9.4T human MRI: Preliminary results (original) (raw)
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In vivo human head MRI at 10.5T: A radiofrequency safety study and preliminary imaging results
Magnetic Resonance in Medicine, 2019
The purpose of this study is to safely acquire the first human head images at 10.5T. Methods: To ensure safety of subjects, we validated the electromagnetic simulation model of our coil. We obtained quantitative agreement between simulated and experimental B + 1 and specific absorption rate (SAR). Using the validated coil model, we calculated radiofrequency power levels to safely image human subjects. We conducted all experiments and imaging sessions in a controlled radiofrequency safety lab and the whole-body 10.5T scanner in the Center for Magnetic Resonance Research. Results: Quantitative agreement between the simulated and experimental results was obtained including S-parameters, B + 1 maps, and SAR. We calculated peak 10 g average SAR using 4 different realistic human body models for a quadrature excitation and demonstrated that the peak 10 g SAR variation between subjects was less than 30%. We calculated safe power limits based on this set and used those limits to acquire T 2-and T * 2-weighted images of human subjects at 10.5T. Conclusions: In this study, we acquired the first in vivo human head images at 10.5T using an 8-channel transmit/receive coil. We implemented and expanded a previously proposed workflow to validate the electromagnetic simulation model of the 8-channel transmit/receive coil. Using the validated coil model, we calculated radiofrequency power levels to safely image human subjects.
NMR in biomedicine, 2015
The performance of multichannel transmit coil layouts and parallel transmission (pTx) RF pulse design was evaluated with respect to transmit B1 (B1 (+) ) homogeneity and specific absorption rate (SAR) at 3 T for a whole body coil. Five specific coils were modeled and compared: a 32-rung birdcage body coil (driven either in a fixed quadrature mode or a two-channel transmit mode), two single-ring stripline arrays (with either 8 or 16 elements), and two multi-ring stripline arrays (with two or three identical rings, stacked in the z axis and each comprising eight azimuthally distributed elements). Three anatomical targets were considered, each defined by a 3D volume representative of a meaningful region of interest (ROI) in routine clinical applications. For a given anatomical target, global or local SAR controlled pTx pulses were designed to homogenize RF excitation within the ROI. At the B1 (+) homogeneity achieved by the quadrature driven birdcage design, pTx pulses with multich...
Improving RF safety in MRI by modifying the electric field distribution
2011 XXXth URSI General Assembly and Scientific Symposium, 2011
In this work we demonstrate that the radiofrequency (RF) electric field in magnetic resonance imaging (MRI) can be modified in order to enhance patient safety. The heating of metallic devices in MRI is directly related to electric field distribution. On the other hand the MR image homogeneity is related to forward polarized component of the magnetic field (transmit sensitivity). In order to prevent heating, electric field-free zones should be generated in the body without significantly altering the transmit sensitivity. For this purpose the linearly polarized birdcage coil is proposed as a metallic device friendly MRI coil. The zero electric field plane of the linear birdcage coil is coincided with the location of the metallic device and the heating is reduced as shown by simulations and experiments. One disadvantage of this approach is, the linear coils generate twice as much whole body average SAR when compared to quadrature birdcage coils. In order to solve this problem simulations are performed to find electromagnetic field solutions with reduced average SAR and uniform transmit sensitivity.
… , IEEE Transactions on, 2005
In this paper, two TEM resonators were evaluated experimentally and numerically at 8 tesla (T) (340 MHz for 1 H imaging). The coils were constructed to be 21.2-cm long (standard) and 11-cm long (a proposed less claustrophobic design). The experimental evaluation was done on a single cadaver using an ultra high field, 8 T, whole-body magnet. The numerical modeling was performed using an in-house finite difference time domain packagethat treats the coil and the load (anatomically detailed human head model) as a single system. The coils were tested with quadrature excitation at different coil alignment positions with respect to human head. For head imaging at 8 T, the overall numerical and experimental results demonstrated that when compared to the longer coil, the shorter coil provides superior signal-to-noise ratio, coil sensitivity, and excite field in the biological regions that lie within both of the coils' structures. A study of the RF (excite/receive fields) homogeneity showed variations in the performance of both coils that are mostly dependant on the region of interest and the position of coil with respect to the head. As such, depending on the application, the shorter coil could be effectively utilized.
In vivo MR imaging with simultaneous RF transmission and reception
Magnetic Resonance in Medicine, 2016
Purpose-To present a practical scheme of a simultaneous RF transmit and receive (STAR) system for MRI, discuss the challenges and solutions, and show preliminary in vivo MR images obtained with this new technique. Methods-A remotely controlled STAR system was built and tested with a transverse electromagnetic (TEM) head coil on a 4T (Oxford, 90 cm-bore) MRI scanner equipped with an Agilent DirectDrive™ console. In vivo head images have been acquired using continuous sweep excitation and acquisition. Results-The bench-test and MR experimental results show our STAR system to have high isolation (60 dB) between transmit and receive with insensitivity to load swings created by head motion. To acquire in vivo head images, ultra-low RF peak power of 50 mW was used. Conclusion-A novel motion-insensitive STAR MRI technique was developed and experimentally tested. The first in vivo MR images using this method were acquired.
