SURFACE RECONSTRUCTION OF EX-VIVO HUMAN V1 THROUGH IDENTIFICATION OF THE STRIA OF GENNARI USING MRI AT 7T

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1 SURFACE RECONSTRUCTION OF EX-VIVO HUMAN V1 THROUGH IDENTIFICATION OF THE STRIA OF GENNARI USING MRI AT 7T Oliver P. Hinds 1, Jonathan R. Polimeni 2, Megan L. Blackwell 3, Christopher J. Wiggins 3, Graham Wiggins 3, André J.W. van der Kouwe 3, Lawrence L. Wald 3, Eric L. Schwartz 1,2,4, and Bruce Fischl 3,5 1 Department of Cognitive and Neural Systems, Boston University, Boston, MA 02215, USA 2 Department of Electrical and Computer Engineering, Boston University, Boston, MA 02215, USA 3 Department of Radiology, MGH, Athinoula A Martinos Center, Harvard Medical School, Charlestown, MA 02129, USA 4 Department of Anatomy and Neurobiology, Boston University School of Medicine, Boston, MA 02118, USA 5 Artificial Intelligence Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139, USA January 11, 2005 Abstract Objective The stria of Gennari provides a definitive anatomical landmark delineating primary visual cortex (V1). Although the stria has been imaged previously both ex-vivo and in-vivo [1; 2], no surface reconstruction was performed. Here, we detected the stria in structural MR images of ex-vivo human brain and reconstructed the full striate surface. Surfaces computed from intact brain samples eliminate image alignment error, providing more accurate reconstructions than those from serial sections. Computer-flattened reconstructions from MRI data allow quantitative comparison with histologically defined area boundaries from physically flattened cortical tissue. Accurate localization of V1 across ex-vivo samples supplies ground-truth data for probabilistic anatomical atlases used for cross-subject registration. Techniques developed for ex-vivo reconstructions may also be applied in-vivo, allowing direct comparison of functionally and structurally determined V1 in the same subject, and facilitating the testing of human visuotopy models. We present a semi-automated method to reconstruct the striate surface of V1 ex-vivo from structural MRI, which addresses several difficulties associated with surface reconstruction of highresolution serial sections a task which is not well supported by current MRI-based surface reconstruction software. Methods We imaged ex-vivo human occipital cortex at 7T with enhanced contrast between gray and white matter, which enabled identification of the stria of Gennari, a stripe of myelinated tissue characteristic of layer IVb of V1. We developed software aiding the manual identification of stria, and therefore V1, in MR images. The software stores vertices identified as points of the striate surface and uses them as input to a surface tiling algorithm. The output of surface tiling is a two-dimensional, manifold triangular mesh representing the striate surface, which is then flattened quasi-isometrically (see Balasubramanian et al., this meeting) for comparison with physical or computerized flattenings of V1. To increase SNR while reducing MR image distortions due to magnetic field inhomogeneities and susceptibility artifacts, we employed a recently developed high-bandwidth, multiecho FLASH pulse sequence [3]. Results and Discussion Examples of flattened V1 are demonstrated, including statistics such as surface area, perimeter, and per-vertex error in flattening. Comparisons with the flattened shape of V1 defined using fmri are shown. Presented at 11th Annual Meeting of the Organization for Human Brain Mapping. Abstract number 140. Contact info: Oliver P. Hinds, Computer Vision and Computational Neuroscience Lab, 677 Beacon St., Boston, MA URL: oph@cns.bu.edu 1

2 Conclusions We present methods for reconstructing the striate surface within V1 in ex-vivo human data using 7T structural MRI. This facilitates comparison of techniques of area identification, development of probabilistic atlases, and testing of two-dimensional models of human visuotopy. References and Acknowledgements [1] V.P. Clark, E. Courchesne, and M. Grafe. In vivo myeloarchitectonic analysis of human striate and extrastriate cortex using magnetic resonance imaging. Cerebral Cortex, 2(5): , [2] E.L. Barbier, S. Marrett, A. Danek, A. Vortmeyer, P. van Gelderen, J. Duyn, P. Bandettini, J. Grafman, and A. P. Koretsky. Imaging cortical anatomy by high-resolution MR at 3.0T: detection of the stripe of Gennari in visual area 17. Magnetic Resonance in Medicine, 48(4): , [3] B. Fischl, D.H. Salat, A.J. van der Kouwe, N. Makris, F. Segonne, B.T. Quinn, and A.M. Dale. Sequence-independent segmentation of magnetic resonance images. NeuroImage, 23 Suppl 1:S69 84, This study was supported by NIH/NIBIB EB

3 Figure 1: Reconstruction of the striate surface of V1 from MR images gathered at 7T acquired using 130 µm isotropic voxels. The stria was manually located, and the surface was reconstructed using a surface tiling algorithm. Four views of a smoothed striate surface are shown. (A) A view of the striate surface looking from medial to lateral into the calcarine sulcus. (B) A view looking down the anteroposterior axis. (C) A view looking ventrally at the dorsal bank of the calcarine sulcus. (D) A view looking posteriorly into the calcarine sulcus. 3

4 Figure 2: Reconstruction of the striate surface shown embedded in three orthogonal slices from the MR data. The surface is unsmoothed and flat shaded to accentuate ridges orthogonal to the slice direction. These ridges demonstrate the difficulty of locating precise boundaries in high-resolution imaging. 4

5 Figure 3: (A) Single MR image that intersects the striate surface. (B) The same MR image, showing the points identified as the striate surface. These points become vertices of the final triangular mesh representation. The colors label independent connected components. 5