185 research outputs found
Long-lived selective spin echoes in dipolar solids under periodic and aperiodic pi-pulse trains
The application of Carr-Purcell-Meiboom-Gill (CPMG) trains for
dynamically decoupling a system from its environment has been extensively
studied in a variety of physical systems. When applied to dipolar solids,
recent experiments have demonstrated that CPMG pulse trains can generate
long-lived spin echoes. While there still remains some controversy as to the
origins of these long-lived spin echoes under the CPMG sequence, there is a
general agreement that pulse errors during the pulses are a necessary
requirement. In this work, we develop a theory to describe the spin dynamics in
dipolar coupled spin-1/2 system under a CPMG() pulse train,
where and are the phases of the pulses. From our
theoretical framework, the propagator for the CPMG() pulse
train is equivalent to an effective ``pulsed'' spin-locking of single-quantum
coherences with phase , which generates a
periodic quasiequilibrium that corresponds to the long-lived echoes. Numerical
simulations, along with experiments on both magnetically dilute, random spin
networks found in C and C and in non-dilute spin systems found in
adamantane and ferrocene, were performed and confirm the predictions from the
proposed theory.Comment: 25 pages, 12 figures, submitted to Physical Review
Cerebral white matter analysis using diffusion imaging
Thesis (Ph. D.)--Harvard-MIT Division of Health Sciences and Technology, 2006.Includes bibliographical references (p. 183-198).In this thesis we address the whole-brain tractography segmentation problem. Diffusion magnetic resonance imaging can be used to create a representation of white matter tracts in the brain via a process called tractography. Whole brain tractography outputs thousands of trajectories that each approximate a white matter fiber pathway. Our method performs automatic organization, or segmention, of these trajectories into anatomical regions and gives automatic region correspondence across subjects. Our method enables both the automatic group comparison of white matter anatomy and of its regional diffusion properties, and the creation of consistent white matter visualizations across subjects. We learn a model of common white matter structures by analyzing many registered tractography datasets simultaneously. Each trajectory is represented as a point in a high-dimensional spectral embedding space, and common structures are found by clustering in this space. By annotating the clusters with anatomical labels, we create a model that we call a high-dimensional white matter atlas.(cont.) Our atlas creation method discovers structures corresponding to expected white matter anatomy, such as the corpus callosum, uncinate fasciculus, cingulum bundles, arcuate fasciculus, etc. We show how to extend the spectral clustering solution, stored in the atlas, using the Nystrom method to perform automatic segmentation of tractography from novel subjects. This automatic tractography segmentation gives an automatic region correspondence across subjects when all subjects are labeled using the atlas. We show the resulting automatic region correspondences, demonstrate that our clustering method is reproducible, and show that the automatically segmented regions can be used for robust measurement of fractional anisotropy.by Lauren Jean O'Donnell.Ph.D
Marek's Disease Virus VP22: Subcellular Localization and Characterization of Carboxyl Terminal Deletion Mutations
AbstractMarek's disease virus (MDV) is an alphaherpesvirus that causes T cell lymphoma and severe immunosuppression in chickens. The MDV UL49 gene, which encodes the tegument viral protein 22 (VP22), has been expressed as a green fluorescent protein (GFP) fusion protein in chicken embryonic fibroblasts to examine its subcellular localization. As with both human herpesvirus 1 and bovine herpesvirus 1VP22-GFP fusion proteins, the MDV VP22-GFP product binds to microtubules and heterochromatin. In addition, the MDV protein also binds to the centrosomes. During mitosis, VP22-GFP binds to sister chromatids, but dissociates from the centrosomes and the microtubules of the mitotic spindle. A series of VP22 carboxy terminal truncation mutants were constructed to define regions responsible for these binding properties. These mutants identified separable domains or motifs responsible for binding microtubules and heterochromatin
Tractography-Based Parcellation of Cerebellar Dentate Nuclei via a Deep Nonnegative Matrix Factorization Clustering Method
As the largest human cerebellar nucleus, the dentate nucleus (DN) functions
significantly in the communication between the cerebellum and the rest of the
brain. Structural connectivity-based parcellation has the potential to reveal
the topography of the DN and enable the study of its subregions. In this paper,
we investigate a deep nonnegative matrix factorization clustering method
(DNMFC) for parcellation of the human DN based on its structural connectivity
using diffusion MRI tractography. We propose to describe the connectivity of
the DN using a set of curated tractography fiber clusters within the
cerebellum. Experiments are conducted on the diffusion MRI data of 50 healthy
adults from the Human Connectome Project. In comparison with state-of-the-art
clustering methods, DN parcellations resulting from DNMFC show better quality
and consistency of parcels across subjects
TractCloud: Registration-free tractography parcellation with a novel local-global streamline point cloud representation
Diffusion MRI tractography parcellation classifies streamlines into
anatomical fiber tracts to enable quantification and visualization for clinical
and scientific applications. Current tractography parcellation methods rely
heavily on registration, but registration inaccuracies can affect parcellation
and the computational cost of registration is high for large-scale datasets.
