Optimization of the phase and modulation depth signal-to-noise ratio for near-infrared spectroscopy of the biological tissue

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.