Functional Near-InfraRed Spectroscopy (NIRS) (fNIRS) is a powerful method for non-invasively measuring cerebral hemodynamics on human subjects. Measurement contamination from superficial tissue which do not represent the brain continues to be an issue. We have proposed the Dual-Slope (DS) approach which is less sensitive to superficial tissue compared to typical Single-Distance (SD) methods. This DS method has been applied to Diffuse Optical Imaging (DOI), designing and constructing a large source-detector array. Previous results suggested that DS phase (Φ) has intrinsically higher sensitivity to the brain compared to SD Intensity (I). To further investigate this finding, on a large population of subjects, a modular DS array is designed. Allowing for collection from different cortical locations during various protocols. These source-detector modules are hexagonal and contain 4 intra-module DS sets. Tessellation greatly expands the number of measurement sets through the creation of inter-module DS sets. In one example, we found a tessellation of 7 modules which generated 94 DS sets. The modules will be used to enable large population DS DOI studies. Here we present one example trace during a 3-back protocol. Examination of the DS traces suggest the expected higher DS Φ sensitivity to cerebral hemodynamics. Further, close observation of the results demonstrate the importance of considering both the Oxy-hemoglobin concentration change (ΔO) and Deoxy-hemoglobin concentration change (ΔD) during such protocols. The results indicated that if one observed only ΔO they would have mis-identified brain activation in the short SD I measurement. Other data-types and ΔD dynamics suggested that the short SD I was dominated by superficial blood-volume instead of the blood-flow dynamics associated with brain activation.
Cerebral hemodynamics, measured with near-infrared spectroscopy, that are coherent with changes in blood pressure can be analyzed with Coherent Hemodynamics Spectroscopy (CHS). Performing diffuse optical imaging during a CHS protocol provides the ability to spatially map cerebral hemodynamics and elucidate their relation to blood flow and blood volume dynamics. Here, we apply frequency-domain dual-slope optical imaging during a CHS protocol to demonstrate the preferential sensitivity to cerebral hemodynamics of dual-slope frequency-domain measurements as compared to traditional single-distance intensity measurements. Specifically, the results show that dual-slope phase measurements recorded hemodynamics that are mostly associated with blood-flow oscillations (as expected in the brain), while single distance intensity measurements recorded hemodynamics that are mostly associated with blood-volume oscillations (as expected in the scalp). Reconstructed dual-slope phase images showed the effect of a spatially variable skull thickness, which can cause heterogeneity within the reconstructed images. Future work will include measurements on multiple subjects and across multiple oscillation frequencies to further investigate the spatial distribution and frequency dependence of cerebral hemodynamic oscillations.
Measurements of absolute optical properties of scattering samples is valuable in the field of bio-medicine and beyond. However, achieving these measurements is complicated by the need for calibration and by the large sample volumes typically needed to meet common diffusion theory models. We propose a method for calibration free absolute measurements of the absorption coefficient (µa) and reduced scattering coefficient (µ 0 s ) in a relatively small volume (the size of a standard cuvette; 45 mm × 10 mm × 10 mm). This method utilizes the previously proposed Self-Calibrating (SC) / Dual-Slope (DS) geometry by placing two light sources on one face of the cuvette and two optical detectors on the opposite face. This leads to the proposal of the Dual-Ratio of the complex Transmittance (Te) (DR{Te}), a method with the same advantages as SC / DS but with less geometric requirements and constraints. Here we confirm that measurement of DR{Te} from a cuvette may be converted to absolute optical properties. We then investigate differences between two choices of forward models for optical measurements in a cuvette, either Monte-Carlo or diffusion theory. A discrepancy between the two was found, which leads to an error of 10 % in µa and no error in µ's when Monte-Carlo was used to generate data and diffusion theory used to invert it. This result highlights the importance of which model is chosen for the inverse problem when this method is implemented in practice. Implementations would need to evaluate different models against ground truths to identify the optimal method for the measurement of absolute optical properties in a cuvette.
We propose novel frequency-domain data types, that show hybrid features to those of phase and AC intensity, i.e., better CNR features than phase data while preserving preferential sensitivity to deep tissue regions (like phase data). We show the CNR features of some of the novel data types in the dual-slope source-detector arrangement in the semi-infinite homogeneous medium and in the two-layer geometries. The results show that these novel data types indeed may have some hybrid features of AC and phase and may have a potential application in imaging of tissue.
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