Browsing by Author "McIlvain, Grace"
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Item Correlated noise in brain magnetic resonance elastography(Magnetic Resonance in Medicine, 2021-10-22) Hannum, Ariel J.; McIlvain, Grace; Sowinski, Damian; McGarry, Matthew D. J.; Johnson, Curtis L.Purpose: Magnetic resonance elastography (MRE) uses phase-contrast MRI to generate mechanical property maps of the in vivo brain through imaging of tissue deformation from induced mechanical vibration. The mechanical property estimation process in MRE can be susceptible to noise from physiological and mechanical sources encoded in the phase, which is expected to be highly correlated. This correlated noise has yet to be characterized in brain MRE, and its effects on mechanical property estimates computed using inversion algorithms are undetermined. Methods: To characterize the effects of signal noise in MRE, we conducted 3 experiments quantifying (1) physiomechanical sources of signal noise, (2) physiological noise because of cardiac-induced movement, and (3) impact of correlated noise on mechanical property estimates. We use a correlation length metric to estimate the extent that correlated signal persists in MRE images and demonstrate the effect of correlated noise on property estimates through simulations. Results: We found that both physiological noise and vibration noise were greater than image noise and were spatially correlated across all subjects. Added physiological and vibration noise to simulated data resulted in property maps with higher error than equivalent levels of Gaussian noise. Conclusion: Our work provides the foundation to understand contributors to brain MRE data quality and provides recommendations for future work to correct for signal noise in MRE.Item Imaging the mechanical properties of the pediatric brain(University of Delaware, 2022) McIlvain, GraceBrain mechanical properties can be measured in vivo using a phase contrast MRI technology known as magnetic resonance elastography (MRE). Mechanical properties describe underlying neural tissue microstructural composition, and they have been found to sensitively describe changes in aging, neurodegenerative disease, and tumors. Interestingly, mechanical properties have recently been found to relate to cognitive function, highlighting the sensitivity of MRE to individual differences. However, brain mechanical properties have not previously been measured in vivo in any pediatric population, as MRE is an inherently long acquisition technique which was previously ill-suited for scanning challenging populations such as children. Pediatric elastography has tremendous potential to aid in understanding neural tissue differences in neurodevelopmental disorders, and to help expand scientific understanding of how tissue mechanical maturation contributes to maturation of cognitive function. The goal of this dissertation is to develop fast acquisition MRE techniques which are specifically tailored for the pediatric population and for the first time, characterize normal regional brain mechanical maturation from childhood to adulthood.Item THE MECHANICAL PROPERTIES OF THE ADOLESCENT BRAIN(University of Delaware, 2017-05) McIlvain, GraceIntroduction 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.Item Quantitative effects of off-resonance related distortion on brain mechanical property estimation with magnetic resonance elastography(NMR in Biomedicine, 2021-09-20) McIlvain, Grace; McGarry, Matthew D. J.; Johnson, Curtis L.Off-resonance related geometric distortion can impact quantitative MRI techniques, such as magnetic resonance elastography (MRE), and result in errors to these otherwise sensitive metrics of brain health. MRE is a phase contrast technique to determine the mechanical properties of tissue by imaging shear wave displacements and estimating tissue stiffness through inverse solution of Navier's equation. In this study, we systematically examined the quantitative effects of distortion and corresponding correction approaches on MRE measurements through a series of simulations, phantom models, and in vivo brain experiments. We studied two different k-space trajectories, echo-planar imaging and spiral, and we determined that readout time, off-resonance gradient strength, and the combination of readout direction and off-resonance gradient direction, impact the estimated mechanical properties. Images were also processed through traditional distortion correction pipelines, and we found that each of the correction mechanisms works well for reducing stiffness errors, but are limited in cases of very large distortion. The ability of MRE to detect subtle changes to neural tissue health relies on accurate, artifact-free imaging, and thus off-resonance related geometric distortion must be considered when designing sequences and protocols by limiting readout time and applying correction where appropriate.