Elsevier

Magnetic Resonance Imaging

Volume 17, Issue 9, November 1999, Pages 1275-1288
Magnetic Resonance Imaging

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

https://doi.org/10.1016/S0730-725X(99)00080-6Get rights and content

Abstract

The purpose of our study is to trace in vivo and during the perinatal period, the brain maturation process with exhaustive measures of the T2 relaxation time values. We also compared regional myelination progress with variations of the relaxation time values and of brain signal. T2 relaxation times were measured in 7 healthy premature newborns at the post-conceptional age of 37 weeks, using a Carr-Purcell-Meiboom-Gill sequence (echo time 60 to 150 ms), on a 2.35 Tesla Spectro-Imaging MR system. A total of 62 measures were defined for each subject within the brain stem, the basal ganglia and the hemispheric gray and white matter. The mean and standard deviation of the T2 values were calculated for each location. Regional T2 values changes and brain signal variations were studied. In comparison to the adult ones, the T2 relaxation time values of both gray and white matter were highly prolonged and a reversed ratio between gray and white matter was found. The maturational phenomena might be regionally correlated with a T2 value shortening. Significant T2 variations in the brainstem (p < 0.02), the mesencephalon (p < 0.05), the thalami (p < 0.01), the lentiform nuclei (p < 0.01) and the caudate nuclei (p < 0.02) were observed at an earlier time than they were visible on T2-weighted images. In the cerebral hemispheres, T2 values increased from the occipital white matter to parietal, temporal and frontal white matter (p < 0.05) and in the frontal and occipital areas from periventricular to subcortical white matter (p < 0.01). Maturational progress was earlier and better displayed with T2 measurements and T2 mapping. During the perinatal period, the measurements and analysis of T2 values revealed brain regional differences not discernible with T2-weighted images. It might be a more sensitive indicator for assessment of brain maturation.

Introduction

The brain is in immature condition at birth, especially in the premature newborn. During the postnatal life, the brain maturation increases, including cell differentiation, neuronal growth and myelination. The myelination process, that occurs mainly during the first two years, follows an ordered and specific course.1, 2 Previous histologic, magnetic resonance imaging (MRI) and magnetic resonance spectroscopic (MRS) studies have shown that myelination seems in precise areas with specific timing related to the functional conditions of the brain.3, 4, 5 Myelination is a major biologic event of great interest in pediatrics. Pathologies during this critical period of cerebral development may lead to neurologic deficiencies. Brain injuries and in particular those related to hypoxia-ischemia, affect regions involved in the process of active maturation.6 Early diagnosis of the damage or of a maturational delay is of great interest due to the importance of after-effect in the psychomotor development.6, 7

Using a combination of T1 and T2-weighted spin echo sequences (SE) and inversion recuperation sequences (IR), MRI is able to display accurately and non-invasively the signal changes generated by the myelination progress.8, 9 The opportunity to follow the advance of the myelination process, according to the gray and white matter differentiation, allows in vivo the assessment of the post-natal development. In the perinatal period, because of immaturity, assessment of brain maturation for the premature newborns is somewhat difficult. The same challenge occurs in detecting white matter brain damage or in evaluating a maturational delay. Furthermore, if the injuries related to a severe anoxia are easily identifiable with the SE or IR MRI, the detection of more subtle damage is often difficult with these sequences.10

In biologic systems, the relaxation times are closely related to the water content and distribution.11, 12 Changes in the brain water status occur as myelination advances. The T2 relaxation time (spin-spin relaxation time) is mainly affected by the interactions between water molecules and tissue macromolecules. It might provide a reliable measure of tissue hydration and might offer new opportunities to trace in vivo the myelination process.13 In pathologic cases, measuring relaxation times may be an effective method to investigate abnormalities undetectable with conventional MR imaging.14, 15

Few studies have dealt with estimation of relaxation times values of gray and white matter in newborns.16, 17, 18, 19 In addition, based on our knowledge, no exhaustive measures have been performed on the premature brain. Quantification of values of the normal premature brain seems essential before undertaking quantitative study of the pathologic population. The purpose of this work was to achieve a cerebral quantitative T2 MR study in healthy premature neonates during the neonatal period. Regional progress of brain maturation with T2 values was evaluated secondarily. Comparison of T2 values with signal and contrast on T2-weighted images was completed to assess possible variations, not displayed with T2-weighted image.

Section snippets

Materials and methods

Using MRI and T2 relaxation time measurements, a quantitative study of brain maturation was performed on 7 preterm newborns ranging in post-menstrual age between 26 to 33 weeks (mean: 30 ± 2 weeks). MRI investigated all the neonates at the post-conceptional age (PCA) of 37 weeks (the post-conceptional age was calculated from post menstrual-age at birth and post-natal age). Once the babies were clinically stable, the examinations were performed during the neonatal period. Newborns were included

Results

The morphologic and signal features of the premature brain reflected a lack of maturation. The morphologic characteristics of the brain at the age of the examination (37 weeks of post conceptional age) were close to those of a full term newborn at birth. The cortex was thin and the gyral and sulcal patterns were just appearing. A lack of gray and white matter differentiation and a pronounced high signal of the cerebral white matter on T2-weighted images characterized the premature brain. On

Discussion

During the perinatal period, we found a prolongation and reversal of T2 times in both gray and white matter compared with child and adult values. The maturational progress seemed to be regionally correlated with a T2 shortening and the decrease of regional T2 values was observed at an early times than it was visible in T2-weighted images. More over, T2 maps revealed an apparent anterior to posterior gradient in cortical and subcortical brain regions and from subcortical to periventricular areas

Acknowledgements

We would like to thank Dr. F. Seguin for his advice on MRI, Pr. J. Laugier for referring the patients, L. Foo Cheung and N. Aas-Rouxparis for their essential help.

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