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.

3D level set reconstruction of model and experimental data in Diffuse Optical Tomography.

The level set technique is an implicit shape-based image reconstruction method that allows the recovery of the location, size and shape of objects of distinct contrast with well-defined boundaries embedded in a medium of homogeneous or moderately varying background parameters. In the case of diffuse optical tomography, level sets can be employed to simultaneously recover inclusions that differ in their absorption or scattering parameters from the background medium. This paper applies the level set method to the three-dimensional reconstruction of objects from simulated model data and from experimental frequency-domain data of light transmission obtained from a cylindrical phantom with tissue-like parameters. The shape and contrast of two inclusions, differing in absorption and diffusion parameters from the background, respectively, are reconstructed simultaneously. We compare the performance of level set recons uction with results from an image-based method using a Gauss-Newton iterative approach, and show that the level set technique can improve the detection and localisation of small, high-contrast targets.

Significance of background optical properties, time-resolved information and optode arrangement in diffuse optical imaging of term neonates.

The significance of accurate knowledge of background optical properties and time-resolved information in reconstructing images of hemodynamic changes in the neonatal brain from diffuse optical imaging data was studied using Monte Carlo (MC) simulation. A segmented anatomical magnetic resonance (MR) image and literature-derived optical properties for each tissue type were used to create a voxel-based anatomical model. Small absorbing perturbations were introduced into the anatomical model to simulate localized hemodynamic responses related to brain activation. Perturbation MC (pMC) was used as the primary method of image reconstruction. For comparison, reconstructions were also performed using the finite element method (FEM) to solve the diffusion approximation (DA) to the radiative transfer equation (RTE). The effect of optode layout was investigated using three different grids. Of the factors studied, the density of the optode grid was found to have the greatest effect on image quality. The use of time-resolved information significantly improved the spatial accuracy with all optode grids. Adequate knowledge and modeling of the optical properties of the background was found to significantly improve the spatial accuracy of the reconstructed images and make the recovery of contrast of absorption changes more consistent over simplified modeling. Localization accuracy of small perturbations was found to be 2-3 mm with accurate a priori knowledge of the background optical properties, when a grid with high optode density (>1 optode cm(-2)) was used.

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.