Quantitative comparison of intrabrain diffusion in adults and preterm and term neonates and infants.

OBJECTIVE: Quantitative measurements of mean water diffusivity (D(av)) were made in human neonates, infants, and adults to assess changes in brain tissue that occur with maturation. SUBJECTS AND METHODS: Values of D(av) were obtained by calculating the average of the diffusion measurements made with diffusion-sensitizing gradients placed along three orthogonal directions. The mean diffusivity, a rotationally invariant determination of apparent diffusion coefficient, was measured in five healthy prematurely born neonates and infants, in 10 healthy term neonates and infants, and in five adults. RESULTS: Values of D(av) were found to decrease with maturation in most parts of the brain. In prematurely born neonates and infants with a postmenstrual age (postgestastional age + postnatal age) under 36 weeks, the average value of D(av) in frontal white matter was 1.90 x 10(-3) mm2 sec(-1). The corresponding value was measured as 1.62 x 10(-3) mm2 sec(-1) in neonates and infants born at term with a postnatal age of no more than 43 days and 0.79 x 10(-3) mm2 sec(-1) in the adult brain. CONCLUSION: Values of D(av) are known to decrease in neonates and young infants in the period immediately after ischemic insult. This decrease and the associated increase in signal intensity seen on diffusion-weighted imaging have been used to monitor ischemic brain injury in neonates and infants. Therefore, the decrease in D(av) that occurs with maturation, which we report in this study, must be considered if quantitative diffusion measurements are used to assess ischemic neonatal brain injury.

MR features of developing periventricular white matter in preterm infants: evidence of glial cell migration.

PURPOSE: MR imaging of the brain is increasingly used in the investigation of the newborn, but little information is available on the normal appearance of the developing brain. We scanned a series of newborn infants in an attempt to define the normal appearance of developing periventricular white matter and to assess how pathologic conditions may modify this appearance. METHODS: Sixty-eight newborn infants, median postmenstrual age (PMA) 34 weeks (range, 24 to 42 weeks), were subdivided into two groups: group A (n = 33), which included those with normal clinical and sonographic examinations, and group B (n = 35), which contained those with evidence of neuroabnormality detected prior to the MR study, either clinically or by cerebral sonography. Images were acquired in two planes on a 1.5-T imager using turbo spin-echo pulse sequences. RESULTS: Symmetric periventricular bands of reduced signal intensity were noted in the frontal periventricular white matter on T2-weighted images in 98% of group A infants and in 97% of group B infants. The number of bands was inversely related to PMA. The reduction in number of bands with increasing PMA was delayed in group B infants. CONCLUSION: The uniform appearance of periventricular bands in a population of healthy infants and their relationship to the infants' maturity is consistent with the results of previous histologic studies. These studies demonstrate the presence of migrating glial cells within the periventricular white matter of infants beyond 20 weeks' gestation, when neuronal migration to the cortex is complete. We postulate that the bands seen on T2-weighted images represent groups of migrating glial cells, providing a further marker of cerebral maturation.

Magnetic resonance of the neonatal brain.

We discuss the advantages of magnetic resonance (MR) technique in the study of the neonatal brain. Major results have been obtained concerning the understanding of the normal appearance of the developing brain. Bands of cells migrating to the cortex have been identified in the frontal periventricular white matter up to late gestation. MR can also identify the disappearance of the subependymal germinal matrix with increasing gestational age. With respect to the lesions of prematurity, subependymal germinal matrix hemorrhages were for the first time identified by MR underneath the posterior horns of the lateral ventricles. Subtle ischemic lesions of the periventricular white matter, not detected by brain ultrasound scan, are also described. Analysis of the lesions of the term neonates showed the role of MR in defining a more precise prognosis of infants who have sustained "birth asphyxia". The lesions can affect the basal ganglia, watershed areas of the white matter, and cortex. MR scans performed in the second week of life seem to show a stronger association with the outcome. Brain ischemic areas of the term neonates presenting with focal or multifocal seizures can also be detected by MR. These infarcted zones are usually located in the perfusion territory of the middle cerebral artery, more often in the left hemisphere. The timing of the scan is an important factor as the conventional MR can be negative in the first two-three days after the seizures. The diffusion-weighted imaging (DWI) is a new MR technique very sensitive to acute ischemic injury, and it may solve the problem of the scan timing.