Leeor Alon

System and SAR Characterization in Parallel RF Transmission

The markedly increased degrees of freedom introduced by parallel radiofrequency transmission presents both opportunities and challenges for specific absorption rate (SAR) management. On one hand they enable E‐field tailoring and SAR reduction while facilitating excitation profile control. On other hand they increase the complexity of SAR behavior and the risk of inadvertently exacerbating SAR by improper design or playout of radiofrequency pulses. The substantial subject‐dependency of SAR in high field magnetic resonance can be a compounding factor. Building upon a linear system concept and a calibration scheme involving a finite number of in situ measurements, this work establishes a clinically applicable method for characterizing global SAR behavior as well as channel‐by‐channel power transmission. The method offers a unique capability of predicting, for any excitation, the SAR and power consequences that are specific to the subject to be scanned and the MRI hardware. The method was validated in simulation and experimental studies, showing promise as the foundation to a prospective paradigm where power and SAR are not only monitored but, through prediction‐guided optimization, proactively managed. Magn Reson Med, 2012.

Specific absorption rate benefits of including measured electric field interactions in parallel excitation pulse design.

Specific absorption rate management and excitation fidelity are key aspects of radiofrequency pulse design for parallel transmission at ultra–high magnetic field strength. The design of radiofrequency pulses for multiple channels is often based on the solution of regularized least‐squares optimization problems for which a regularization term is typically selected to control the integrated or peak pulse waveform amplitude. Unlike single‐channel transmission, the specific absorption rate of parallel transmission is significantly influenced by interferences between the electric fields associated with the individual transmission elements, which a conventional regularization term does not take into account. This work explores the effects upon specific absorption rate of incorporating experimentally measurable electric field interactions into parallel transmission pulse design. Results of numerical simulations and phantom experiments show that the global specific absorption rate during parallel transmission decreases when electric field interactions are incorporated into pulse design optimization. The results also show that knowledge of electric field interactions enables robust prediction of the net power delivered to the sample or subject by parallel radiofrequency pulses before they are played out on a scanner. Magn Reson Med, 2011.

Method for in situ characterization of radiofrequency heating in parallel transmit MRI.

In ultra‐high‐field magnetic resonance imaging, parallel radiofrequency (RF) transmission presents both opportunities and challenges for specific absorption rate management. On one hand, parallel transmission provides flexibility in tailoring electric fields in the body while facilitating magnetization profile control. On the other hand, it increases the complexity of energy deposition as well as possibly exacerbating local specific absorption rate by improper design or delivery of RF pulses. This study shows that the information needed to characterize RF heating in parallel transmission is contained within a local power correlation matrix. Building upon a calibration scheme involving a finite number of magnetic resonance thermometry measurements, this work establishes a way of estimating the local power correlation matrix. Determination of this matrix allows prediction of temperature change for an arbitrary parallel transmit RF pulse. In the case of a three transmit coil MR experiment in a phantom, determination and validation of the power correlation matrix were conducted in less than 200 min with induced temperature changes of <4°C. Further optimization and adaptation are possible, and simulations evaluating potential feasibility for in vivo use are presented. The method allows general characteristics indicative of RF coil/pulse safety determined in situ. Magn Reson Med, 2013.

Maximum efficiency radiofrequency shimming: Theory and initial application for hip imaging at 7 tesla

Radiofrequency shimming with multiple channel excitation has been proposed to increase the transverse magnetic field uniformity and reduce specific absorption rate at high magnetic field strengths (≥7 T) where high‐frequency effects can make traditional single channel volume coils unsuitable for transmission. In the case of deep anatomic regions and power‐demanding pulse sequences, optimization of transmit efficiency may be a more critical requirement than homogeneity per se. This work introduces a novel method to maximize transmit efficiency using multiple channel excitation and radiofrequency shimming. Shimming weights are calculated in order to obtain the lowest possible net radiofrequency power deposition into the subject for a given transverse magnetic field strength. The method was demonstrated in imaging studies of articular cartilage of the hip joint at 7 T. We show that the new radiofrequency shimming method can enable reduction in power deposition while maintaining an average flip angle or adiabatic condition in the hip cartilage. Building upon the improved shimming, we further show that the signal‐to‐noise ratio in hip cartilage at 7 T can be substantially greater than that at 3 T, illustrating the potential benefits of high field hip imaging. Magn Reson Med, 2013.

A Method for Safety Testing of Radiofrequency/Microwave-Emitting Devices Using MRI

Strict regulations are imposed on the amount of radiofrequency (RF) energy that devices can emit to prevent excessive deposition of RF energy into the body. In this study, we investigated the application of MR temperature mapping and 10‐g average specific absorption rate (SAR) computation for safety evaluation of RF‐emitting devices.

A flexible nested sodium and proton coil array with wideband matching for knee cartilage MRI at 3T

We describe a 2 × 6 channel sodium/proton array for knee MRI at 3T. Multielement coil arrays are desirable because of well‐known signal‐to‐noise ratio advantages over volume and single‐element coils. However, low tissue–coil coupling that is characteristic of coils operating at low frequency can make the potential gains from a phased array difficult to realize.

Subject- and resource-specific monitoring and proactive management of parallel radiofrequency transmission.

Develop a practical comprehensive package for proactive management of parallel radiofrequency (RF) transmission.

Synthesized tissue-equivalent dielectric phantoms using salt and polyvinylpyrrolidone solutions: Synthesized Tissue-Equivalent Dielectric Properties Using a Water-Soluble Polymer

To explore the use of polyvinylpyrrolidone (PVP) for simulated materials with tissue‐equivalent dielectric properties.

Heat equation inversion framework for average SAR calculation from magnetic resonance thermal imaging.

Deposition of radiofrequency (RF) energy can be quantified via electric field or temperature change measurements. Magnetic resonance imaging has been used as a tool to measure three dimensional small temperature changes associated with RF radiation exposure. When duration of RF exposure is long, conversion from temperature change to specific absorption rate (SAR) is nontrivial due to prominent heat-diffusion and conduction effects. In this work, we demonstrated a method for calculation of SAR via an inversion of the heat equation including heat-diffusion and conduction effects. This method utilizes high-resolution three dimensional magnetic resonance temperature images and measured thermal properties of the phantom to achieve accurate calculation of SAR. Accuracy of the proposed method was analyzed with respect to operating frequency of a dipole antenna and parameters used in heat equation inversion. Bioelectromagnetics. 37:493-503, 2016.

Transverse slot antennas for high field MRI: Transverse Slot Antennas for High Field MRI

Introduce a novel coil design using an electrically long transversely oriented slot in a conductive sheet.