Improving delineation of the corticospinal tract in the monkey brain scanned with conventional DTI by using a compressed sensing based algorithm

Abstract

Background: The corticospinal tract (CST) is a major tract for motor function. It can be impaired by stroke. Its degeneration is associated with stroke outcome. Diffusion tensor imaging (DTI) tractography plays an important role in assessing fiber bundle integrity. However, it is limited in detecting crossing fibers in the brain. The crossing fiber angular resolution of intra-voxel structure (CFARI) algorithm shows potential to resolve complex fibers in the brain. The objective of the present study was to improve delineation of CST pathways in monkey brains scanned by conventional DTI.

Methods: Healthy rhesus monkeys were scanned by diffusion MRI with 128 diffusion encoding directions to evaluate the CFARI algorithm. Four monkeys with ischemic occlusion were also scanned with DTI (b = 1000 s/mm², 30 diffusion directions) at 6, 48, and 96 hours post stroke. CST fibers were reconstructed with DTI and CFARI-based tractography and evaluated. A two-way repeated MANOVA was used to determine significances of changes in DTI indices, tract number, and volumes of the CST between hemispheres or post-stroke time points.

Results: CFARI algorithm revealed substantially more fibers originated from the ventral premotor cortex in healthy and stroke monkey brains than DTI tractography. In addition, CFARI showed better sensitivity in detecting CST abnormality than DTI tractography following stroke.

Conclusion: CFARI significantly improved delineation of the CST in the brain scanned by DTI with 30 gradient directions. It showed better sensitivity in detecting abnormity of the CST following stroke. Preliminary results suggest that CFARI could facilitate prediction of function outcomes after stroke.

Keywords: CFARI; compressed sensing; diffusion tensor imaging; fiber tracking; nonhuman primate; stroke.

Figure 1.

Flow chart showing fiber tractography processing of conventional diffusion tensor imaging (DTI) data using the crossing fiber angular resolution of intra-voxel structure (CFARI) algorithm.

Figure 2.

Fiber maps in cortical regions comprising dorsal/ventral premotor cortices overlapped on coronal T1-weighted images of a rhesus monkey brain. Fibers were delineated using conventional DTI/QBI tractography and CFARI algorithm with datasets from different diffusion-encoding schemes. Numbers of diffusion encoding directions in inner (b = 1000 s/mm²) and outer (b = 1700 s/mm²) shells are separated with a comma and exhibited for each scheme.

Figure 3.

Corticospinal tract of a healthy monkey brain delineated using A) traditional DTI with b = 1000 s/mm² and 32 gradient directions, B) traditional DTI data reconstructed using CFARI approach, and C) q-ball imaging (QBI) for dataset with b = 1700 s/mm² and 128 gradient directions. Obvious differences of delineated fibers by DTI, CFARI, and QBI approaches are illustrated in shaded areas.

Figure 4.

Longitudinal alteration of the corticospinal tract (CST) in a stroke monkey brain (ID: RRI3) (overlaid on coronal diffusion-weighted images) following ischemic occlusion at Day 0, Day 2, and Day 4. CST pathways were constructed with conventional DTI (top) (b = 1000s/mm2, 30 gradient directions) and CFARI-based (bottom) tractography.

Figure 5.

A graphical representation of temporal evolution of fractional anisotropy (FA) (a), mean diffusivity (MD) (b), tract number (c), and tract volume (d) of the corticospinal tract (CST) in the contralateral (Con.) or ipsilateral (Ips.) hemisphere of a monkey brain following an ischemic stroke. *Significant differences (Bonferroni correction; p < 0.05) between two hemispheres using the same tractography (DTI or CS-DTI) at each time point. §, §§: significant difference (Bonferroni correction; ^§ p < 0.05, ^§§ p < 0.01) between post-occlusion time points using the same (DTI or CS-DTI) tractography in the same hemisphere (contralateral or ipsilateral).

Figure 6.

Bielschowsky’s silver staining revealing degenerative white matter fiber bundles in a monkey brain (RRI3) at 96 hours post stroke. Bar = 3mm. Arrows mark white matter fiber bundles with or without stroke lesion.