Time-resolved reconstruction of motion, force, and stiffness using Spectro-Dynamic MRI

Measuring the dynamics and mechanical properties of muscles and joints is important to understand the (patho)physiology of muscles. However, acquiring dynamic time-resolved MRI data is challenging. We have previously developed Spectro-Dynamic MRI which allows the characterization of dynamical systems at a high spatial and temporal resolution directly from k-space data. This work presents an extended Spectro-Dynamic MRI framework that reconstructs 1) time-resolved MR images, 2) time-resolved motion fields, 3) dynamical parameters, and 4) an activation force, at a temporal resolution of 11 ms. An iterative algorithm solves a minimization problem containing four terms: a motion model relating the motion to the fully-sampled k-space data, a dynamical model describing the expected type of dynamics, a data consistency term describing the undersampling pattern, and finally a regularization term for the activation force. We acquired MRI data using a dynamic motion phantom programmed to move like an actively driven linear elastic system, from which all dynamic variables could be accurately reconstructed, regardless of the sampling pattern. The proposed method performed better than a two-step approach, where time-resolved images were first reconstructed from the undersampled data without any information about the motion, followed by a motion estimation step

Visualization of metasurface eigenmodes with magnetic resonance imaging

The ability to control the electromagnetic near field with metasurfaces offers potential applications over the frequency range from radio frequency to optical domains. In this work, we show an essential feature of metasurfaces, subwavelength field confinement via excitation of a large number of eigenstates in a narrow frequency range, and demonstrate an innovative way of visualizing profiles of metasurface eigenmodes with the aid of a magnetic resonance imaging (MRI) system. We show that by tuning different eigenmodes of the metasurface to the Larmor frequency, we can passively tailor the near-field distribution to adjust the desired pattern of radio-frequency excitation in a MRI experiment. Our work demonstrates a practical nonperturbed rapid way of imaging metasurface eigenmodes

Radiofrequency fields in hyperthermia and MRI : Exploiting their similarities for mutual benefit

Hyperthermia treatment planning aims to calculate and optimize the thermal dose of hyperthermia treatments for individual patient cases. For this purpose extensive electromagnetic and thermal modelling techniques have been successfully developed over the last years. Unfortunately, means to monitor and verify a pre-calculated treatment plan during treatment are limited. MR imaging is an excellent imaging modality to visualize anatomy and physiology. As field strengths increase, problems with respect to greater RF field inhomogeneities and elevated RF power deposition pose technical challenges. This thesis demonstrates that hyperthermia treatment planning can be verified with MR imaging and that MR imaging can be optimized with hyperthermia treatment planning

Ultra fast electromagnetic field computations for RF multi-transmit techniques in high field MRI

A new, very fast, approach for calculations of the electromagnetic excitation field for MRI is presented. The calculation domain is divided in different homogeneous regions, where for each region a general solution is obtained by a summation of suitable basis functions. A unique solution for the electromagnetic field is found by enforcing the appropriate boundary conditions between the different regions. The method combines the speed of an analytical method with the versatility of full wave simulation methods and is validated in the pelvic region against FDTD simulations at 3 and 7 T and measurements at 3 T. The high speed and accurate reproduction of measurements and FDTD calculations are believed to offer large possibilities for multi-transmit applications, where it can be used for on-line control of the global and local electric field and specific absorption rate (SAR) in the patient. As an example the method was evaluated for RF shimming with the use of 7 T simulation results, where it was demonstrated that the magnetic excitation field could be homogenized, while both the local and average SAR were reduced by 38% or more. © 2009 Institute of Physics and Engineering in Medicine

First-Order Induced Current Density Imaging and Electrical Properties Tomography in MRI

Radiolog

RF peak power reduction in CAIPIRINHA excitation by interslice phase optimization

The purpose of this work was to show that the overall peak power of RF pulses for CAIPIRINHA excitation can be substantially reduced by applying interslice phase relaxation. The optimal phases are scan dependent and can be quickly calculated by the proposed method. The multi-band RF pulse design is implemented as the minimization of a linear objective function with quadratic constraints. The interslice phase is considered to be a variable for optimization. In the case of a phase cycling scheme (CAIPIRINHA), the peak power is considered over all pulses. The computation time (about 1s) is compatible with online RF pulse design. It is shown that the optimal interslice phases depend on the CAIPIRINHA scheme used and that RF peak power is reduced when the CAIPIRINHA phase cycling is taken into account in the optimization. The proposed method is extremely fast and results in RF pulses with low peak power for CAIPIRINHA excitation. The MATLAB implementation is given in the appendix; it allows for online determination of scan-dependent phase parameters. Furthermore, the method can be easily extended to pTx shimming systems in the context of multi-slice excitations, and this possibility is included in the software. Copyright (c) 2015 John Wiley & Sons, Ltd