Experimental investigation of NIRS spatial sensitivity

Amol V Patil,Reinhard Grebe,Hamid Abrishami Moghaddam,Fabrice Wallois,Javad Safaie

doi:10.1364/boe.2.001478

Amol V Patil, Reinhard Grebe + Show 3 more

Open Access

https://doi.org/10.1364/boe.2.001478

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Abstract

Near infrared spectroscopy (NIRS) is regarded as a potential medical diagnostic technique for investigation of hemodynamic changes. However, uncertainties pertaining to the origin of NIRS signals have hampered its clinical interpretation. The uncertainities in NIRS measurements especially in case of living tissues are due to lack of rigorous combined theoretical-experimental studies resulting in clear understanding of the origin of NIRS signals. For their reliable interpretation it is important to understand the relationship between spatial changes in optical properties and corresponding changes in the NIRS signal. We investigated spatial sensitivity of near infrared optical measurements using an experimental approach. It uses a liquid optical phantom as tissue equivalent, which is explored under robot-control by a small, approximately point like perturbation of desired optical properties, and a NIRS instrument for trans-illumination/reflection measurements. The experimentally obtained sensitivity has been analyzed and compared with numerical simulations. In preliminary experiments we investigated the influence of various optical properties of the medium and of source/detector distances on the spatial sensitivity distribution. The acquired sensitivity maps can be used to define characteristic parameters. As an example, we used a 25% threshold to define a penetration depth measure which provides values in good accordance with published ones. To the best of our knowledge this is the first experimental study of NIRS spatial sensitivity. The presented method will allow in depth experimental investigation of the influence of various conditions pertaining to medium such as optical properties of tissue (scattering and absorption) and of the source/detector configuration.

Highlights

Near infrared spectroscopy (NIRS) was first applied for in-vivo use in 1977 [1]
Experimental approaches using a tissue equivalent optical phantom [21,22,23] provided valuable insights in NIRS signal properties such as effects related to the redistribution of blood volume as resulting from changes in local vessel diameters affecting the optical properties of various tissue layers [24]
For tissue like medium there exists an excellent theoretical framework concerning the spatial sensitivity of NIR optical measurements [26]

Summary

Introduction

NIRS was first applied for in-vivo use in 1977 [1]. Since this discovery of the NIRS window (700–1300 nm) into the body, it has been widely used for measurement of tissue oxygenation. For tissue like medium there exists an excellent theoretical framework concerning the spatial sensitivity of NIR optical measurements [26]. These spatial sensitivity maps have until now neither been experimentally investigated nor explicitly verified. Another set of experiments has been dedicated to the influence of the distance between source and detector on light propagation in a certain medium and on the sensitivity map (Fig. 11). This becomes even more obvious when comparing sensitivity on the perpendicular bisectors of the source-detector connection (Fig. 12 upper part)

Results

Discussion

Conclusion