Recently, deep-learning-based methods have been proposed for tractography
parcellation using various types of representations for streamlines. However,
these methods only focus on the information from a single streamline, ignoring
geometric relationships between the streamlines in the brain. We propose
TractCloud, a registration-free framework that performs whole-brain
tractography parcellation directly in individual subject space. We propose a
novel, learnable, local-global streamline representation that leverages
information from neighboring and whole-brain streamlines to describe the local
anatomy and global pose of the brain. We train our framework on a large-scale
labeled tractography dataset, which we augment by applying synthetic transforms
including rotation, scaling, and translations. We test our framework on five
independently acquired datasets across populations and health conditions.
TractCloud significantly outperforms several state-of-the-art methods on all
testing datasets. TractCloud achieves efficient and consistent whole-brain
white matter parcellation across the lifespan (from neonates to elderly
subjects, including brain tumor patients) without the need for registration.
The robustness and high inference speed of TractCloud make it suitable for
large-scale tractography data analysis. Our project page is available at
https://tractcloud.github.io/.Comment: MICCAI 202
Superficial White Matter Analysis: An Efficient Point-cloud-based Deep Learning Framework with Supervised Contrastive Learning for Consistent Tractography Parcellation across Populations and dMRI Acquisitions
Diffusion MRI tractography is an advanced imaging technique that enables in
vivo mapping of the brain's white matter connections. White matter parcellation
classifies tractography streamlines into clusters or anatomically meaningful
tracts. It enables quantification and visualization of whole-brain
tractography. Currently, most parcellation methods focus on the deep white
matter (DWM), whereas fewer methods address the superficial white matter (SWM)
due to its complexity. We propose a novel two-stage deep-learning-based
framework, Superficial White Matter Analysis (SupWMA), that performs an
efficient and consistent parcellation of 198 SWM clusters from whole-brain
tractography. A point-cloud-based network is adapted to our SWM parcellation
task, and supervised contrastive learning enables more discriminative
representations between plausible streamlines and outliers for SWM. We train
our model on a large-scale tractography dataset including streamline samples
from labeled SWM clusters and anatomically implausible streamline samples, and
we perform testing on six independently acquired datasets of different ages and
health conditions (including neonates and patients with space-occupying brain
tumors). Compared to several state-of-the-art methods, SupWMA obtains highly
consistent and accurate SWM parcellation results on all datasets, showing good
generalization across the lifespan in health and disease. In addition, the
computational speed of SupWMA is much faster than other methods.Comment: 12 pages, 7 figures. Extension of our ISBI 2022 paper
(arXiv:2201.12528) (Best Paper Award Finalist
Research using population-based administration data integrated with longitudinal data in child protection settings: A systematic review
Introduction: Over the past decade there has been a marked growth in the use of linked population administrative data for child protection research. This is the first systematic review of studies to report on research design and statistical methods used where population-based administrative data is integrated with longitudinal data in child protection settings.
Methods: The systematic review was conducted according to Preferred Reporting Items for Systematic Review and Meta-Analyses (PRISMA) statement. The electronic databases Medline (Ovid), PsycINFO, Embase, ERIC, and CINAHL were systematically searched in November 2019 to identify all the relevant studies. The protocol for this review was registered and published with Open Science Framework (Registration DOI: 10.17605/OSF.IO/96PX8)
Results: The review identified 30 studies reporting on child maltreatment, mental health, drug and alcohol abuse and education. The quality of almost all studies was strong, however the studies rated poorly on the reporting of data linkage methods. The statistical analysis methods described failed to take into account mediating factors which may have an indirect effect on the outcomes of interest and there was lack of utilisation of multi-level analysis.
Conclusion: We recommend reporting of data linkage processes through following recommended and standardised data linkage processes, which can be achieved through greater co-ordination among data providers and researchers
- …