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

Three-Dimensional Magnetic Resonance Fingerprinting in Neonates: Quantifying Regional Difference and Maturation in the Brain

OBJECTIVES

Magnetic resonance fingerprinting (MRF) allows the simultaneous measurement of multiple tissue properties in a single acquisition. Three-dimensional (3D) MRF with high spatial resolution can be used for neonatal brain imaging. The aim of this study is to apply 3D MRF to neonates and show regional differences and maturation in the brain.

MATERIALS AND METHODS

In this prospective study, 3D MRF using hybrid radial-interleaved acquisition was performed on phantoms and neonates from December 2019 to October 2020. For the reconstruction of 3D MRF, singular value decomposition was applied to reduce reconstruction time, and the iterative reconstruction technique was applied to improve image quality. The accuracies of T1 and T2 values derived from 3D MRF were evaluated in a phantom experiment. Regional T1 and T2 values were obtained from neonates' brain T1 and T2 maps derived from 3D MRF. Regional T1 and T2 values were compared, and their changes according to corrected gestational age were evaluated.

RESULTS

The acquisition time for 3D MRF with a spatial resolution of 0.7 × 0.7 × 2 mm3 was less than 5 minutes. The phantom study showed high correlation between T1 and T2 values derived from 3D MRF and those from conventional spin echo sequences (T1, R2 = 0.998, P < 0.001; T2, R2 = 0.998, P < 0.001). Three-dimensional MRF was performed in 25 neonates (15 boys, 10 girls; median corrected gestational age, 263 days; interquartile range, 10 days). In neonates, T1 and T2 values differed in the frontal (median [interquartile range], 2785 [2684-2888] milliseconds and 189.8 [176.7-222.9] milliseconds), parietal (2849 [2741-2950] milliseconds and 191.6 [167.5-232.9] milliseconds), and occipital white matter (2621 [2513-2722] milliseconds and 162.9 [143.5-186.1] milliseconds), showing lower values in occipital white matter (P < 0.001). Regional T1 values showed a negative relationship with corrected gestational age (coefficient, -0.775 to -0.480; P < 0.05).

CONCLUSIONS

Fast and high spatial resolution 3D MRF was applied to neonates. T1 and T2 maps derived from 3D MRF enabled the quantification of regional differences and maturation in the neonatal brain.

Age-specific optimization of T1-weighted brain MRI throughout infancy

The infant brain undergoes drastic morphological and functional development during the first year of life. Three-dimensional T1-weighted Magnetic Resonance Imaging (3D T1w-MRI) is a major tool to characterize the brain anatomy, which however, manifests inherently low and rapidly changing contrast between white matter (WM) and gray matter (GM) in the infant brains (0-12 month-old). Despite the prior efforts made to maximize tissue contrast in the neonatal brains (≤1 months), optimization of imaging methods in the rest of the infancy (1-12 months) is not fully addressed, while brains in the latter period exhibit even more challenging contrast. Here, we performed a systematic investigation to improve the contrast between cortical GM and subcortical WM throughout the infancy. We first performed simultaneous T1 and proton density mapping in a normally developing infant cohort at 3T (n = 57). Based on the evolution of T1 relaxation times, we defined three age groups and simulated the relative tissue contrast between WM and GM in each group. Age-specific imaging strategies were proposed according to the Bloch simulation: inversion time (TI) around 800 ms for the 0-3 month-old group, dual TI at 500 ms and 700 ms for the 3-7 month-old group, and TI around 700 ms for 7-12 month-old group, using a centrically encoded 3D-MPRAGE sequence at 3T. Experimental results with varying TIs in each group confirmed improved contrast at the proposed optimal TIs, even in 3-7 month-old infants who had nearly isointense contrast. We further demonstrated the advantage of improved relative contrast in segmenting the neonatal brains using a multi-atlas segmentation method. The proposed age-specific optimization strategies can be easily adapted to routine clinical examinations, and the improved image contrast would facilitate quantitative analysis of the infant brain development.

Structure and function: how to connect?

INTRODUCTION

The majority, but not all, of very preterm-born infants have difficulties with a variety of cognitive functions as children. It is critical to be able to predict as early as possible those who will have difficulties, to be able to direct appropriate interventions.

METHODS

We are conducting multimodal structural and functional MRI studies in very preterm-born infants and following them with behavioural and neuroimaging assessments until 4 years of age. We are also completing structural and more complex functional imaging in school-aged very preterm-born children.

RESULTS

A number of MRI measures between preterm and term age correlate with outcome at 2 years of age. Functional and structural differences are also seen at school age; examples from these various studies are presented.

CONCLUSION

Structural and functional studies in preterm-born versus term-born infants and children, particularly if completed longitudinally, provide important information on the evolution of brain-behaviour correlates and can help predict outcome in this high-risk population.

Quantitative MRI in the very preterm brain: assessing tissue organization and myelination using magnetization transfer, diffusion tensor and T₁ imaging

Magnetization transfer ratio (MTR), diffusion tensor imaging (DTI) parameters and T(1) relaxometry values were used to create parametric maps characterizing the tissue microstructure of the neonatal brain in infants born very premature (24-32 gestational weeks) and scanned at preterm and term equivalent age. Group-wise image registration was used to determine anatomical correspondence between individual scans and the pooled parametric data at the preterm and term ages. These parametric maps showed distinct contrasts whose interrelations varied across brain regions and between the preterm and term period. Discrete patterns of regional variation were observed for the different quantitative parameters, providing evidence that MRI is sensitive to multiple independent aspects of brain maturation. MTR values showed a marked change in the pattern of regional variation at term equivalent age compared to the preterm period such that the ordinal ranking of regions by signal contrast changed. This was unlike all other parameters where the regional ranking was preserved at the two time points. Interpreting the data in terms of myelination and structural organization, we report on the concordance with available histological data and demonstrate the value of quantitative MRI for tracking brain maturation over the neonatal period.

Microglial reaction in axonal crossroads is a hallmark of noncystic periventricular white matter injury in very preterm infants

Disabilities after brain injury in very preterm infants have mainly been attributed to noncystic periventricular white matter injury (PWMI). We analyzed spatiotemporal patterns of PWMI in the brains of 18 very preterm infants (25-29 postconceptional weeks [pcw]), 7 preterm infants (30-34 pcw), and 10 preterm controls without PWMI. In very preterm infants, we examined PWMI in detail in 2 axonal crossroad areas in the frontal lobe: C1 (lateral to the lateral angle of the anterior horn of the lateral ventricle, at the exit of the internal capsule radiations) and C2 (above the corpus callosum and dorsal angle of the anterior horn). These brains had greater microglia-macrophage densities and activation but lesser astroglial reaction (glial fibrillary acidic protein and monocarboxylate transporter 1 expression) than in preterm cases with PWMI. In preterm infants, scattered necrotic foci were rimmed by axonal spheroids and ionized calcium binding adaptor molecule 1-positive macrophages. Diffuse lesions near these foci consisted primarily of hypertrophic and reactive astrocytes associated with fewer microglia. No differences in Olig2-positive preoligodendrocytes between noncystic PWMI and control cases were found. These data show that the growing axonal crossroad areas are highly vulnerable to PWMI in very preterm infants and highlight differences in glial activation patterns between very preterm and preterm infants.