Medicine and Health Sciences Commons^™

Optimization Of A Boundary Element Approach To Electromagnet Design With Application To A Host Of Current Problems In Magnetic Resonance Imaging, Chad T. Harris

Electronic Thesis and Dissertation Repository

Magnetic resonance imaging (MRI) has proven to be a valuable methodological approach in both basic research and clinical practice. However, significant hardware advances are still needed in order to further improve and extend the applications of the technique. The present dissertation predominantly addresses gradient and shim coil design (sub-systems of the MR system).

A design study to investigate gradient performance over a set of surface geometries ranging in curvature from planar to a full cylinder using the boundary element (BE) method is presented. The results of this study serve as a guide for future planar and pseudo-planar gradient systems for …

Imaging Breast Cancer Progression And Lymph Node Metastases In Murine Models Using Mri And Magnetic Nanoparticles, Vasiliki Economopoulos

Electronic Thesis and Dissertation Repository

Most breast cancer related deaths are caused by the spread or metastasis of the primary tumor to distant sites in the body. The lymph nodes are one of the first places where metastases can be detected and are frequently examined for macroscopic metastases to help determine course of treatment for patients. However, little is known about the significance of microscopic metastases and disseminated individual cancer cells within the nodes. The goal of this work was to use MRI to monitor the development of primary tumors and lymphatic metastases in models of breast cancer.

In this thesis, we examined the MRI …

A Multivariate Method To Determine The Dimensionality Of Neural Representation From Population Activity., Jörn Diedrichsen, Tobias Wiestler, Naveed Ejaz

Brain and Mind Institute Researchers' Publications

How do populations of neurons represent a variable of interest? The notion of feature spaces is a useful concept to approach this question: According to this model, the activation patterns across a neuronal population are composed of different pattern components. The strength of each of these components varies with one latent feature, which together are the dimensions along which the population represents the variable. Here we propose a new method to determine the number of feature dimensions that best describes the activation patterns. The method is based on Gaussian linear classifiers that use only the first d most important pattern …

Skill Learning Strengthens Cortical Representations Of Motor Sequences., Tobias Wiestler, Jörn Diedrichsen

Brain and Mind Institute Researchers' Publications

Motor-skill learning can be accompanied by both increases and decreases in brain activity. Increases may indicate neural recruitment, while decreases may imply that a region became unimportant or developed a more efficient representation of the skill. These overlapping mechanisms make interpreting learning-related changes of spatially averaged activity difficult. Here we show that motor-skill acquisition is associated with the emergence of highly distinguishable activity patterns for trained movement sequences, in the absence of average activity increases. During functional magnetic resonance imaging, participants produced either four trained or four untrained finger sequences. Using multivariate pattern analysis, both untrained and trained sequences could …

Two Distinct Ipsilateral Cortical Representations For Individuated Finger Movements., Jörn Diedrichsen, Tobias Wiestler, John W Krakauer

Brain and Mind Institute Researchers' Publications

Movements of the upper limb are controlled mostly through the contralateral hemisphere. Although overall activity changes in the ipsilateral motor cortex have been reported, their functional significance remains unclear. Using human functional imaging, we analyzed neural finger representations by studying differences in fine-grained activation patterns for single isometric finger presses. We demonstrate that cortical motor areas encode ipsilateral movements in 2 fundamentally different ways. During unimanual ipsilateral finger presses, primary sensory and motor cortices show, underneath global suppression, finger-specific activity patterns that are nearly identical to those elicited by contralateral mirror-symmetric action. This component vanishes when both motor cortices are …

Decoding The Neural Mechanisms Of Human Tool Use., Jason P Gallivan, D Adam Mclean, Kenneth F Valyear, Jody C Culham

Brain and Mind Institute Researchers' Publications

Sophisticated tool use is a defining characteristic of the primate species but how is it supported by the brain, particularly the human brain? Here we show, using functional MRI and pattern classification methods, that tool use is subserved by multiple distributed action-centred neural representations that are both shared with and distinct from those of the hand. In areas of frontoparietal cortex we found a common representation for planned hand- and tool-related actions. In contrast, in parietal and occipitotemporal regions implicated in hand actions and body perception we found that coding remained selectively linked to upcoming actions of the hand whereas …

Delta Relaxation Enhanced Magnetic Resonance - Development And Application Of A Field-Cycling Contrast Mechanism, Yonathan Araya

Electronic Thesis and Dissertation Repository

Delta relaxation enhanced magnetic resonance (dreMR) is a novel imaging method capable of producing contrast proportional only to the concentration of the bound form of the targetable contrast agent using a dynamic field-cycling technique. The characteristic high relaxivity magnetic field dependence of bound paramagnetic contrast agents enables suppression of tissue contrast from unbound agents and unenhanced tissue, thereby increasing probe specificity. The dreMR technique requires an auxiliary actively shielded field-shifting insert electromagnet to modulate the strength of the main clinical magnetic field as a function of time during the relaxation and evolution periods of a pulse sequence.

Ablavar (approved for …

A Magnetic-Resonance-Imaging-Compatible Remote Catheter Navigation System., Mohammad Ali Tavallaei, Yogesh Thakur, Syed Haider, Maria Drangova

Medical Biophysics Publications

A remote catheter navigation system compatible with magnetic resonance imaging (MRI) has been developed to facilitate MRI-guided catheterization procedures. The interventionalist's conventional motions (axial motion and rotation) on an input catheter - acting as the master - are measured by a pair of optical encoders, and a custom embedded system relays the motions to a pair of ultrasonic motors. The ultrasonic motors drive the patient catheter (slave) within the MRI scanner, replicating the motion of the input catheter. The performance of the remote catheter navigation system was evaluated in terms of accuracy and delay of motion replication outside and within …

Errors In Device Localization In Mri Using Z-Frames., Jeremy Cepek, Blaine A Chronik, Aaron Fenster

Brain and Mind Institute Researchers' Publications

The use of a passive MRI-visible tracking frame is a common method of localizing devices in MRI space for MRI-guided procedures. One of the most common tracking frame designs found in the literature is the z-frame, as it allows six degree-of-freedom pose estimation using only a single image slice. Despite the popularity of this design, it is susceptible to errors in pose estimation due to various image distortion mechanisms in MRI. In this paper, the absolute error in using a z-frame to localize a tool in MRI is quantified over various positions of the z-frame relative to the MRI isocenter, …