Université Libre de Bruxelles Faculté de Médecine
In vivo diffusion tensor imaging in infants:
assessment of brain development and correlation with language abilities in childhood
Alec AEBY
Travail présenté en vue de l’obtention du grade de Docteur en Sciences Médicales
Promoteur : Patrick Van Bogaert Co-Promoteur : Xavier De Tiège Année académique: 2012–2013
Avant-Propos
Au cours de ces nombreuses années passées, entre autres, à réaliser cette thèse de doctorat de nombreuses personnes m’ont offert leur soutien.
Le Professeur Patrick Van Bogaert, promoteur de ce travail, m’a tout d’abord donné le gout de la neurologie pédiatrique lorsque j’ai travaillé à ses côtés au cours de ma formation en pédiatrie. Il m’a fait bénéficier de son sens clinique remarquable et soutenu activement dans plusieurs projets d’articles scientifiques, dans une ambiance de travail stimulante et respectueuse où je me suis tout de suite senti à ma place.
Il m’a ensuite proposé de le rejoindre dans son équipe, une fois ma formation en pédiatrie achevée, pour suivre la formation en neurologie pédiatrique et devenir son adjoint. C’est alors que nous avons décidé d’étudier la valeur pronostique de l’imagerie par résonance magnétique (IRM) cérébrale avec séquence tenseur de diffusion sur le pronostic neurologique du prématuré. Cofondateur avec le Professeur Serge Goldman du Laboratoire de Cartographie Fonctionnelle du Cerveau (LCFC) au sein duquel j’ai réalisé cette thèse, le Professeur Van Bogaert a supervisé la réalisation de ce travail. Ses critiques constructives, son sens de la synthèse et de l’innovation ont guidé mes travaux de recherche jusqu’à l’aboutissement de ce travail.
C’est dans son équipe que j’ai rencontré le Docteur De Tiège, co-promoteur de ma thèse, qui était alors neurologue en formation et Doctorant chez le Professeur Van Bogaert. Au sein du LCFC, son expertise en neuroanatomie et en neuroimagerie a été le pilier fondateur de l’aboutissement de ma thèse. Le Docteur De Tiège est un chercheur brillant, curieux, rigoureux et généreux. Les nombreuses discussions que nous avons eues ont chaque fois fait progresser mon travail.
Je tiens ici à les remercier, ainsi que le Professeur Serge Goldman, qui m’a toujours soutenu dans l’avancement de mes recherches, pour le soutien sans faille qu’ils m’accordent chaque jour et qui me permet d’évoluer en confiance dans un environnement stimulant et amical.
Le Professeur Thierry Metens, responsable technique de l’IRM cérébrale, m’a
discuter des subtilités de l’imagerie cérébrale ou m’aider dans mes calculs statistiques. Avec Vincent Denolin, Docteur en physique comme lui, ils ont déterminé les paramètres d’acquisitions des séquences d’imagerie par tenseur de diffusion, qui n’existaient pas encore pour les nouveau-nés et offert un soutien précieux dans la méthodologie d’analyse des données d’IRM.
Le Professeur Danielle Balériaux et le Docteur Philippe David ont analysé les séquences d’IRM conventionnelle et offert leur aide dans la résolution des multiples problèmes pratiques rencontrés dans le service d’IRM, qu’ils en soient chaleureusement remerciés. Merci également au Dr Yan Liu, qui a reconstruit les faisceaux de fibres par tractographie dans ma première étude.
Ma reconnaissance envers le Professeur Anne Pardou, Chef de Service de Néonatologie et ses Adjoints, les Docteurs Danièle Vermeylen et Yves Hennequin qui m’ont aidé à établir le protocole qui permet dorénavant de réaliser les IRM cérébrales lors d’une période de sommeil induite naturellement est également importante. Ensemble, et c’est un des premiers résultats de cette étude, nous avons réussi à ne plus devoir administrer de médicament sédatif chez le nouveau-né qui bénéficie d’une IRM cérébrale. Merci aussi au Professeur Bart Van Overmeire, qui a pris la succession du Professeur Anne Pardou, et m’a chaleureusement soutenu dans ma thèse comme dans mes autres projets scientifiques en neurologie néonatale.
Je voudrais aussi exprimer ici toute ma gratitude à l’ensemble des personnes que j’ai rencontré au cours des ces années et qui m’ont permis de réaliser ces travaux au quotidien. Merci à Marylise Creuzil, psychologue, qui a réalisé les tests neuropsychologiques chez les anciens prématurés. Merci aux infirmières du service néonatal qui font tout leur possible pour que les nouveau-nés arrivent calmes avant de réaliser leur IRM. Merci aux techniciens de l’IRM cérébrale, qui sont toujours prêts à insérer un patient dans leur programme clinique chargé. Merci à mes collègues, Neuropédiatres et Pédiatres, les Docteurs Catherine Wetzburger, Vanessa Wermenbol, Dominique Detemmerman, Christophe Fricx et Yaël Weinblum pour leur compétence clinique et leurs qualités humaines exceptionnelles. Merci également aux infirmiéres(-iers) de la consultation et aux techniciens EEG, toujours prêts à me rendre service, travailler tous les jours à leur côté est un privilège. Un énorme remerciement également aux parents et aux enfants qui ont participé à ces études,
sans eux rien n’aurait été possible.
Pour conclure, je souhaite bien évidemment remercier ma famille et en particulier ma femme, Florence. Sans ses encouragements et ses conseils avisés, cette thèse n’aurait pas vu le jour. Tous les jours grâce à toi ma vie est joyeuse, heureuse et intense.
Merci également à mes fils, Félix, Marius et Jules : votre joie de vivre est une source de bonheur inépuisable. Merci également à mes parents et mes beaux-parents, toujours soutenants, généreux, enthousiastes et positifs. Merci également à Marjorie Gassner qui a relu attentivement ma première étude et ce manuscrit.
Remerciements
Je désire également remercier le Fonds de la Recherche Scientifique (FRS-FNRS) pour son soutien financier ainsi que CAP 48 grâce à qui le suivi systématique des prématurés a pu être mis en place.
