variations of the optical properties in the brain are derived from the intensity and the first and second moments of the time-of-flight distributions. A typical result of a brain-stimulation experiment is shown in the figure below. As can be seen from the Mean Time of Flight curves, the variations are on the order of a few picoseconds. Therefore, a timing stability of the TCSPC modules of better than a picosecond is required to correctly record the response of the brain to the stimulation.

Left to right: Intensity change, change in the mean time of flight, and change in the variance of the time of flight over the stimulation period. The horizontal bars indicate the periods of stimulation. From bh TCSPC Handbook, Data courtesy of Adam Liebert, Ibib Warshaw.

Depth-resolved intra- and extra-cerebral changes of the oxy- and deoxyhemoglobin concentrations calculated from the data at both wavelength, and all source and detector positions of one hemisphere are shown in the figure below.

Intra- and extra-cerebral changes of oxy- and deoxyhemoglobin concentrations

For more information, related applications and references, please see:

The bh TCSPC Handbook, chapter ‘Diffuse Optical Tomography: DOT, NIRS and fNIRS’

 

References Related to fNIRS
More references in W. Becker, The bh TCSPC Handbook 7ed. (2017)

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