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Micro-Magnetic Resonance Elastography
Soft tissue mechanobiology has as its goal the understanding of
the mechanisms by which physical forces regulates cell and tissue
growth, differentiation and divisions. In order to apply stresses
to living system we must develop a safe and noninvasive method for
characterizing the elements of the stress-strain tensor. Mechanical
testing of excised tissue samples, typically cartilage, muscle fibers
and ligaments reveals the complexity of this task. Recent advances
in MR imaging techniques offer the potential of performing controlled
stress-strain measurements on living tissue. These techniques -
termed MR-elastography- are currently under active investigation
at many clinical and biomedical research centers for potential diagnostic
applications. We are currently developing micro-magnetic resonance
elastography (mMRE) in which a miniature mechanical piezoelectric
actuator with localized excitation is integrated with the RF micro-coil
to visualize the three dimensional shear waves in samples at sub-millimeter
resolution, voxel dimension of less than 100 mm x 100 mm x 200 mm
MR Characterization of Tissue-engineered Constructs
Tissue engineering employs biocompatible scaffolds seeded with stem
cells to initiate tissue regeneration that mimics natural healing.
Mesenchymal stem cells (MSCs) extracted from bone marrow can serve
as progenitor cells which differentiate into specific types of tissues
such as bone, adipose tissue, cartilage and muscle. It is important
to monitor the growth and maturation of the regenerating tissues
to ensure proper development of tissue-engineered constructs. High
resolution MRI can be used to visualize tissue-engineered constructs
in a non-invasive manner and MRE has the potential to assess dynamic
biomechanical changes in the developing tissue. Figure below shows
MR axial image of constructs cultured for 6 weeks.
Fractional Analysis of the Bloch Equations
A fractional model provides a more complete model for biological
processes. It expands the conventional integer order derivative
models of complex system behavior to emulate a wider range of relaxation
processes. It is believed that a fractional model will provide a
deeper understanding of the behavior of the magnetization vector
and the relaxation processes.
Microimaging
With the recent accusation of a Bruker Micro5 imaging accessory
for use with the RRC 500 MHz (11.75 T) magnet, high resolution microimages
of small in vitro and ex vivo biological systems will
soon be generated. Having gradient strengths of up to 192 G/cm,
this system will provide the ability to create detailed diffusion
maps for single cells and study compartimental diffusion over very
small lengths. Furthermore, this microimaging upgrade coupled with
home-built microcoils will allow us to perform localized spectroscopy
on biological constructs. Current study is being directed toward
Xenopus laevis oocytes and implantable biocapsules for diabetes
therapy. Both of these specimen presently are used in biochemical
and transgenic investigations
Scroll microcoils
These RF microcoils consist of a single ribbon of conductor
that is wound upon itself in the manner of a paper scroll. This
novel geometry can be patterned using standard microfabrication
techniques that allow for flexibility in construction and greatly
reduced dimensions. Current efforts are directed at evaluating the
line broadening and signal-to-noise characteristics of these coils.
It is hoped that these coils can be miniaturized to investigate
mammalian cells and constructs.
Spiral planar microcoils
Using the principles of the Archimedes' spiral, these planar
NMR coils are being studied to gauge their usefulness in both high
resolution spectroscopy and imaging. With a purely analytical solution,
these coils offer an ideal system to evaluate our techniques for
predicting spectral resolution and sensitivity. Furthermore, efforts
are under way to create a miniaturized array of spiral microcoils
upon which certain cells can be cultured and investigated.
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