MR assessment of the brain maturation during the perinatal period: quantitative T2 MR study in premature newborns

Neurodevelopmental outcome in preterm infants with intraventricular hemorrhages: the potential of quantitative brainstem MRI

OBJECTIVES

This retrospective study aimed to identify quantitative magnetic resonance imaging markers in the brainstem of preterm neonates with intraventricular hemorrhages. It delves into the intricate associations between quantitative brainstem magnetic resonance imaging metrics and neurodevelopmental outcomes in preterm infants with intraventricular hemorrhage, aiming to elucidate potential relationships and their clinical implications.

MATERIALS AND METHODS

Neuroimaging was performed on preterm neonates with intraventricular hemorrhage using a multi-dynamic multi-echo sequence to determine T1 relaxation time, T2 relaxation time, and proton density in specific brainstem regions. Neonatal outcome scores were collected using the Bayley Scales of Infant and Toddler Development. Statistical analysis aimed to explore potential correlations between magnetic resonance imaging metrics and neurodevelopmental outcomes.

RESULTS

Sixty preterm neonates (mean gestational age at birth 26.26 ± 2.69 wk; n = 24 [40%] females) were included. The T2 relaxation time of the midbrain exhibited significant positive correlations with cognitive (r = 0.538, P < 0.0001, Pearson's correlation), motor (r = 0.530, P < 0.0001), and language (r = 0.449, P = 0.0008) composite scores at 1 yr of age.

CONCLUSION

Quantitative magnetic resonance imaging can provide valuable insights into neurodevelopmental outcomes after intraventricular hemorrhage, potentially aiding in identifying at-risk neonates. Multi-dynamic multi-echo sequence sequences hold promise as an adjunct to conventional sequences, enhancing the sensitivity of neonatal magnetic resonance neuroimaging and supporting clinical decision-making for these vulnerable patients.

Impact of Prematurity on the Tissue Properties of the Neonatal Brain Stem: A Quantitative MR Approach

BACKGROUND AND PURPOSE

Preterm birth interferes with regular brain development. The aim of this study was to investigate the impact of prematurity on the physical tissue properties of the neonatal brain stem using a quantitative MR imaging approach.

MATERIALS AND METHODS

A total of 55 neonates (extremely preterm [n = 30]: <28 + 0 weeks gestational age; preterm [n = 10]: 28 + 0-36 + 6 weeks gestational age; term [n = 15]: ≥37 + 0 weeks gestational age) were included in this retrospective study. In most cases, imaging was performed at approximately term-equivalent age using a standard MR protocol. MR data postprocessing software SyMRI was used to perform multidynamic multiecho sequence (acquisition time: 5 minutes, 24 seconds)-based MR postprocessing to determine T1 relaxation time, T2 relaxation time, and proton density. Mixed-model ANCOVA (covariate: gestational age at MR imaging) and the post hoc Bonferroni test were used to compare the groups.

RESULTS

There were significant differences between premature and term infants for T1 relaxation time (midbrain: P < .001; pons: P < .001; basis pontis: P = .005; tegmentum pontis: P < .001; medulla oblongata: P < .001), T2 relaxation time (midbrain: P < .001; tegmentum pontis: P < .001), and proton density (tegmentum pontis: P = .004). The post hoc Bonferroni test revealed that T1 relaxation time/T2 relaxation time in the midbrain differed significantly between extremely preterm and preterm (T1 relaxation time: P < .001/T2 relaxation time: P = .02), extremely preterm and term (T1 relaxation time/T2 relaxation time: P < .001), and preterm and term infants (T1 relaxation time: P < .001/T2 relaxation time: P = .006).

CONCLUSIONS

Quantitative MR parameters allow preterm and term neonates to be differentiated. T1 and T2 relaxation time metrics of the midbrain allow differentiation between the different stages of prematurity. SyMRI allows for a quantitative assessment of incomplete brain maturation by providing tissue-specific properties while not exceeding a clinically acceptable imaging time.

Synthetic MRI demonstrates prolonged regional relaxation times in the brain of preterm born neonates with severe postnatal morbidity

BACKGROUND

To identify preterm infants at risk for neurodevelopment impairment that might benefit from early neurorehabilitation, early prognostic biomarkers of future outcomes are needed.

OBJECTIVE

To determine whether synthetic MRI is sensitive to age-related changes in regional tissue relaxation times in the brain of preterm born neonates when scanned at term equivalent age (TEA, 37-42 weeks), and to investigate whether severe postnatal morbidity results in prolonged regional tissue relaxation times.

MATERIALS AND METHODS

This retrospective study included 70 very preterm born infants scanned with conventional and synthetic MRI between January 2017 and June 2019 at TEA. Infants with severe postnatal morbidity were allocated to a high-risk group (n = 22). All other neonates were allocated to a low-risk group (n = 48). Linear regression analysis was performed to determine the relationship between relaxation times and postmenstrual age (PMA) at scan. Analysis of covariance was used to evaluate the impact of severe postnatal morbidity in the high-risk group on T1 and T2 relaxation times. Receiver operating characteristic (ROC) curves were plotted and analysed with area under the ROC curve (AUC) to evaluate the accuracy of classifying high-risk patients based on regional relaxation times.

RESULTS

A linear age-related decrease of T1 and T2 relaxation times correlating with PMA at scan (between 37 and 42 weeks) was found in the deep gray matter, the cerebellum, the cortex, and the posterior limb of the internal capsule (PLIC) (p < .005 each), but not in the global, frontal, parietal, or central white matter. Analysis of covariance for both risk groups, adjusted for PMA, revealed significantly prolonged regional tissue relaxation times in neonates with severe postnatal morbidity, which was best illustrated in the central white matter of the centrum semiovale (T1 Δ = 11.5%, T2 Δ = 13.4%, p < .001) and in the PLIC (T1 Δ = 9.2%, T2 Δ = 6.9%, p < .001). The relaxation times in the PLIC and the central white matter predicted high-risk status with excellent accuracy (AUC range 0.82-0.86).

CONCLUSION

Synthetic MRI-based relaxometry in the brain of preterm born neonates is sensitive to age-related maturational changes close to TEA. Severe postnatal morbidity correlated with a significant delay in tissue relaxation. Synthetic MRI may provide early prognostic biomarkers for neurodevelopment impairment.

Development of Brain Networks In Utero: Relevance for Common Neural Disorders

Magnetic resonance imaging, histological, and gene analysis approaches in living and nonliving human fetuses and in prematurely born neonates have provided insight into the staged processes of prenatal brain development. Increased understanding of micro- and macroscale brain network development before birth has spurred interest in understanding the relevance of prenatal brain development to common neurological diseases. Questions abound as to the sensitivity of the intrauterine brain to environmental programming, to windows of plasticity, and to the prenatal origin of disorders of childhood that involve disruptions in large-scale network connectivity. Much of the available literature on human prenatal neural development comes from cross-sectional or case studies that are not able to resolve the longitudinal consequences of individual variation in brain development before birth. This review will 1) detail specific methodologies for studying the human prenatal brain, 2) summarize large-scale human prenatal neural network development, integrating findings from across a variety of experimental approaches, 3) explore the plasticity of the early developing brain as well as potential sex differences in prenatal susceptibility, and 4) evaluate opportunities to link specific prenatal brain developmental processes to the forms of aberrant neural connectivity that underlie common neurological disorders of childhood.

