3 Introduction to Brain Imaging
49
the limitation of this technique for predicting the necessity of a specific brain region
for carrying out certain functions.
3.1.1.3 Diffusion Tensor Imaging (DTI)
Diffusion tensor imaging (DTI) is a relatively newer method to detect the orientation
and integrity of white matter fibres in vivo [11, 97]. DTI has generated much interest
because it currently is the only approach available to noninvasively study the threedimensional architecture of white matter tracts [79]. DTI is sensitive to the diffusion
of water molecules in the direction of the field gradient. Diffusion is anisotropic
in white matter tracts because the axonal membranes and myelin sheaths limit the
motion of water molecules to the orientation parallel to the fibre. Thus, the direction
of maximum diffusivity has been shown to coincide with the white matter fibre tract
orientation [80]. A minimum of six diffusion-encoded measurements are required to
accurately describe the diffusion tensor (mathematical 3D model of diffusion), but
using more directions will improve the accuracy of the tensor measurement [91].
Fibre tractography is a post-processing derivation of the diffusion data whereby
three-dimensional trajectories of white matter tracts are reconstructed based on the
estimates of fibre orientation in each voxel. The visualisation in three dimensions
can assist in identifying specific fibre tracts.
3.1.1.4 Ultrahigh Magnetic Fields
At ultrahigh magnetic fields (7 Tesla and above), the intrinsic signal-to-noise ratio
(SNR) of MRI and the BOLD signal change of fMRI are increased, enabling
even higher-resolution anatomic and functional imaging compared to traditional
field strengths (1.5 and 3 Tesla). The enhanced imaging can afford finer detail of
the brain’s functional organisation and improved detection of subtle anatomic and
functional abnormalities associated with neurologic disorders. However, at ultrahigh
magnetic fields, increased vulnerability to image artifacts and restrictions on image
acquisition time pose limitations on the spatial coverage of the brain and require
the development of customised acquisition sequences and specialised hardware
(recently reviewed in [8, 55, 129]).
3.1.2 Electrophysiological Brain Imaging Techniques
3.1.2.1 Electroencephalography (EEG) and Magnetoencephalography
(MEG)
Electrophysiological techniques to probe the electrical activity of the brain directly
(i.e. not via the haemodynamic reactivity) and noninvasively were introduced in
49
the limitation of this technique for predicting the necessity of a specific brain region
for carrying out certain functions.
3.1.1.3 Diffusion Tensor Imaging (DTI)
Diffusion tensor imaging (DTI) is a relatively newer method to detect the orientation
and integrity of white matter fibres in vivo [11, 97]. DTI has generated much interest
because it currently is the only approach available to noninvasively study the threedimensional architecture of white matter tracts [79]. DTI is sensitive to the diffusion
of water molecules in the direction of the field gradient. Diffusion is anisotropic
in white matter tracts because the axonal membranes and myelin sheaths limit the
motion of water molecules to the orientation parallel to the fibre. Thus, the direction
of maximum diffusivity has been shown to coincide with the white matter fibre tract
orientation [80]. A minimum of six diffusion-encoded measurements are required to
accurately describe the diffusion tensor (mathematical 3D model of diffusion), but
using more directions will improve the accuracy of the tensor measurement [91].
Fibre tractography is a post-processing derivation of the diffusion data whereby
three-dimensional trajectories of white matter tracts are reconstructed based on the
estimates of fibre orientation in each voxel. The visualisation in three dimensions
can assist in identifying specific fibre tracts.
3.1.1.4 Ultrahigh Magnetic Fields
At ultrahigh magnetic fields (7 Tesla and above), the intrinsic signal-to-noise ratio
(SNR) of MRI and the BOLD signal change of fMRI are increased, enabling
even higher-resolution anatomic and functional imaging compared to traditional
field strengths (1.5 and 3 Tesla). The enhanced imaging can afford finer detail of
the brain’s functional organisation and improved detection of subtle anatomic and
functional abnormalities associated with neurologic disorders. However, at ultrahigh
magnetic fields, increased vulnerability to image artifacts and restrictions on image
acquisition time pose limitations on the spatial coverage of the brain and require
the development of customised acquisition sequences and specialised hardware
(recently reviewed in [8, 55, 129]).
3.1.2 Electrophysiological Brain Imaging Techniques
3.1.2.1 Electroencephalography (EEG) and Magnetoencephalography
(MEG)
Electrophysiological techniques to probe the electrical activity of the brain directly
(i.e. not via the haemodynamic reactivity) and noninvasively were introduced in