Many musculoskeletal soft tissues,
such as tendons, ligaments, meniscus and cartilage are inhomogeneous. Hence,
during mechanical loading it is likely that a nonuniform strain pattern occurs
within the tissue. These nonuniform strain patterns may assist in successful
load transmission and minimize rupture of the tissue during physiological
loading. Determination of local material properties will likely be important
for successful function and design of tissue engineered replacements. In the
late 1980’s uniaxial tensile tests were conducted using a video camera in
conjunction with surface markers to document local strain distributions on the
surface of ligaments. Photoelasticity has also been used to document local
strain patterns.  

As Magnetic Resonance Imaging (MRI) technology has improved
in the recent decade, its utility in determining local strains, noninvasively, within
soft tissues has evolved. MRI will likely influence the biomechanics community
with the capability to assess in vivo,
3-dimensional tissue strains.

One such MRI-based technique is DENSE-FSE or displacement
encoding with stimulated echoes and a fast spin echo readout. The goal of this
work was to acquire images with high spatial resolution in reasonable imaging
times. This approach requires that the sample first be loaded to reach
steady-state to avoid motion artifacts. Images are collected while the sample
is cyclically loaded requiring a repeatable loading cycle to allow for
sufficient time to collect the MR images. Hence, for a single, fast
displacement test that is not repeatable, this approach may not work. Reducing
the required resolution would shorten the imaging time.

Neu, C. P. and
J. H. Walton (2008). "Displacement encoding for the measurement of
cartilage deformation." Magn Reson Med 59(1): 149-55.

Gilchrist et al., presents a texture correlation algorithm using
first-order displacement mapping terms with MR images. This approach eliminates
the need for "tags" or "markers". However, the technique is
heavily dependent on image contrast/texture and thus is sensitive to noise.

Gilchrist, C.
L., J. Q. Xia, et al. (2004). "High-resolution determination of soft
tissue deformations using MRI and first-order texture correlation." IEEE
Trans Med Imaging 23(5): 546-53.

Finite element warping
has also been used to track local displacements on MR images. While this
approach eliminates the need for markers in the tissue, or MR tags, it does assume
the discretized template to be a hyperelastic material in determining fiber
stretch. Cine-MRI and deformable image registration uses differences in image
intensities between a reference position image and a loaded position image to
generate a body force that deforms a FE representation of the template so that
it matches the target. This requires a priori assumptions about material
properties and constitutive behavior of the material and only works for static
loading.

Phatak, N. S., Q. Sun,
et al. (2007). "Noninvasive determination of ligament strain with
deformable image registration." Ann Biomed Eng 35(7): 1175-87.

Major technical advances in MRI have allowed the above
approaches to be developed for non-invasive measurement of soft tissue
deformations. With continued advances in MRI technology that may increase
resolution and shorten loading times, these approaches and others may enable
dynamic three-dimensional collection of physiologically loaded soft tissues
both in vivo and in vitro.