Postnatal Guinea Pig Brain Development, as Revealed by Magnetic Resonance and Diffusion Kurtosis Imaging

This study used in vivo magnetic resonance imaging (MRI) to identify age dependent brain structural characteristics in Dunkin Hartley guinea pigs. Anatomical T₂-weighted images, diffusion kurtosis (DKI) imaging, and T₂ relaxometry measures were acquired from a cohort of male guinea pigs from postnatal day (PND) 18–25 (juvenile) to PND 46–51 (adolescent) and PND 118–123 (young adult). Whole-brain diffusion measures revealed the distinct effects of maturation on the microstructural complexity of the male guinea pig brain. Specifically, fractional anisotropy (FA), as well as mean, axial, and radial kurtosis in the corpus callosum, amygdala, dorsal-ventral striatum, and thalamus significantly increased from PND 18–25 to PND 118–123. Age-related alterations in DKI measures within these brain regions paralleled the overall alterations observed in the whole brain. Age-related changes in FA and kurtosis in the gray matter-dominant parietal cerebral cortex and dorsal hippocampus were less pronounced than in the other brain regions. The regional data analysis revealed that between-age changes of diffusion kurtosis metrics were more pronounced than those observed in diffusion tensor metrics. The age-related anatomical differences reported here may be important determinants of the age-dependent neurobehavior of guinea pigs in different tasks.

Newborns and preterm infants at term equivalent age: A semi-quantitative assessment of cerebral maturity

BACKGROUND AND PURPOSE

Currently available MRI scoring systems of cerebral maturation in term and preterm infant at term equivalent age do not include the changes of transient fetal compartments that persist to term age. We studied the visibility and the pattern of these structures in healthy term newborns compared to preterm infants at term equivalent age in order to investigate if they can be included in a new MRI score system. We hypothesized that transient fetal compartments are different in both groups, and that these differences can be characterized using the clinical T2-weighted MRIs.

MATERIALS AND METHODS

Using 3T MRI T2-weighted brain sequences of 21 full-term and 41 preterm infants (< 32 weeks), scanned at term equivalent age, 3 raters independently scored the maturation level of 3 transient fetal compartments: the periventricular crossroads, von Monakow segments of the white matter, and the subplate compartment. These 3 new items were included in a scoring system along with validated parameters of brain maturation (germinal matrix, bands of migration, subarachnoid space and quality of gyrification). A cumulative maturity score was calculated separately for both groups of newborns by adding together each item. More mature were the brain structures, higher was the cumulative maturity score.

RESULTS

Cumulative maturity score distinguished full-term from preterm infants (mean score 41/60 ± 1.4 versus 37/60 ± 2.5 points, p < 0.001), with an increase of 0.5 points for each supplemental gestational week at birth (r = 0.5, 95% CI 0.5 - 0.85). While a majority of transient fetal compartments were less mature in preterm group at term equivalent age, von Monakow segments of the white matter and subplate compartment presented a more advanced maturational stage in the preterm group compared to the term group. No subject had all scored items in the most mature state. Except a slight intra-rater agreement for von Monakow segment II, inter- and intra-rater agreements were moderate to excellent indicating the potential of the developed scoring system in routine clinical practice.

CONCLUSION

Brain transient fetal structures can be assessed on regular T2-weighted MRI in newborns. Their appearance differs between term and preterm babies. However our results suggest a more complex situation, with both delayed and accelerated maturation pattern in preterm infants. It remains to be determined if these differences could be biomarkers of the future neurodevelopment of preterm infants.

Age-Related Changes in Tissue Value Properties in Children: Simultaneous Quantification of Relaxation Times and Proton Density Using Synthetic Magnetic Resonance Imaging

OBJECTIVES

The properties of brain tissue undergo dynamic changes during maturation. T1 relaxation time (T1), T2 relaxation time (T2), and proton density (PD) are now simultaneously quantifiable within a clinically acceptable time, using a synthetic magnetic resonance imaging (MRI) sequence. This study aimed to provide age-specific reference values for T1, T2, and PD in children, using synthetic MRI.

MATERIALS AND METHODS

We included 89 children (median age, 18 months; range, 34 weeks of gestational age to 17 years) who underwent quantitative MRI, using a multidynamic, multiecho sequence on 3 T MRI, between December 2015 and November 2016, and had no abnormal MRI/neurologic assessment findings. T1, T2, and PD were simultaneously measured in each of the 22 defined white matter and gray matter regions of interest. The measured values were plotted against age, and a curve fitting model that best explained the age dependence of tissue values was identified. Age-specific regional tissue values were calculated using a fit equation.

RESULTS

The tissue values of all brain regions, except cortical PD, decreased with increasing age, and the robust negative association was best explained by modified biexponential model of the form Tissue values = T1 × exp (-C1 × age) + T2 × exp (-C2 × age). The quality of fit to the modified biexponential model was high in white matter and deep gray matter (white matter, R = 97%-99% [T1], 88%-95% [T2], 88%-97% [PD]; deep gray matter, R = 96%-97% [T1], 96% [T2], 49%-88% [PD]; cortex, 70%-83% [T1], 87%-90% [T2], 5%-27% [PD]). The white matter and deep gray matter changed the most dynamically within the first year of life.

CONCLUSIONS

Our study provides age-specific regional reference values, from the neonate to adolescent, of T1, T2, and PD, which could be objective tools for assessment of normal/abnormal brain development using synthetic MRI.

MR fingerprinting enables quantitative measures of brain tissue relaxation times and myelin water fraction in the first five years of life

Quantitative assessments of normative brain development using MRI are of critical importance to gain insights into healthy neurodevelopment. However, quantitative MR imaging poses significant technical challenges and requires prohibitively long acquisition times, making it impractical for pediatric imaging. This is particularly relevant for healthy subjects, where imaging under sedation is not clinically indicated. MR Fingerprinting (MRF), a novel MR imaging framework, provides rapid, efficient, and simultaneous quantification of multiple tissue properties. In this study, a 2D MR Fingerprinting method was developed that achieves a spatial resolution of 1 × 1 × 3 mm³ with rapid and simultaneous quantification of T₁, T₂ and myelin water fraction (MWF). Phantom experiments demonstrated that accurate measurements of T₁ and T₂ relaxation times were achieved over a wide range of T₁ and T₂ values. MRF images were acquired cross-sectionally from 28 typically developing children, 0 to five years old, who were enrolled in the UNC/UMN Baby Connectome Project. Differences associated with age of R1 (=1/T₁), R2 (=1/T₂) and MWF were obtained from several predefined white matter regions. Both R1 and R2 exhibit a marked increase until ∼20 months of age, followed by a slower increase for all WM regions. In contrast, the MWF remains at a negligible level until ∼6 months of age for all predefined ROIs and gradually increases afterwards. Depending on the brain region, rapid increases are observed between 6 and 12 months to 6-18 months, followed by a slower pace of increase in MWF. Neither relaxivities nor MWF were significantly different between the left and right hemispheres. However, regional differences in age-related R1 and MWF measures were observed across different white matter regions. In conclusion, our results demonstrate that the MRF technique holds great potential for multi-parametric assessments of normative brain development in early childhood.