TABLE of CONTENTS
Glossary………Page 8
Summary………...Page 9
Introduction...Page 12
1. Brain development
1.1 Before the third trimester of gestation 1.2 After the third trimester of gestation
1.2.1 Organizational events 1.2.1.1 Subplate neurons
1.2.1.2 Lamination and neurite outgrowth 1.2.1.3 Axonal development
1.2.1.4 Synaptic development and regressive events 1.2.1.5 Glial proliferation and differentiation 1.2.2 Cortical folding
1.2.3 Myelination 2. Preterm birth
2.1 Age terminology during the perinatal period 2.2 Epidemiology
2.3 Neurodevelopment 2.4 Brain injury
2.4.1 Periventricular leukomalacia (PVL) 2.4.1.1 Upstream mechanisms
2.4.1.1.1 Ischemia
2.4.1.1.2 Infection/inflammation 2.4.1.2 Downstream mechanisms
2.4.2 Germinal matrix haemorrhage–intraventricular haemorrhage (GMH-IVH) with periventricular haemorrhagic infarction (PHI)
3. Brain imaging
3.1 Practical issue with MR imaging in the term and preterm infant
3.2 Conventional imaging of the neonatal brain 3.2.1 Technique
3.2.2 Assessment of brain maturation in the fetus and normal term infant 3.2.3 Assessment of brain maturation in the preterm infant
3.2.4 Assessment of brain injury in the preterm infant 3.2.5 Clinical application
3.2.5.1 Brain MRI performed during the neonatal period 3.2.5.1.1 The Woodward score
3.2.5.2 Brain MRI performed during childhood and adolescence 3.3 Diffusion imaging
3.3.1 Diffusion Weighted Imaging (DWI) 3.3.2 Diffusion Tensor Imaging (DTI)
3.3.2.1 DTI indices maps
3.3.2.2 Fiber bundle tractography
3.3.3 Assessment of brain development with DWI/DTI 3.3.4 Assessment of brain injury with DWI/DTI 3.3.5 Clinical application
Objectives...Page 55
Patients and Methods...Page 57
1. Patients
2. Methods of investigation 2.1 Brain MRI data
2.1.1 MRI data acquisition 2.1.2 DTI indices calculation 2.1.3 Diffusion tensor tractography 2.2 Statistical parametric mapping (SPM)
2.2.1 The general linear model 2.2.2 SPM analysis of DTI data
2.2.2.1 Step 1: Visual inspection 2.2.2.2 Step 2: Realignment
2.2.2.4 Step 4: Image normalization and smoothing 2.2.2.5 Step 5: Quality assessment of DTI maps 2.2.3 Experimental design
2.2.3.1 Subtractive designs 2.2.3.2 Parametric designs
2.3. Bayley Scales of Infant and Toddler Development, Third Edition 2.3.1. Description
2.3.1.1 Scales
2.3.1.2 Administration and scoring procedures 2.3.1.3 Validity
Results...Page 68
1. Study 1: Maturation of thalamic radiations between 34 and 41 weeks post menstrual age: A combined voxel-based study and probabilistic tractography with diffusion tensor imaging.
1.1 Introduction
1.2 Material and methods 1.3 Results
2. Study 2: Nonlinear microstructural changes in the right superior temporal sulcus and lateral occipitotemporal gyrus between 35 and 43 weeks in the preterm brain.
2.1 Introduction
2.2 Material and methods 2.3 Results
3. Study 3: Language development at 2 years is correlated to brain microstructure in the left superior temporal gyrus at term equivalent age: a diffusion tensor imaging study.
3.1 Introduction
3.2 Material and methods 3.3 Results
Discussion………Page 93 Conclusion and perspectives……….Page 106 References………...Page 109
GLOSSARY
• ADC: apparent diffusion coefficient
• ATR: anterior thalamic radiations
• CC: corpus callosum
• CP: cerebral palsy
• CNS: central nervous system
• CR: callosal radiations
• CSF: cerebrospinal fluid
• CST: corticospinal tract
• DEHSI: diffuse excessive hypersignal intensity
• DQ: developmental quotient
• DTI: diffusion tensor imaging
• DWI: diffusion weighted imaging
• EPI: echo planar imaging
• FA: fractional anisotropy
• GA: gestational age
• GE: ganglionic eminence
• GABA: gamma-aminobutyric acid
• GLM: general linear model
• GM: gray matter
• GRF: gaussian random field
• GMH-IVH: germinal matrix haemorrhage – intraventricular haemorrhage
• IQ: intelligence quotient
• INF γ: interferon γ
• λ//: longitudinal diffusivity
• λ⊥: transverse diffusivity
• LOTG: lateral occipitotemporal gyrus
• NIRS: near infrared spectroscopy
• MBP: myelin binding protein
• MEG: magnetoencephalography
• MD: mean diffusivity
• MRI: magnetic resonance imaging
• NOS: nitrogen oxygen species
• PHI: periventricular haemorrhagic infarction
• PLIC: posterior limb of the internal capsule
• PMA: post-menstrual age (definition p 24)
• Pre-OL: pre-oligodendrocytes and immature oligodendrocytes
• PTR: posterior thalamic radiations
• PVL: periventricular leukomalacia
• ROI: region of interest
• ROS: reactive oxygen species
• SPM: statistical parametric mapping
• SI: signal intensity
• STG: superior temporal gyrus
• STR: superior thalamic radiations
• STS: superior temporal sulcus
• SVC: small volume corrected
• SVZ: subventricular zone
• TNF: tumor necrosis factor
• TR: thalamic radiations
• VZ: ventricular zone
• WI: weighted-imaging
SUMMARY
Rapid and important cerebral developmental changes occur between the third trimester of gestation and the first postnatal months (Sidman and Rakic, 1982).
Assessment of these changes in term and preterm infants is of great interest, as it provides insights into early brain development but also how early birth may affect normal brain development (Mewes et al., 2006).
Conventional brain magnetic resonance imaging (MRI) is a useful technique to provide structural information on brain development, and several studies have correlated brain structure modifications with specific learning or behavioral problems (Peterson et al., 2003, Woodward et al., 2005, Kapellou et al., 2006, Woodward et al., 2006). Nevertheless, this technique is not sensitive enough to evidence subtle microstructural changes.
Diffusion tensor imaging (DTI), which assesses and quantifies water diffusion in biological tissues at a microstructural level, may provide unique clues to the structure and geometric organization of the cerebral tissues (Le Bihan et al., 2001). DTI takes advantage of the fact that, in the brain, water molecules diffuse more easily in the direction of the fibers than orthogonally to study cortex and white matter (WM). DTI indices like fractional anisotropy (FA), which expresses the fraction of the magnitude of the diffusion tensor attributable to anisotropic diffusion, mean diffusivity (MD), which corresponds to the directionally averaged magnitude of water diffusion, and longitudinal and transverse diffusivity (λ// and λ⊥), which express respectively the parallel and perpendicular diffusion of water molecules, are used to indirectly quantify brain microstructure and evaluate brain damage (Hüppi et al., 1998, Miller et al., 2002, Ment et al., 2009, Liu et al., 2012).
Most previously published studies in neonates limited their analysis to particular zones of the WM, using regions of interest (ROI) to select regions where DTI values are expected to change. Approaches on the basis of ROIs have well-known limitations because strong a priori hypotheses about localization and extent of the effects of interest have to be made (Giuliani et al., 2005). Voxel-based methods of
neuroimaging data analysis, such as statistical parametric mapping (SPM), do not have such limitations and have been successfully applied to study age-related DTI changes in adults, DTI differences between preterm and infants at term equivalent age, and brain structural asymmetries in infants (Ashtari et al., 2007, Snook et al., 2007, Gimenez et al., 2008, Dubois et al., 2010).
Studies correlating DTI indices at term equivalent age with later neurodevelopment are scarce and their analysis is limited to the WM, without exploring the cortex (Arzoumanian et al., 2003). Moreover, they use neuropsychological testing where language evaluation is combined with cognitive and motor scales to give an overall cognitive score (Krishnan et al., 2007, Rose et al., 2007, Rose et al., 2009).
The aims of this work were, using a voxel-based analysis of DTI sequences, 1) to evidence new brain regions that experience microstructural modifications along post- menstrual age (PMA) during early development of the human brain, and 2) to correlate regional brain microstructure at term equivalent age with subsequent cognitive, motor and language development at two years corrected age in a population of preterm infants.
