The contribution of water to the quantitative determination of hemoglobin concentration and saturation by near-infrared spectroscopy in turbid media was investigated. The study consisted of in vitro measurements of an aqueous suspension containing Liposyn, bovine blood, and yeast buffered at pH 7.2. The optical coefficients of the medium (μa∼0.03 to 0.08 cm-1, μ′s ∼ 6 cm-1 at wavelengths of 715 and 825 nm) were similar to those of biological tissue in the near-infrared, and the hemoglobin concentration was about 23 μM. It was possible to reversibly saturate and desaturate hemoglobin in the full range of 0 to 100% by flowing either oxygen or nitrogen through the suspension. In these experimental conditions, water absorption must be taken into account to obtain accurate oxyhemoglobin concentrations and low hemoglobin saturation values. By contrast, the water correction has a small effect on the determination of deoxyhemoglobin concentration and high hemoglobin saturation values. By extrapolating the result to physiological conditions, where water content is lower and hemoglobin content is higher than in the experimental conditions, it was concluded that water absorption should have a smaller effect on the determination of hemoglobin concentrations and saturation in tissues at the wavelengths used in this study. In particular, for hemoglobin concentrations larger than 100 μM, the water correction is less than 5% at saturation values higher than 50%. © 1997 Society of Photo-Optical Instrumentation Engineers.

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.

We present a re-engineered frequency-domain tissue oximeter operating in the near-infrared spectral region. This instrument is based on the multi-distance measurement protocol, which we have implemented in our original design by multiplexing multiple light sources. The new instrument uses intensity modulated (110 MHz) laser diodes emitting at 750 and 840 nm. The laser diodes are coupled to glass optical fibers (600 μm core diameter). The average light intensity delivered to the tissue is about 3 mW. The multiplexing electronics are based on solid state switches that allow for acquisition times per point as short as tens of milliseconds. Our tests on phantoms and in vivo with the new oximeter have shown significant improvement in terms of stability, reliability, and reproducibility with respect to the original prototype. Furthermore, by using optical fibers we achieve a high versatility in the design of the measuring probe, permitting custom design for various tissue contours and different measurements. To verify the improved performance of the new oximeter, we have performed an in vivo test consisting of monitoring the hemoglobin saturation (Y) and concentration (THC) on the calf of 18 healthy volunteers during walking and running routines.

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.