Epilepsy-predictive magnetic resonance imaging changes following experimental febrile status epilepticus: Are they translatable to the clinic?

OBJECTIVE

A subset of children with febrile status epilepticus (FSE) are at risk for development of temporal lobe epilepsy later in life. We sought a noninvasive predictive marker of those at risk that can be identified soon after FSE, within a clinically realistic timeframe.

METHODS

Longitudinal T₂ -weighted magnetic resonance imaging (T₂ WI MRI) of rat pups at several time points after experimental FSE (eFSE) was performed on a high-field scanner followed by long-term continuous electroencephalography. In parallel, T₂ WI MRI scans were performed on a 3.0-T clinical scanner. Finally, chronic T₂ WI MRI signal changes were examined in rats that experienced eFSE and were imaged months later in adulthood.

RESULTS

Epilepsy-predicting T₂ changes, previously observed at 2 hours after eFSE, persisted for at least 6 hours, enabling translation to the clinic. Repeated scans, creating MRI trajectories of T₂ relaxation times following eFSE, provided improved prediction of epileptogenesis compared with a single MRI scan. Predictive signal changes centered on limbic structures, such as the basolateral and medial amygdala. T₂ WI MRI changes, originally described on high-field scanners, can also be measured on clinical MRI scanners. Chronically elevated T₂ relaxation times in hippocampus were observed months after eFSE in rats, as noted for post-FSE changes in children.

SIGNIFICANCE

Early T₂ WI MRI changes after eFSE provide a strong predictive measure of epileptogenesis following eFSE, on both high-field and clinical MRI scanners. Importantly, the extension of the acute signal changes to at least 6 hours after the FSE enables its inclusion in clinical studies. Chronic elevations of T₂ relaxation times within the hippocampal formation and related structures are common to human and rodent FSE, suggesting that similar processes are involved across species.

T2 Relaxometry MRI Predicts Cerebral Palsy in Preterm Infants

BACKGROUND AND PURPOSE

T2-relaxometry brain MR imaging enables objective measurement of brain maturation based on the water-macromolecule ratio in white matter, but the outcome correlation is not established in preterm infants. Our study aimed to predict neurodevelopment with T2-relaxation values of brain MR imaging among preterm infants.

MATERIALS AND METHODS

From January 1, 2012, to May 31, 2015, preterm infants who underwent both T2-relaxometry brain MR imaging and neurodevelopmental follow-up were retrospectively reviewed. T2-relaxation values were measured over the periventricular white matter, including sections through the frontal horns, midbody of the lateral ventricles, and centrum semiovale. Periventricular T2 relaxometry in relation to corrected age was analyzed with restricted cubic spline regression. Prediction of cerebral palsy was examined with the receiver operating characteristic curve.

RESULTS

Thirty-eight preterm infants were enrolled for analysis. Twenty patients (52.6%) had neurodevelopmental abnormalities, including 8 (21%) with developmental delay without cerebral palsy and 12 (31.6%) with cerebral palsy. The periventricular T2-relaxation values in relation to age were curvilinear in preterm infants with normal development, linear in those with developmental delay without cerebral palsy, and flat in those with cerebral palsy. When MR imaging was performed at >1 month corrected age, cerebral palsy could be predicted with T2 relaxometry of the periventricular white matter on sections through the midbody of the lateral ventricles (area under the receiver operating characteristic curve = 0.738; cutoff value of >217.4 with 63.6% sensitivity and 100.0% specificity).

CONCLUSIONS

T2-relaxometry brain MR imaging could provide prognostic prediction of neurodevelopmental outcomes in premature infants. Age-dependent and area-selective interpretation in preterm brains should be emphasized.

Measuring in vivo cerebral maturation using age-related T2 relaxation times at 3T

OBJECTIVE

To examine age-related changes in T₂ relaxation times during infancy and childhood in order to assess T₂ values obtained from routine MRI as a biomarker.

METHODS

From our pool of clinical pediatric MRI examinations at 3T all patients with normal conventional MRI scans were retrospectively selected. Depending on their clinical findings the identified 99 patients (0-199months) were divided into 43 healthy controls and 56 diseased children with various clinical abnormalities (developmental delay, epilepsy, prematurity, and deafness). T₂ maps based on routinely performed triple echo turbo spin echo sequences were created. T₂ values were measured in 22 brain regions to determine age-related changes. We also investigated whether such changes differ between healthy and diseased children.

RESULTS

Age significantly reduced T₂ relaxation times across all regions (p<0.05), but health status had no impact. With increasing age, T₂ values decreased continuously, with declines faster over the first 10months and slower thereafter. Early rapid and later slow decline was similar in healthy and diseased groups.

CONCLUSIONS

Using T₂ maps based on clinical MRI data we could determine age-related T₂ relaxation times in 22 brain regions during infancy and childhood. Our data have relevance for future investigator independent T₂ relaxation time measurements in determining whether T₂ values are within the normal range or should be considered as potentially pathologic.

Age-related T2 relaxation times at 3 Tesla as a biomarker of infratentorial brain maturation

PURPOSE

The purpose of this study was to examine age-related, infratentorial changes in T₂ relaxation times during infancy and childhood using routine MRI data at 3 Tesla.

METHODS

One hundred patients (0-199 months) without signal abnormalities on conventional MRI were retrospectively selected from our pool of pediatric MRI examinations. T₂ maps based on our routinely acquired triple-echo turbo spin-echo (TSE) sequence were created. Based on their clinical symptoms, the children were divided into 43 controls and 57 diseased children with different clinical diseases. T₂ relaxation times were measured in 15 infratentorial brain regions (medullary pyramid, ventral and dorsal pons, middle cerebellar peduncle, dentate nucleus, medial and lateral cerebellar hemisphere each on both sides, and in the cerebellar vermis) investigating age-related changes. Secondly, this study examined whether those changes in T₂ values differed between healthy and diseased children.

RESULTS

Age significantly reduced T₂ relaxation time in all infratentorial brain regions (p < 0.05). With increasing age, the T₂ relaxation times decreased continuously, faster in the first 9 months and slower thereafter. Overall, controls did not differ significantly from diseased children (p > 0.05) apart from the dentate nucleus and cerebellar hemispheres in terms of rapid decline (larger in controls) and the right dorsal pons and left pyramid in terms of slow decline (larger in diseased children). In both groups, the later slow decline was almost negligible.

CONCLUSIONS

Using T₂ maps, it was possible to determine age-related T₂ relaxation times in the different infratentorial brain regions in this preliminary study. Between neurologically healthy controls and diseased children, no significant differences in T₂ relaxation times could be found overall in the studied regions.

Validation of a brain MRI relaxometry protocol to measure effects of preterm birth at a flexible postnatal age

BACKGROUND

Magnetic resonance imaging (MRI) is a useful tool to study brain growth and organization in preterm neonates for clinical and research purposes, but its practicality can be limited by time and medical constraints. The aim of this study was to determine if MRI relaxometry of the deep nuclei, as opposed to white matter, would reflect the influence of gestational age at birth on structures essential to motor development, regardless of postnatal age at the time of imaging.

