4 Brain Tissue Mechanical Properties
77
attempted to create ‘maps’ of regional brain properties in healthy adults using MRE
[24, 25], although the regional differences are not large and regions and values
reported vary between groups. The most consistent finding is that the cerebellum
is considerably less stiff than the cerebral hemispheres [25, 26], possibly due to its
finer structure [26]. There has also been progress towards estimating differences
in white and grey matter and also in methods to estimate anisotropy of white
matter tracts. Although consensus remains to be reached on the precise quantitative
differences, it is reasonably clear that the differences in white and grey matter shear
moduli are likely fairly small and that the degree of mechanical anisotropy is also
modest in most brain regions.
Vappou et al. [27] have published a direct comparison of rheometry data on
ex vivo brain tissue with MRE measurements, but it is difficult to draw direct
conclusions about the validity of MRE on the basis of their work, since their testing
frequencies did not overlap for the two methods, their MRE shear moduli were
estimated from a simple wavelength-based formula rather than full inversion of
the wave equation, and their rheometry testing was conducted at a shear strain
of 0.5%, which the above discussion suggests may have been beyond the linear
viscoelastic limit, and thus have slightly underestimated the shear modulus. Further
rigorous validation of MRE is required before the absolute values estimated from
this technique can be considered quantitatively reliable or results from different
analytical techniques can be compared. Studies performed using different MRE
techniques still tend to report differing shear modulus values at similar frequencies
[28], although general trends towards higher moduli at higher frequencies are
consistent with rheometry studies.
Brain MRE has recently begun to be used in research studies in clinical
populations, including demyelination [29, 30], dementia [31, 32], cerebrospinal
fluid flow disorders such as hydrocephalus [33–35], and brain cancers [36–38].
These studies, while intriguing, remain to be repeated in independent cohorts,
and typically show substantial overlap between patient and control groups, and
occasionally have contradictory results, which may indicate that they are limited
in their diagnostic power using current methods.
Despite these issues, MRE has great promise as a relatively noninvasive method
of measuring in vivo human tissue properties, which is impossible using other more
traditional techniques.
Ultrasound has also been used to estimate brain tissue properties, in the linear
viscoelastic (small amplitude) range ex vivo. Lippert et al. (2004) used the ‘wave in
a tube’ technique, where a sample is placed in a tube and an ultrasonic (100 kHz–
10 MHz) waves passed through the sample. By measuring this wave propagation,
the wave speed in the tube is estimated and the linear viscoelastic shear modulus
(G*) extracted. Lippert et al. [39] estimated the shear modulus for juvenile ovine
brain tissue samples to be in the range of 140–400 MPa, where the larger values are
associated with the higher frequencies. These values are orders of magnitude larger
than values from lower frequencies, and somewhat higher than simple extrapolation
of the power-law behaviour measured at lower frequencies (e.g. the data shown
in Fig. 4.1) would predict. Atay et al. [40] measured mouse brain shear modulus
77
attempted to create ‘maps’ of regional brain properties in healthy adults using MRE
[24, 25], although the regional differences are not large and regions and values
reported vary between groups. The most consistent finding is that the cerebellum
is considerably less stiff than the cerebral hemispheres [25, 26], possibly due to its
finer structure [26]. There has also been progress towards estimating differences
in white and grey matter and also in methods to estimate anisotropy of white
matter tracts. Although consensus remains to be reached on the precise quantitative
differences, it is reasonably clear that the differences in white and grey matter shear
moduli are likely fairly small and that the degree of mechanical anisotropy is also
modest in most brain regions.
Vappou et al. [27] have published a direct comparison of rheometry data on
ex vivo brain tissue with MRE measurements, but it is difficult to draw direct
conclusions about the validity of MRE on the basis of their work, since their testing
frequencies did not overlap for the two methods, their MRE shear moduli were
estimated from a simple wavelength-based formula rather than full inversion of
the wave equation, and their rheometry testing was conducted at a shear strain
of 0.5%, which the above discussion suggests may have been beyond the linear
viscoelastic limit, and thus have slightly underestimated the shear modulus. Further
rigorous validation of MRE is required before the absolute values estimated from
this technique can be considered quantitatively reliable or results from different
analytical techniques can be compared. Studies performed using different MRE
techniques still tend to report differing shear modulus values at similar frequencies
[28], although general trends towards higher moduli at higher frequencies are
consistent with rheometry studies.
Brain MRE has recently begun to be used in research studies in clinical
populations, including demyelination [29, 30], dementia [31, 32], cerebrospinal
fluid flow disorders such as hydrocephalus [33–35], and brain cancers [36–38].
These studies, while intriguing, remain to be repeated in independent cohorts,
and typically show substantial overlap between patient and control groups, and
occasionally have contradictory results, which may indicate that they are limited
in their diagnostic power using current methods.
Despite these issues, MRE has great promise as a relatively noninvasive method
of measuring in vivo human tissue properties, which is impossible using other more
traditional techniques.
Ultrasound has also been used to estimate brain tissue properties, in the linear
viscoelastic (small amplitude) range ex vivo. Lippert et al. (2004) used the ‘wave in
a tube’ technique, where a sample is placed in a tube and an ultrasonic (100 kHz–
10 MHz) waves passed through the sample. By measuring this wave propagation,
the wave speed in the tube is estimated and the linear viscoelastic shear modulus
(G*) extracted. Lippert et al. [39] estimated the shear modulus for juvenile ovine
brain tissue samples to be in the range of 140–400 MPa, where the larger values are
associated with the higher frequencies. These values are orders of magnitude larger
than values from lower frequencies, and somewhat higher than simple extrapolation
of the power-law behaviour measured at lower frequencies (e.g. the data shown
in Fig. 4.1) would predict. Atay et al. [40] measured mouse brain shear modulus