A dedicated eight‐channel receive RF coil array for monkey brain MRI at 9.4 T
NMR in Biomedicine, 2020
The neuroimaging of nonhuman primates (NHPs) realised with magnetic resonance imaging (MRI) plays an important role in understanding brain structures and functions, as well as neurodegenerative diseases and pathological disorders. Theoretically, an ultrahigh field MRI (≥7 T) is capable of providing a higher signal‐to‐noise ratio (SNR) for better resolution; however, the lack of appropriate radiofrequency (RF) coils for 9.4 T monkey MRI undermines the benefits provided by a higher field strength. In particular, the standard volume birdcage coil at 9.4 T generates typical destructive interferences in the periphery of the brain, which reduces the SNR in the neuroscience‐focused cortex region. Also, the standard birdcage coil is not capable of performing parallel imaging. Consequently, extended scan durations may cause unnecessary damage due to overlong anaesthesia. In this work, assisted by numerical simulations, an eight‐channel receive RF coil array was specially designed and manufac...
A 16-Channel Dipole Antenna Array for Human Head Magnetic Resonance Imaging at 10.5 Tesla
Sensors, 2021
For ultra-high field and frequency (UHF) magnetic resonance imaging (MRI), the associated short wavelengths in biological tissues leads to penetration and homogeneity issues at 10.5 tesla (T) and require antenna transmit arrays for efficiently generated 447 MHz B1+ fields (defined as the transmit radiofrequency (RF) magnetic field generated by RF coils). Previously, we evaluated a 16-channel combined loop + dipole antenna (LD) 10.5 T head array. While the LD array configuration did not achieve the desired B1+ efficiency, it showed an improvement of the specific absorption rate (SAR) efficiency compared to the separate 8-channel loop and separate 8-channel dipole antenna arrays at 10.5 T. Here we compare a 16-channel dipole antenna array with a 16-channel LD array of the same dimensions to evaluate B1+ efficiency, 10 g SAR, and SAR efficiency. The 16-channel dipole antenna array achieved a 24% increase in B1+ efficiency in the electromagnetic simulation and MR experiment compared to ...
Evolution of UHF Body Imaging in the Human Torso at 7T
Topics in Magnetic Resonance Imaging, 2019
The potential value of ultrahigh field (UHF) magnetic resonance imaging (MRI) and spectroscopy to biomedical research and in clinical applications drives the development of technologies to overcome its many challenges. The increased difficulties of imaging the human torso compared with the head include its overall size, the dimensions and location of its anatomic targets, the increased prevalence and magnitude of physiologic effects, the limited availability of tailored RF coils, and the necessary transmit chain hardware. Tackling these issues involves addressing notoriously inhomogeneous transmit B 1 (B 1 þ) fields, limitations in peak B 1 þ , larger spatial variations of the static magnetic field B 0 , and patient safety issues related to implants and local RF power deposition. However, as research institutions and vendors continue to innovate, the potential gains are beginning to be realized. Solutions overcoming the unique challenges associated with imaging the human torso are reviewed as are current studies capitalizing on the benefits of UHF in several anatomies and applications. As the field progresses, strategies associated with the RF system architecture, calibration methods, RF pulse optimization, and power monitoring need to be further integrated into the MRI systems making what are currently complex processes more streamlined. Meanwhile, the UHF MRI community must seize the opportunity to build upon what have been so far proof of principle and feasibility studies and begin to further explore the true impact in both research and the clinic.
Exploring the limits of RF shimming for high-field MRI of the human head
Magnetic Resonance in Medicine, 2006
Several methods have been proposed for overcoming the effects of radiofrequency (RF) magnetic field inhomogeneity in high-field MRI. Some of these methods rely at least in part on the ability to independently control magnitude and phase of different drives in either one multielement RF coil or in different RF coils in a transmit array. The adjustment of these drive magnitudes and phases alone to create uniform RF magnetic (B 1 ) fields has been called RF shimming, and has certain limits at every frequency as dictated by possible solutions to Maxwell's equations. Here we use numerical calculations to explore the limits of RF shimming in the human head. We found that a 16-element array can effectively shim a single slice at frequencies up to 600 MHz and the whole brain at up to 300 MHz, while an 80-element array can shim the whole brain at up to 600 MHz. Magn Reson Med 56:918 -922, 2006.
Progress in high field MRI at the University of Florida
Magma: Magnetic Resonance Materials in Physics, Biology, and Medicine, 2001
In this article we report on progress in high magnetic field MRI at the University of Florida in support of our new 750MHz wide bore and 11.7T/40cm MR instruments. The primary emphasis is on the associated rf technology required, particularly high frequency volume and phased array coils. Preliminary imaging results at 750MHz are presented. Our results imply that the pursuit of even higher fields seems warranted.