RESULTS

This was a prospective observational study of infants without brain injury on conventional neuroimaging who were cared for in the neonatal intensive care unit (NICU) at Vanderbilt. Infants were studied using MRI relaxometry within a 2-month window of postmenstrual term age. In 45 infants, white matter MRI T1 relaxation times were influenced by both gestational age and postnatal age at imaging time (R(2) = 0.19 for gestational age vs. R(2) = 0.34 adjusting for both gestational age and age at imaging; all p < 0.01). Similar results were obtained with T2 relaxation times. In contrast, globus pallidus T1 reflected gestational age but was minimally affected by postnatal age (R(2) = 0.50 vs. 0.57, p < 0.001).

CONCLUSIONS

The results obtained using this imaging protocol are consistent with the slow maturation of the globus pallidus, essential to normal development of complex motor programs into adulthood. Globus pallidus MRI relaxometry measures the impact of gestational age at birth on brain development independent of postnatal age in preterm infants and should prove useful for predictive modeling in a flexible time-window around postmenstrual term age.

Reprint of "Quantitative evaluation of brain development using anatomical MRI and diffusion tensor imaging"

The development of the brain is structure-specific, and the growth rate of each structure differs depending on the age of the subject. Magnetic resonance imaging (MRI) is often used to evaluate brain development because of the high spatial resolution and contrast that enable the observation of structure-specific developmental status. Currently, most clinical MRIs are evaluated qualitatively to assist in the clinical decision-making and diagnosis. The clinical MRI report usually does not provide quantitative values that can be used to monitor developmental status. Recently, the importance of image quantification to detect and evaluate mild-to-moderate anatomical abnormalities has been emphasized because these alterations are possibly related to several psychiatric disorders and learning disabilities. In the research arena, structural MRI and diffusion tensor imaging (DTI) have been widely applied to quantify brain development of the pediatric population. To interpret the values from these MR modalities, a "growth percentile chart," which describes the mean and standard deviation of the normal developmental curve for each anatomical structure, is required. Although efforts have been made to create such a growth percentile chart based on MRI and DTI, one of the greatest challenges is to standardize the anatomical boundaries of the measured anatomical structures. To avoid inter- and intra-reader variability about the anatomical boundary definition, and hence, to increase the precision of quantitative measurements, an automated structure parcellation method, customized for the neonatal and pediatric population, has been developed. This method enables quantification of multiple MR modalities using a common analytic framework. In this paper, the attempt to create an MRI- and a DTI-based growth percentile chart, followed by an application to investigate developmental abnormalities related to cerebral palsy, Williams syndrome, and Rett syndrome, have been introduced. Future directions include multimodal image analysis and personalization for clinical application.

Quantitative evaluation of brain development using anatomical MRI and diffusion tensor imaging

The development of the brain is structure-specific, and the growth rate of each structure differs depending on the age of the subject. Magnetic resonance imaging (MRI) is often used to evaluate brain development because of the high spatial resolution and contrast that enable the observation of structure-specific developmental status. Currently, most clinical MRIs are evaluated qualitatively to assist in the clinical decision-making and diagnosis. The clinical MRI report usually does not provide quantitative values that can be used to monitor developmental status. Recently, the importance of image quantification to detect and evaluate mild-to-moderate anatomical abnormalities has been emphasized because these alterations are possibly related to several psychiatric disorders and learning disabilities. In the research arena, structural MRI and diffusion tensor imaging (DTI) have been widely applied to quantify brain development of the pediatric population. To interpret the values from these MR modalities, a "growth percentile chart," which describes the mean and standard deviation of the normal developmental curve for each anatomical structure, is required. Although efforts have been made to create such a growth percentile chart based on MRI and DTI, one of the greatest challenges is to standardize the anatomical boundaries of the measured anatomical structures. To avoid inter- and intra-reader variability about the anatomical boundary definition, and hence, to increase the precision of quantitative measurements, an automated structure parcellation method, customized for the neonatal and pediatric population, has been developed. This method enables quantification of multiple MR modalities using a common analytic framework. In this paper, the attempt to create an MRI- and a DTI-based growth percentile chart, followed by an application to investigate developmental abnormalities related to cerebral palsy, Williams syndrome, and Rett syndrome, have been introduced. Future directions include multimodal image analysis and personalization for clinical application.

Multimodal magnetic resonance imaging assessment of white matter aging trajectories over the lifespan of healthy individuals

BACKGROUND

Postmortem and volumetric imaging data suggest that brain myelination is a dynamic lifelong process that, in vulnerable late-myelinating regions, peaks in middle age. We examined whether known regional differences in axon size and age at myelination influence the timing and rates of development and degeneration/repair trajectories of white matter (WM) microstructure biomarkers.

METHODS

Healthy subjects (n = 171) 14-93 years of age were examined with transverse relaxation rate (R(2)) and four diffusion tensor imaging measures (fractional anisotropy [FA] and radial, axial, and mean diffusivity [RD, AxD, MD, respectively]) of frontal lobe, genu, and splenium of the corpus callosum WM (FWM, GWM, and SWM, respectively).

RESULTS

Only R(2) reflected known levels of myelin content with high values in late-myelinating FWM and GWM regions and low ones in early-myelinating SWM. In FWM and GWM, all metrics except FA had significant quadratic components that peaked at different ages (R(2) < RD < MD < AxD), with FWM peaking later than GWM. Factor analysis revealed that, although they defined different factors, R(2) and RD were the metrics most closely associated with each other and differed from AxD, which entered into a third factor.

CONCLUSIONS

The R(2) and RD trajectories were most dynamic in late-myelinating regions and reflect age-related differences in myelination, whereas AxD reflects axonal size and extra-axonal space. The FA and MD had limited specificity. The data suggest that the healthy adult brain undergoes continual change driven by development and repair processes devoted to creating and maintaining synchronous function among neural networks on which optimal cognition and behavior depend.

Automated detection of white matter signal abnormality using T2 relaxometry: application to brain segmentation on term MRI in very preterm infants

Hyperintense white matter signal abnormalities, also called diffuse excessive high signal intensity (DEHSI), are observed in up to 80% of very preterm infants on T2-weighted MRI scans at term-equivalent age. DEHSI may represent a developmental stage or diffuse microstructural white matter abnormalities. Automated quantitative assessment of DEHSI severity may help resolve this debate and improve neonatal brain tissue segmentation. For T2-weighted sequence without fluid attenuation, the signal intensity distribution of DEHSI greatly overlaps with that of cerebrospinal fluid (CSF) making its detection difficult. Furthermore, signal intensities of T2-weighted images are susceptible to magnetic field inhomogeneity. Increased signal intensities caused by field inhomogeneity may be confused with DEHSI. To overcome these challenges, we propose an algorithm to detect DEHSI using T2 relaxometry, whose reflection of the rapid changes in free water content provides improved distinction between CSF and DEHSI over that of conventional T2-weighted imaging. Moreover, the parametric transverse relaxation time T2 is invulnerable to magnetic field inhomogeneity. We conducted computer simulations to select an optimal detection parameter and to validate the proposed method. We also demonstrated that brain tissue segmentation is further enhanced by incorporating DEHSI detection for both simulated preterm infant brain images and in vivo in very preterm infants imaged at term-equivalent age.

