The optics of deep optical imaging in tissues using long wavelengths

The ability of long wavelength (>1150 nm) photons to penetrate into a tissue or through a thin tissue section is modeled using Monte Carlo simulations based on the optical properties of breast and brain and is compared with the experiments of Shi et al. [1]. Conclusions are as follows: (1) Longer wavelengths are disadvantageous when imaging with multiply scattered photons due to increased water absorption. (2) Longer wavelengths are advantageous when using a subset of near-ballistic photons with shorter pathlengths to reach a target depth or detector. (2a) This subset increases with a lower scattering coefficient ms, which coincides with a lower anisotropy of scatter, g, when the experimental value for sm = ms(1 − g) is considered to be constant. Longer wavelengths (e.g., 1700 nm) that exceed the sub-μm size distribution of scatterers within a soft tissue yield lower g and ms. (2b) The narrow versus broad balance of the scattering function (g2) can vary even though the forward versus backward balance (g or g1) is held constant. An increasingly narrow scatter function increases photon penetration. (2c) At long wavelengths, the absorption coefficient matches or exceeds the reduced scattering coefficient in magnitude, causing multiply scattered photons to be selectively absorbed relative to near-ballistic photons with the shortest path to a target depth, which improves imaging as reported by Yoo et al. [3]. 4.1 IntroductionThere have been tremendous strides in a variety of optical imaging modalities, some of which, e.g., absorption, reflectance, Raman scattering, IR spectroscopy, photoacoustics, might benefit by using longer wavelengths for deep imaging [1]. Deep imaging involves imaging that does not use diffuse multiply scattered photons, but instead uses near-ballistic photons whose trajectories within a tissue are only partially deflected by scattering. Deep imaging should be sensitive to the optical absorption by tissue water and the optical scattering by tissue structure. At long wavelengths above ~1150 nm, the absorption coefficient of water is comparable to or greater than the reduced scattering coefficient of soft tissues (ma ≥ sm). This chapter considers the mechanisms involved in deep imaging using such long wavelengths.