Institutionen för klinisk neurovetenskap / Department of Clinical Neuroscience
Abstract
With diffusion MR imaging, the Brownian motion (or self-diffusion) of
water molecules is measured. In the corresponding ADC (Apparent Diffusion
Coefficient) image, each image element (voxel) represents the average
diffusion. In MRI, the diffusion is measured with diffusion gradients
along certain directions in space. The diffusion-weighting gradients are
often incorporated in an echo planar imaging (EPI) pulse sequence.
Depending on the type of tissue being imaged, the measured diffusion may
be isotropic, i.e. equal for an directions of the diffusion gradient, as
Seen, e.g., in grey matter and the cerebrospinal fluid (CSF). In
contrast, in white matter, the diffusion is higher along the nerve fibres
than across because water molecules moving along fibres are not hindered
- this is referred to as anisotropic diffusion. In diffusion tensor
imaging (DTI), the diffusion, for each voxel, is represented by an
ellipsoid with a certain shape and orientation. For e.g. grey matter the
ellipsoid is a sphere, whereas for white matter it is elongated in one
direction like a cigar. From the diffusion tensor the mean diffusion
(size of the ellipsoid) and the degree of anisotropy (shape of the
ellipsoid) may be calculated.
In this thesis different strategies to improve the image quality and
reliability of the DTI data and diffusion anisotropy maps are presented.
These include simulation studies to determine 1) which anisotropy index
is most insensitive to noise and 2) which diffusion scheme (i.e. which
set of diffusion gradients to use in the scanning process) minimises the
variance and bias of the calculated DTI data. The simulation results were
also complemented with data from phantoms and volunteers. Additionally,
two major types of image artefacts in diffusion weighted single-shot echo
planar imaging (DW SS-EPI) and means of correcting them have been
investigated. The first artefact is signal dropout, predominantly in the
mid-lower part of the brain, due to brain motion. The second is eddy
currents induced by the diffusion gradients that cause the DW images to
be distorted differently depending on the direction of the diffusion
gradient. The distortions are translation, scaling and shear effects in
the phase encoding direction of the image. These distortions, together
with patient motion, result in anatomical mismatch between the different
DW images used for the calculation of the diffusion tensor data. A new
distortion correction method that corrects for this mismatch has been
developed. DTI has been performed in a study comparing schizophrenic
patients and normal controls with respect to diffusion anisotropy, mean
diffusion and morphological differences