Effects of improper source coupling in frequency-domain near-infrared spectroscopy.

Currently, there is no widely used method to assess the reliability of contact between optodes and tissue in near-infrared spectroscopy (NIRS). In this study we observe a high linear dependence (R(2) approximately 0.99) of the logarithmic modulation amplitude (ln(I(AC))), average intensity (ln(I(DC))) and phase (phi) on the source-detector distance (SDD) ranging from approximately 20 to 50 mm on human forehead measurements. The regression of phi is clearly reduced in measurements where light leakage occurs, mainly due to insufficient contact between the source optode and tissue. Utilizing this observation, a novel criterion to detect light leakage is developed. The criterion is applied to study the reliability of hemodynamic responses measured on the human forehead when breathing carbon dioxide-enriched air and during hyperventilation. The contrast of the signals is significantly lower in measurements which were adversely affected by light leakage. Furthermore, such unreliable signals at SDDs >or= 50 mm correlate significantly (for [HbO2] p < 0.01 and for [HbR] p < 0.001) better with the signals measured at SDDs < 20 mm. Using this method, poor contact between the source optode and tissue can be detected and corrected before the actual measurement, which enables us to avoid the acquisition of low contrast cortical signals.

Auditory hemodynamic studies of newborn infants using near-infrared spectroscopic imaging.

The noninvasive study of tissue blood volume and oxygenation using near-infrared light is a new and actively developing technology. We have used near-infrared spectroscopic imaging (NIRSI) to study hemodynamic responses on the auditory cortices evoked by auditory stimulation. Ten healthy newborn infants were studied. The otoacoustic emission hearing test was performed for each infant. Pulse oximetry was used to monitor the heart rate during the measurement, video recording was used to monitor motion artifacts, and the eye movements were noted in order to determine sleep stage. A 16-channel frequency-domain optical imaging system developed in our laboratory was used for NIRSI measurements. The stimuli were presented in trains of seven 1 kHz beeps with 700-ms inter-stimulus intervals. The stimulus trains were separated by 25-s silent periods in order to allow for the hemodynamic delay. In 3/8 cases, we obtained a clear bilateral increase in [HbO/sub 2/], and in two additional cases, a clear response on one hemisphere. The mean change in [HbO/sub 2/] was +0.9+/-0.9muM and the mean change in [Hb] was -0.3+/-0.4muM for those channels producing the largest response for each subject. No statistically significant response was found in 3/8 cases.