MR connectomics: a conceptual framework for studying the developing brain

THE COMBINATION OF ADVANCED NEUROIMAGING TECHNIQUES AND MAJOR DEVELOPMENTS IN COMPLEX NETWORK SCIENCE, HAVE GIVEN BIRTH TO A NEW FRAMEWORK FOR STUDYING THE BRAIN: "connectomics." This framework provides the ability to describe and study the brain as a dynamic network and to explore how the coordination and integration of information processing may occur. In recent years this framework has been used to investigate the developing brain and has shed light on many dynamic changes occurring from infancy through adulthood. The aim of this article is to review this work and to discuss what we have learned from it. We will also use this body of work to highlight key technical aspects that are necessary in general for successful connectome analysis using today's advanced neuroimaging techniques. We look to identify current limitations of such approaches, what can be improved, and how these points generalize to other topics in connectome research.

Regional and hemispheric asymmetries of cerebral hemodynamic and oxygen metabolism in newborns

Understanding the evolution of regional and hemispheric asymmetries in the early stages of life is essential to the advancement of developmental neuroscience. By using 2 noninvasive optical methods, frequency-domain near-infrared spectroscopy and diffuse correlation spectroscopy, we measured cerebral hemoglobin oxygenation (SO(2)), blood volume (CBV), an index of cerebral blood flow (CBF(i)), and the metabolic rate of oxygen (CMRO(2i)) in the frontal, temporal, and parietal regions of 70 premature and term newborns. In concordance with results obtained using more invasive imaging modalities, we verified both hemodynamic (CBV, CBF(i), and SO(2)) and metabolic (CMRO(2i)) parameters were greater in the temporal and parietal regions than in the frontal region and that these differences increased with age. In addition, we found that most parameters were significantly greater in the right hemisphere than in the left. Finally, in comparing age-matched males and females, we found that males had higher CBF(i) in most cortical regions, higher CMRO(2i) in the frontal region, and more prominent right-left CBF(i) asymmetry. These results reveal, for the first time, that we can detect regional and hemispheric asymmetries in newborns using noninvasive optical techniques. Such a bedside screening tool may facilitate early detection of abnormalities and delays in maturation of specific cortical areas.

Age-related slowing in cognitive processing speed is associated with myelin integrity in a very healthy elderly sample

Performance on measures of cognitive processing speed (CPS) slows with age, but the biological basis associated with this cognitive phenomenon remains incompletely understood. We assessed the hypothesis that the age-related slowing in CPS is associated with myelin breakdown in late-myelinating regions in a very healthy elderly population. An in vivo magnetic resonance imaging (MRI) biomarker of myelin integrity was obtained from the prefrontal lobe white matter and the genu of the corpus callosum for 152 healthy elderly adults. These regions myelinate later in brain development and are more vulnerable to breakdown due to the effects of normal aging. To evaluate regional specificity, we also assessed the splenium of the corpus callosum as a comparison region, which myelinates early in development and primarily contains axons involved in visual processing. The measure of myelin integrity was significantly correlated with CPS in highly vulnerable late-myelinating regions but not in the splenium. These results have implications for the neurobiology of the cognitive changes associated with brain aging.

Optimized T1- and T2-weighted volumetric brain imaging as a diagnostic tool in very preterm neonates

BACKGROUND

T1- and T2-W MR sequences used for obtaining diagnostic information and morphometric measurements in the neonatal brain are frequently acquired using different imaging protocols. Optimizing one protocol for obtaining both kinds of information is valuable.

OBJECTIVE

To determine whether high-resolution T1- and T2-W volumetric sequences optimized for preterm brain imaging could provide both diagnostic and morphometric value.

MATERIALS AND METHODS

Thirty preterm neonates born between 24 and 32 weeks' gestational age were scanned during the first 2 weeks after birth. T1- and T2-W high-resolution sequences were optimized in terms of signal-to-noise ratio, contrast-to-noise ratio and scan time and compared to conventional spin-echo-based sequences.

RESULTS

No differences were found between conventional and high-resolution T1-W sequences for diagnostic confidence, image quality and motion artifacts. A preference for conventional over high-resolution T2-W sequences for image quality was observed. High-resolution T1 images provided better delineation of thalamic myelination and the superior temporal sulcus. No differences were found for detection of myelination and sulcation using conventional and high-resolution T2-W images.

CONCLUSION

High-resolution T1- and T2-W volumetric sequences can be used in clinical MRI in the very preterm brain to provide both diagnostic and morphometric information.

Neuroimaging of cortical development and brain connectivity in human newborns and animal models

Significant human brain growth occurs during the third trimester, with a doubling of whole brain volume and a fourfold increase of cortical gray matter volume. This is also the time period during which cortical folding and gyrification take place. Conditions such as intrauterine growth restriction, prematurity and cerebral white matter injury have been shown to affect brain growth including specific structures such as the hippocampus, with subsequent potentially permanent functional consequences. The use of 3D magnetic resonance imaging (MRI) and dedicated postprocessing tools to measure brain tissue volumes (cerebral cortical gray matter, white matter), surface and sulcation index can elucidate phenotypes associated with early behavior development. The use of diffusion tensor imaging can further help in assessing microstructural changes within the cerebral white matter and the establishment of brain connectivity. Finally, the use of functional MRI and resting-state functional MRI connectivity allows exploration of the impact of adverse conditions on functional brain connectivity in vivo. Results from studies using these methods have for the first time illustrated the structural impact of antenatal conditions and neonatal intensive care on the functional brain deficits observed after premature birth. In order to study the pathophysiology of these adverse conditions, MRI has also been used in conjunction with histology in animal models of injury in the immature brain. Understanding the histological substrate of brain injury seen on MRI provides new insights into the immature brain, mechanisms of injury and their imaging phenotype.

Frequently encountered cranial ultrasound features in the white matter of preterm infants: correlation with MRI

BACKGROUND

Bilateral symmetrical echogenic and echolucent areas in the white matter are frequently seen on the cranial ultrasound scans of apparently well preterm infants without overt pathology.

AIM

To determine whether these features reflect maturational processes as seen on MRI.

METHODS

Preterm and term-born infants without overt pathology on contemporaneous brain ultrasound and MRI were studied. Ultrasound scans were compared with T(2)-weighted MRI to identify MR correlates for the bilateral and symmetrical echogenic and echolucent phenomena in the white matter seen on ultrasound.

RESULTS

Forty-four sets of scans (26 preterm, 8 term-born infants) were assessed. Echogenic features were better and more frequently seen on early ultrasound as compared to nearer term age. Echogenic blushes in the white matter correlated well with high signal intensity areas and echogenic lines with low signal intensity lines on MRI. Echolucent areas correlated with the site of the internal capsule and the myelinated posterior pons. The subplate was not reliably identified.

CONCLUSION

Many echogenic and echolucent features in the white matter of well preterm and some term-born infants correlated well with areas of differing signal intensity on MRI. They most likely reflect normal maturational processes but the echogenic hemispheric features may represent delayed or abnormal maturation.

