Biomedical Engineering Reference
In-Depth Information
to the attenuation coeffi cient measured at a similar location within the specimen. Because the
attenuation coeffi cient correlates to the material density, the resultant 2-D maps reveal the mate-
rial phases within the specimen, depending on the scanning resolution, which ranges from 1 to
50 μm. A 3-D reconstruction allows a close-up view of any specifi c location, thus the observation
of pore shape, the measurement of pore size, and the thickness of strut or wall can be conducted
in these close-ups.
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A scaffold made from polymeric materials has low x-ray attenuation, but the contrast between
air and polymer is suffi ciently high to differentiate them. Therefore, µ-CT has extra applications for
the assessment of scaffolds' architecture, such as pore size, porosity, pore interconnectivity, quanti-
fi cation of microarchitectural parameters, and correlation with compressive mechanical properties
of scaffolds.
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In the context of TE, µ-CT scanning has mainly been used for monitoring mineral-
ization within 3-D scaffolds
in vitro
, since bones have higher x-ray attenuation coeffi cient than soft
tissue or polymeric materials. Numerous studies have reported the mineralization degree within
the 3-D construct by µ-CT scanning method. A shortcoming of µ-CT scanning is that in most of
the cases, the analysis has to be performed in a dry state since most scaffold materials have similar
attenuation coeffi cient to water. This renders the online monitoring of tissue turnover fairly diffi cult
to accomplish. Even though the online follow-up of scaffolds mineralization by µ-CT has been
reported,
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one should bear in mind that it was achieved by using low-voltage x-rays during short
periods of time. Such conditions may not be suffi cient to achieve the degree of detail required in
some cases. Furthermore, the cellular response to such harsh conditions in long-term studies has
not been reported so far.
3.4.3 O
PTICAL
C
OHERENCE
T
OMOGRAPHY
Optical coherence tomography (OCT)
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has recently emerged as a promising imaging technique,
mainly for medical applications. The original development of OCT was for transparent tissues,
such as cornea and retinal tissues.
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The current OCT technology enables nontransparent soft and
hard tissues to be examined
in vivo
,
137
for example, the skin,
138
gastrointestinal tract,
139
nervous
systems,
140
cartilage,
141
and respiratory tract.
142
OCT is an imaging modality that can be used to
study tissues or biological systems
in vivo
with near-histological, ultrahigh resolution. OCT is an
interferometric technique. The light backscattered from the sample interferes with the light from
the reference arm when the delay is within the coherence length of the source. The variation of
the optical path in the reference arm allows in-depth scanning of the sample. Clearly, its features
provide enormous potential to overcome a number of limitations currently experienced in TE for
monitoring scaffold architecture and also tissue-engineered constructs.
In the last decade, the instrumentation of OCT has been continuously investigated and devel-
oped. The resolution, the penetration depth, and functionality of OCT have been improved dramati-
cally. The image penetration depth of OCT determined by the scattering coeffi cient of the sample
can be up to 2-3 mm in tissue. Depending on the light source, image resolutions of 1-15 µm have
been achieved, which are at the same order of cell dimension, showing the potential to view cell
morphology by this modality. Generally, a set of computer-controlled mirrors move the beam over
the sample in
x
- and
y
-directions, allowing a 3-D reconstruction of the specimen. Most importantly,
analysis with OCT can be realized in real time and in a nondestructive manner, without the need
for excision and processing of specimens, providing a great amount of quantitative data regarding
the tissue morphology.
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Some studies that focus the use of OCT in TE have been reported.
144
Yang et al.
145
have used
this technique to image macrostructural morphology and delineate the morphology of cells and
constructs in a developing
in vitro
engineered bone tissue. The authors have shown the potential
for the use of OCT in noninvasive monitoring of cellular activities in 3-D developing engineered
tissues. Since OCT provides both quantitative and qualitative data, it can further be adjusted to
monitor scaffolds' degradation or erosion.
