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

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

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

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

No abstract available

Acoustic noise reduction in the NexGen 7 T scanner

Purpose: Driven by the Lorentz force, acoustic noise may arguably be the next physiological challenge associated with ultra-high field MRI scanners and powerful gradient coils. This work consisted of isolating and mitigating the main sound pathway in the NexGen 7 T scanner equipped with the investigational Impulse head gradient coil. Methods: Sound pressure level (SPL) measurements were performed with and without the RF coil to assess its acoustic impact. Vibration measurements were carried out on the gradient coil, the RF coil, and on the patient table to distinguish the different vibration mechanisms and pathways. Vibrations of the RF coil were modified by either making contact with the patient bore liner with padding material or by changing directly the RF shield with phosphor bronze mesh material. Results: SPL and vibration measurements demonstrated that eddy-currents induced in the RF shield were the primary cause of acoustic noise. Replacing the conventional solid copper shield with phosphor bronze mesh material altered the vibrations of the RF shield and decreased SPL by 6 to 8 dB at the highest frequencies in EPI, depending on the gradient axis, while boosting the transmit B1+ field by 15%. Padding led to slightly less sound reduction on the X and Z gradient axes, but with minimal impact for the Y axis. Conclusion: This study demonstrates the potential importance of eddy-current induced vibrations in the RF coil in terms of acoustic noise and opens new horizons for mitigation measures

A 3D surface coil with deep learning‐based noise reduction for parotid gland imaging at 7T

Background: Background: Parotid gland neoplasms occur near the facial nerve. Hence, it is crucial to determine whether the malignant neoplasms involve the facial nerve and whether sacrifice of the nerve in surgery is necessary. Furthermore, while 20% of all neoplasms are malignant, the most common benign neoplasm, pleomorphic adenoma, has a risk for malignant transformation, making early detection and treatment essential. 7T magnetic resonance imaging offers increased signal‐to‐noise ratio (SNR) and sensitivity. Aim: In this work, we address imaging the parotid gland since it remains challenging at 7T because of its spatial location. Materials and Methods: Here, we present a novel three‐dimensional surface coil (3D Coil) architecture that offers increased depth penetration and SNR compared to the single channel surface coil. We further developed a deep learning (DL)‐based noise reduction method that receives inputs from three elements of the 3D Coil. Results: The 3D coil with DL‐based denoising method offers twice the SNR compared to the single channel surface coil for parotid gland imaging at 7T. Discussion and Conclusion: The proposed 3D Coil and DL‐based noise reduction method offers a promising way of achieving higher SNR for parotid salivary gland imaging at 7T, paving the road for clinical applications

A 32‐channel high‐impedance honeycomb‐shaped receive array for temporal lobes exploration at 11.7T

Purpose: The newly operational 11.7T Iseult scanner provides an improved global SNR in the human brain. This gain in SNR can be pushed even further locally by designing region‐focused dense receive arrays. The temporal lobes are particularly interesting to neuroscientists as they are associated with language and concept recognition. Our main goal was to maximize the SNR in the temporal lobes and provide high‐acceleration capabilities for fMRI studies. Methods: We designed and developed a 32‐channel receive array made of non‐overlapped hexagonal loops. The loops were arranged in a honeycomb pattern and targeted the temporal lobes. They were placed on a flexible neoprene cap closely fitting the head. A new stripline design with a high impedance was proposed and applied for the first time at 11.7T. Specific homebuilt miniaturized low‐impedance preamplifiers were directly mounted on the loops, providing preamplifier decoupling in a compact and modular design. Using an anatomical phantom, we experimentally compared the SNR and parallel imaging performance of the region‐focused cap to a 32‐channel whole‐brain receive array at 11.7T. Results: The experimental results showed a 1.7‐time higher SNR on average in the temporal lobes compared to the whole brain receive array. The g‐factor is also improved when undersampling in the antero‐posterior and head‐foot directions. Conclusion: A significant SNR boost in the temporal lobes was demonstrated at 11.7T compared to the whole‐brain receive array. The parallel imaging capabilities were also improved in the temporal lobes in some acceleration directions. Conclusion: A significant SNR boost in the temporal lobes was demonstrated at 11.7T compared to the whole-brain receive array. The parallel imaging capabilities were also improved in the temporal lobes in some acceleration directions