T(2) relaxometry of normal pediatric brain development

PURPOSE

To establish normal age-related changes in the magnetic resonance (MR) T(2) relaxation time constants of brain using data collected as part of the National Institutes of Health (NIH) MRI Study of Normal Brain Development.

MATERIALS AND METHODS

This multicenter study of normal brain and behavior development provides both longitudinal and cross-sectional data, and has enabled us to investigate T(2) evolution in several brain regions in healthy children within the age range of birth through 4 years 5 months. Due to the multicenter nature of the study and the extended period of data collection, periodically scanned inanimate and human phantoms were used to assess intra- and intersite variability.

RESULTS

The main finding of this work, based on over 340 scans, is the identification and parameterization of the monoexponential evolution of T(2) from birth through 4 years 5 months of age in various brain structures.

CONCLUSION

The exponentially decaying T(2) behavior is believed to reflect the rapid changes in water content as well as myelination during brain development. The data will become publicly available as part of a normative pediatric MRI and clinical/behavioral database, thereby providing a basis for comparison in studies assessing normal brain development, and studies of deviations due to various neurological, neuropsychiatric, and developmental disorders.

Study of the development of fetal baboon brain using magnetic resonance imaging at 3 Tesla

Direct observational data on the development of the brains of human and nonhuman primates is on remarkably scant, and most of our understanding of primate brain development is extrapolated from findings in rodent models. Magnetic resonance imaging (MRI) is a promising tool for the noninvasive, longitudinal study of the developing primate brain. We devised a protocol to scan pregnant baboons serially at 3 T for up to 3 h per session. Seven baboons were scanned 1-6 times, beginning as early as 56 days post-conceptional age, and as late as 185 days (term approximately 185 days). Successful scanning of the fetal baboon required careful animal preparation and anesthesia, in addition to optimization of the scanning protocol. We successfully acquired maps of relaxation times (T(1) and T(2)) and high-resolution anatomical images of the brains of fetal baboons at multiple time points during the course of gestation. These images demonstrated the convergence of gray and white matter contrast near term, and furthermore demonstrated that the loss of contrast at that age is a consequence of the continuous change in relaxation times during fetal brain development. These data furthermore demonstrate that maps of relaxation times have clear advantages over the relaxation time weighted images for the tracking of the changes in brain structure during fetal development. This protocol for in utero MRI of fetal baboon brains will help to advance the use of nonhuman primate models to study fetal brain development longitudinally.

Optimization of 3D MP-RAGE for neonatal brain imaging at 3.0 T

Three-dimensional (3D) magnetic resonance imaging (MRI) has shown great potential for studying the impact of prematurity and pathology on brain development. We have investigated the potential of optimized T1-weighted 3D magnetization-prepared rapid gradient-echo imaging (MP-RAGE) for obtaining contrast between white matter (WM) and gray matter (GM) in neonates at 3 T. Using numerical simulations, we predicted that the inversion time (TI) for obtaining strongest contrast at 3 T is approximately 2 s for neonates, whereas for adults, this value is approximately 1.3 s. The optimal neonatal TI value was found to be insensitive to reasonable variations of the assumed T1 relaxation times. The maximum theoretical contrast for neonates was found to be approximately one third of that for adults. Using the optimized TI values, MP-RAGE images were obtained from seven neonates and seven adults at 3 T, and the contrast-to-noise ratio (CNR) was measured for WM versus five GM regions. Compared to adults, neonates exhibited lower CNR between cortical GM and WM and showed a different pattern of regional variation in CNR. These results emphasize the importance of sequence optimization specifically for neonates and demonstrate the challenge in obtaining strong contrast in neonatal brain with T1-weighted 3D imaging.

What's behind the decline? The role of white matter in brain aging

The specific molecular events that underlie the age-related loss of cognitive function are poorly understood. Although not experimentally substantiated, age-dependent neuronal loss has long been considered central to age-related cognitive decline. More recently, age-related changes in brain white matter have taken precedence in explaining the steady decline in cognitive domains seen in non-diseased elderly. Characteristic alterations in the ultrastructure of myelin coupled with evidence of inflammatory processes present in the white matter of several different species suggest that specific molecular events within brain white matter may better explain observed pathological changes and cognitive deficits. This review focuses on recent evidence highlighting the importance of white matter in deciphering the course of "normal" brain aging.

Comparative prognostic utilities of early quantitative magnetic resonance imaging spin-spin relaxometry and proton magnetic resonance spectroscopy in neonatal encephalopathy

OBJECTIVE

We sought to compare the prognostic utilities of early MRI spin-spin relaxometry and proton magnetic resonance spectroscopy in neonatal encephalopathy.

METHODS

Twenty-one term infants with neonatal encephalopathy were studied at a mean age of 3.1 days (range: 1-5). Basal ganglia, thalamic and frontal, parietal, and occipital white matter spin-spin relaxation times were determined from images with echo times of 25 and 200 milliseconds. Metabolite ratios were determined from an 8-mL thalamic-region magnetic resonance spectroscopy voxel (1H point-resolved spectroscopy; echo time 270 milliseconds). Outcomes were assigned at age 1 year as follows: (1) normal, (2) moderate (neuromotor signs or Griffiths developmental quotient of 75-84), (3) severe (functional neuromotor deficit or developmental quotient <75 or died). Predictive efficacies for differentiation between normal and adverse (combined moderate and severe) outcomes were compared by receiver operating characteristic curve analysis and logistic regression.

RESULTS

Thalamic and basal ganglia spin-spin relaxation times correlated positively with outcome and predicted adversity. Although thalamic and basal ganglia spin-spin relaxation times were prognostic of adversity, magnetic resonance spectroscopy metabolite ratios were better predictors, and, of these, lactate/N-acetylaspartate was most accurate.

CONCLUSIONS

Deep gray matter spin-spin relaxation time was increased in the first few days after birth in infants with an adverse outcome. Proton magnetic resonance spectroscopy was more prognostic than spin-spin relaxation time, with lactate/N-acetylaspartate the best measure. Nevertheless, both techniques were useful for early prognosis, and the potential superior spatial resolution of spin-spin relaxometry may define better the precise anatomic pattern of injury in the early days after birth.

Diffusion Tensor Imaging and Tractography of Human Brain Development

Over the past decade, diffusion tensor imaging (DTI) has offered researchers and clinicians a new noninvasive window into the developing human brain, from preterm infants through adolescents and young adults. DTI improves on conventional MR imaging, such as T1-weighted and T2-weighted sequences, through its sensitivity to many microstructural features of neural organization. This has enabled visualization of the early cerebral laminar architecture in premature infants, of developing white matter before myelination, and of the microarchitecture of the cerebral cortex during preterm maturation. DTI provides reproducible quantitative measures, such as mean diffusivity and fractional anisotropy, that reflect the underlying tissue properties of gray matter and white matter and may therefore become useful as developmental milestones for the improved assessment of abnormal brain maturation. Furthermore, three-dimensional fiber tractography based on DTI can reveal the developing axonal connectivity of the human brain as well as aberrant connectivity in structural brain malformations. In this article, applications of DTI and fiber tractography to the study of human brain development are reviewed. The new insights into brain maturation afforded by DTI promise to improve the diagnostic evaluation of an array of congenital, metabolic, and neurodevelopmental disorders.