We first investigated DTI changes in a population of 22 healthy preterm and 6 term infants covering the life period between 34 and 41 weeks PMA, and found that, besides the already-evidenced FA increase in the corticospinal tract (CST) and callosal radiations, the thalami and the thalamic radiations experienced linear microstructural changes. These changes were interpreted as a marker of regression of cytoplasmic arborization and proliferation of immature oligodendrocytes that wrapped around the axons well before the appearance of myelin (Aeby et al., 2009).
Then we looked for nonlinear DTI changes, considering that many of biological processes that occur during development follow a nonlinear course. This yielded negative results, probably due to the small sample size. Therefore, in a second study, we searched for regional linear and nonlinear microstructural changes with PMA throughout the brain in a larger population (65 patients) composed exclusively of preterm neonates scanned between 35 and 43 weeks PMA. This study confirmed the linear FA changes with age previously described and, more importantly, evidenced nonlinear changes in brain structures around the right posterior superior temporal
decrease between 34 and 39 weeks followed by FA increase from 40 weeks to 43 weeks. The right STS belongs to the speech-processing network and is implicated in prosody but also in inter-individual communicative behavior and face processings in close association with the right LOTG. We suggest that the microstructural modifications in brain structures around the right STS and in the LOTG observed between 35 and 43 weeks of gestation in preterm infants could contribute to the functional maturation of these brain regions with increasing age, in a period of life where voices, prosody and faces represent extremely salient stimuli (Aeby et al., 2012).
In the second part of the thesis, we tested the hypothesis that abnormal local brain microstructure of preterm infants at term equivalent age would affect neurodevelopmental abilities at age 2 years. Therefore, we searched throughout the whole brain to correlate changes of the Bayley-III scores (cognitive, motor and language composite scores) with the regional distribution of MD, FA, λ// and λ⊥. We found that language abilities are negatively correlated to MD, λ// and λ⊥in the left superior temporal gyrus (STG) in preterm infants. These findings suggest that higher MD, λ// and λ⊥ values at term- equivalent age in the left STG are associated with poorer language scores in later childhood. Consequently, this highlights the key role of the left STG for the development of language abilities in children and suggests that brain DTI might be an interesting tool to assess on an individual basis the development of language in the preterm.
To sum up, in this thesis, we showed that, besides the already-evidenced FA increase in the CST and corpus callosum, the thalami and the thalamic radiations experience linear microstructural changes in the early development of the human brain. We further showed that FA changes nonlinearly with age in brain structures around the right STS and in the right LOTG, which are key regions in verbal and non-verbal communicative behavior. We also showed that voxel-based DTI analysis is able to evidence microstructural changes in the lSTG that are negatively correlated with language development at two years in the preterm at the group level. These results highlight the key role of the lSTG in the development of language in the preterm and suggest that brain DTI might be an interesting tool to predict the development of language on an individual basis.
INTRODUCTION
The third trimester of gestation is a crucial period for brain development. Indeed, during this period of life, the cortical folding occurs and the white matter (WM) experiences profound organizational and maturational changes. Exploring these processes in vivo in humans is of great clinical importance as many neurological and neurobehavioral disorders have their origin in early structural and functional cerebral maturation disturbances (Sidman and Rakic, 1982). Although feasible (Zanin et al., 2011), fetal magnetic resonance imaging (MRI) still remains a technical challenge because of the many sources of artifacts, mainly related to fetal and maternal motions (Kasprian et al., 2008, Kasprian et al., 2011). Studying brain development with conventional brain MRI in term and preterm infants is less prone to these artifacts and provides insights into early brain development and how early birth may affect normal brain development (Mewes et al., 2006). Moreover, it also enables to document brain lesions in the preterm and correlates them with later neurodevelopment (Woodward et al., 2006). Nevertheless, this technique is limited in qualitative assessment and does not allow to evaluate the microstructural organization of the tissue studied. This is possible with another MR modality, diffusion tensor imaging (DTI), which assesses water diffusion in biological tissues at a microstructural level, providing quantitative microstructural assessment and geometric organization of the cerebral tissues (Le Bihan et al., 2001). This non- invasive imaging technique might bring new insights into our understanding of normal brain development, its alteration related to early birth, and the functional outcome in prematurely born infants. Therefore, the aim of this PhD thesis was to evidence, using an original DTI voxel-based analysis (see methods 2.2), the brain regions experiencing significant microstructural modification along post-menstrual age (PMA) and to explore if DTI at term equivalent age, is predictive of later neurodevelopment. To bring this work in perspective with the current knowledge on normal and pathological brain development, I will first describe the brain development focusing on the third trimester of gestation, which is the period that has been studied in this work. I will then review the epidemiology, neurodevelopmental disabilities and the physiopathology of brain injury in the preterm. Finally, I will analyse the assessment of brain development and the prognostic value of
1.Brain development
1.1.Before the third trimester of gestation
During the first trimester, the neural tube differentiates with the formation of the brain and spinal cord regions down to the lower sacral levels (neurulation). Then, at around 6 weeks of gestation, the forebrain prosencephalic structures develop and cleaves, with the formation of the telencephalon (i.e. the cerebral hemispheres), diencephalon and rhombencephalon (ventral induction) (Suzuki, 2007).
At the end of the first trimester and during the second trimester, neurons and glial cells that originate from progenitor cells in the proliferative zones of the dorsal telencephalon (i.e. the ventricular (VZ) and subventricular zones (SVZ)) first proliferate and then migrate to the cortex (Sidman and Rakic, 1973).
There are two main populations of neurons in the cerebral cortex: projection (or pyramidal) neurons which are glutamatergic and excitatory, and interneurons which are gamma-aminobutyric acid (GABA)-ergic and inhibitory. All cortical projection neurons originate from progenitors located in the proliferative zone of the dorsal telencephalon like some of the cortical interneurons. Nevertheless, the majority of the cortical interneurons originate from progenitors located in the ganglionic eminences (GE) of the ventral telencephalon (Faux et al., 2012).
There are two basic modes of cell migration: radial (figure 1) and tangential (figure 2) migration. Glial cells, pyramidal neurons and interneurons originating from the VZ/SVZ migrate radially (figure 1) while cortical interneurons originating from the GE neurons first migrate tangentially to the cortex over a long distance and finally encompass a short-range radial migration into the cortical plate (figure 2) (Faux et al., 2012).
Figure 1: Radial migration: cells are generated by radial glial progenitors, and the clonally related neuron migrates along the parent radial glial fiber (Volpe, 2008).
Figure 2: The proliferation and migration of GABAergic interneurons from the subventricular zone (SVZ) and ventral germinative epithelium of the ganglionic eminence (GE) are shown.
Neurons from the SVZ (blue) migrate radially to the cortex and from the GE (green), tangentially and then radially, to the cortex. The migrating stream of interneurons from the GE to the dorsal thalamus is also shown. GP=globus pallidus.
MN=migrating neurons. P=putamen (Volpe, 2009).
These processes will lead finally, by 20 to 24 weeks of gestation, to the formation of the 6 layers cortex with its full complement of neurons (figure 3) (Volpe, 2009).
Figure 3: Schematic summary of the development of the human prefrontal cortex. At the earliest age studied (10.5 weeks), the preplate zone has been split by the early- arriving neurons of the cortical plate into neurons of the marginal zone (MZ) above and of the subplate zone (SP) below. Note the exuberant neuronal development of the subplate zone into the third trimester of gestation. CP, cortical plate; IZ, intermediate zone; SPL, subplate lower; SPP, subplate-preplate; SPu, subplate zone upper; SV, subventricular zone; V, ventricular zone; WM, white matter (Mrzljak et al., 1988).
