FUNCTIONAL MR IMAGING IN PEDIATRICS

The NIH MRI study of normal brain development (Objective-2): newborns, infants, toddlers, and preschoolers

The Magn. Reson. Imaging (MRI) study of normal brain development currently conducted by the Brain Development Cooperative Group represents the most extensive MRI study of brain and behavioral development from birth through young adulthood ever conducted. This multi-center project, sponsored by four Institutes of the National Institutes of Health, uses a combined longitudinal and cross-sectional design to characterize normal, healthy brain and behavioral development. Children, ages newborn through 18-plus years of age, receive comprehensive behavioral, neurological and multimodal MRI evaluations via Objective-2 (birth through 4-years 5-months of age) and Objective-1 (4-years 6-months through 18 years of age and older). This report presents methods (e.g., neurobehavioral assessment, brain scan) and representative preliminary results (e.g., growth, behavior, brain development) for children from newborn through 4-years 5-months of age. To date, 75 participants from birth through 4-years 5-months have been successfully brain scanned during natural sleep (i.e., without sedation); most with multiple longitudinal scans (i.e., 45 children completing at least three scans, 22 completing four or more scans). Results from this younger age range will increase our knowledge and understanding of healthy brain and neurobehavioral development throughout an important, dynamic, and rapid growth period within the human life span; determine developmental associations among measures of brain, other physical characteristics, and behavior; and facilitate the development of automated, quantitative MR image analyses for neonates, infants and young children. The correlated brain MRI and neurobehavioral database will be released for use by the research and clinical communities at a future date.

Functional MRI of the newborn

In order to provide accurate prognosis and developmental intervention to newborns, new methods of assessing cerebral functions are needed. The non-invasive technique of functional magnetic resonance imaging (fMRI) can be considered as the leading technique for functional exploration of the infant's brain. Several studies have previously applied fMRI in both healthy and diseased newborns with different sensory and cognitive tasks. In this chapter, the methodological issues that are proper to the use of fMRI in the newborn are detailed. In addition, an overview of the major findings of previous fMRI studies is provided, with a focus on notable differences from those in adult subjects. More specifically, the functional responses and the localization of cortical activations in healthy and diseased newborns are discussed. We expect a rapid expansion of this field and the establishment of fMRI as a valid clinical diagnostic tool in the newborn.

The role of functional neuroimaging in pediatric brain injury

The aim of this article is to review empirical studies published in the last 10 years that used various functional neuroimaging techniques to assess pediatric patients with brain injury. Overall, these studies have demonstrated the ability of functional neuroimaging to offer unique information concerning the diagnosis, clinical outcome, and recovery mechanisms after pediatric brain injury. Future research using functional neuroimaging is recommended to better understand the functional reorganization and neurodevelopmental consequences resulting from brain injury. Such research might allow clinicians to design tailored early-intervention and rehabilitation programs to maximize the recovery process for pediatric patients. Limitations and advantages associated with the use of functional neuroimaging in pediatric populations are discussed.

Breath holding reveals differences in fMRI BOLD signal in children and adults

Application of fMRI to studies of cognitive development is of growing interest because of its sensitivity and non-invasive nature. However, interpretation of fMRI results in children is presently based on vascular dynamics that have been studied primarily in healthy adults. Comparison of the neurological basis of cognitive development is valid to the extent that the neurovascular responsiveness between children and adults is equal. The present study was designed to detect age-related vascular differences that may contribute to altered BOLD fMRI signal responsiveness. We examined BOLD signal changes in response to breath holding, a global, systemic state change in brain oxygenation. Children exhibited greater percent signal changes than adults in grey and white matter, and this was accompanied by an increase in noise. Consequently, the volume of activation exceeding statistical threshold was reduced in children. The reduced activation in children was well modeled by adding noise to adult data. These findings raise the possibility that developmental differences in fMRI findings between children and adults could, under some circumstances, reflect greater noise in the BOLD response in the brains of children than adults. BOLD responses varied across brain regions, but showed similar regional variation in children and adults.

