Diffusion Tensor Imaging (DTI) is a magnetic resonance imaging (MRI) modality which can significantly improve our understanding of the brain structures and neural connectivity. DTI measures are thought to be representative of brain tissue microstructure and are particularly useful for examining organized brain regions, such as white matter tract areas. DTI measures the water diffusion tensor using diffusion weighted pulse sequences which are sensitive to microscopic random water motion. The resulting diffusion weighted images (DWI) display and allow quantification of how water diffuses along axes or diffusion encoding directions. This can help to measure and quantify the tissue’s orientation and structure, making it an ideal tool for examining cerebral white matter and neural fiber tracts. In this chapter the authors discuss the theoretical aspects of DTI, the information that can be extracted from DTI data, and the use of the extracted information for the reconstruction of fiber tracts and the diagnosis of a disease. In addition, a review of known fiber tracking algorithms is presented.
Diffusion is the random translation or Brownian motion of molecules which is driven by internal thermal energy. The mobility of molecules can be characterized by a physical constant, the diffusion coefficient D. The random motion of molecules, in the presence of a strong magnetic gradient results in MR signal loss as a consequence of the dephasing of spin coherence. The signal attenuation depends on D and on the b-factor and is given by (Basser, 1994; Basser, 1998; Bihan, 2001; Le Bihan, 1991; Mattiello, 1994; Mattiello, 1997; Melherm 2002): (1)
The b-value expresses the degree of diffusion weighting and it is determined by the type of the sensitizing gradient scheme implemented in the MR experiment. It is given by: (2)
is the gyromagnetic ration, G
is the strength of gradients,
is the duration of gradients and
is the time between gradients.
Key Terms in this Chapter
Relative Anisotropy: The ratio of the anisotropic part of tensor D to its isotropic part.
Diffusion: The spontaneous movement of particles from an area of high concentration to an area of low concentration in a given volume of fluid. Diffusion MRI is a specific Magnetic Resonance Imaging (MRI) modality which produces in vivo images of biological tissues weighted by the local micro structural characteristics of water diffusion.
Echo Planar Imaging: A technique of planar imaging in which a complete planar image is obtained from one selective excitation pulse.
Volume Ratio: The ratio of the ellipsoid volume to the volume of a sphere.
Mean Diffusivity: A measure of the average molecular motion independent of any tissue directionality. It is affected by the cellular size and integrity.
Fractional Anisotropy: One of the commonly used measures of deviation from isotropy and reflects the degree of alignment of cellular structures within fiber tracts, as well as their structural integrity.
Diffusion Tensor Imaging: A magnetic resonance imaging (MRI) technique which enables the measurement of the restricted diffusion of water in tissue in order to produce neural tract images. Its common application is in the imaging of white matter where the location, orientation, and anisotropy of the tracts can be measured.
Signal to Noise Ratio: Used to describe the relative contributions to a detected signal of the true signal and random superimposed signals (noise).
Brownian Motion: (named in honor of the botanist Robert Brown) is the random movement of particles suspended in a liquid or gas or the mathematical model used to describe such random movements, often called a particle theory.
Navigator Echoes: Additional spin or gradient echoes used to monitor changes in the position of the sample during the scan time.