1.2 After the third trimester of gestation
After this brief summary on brain development before the third trimester, I will focus on brain development during the third trimester of pregnancy, which is the period that we have explored in this work. I have divided this period into three chapters:
organizational events, cortical folding and myelination.
1.2.1 Organizational events
Organizational events occur during a peak time period from the sixth month of gestation to several years after birth. The major developmental features include:
(1) Subplate neurons
(2) Lamination and neurite outgrowth (3) Axonal development
(4) Synaptic development and regressive events (5) Glial proliferation and differentiation.
1.2.1.1 Subplate neurons
Subplate neurons, which originate from the germinative zones and migrate to the primitive marginal zone at 7 weeks of gestation, are most prominent between 22 and 34 weeks gestation in the subplate zone (figure 3), which explains their vulnerability in the preterm. The functions of subplate neurons are wide: (1) they are a site for synaptic contact for “waiting” thalamo-cortical (24-32 weeks) and cortico-cortical afferents before the formation of the cortical plate, (2) they are a functional link between “waiting” afferents and cortical targets, (3) they have an axonal guidance function for ascending and descending axons to, respectively, their cortical and subcortical targets (Ghosh et al., 1990, Ghosh and Shatz, 1992). After 33 weeks, the subplate zone gradually dissolves and disappears around the 38th weeks of gestation in humans (Kostovic and Jovanov-Milosevic, 2006).
1.2.1.2 Lamination and neurite outgrowth
The lamination (i.e. alignment, orientation and layering of cortical neurons) occurs as neuronal migration ceases. Then, neurite outgrowth (i.e. elaboration of dendritic and axonal ramifications) next becomes the dominant developmental activity in the cerebral cortex. There is a progressive enrichment of dendritic and axonal plexus (Mrzljak et al., 1990) (figure 3). Accompanying that processes are the development of neurofibrils and an increased size of endoplasmic reticulum within the cytoplasm of cells (Mrzljak et al., 1988, Kostovic, 1990). A striking increase in cerebral cortical volume accompanies these developmental changes in cortical neurons during the premature period.
1.2.1.3 Axonal development
Axonal development is remarkably exuberant in the developing cerebrum over the last trimester of gestation and in the early postnatal period (figure 4). Thalamocortical axons are prominent in the cerebral WM to the region of the subplate at 23 weeks
ingrowth of thalamocortical axons in the cortical plate with, for the first time, formation of synapses in the deep cortical plate (Haynes et al., 2005). Between 33-35 weeks, thalamocortical fibres, together with other projection fibres, build up the corona radiata and adapt their course to the process of gyration. After 35 weeks, following a resolution of the subplate zone, there is no further growth of long afferents and cortico-cortical pathways. At the same time, a retraction of a number of exuberant callosal axons begins. In addition, two other processes occur in this developmental period: axonal arborization within the cortical plate and growth of short cortico-cortical connections. In conclusion, in the preterm, the active growth of the axonal pathways explains their vulnerability (Kostovic and Jovanov-Milosevic, 2006).
Figure 4: Cerebrum in coronal section at 28 weeks’
gestation showing critical events in cortical development: (A) The axons (green) emanate from the thalamus (T; projection fibres), corpus callosum (CC; commissural fibres), and cortex (association fibres), which synapse initially on subplate neurons (SPNs). SPNs send axons to the cortex and promote cortical development before the thalamo-cortical and cortico-cortical fibres enter the cortex. From the cortex, axons (blue) descend to the thalamus, basal ganglia, and corticospinal (and corticopontine) tracts.
1.2.1.4 Synaptic development and regressive events
Synaptic formation differs among brain regions in the human brain: for example, in the hippocampus, synapses are abundant as early as at 15 weeks of gestation while in the somatosensory cortex synapses become abundant in the deep cortical plate between 28 and 32 weeks (Kostovic and Jovanov-Milosevic, 2006). Beside synapse formation, two other processes are crucial for the establishment of the brain architecture: axon pruning and programmed neuronal cell death (Vanderhaeghen and Cheng, 2010). Axon pruning enables removal of exuberant or misguided axon branches in the absence of cell death, whereas other appropriate connections of the
same neuron are maintained. In contrast, cell death removes the entire neuron and ultimately leads to the loss of all neurites associated with the dying parent neuron.
These processes are activated by the interaction of the environment with genetically controlled cellular mechanisms and may explain for example the demonstration in both human infants and experimental models that, after neonatal cerebral lesions, ispilateral corticospinal tract (CST) projections can remain and could improve the functional deficit (Barth and Stanfield, 1990). Even if these processes are mainly postnatal (Changeux et al., 1973), one pathological study has shown that sensory–
motor and association cortical areas of the brain start to experience significant cell loss (up to 70%) after 32 weeks of gestation, suggesting that programmed neuronal death may play a major role in early human cortex development (Rabinowicz et al., 1996).
1.2.1.5 Glial proliferation and differentiation
Astrocytes, oligodendrocytes and microglia are the major glial cells of the central nervous system (CNS).
Astrocytes are generated primarily before oligodendrocytes (Kinney and Back, 1998).
The progenitors of both astrocytes and oligodendrocytes are initially cells of the SVZ.
Proliferation of glia, unlike that of neurons, may also occur locally during and after migration. Astrocytes play a variety of complex nutritive and supportive roles in relation to neuronal homeostasis, minimize the consequences of metabolic and structural insults and modulate inflammation, immune responses, production of trophic and neuroprotective factors and tissue remodeling after injury (Gressens et al., 1992).
Oligodendroglial proliferation and differentiation are crucial for myelination and will be discussed later.
Microglia include the resident immune cells of the brain and originate principally from bone marrow-derived monocytes. These cells enter the CNS in the cerebrum during the second trimester within the marginal zone, the cortical plate and subplate and the VZ/SVZ, and migrate from the VZ/SVZ to the cerebral WM (20 to 35 weeks) and then to the cerebral cortex (Billiards et al., 2006). Microglia plays key roles during brain development involving vascularization (Rezaie and Male, 2002, Rezaie
myelination (Hamilton and Rome, 1994). Moreover, when activated by insults such as hypoxia-ischemia or infection-inflammation, they liberate cytokines and reactive oxygen (ROS) and nitrogen oxygen species (NOS), which could injure differentiating oligodendrocytes of the premature infant or neurons of the term infant but also axons and subplate neurons (for details, see 2.4 Brain injury) (Billiards et al., 2006).
1.2.2 Cortical folding
The mechanisms underlying the formation and distribution of cortical folding are assumed to be a combination of genetic control (Rakic, 2004, Van Essen et al., 2006) and mechanical constraints (Van Essen, 1997). Indeed, as the cortex is a closed surface with glial and axonal fibers pulling radially, an increase of the cortical volume, associated with the early genetic architectonic regionalization of the cerebral cortex, leads to the development of convolutions and to their distribution (Toro and Burnod, 2005). The first convolutions to develop, the so-called primary convolutions, are more or less invariant in their location and configuration and begin to develop at mid-gestation, like the calcarine and central sulcus. Most of the primary gyri are already well defined at 26 - 28 weeks of gestation (Chi et al., 1977). Even if during the last trimester no new primary gyri develop, they become more prominent and more deeply infolded. Moreover, there is subsequent development of secondary and even tertiary gyri which are less deep (table1) (Chi et al., 1977).
