Optimization of the signal-to-noise ratio of frequency-domain instrumentation for near-infrared spectro-imaging of the human brain

Abstract

Frequency-domain near-infrared spectro-imaging offers significant advantages over the continuous-wave method i n human brain applications. However, the drawback of existing instruments is a low signalto-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 two studies: one performed experimentally using a subnanosecond pulsed light source and a spherical absorbing inhomo geneity immersed in a highly scattering solution, and the other performed numerically using Monte Carlo simulations of light transport in an MRIbased digital phantom of the adult human head. 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 human head geometry an increase of the modulation frequency from 100 to 500 MHz can increase the phase SNR 2-3 times, and the modulation depth SNR up to 10 times.

A spatial and temporal comparison of hemodynamic signals measured using optical and functional magnetic resonance imaging during activation in the human primary visual cortex

“…The optical fibers were "ferruled" using plastic tubing, and the frame of the optical probe was constructed from polyurethane which has appropriate mechanical properties and produces insignificant artifacts in the MR images. The topology of the probe was designed so that the optical channels overlapped and the distribution of source-detector distances covered the optimal range (Toronov et al, 2003a), between approximately 20 and 30 mm, as shown in Fig. 1.…”

“…This allows comparisons of the spatial and temporal characteristics of the fMRI signals with optically-derived hemodynamic changes to clarify the contributions of different physiological parameters to the fMRI signals. Several studies of functional activations have been performed in the human motor cortex by simultaneous fMRI and NIRS (Toronov et al, , 2001b(Toronov et al, , 2003a. In particular it was found that, in the human motor cortex, the temporal dependence of the deoxyhemoglobin concentration was the factor most closely related to the BOLD fMRI signal.…”

An ideal-observer framework to investigate signal detectability in diffuse optical imaging

“…We have performed our analysis with a Poisson noise model, since this is often the major noise source in DOT systems [19,34]. Our analysis can be extended to detectors that have other noise sources.…”

“…The major noise source in DOT is often the Poisson-distributed shot noise [19,34]. Letḡ m (μ μ μ) denote the noiseless mean image data as a function of the scattering and absorption coefficient vectors μ μ μ = {μ s μ s μ s ,μ a μ a μ a }.…”

Spatial and temporal hemodynamic study of human primary visual cortex using simultaneous functional MRI and diffuse optical tomography

“…The topology of optodes is designed so that the optical channels are overlapped and the distribution of source-detector distances evenly covers the optimal range -between approximately 20 and 30 mm [4]. The optical probe has 16 pairs of 400-µm-diameter core plastic clad multimode silica source fibers (FT-400-EMT, Thorlabs) and 4 detector fiber bundles (Fiberoptics Technology).…”