THE MECHANICAL PROPERTIES OF THE ADOLESCENT BRAIN
Date
2017-05
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
Introduction
The mechanical properties of the brain, as imaged by magnetic resonance elastography (A. Manduca, 2001), have emerged as sensitive measures of neural tissue structure. Studies of the adult brain have revealed a high sensitivity to microstructural health in many neurodegenerative conditions and, recently, a strong structure-function relationship between hippocampal viscoelasticity and memory performance (Schwarb, 2016). However, there are currently no MRE studies that have characterized the stiffness of adolescent brains. This work seeks to address this critical gap in the literature to provide the first in vivo measurements of the adolescent human brain, and compare with previously reported values for the healthy adult brain. Ultimately, these MRE measurements can provide a novel, sensitive approach to studying how the brain matures, and potentially determine structure-function relationships in the developing brain.
Methods
A sample of N=46 healthy, adolescent children (20/26 M/F; age 12-14) completed an MRI scan session on a Siemens 3T T rio scanner, which included high-resolution MRE (2.0 mm resolution; Johnson, 2016) and T1-weighted anatomical (MPRAGE; 0.9 mm resolution) scans. Whole-brain MRE displacement data at 50 Hz was used to create maps of viscoelastic shear stiffness through the nonlinear inversion algorithm (NLI; McGarry, 2012). Regional stiffness was quantified for comparison with literature values of adult brain stiffness by creating ROIs in two ways. (1)ROIs of the cerebrum, cerebellum, and individual lobes, which are regions reported in MRE of the adult brain by Murphy (2013), were created from the WFU PickAtlas (Maldjian,2003). Atlas masks were registered from standard space to the MRE data in FSL (Jenkinson, 2012). (2) ROIs of subcortical structures (amygdala, hippocampus, pallidum, putamen, caudate, and thalamus), as analyzed with MRE by Johnson (2016), were determined by segmentation of the MPRAGE by FIRST (Patenaude, 2011), and similarly registered to the MRE data.
Results
The stiffness values for regional brain lobes in adolescents was compared to the values for adults as reported by Murphy (2013) (Fig. 1). Both the adolescent cerebrum and cerebellum showed similar average stiffness values as in the adult brain, with adolescent brain with differences of -0.3% and 1.7%, respectively. The four main lobes of the cerebrum (frontal, temporal, parietal, and occipital) are all softer in adolescents with differences between -5% and -13%. Interestingly, the region comprising deep gray and white matter was 7.5% stiffer in adolescents.
To further examine the regions central to the cerebrum, six subcortical structures were examined and compared to the adult values reported by Johnson (2016) (Fig. 2). In this case, the caudate and the thalamus were very similar in adults and adolescents, -0.8% and 0.5% difference; the pallidum and the putamen were much stiffer in adolescents 8.4% and 6.9% respectively; and the amygdala and the hippocampus were much softer -18.3% and -10.8%.
Conclusions
This is the first report of the mechanical properties of the human adolescent brain measured in vivo with MRE. By comparing regional stiffness values with adult brain values from literature, a difference was able to be observed between adolescents and adults. Analysis of lobes suggested a gradient of stiffness from higher at the center
of the brain to lower at the periphery; while subcortical regions suggest clustering of stiffer or softer structures based on anatomical location. It is likely that these findings of stiffness in the adolescent brain relative to the adult brain reflect patterns of development as the brain matures to adulthood, similar to previous reports of age-dependent white and gray matter structure (Toga, 2006). MRE of the adolescent brain can be used to identify trends relating to the development of brain structure and potentially provide insight into behavior and social development through sensitive structure-function relationships.
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Keywords
Biomedical Engineering, adolescent brain