Table 1: regional development of fissures, sulci and gyri, adapted from (Chi et al., 1977).
Fissures and sulci Gyri
Interhemispheric fissure 10wk Gyrus rectus 16wk
Transverse cerebral fissure 10wk Insula 18wk
Callosal sulcus 14wk Superior temporal gyrus 23wk
Sylvian fissure 14wk Prerolandic gyrus 24wk
Parietooccipital fissure 16wk Superior frontal gyrus 25wk
Calcarine fissure 16wk Postrolandic gyrus 25wk
Rolandic sulcus 20wk Middle temporal gyrus 26wk
Superior temporal sulcus 23wk Middle frontal gyrus 27wk
Collateral sulcus 23wk Fusiform gyrus 27wk
Prerolandic sulcus 24wk Superior occipital gyri 27wk
Postrolandic sulcus 25wk Inferior occipital gyri 27wk
Middle temporal sulcus 26wk Cuneus 27wk
Lateral occipital sulcus 27wk Angular gyrus 28wk
Inferior temporal sulcus 30wk Occipitotemporal gyrus 30wk
Inferior temporal gyrus 30wk
Transverse temporal gyrus 31wk
Paracentral gyri 35wk
1.2.3 Myelination
Myelination is characterized by the acquisition of the highly specialized myelin membrane around axon, begins in the second trimester of gestation and continues in adult life (Kinney HC, 1939).
The four sequential stages of oligodendroglial maturation include:
1) The oligodendroglial progenitor
2) The pre-oligodendrocyte (or late oligodendroglial progenitor; positive for monoclonal antibody O4 i.e. sulfatide)
3) The immature oligodendrocyte (positive for monoclonal antibodies O4 and O1 i.e. galactocerebroside)
4) The mature myelin-producing oligodendrocyte (positive for myelin basic
Figure 5: progression of the oligodendroglial lineage (OL) through the four major stages. The predominant forms in the premature infant are the O4+O1- and O4+O1+
forms (Volpe, 2008)
The progenitor originates in the proliferative VZ/SVZ and migrates into the cerebral WM where differentiation proceeds to pre-oligodendrocytes that predominate in cerebral WM at 28 weeks and account for 90% of the total oligodendroglial population (Back et al., 2001). At 28–40 weeks of gestation, the pre-oligodendrocytes begin their differentiation to immature oligodendrocytes, which account for approximately 30% of the total oligodendrocyte population during the later premature period and about 50% by term (Rivkin et al., 1995). These differentiating forms ensheath axons in preparation for full differentiation to myelin-producing oligodendrocytes (figure 5). Mature, myelin-basic-protein expressing ultimately myelin-producing oligodendrocytes does not become abundant in the cerebral WM until after term. Pre-oligodendrocytes and immature oligodendrocytes will be further named pre-OLs (see 2.4 page 27). Programmed cell death is an important feature of oligodendroglial development with approximately 50 % of oligodendroglia that will undergo apoptosis during development (Barres et al., 1992).
Myelination begins in the peripheral nervous system, where motor roots myelinate before sensory roots. Still before birth, myelin appears in the CNS in the brainstem and cerebellum in components of some major sensory systems (e.g. medial lemniscus for somesthesic stimuli; lateral lemniscus, trapezoid body and brachium of the inferior colliculus for auditory stimuli) and in components of some major motor
systems (e.g. CST in the midbrain and pons and superior cerebral peduncle) (Yakovlev and Lecours 1967, Gilles, 1976). In general and in contrast to the peripheral nervous system, myelination in the central sensory systems tends to precede that in central motor systems (Yakovlev and Lecours 1967). Myelination within the cerebral hemispheres and particularly in regions involved in higher-level associative functions and sensory discriminations (e.g. associative areas, cerebral commissures) occurs after birth and continues to progress over decades. The median age when mature myelin is reached is depicted in table 2.
Table 2: median age when mature myelin is reached (Brody et al., 1987).
Brain region Post frontoparieto- occipital sites
Ant frontotemporal sites
Internal capsule Post limb, 4 wks* Anterior limb, 47wks*
Sensory radiations Optic radiation, 12wks Heschl gyrus, 48wks
Corpus callosum Body, 20wks Rostrum, 47wks
Splenium, 25wks
Central WM Postcentral gyrus, 30wks Temporal lobe, 79wks Posterior frontal, 40wks Temporal pole, 82wks Posterior parietal, 59wks Frontal pole, 79wks Occipital pole, 47wks
* Age shown is post-term age at which 50% of infants attain mature myelin (Brody et al., 1987).
Five major general rules can be derived from the study of Brody et al. (1987) (see figure 6):
(1) proximal pathways myelinate before distal pathways (2) sensory pathways myelinate before motor pathways
(3) projection pathways myelinate before cerebral associative pathways (4) central cerebral sites myelinate before cerebral poles and
(5) occipital poles myelinate before frontotemporal poles .
Overall, the more rapid changes in myelination occurred within the first 8 postnatal months (Kinney et al., 1988).
Figure 6: Progression of myelination. This drawing of the cerebrum depicts the progression of myelination in telencephalic sites from the central sulcus outward to the poles, with the posterior sites preceding the anterior frontotemporal sites (Volpe, 2008).
2. Preterm birth
The third trimester of gestation is precisely the period when preterm birth occurs. In humans, preterm birth (Latin: partus praetemporaneus or partus praematurus) is the birth of a baby of less than 37 weeks gestational age (GA).
Preterm births can be classified according to GA: about 5% of preterm births occur at less than 28 weeks (extreme prematurity), about 15% at 28–31 weeks (severe prematurity), about 20% at 32–33 weeks (moderate prematurity) and 60–70% at 34–
36 weeks (near term).
2.1 Age terminology during the perinatal period
Consistent definitions to describe age in neonates and infants are needed to compare outcomes. Therefore, the American Academy of Pediatrics recommends the use of a standard terminology (figure 7) (Engle et al., 2004):
Gestational age (GA): time elapsed between the 1st day of the last menstrual period and the day of delivery.
Chronological age: time elapsed after birth.
Corrected age: age of the child born preterm from the expected date of delivery.
Postmenstrual age (PMA): time elapsed between the first day of the last menstrual.
period and birth (GA) plus the time elapsed after birth (chronological age).
Figure 7: age terminology during the perinatal period (Engle et al., 2004).
2.2 Epidemiology
In industrialized countries, 5–12% of infants are born preterm, and the rate has been increasing since the early 1980s (MacDorman and Kirmeyer, 2009) (figure 8).
Preterm births account for 75% of perinatal deaths, with over two thirds of these arising in the preterm infants who are delivered before 32 weeks’ gestation (Slattery et al., 2008).
Figure 8: percentage of preterm birth in industrialized countries in 2004 (MacDorman and Mathews, 2009)
The factors that contribute to preterm birth differ between studies but generally include one or more of those shown in table 3 (Slattery and Morrison, 2002).
