Biomedical Engineering Reference
In-Depth Information
6.5 Discussion
The enabling aspect of the SHG contrast results from the quasicoherence of the process as well as the
intrinsic symmetry constraints, yielding sensitivity to morphological and physical properties that may
in general be different for normal and diseased tissues. For example, the axial directionality response
(Figures 6.6 and 6.10 for the oim and ovary work, respectively) arises, in part, from the initial emission
directionality, which is directly related to the fibrillar assembly of the tissue in terms of fibril size as well
as organization. The measured
F/B
versus depth is also governed by the secondary filter effects on the
generated signal, which are statistically different for the oim and WT. The axial dependence of the SHG
intensity (Figures 6.8 and 6.12) provides an additional piece of the metric for tissue characterization as
it is governed by SHG conversion efficiency, as well as the primary and secondary filter effects during
subsequent propagation. The conversion efficiency is directly related to the organization of the tissue,
such that at the same collagen concentration, uniformly aligned fibrils will yield a larger second-order
response, χ
(2)
than a more random assembly. We stress it is not possible to generally associate more ran-
domness with diseased states. While the collagen in the OI case was more random and produced weaker
SHG (~threefold), the ECMs in the malignant ovaries were more organized, of higher density, and had
fourfold brighter SHG intensity
We also point out that the extent or regularity in the order, or in the other limit, the randomness, is
not directly reflected in the scattering coefficient, which is essentially a measure of density. While the
scattering anisotropy,
g
, is related to the order, the SHG conversion efficiency is of more direct relevance
due to the inherent need for nonrandom assembly to satisfy the second-order asymmetry constraint.
However, the simulations still require bulk optical parameters at both the fundamental and second
harmonic wavelengths. Thus, we submit that when taken together, the SHG signatures (initial emission
direction, conversion efficiency, and subsequent propagation) more directly and completely reflect the
tissue organization than possible by consideration of the bulk optical parameters or SHG properties
alone. Lastly, we note that the Monte Carlo simulations are essential in decoupling the factors that give
rise to the 3D SHG data. This not only provides physical insight into tissue structure but may also iden-
tify the most sensitive factors in discriminating normal and diseased tissues and thus have diagnostic/
prognostic value.
While we utilized this analysis for the murine model of OI and human ovarian cancer, we submit that
the approach is a general means to analyze tissue structures. For example, we used this same combina-
tion of experiments (imaging and measurement of bulk optical properties) simulations to investigate
the mechanism of optical clearing in skeletal muscle and tendon (see Chapter 8). In those efforts, we
showed consistency with the extent of clearing and the reduction of scattering in thick tissue slices.
While experimentally and computationally intensive, our integrated approach should be applicable for
comparative analysis between any type of tissues that are composed of collagen. As a result, a wide range
of pathologies such as cancer, connective tissue disorders, fibrosis, skin damage, and pathologies of the
cornea could be analyzed. In contrast, signal processing schemes to date rely on changes in fiber align-
ment to provide quantitative discrimination between tissues and it remains to be seen how generalizable
more advanced approaches like texture analysis with feature selection will be.
Through our exploratory studies derived from a basic science perspective, we have identified a col-
lection of physical/structural properties of the ECM that change in both the OI model and in human
ovarian cancer. While these were
ex vivo
studies with no opportunity for follow-up, the methods could
be used to monitor the status of disease and/response to treatment. For example, for human OI patients,
we foresee the method as being especially useful in monitoring the status of individual patients relative
to their initial screen, where patients would already have a genetic profile. Thus, imaging several areas
of skin would provide a reference point for future screenings. Additionally, this approach may permit
monitoring the efficacy of treatment. For example, the effect of treatment with bis-phosphonates has
been typically performed by bulk bone density and mineralization measurements [46]. Perhaps more
insight into the action of such drugs can be gained by analyzing the fibrillar structure of the matrix at
