Biomedical Engineering Reference
In-Depth Information
fluorescent beads in the same image plane because the
F
i
/
B
i
of the beads was known from a separate
measurement [1,27].
Since altered collagen production and degradation are hallmarks of reactive stroma [2-4], measurable
differences in collagen
F
i
/
B
i
ratios might be expected in tumor versus healthy tissue. Thus, these results
might seem somewhat surprising, until we consider the finding that
F
i
/
B
i
ratios also did not vary with
apparent fiber diameter [1] (also see the comparative discussion of
F
i
/
B
i
and
F
d
/
B
d
ratios in cancer tissue,
in this section below). A uniform
F
i
/
B
i
despite a changing fiber diameter suggests that the SHG-generating
collagen features maintain a stable axial length/λ
SHG
ratio despite variations in the overall fiber thickness.
At least two possible models may account for how this could occur: (1) Tumor collagen fibrils might only
be appropriately ordered for strong SHG emission within an exterior “shell” region of relatively constant
thickness (and thus relatively uniform
F
i
/
B
i
), with an SHG-deficient center region that varies to affect the
overall fibril diameter (as was found in the rat-tail model [27]), or (2) tumor collagen fibrils produce strong
SHG emission throughout, and are a collection of similar-diameter (i.e., uniform
F
i
/
B
i
), rod-like collagen
“building blocks.” In each case, the uniform
F
i
/
B
i
fibrils assemble in varying numbers to determine vis-
ible collagen fiber thickness [1]. In the Han study, further analysis of the
F
i
/
B
i
ratios (that averaged ~34 for
all tissues), along with confirmatory electron microscopy, found that both healthy and tumorous murine
mammary tissue were composed of core ~70 nm diameter collagen rods (i.e., apparent “building blocks”)
whose diameter remained constant, but which aggregated to form larger diameter fibrils, as per the sec-
ond model above [1]. These results are interesting because they inform us that at least in this particular
animal cancer model, some base level of collagen organizational structure as measured by
F
i
/
B
i
does not
change significantly between normal and cancerous tissues, despite clearly visible higher-order changes in
collagen morphology. Additional studies will help determine whether differences in collagen
F
i
/
B
i
can be
detected in normal versus tumor tissue, across a wider range of animal and human cancer models.
In another study, Nadiarnykh et al. [48] found that away from the surface epithelium, human ovarian
cancer tissue contained fewer cells, and more dense and more regularly patterned collagen, as com-
pared to the control ovarian tissue. These morphologic differences manifested as several quantitative
changes related to scattering properties of collagen SHG, as follows. First, the increased density in can-
cer tissue was reflected, as expected, in higher scattering coefficients (μ
s
) (as calculated by Monte Carlo
simulations), which is the inverse of the MFP, or the distance a photon will travel before undergoing a
direction-changing scattering collision [48]. The readers can refer to Refs. [28,48] for further details on
how these μ
s
values were obtained. Second,
F
d
/
B
d
ratios were decreased in human ovarian cancer tissue
compared to normal controls, which reflected decreased
F
d
SHG in the cancer tissues, consistent with
a
higher
μ
s
(and density) in this population such that initially forward-directed (
F
i
) SHG photons were
more likely to be scattered in different directions. Moreover, as expected for
detected F
/
B
,
F
d
/
B
d
ratios
for each condition increased with increasing tissue depth [48], which photon diffusion theory informs
is consistent with less scattering/redirection of the
F
i
SHG as the distance from the focal point to the
forward tissue boundary becomes smaller, thus increasing
F
d
SHG (and decreasing
B
d
) and therefore
increasing
F
d
/
B
d
with depth into the tissue [48,50]. Overall, the authors found these results support the
idea that collagen's structural and organizational changes in cancer likely arise from new synthesis of
collagen, rather than from reorganization of the existing collagen pools.
17.1.3 technical Advances for Measuring
F/B
The reports described above have advanced methods for determining
F
/
B
SHG in malignant tissues,
to evaluate the significance of these parameters as they predict collagen structure and organization
in cancer diseased versus undiseased states. A series of studies, some of which were discussed above,
have established elegant methods for determining
F
d
/
B
d
in biologic tissues as influenced by tissue depth
and scattering properties [28,29,47-49,51], as well as by calibration methods to extract
F
i
/
B
i
from mea-
sured
F
d
/
B
d
[1,27]. These studies were enabled by an abundance of prior work that has laid the founda-
tion for deciphering the meaning of
F
/
B
ratios as they pertains to collagen's organizational structure