Table 3: Causes of preterm births
Causes Frequency
Preterm labour 31-50%
Multiple pregnancy 12-28%
Preterm prelabour rupture of membranes 6-40%
Hypertensive disorders of pregnancy 12%
Intrauterine growth restriction 2-4%
Antepartum haemorrhage 6-9%
Cervical incompetence, uterine malfo 8-9%
2.3 Neurodevelopment
Children who were born extremely preterm are at a higher risk of developing deafness (odds ratio, OR=32), blindness (OR= 80) and seizures (OR=20) (Wood et al., 2000, Marlow et al., 2005, Crump et al., 2011).
Already during early childhood, several studies have revealed significant delay in cognitive development: in the EPICURE study, the cohort of extremely preterm children had a mean development index of more than one standard deviation below the normative mean, reflecting significant cognitive delay (Wood et al., 2000) and results were similar for the Victorian Infant Collaborative Study Group (Doyle and Victorian Infant Collaborative Study, 2004). At school age, several meta-analyses have shown that children born preterm are at risk for reduced cognitive test scores, and that their immaturity at birth is directly proportional to the mean cognitive scores at school age. Preterm-born children also show an increased incidence of attention deficit hyperactivity disorder, autism, internalizing behaviors (anxiety, depression, phobias, autism) and poor executive function (Bhutta et al., 2002, Anderson et al., 2003, Aarnoudse-Moens et al., 2009). They also tend to have difficulties in learning, notably in reading, spelling and applying mathematical concepts (Anderson et al., 2003).
Motor impairments are also more frequent in extremely preterm infants. Nearly half of them show a pattern of motor development during their first year of life which is different from that expected for infants born at term, even if few of these infants will develop cerebral palsy (CP) (Bax et al., 2005). Nevertheless, for these infants there is a tendency, although not statistically significant, to find a higher rate of developing minor motor impairments in childhood (Pedersen et al., 2000). In another study, this pattern of minor motor impairment has been found to be more prevalent in extremely preterm infants (19 to 40.5/100) than in the general population (5 to 10/100) (Williams et al., 2010). Moreover, 5 to 15% of children will have CP, with a spectrum of gross motor functional levels and performance developmental indices scores that range from significant to near-normal function and ability, with 30% of CP patients that will be ambulatory at 2 years (Pharoah et al., 1998, Vohr et al., 2005).
Language delay is a common problem for preterm children (Aram et al., 1991,
months, nearly 25% of extremely preterm children will have a speech delay (Wood et al., 2000). At early school-age, extremely preterm children score significantly lower in phonological processing, vocabulary, verbal comprehension and verbal memory (Taylor et al., 1995). By middle school-age, these infants will still exhibit language deficits, performing substantially lower on tests assessing the ability to recall sentences and comprehend and follow directions (Taylor et al., 2000). In adolescence, deficits are observed in higher-order language skills (identifying synonyms, pragmatic language and fluent word generation) (Taylor et al., 2004 ). Thus, based on these observations, we can conclude that language difficulties exhibited by children born extremely preterm are substantial and persist throughout childhood (Anderson and Doyle, 2008).
Consequently, at school age, they experience problems across most educational domains and are 50% more likely to be enrolled in special education classes compared with term-born children (Chaikind and Corman, 1991).
2.4 Brain injury
These neurological deficits are the consequence of brain injury secondary to prematurity. The neuropathological correlates of this encephalopathy include various lesions, most notably periventricular leukomalacia (PVL), and accompanying neuronal/ axonal deficits that involve the cerebral WM, thalamus, basal ganglia, cerebral cortex, brainstem, and cerebellum. Severe germinal matrix haemorrhage – intraventricular haemorrhage (GMH-IVH), particularly with periventricular haemorrhagic infarction (PHI) although an important, albeit quantitatively less common lesion in premature infants (Volpe, 2009) is also associated with neurological deficits.
2.4.1 Periventricular leukomalacia (PVL)
PVL refers to injury to the cerebral WM with two components: focal and diffuse (Volpe, 2009). The focal component consists of localized necrosis deep in periventricular WM with loss of all cellular components and can be macroscopic in size, evolving over several weeks to multiple cystic lesions (i.e. “cystic PVL”). This severe lesion is observed in less than 5% of extremely preterm infants in modern
intensive care units and accounts for a small minority of PVL (Woodward et al., 2006). Much more frequently, focal necrosis is microscopic in size and evolves over several weeks to glial scars that are not readily seen by neuroimaging. This form of PVL, which accounts for the majority of cases, is termed “non-cystic PVL” (figure 9).
In PVL, the primary event is most likely a destructive process (injury) and the subsequent trophic/maturational disturbances are secondary (Volpe, 2009).
The principal pathogenic factors in PVL appear to be cerebral ischaemia, maternal intrauterine (or neonatal) infection and fetal (or neonatal) systemic inflammation.
These upstream mechanisms activate downstream mechanisms—that is, cytokines, excitotoxicity and free radical attack by ROS, RNS—that lead to death of the vulnerable pre-OL.
2.4.1.1 Upstream mechanisms 2.4.1.1.1 Ischemia
Premature infants have a propensity to develop cerebral ischaemia, especially in the WM. The first reason is linked to vascular anatomical factors. Indeed, the deep focal necrotic lesions in PVL occur in periventricular arterial end zones of long penetrating vessels derived mainly from the middle cerebral arteries. These vessels run from the pial surface and terminate in the deep periventricular WM. The terminations of these long penetrators essentially form distal arterial fields and are most sensitive to falls in cerebral perfusion (Takashima et al., 1978). Active development of this periventricular vasculature occurs during the last 16 weeks of human gestation (Ballabh et al., 2004). Thus, in the most immature infants, a lesser degree of ischaemia may cause focal necrotic lesions.
Figure 9: Cystic and non-cystic periventricular leukomalacia (PVL). Coronal sections from the brain of a 28-week-old premature infant. The dorsal cerebral subventricular zone (SVZ), the ventral germinative epithelium of the ganglionic eminence (GE), thalamus (T), and putamen (P)/globus pallidus (GP) are shown. (A) The focal necrotic lesions in cystic PVL (small circles) are macroscopic in size and evolve to cysts. The focal necrotic lesions in non-cystic PVL (black dots) are microscopic in size and evolve to glial scars. The diffuse component of both cystic and non-cystic PVL (pink) is characterized by the cellular changes (Volpe, 2009).
The second cause for developing WM lesions is impaired regulation of cerebral blood flow (CBF). There is accumulating evidence that the brain of the preterm infant often shows impaired cerebrovascular autoregulation in response to changes in blood pressure. The resulting inability to maintain CBF in the face of even minor falls in systemic blood pressure (as often occurs in preterm infants) might lead to ischaemia in the vulnerable arterial end and border zones described above (Khwaja and Volpe, 2008).
2.4.1.1.2 Infection/inflammation
Infection and inflammation (including ischaemia-induced inflammation) together
represent the second major upstream mechanism leading to injury or death of pre- OLs (fig 10). A number of epidemiological studies have shown an association between maternal/fetal infection and sonographically detectable PVL or cerebral palsy (Leviton et al., 2005). Both outcomes are increased in the presence of infections of the decidua, placenta and amniotic fluid, fetal vasculitis, and postnatal sepsis (Inder et al., 2003). Infection/inflammation is associated with production of cytokines (especially tumor necrosis factor (TNF) α and interferon (INF) γ). Raised concentration of TNF α have been found in the CSF of infants with MRI-defined WM injury. Moreover, it has been shown that INF γ is directly toxic to pre-OLs, an effect potentiated by TNF α, suggesting an important role of TNF α and INF γ in the pathogenesis of WM lesions in the preterm (Khwaja and Volpe, 2008).
