Gheorghe Postelnicu

Combined Volumetric and Surface Registration

In this paper, we propose a novel method for the registration of volumetric images of the brain that optimizes the alignment of both cortical and subcortical structures. In order to achieve this, relevant geometrical information is extracted from a surface-based morph and diffused into the volume using the Navier operator of elasticity, resulting in a volumetric warp that aligns cortical folding patterns. This warp field is then refined with an intensity driven optical flow procedure that registers noncortical regions, while preserving the cortical alignment. The result is a combined surface and volume morph (CVS) that accurately registers both cortical and subcortical regions, establishing a single coordinate system suitable for the entire brain.

fMRI mapping of a morphed continuum of 3D shapes within inferior temporal cortex.

Here, we mapped fMRI responses to incrementally changing shapes along a continuous 3D morph, ranging from a head (“face”) to a house (“place”). The response to each shape was mapped independently by using single-stimulus imaging, and stimulus shapes were equated for lower-level visual cues. We measured activity in 2-mm samples across human inferior temporal cortex from the fusiform face area (FFA) (apparently selective for faces) to the parahippocampal place area (PPA) (apparently selective for places), testing for (i) incremental changes in the topography of FFA and PPA (predicted by the continuous-mapping model) or (ii) little or no response to the intermediate morphed shapes (predicted by the category model). Neither result occurred; instead, we found approximately linearly graded changes in the response amplitudes to graded-shape changes, without changes in topography—similar to visual responses in different lower-tier cortical areas.

Geometry driven volumetric registration

In this paper, we propose a novel method for the registration of volumetric images of the brain that attempts to maximize the overlap of cortical folds. In order to achieve this, relevant geometrical information is extracted from a surface-based morph and is diffused throughout the volume using the Navier operator of elasticity. The result is a volumetric warp that aligns the folding patterns.

The ‘Parahippocampal Place Area’ responds selectively to high spatial frequencies in humans and monkeys

Defining the exact mechanisms by which the brain processes visual objects and scenes remains an unresolved challenge. Valuable clues to this process have emerged from the demonstration that clusters of neurons (‘‘modules’’) in inferior temporal cortex apparently respond selectively to specific categories of visual stimuli, such as places/scenes. However, the higher-order ‘‘category-selective’’ response could also reflect specific lower-level spatial factors. Here we tested this idea in multiple functional MRI experiments, in humans and macaque monkeys, by systematically manipulating the spatial content of geometrical shapes and natural images. These tests revealed that visual spatial discontinuities (as reflected by an increased response to high spatial frequencies) selectively activate a well-known place-selective region of visual cortex (the ‘‘parahippocampal place area’’) in humans. In macaques, we demonstrate a homologous cortical area, and show that it also responds selectively to higher spatial frequencies. The parahippocampal place area may use such information for detecting object borders and scene details during spatial perception and navigation. Citation: Rajimehr R, Devaney KJ, Bilenko NY, Young JC, Tootell RBH (2011) The ‘‘Parahippocampal Place Area’’ Responds Preferentially to High Spatial Frequencies in Humans and Monkeys. PLoS Biol 9(4): e1000608. doi:10.1371/journal.pbio.1000608 Academic Editor: David Whitney, University of California Davis, United States of America Received July 15, 2010; Accepted February 25, 2011; Published April 5, 2011 Copyright: 2011 Rajimehr et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This research was supported by the US National Institutes of Health (NIH Grants R01 MH67529 and R01 EY017081 to RBHT), the Martinos Center for Biomedical Imaging, the NCRR, and the MIND Institute. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. Abbreviations: c/deg, cycles/degree; FFA, fusiform face area; FFT, fast Fourier transform; fMRI, functional magnetic resonance imaging; IT, inferior temporal; mFFA, monkey homolog of fusiform face area; mPPA, monkey homolog of parahippocampal place area; PPA, parahippocampal place area; SF, spatial frequency * E-mail: reza@nmr.mgh.harvard.edu

Entorhinal verrucae correlate with surface geometry

Entorhinal verrucae are unique, small elevations on the surface of entorhinal cortex, formed due to distinctive clustering of large neurons in entorhinal layer II. In Alzheimer’s disease, the verrucae atrophy as a result of neurofibrillary tangle formation and concomitant neuronal loss. Previously, we found significant decreases in verrucae height, width, surface area, and volume even in the mildest stage of Alzheimer’s disease. In this report, we introduce a new method for analyzing verrucae prominence using measures of their curvature. Smoothed surfaces and curvatures were generated using FreeSurfer (http://surfer.nmr.mgh.harvard.edu) from 100 μm3ex vivo MRI isosurfaces. We examined the positive and negative components of mean curvature AreaNorm(H+/-) and Gaussian curvature AreaNorm(K+/−) in entorhinal cortex. A significant difference was found between entorhinal (n=10) and non-entorhinal cortices (n=9) for both AreaNorm(H+/-) and AreaNorm(K+/−). We also validated our curvature analysis through a comparison with previously published verrucae measures derived from manual labels of individual verrucae. A significant positive correlation was found between mean verrucae height and AreaNorm(H+/-). Both mean verrucae height and volume were significantly positively correlated with AreaNorm(K+/−). These results demonstrate that K and H are accurate metrics for detecting the presence or absence of entorhinal verrucae. Curvature analysis may be a useful and sensitive technique for detecting local surface changes in entorhinal cortex.

P2-333: High resolution structural magnetic resonance images of the entorhinal cortex predict the presence of verrucae in 3D reconstruction volumes in control brains

Despite the neuroimaging tools that can provide data in vivo to support a diagnosis of probable Alzheimer’s disease (AD), a diagnosis of definite AD currently rests on the presence and spatial organization of neuropathological markers within specific cytoarchitectural areas including limbic, temporal, occipital cortices in post-mortem brain tissue. Histopathological examination of brain tissue is done using serial sections; thus, spatial topography of pathologic findings is challenging to reconstruct threedimensionally. We have developed novel methods to image ex vivo control brains using ultra high resolution MRI that provides a powerful tool to evaluate cytoarchitecture of medial temporal lobe areas. Images were collected on a 7T whole body MRI scanner based on a Siemens Sonata platform (Siemens Medical Systems, Erlangen, Germany) using either a surface coil or a solenoid coil (28.5mmx44mm,4 turn). A conventional 3D spoiled gradient echo sequence was used with 100um resolution for the block data and 120um for surface coil. For 100um data, fast-low-angle-shot images were acquired with a fixed TE 7.8ms and TR 20ms, with flip angles of 10, 20 or 30 degrees. Our ex vivo studies illustrated that layer II islands are visible in the entorhinal cortex (EC) and that when these MR volumes are reconstructed, the verrucae are observed on the surface of the EC. In these data, the presence or absence of EC islands in MR images reflects the presence or absence of verrucae in the MR reconstructions (n 5), and thus, EC islands in the MR images predict the presence of verrucae with reconstruction. Furthermore, the perforant pathway was studied in one case and imaged on 4.7Tesla (Siemens) scanner in a solenoid coil for diffusion tensor imaging. The perforant pathway’s trajectory was generated with DTI Studio (John Hopkins University) and were mapped 1) crossing the hippocampal fissure and 2) coursing around the hippocampal fissure. These data are the preliminary step that will lead to novel insights for the MR signal properties of control brains and AD and ultimately could provide the foundation for in vivo MRI techniques to detect AD neuropathology.