Differential brain growth in the infant born preterm: current knowledge and future developments from brain imaging

Preterm birth is associated with a high prevalence of neuropsychiatric impairment in childhood and adolescence, but the neural correlates underlying these disorders are not fully understood. Quantitative magnetic resonance imaging techniques have been used to investigate subtle differences in cerebral growth and development among children and adolescents born preterm or with very low birth weight. Diffusion tensor imaging and computer-assisted morphometric techniques (including voxel-based morphometry and deformation-based morphometry) have identified abnormalities in tissue microstructure and cerebral morphology among survivors of preterm birth at different ages, and some of these alterations have specific functional correlates. This chapter reviews the literature reporting differential brain development following preterm birth, with emphasis on the morphological changes that correlate with neuropsychiatric impairment.

Neonatal Brain: Regional Variability of in Vivo MR Imaging Relaxation Rates at 3.0 T—Initial Experience

PURPOSE

To retrospectively investigate regional in vivo magnetic resonance (MR) imaging transverse and longitudinal relaxation rates at 3.0 T in neonatal brain, the relationship between these rates, and their potential use for gray matter (GM) versus white matter (WM) tissue discrimination.

MATERIALS AND METHODS

Informed parental consent for performance of imaging procedures was obtained in each infant. Informed consent for retrospective image analysis was not required; ethics approval was obtained from institutional review board. At 3.0 T, R1 and R2 were measured in brain regions (frontal WM, posterior WM, periventricular WM, frontal GM, posterior GM, basal ganglia, and thalamus) in 13 infants with suspected neurologic abnormality (two term, 11 preterm). Maps of R1 and R2 were acquired with T1 by multiple readout pulses and segmented spin-echo echo-planar imaging sequences, respectively. Accuracy of R1 and R2 map acquisition methods was tested in phantoms by comparing them with inversion-recovery and spin-echo sequences, respectively. Statistical analysis included linear regression analysis to determine relationship between R1 and R2 and Wilcoxon signed rank test to investigate the potential for discrimination between GM and WM.

RESULTS

In phantoms, R1 values measured with T1 by multiple readout pulses sequence were 3%-8% lower than those measured with inversion recovery sequence, and R2 values measured with segmented echo-planar sequence were 1%-8% lower than those measured with spin-echo sequence. A strong correlation of 0.944 (P < .001) between R1 and R2 in neonatal brain was observed. For R2, relative differences between GM and WM were larger than were those for R1 (z = -2.366, P < .05). For frontal GM and frontal WM, (R2(GM) - R2(WM))/R2(WM) yielded 0.8 +/- 0.2 (mean +/- standard deviation) and (R1(GM) - R1(WM))/R1(WM) yielded 0.3 +/- 0.09.

CONCLUSION

Results at 3.0 T indicate that R1 decreases with increasing field strength, while R2 values are similar to those reported at lower field strengths. For neonates, R2 image contrast may be more advantageous than R1 image contrast for differentiation between GM and WM.

Normal Brain Maturation Characterized With Age-Related T2 Relaxation Times: An Attempt to Develop a Quantitative Imaging Measure for Clinical Use

OBJECTIVES

We studied age-related changes in T2 relaxation times from the normal maturating human brain under routine clinical MR examination conditions.

MATERIALS AND METHODS

In 70 healthy subjects aged between 3 weeks and 39 years, T2 maps of the brain in which the intensity of each pixel corresponded to T2 relaxation times were generated based on magnetic resonance imaging data collected with a triple spin echo sequence. T2 relaxation times in white matter (WM) and gray matter (GM) were measured in 6 distinctive regions of interest of the T2 maps. The age dependence of the T2 values was mathematically simulated using a biexponential function.

RESULTS

T2 values were largest at the age of 3 weeks (maximum: approximately 400 milliseconds for WM and 200 milliseconds for GM) and decreased continuously with increasing age, faster in the first few months and slower thereafter, until values achieved between 95 and 110 milliseconds for WM and 88 and 95 milliseconds for GM in adults. The relationship between T2 values and age could be well simulated using a biexponential function (R > 0.92).

CONCLUSIONS

T2 relaxation time correlates well with the progress of brain maturation. The used biexponential function reflects the dynamic development of myelination in newborns and young children as well as the maturation of myelination during adolescence and could be used to develop a "normal" reference for neuroradiological diagnoses.

Heterogeneous age-related breakdown of white matter structural integrity: implications for cortical "disconnection" in aging and Alzheimer's disease

Human and non-human primate data suggest that the structural integrity of myelin sheaths deteriorates during normal aging, especially in the late-myelinating association regions and may result in "disconnection" of widely distributed neural networks. Magnetic resonance imaging (MRI) was used to assess the heterogeneity of this process and its impact on brain aging and Alzheimer's disease (AD) by evaluating early- and later-myelinating regions of the corpus callosum, the splenium (Scc) and genu (Gcc), respectively. Calculated transverse relaxation rates (R2), an indirect measure of white matter structural integrity for the Gcc and Scc, were examined. The relationship between age and R2 differed in the two regions. A quadratic (inverted U) function with an accelerating rate of decline beginning at age 31 best represented the Gcc pattern while the Scc decline was three-fold smaller, gradual, and linear. These data suggest that the severity of age-related myelin breakdown is regionally heterogeneous, consistent with the hypothesis that differences in myelin properties make later-myelinating regions more susceptible to this process. In AD this process is globally exacerbated, consistent with an extracellular deleterious process such as amyloid beta-peptide toxicity. Non-invasive measures such as R2 may be useful in primary prevention studies of AD.

Novel MR contrasts to reveal more about the brain

Twenty percent of patients with refractory focal epilepsy have an undetermined etiologic basis for their epilepsy despite extensive investigation, including optimal MR imaging. Surgical treatment of this group is associated with a less favorable postoperative outcome. Even with improvements in imaging techniques, a proportion of these patients will remain "MR imaging-negative." It is likely, however, that some of the discrete macroscopic focal lesions that are currently occult will be identified by imaging techniques interrogating different microstructural characteristics. Furthermore, these methods may provide pathologic specificity when used in combination. The description and application of these techniques in epilepsy are the focus of this article.

A newborn piglet study of moderate hypoxic-ischemic brain injury by 1H-MRS and MRI

Cerebral hypoxia-ischemia (HI) is an important cause of perinatal brain damage in the term newborn. The areas most affected are the parasagittal regions of the cerebral cortex and, in severe situations, the basal ganglia. The aim of this study was to show that the newborn piglet model can be used to produce neuropathology resulting from moderate HI insult and to monitor damage for 7 days. Two acute cerebral HI were induced in newborn Large White piglets by reducing the inspired oxygen fraction to 4% and occluding the carotid arteries. Newborn piglets were resuscitated, extubated and monitored for 7 days. (31)P magnetic resonance spectroscopy (MRS) offers the ability to monitor the severity of the HI insults. Lactate (Lac) was detected in the HI group at 2 h, 3 days and 5 days after insult by (1)H MRS. Lac/n-acetylaspartate and Lac/choline and Lac/creatine ratios increased significantly (p < 0.01) in the HI group 2 h after HI insults and remained high over 7 days. For the HI group, mean T(2) values increased significantly in the parietal white matter (subcortical) for 5 days after HI insult [117.5 (+/-7.4) to 158.5 (+/-19.2) at T+3 days, 167.7 (+/-15.4) at T+5 days and 160.9 (+/-10.1) at T+7 days (p < 0.01)]. This newborn piglet model of moderate HI brain injury with reproducible cerebral damage could be use as reference for the study of neuroprotective strategy for a period of 7 days.