The influence of cortical maturation on the BOLD response: an fMRI study of visual cortex in children

We performed blood oxygenation level-dependent (BOLD) functional MR imaging in 11 children younger than 5 y of age and 10 children older than 5 y of age. All but five of the children in the older age group were tested under light anesthesia. We examined the cerebral oxidative metabolism (CMRO(2)) associated with the processing of a flashed and a reversing checkerboard stimulus. These stimuli had been shown in a previous study to induce identical vascular responses. The reversing checkerboard activated twice the neuronal population of the flashed checkerboard, doubling the CMRO(2) associated with it. We compared the extent of activation for the positive BOLD response and found that it did not differ between the different age groups. We estimated the oxidative metabolism by examining the change in the local deoxyhemoglobin (HbR) concentration using Delta R2*. Because both stimuli induced the same vascular response, any increase in oxygen requirement would have to be met by the identical blood volume. Increasing CMRO(2) will therefore result in an increase in the oxygen extraction fraction (OEF), which raises the local HbR concentration. In the younger children, both checkerboard stimuli produced identical, high HbR concentrations. In the older children, the HbR concentration to the flashed stimulus was significantly lower than to the reversing stimulus. We conclude that, for identical stimuli, the oxidative energy requirement associated with the cortical processing is higher in young children than in older children because the presence of superfluous synaptic connections in the immature visual system activates a larger neuronal population.

What effect does measuring children under anesthesia have on the blood oxygenation level-dependent signal? A functional magnetic resonance imaging study of visual cortex

We performed functional magnetic resonance measurements involving visual stimuli on 10 children. Half of the children were measured awake, the other half were measured under light Sevoflurane anesthesia corresponding to 0.5 mean alveolar concentration. Each child was presented with a flashed and a reversing checkerboard, which previous investigations have shown to induce identical increases in cerebral blood flow. The latter stimulus activated double the number of neurons as the former so that cerebral metabolic rate of oxygen consumption (CMRO(2)) was doubled, leading to an effective rise of the oxygen extraction fraction. We measured the extent of activation by counting the number of activated pixels and assessed the change in CMRO(2) by measuring the change in the local deoxyhemoglobin (HbR) concentration, using change in spin relaxivity. In both groups of children, the extent of activation was larger for the flashed than the reversing checkerboard, although the absolute number of activated voxels was smaller for the children who were measured under anesthesia. The HbR concentration was significantly higher during the presentation of the reversing compared with the flashed checkerboard. The relative change in the HbR concentration to the flashed and reversing checkerboard was the same in the children who were measured under anesthesia as in the children who were measured awake. We conclude that light levels of anesthesia may reduce the extent of activation but does not unduly influence either CMRO(2) or cerebral blood flow, thus preserving the blood oxygenation level-dependent signal amplitude.

Decrease in haemoglobin oxygenation during absence seizures in adult humans

Near-infrared spectroscopy (NIRS) is a noninvasive method that allows the assessment of activation-induced cortical oxygenation changes in humans. It has been demonstrated that an increase in oxygenated and a decrease in deoxygenated haemoglobin can be expected over an area activated by functional stimulation. Likewise, an inverse oxygenation pattern has been shown to be associated with cortical deactivation. The aim of the current study was to determine the oxygenation changes that occur during absence seizures. We performed ictal NIRS simultaneously with video-EEG telemetry in three adult patients with typical absence seizures. NIRS probes were placed over the frontal cortex below the F1/F2 leads. During all absence seizures studied, pronounced changes in cerebral Hb-oxygenation were noted and there were no changes in the interval. We observed a reproducible decrease in [oxy-Hb] and an increase in [deoxy-Hb] during absence seizures indicating a reduction of cortical activity. Oxygenation changes started several seconds after the EEG-defined absence onset and outlasted the clinically defined event by 20-30 s.

Functional magnetic resonance imaging: a review of methodological aspects and clinical applications

This paper gives an overview of the recent literature on methodological developments of functional magnetic resonance imaging (fMRI) and recent trends in clinical applications. With the recent introduction of high-field systems and methodological developments leading to more robust signal behavior, fMRI is in a transition state from a research modality for use by experts to a standard procedure with useful applications in patient management. Compared to the use in neuroscientific research, which is often based on BOLD techniques alone, the application in patients is distinguished by a multiparametric characterization of the brain using a combination of several techniques. Neuronal fiber tracking based on diffusion anisotropy measurements, in particular, has already turned out to provide relevant supplementary information to the BOLD-based cortical activation maps.

