Hybrid-State Free Precession in Nuclear Magnetic Resonance

The dynamics of large spin-1/2 ensembles in the presence of a varying magnetic field are commonly described by the Bloch equation. Most magnetic field variations result in unintuitive spin dynamics, which are sensitive to small deviations in the driving field. Although simplistic field variations can produce robust dynamics, the captured information content is impoverished. Here, we identify adiabaticity conditions that span a rich experiment design space with tractable dynamics. These adiabaticity conditions trap the spin dynamics in a one-dimensional subspace. Namely, the dynamics is captured by the absolute value of the magnetization, which is in a transient state, while its direction adiabatically follows the steady state. We define the hybrid state as the co-existence of these two states and identify the polar angle as the effective driving force of the spin dynamics. As an example, we optimize this drive for robust and efficient quantification of spin relaxation times and utilize it for magnetic resonance imaging of the human brain

Maximizing information on the environment by dynamically controlled qubit probes

We explore the ability of a qubit probe to characterize unknown parameters of its environment. By resorting to quantum estimation theory, we analytically find the ultimate bound on the precision of estimating key parameters of a broad class of ubiquitous environmental noises ("baths") which the qubit may probe. These include the probe-bath coupling strength, the correlation time of generic bath spectra, the power laws governing these spectra, as well as their dephasing times T2. Our central result is that by optimizing the dynamical control on the probe under realistic constraints one may attain the maximal accuracy bound on the estimation of these parameters by the least number of measurements possible. Applications of this protocol that combines dynamical control and estimation theory tools to quantum sensing are illustrated for a nitrogen-vacancy center in diamond used as a probe.Comment: 8 pages + 6 pages (appendix), 3 Figure

Methodological considerations for neuroimaging in deep brain stimulation of the subthalamic nucleus in Parkinson’s disease patients

Deep brain stimulation (DBS) of the subthalamic nucleus is a neurosurgical intervention for Parkinson’s disease patients who no longer appropriately respond to drug treatments. A small fraction of patients will fail to respond to DBS, develop psychiatric and cognitive side-effects, or incur surgery-related complications such as infections and hemorrhagic events. In these cases, DBS may require recalibration, reimplantation, or removal. These negative responses to treatment can partly be attributed to suboptimal pre-operative planning procedures via direct targeting through low-field and low-resolution magnetic resonance imaging (MRI). One solution for increasing the success and efficacy of DBS is to optimize preoperative planning procedures via sophisticated neuroimaging techniques such as high-resolution MRI and higher field strengths to improve visualization of DBS targets and vasculature. We discuss targeting approaches, MRI acquisition, parameters, and post-acquisition analyses. Additionally, we highlight a number of approaches including the use of ultra-high field (UHF) MRI to overcome limitations of standard settings. There is a trade-off between spatial resolution, motion artifacts, and acquisition time, which could potentially be dissolved through the use of UHF-MRI. Image registration, correction, and post-processing techniques may require combined expertise of traditional radiologists, clinicians, and fundamental researchers. The optimization of pre-operative planning with MRI can therefore be best achieved through direct collaboration between researchers and clinicians

SAR Prediction and SAR Management for Parallel Transmit MRI

Parallel transmission enables control of the RF field in high-field Magnetic Resonance Imaging (MRI). However, the approach has also caused concerns about the specific absorption rate (SAR) in the patient body. The present work provides new concepts for SAR prediction. A novel approach for generating human body models is proposed, based on a water-fat separated MRI pre-scan. Furthermore, this work explores various approaches for SAR reduction

Imaging cerebrovascular health using 7T MRI

Magnetic resonance imaging is a valuable clinical tool for the visualization of intracranial vasculature. Without exposing patients to ionizing radiation or intravenous contrasts, it can provide multi-modal structural information about the shape, structure, and function of the various vessels involved in stroke and dementia. However, imaging methods are limited by the achieved contrasts and resolutions, as well as the required scan times. Ultra-high field 7T MRI offers increased signal-to-noise ratio and desirable changes in relaxation parameters, therefore promising substantial improvements to existing neurovascular MRI approaches such as MR angiography (MRA) and MR vessel wall imaging (VWI). However, 7T MRI also introduces increased specific absorption rates and reduced homogeneity and extent of the transmit B1 field. Because of the latter, the first research chapter in this thesis (Chapter 3) studies the possibility to increase the extent of this 7T B1+ field into the feeding arteries in the neck using parallel transmission (pTx). The second research chapter (Chapter 4) aims to improve the accelerated acquisition of high-resolution MRA using compressed sensing reconstruction. This facilitates the visualization of the small intracranial arteries which are involved in lacunar infarcts and vascular dementia, which can be achieved within clinical scan times. The final parts of this thesis (Chapters 5-7) focus on a specific intracranial VWI sequence called DANTE-SPACE. A simulation framework for the sequence is first presented in Chapter 5. This framework includes various additional processes such as (pulsatile) tissue motion and B1+ variations to accurately represent the intra- and extra-vascular contrast mechanisms. The simulations are then used for the optimization and comparison of the T2-weighted DANTE-SPACE sequence at 3T, 7T without pTx, and 7T with pTx. The optimizations aim to maximize the contrast between both the blood within and the cerebrospinal fluid surrounding intracranial vessel walls, and the comparison between different field strengths provides a first quantitative indication of the added value of ultra- high field MRI for the DANTE-SPACE sequence