Radio Frequency Coils for Ultra-high Field Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) has become a powerful tool not only to analyze the anatomical structures of the human body non invasively but also to investigate brain activity with functional MRI. The promise of increase in signal to noise ratio and spectral resolution proportional to the main magnetic field strength motivated a few research laboratories to pursue even higher field strengths. The 9.4T whole body human scanner and the 16.4T animal scanner installed at the Max Planck Institute for Biological Cybernetics, Tuebingen were, for many years, the worlds strongest magnets in their respective categories. In addition to the strong magnets, radio frequency (RF) coils are also equally important in realising the benefits offered by the high field MRI scanners. The aim of this thesis work is to develop optimized RF coils and RF hardware for ultra-high high field MRI

Ultra-high field MRI: parallel-transmit arrays and RF pulse design

This paper reviews the field of multiple or parallel radiofrequency (RF) transmission for magnetic resonance imaging (MRI). Currently the use of ultra-high field (UHF) MRI at 7 tesla and above is gaining popularity, yet faces challenges with non-uniformity of the RF field and higher RF power deposition. Since its introduction in the early 2000s, parallel transmission (pTx) has been recognized as a powerful tool for accelerating spatially selective RF pulses and combating the challenges associated with RF inhomogeneity at UHF. We provide a survey of the types of dedicated RF coils used commonly for pTx and the important modeling of the coil behavior by electromagnetic (EM) field simulations. We also discuss the additional safety considerations involved with pTx such as the specific absorption rate (SAR) and how to manage them. We then describe the application of pTx with RF pulse design, including a practical guide to popular methods. Finally, we conclude with a description of the current and future prospects for pTx, particularly its potential for routine clinical use

A nested eight-channel transmit array with open-face concept for human brain imaging at 7 tesla

Purpose: Parallel transmit technology for MRI at 7 tesla will significantly benefit from high performance transmit arrays that offer high transmit efficiency and low mutual coupling between the individual array elements. A novel dual-mode transmit array with nested array elements has been developed to support imaging the human brain in both the single-channel (sTx) and parallel-transmit (pTx) excitation modes of a 7 tesla MRI scanner. In this work, the design, implementation, validation, specific absorption rate (SAR) management, and performance of the head coil is presented. Methods: The transmit array consisted of a nested arrangement to improve decoupling between the second-neighboring elements. Two large cut-outs were introduced in the RF shield for an open-face design to reduce claustrophobia and to allow patient monitoring. A hardware interface allows the coil to be used in both the sTx and pTx modes. SAR monitoring is done with virtual observation points (VOP) derived from human body models. The transmit efficiency and coverage is compared with the commercial single-channel and parallel-transmit head coils. Results: Decoupling inductors between the second-neighboring coil elements reduced the coupling to less than −20 dB. Local SAR estimates from the electromagnetic (EM) simulations were always less than the EM-based VOPs, which in turn were always less than scanner predictions and measurements for static and dynamic pTx waveforms. In sTx mode, we demonstrate improved coverage of the brain compared to the commercial sTx coil. The transmit efficiency is within 10% of the commercial pTx coil despite the two large cut-outs in the RF shield. In pTx mode, improved signal homogeneity was shown when the Universal Pulse was used for acquisition in vivo. Conclusion: A novel head coil which includes a nested eight-channel transmit array has been presented. The large cut-outs improve patient monitoring and reduce claustrophobia. For pTx mode, the EM simulation and VOP-based SAR management provided greater flexibility to apply pTx methods without the limitations of SAR constraints. For scanning in vivo, the coil was shown to provide an improved coverage in sTx mode compared to a standard commercial head coil

Validation and Safety Approval of a Dual-Mode Head Coil for pTx Applications In Vivo at 7 Tesla

Following regulatory approval of single-transmit MRI at 7 tesla, there is a rapidly growing interest in clinical MRI at this field strength. However, the wider use of diagnostic MRI at 7T will require imaging in parallel-transmit (pTx) mode to reduce B1+ inhomogeneity. Previous work introduced a dual-mode head coil that operates in both transmit modes and this study investigates the use of this coil for the pTx case. It also describes the safety procedure that was followed to ensure safe operation for human scanning using the real-time SAR supervision on a commercial scanner

Electromagnetic and RF pulse design simulation based optimization of an eight-channel loop array for 11.7T brain imaging

