Simultaneous use of linear and nonlinear gradients for B1 + inhomogeneity correction

The simultaneous use of linear spatial encoding magnetic fields (L-SEMs) and nonlinear spatial encoding magnetic fields (N-SEMs) in B1 + inhomogeneity problems is formulated and demonstrated with both simulations and experiments. Independent excitation k-space variables for N-SEMs are formulated for the simultaneous use of L-SEMs and N-SEMs by assuming a small tip angle. The formulation shows that, when N-SEMs are considered as an independent excitation k-space variable, numerous different k-space trajectories and frequency weightings differing in dimension, length, and energy can be designed for a given target transverse magnetization distribution. The advantage of simultaneous use of L-SEMs and N-SEMs is demonstrated by B1 + inhomogeneity correction with spoke excitation. To fully utilize the independent k-space formulations, global optimizations are performed for 1D, 2D RF power limited, and 2D RF power unlimited simulations and experiments. Three different cases are compared: L-SEMs alone, N-SEMs alone, and both used simultaneously. In all cases, the simultaneous use of L-SEMs and N-SEMs leads to a decreased standard deviation in the ROI compared with using only L-SEMs or N-SEMs. The simultaneous use of L-SEMs and N-SEMs results in better B1 + inhomogeneity correction than using only L-SEMs or N-SEMs due to the increased number of degrees of freedom. Copyright © 2017 John Wiley & Sons, Ltd

Magnetic resonance spectroscopic imaging using parallel transmission at 7T

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 123-130).Conventional magnetic resonance spectroscopic imaging (MRSI), also known as phase-encoded (PE) chemical shift imaging (CSI), suffers from both low signal-to-noise ratio (SNR) of the brain metabolites, as well as inflexible tradeoffs between acquisition time and spatial resolution. In addition, although CSI at higher main field strengths, e.g. 7 Tesla (T), offers improved SNR over clinical 1.5T or 3.OT scanners, the realization of these benefits is limited by severe inhomogeneities of the radio frequency (RF) excitation magnetic field (B,+), which is responsible for significant signal variation within the volume of interest (VOI) resulting in spatially dependent SNR losses. The work presented in this dissertation aims to provide the necessary means for using spectroscopic imaging for reliable and robust whole brain metabolite detection and quantification at high main field strengths. It addresses the challenges mentioned above by improving both the excitation and the readout components of the CSI acquisition. The long acquisition times of the PE CSI are significantly shortened (at least 20 fold) by implementing the time-efficient spiral CSI algorithm, while the B1 non-uniformities are corrected for using RF pulses designed for new RF excitation hardware at 7T, so-called parallel transmission (pTx). The B1 homogeneity of the pTx excitations improved at least by a factor of 4 (measured by the normalized spatial standard deviations) compared to conventional single channel transmit systems. The first contribution of this thesis describes the implementation of spiral CSI algorithm for online gradient waveform design and spectroscopic image reconstruction with standard clinical excitation protocols and applied in studies of Late-Onset Tay- Sachs (LOTS), adrenoleukodystrophy (ALD) and brain tumors. A major contribution of this thesis is pTx excitation design for CSI to provide spectral-spatial mitigation of the B1+ inhomogeneities at 7T. Novel pTx RF designs are proposed and demonstrated to yield excellent flip angle mitigation of the brain metabolites, and also enable improved suppression of the undesired water and lipid signals. A major obstacle to the deployment of 7T pTx applications for clinical imaging is the monitoring and management of local specific absorption rate (SAR). This thesis also proposes a pTx SAR monitoring system with real-time RF monitoring and shut-off capabilities.by Borjan Aleksandar Gagoski.Ph.D

RF Pulse Design for Parallel Transmission in Ultra High Field Magnetic Resonance Imaging

Magnetic Resonance Imaging (MRI) plays an important role in visualizing the structure and function of the human body. In recent years, ultra high magnetic field (UHF) MRI has emerged as an attractive means to achieve significant improvements in both signal-to-noise ratio (SNR) and contrast. However, in vivo imaging at UHF is hampered by the presence of severe B1 and B0 inhomogeneities. B1 inhomogeneity leads to spatial non-uniformity excitation in MR images. B0 inhomogeneity, on the other hand, produces blurring, distortions and signal loss at tissue/air interfaces. Both of them greatly limit the applications of UHF MRI. Thus mitigating B1 and B0 inhomogeneities is central in making UHF MRI practical for clinical use. Tailored RF pulse design has been demonstrated as a feasible means to mitigate the effects of B1 and B0 inhomogeneities. However, the primary limitation of such tailored pulses is that the pulse duration is too long for practical clinical applications. With the introduction of parallel transmission technology, one can shorten the pulse duration without sacrificing excitation performance. Prior reports in parallel transmission were formulated using linear, small-tip-angle approximation algorithms, which are violated in the regime of nonlinear large-tip-angle excitation. The overall goal of this dissertation is to develop effective and fast algorithms for parallel transmission UHF RF pulses design. The key contributions of this work include 1) a novel large-tip-angle RF pulse design method to achieve significant improvements compared with previous algorithms; 2) implementing a model-based eddy current correction method to compensate eddy current field induced on RF shield for parallel transmission and leading to improved excitation and time efficiency; 3) developing new RF pulse design strategy to restore the lost signal over the whole brain and increase BOLD contrast to brain activation in T2*-weighted fMRI at UHF. For testing and validation, these algorithms were implement on a Siemens 7T MRI scanner equipped with a parallel transmission system and their capabilities for ultra high field MRI demonstrated, first by phantom experiments and later by in vivo human imaging studies. The contributions presented here will be of importance to bring parallel transmission technology to clinical applications in UHF MRI

Numerical field simulation for parallel transmission in MRI at 7 tesla

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 41-42).Parallel transmission (pTx) is a promising improvement to coil design that has been demonstrated to mitigate B1* inhomogeneity, manifest as center brightening, for high-field magnetic resonance imaging (MRI). Parallel transmission achieves spatially-tailored pulses through multiple radiofrequency (RF) excitation coils that can be activated independently. In this work, simulations of magnetic fields in numerical phantoms using an FDTD solver are used to estimate the excitation profiles for an 8-channel RF head coil. Each channel is driven individually in the presence of a dielectric load, and the excitation profiles for all channels are combined post-processing into a B1+ profile of the birdcage (BC) mode. The B1 profile is calculated for a dielectric sphere phantom with material properties of white matter at main magnetic field strengths of 3T and 7T to demonstrate center brightening associated with head imaging at high magnetic field strengths. Measurements of a circular ROI centered in the image show more B1+ inhomogeneity at 7T than at 3T. The B1* profile is then simulated for a numerical head phantom with spatially segmented tissue compartments at 7T. Comparison of the simulated and in vivo B1* profiles at 7T shows agreement in the B1 inhomogeneity. The results provide confidence in numerical simulation as a means to estimate magnetic fields for human imaging. This work will allow further numerical simulations to model the propagation of electric fields within the body, ultimately to provide an estimate of heat deposition in tissue, quantified by the specific absorption rate (SAR), which is a limiting factor of the use of high-field MRI in the clinical setting.by Jessica A. Bernier.S.M