title: Variability of homotopic and heterotopic callosal connectivity in partial agenesis of the corpus callosum: a 3T diffusion tensor imaging and Q-ball tractography study.
abstract: OBJECTIVE
  Little is known about the anatomic connectivity of callosal axons in individuals with partial agenesis of the corpus callosum (pAgCC). We used tractography based on both diffusion tensor imaging (DTI) and high angular resolution diffusion imaging (HARDI) to investigate interhemispheric white matter connectivity in pAgCC.
  
  
  METHODS
  DTI and HARDI were performed at 3T on 6 individuals with pAgCC and 8 control subjects. For HARDI analysis, a Q-ball reconstruction method capable of visualizing multiple intravoxel fiber orientations was used. In both DTI and HARDI, whole-brain 3D fiber tractography was performed by using deterministic streamline algorithms. Callosal fibers were then segmented to identify separately connections between homologous cortical regions (homotopic fibers) and nonhomologous regions (heterotopic fibers) by using manually drawn regions of interest.
  
  
  RESULTS
  In control individuals, we observed densely connected homotopic fibers. However, in individuals with pAgCC, we identified not only homotopic connections but also heterotopic connections in 4 of 6 subjects. Furthermore, the observed homotopic connections in pAgCC did not necessarily correlate with the position or size of the residual callosum. The nature of homotopic and heterotopic connectivity varied considerably among subjects with pAgCC, and HARDI recovered more callosal fibers than DTI.
  
  
  CONCLUSIONS
  Individuals with pAgCC demonstrate a remarkable diversity of callosal connectivity, including a number of heterotopic tracts that are absent in healthy subjects. The patterns of their callosal connections cannot be predicted from the appearance of their callosal fragments on conventional MR imaging. More tracts and more extensive fibers within tracts are recovered with HARDI than with DTI.
authors.affiliation: Department of Radiology and Biomedical Imaging, University of California, San Francisco, San Francisco, CA 94143-0628, USA.
authors.forename: M
authors.initials: M
authors.lastname: Wahl
authors.forename: Z
authors.initials: Z
authors.lastname: Strominger
authors.forename: R J
authors.initials: RJ
authors.lastname: Jeremy
authors.forename: A J
authors.initials: AJ
authors.lastname: Barkovich
authors.forename: M
authors.initials: M
authors.lastname: Wakahiro
authors.forename: E H
authors.initials: EH
authors.lastname: Sherr
authors.forename: P
authors.initials: P
authors.lastname: Mukherjee
mesh_terms: D055673:Acrocallosal Syndrome / Q000473:pathology*; D000293:Adolescent; D000328:Adult; D000368:Aged; D001927:Brain Diseases / Q000473:pathology*; D002540:Cerebral Cortex / Q000473:pathology; D002828:Choristoma / Q000473:pathology*; D038524:Diffusion Magnetic Resonance Imaging / Q000379:methods*; D005260:Female; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008297:Male; D008875:Middle Aged; D008959:Models, Neurological; D009413:Nerve Fibers, Myelinated / Q000473:pathology; D009434:Neural Pathways / Q000473:pathology*; D055815:Young Adult