Biomedical Engineering Reference
In-Depth Information
β varies between -0.5 and 1, and
I
SHGpar
and
I
SHGperp
correspond to the intensities of the SHG signals
polarized parallel and perpendicular to the laser polarization, respectively. A similar PA parameter
has also been expressed as PA = (
I
SHGpar
-
I
SHGperp
)/(
I
SHGpar
+
I
SHGperp
), where PA varies between 0 and 1
[63,64]. In both cases, β or PA = 0 corresponds to less ordered collagen, and β or PA = 1 corresponds
to more ordered collagen [46,51,60,63,64]. Third, we can obtain the helical pitch angle of collagen
(see the Appendix, Figure A.2, and description) [36,37,39,65]. Finally, we can obtain the “tilt” angle
of individual collagen triple helices relative to the collagen fibril axis (see the Appendix, Figure A.3,
and description) [66]. These latter two properties are particularly intriguing because such measure-
ments may enable us to determine by optical imaging approaches, whether fundamental molecular
properties of collagen are altered by different disease states, including cancer. Overall, we see that
SHG intensity under varied polarization states contains not just information with regard to the ori-
entation of the fibril in the XY plane, but from this, we can also extract information regarding the
molecular structure of collagen, which together make this a powerful optical imaging modality for
investigating cancer. See the Appendix for mathematical derivations and more detailed discussions
of these concepts.
17.2.2 SHG Polarization Measurements in tumor tissue
Contrary to the relative abundance of studies on forward and backward SHG in tumors, to our knowl-
edge, only a few studies have carried out polarization-related SHG measurements specifically on tumor
tissue. Using PA of collagen SHG to calculate θ (see the Appendix), Han et al. [1] found no differences in
θ between normal mouse mammary fat pad, and mouse mammary tumors. On the one hand, perhaps,
this is not surprising because one might expect the fundamental aspects of collagen's molecular struc-
ture (e.g., helical pitch angle) to remain unchanged even in diseased tissue, with any SHG-identifiable
changes between normal and tumor tissue resulting from higher-order changes in collagen's organiza-
tion. On the other, it would be intriguing if disease states
did
alter the fundamental aspects of collagen's
molecular structure such as pitch angle, and thus, further measurements of θ in a wider range of cancer
tissues should prove informative.
Other SHG anisotropy studies of normal versus tumor tissues have looked at related polarization
parameters. A series of studies from one group found differences in the
d
22
coefficient (a measure of col-
lagen's second-order nonlinear susceptibility) [63,67] and in the PA parameter (PA, as defined above)
[63], between human tumor tissues xenografted into mice (i.e., human tumor cell lines injected into
mice) and normal mouse tissue taken from the same anatomical region. The authors were able to attri-
bute changes in these parameters to observed changes in collagen amount or organization between the
normal and tumor tissues (xenografted human tumors had much more sparse collagen compared to
anatomically matched normal mouse tissue), suggesting that these measures may provide a quantifiable
means of discriminating collagen organizational changes in cancer.
In a related extensive study of normal versus cancerous human ovarian tissue, Nadiarnykh et al. [48]
found a higher anisotropy parameter (β, as defined above) in cancerous versus normal tissue, which
they attributed to more organized and aligned collagen fibers in the cancer conditions. This finding
was additionally supportive of and consistent with the interpretation of their
F
/
B
data, as discussed
above.
Curiously, at least one study has found that SHG-calculated θ values may differ in collagen from dif-
ferent species [1], and for example, the above
d
22
coefficient is sensitive to dispersion and thus collagen
packing and density [63,67], which in turn is often tissue specific [63] and could conceivably vary across
species. In light of these intriguing results, it would be informative to undertake more comprehensive
studies as to how these and related anisotropy parameters (and
F
/
B
ratios) may vary in collagen taken
from different anatomical regions and from different species, thus facilitating translation of the existing
data across experimental models, cancer types, and species.