2.4.1.2 Downstream mechanisms
Hypoxia-ischemia as well as infection-inflammation, besides a direct toxic effect on pre-OLs, will also activate microglia. Microglia, that include the resident immune cells of the brain, will liberate cytokines but also glutamate and ROS/RNS. Several studies have shown that pre-OL’s but also axons and subplate neurons (Billiards et al., 2006) are vulnerable to free radical attack and excitotoxicity that will injure WM in the preterm (figure 10).
The final result is a deficit of mature oligodendroglia and a consequent impairment of myelination, the hallmark of PVL. However, as oligodendrocytes have a critical trophic role for axonal development, pre-OLs injury could also lead to failure of axonal development and ultimately axonal degeneration. The consequence of axonal deficiency is diminished cerebral cortical and thalamic/basal ganglia volumes secondary to retrograde and anterograde (trans-synaptic) effects (i.e. projection fibers to and from the cortex, the thalamus and the basal ganglia).
Several studies have also highlighted a particular vulnerability of axons (Haynes et al., 2008), thalamus (Ligam et al., 2009), subplate neurons (Robinson et al., 2006) and SVZ (Robinson et al., 2006) in the pathophysiology of PVL.
Figure 10: Mechanisms in the pathogenesis of PVL (Khwaja and Volpe, 2008)
2.4.2 Germinal matrix haemorrhage (GMH)
The basic lesion in GMH is bleeding in the subependymal germinal matrix. This region is represented by the GE described earlier (see 1.1). Over the final 12 to 16 weeks of gestation, this matrix becomes less prominent and is exhausted by term.
This region is highly cellular and richly vascularized with an elaborated capillary bed.
Several factors may favor the occurrence of GMH: intravascular factors (fluctuating CBF, increase CBF, increase cerebral venous pressure, decrease CBF followed by reperfusion, platelet and coagulation disturbance) and vascular factors (tenuous capillary integrity, vulnerability to hypoxic-ischemic events).
GMH may greatly vary in severity, from no intraventricular hemorrhage (IVH) (grade I) to GMH with IVH (grades II and III) and the severe form GMH-IVH with post-hemorrhagic infarction (PHI) (grade IV). This latter form accounts for most of the cases experiencing neurological disability secondary to GMH-IVH. Although rare (4-5 % of all premature infants with extreme prematurity), the incidence is higher in the most immature infants (Murphy et al., 2002). The haemorrhage destroys the germinal matrix and the associated venous infarction (the PHI) destroys the dorsal telencephalic SVZ and the surrounding WM, including pre-OL and axons. The latter result in large areas of tissue loss, thalamocortical fibers interruption and perturbation of the cortical development.
3. Brain imaging
The most widely used imaging technique for evaluating these brain lesions in the preterm is cranial ultrasound. Indeed, it can be performed bedside and will detect major brain abnormalities (haemorrhage, cysts, ventricular dilatation). Nevertheless, non-cystic PVL and diffuse WM injury, which are the most common form of WM lesion in the preterm, are not readily seen by ultrasound although MRI has a higher sensitivity for the detection of these lesions (Woodward et al., 2006). Therefore, brain MRI is essential for imaging accurately the neonatal brain. The two greatest obstacles that are present when imaging infants and children with MRI are the proper preparation of the subject and the selection of the right combination of imaging parameters to achieve the desired result. MR systems are designed to accommodate the adult population; however, with a little effort and careful planning they can be successfully used to scan newborns and infants.
3.1 Practical issue with MR imaging in the term and preterm infant
Infants who undergo MRI often require sedation to minimize motion artifacts.
Nevertheless, in infants of less than 1 month corrected age, it is possible, with particular precautions, to minimize movements during data acquisition and to perform MRI without sedation.
In our institution, we developed an original MRI data acquisition protocol that allows to obtain quality MR imaging in unsedated infants when we started our study on DTI in infants in 2005 (table 1).
Table 1 : patient preparation
• Perform MRI early in the morning (8:00 pm)
• Give a bath and feed the infant 45 minutes before exam to favor natural sleep
• Perform metal check prior entrance in the scanner room
• When asleep, settle the subject in a vacuum-immobilization pillow to minimize movements during the procedure
• Place ear-muffs to minimize noise-discomfort
Effective immobilization of the infant is vital in order to obtain high-quality MR images. Therefore, we used a Vac-Lok immobilization pillow, which contains small polystyrene balls (figure 11). Air is evacuated with suction to fit perfectly around the baby’s head and body. This bag also helps to muffle sound. In order to maintain the infant’s temperature, he or she is swaddled in blankets and the temperature of the room is set at 24°C. Oxygen saturation and heart rate is monitored throughout MRI examination thanks to MR compatible equipment.
Figure 11: Infant wrapped in Vac-Lok immobilization pillow for MRI (personal data)
3.2 Conventional imaging of the neonatal brain using Magnetic Resonance Imaging 3.2.1 Technique
The neonatal brain has a higher water content (92-95%) than the adult brain (82-85%) and therefore T1 and T2 values are greater (Johnson et al., 1983). As a result, the
pulse sequences need to be adjusted to allow for the different MR properties of the immature brain. MR imaging uses the resonance of protons to generate images.
Proton spins are excited by a radiofrequency pulse of an appropriate frequency and then induce radiofrequency signal in a coil located close to the tissue of interest. The signal decreases with an exponential curve characterized by the transverse relaxation time T2. The signal amplitude is also influenced by the longitudinal relaxation time T1. T1-weighted images (WI) have low gray matter (GM)/WM contrast but are useful in assessing haemorrhage, brain swelling in hypoxic-ischemic encephalopathy and for assessing tissue enhancement after administration of contrast. Inversion recovery (IR) pulse sequences are used to give heavy T1 WI. IR sequences offer an extremely high contrast between GM and WM, and also in the WM between myelinated and unmyelinated tissues. T2-WI has also good GM/WM contrast and is useful for demonstrating myelination in the premature brain (Counsell et al., 2002, Rutherford, 2002).
3.2.2 Assessment of brain maturation in the fetus and normal term infant
In the neonatal brain, unmyelinated WM has a low signal intensity (SI) on T1-WI and high SI on T2-WI. Cerebrospinal fluid (CSF) is hypointense on T1-WI and hyperintense on T2-WI (figure 12).
MRI will illustrate identical anatomic sequences as histology, but with a time delay compared to histological studies (Girard et al., 1991). As myelination proceeds, the water content of WM decreases, causing a reduction in SI on T2-WI. There is a corresponding increase in glycolipids, cholesterol and proteins, which causes an increase in SI on T1-WI (Barkovich, 2000). Gyration of the cortex on fetal MRI starts with the appearance of a shallow identation of the fetal brain in the temporal regions at 18 weeks GA. At the same time, the anterior cingular fissure and the parietooccipital fissure occur. The central sulcus begins to be visible at 24 weeks GA.
Fissure then become deeper, tighter and gyri bulge into the subarachnoid space. Until 35 weeks GA, all primary and most of the secondary sulci are present (Garel et al., 2001, Prayer et al., 2006).
