In this paper we discuss temporal and spatial patterns of brain hemodynamics under rest and motor stimulation conditions obtained by functional magnetic resonance imaging and simultaneous fast multi-channel near-infrared spectro-imaging in the human motor cortex. Our data indicate that the main difference between the brain hemodynamics under the repetitive stimulation and the rest conditions is not in the appearance of hemoglobin concentration changes during the stimulations (since fluctuations occur at rest as well), but in their more regular, Le. phase-synchronous with the stimulation behavior.

Frequency-domain near-infrared spectroscopy and imaging offer significant advantages over the continuous-wave method in human brain applications. However, the drawback of existing instruments is a low signal-to-noise ratio for measured phase and modulation depth changes caused by cerebral activation. In this paper we show that in the case of the geometry specific for the activated area in the human brain, the SNR can be significantly improved by increasing the modulation frequency. We present the results of a study performed experimentally using a sub-nanosecond pulsed light source and a spherical absorbing object immersed in a highly scattering solution. We show that changes caused by the absorbing inhomogeneity in both phase and modulation depth increase with frequency and reach maximum values at frequencies between 400 and 1400 MHz, depending on the particular source-detector distance. We also show that for the semi-infinite geometry an increase of the modulation frequency from 100 to 500 MHz can increase the phase signal-to-noise ratio 2-3 times, and the modulation depth SNR up to 10 times.

The aim of our study was to explore the possibility of detecting hemodynamic changes in the brain using frequency-domain near-infrared spectroscopy by exploiting the phase of the intensity modulated optical signal. To obtain optical signals with the highest possible signal-to-noise ratio, we performed simultaneous NIRS-fMRI measurements, with subsequent correlation of the time courses of both measurements. The cognitive paradigm used arithmetic calculations, with optical signals acquired with sensors placed on the forehead. In three subjects we demonstrated correlation between the hemodynamic signals obtained using NIRS and BOLD fMRI.

Although many results on the near infrared spectroscopy and imaging of the human brain have been already published, the signal-to-noise ratio (SNR) of the instrumentation used in these studies has never been systematically optimized for the anatomy of the human head. Assuming a typical anatomical structure of the adult human head, we have studied the SNR of the frequency-domain instrument as a function of the modulation frequency and the source-detector distance. The study was performed both experimentally (using a pulsed laser system) and numerically (using the diffusion approximation and Monte Carlo simulations). For the specified geometry we have found that changes caused by non-homogeneities in the phase and in the ratio of the modulation amplitude to the mean intensity significantly increase with the modulation frequency and exhibit peaks between 400 and 1100 MHz, depending on the source-detector distance. Assuming the shot noise, we have found that the corresponding SNR peaks between 400 and 600 MHz. Increase in the source-detector distance results in the SNR increase only for distances below 30 mm. At larger source-detector distances the higher noise decreases the SNR. Our results show that increasing modulation frequency only to 300 MHz offers a very significant improvement in the functional near-infrared spectro-imaging of the adult human brain compared to the 100 MHz instrument.