Biomedical Engineering Reference
In-Depth Information
TABLE 6.1 Bulk Optical Parameters for Oim and WT Skin Measured at the Fundamental
and SHG Wavelengths
Oim 457 nm Oim 900 nm WT 457 nm WT 900 nm
μ s (cm −1 ) 177 ± 17 130 ± 13 302 ± 45 106 ± 19
μ a (cm −1 ) 1.5 ± 0.1 1.9 ± 0.1 1.8 ± 0.4 1.1 ± 0.3
g 0.65 ± 0.04 0.80 ± 0.02 0.80 ± 0.02 0.83 ± 0.01
μ s (cm −1 ) 57 ± 9 26 ± 3 61 ± 10 18 ± 3
Source: Reprinted from Biophys. J . 94, Lacomb, R., O. Nadiarnykh, and P. J. Campagnola, Quantitative
SHG imaging of the diseased state osteogenesis imperfecta: Experiment and simulation, 4504-4514,
Copyright (2008), with permission from Elsevier.
of 0.65 ± 0.04 and 0.80 ± 0.02. These values at 457 nm are statistically significantly different ( p = 0.03),
whereas at the fundamental wavelength, they are statistically similar ( p = 0.17). We also point out that
the measured g values are to be considered effective, as the 100-micron-thick biopsies include both der-
mal and adipose layers and can support 1-2 scattering events; thus, the actual values might be somewhat
higher. This is because the convolution of single scattering with additional scattering events results in a
broader distribution of measured angles and lower g . However, as we have not observed significant dif-
ferences in extracted values for tissues 100-200 microns in thickness, we believe the extracted g values
are attributed to a dominant contribution from the dermis. Moreover, the measurements were made to
provide a comparison between the tissues, and as they were performed in a self-consistent manner, this
does not affect the subsequent analysis.
he dual integrating sphere setup was used to extract μ′ and μ a . The analysis was achieved by using a
multilayer inverse Monte Carlo simulation that also considered the scattering within the adipose layer
underlying the dermis. Values for the adipose were measured separately in the absence of the dermis and
verified by comparison with published values [39]. By this analysis, we find reduced scattering coefficients
(shown in Table 6.1) that are in the same range as those determined in human skin by Tuchin [40]. We
observe insignificant absorption at these wavelengths, which are on the red side above the type I collagen
autofluorescence band. The μ a values are to be considered upper bounds, as these measurements were
near the noise floor. Thus, the mean free path (MFP) is effectively the inverse of scattering coefficient.
Using separately measured g , values for μ s are determined by Equation 6.1. We observe that the oim
tissue is statistically less scattering ( p = 0.009) and more isotropic ( p = 0.03) than the WT at the SHG
wavelength and we interpret these results to be indicative that the matrix of the former is less densely
packed. This is consistent with the smaller, shorter fibrils observed for the oim skin in Figure 6.5. We
suggest that this approach relating morphological disorder based on bulk optical parameters in con-
junction with the SHG image data is further consistent with the clinical presentation of a weakened
matrix for the oim tissue. We note that these values at 900 nm were not statistically different, indicating
that a descriptive metric of tissues must consist of more information than single wavelength measure-
ments of the bulk optical parameters. In the next two sections, we show how a combination of 3D SHG
imaging, the bulk optical parameters, and Monte Carlo simulations provides such a metric.
6.4.1.2 Forward/Backward SHG versus Depth: experiment and Simulation
Here, we use the measured forward-backward intensity ratio of the SHG signal as part of the overall
metric to differentiate oim and WT skin. This measurement arises from the SHG directional emission
creation ratio ( F SHG /B SHG ) and the secondary filter effects on the subsequent SHG propagation through
the tissue. Through simulation, we will extract the respective values of F SHG /B SHG , which as described
previously are characteristic of the tissue but cannot be directly measured.
The experimentally measured F/B (composed of both components) versus depth plots for oim and WT
skin are shown in Figure 6.6. These data result from 14 mice (7 each), with 5-10 3D stacks acquired from
separate regions of the dermis. We observe that at all depths the oim is more forward directed, which we
 
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