Optical Characterization of Two-Layered Turbid Media for Non-Invasive, Absolute Oximetry in Cerebral and Extracerebral Tissue

Abstract

We introduce a multi-distance, frequency-domain, near-infrared spectroscopy (NIRS) method to measure the optical coefficients of two-layered media and the thickness of the top layer from diffuse reflectance measurements. This method features a direct solution based on diffusion theory and an inversion procedure based on the Levenberg-Marquardt algorithm. We have validated our method through Monte Carlo simulations, experiments on tissue-like phantoms, and measurements on the forehead of three human subjects. The Monte Carlo simulations and phantom measurements have shown that, in ideal two-layered samples, our method accurately recovers the top layer thickness (L), the absorption coefficient (µa) and the reduced scattering coefficient (µ′s) of both layers with deviations that are typically less than 10% for all parameters. Our method is aimed at absolute measurements of hemoglobin concentration and saturation in cerebral and extracerebral tissue of adult human subjects, where the top layer (layer 1) represents extracerebral tissue (scalp, skull, dura mater, subarachnoid space, etc.) and the bottom layer (layer 2) represents cerebral tissue. Human subject measurements have shown a significantly greater total hemoglobin concentration in cerebral tissue (82±14 µM) with respect to extracerebral tissue (30±7 µM). By contrast, there was no significant difference between the hemoglobin saturation measured in cerebral tissue (56%±10%) and extracerebral tissue (62%±6%). To our knowledge, this is the first time that an inversion procedure in the frequency domain with six unknown parameters with no other prior knowledge is used for the retrieval of the optical coefficients and top layer thickness with high accuracy on two-layered media. Our absolute measurements of cerebral hemoglobin concentration and saturation are based on the discrimination of extracerebral and cerebral tissue layers, and they can enhance the impact of NIRS for cerebral hemodynamics and oxygenation assessment both in the research arena and clinical practice.

Comparison of a layered slab and an atlas head model for Monte Carlo fitting of time-domain near-infrared spectroscopy data of the adult head

“…Furthermore, note that the range of scattering values we studied (4 to 20 cm −1 ) is broader than that typically reported in the literature. Similarly, we (data not shown) and others 31,49 have observed that while TD or FD two-layer models enable reasonable recovery of μ a1 , μ a2 , and μ s1 0 on layered media, in contrast, μ s2 0 can only be estimated with high uncertainty due to the low sensitivity of the TPSF to this parameter.…”

“…30 Paralleling the TD approach, Hallacoglu et al demonstrated the improvement afforded by the FD two-layer geometry 17 with Monte Carlo and phantom data, and applied the technique to in vivo data obtained on the adult head. 31 However, in the few in vivo studies using a layered model of the adult head, 30,31 there was no validation of the retrieved brain values by complementary modalities. More generally, the performances of the twolayer analytical methods have only been assessed through simulations or phantom measurements on simple two-layer slab geometries and not on realistic head structures.…”

Assessment of a multi-layered diffuse correlation spectroscopy method for monitoring cerebral blood flow in adults

“…In clinical practice, the thicknesses of the scalp and skull could be obtained from medical images or possibly measured directly using ultrasound. Alternately, the total scalp/skull thickness could be included as an additional fitting parameter, similar to an approach proposed for measuring cerebral oxygenation by multi-distance frequencydomain NIRS [43]. This would likely require more SDDs than used in the current study and careful attention to the shortest SDD in order to avoid violating the Siegert relationship [44].…”

In-vivo multilaboratory investigation of the optical properties of the human head

“…Monte Carlo) were used. The local approach has been also explored by Hallacoglu et al [27] with a frequencydomain setup. The added value of this work, besides the higher number of volunteers involved, is the application of the different experimental techniques to the same subjects.…”