Technical Note
A note on laser penetration in nanoshell deposited tissue

https://doi.org/10.1016/j.ijheatmasstransfer.2009.02.014Get rights and content

Abstract

The radiative intensity profiles of biological tissues infused with gold nanoshells and illuminated with collimated radiation are compared. A one-dimensional radiative transport model using the P1 approximation is used to calculate the collimated and diffuse components of incident radiation in a series of semi-infinite slabs representing nine distinct human tissue media. Each tissue model is subject to 633-nm collimated radiation on one end under four concentrations of nanoshell embedment, with theoretical nanoshells tuned to reach peak absorption at 630 nm. The penetration depth each component is shown to be highly influenced by the base tissue extinction, and the profile morphology can be visibly categorized into one of two groups: low scattering, with scattering coefficients between 4000 and 8000 m−1, and high scattering, with scattering coefficients in the range of 30,000 and 40,000 m−1. Increasing nanoshell density up to 7 × 1016 m−3 for both types is shown to decrease penetration depth and lower radiative intensity for the diffuse component while not affecting the penetration depth or intensity of the collimated component.

Introduction

Gold nanoshells have the potential to be an incredible asset to photothermal therapy for induced hyperthermia in the treatment of cancers. It has been shown in model and experiment that increased concentrations of gold nanoshell particles in biological tissues exposed to laser radiation lead to shorter heating times and increased precision in the targeting of cancers for therapeutic heating [1], [2]. The degree of nanoshell addition necessary for hyperthermia and its effect on light penetration and subsequent heating have also been shown in [1] to be highly dependent on the optical and thermal properties of the host tissue alone. This leads to the need for catering specific therapies for each of the different tissue types, and possibly, tissue lengths needed for a given tumor size.

The purpose of this paper is to illustrate the different radiative intensity profiles resulting from the application of the same collimated laser source to different tissue media. The P1 approximation of radiative transfer theory is used to take the absorption and scattering properties of a given one-dimensional, gray, participating medium and calculate the collimated and diffuse components of incident radiation resulting from a beam of laser radiation penetrating from one end. The changes in intensity profiles resulting from the addition of nanoshells are also shown for each tissue. These profiles serve to illustrate the relative amounts of incident radiation delivered throughout the tissue by means of continued collimated radiation by the illuminating laser and by diffuse radiation resulting from the outscattering and subsequent inscattering by tissue surrounding the one-dimensional path of study. Knowledge of radiative intensity as a function of depth in the medium is pertinent to any therapy or imaging technique that uses lasers in the human body. Furthermore, a rudimentary understanding of different tissue media, their optical properties, and how these properties comparatively result in different optical penetration is highly advantageous to researchers and oncologists alike.

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Analysis

Previous studies have shown that the optical properties of a participating medium can be altered by adding a homogenous array of gold nanoshells to its matrix [3]. As light enters a nanoshell-embedded slab, propagating light is absorbed and scattered by both the host medium and the embedded nanoshells. The extent of absorption and scattering made by these two constituents can be analyzed linearly, since radiative transfer theory shows that the radiative intensity of a propagating light through

Results

The incident radiation profiles of nine tissues resulting from a 10,000 W/m2 exposure are presented in Fig. 2. Here the intensity is measured in a non-dimensional term, Φ(τλ), composed of the ratio of the local incident radiation divided by the input laser power. The tissues are ordered in order of similarity in shape and length of optical penetration.

Upon initial inspection, we see that subcutaneous fat, adult brain gray matter, skeletal muscle, and rib bone, (a)–(d), each have moderately broad

Conclusion

The one-dimensional radiative intensity profiles of nine tissues following identical exposure to collimated radiation have been simulated using a P1 approximation-based code. Of the tissues presented, four exhibit optical properties characterizing them as low scattering, and five possess optical properties characterizing them as high scattering. It has been shown that the low scattering tissues exhibit a broad, lengthened penetration profile with decreasing intensity and penetration depth

Acknowledgements

This work was funded by the Alliances for Graduate Education and the Professoriate (AGEP) program through the National Science Foundation Grant HRD-0450363.

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