Tractography Using Diffusion Tensor Imaging (DTI) in 7T MRI

i. Department of Radiology and Biomedical Imaging, UCSF, San Francisco, California 94107.

Hendry S. Cahaya,i Aditi Guha,i Tzu-Cheng Lee,i Cheng-Liang Liu,i and Zhixiang Zhang i

BackgroundWater diffusion in the brain tissue is affected by the presence of structural barriers to translational motion such as cell membranes and myelinated fibers. Quantitatively, the restricted movement of water in tissue, can be modeled with an ellipsoidal shape tensor that is elongated along the axis of fastest diffusion but contracted along directions where diffusion is restricted.

Using the estimated diffusion tensor at each voxel, the magnitude and the direction of diffusion can be derived from its eigenvalues and eigenvectors respectively. Quantitative brain map can then be evaluated from the measures of diffusion anisotropy, including fractional anisotropy (FA) and the apparent diffusion coefficient (ADC). The ADC value represents the presence of barriers to free diffusion in the volume. The FA value which ranges from 0 to 1, represents the direction of the diffusion movement. When diffusion is isotropic (no preferred direction), FA value is 0. Otherwise, an FA value of 1 represents a unidirectional movement (anisotropic diffusion). To complement the quantitative anisotropy brain map, a colored three-dimensional white matter tractography is generated to represent the diffusion intensity and direction.

In this study, we explored the power of diffusion tensor imaging using a 7T MRI. We narrowed our DTI analysis on three main fiber tracts, namely corpus callosum (CC), cortico-spinal tract (CST) and arcuate fasciculus (AF). Values obtain from FA and ADC were then compared with published literature. We also generated three-dimensional tractography in tandem to harness our finding with substantial anatomical insights.

Summary• The random motion of water in tissues in influenced by various cellular structures such as cell membranes, cytoskeletal proteins and

other cellular macrostructures.

• DTI is a measurement of water diffusion in a living brain that takes into account various structural contributions that might impact the overall water diffusional process.

• DTI is sensitive to the tissue integrity, orientation and connectivity of the underlying nerve fibers.

• From the estimated diffusion tensor at each volume, two metrics (FA and ADC) are obtained.

• Apparent diffusion coefficient (ADC) reports the presence of barriers to free diffusion in the volume. Fractional anisotropy (FA) reports the diffusional direction.

• Lower fractional anisotropy and higher mean diffusivity often indicate impaired fiber integrity, but not always; depending on the cellular basis for the changes, abnormally high or low values may indicate dysfunction or may even confer an advantage.

• Tractography based on DTI allows visualization of the white matter fiber bundles (tracts).

• Current DTI based tractography may still require a more fined-tuned fiber tracking algorithm in order to resolve fiber orientation heterogeneity for better visualization of crossing and dispersing white matter tracts.

• DTI and tractography are common tools to determine preclinical sign of disease from brain tumors to acute stroke. These tools are also used to discover genetic and environmental factors that affect disease progression and to evaluate treatment efficacy in longitudinal study.

Results

MethodsSubjectsFour normal subjects were recruited in the study (age range: 26~32 years; mean± SD: 28±2.708 years; all right-handed male).

Scan informationMRI scans were performed on a 7.0 T MRI scanner (GE MR950). Both T2-weighted structure images and diffusion tensor imaging (DTI) were acquired with the same slice orientation and the same scan range. The T2-weighted images were acquired using CUBE T2 COR, TR/TE = 5950/62.5ms, image size = 512×512, and slice thickness = 1mm. DTI was performed using a twice refocused balanced echo diffusion echo planar imaging with 55 diffusion-encoding gradient vectors, bmax = 2000 s/mm2 and TR/TE = 9000/83.5ms, image size = 256×256. DTI with spatial resolution of 1 mm3 and 4256 transaxial slices were obtained within 20 minutes.

Tractography reconstruction and parameter analysis Tractography was reconstructed using a streamline-based algorithm adapted for diffusion data using in-house software (DSI studio, http://dsi-studio.labsolver.org/). One of the decomposed fiber vectors in the white matter pixels was used as a seed vector to produce one fiber tract. A step of 0.5 pixels was tracked forward from the seed vector, and all vectors in the nearest voxels were evaluated to determine the proceeding orientation for the next step. A vector with closest orientation to the starting vector was chosen if it was within 45° deviation. Tracking stopped if no such vectors were found in the nearest voxels. The procedure of the fiber tracking was iterated by randomly selecting the seed vectors in the pixels covering each association fiber tract. Tractography was reconstructed based on a simple algorithm adapted for DTI data. Three main white matter tracts were reconstructed, including association fiber: arcuate fasciculus (AF); commissural fiber: corpus callosum (CC); projection fiber: corticospinal tract (CST). The tract-specific analysis was performed based on the mean path of individual fiber tract by the software. Fiber integrity along three fiber tracts was quantified in terms of FA. Last, ADC value also calculated to present the properties of diffusion occurring within a particular voxel (volume picture element).

A. B.

C. D.

The column chart of the FA & ADC value in terms of different brain fibers and subjects: (A) the statistic FA value among different brain fibers. (B) the statistic FA value among different subjects. (C) the statistic ADC value among different brain fibers. (D) the statistic ADC value among different subjects.

A. B.

Box chart of the FA value in terms of different brain fibers and subjects: the middle square represents the mean value, the up and down edges of the rectangle represents one standard deviation, the middle bar means the medium value, and the up and down rods represents the max and min value. (A) the statistic FA value among different brain fibers. (B) the statistic ADC value among different brain fibers.

Functional anisotropy (FA) and apparent diffusion coefficient (ADC) values of three brain fibers from four subjects. Neural fibers selected are corpus callosum (CC), cortico-spinal tract (CST Left and CST Right), and arcuate fasciculus (AF Left and AF Right).

Brain Tractography

Top panel: three fiber tracts were selected for our study. They were corpus callosum (red), arcuate fasciculus (green) and cortico-spinal tract (blue). Bottom panel: We encountered an incidental finding from one of the volunteers. An arachnoid cyst was seen on the right anterior frontal lobe. The cyst has the same signal intensity as CSF. The right side fiber projections that would normally radiate out appeared to be compressed, but not shortened. This finding nicely shows the concept of brain plasticity.