Figure 12: Cerebral appearance in the term neonate on T1- and T2-WI. The anterior brainstem at the level of the pons is not yet fully myelinated and appears of low signal intensity on T1-WI (A) and bright signal on T2-WI (D) compared to the remaining areas of the brainstem. Myelination is also present in the posterior limb of the internal capsule with a bright signal on T1-WI (B). Note that myelination is present in the thalami (E, hyposignal on T2-WI) and in the white matter underlying the central sulcus (F, hyposignal on T2-WI and hypersignal on T1-WI) (personal data).
3.2.3 Assessment of brain maturation in the preterm infant
In a study of 26 very preterm infants (< 30 weeks GA), hyperintense T1 SI or hypointense T2 SI to unmyelinated WM, suggestive of myelin, has been shown in several regions of the brainstem, decussation of the superior cerebellar peduncles, medial lemnisci, lateral lemnisci at <28 weeks PMA (Counsell et al., 2002). From this GA, myelination was not visualized at any new site until 36 weeks PMA, when SI suggestive of myelin presence was visualized in the corona radiata, posterior limb of the internal capsule (PLIC), CST of the precentral and postcentral gyri, and lateral geniculate bodies (Counsell et al., 2002).
The germinal matrix is visible up to 32 weeks PMA as a prominent structure at the margins of the lateral ventricles. After this age, small residual areas of the germinal
matrix has a high SI on T1-WI and low SI on T2-WI. The cerebral cortex has high SI on T1-WI and low SI on T2-WI. The very premature brain has little sulcation and gyration at 24 weeks PMA but this evolves rapidly.
By applying dedicated post-processing tools to MRI data acquired shortly after birth over a developmental period critical for the human cortex maturation (26-36 weeks PMA), Dubois et al. noninvasively investigated the sulcal emergence in 35 preterm newborns. They observe a trend towards lower cortical surface, smaller cortex, and WM volumes, but equivalent sulcation in females compared with males and could evidence interhemispherical asymmetries, with a larger right superior temporal sulcus (STS) than the left (Dubois et al., 2008a). Moreover, in a voxel-based analyses of cortical and WM masks over a group of 25 newborns from 26 to 36 weeks PMA, asymmetries were observed, with a deeper STS on the right side, a larger posterior region of the sylvian fissure on the left side, close to planum temporale and a larger anterior region of the sylvian fissure on the left side, close to Broca's region (Dubois et al., 2010).
3.2.4 Assessment of brain injury in preterm infant
The developing brain is highly susceptible to injury including PVL, GMH-IVH+/- PHI.
Brain MRI performed at term-equivalent age allows the detection of the 3 components of WM injury (cystic PVL, non-cystic PVL and diffuse WM injury or DEHSI see below) (figure 13). Several studies have shown that, in 75% of the preterm infants born < 30 weeks GA, there are several areas of excessive T2 SI (Diffuse Excessive Hypersignal Intensity or DEHSI) within the cerebral WM (Maalouf et al., 1999, Counsell et al., 2003, Krishnan et al., 2007) that probably represent a form of diffuse WM disease in the preterm infant (figure 13) (Counsell et al., 2003). These changes are most marked in the periventricular WM and decrease with age. Even in patients without overt brain lesion, several studies have shown that preterm infants have, compared with term control infants, global and regional decreases in cortical GM and deep GM, less myelinated WM, smaller corpus callosum and substantial ventriculomegaly (Huppi et al., 1998, Peterson et al., 2000, Boardman et al., 2007).
Figure 13: Periventricular leukomalacia. Infant born at 33 weeks gestational age.
Brain MRI performed at 38 weeks PMA revealing multiple periventricular cysts (light blue arrows), scars corresponding to non-cystic PVL (red arrow) and DEHSI (dark blue arrows) on T2 WI (personal data).
In older children, PVL is also associated with reduction in WM, reduction in myelination and angular dilatation of the posterior lateral ventricles (figure 14).
GMH-IVH is revealed by MRI as low signal on T2-WI and high SI on T1-WI. It can be differentiated from the normal germinal layer by its irregular appearance and its slightly more hypointense SI on T2-WI. PHI is a consequence of large IVH obstructing the terminal veins, and results in interruption of projection and association fibres as well as oligodendroglial damage, which disrupts myelination. It is revealed by MRI as a fanshaped lesion of low SI on T2-WI. In surviving infants, a porencephalic cyst usually develops at the site of the lesion (Volpe, 2009).
Figure 14: Periventricular leukomalacia (PVL). Infant born at 26 weeks gestation.
Spastic diplegia diagnosed at 6 months corrected age with brain MRI revealing PVL (data not shown). MRI at 10 years old (A) T1-WI: there is cyst formation around both ventricles (arrows). (B) T1-WI: there is an angular dilatation of the posterior lateral ventricle (personal data).
3.2.5 Clinical application
3.2.5.1 Brain MRI performed during the neonatal period
In the preterm, brain MRI performed at term-equivalent-age is more predictive of later neurodevelopment than early MRI (Dyet et al., 2006).
One study on preterm neonates has correlated WM volumes in the sensorimotor and midtemporal regions with measures of neurodevelopmental outcome (mental development index of the Bayley Scales of Infant Development II) (Peterson et al., 2003). In infants with PVL, abnormal signal in the PLIC at term-equivalent age is associated with the presence of motor impairment at one year (Roelants-van Rijn et al., 2001).
In infants with GMH-IVH with PHI, the appearance of myelin in the PLIC at term
infants (De Vries et al., 1999).
Several studies have linked the progression of cortical folding with behavioral functions at term equivalent age. These studies suggested that smaller cortical volumes and impaired sulcation were associated with impaired behavioral functions in preterm infants (Dubois et al., 2008a, Vasileiadis et al., 2009).
3.2.5.1.1 The Woodward score, an important Brain MRI score to assess the prognosis of very preterm infants
In a study on 167 very preterm infants, Woodward et al. (2006) show that abnormal findings on MRI at term equivalent strongly predict adverse neurodevelopmental outcomes at two years of age (Woodward et al., 2006). Neurological development was evaluated with the Bayley-II. The WM abnormality was graded according to five scales, which assessed the nature and extent of WM signal abnormality, the loss in the volume of periventricular WM and the extent of any cystic abnormalities, ventricular dilatation or the thinning of the corpus callosum. The GM abnormality was graded according to three scales, which assessed the extent of GM signal abnormality, the quality of gyral maturation and the size of subarachnoid space.
Composite WM and GM scores were created and used to categorize infants according to the extent of their cerebral abnormalities. The categories of WM abnormality were none (a score of 5 to 6), mild (a score of 7 to 9), moderate (a score of 10 to 12) and severe (a score of 13 to 15) (figure 15). GM was categorized as normal (a score of 3 to 5) or abnormal (a score of 6 to 9). This study has shown that increasing severity of WM abnormalities on MRI at term equivalent age is associated with poorer performance on the cognitive and psychomotor scales of the BSID-II as well as increasing risk of severe cognitive delay, severe motor delay, CP and neurosensory impairment (i.e. hearing or visual impairment). Preterm infants with GM abnormalities at term equivalent had poorer scores on the cognitive index and the psychomotor index of the Bayley-II and had higher risks of severe cognitive delay, severe motor delay and CP.
