5 Modelling of the Brain for Injury Simulation and Prevention
109
axis of protons due to different stimulations, MRI signals offer a three-dimensional
(3D), noninvasive, and radiation-free technique for the distinction between the grey
and white matter for TBI detections and can also be used for diagnosing brain
tumours.
The diffusion tensor imaging (DTI) is a diffusion-weighted MRI technique aimed
at providing the measurements of the 3D magnitude, orientation, and extent of
anisotropy of water (e.g. [1] and Hulkower et al. [37]). More specifically, a method
called the DTI tractography is now a popular method for 3D visualisation of white
matter connectivity patterns. When an axon is injured, it is hypothesised that the
diffusion along its axis will decrease, while the diffusion perpendicular to the same
axis will increase [3]. Because white matter is largely parallel to each other in several
brain regions, DTI is considered to be a powerful technique for the detection of DAI,
despite that large, longitudinal studies utilising standardised protocols have not been
conducted to determine the efficacy of DTI as a tool for TBI patient care [37].
Because white matter represents axons, some researchers have contemplated the
use of orthotropic properties to highlight its directional dependency (e.g. [6, 21, 40,
85]) and to better predict the location and severity of DAI. In our opinion, many
practical issues need to be resolved before this approach is used. First, neuronal
tissue is anatomically divided into grey matter and white matter. Grey matter is
made up mostly by cell bodies or neurons, while white matter is made up mostly
by myelinated axons. Due to this morphological variation, mechanical properties of
grey and white matter are expected to be different. Unfortunately, data reported in
literature are not consistent in terms of which brain matter is stiffer than the other
(see Chap. 4).
Second, there is a lack of mechanical property data on directional dependency of
axons to justify the use of such a computationally expensive representation. Third,
the accuracy of DTI tractography for predicting the true patterns of white matter
course is still being debated. For example, Thomas et al. [94] reported an inherent
limitation in using tractography for ‘determining long-range anatomical projections
based on voxel-averaged estimates of local fibre orientation’. Similarly, Mandelli et
al. [60] found that DTI has poor sensitivity in defining the motor pathways in lateral
region.
Fourth, some of the FE models utilised a coarse mesh with typical element
size of 5 mm or larger. Fig. 5.3 shows axonal tracts of a whole brain overlaid by
a 5 mm FE grid. Because a typical axon has a diameter of 1 micrometre (μm),
numerous axonal fibres are needed to form an axonal tract, and numerous axonal
tracts are needed to form an ‘element’ in an FE model. A typical high-resolution DTI
tractography has a voxel size of around 3 mm 3 . Projecting these voxels to a coarsemesh model with an element size of 5 mm (125 mm 3 in volume) would need a lot
of averaging to define the ‘axonal direction’ in an element. As such, the computed
‘axonal direction’ could vary greatly from one element to the other in the same
neighborhood. We believe that comprehensive and accurate representation of axonal
structures is necessary for ensuring accurate prediction of axonal injuries. However,
including the representation of axonal tracts is not recommended in coarse-mesh
model. When accurate representation of axonal structures cannot be guaranteed,
109
axis of protons due to different stimulations, MRI signals offer a three-dimensional
(3D), noninvasive, and radiation-free technique for the distinction between the grey
and white matter for TBI detections and can also be used for diagnosing brain
tumours.
The diffusion tensor imaging (DTI) is a diffusion-weighted MRI technique aimed
at providing the measurements of the 3D magnitude, orientation, and extent of
anisotropy of water (e.g. [1] and Hulkower et al. [37]). More specifically, a method
called the DTI tractography is now a popular method for 3D visualisation of white
matter connectivity patterns. When an axon is injured, it is hypothesised that the
diffusion along its axis will decrease, while the diffusion perpendicular to the same
axis will increase [3]. Because white matter is largely parallel to each other in several
brain regions, DTI is considered to be a powerful technique for the detection of DAI,
despite that large, longitudinal studies utilising standardised protocols have not been
conducted to determine the efficacy of DTI as a tool for TBI patient care [37].
Because white matter represents axons, some researchers have contemplated the
use of orthotropic properties to highlight its directional dependency (e.g. [6, 21, 40,
85]) and to better predict the location and severity of DAI. In our opinion, many
practical issues need to be resolved before this approach is used. First, neuronal
tissue is anatomically divided into grey matter and white matter. Grey matter is
made up mostly by cell bodies or neurons, while white matter is made up mostly
by myelinated axons. Due to this morphological variation, mechanical properties of
grey and white matter are expected to be different. Unfortunately, data reported in
literature are not consistent in terms of which brain matter is stiffer than the other
(see Chap. 4).
Second, there is a lack of mechanical property data on directional dependency of
axons to justify the use of such a computationally expensive representation. Third,
the accuracy of DTI tractography for predicting the true patterns of white matter
course is still being debated. For example, Thomas et al. [94] reported an inherent
limitation in using tractography for ‘determining long-range anatomical projections
based on voxel-averaged estimates of local fibre orientation’. Similarly, Mandelli et
al. [60] found that DTI has poor sensitivity in defining the motor pathways in lateral
region.
Fourth, some of the FE models utilised a coarse mesh with typical element
size of 5 mm or larger. Fig. 5.3 shows axonal tracts of a whole brain overlaid by
a 5 mm FE grid. Because a typical axon has a diameter of 1 micrometre (μm),
numerous axonal fibres are needed to form an axonal tract, and numerous axonal
tracts are needed to form an ‘element’ in an FE model. A typical high-resolution DTI
tractography has a voxel size of around 3 mm 3 . Projecting these voxels to a coarsemesh model with an element size of 5 mm (125 mm 3 in volume) would need a lot
of averaging to define the ‘axonal direction’ in an element. As such, the computed
‘axonal direction’ could vary greatly from one element to the other in the same
neighborhood. We believe that comprehensive and accurate representation of axonal
structures is necessary for ensuring accurate prediction of axonal injuries. However,
including the representation of axonal tracts is not recommended in coarse-mesh
model. When accurate representation of axonal structures cannot be guaranteed,