Atlas-Free Surface Reconstruction of the Cortical Grey-White Interface in Infants

BACKGROUND: The segmentation of the cortical interface between grey and white matter in magnetic resonance images (MRI) is highly challenging during the first post-natal year. First, the heterogeneous brain maturation creates important intensity fluctuations across regions. Second, the cortical ribbon is highly folded creating complex shapes. Finally, the low tissue contrast and partial volume effects hamper cortex edge detection in parts of the brain. METHODS AND FINDINGS: We present an atlas-free method for segmenting the grey-white matter interface of infant brains in T2-weighted (T2w) images. We used a broad characterization of tissue using features based not only on local contrast but also on geometric properties. Furthermore, inaccuracies in localization were reduced by the convergence of two evolving surfaces located on each side of the inner cortical surface. Our method has been applied to eleven brains of one- to four-month-old infants. Both quantitative validations against manual segmentations and sulcal landmarks demonstrated good performance for infants younger than two months old. Inaccuracies in surface reconstruction increased with age in specific brain regions where the tissue contrast decreased with maturation, such as in the central region. CONCLUSIONS: We presented a new segmentation method which achieved good to very good performance at the grey-white matter interface depending on the infant age. This method should reduce manual intervention and could be applied to pathological brains since it does not require any brain atlas

Segmentation steps and speeds of surface deformations.

<p>Upper row: a: MR axial slice of a 9-week-old infant; from <i>b</i> to <i>d</i>: brain mask in <i>grey</i> and detection outputs in <i>white</i>; <i>b:</i> MC^WM; <i>c:</i> TH^WM; <i>d:</i> TH^GM; <i>e:</i> feature field with cortical grey matter in <i>black</i> and white matter in <i>white</i>; tissue contrast is enhanced over the brain; <i>f</i>: white matter segmentation. Lower row: Speed variations in second round deformation according to feature value <i>f</i> and the number of neighbors <i>n</i>. S_high: high speed; S_low: low speed; S_verylow: very low speed (outer surface only); When contrast is weak, i.e., f in [F^GM, F^WM], <i>S_i(f)</i> and <i>S_o(f)</i> speeds are finely tuned according to both feature and neighbor configuration. Speed equations are given in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0027128#pone-0027128-t001" target="_blank">table 1</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0027128#pone-0027128-t002" target="_blank">2</a> for the inner and outer surfaces, respectively.</p

Description of morphological operators over a MRI axial slice including a cortical gyrus.

<p>A. Case of top hat detection of grey matter. A morphological closing is first applied to the image intensity using square elements such as the axial one shown. Results of the closing operation, namely <i>Closing(I)</i>, as well as the intensity profile (<i>I</i>), are depicted on the top left along a given sagittal cut. Top hat detection, i.e., <i>TH(I)</i>, which is the subtraction of <i>I</i> from <i>Closing(I)</i>, is shown on the top right. The detection map over the axial slice is shown with <i>red-white</i> color table. B. Case of curvature detection of white matter. Three portions of isointensity surfaces are shown on the left: a grey matter surface in <i>black</i>, an intermediate surface in <i>grey</i> and a white matter surface in <i>light grey</i>. When the isointensity surface is folding up such as the white matter one, curvature reaches minimal (negative) values. These curvature minima make up ridge lines, such as the gyral one shown on the curvature map with <i>blue-white</i> color table.</p

Speed of the outer surface.

<p>Speed (S_o) as a function of the number of neighbors (regularization) and feature value (f) with . See text for details.</p

Speed of the inner surface.

<p>Speed (S_i) as a function of the number of neighbors (regularization) and feature value (f) with . See text for details.</p

Frequency distributions (histograms) of brain tissue signal intensity according to age (3, 9, 14 and 16 week-old).

<p>Separate histogram modes of brain tissue disappear (<i>black</i> arrows) as the grey-white matter contrast decreases due to on-going maturation. GM: grey matter; WM: white matter.</p

New human-specific brain landmark: the depth asymmetry of superior temporal sulcus

Identifying potentially unique features of the human cerebral cortex is a first step to understanding how evolution has shaped the brain in our species. By analyzing MR images obtained from 177 humans and 73 chimpanzees, we observed a human-specific asymmetry in the superior temporal sulcus at the heart of the communication regions and which we have named the “superior temporal asymmetrical pit” (STAP). This 45-mm-long segment ven- tral to Heschl’s gyrus is deeper in the right hemisphere than in the left in 95% of typical human subjects, from infanthood till adult- hood, and is present, irrespective of handedness, language later- alization, and sex although it is greater in males than in females. The STAP also is seen in several groups of atypical subjects includ- ing persons with situs inversus, autistic spectrum disorder, Turner syndrome, and corpus callosum agenesis. It is explained in part by the larger number of sulcal interruptions in the left than in the right hemisphere. Its early presence in the infants of this study as well as in fetuses and premature infants suggests a strong genetic influence. Because this asymmetry is barely visible in chimpanzees, we recommend the STAP region during midgestation as an impor- tant phenotype to investigate asymmetrical variations of gene expression among the primate lineage. This genetic target may provide important insights regarding the evolution of the crucial cognitive abilities sustained by this sulcus in our species, namely communication and social cognition