Diffusion Tensor Imaging

Excerpt

Advanced magnetic resonance (MR) neuroimaging modalities are becoming more available and useful as their value in the diagnosis and prognosis of central nervous system diseases is more fully studied and understood. Specifically, diffusion tensor imaging (DTI) has become increasingly studied and utilized in recent years. It has become incorporated by many radiologists into routine clinical practice, with most research performed on traumatic brain injury (TBI). DTI is a variant of diffusion-weighted imaging (DWI) that utilizes a tissue water diffusion rate for image production. The first application of DWI to the human brain was performed in 1986 and since has become the gold standard for detecting acute stroke. DTI does not require contrast and is available on almost all modern MR scanners with relatively quick scan times for this sequence.

Random thermal motion, or Brownian motion, is water molecular diffusion in three-dimensional (3D) space. Isotropy is defined as uniformity in all directions, and when applied to water molecules, isotropy occurs when the diffusion of water is entirely uninhibited (such as water movement in a glass of water). Anisotropy is when there is a directionality in the diffusion of water present, and the movement of water is no longer random (such as water movement along straws placed in a glass). The greater the anisotropy, the more directional and linear the diffusion of water molecules. Water molecules will diffuse differently through space depending on the tissue type, components, structure, architecture, and integrity; these principles allow clinically significant imaging to occur, particularly the DTI. The latter measures the movement of water along axons, analogous to the straws in a glass of water.

As early as May 2002, medical literature reported that DTI showed abnormalities in patients who suffered from mild brain trauma as compared to normal control subjects. "This study included five patients with mild traumatic brain injury (three men and two women) and ten volunteers with no known neurological disorders (five men and five women)." This study reported abnormalities in the patients with a mild brain injury that were absent in the control subjects or the uninvolved sides of the injured patients' brains: "Patients displayed a significant reduction of diffusion anisotropy in several regions compared with the homologous ones in the contralateral hemisphere. Such differences were not observed in the control subjects. Significant reduction of diffusion anisotropy was also detected when diffusion tensor results from the patients were compared with those of the controls."

DWI uses volume elements (voxels) as a statistical method for data collection. When a voxel contains scalar values constituting a vector, it is known as a tensor, where DTI receives its name and explains the additional information provided through DTI. DTI MR settings can measure the diffusion of water along an axon in many directions; 6, 9, 33, and 90 directions are typical parameters used, with 33 directions and above increasing confidence in the accuracy. Ninety directions typically require upwards of 20 additional minutes in the MR scanner. Therefore, it may not be suitable for routine clinical practice. In effect, DTI will provide an indirect method of assessing neuroanatomy structure on a microscopic level using water molecules' degree of anisotropy and structural orientation within a voxel. Therefore, the principal application for DTI is in the imaging of white matter, where the orientation, location, and anisotropy of the tracts can be measured and evaluated. The architecture of the axons in parallel bundles and their myelin sheaths facilitate the diffusion of the water molecules preferentially along their main direction.

There are several measures calculated using DTI that can provide quantitative power. One of the most widely used DTI measures is fractional anisotropy (FA). Others include mean diffusivity or apparent diffusion coefficient (ADC), radial (perpendicular) diffusivity, and axial (parallel) diffusivity. DTI uses mean diffusivity for the rate of molecular diffusion, FA for the summative direction of the diffusion, which provides a prominent vector, axial diffusivity for the rate of diffusion parallel to the main vector, and radial diffusivity for the rate of diffusion perpendicular to the main vector.

FA quantifies the directionality of diffusivity in a summative manner and is highly sensitive to change in microstructure; however, it can be nonspecific to the cause of change. Mean diffusivity quantifies cellular and membrane density, whereas an increase in mean diffusivity indicates disease processes such as edema or necrosis. Radial diffusivity quantifies myelin neuropathology and increases with demyelination. Axial diffusivity quantifies axonal degeneration and increases with brain maturation.

FA values are numerical values based on the anisotropy of water along the axon, which reflects the health of the axon. Abnormal FA values indicate axonal damage. FA values can be calculated utilizing the region of interest (ROI) method, whole-brain analysis (Voxel-Based analysis), or tract-based spatial statistics. The whole-brain analysis is gaining popularity due to its automation and ability to analyze more tracts. The ROI method, where the regions to be analyzed are traced by a technologist and then analyzed by a computer, remains reliable and replicable. One of the more common and standardized ROI methods is the segmented corpus callosal values. Being the largest axonal tract in the brain, damage to the corpus callosum is well described following head trauma and other pathologies (see Image. DTI of Corpus Callosum).

FA values can vary depending on which of the above 3 analyzing methods is used and other factors such as MR technique and type of post-processing performed. Utilizing a standardized technique, FA values are highly reproducible and not technologist-dependent. They can be subjectively interpreted by a radiologist and roughly compared to select values in the literature.

Pediatric normal values are slightly less than those of adults. However, most changes occur by age 5, and 90% of adult FA values are achieved by 11 years of age in the corpus callosum. After adulthood, FA values tend to decrease with age linearly.

Additionally, FA values comparison across different scanners is now possible, even if those scanners utilize different techniques. This is achieved using 'human phantom phenomena' where a single subject is scanned on 2 different scanners, enabling a comparison between scanners by a scaling factor or even to normative databases performed on a different scanner(s).

Three-dimensional reconstructions of the tensor tracts are accomplished with computer modeling and can beautifully illustrate the fiber tracts identify pathology, and aid neurosurgeons (see Image. 3D Reconstructed DTI Image of Patient with Bilateral Post-Traumatic Frontal Contusions Compared to a Normal 3D DTI).