Purpose: Optimization of transmit array performance is crucial in ultra-high-field MRI scanners such as 11.7T because of the increased RF losses and RF nonuniformity. This work presents a new workflow to investigate and minimize RF coil losses, and to choose the optimum coil configuration for imaging. Methods: An 8-channel transceiver loop-array was simulated to analyze its loss mechanism at 499.415 MHz. A folded-end RF shield was developed to limit radiation loss and improve the B+ 1 efficiency. The coil element length, and the shield diameter and length were further optimized using electromagnetic (EM) simulations. The generated EM fields were used to perform RF pulse design (RFPD) simulations under realistic constraints. The chosen coil design was constructed to demonstrate performance equivalence in bench and scanner measurements. Results: The use of conventional RF shields at 11.7T resulted in significantly high radiation losses of 18.4%. Folding the ends of the RF shield combined with optimizing the shield diameter and length increased the absorbed power in biological tissue and reduced the radiation loss to 2.4%. The peak B+ 1 of the optimal array was 42% more than the reference array. Phantom measurements validated the numerical simulations with a close match of within 4% of the predicted B+ 1 . Conclusion: A workflow that combines EM and RFPD simulations to numerically optimize transmit arrays was developed. Results have been validated using phantom measurements. Our findings demonstrate the need for optimizing the RF shield in conjunction with array element design to achieve efficient excitation at 11.7T

First In Vivo Images from an InHouse Parallel Transmit (pTx) Coil for MRI at 7 Tesla

No abstract available

Volumetric imaging with homogenised excitation and static field at 9.4 T

Objectives: To overcome the challenges of B and RF excitation inhomogeneity at ultra-high field MRI, a workflow for volumetric B and flip-angle homogenisation was implemented on a human 9.4 T scanner. Materials and methods: Imaging was performed with a 9.4 T human MR scanner (Siemens Medical Solutions, Erlangen, Germany) using a 16-channel parallel transmission system. B- and B-mapping were done using a dual-echo GRE and transmit phase-encoded DREAM, respectively. B shims and a small-tip-angle-approximation kT-points pulse were calculated with an off-line routine and applied to acquire T- and T -weighted images with MPRAGE and 3D EPI, respectively. Results: Over six in vivo acquisitions, the B-distribution in a region-of-interest defined by a brain mask was reduced down to a full-width-half-maximum of 0.10\ua0±\ua00.01\ua0ppm (39\ua0±\ua02\ua0Hz). Utilising the kT-points pulses, the normalised RMSE of the excitation was decreased from CP-mode’s 30.5\ua0±\ua00.9 to 9.2\ua0±\ua00.7\ua0% with all B \ua0voids eliminated. The SNR inhomogeneities and contrast variations in the T- and T -weighted volumetric images were greatly reduced which led to successful tissue segmentation of the T-weighted image. Conclusion: A 15-minute B- and flip-angle homogenisation workflow, including the B- and B-map acquisitions, was successfully implemented and enabled us to reduce intensity and contrast variations as well as echo-planar image distortions in 9.4 T images

The effects of RF coils and SAR supervision strategies for clinically applicable nonselective parallel-transmit pulses at 7 T

Purpose: To investigate the effects of using different parallel-transmit (pTx) head coils and specific absorption rate (SAR) supervision strategies on pTx pulse design for ultrahigh-field MRI using a 3D-MPRAGE sequence. Methods: The PTx universal pulses (UPs) and fast online-customized (FOCUS) pulses were designed with pre-acquired data sets (B0, B1+ maps, specific absorption rate [SAR] supervision data) from two different 8 transmit/32 receive head coils on two 7T whole-body MR systems. For one coil, the SAR supervision model consisted of per-channel RF power limits. In the other coil, SAR estimations were done with both per-channel RF power limits as well as virtual observation points (VOPs) derived from electromagnetic field (EMF) simulations using three virtual human body models at three different positions. All pulses were made for nonselective excitation and inversion and evaluated on 132 B0, B1+, and SAR supervision datasets obtained with one coil and 12 from the other. At both sites, 3 subjects were examined using MPRAGE sequences that used UP/FOCUS pulses generated for both coils. Results: For some subjects, the UPs underperformed when simulated on a different coil from which they were derived, whereas FOCUS pulses still showed acceptable performance in that case. FOCUS inversion pulses outperformed adiabatic pulses when scaled to the same local SAR level. For the self-built coil, the use of VOPs showed reliable overestimation compared with the ground-truth EMF simulations, predicting about 52% lower local SAR for inversion pulses compared with per-channel power limits. Conclusion: FOCUS inversion pulses offer a low-SAR alternative to adiabatic pulses and benefit from using EMF-based VOPs for SAR estimation