Multi-Fiber Tractography Visualizations for Diffusion MRI Data

<div><p>In recent years, several new diffusion MRI approaches have been proposed to explore microstructural properties of the white matter, such as Q-ball imaging and spherical deconvolution-based techniques to estimate the orientation distribution function. These methods can describe the estimated diffusion profile with a higher accuracy than the more conventional second-rank diffusion tensor imaging technique. Despite many important advances, there are still inconsistent findings between different models that investigate the “crossing fibers” issue. Due to the high information content and the complex nature of the data, it becomes virtually impossible to interpret and compare results in a consistent manner. In this work, we present novel fiber tractography visualization approaches that provide a more complete picture of the microstructural architecture of fiber pathways: multi-fiber hyperstreamlines and streamribbons. By visualizing, for instance, the estimated fiber orientation distribution along the reconstructed tract in a continuous way, information of the local fiber architecture is combined with the global anatomical information derived from tractography. Facilitating the interpretation of diffusion MRI data, this approach can be useful for comparing different diffusion reconstruction techniques and may improve our understanding of the intricate white matter network.</p> </div

Example comparison of hyperstreamlines created using two HARDI methods.

<p>Hyperstreamline examples to compare different methods. The reconstructed arcuate fasciculus (AF) is shown for QBI (a) and for CSD (b). Single tracts from the QBI bundle and CSD bundle are shown as a multi-fiber hyperstreamline in c) and d), respectively. This visualization, both in cross-sectional shape as well as color-encoding, highlights an important difference between the two used methods. For CSD (d), clear left-right fiber populations (red) can be seen crossing the AF (indicated by the arrow), whereas QBI (c) does not display these populations: these lateral projections of the corpus callosum (latCC) could not be detected with QBI. Verification can be seen in e) and f), where the AF is shown in green and the latCC in red (seeded lateral to the AF it should target the corpus callosum, as seen in f).</p

Hyperstreamlines and streamribbons for a tract from the cingulum.

<p>Superior segment of the bilateral cingulum bundles (a). From these full bundles – shown in b) as a yellow haze for anatomical reference – one tract has been selected at the center of each bundle (shown in green in b) and one tract at the interface of the corpus callosum (red in b). These “center” (c and d) and “edge” (e and f) tracts have been with the fODF glyphs along their trajectories (c and e) and as hyperstreamlines (d & f). Strong difference between the magnitudes of the left-right populations can be observed between (c) - (d) and (e) - (f). From the ODFs in c) it is difficult to interpret the continuity of orientation and magnitude of the left-right populations; whereas this is very easy from the hyperstreamlines in (d).</p

Arcuate fasciculus visualized as streamtubes with different color-encoding.

<p>In a), the arcuate fasciculus (AF) is shown with its conventional streamtube representation and with color-encoding according to the tract direction. The multi-fiber hyperstreamline and streamribbon with the orientation color-encoding according to the ODF are shown in b) and c), respectively. Standard streamtubes have been visualized in d), but then using the ODF-based color-encoding used in b).</p

The difference between discrete and continuous visualization.

<p>A coronal overview of the tract to be visualized is seen in a), with the main eigenvector at discrete locations along the tract (b) and the tract itself (c). Analogous to b) and c), d) and e) show the fODF glyphs at discrete locations along the tract and the multi-fiber hyperstreamline, respectively. The continuity is difficult to grasp from the glyphs alone (b and d), whereas the continuous representations (c and e) give a more intuitive feel, are easier to interpret, and represent a more complete picture of the underlying WM pathway.</p

Hyperstreamlines and streamribbons in a simulated crossing fiber phantom.

<p>The global topology of a simulated fiber phantom is illustrated in a), with streamtubes color-encoded by the tract direction. The configuration was designed to contain distinct regions where two and three fiber populations intersect. fODF glyphs in this region clearly show these crossing populations (b). A single tract from the blue fiber bundle is shown in c) as tensor-based hyperstreamline and in d) as tensor-based streamribbon. In e), local fODF glyphs are shown along its trajectory, and a step-by-step creation of the hyperstreamline that envelops these glyphs. In f), the cylindrical glyph objects represent the distinct fODF peaks and the streamribbon is a continuous visualization of these peaks. </p

Hyperstreamlines and streamribbons for a tract from the corpus callosum.

<p>Axial (a) and coronal (b) slices of the brain of the two subjects (subject one: a) and c); subject two: b) and d)) are shown, with a tract from the lateral projections of the corpus callosum (latCC) visualized as multi-fiber hyperstreamline. The boxed regions in a) and b) are enlarged in c) and d), respectively, showing the construction procedures of the hyperstreamline and streamribbon. Starting from the middle of the brain, the fiber tracts first cross the cortico-spinal tracts (CST) before intersecting the arcuate fasciculus (AF) more laterally. From the visualizations in c) and d), regions with three distinct fiber populations can be seen along the tracts, i.e., where the AF and CST both cross the latCC pathway. This is highlighted when color-encoding the hyperstreamline by the number of ODF peaks (red arrow).</p

Visualization of local tract orientations crossing a fiber tract.

<p>A single tract from the arcuate fasciculus (AF), the same as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0081453#pone-0081453-g005" target="_blank">Figures 5</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0081453#pone-0081453-g006" target="_blank">6</a>. a) Tract shown as a thick streamtube with fibers crossing this tract of interest shown as thinner streamtubes. b) Tract shown as multi-fiber hyperstreamline.</p

Conceptual difficulties of tensor-based visualizations of multi-fiber tractography data.

<p>In a) and b), two single fiber populations with their respective diffusion ellipsoids are shown. When these two coexist within one voxel (c), the first eigenvector (ε₁ brown line) no longer corresponds to any of the underlying fiber orientations (white lines). d) The second (ε₂ green) and third (ε₃ blue) eigenvectors do not have a physical meaning with respect to the underlying fiber populations.</p

Difference between hyperstreamlines and streamribbons.

<p>A single tract-of-interest from a simulated fiber configuration (a) is created as hyperstreamline (b-e) and streamribbon (c,f). In b), the fODF at a location along this tract (white tube); c) fODF shown semi-transparently with its peak orientations; d) fODF shown with the plane perpendicular to the tract orientation, delineating the amplitude of the hyperstreamline; e) the hyperstreamline; f) streamribbons are created of the fODF peaks that do not correspond to the tract orientation.</p