Simultaneous measurement of time-domain fNIRS and physiological signals during a cognitive task

Functional near-infrared spectroscopy (fNIRS) is a commonly used technique to measure the cerebral vascular response related to brain activation. It is known that systemic physiological processes, either independent or correlated with the stimulation task, can influence the optical signal making its interpretation challenging. The aim of the present work is to investigate the impact of task-evoked changes in the systemic physiology on fNIRS measurements for a cognitive paradigm. For this purpose we carried out simultaneous measurements of time-domain fNIRS on the forehead and systemic physiological signals, i.e. mean blood pressure, heart rate, respiration, galvanic skin response, scalp blood flow (flux) and red blood cell (RBC) concentration changes. We performed measurements on 15 healthy volunteers during a semantic continuous performance task (CPT). The optical data was analyzed in terms of depth-selective moments of distributions of times of flight of photons through the tissue. In addition, cerebral activation was localized by a subsequent fMRI experiment on the same subject population using the same task. We observed strong non-cerebral task-evoked changes in concentration changes of oxygenated hemoglobin in the forehead. We investigated the temporal behavior and mutual correlations between hemoglobin changes and the systemic processes. Mean blood pressure (BP), galvanic skin response (GSR) and heart rate exhibited significant changes during the activation period, whereby BP and GSR showed the highest correlation with optical measurements

Non-contact time-resolved diffuse reflectance imaging at null source-detector separation

We report results of the proof-of-principle tests of a novel non-contact tissue imaging system. The system utilizes a quasi-null source-detector separation approach for time-domain near-infrared spectroscopy, taking advantage of an innovative state-of-the-art fast-gated single photon counting detector. Measurements on phantoms demonstrate the feasibility of the non-contact approach for the detection of optically absorbing perturbations buried up to a few centimeters beneath the surface of a tissue-like turbid medium. The measured depth sensitivity and spatial resolution of the new system are close to the values predicted by Monte Carlo simulations for the inhomogeneous medium and an ideal fast-gated detector, thus proving the feasibility of the non-contact approach for high density diffuse reflectance measurements on tissue. Potential applications of the system are also discussed. © 2011 Optical Society of America

Separation of indocyanine green boluses in the human brain and scalp based on time-resolved in-vivo fluorescence measurements

Non-invasive detection of fluorescence from the optical tracer indocyanine green is feasible in the adult human brain when employing a time-domain technique with picosecond resolution. A fluorescence-based assessment may offer higher signal-to-noise ratio when compared to bolus tracking relying on changes in time-resolved diffuse reflectance. The essential challenge is to discriminate the fluorescence originating from the brain from contamination by extracerebral fluorescence and hence to reconstruct the bolus kinetics; however, a method to reliably perform the necessary separation is missing. We present a novel approach for the decomposition of the fluorescence contributions from the two tissue compartments. The corresponding sensitivity functions pertaining to the brain and to the extracerebral compartment are directly derived from the in-vivo measurement. This is achieved by assuming that during the initial and the late phase of bolus transit the fluorescence signal originates largely from one of the compartments. Solving the system of linear equations allows one to approximate time courses of a bolus for each compartment. We applied this method to repetitive measurements on two healthy subjects with an overall 34 boluses. A reconstruction of the bolus kinetics was possible in 62% of all cases

Cerebral perfusion in acute stroke monitored by time-domain near-infrared reflectometry

Though potentially relevant for monitoring of acute stroke, even specialized stroke units do not provide continuous methods to determine cerebral perfusion at the bedside. We present patient measurements on cerebral perfusion in ischemic stroke applying optical bolus tracking. To this end, our portable time-domain near-infrared reflectometer has been optimized and technically approved for clinical studies by a notified body. We used data analysis based on statistical moments of measured time-of-flight distributions of photons. Selective sensitivity to deep absorption changes and a suitable representation of cerebral signals is associated with the suppression of movement artifacts in severely affected patients. The proposed technique offers a unique possibility for a frequently repeatable monitoring of cerebral blood flow during acute and subacute cerebral ischemia directly at the bedside

Non-contact time-domain scanning brain imager: results of proof ofprinciple tests

We report on the development of a scanning non-contact brain imager, based on a novel technique in time-resolved nearinfrared spectroscopy, i.e. the null source-detector distance approach. Our concept is designed to image an area of about 10 cm2 with small adjustable scanning steps, i.e. a high density of mapping points can be realized. The feasibility of the proposed method was tested with a single-point confocal optical setup without beam scanning so far. A set of test measurements was performed on a liquid phantom with a small black polyvinyl chloride (PVC) cylinder as a target, which was translated in X direction to emulate the optical scanning and estimate lateral spatial resolution, and in Z direction to estimate the depth sensitivity of the instrument. The problem of dominance of early photons at null sourcedetector separation was solved by applying a fast time-gated detector to detect late only photons. Two fast-gated detectors, a newly developed state-of-art time-gated single-photon avalanche photodiode (tgSPAD) and commercially available fast-gated intensified CCD (iCCD) camera, were compared against each other. It was shown that, due to its better dynamic range, the tgSPAD is capable to detect later photons than the iCCD camera, and hence, a scanning system equipped with the time-gated SPAD has better depth sensitivity. Thus the time-gated SPAD is the detector of choice for further development of the non-contact confocal brain scanner

Non-contact time-resolved diffuse reflectance imaging at null source-detector separation

We report results of the proof-of-principle tests of a novel noncontact tissue imaging system. The system utilizes a quasi-null sourcedetector separation approach for time-domain near-infrared spectroscopy, taking advantage of an innovative state-of-the-art fast-gated single photon counting detector. Measurements on phantoms demonstrate the feasibility of the non-contact approach for the detection of optically absorbing perturbations buried up to a few centimeters beneath the surface of a tissuelike turbid medium. The measured depth sensitivity and spatial resolution of the new system are close to the values predicted by Monte Carlo simulations for the inhomogeneous medium and an ideal fast-gated detector, thus proving the feasibility of the non-contact approach for high density diffuse reflectance measurements on tissue. Potential applications of the system are also discussed

Multi-laboratory investigation of the optical properties of the human head

The optical properties of the human head in the range from 600 nm to 1100 nm have been non-invasively in-vivo investigated by various research groups using different diffuse optics techniques and data analysis methods