Quantitative T2 in the occipital lobe: the role of the CPMG refocusing rate

PURPOSE

To investigate the dependence of occipital gray and white matter T(2) on the Carr-Purcell-Meiboom-Gill (CPMG) refocusing interval, thereby testing the basis of a novel functional magnetic resonance imaging (fMRI) method for blood volume quantification, and addressing recent questions surrounding T(2) contrast in the occipital lobe.

MATERIALS AND METHODS

A CPMG sequence with 1 x 1 x 5 mm(3) resolution was used to quantify T(2) in a single axial slice at the midlevel of the occipital lobe in 23 healthy adult volunteers. Refocusing intervals of 8, 11, and 22 msec were compared. A Bayesian classifier was used to classify a 1 x 1 x 1 mm(3) T(1)-weighted three-dimensional data set into gray matter, white matter, and cerebrospinal fluid, with an average 95% a posteriori probability used as the threshold for inclusion into a tissue-specific region of interest (ROI).

RESULTS

The usual T(2) contrast between the gray and white matter (i.e., T(2GM) > T(2WM)) was observed, with a highly significant effect of tissue type on the estimated T(2) (P < 10(-5)). The observed T(2) gradually decreased with increasing refocusing interval, for a decrease of 3.3 +/- 1.5 msec in gray matter and 3.0 +/- 1.5 msec in white matter between the 8 and 22 msec refocusing interval acquisitions.

CONCLUSION

The observed T(2) shortening is consistent with the effect of the dramatic decrease in T(2) of partly deoxygenated blood on this range of refocusing rates.

Magnetic resonance imaging of preterm brain injury

Magnetic resonance imaging (MRI) has proved to be a valuable tool for monitoring development and pathology in the preterm brain. This imaging modality is useful for assessing numerous pathologies including periventricular leukomalacia, intraventricular haemorrhage/germinal layer haemorrhage, and periventricular haemorrhagic infarction, and can help to predict outcome in these infants. MRI has also allowed the detection of posterior fossa lesions, which are not easily seen with ultrasound. Additionally, and perhaps most relevant, quantitative MR studies have shown differences between the normal appearing preterm brain at term equivalent age and term born infants, confirming that the brain develops differently in the ex utero environment. Further studies using quantifiable MR techniques will improve our understanding of the effects of the ex utero environment, including aspects of neonatal intensive care on the developing brain.

Age-related morphological impairments in the rat hippocampus following developmental lead exposure: an MRI, LM and EM study

Lead is one of the most common neurotoxic metals present in our environment. Chronic developmental lead exposure is known to be associated with cognitive dysfunction in children. Functional and morphological impairment of the rat brain has also been reported in the hippocampus (Hi) following developmental lead exposure. The present study was carried out to further investigate age-related morphological impairments in the rat Hi following developmental lead exposure with three methods: (1) magnetic resonance imaging (MRI); (2) light microscopy (LM); and (3) electron microscopy (EM) techniques. Neonatal Wistar rats were exposed to lead from parturition to weaning via milk of dams drinking a 0.2% lead acetate solution. Age-related morphological alternations were investigated in the Hi of lead-exposed rats at various postnatal ages: postnatal day (PND) 17, 30 and 90. The MRI signal intensities (SIs) in the left, right, superior and inferior hippocampal regions of control and lead-exposed rats were analyzed. Compared with controls, the SIs of the four hippocampal regions of interest were significantly increased in lead-exposed rats at PND 17, 30 and 90. Moreover, the lead-induced impairment of the Hi showed an age-related decline and a specific topographical pattern. The impairment of inferior hippocampal regions was more severe than that of superior regions in lead-exposed rats at PND 17 and 30, while no significant difference of SIs was observed between left and right hippocampal regions in the three age groups, and between superior and inferior regions in the PND 90 lead-exposed rats. The LM observations indicated that the morphological injury of hippocampal neurons in lead-exposed rats was also age-related. The EM observations revealed that the endoplasmic reticular, Golgi complex and mitochondria of hippocampal CA1 and dentate gyrus neurons in lead-exposed rats were damaged. These results demonstrate that lead-induced morphological impairments of the rat Hi follow a specific age- and site-related pattern.

Brain metabolite composition during early human brain development as measured by quantitative in vivo 1H magnetic resonance spectroscopy

Biochemical maturation of the brain can be studied noninvasively by (1)H magnetic resonance spectroscopy (MRS) in human infants. Detailed time courses of cerebral tissue contents are known for the most abundant metabolites only, and whether or not premature birth affects biochemical maturation of the brain is disputed. Hence, the last trimester of gestation was observed in infants born prematurely, and their cerebral metabolite contents at birth and at expected term were compared with those of fullterm infants. Successful quantitative short-TE (1)H MRS was performed in three cerebral locations in 21 infants in 28 sessions (gestational age 32-43 weeks). The spectra were analyzed with linear combination model fitting, considerably extending the range of observable metabolites to include acetate, alanine, aspartate, cholines, creatines, gamma-aminobutyrate, glucose, glutamine, glutamate, glutathione, glycine, lactate, myo-inositol, macromolecular contributions, N-acetylaspartate, N-acetylaspartylglutamate, o-phosphoethanolamine, scyllo-inositol, taurine, and threonine. Significant effects of age and location were found for many metabolites, including the previously observed neuronal maturation reflected by an increase in N-acetylaspartate. Absolute brain metabolite content in premature infants at term was not considerably different from that in fullterm infants, indicating that prematurity did not affect biochemical brain maturation substantially in the studied population, which did not include infants of extremely low birthweight.

Inverse T(2) contrast at 1.5 Tesla between gray matter and white matter in the occipital lobe of normal adult human brain

T(2) of cortical gray matter is generally assumed to be longer than that of white matter. It is shown here that this is not the case in the occipital lobe, but that this effect is often obscured at lower resolution and concealed in standard T(2)-weighted images. Using a high-resolution (1 x 1.3 x 2 mm(3)) segmented EPI Carr-Purcell-Meiboom-Gill sequence, T(2) relaxation times of the brain were measured at 1.5 T for eight healthy adult volunteers. The average T(2) values of cortical gray and white matter were found to be 88 +/- 2 and 84 +/- 3 msec in the frontal lobe, 84 +/- 2 and 83 +/- 3 msec in the parietal lobe, and 79 +/- 1 and 87 +/- 3 msec in the occipital lobe, respectively. This unexpected occipital T(2) contrast between gray and white matter is attributed to regional differences in iron concentration.