Optimization of the frequency-domain instrument for the near-infrared spectro-imaging of the human brain

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