Diffusion tensor imaging can be used to indirectly assess brain microstructure

Diffusion tensor imaging is a relative newcomer in MR imaging. The ability to measure the molecular movement of water within cellular structures was first described less than 20 years ago.

Despite problems in the selection of appropriate imaging sequences and methods of visualization, the technique holds wide potential applications in depicting brain diseases ranging from stroke to Alzheimer's disease and psychiatric disorders such as schizophrenia.

"The idea was to measure the molecular movement of water in tissue. The big discovery came when this technique was used in patients with neurological disorders because water diffuses faster in the direction of fibers in the brain," said Dr. Denis Le Bihan, director of the laboratory of anatomy and functional neurology at Service Hospitalier Frederic Joliot in Orsay, France.

He was the moderator at Saturday's New Horizons session. In an overview of the principles underlying diffusion tensor imaging, Dr. Olaf Dietrich from Ludwig-Maximilian University in Munich explained that diffusion is microscopic stochastic motion induced by thermal energy.

DTI does not capture the process of diffusion from one medium to another or of one substance into another. It chronicles the diffusion of water molecules within themselves.

The aspect of DTI of most interest to neuroimaging is the diffusion coefficient, which reflects diffusion distance over time. A sphere or ellipsoid, which is mathematically described as a tensor, simplifies the visualization of moving water molecules.

In a 3 x 3 configuration, a diffusion tensor matrix yields several specific assessments, including the mean diffusivity (MD) or the apparent diffusion coefficient (ADC), which computes the average degree of diffusion of water molecules over all spatial directions, as well as mathematical vectors and values that describe anisotropy, such as fractional anistrophy (FA).

The use of DTI has been hampered by prolonged echo times, low signal-to-noise ratio, and a high degree of motion sensitivity. Parallel imaging has overcome some of these problems by reducing TE as well as echo trains and susceptibility artifacts.

Also complicating the use of DTI is the large amount of data available for analysis and the difficulty of choosing the best method of studying them, Dietrich said.

In research and clinical settings, DTI has generated information on tissue infrastructure and indications of inflammation, tissue degeneration, and neurodevelopmental abnormalities, said Dr. Marco Bozzali from Don C. Gnocchi Foundation in Milan.

The first application of the technique was in stroke - hours after the acute episode, a diffusion-perfusion mismatch could identify an ischemic penumbra, which could be addressed by treatment.

Additional information has been provided by anistropic maps that reflect the progression of the ischemic lesion, predict the patient's potential outcome, and distinguish between gray and white matter involvement.

Histograms evaluating large portions of tissue reveal significant increases in MD in individuals with Alzheimer's disease when compared with controls as well as a significant reduction in regional peak MD heights, which indicates nerve loss in gray matter and Wallerian degeneration in white matter.

FA in various regions of interest indicates that white matter pathology selectively involves areas of the brain associated with certain cortices but spares white matter serving visual and motor functions.

Microstructural changes in white matter therefore may contribute to cognitive decline, Bozzali said.

In individuals with schizophrenia, changes in FA in various parts of the brain have been consistent with the presence of neurodevelopmental disorder and indicated possible structural disconnectivity.

"Although MR is a powerful tool in the brain, we are interested in obtaining more information, such as tissue microstructure," he said. "Quantitative MR, including DTI, offers an indirect measure."