Developmental and regional changes in the neurochemical profile of the rat brain determined by in vivo 1H NMR spectroscopy

Sixteen metabolites were quantified from 11-24 micro l volumes in three different brain regions (hippocampus, striatum, and cerebral cortex) during postnatal development. Rat pups from the same litter were repeatedly measured on postnatal days 7, 10, 14, 21, and 28 using a completely noninvasive and longitudinal study design. Metabolite quantification was based on ultra-short echo-time (1)H NMR spectroscopy at 9.4 T and LCModel processing. Most of the brain metabolites were quantified with Cramer-Rao lower bounds (CRLB) less than 20%, which corresponded to an estimated concentration error <0.2 micro mol/g. Taurine and total creatine were quantified with CRLB < or = 5% from all 114 processed spectra. The resulting high reliability and reproducibility revealed significant regional and age-related changes in metabolite concentrations. The most sensitive markers for developmental and regional variations between hippocampus, striatum, and cerebral cortex were N-acetylaspartate, myo-inositol, taurine, glutamate, and choline compounds. Absolute values of metabolite concentrations were in very good agreement with previously published in vitro results based on chromatographic measurements of brain extracts. The current data may serve as a reference for studies focused on developmental defects and pathologies using neonatal rat models.

Functional organization of the brain with malformations of cortical development

We examined the localization of cerebral functions in 28 patients with focal epilepsy and malformations of cortical development (MCDs). Polymicrogyria occurred in nine, hemimegalencephaly in four, heterotopia in eight, and focal cortical dysplasia (FCD) in nine cases. We used simple (sensomotor, visual) or complex (language, memory) functional magnetic resonance imaging (fMRI) paradigms. Two thirds of MCDs were activated by simple fMRI paradigms, whereas they less frequently showed activity during complex cognitive fMRI paradigms. During simple paradigms, all disturbances of cortical organization (polymicrogyria, schizencephaly, and mild-type FCD) showed activity, whereas other MCDs (disturbances of earlier steps of cortical development: hemimegalencephaly, Taylor-type FCD, and heterotopia) showed activity in only 44% (p < 0.01). The association between the pathophysiology and morphology of MCDs confirms the recently proposed classification system. Both focal neurological signs (p < 0.05) and focal electroencephalogram slowing (p < 0.05) independently correlated with MCD inactivity, confirming that fMRI showed neuronal functions of MCDs. Conclusively, fMRI visualizes the MCD functions and their relationship to the eloquent cortex, providing useful information before epilepsy surgery. Surgery of cortical organization disturbances should be cautiously performed because these malformations may participate to some degree in brain functions.

Beyond the visible--imaging the human brain with light

Optical approaches to investigate cerebral function and metabolism have long been applied in invasive studies. From the neuron cultured to the exposed cortex in the human during neurosurgical procedures, high spatial resolution can be reached and several processes such as membrane potential, cell swelling, metabolism of mitochondrial chromophores, and vascular response can be monitored, depending on the respective preparation. The authors focus on an extension of optical methods to the noninvasive application in the human. Starting with the pioneering work of Jöbsis 25 years ago, near-infrared spectroscopy (NIRS) has been used to investigate functional activation of the human cerebral cortex. Recently, several groups have started to use imaging systems that allow the generation of images of a larger area of the subject's head and, thereby, the production of maps of cortical oxygenation changes. Such images have a much lower spatial resolution compared with the invasively obtained optical images. The noninvasive NIRS images, however, can be obtained in undemanding set-ups that can be easily combined with other functional methods, in particular EEG. Moreover, NIRS is applicable to bedside use. The authors briefly review some of the abundant literature on intrinsic optical signals and the NIRS imaging studies of the past few years. The weaknesses and strengths of the approach are critically discussed. The authors conclude that NIRS imaging has two major advantages: it can address issues concerning neurovascular coupling in the human adult and can extend functional imaging approaches to the investigation of the diseased brain.