Published ahead of print on November 11, 2008
doi: 10.3174/ajnr.A1361
Variability of Homotopic and Heterotopic Callosal Connectivity in Partial Agenesis of the Corpus Callosum: A 3T Diffusion Tensor Imaging and Q-Ball Tractography Study
M. Wahla,b,
Z. Stromingerb,
R.J. Jeremyc,
A.J. Barkovicha,b,c,
M. Wakahirob,
E.H. Sherrb and
P. Mukherjeea
a Department of Radiology and Biomedical Imaging, University of California, San Francisco, San Francisco, Calif
b Department of Neurology, University of California, San Francisco, San Francisco, Calif
c Department of Pediatrics, University of California, San Francisco, San Francisco, Calif
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Fig 1. Callosal tract segmentation procedure, shown for a control subject (A–F) and for pAgCC subject 5 (G–L). A region of interest is first placed over the entire callosum, and tracts are colored according to their direction with the standard red-blue-green convention used for DTI color maps (A and G). Individual tracts are then segmented by using 2 additional regions of interest defining lobar regions in each hemisphere. For the control subject, homotopic anterior (B) and posterior (C) frontal, parietal (D), and occipitotemporal (E) tracts are segmented. For the subject with pAgCC, homotopic anterior frontal (H) and occipitotemporal (I) tracts and bilateral heterotopic frontal occipitotemporal (J and K) tracts are isolated. In K, an exclusion region of interest (pink) is used to eliminate homotopic occipitotemporal connections. All segmented tracts are then displayed together (F and L). All 3D tracts are shown projected onto axial sections.
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Fig 2. T1-weighted anatomic images and DTI tractography of 6 subjects with pAgCC (top panels) and 2 representative controls (bottom panel). Axial (left) and midline sagittal (middle) T1 sections are shown for each subject. Callosal fragments are identified with yellow arrows, whereas heterotopic fibers visible on T1-weighted images are denoted by red arrows. Midline sagittal DTI color maps are shown with segmented callosal fibers (right). For subjects with pAgCC, connectivity ranged from anterior frontal connections (subject 3) to only posterior frontal and occipitotemporal connections (subject 4). One individual (subject 5) displayed a discontinuous set of homotopic callosal connections, with anterior frontal and occipitotemporal connectivity without any posterior frontal or parietal connections. Control subjects (not shown) displayed similar callosal morphology and tractography results. Tracts are segmented and colored according to their cortical projections: homotopic anterior frontal, blue; homotopic posterior frontal, orange; homotopic parietal, pink; homotopic occipitotemporal, green; heterotopic left anterior-right posterior, yellow; heterotopic right anterior-left posterior, red.
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Fig 3. Q-ball tractography of subjects with pAgCC. All homotopic and heterotopic segmented tracts are shown on both axial (top) and midline sagittal (bottom) projections, with the subject number indicated in the upper left corner of the axial images. Fibers are colored as in Fig 2, with pink and purple fibers for subject 6 representing anterior frontal-temporal heterotopic connections.
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Fig 4. Q-ball tractography of heterotopic callosal connections. Isolated heterotopic connections are displayed for subjects 3–6. Fibers are shown projected on an axial section (left). A magnified view of the callosal fragment is also shown in the midsagittal plane (right) to demonstrate that all heterotopic connections are well isolated from other homotopic fibers.
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Fig 5. Comparison of DTI and QBI tractography. For a control subject (top) and a subject with pAgCC (subject 5, bottom), segmented tracts are shown by using DTI tractography performed on a DTI acquisition at b = 1000 s/mm2 (A and D) and on a HARDI acquisition at b = 3000 s/mm2 (B and E). QBI tractography is shown for the same HARDI acquisition at b = 3000 s/mm2 (C and F). QBI tractography yields more extensive fibers, including more lateral frontal fibers and denser temporal fibers in the control subject and an additional heterotopic fiber in the subject with pAgCC (red in F) not recovered by DTI tractography.
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