Research - Neurofunctional Imaging
ASYMMETRY OF SUBINSULAR ANISOTROPY BY IN VIVO DIFFUSION TENSOR IMAGING
Yue Cao, Jie Huang, Kevin L. Berger, and Mark DeLano
Department of Radiology, Michigan State University, East Lansing, Michigan,
USA
INTRODUCTION
A cortical asymmetry has been described in human auditory cortex (the planum temporal) and Wernicke’s area [1,2]. The asymmetry of the Wernicke’s area mainly concerns the size of the cortex, usually larger in the left side that specializes in speech and language, implying an association of the cortical asymmetry with the hemispheric functional lateralization. Similarly, axonal fiber bundles that interconnect the cortical areas might be developed asymmetrically, either in size, degree of myelination, and/or organization. Applying in vivo diffusion tensor imaging [3-5], we investigated anisotropy in the external/extreme capsule beneath the insular cortex, which may provide connectivity between the inferior frontal cortex and the temporal/parietal cortex.
METHODS AND MATERIALS
Subjects: Seven normal right-handed native English speakers (1 female, 6 male, ages 23 to 70 years) participated in the study. The handedness of the subjects was assessed by the Edinburgh Handedness Inventory [6].
MRI: Axial diffusion tensor images of a whole head were acquired on a GE 1.5 T clinical scanner using a spin echo Echo-Planar-Imaging pulse sequence (TE/TR = 69.3/10000 ms, in-plane resolution 2.3 mm, slice thickness 3 mm, gap 0 mm, 46 sections, and NEX 6). The diffusion encoding was accomplished in six non-colinear directions with b value 1000 s/mm2.
DATA ANALYSIS
Images were assessed and corrected for possible in-plane translation and rotation. After six diffusion tensor elements were calculated by singular value decomposition [4], the trace of the diffusion tensor (Dtr) and the relative anisotropy (RA) were computed [7]. The analysis in the region of interest (ROI) was applied to the RA, Dtr, and T2-weighted (T2W) images. The ROI was chosen in the space that is lateral to putamen and medial to insular cortex corresponding to the external capsule, extreme capsule and claustrum, which are not resolved. The ROIs were selected on the slices where the voxels between the putamen and insular cortex were resolved on the T2W images (Figure 1). Then, the data obtained from the multiple slices were pooled
Figure 1 illustrates the T2W and relative anisotropic images
of a right-handed subject in the respective left and middle panels. Voxels
in cyan and blue showed the selected ROIs. The right panel shows an overlay
of the RA on the T2W image and greater anisotropy within the left subinsular
white matter.
RESULTS AND DISCUSSION
All subjects were strongly right-handed, the handedness index [6] varied between 100% and 82%. The relative anisotropy (RA) of diffusion was markedly asymmetric between the left and right external/extreme capsule. The average difference of the RA between the left and right ROIs was 0.14 + 0.014 (p<0.0005), see Table. This asymmetry was also demonstrated in diffusion trace (Dtr) (p<0.005) with a greater average diffusion of water molecules on the right side than on the left (Table). The first eigenvalue of the diffusion tensor was significantly greater on the left than on the right side (p<0.0005), whereas the second and third eigenvalues were significantly smaller on the left side than on the right side (p<0.0005) (Figure 2).
Table: Trace and Relative Anisotropy of Diffusion
RA(L) |
RA(R) |
ΔRA |
Dtr (L) |
Dtr(R) |
ΔDtr |
|
1 |
0.38 |
0.26 |
0.12 |
2.220 |
2.284 |
-0.064 |
2 |
0.42 |
0.25 |
0.18 |
2.126 |
2.224 |
-0.097 |
3 |
0.39 |
0.25 |
0.14 |
2.092 |
2.195 |
-0.103 |
4 |
0.35 |
0.22 |
0.13 |
2.222 |
2.272 |
-0.050 |
5 |
0.37 |
0.25 |
0.12 |
2.239 |
2.259 |
-0.020 |
6 |
0.36 |
0.23 |
0.14 |
2.056 |
2.170 |
-0.113 |
7 |
0.32 |
0.18 |
0.15 |
2.374 |
2.399 |
-0.025 |
mean |
0.37 |
0.23 |
0.14 |
2.189 |
2.258 |
-0.067 |
sem |
0.01 |
0.01 |
0.01 |
0.041 |
0.028 |
0.014 |
p |
0.0005 |
0.005 |
Figure 2. The eigenvalues of the diffusion tensor differed
significantly between the left and right sides (p<0.0005, pair-t test).
The relative contribution to the anisotropic measurements of the three structures of interest– external capsule, extreme capsule and claustrum--cannot be resolved given the spatial resolution of diffusion tensor imaging. Thus, the MRI measurements in the interested space may arise from any or all these structures, and partial volume effects might contribute to absolute values of the measurements. However, the relative size differences of the subinsular white matter, which have been investigated by using high resolution T1 and T2 weighted images, cannot explain the asymmetry between left and right structures. The marked asymmetry of the RA and eigenvalues of water diffusion in subinsular white matter suggests that the left and right external/extreme capsule may develop differently in size and/or organization, associated with speech-language hemispheric lateralization and right-handedness.
REFERENCES
- Geschwind N, et al. Science, 161:186-187, 1968.
- Galaburda, AM, et al. Arch Neurol, 35:812-817, 1978.
- Le Bihan D, et al, Radiology, 161:401-407, 1986.
- Basser PJ, et al, J Magn Reson B, 103:247-254, 1994.
- Nakada T, et al, Neurosci Res, 22:389-398, 1995.
- Olfield RC, Neuropsychologia, 9:97-113, 1971.
- Ulug Am, et al, J Magn Reson Imag, 9:804-813, 1999.
