Performance and safety assessment of an integrated transmit array for body imaging at 7 T under consideration of specific absorption rate, tissue temperature, and thermal dose

Fiedler, Thomas M.; Orzada, Stephan; Flöser, Martina; Rietsch, Stefan H. G.; Schmidt, Simon; Stelter, Jonathan; Wittrich, Marco; Quick, Harald H.; Bitz, Andreas K.; Ladd, Mark E.

doi:10.1002/nbm.4656

Cited by 11 publications

(10 citation statements)

References 30 publications

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“…Regulatory-cleared parallel-transmit systems 66,67 will be required to solve this issue. Promising steps toward a 7 T parallel-transmit body coil were recently reported 68 . Post–data acquisition attempts to homogenize signal intensity seem moderately successful 69 …”

Section: Technological Challenges and Image Optimizationmentioning

confidence: 99%

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7 T Musculoskeletal MRI

2022

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This review summarizes the current state-of-the-art of musculoskeletal 7 T magnetic resonance imaging (MRI), the associated technological challenges, and gives an overview of current and future clinical applications of 1H-based 7 T MRI. The higher signal-to-noise ratio at 7 T is predominantly used for increased spatial resolution and thus the visualization of anatomical details or subtle lesions rather than to accelerate the sequences. For musculoskeletal MRI, turbo spin echo pulse sequences are particularly useful, but with altered relaxation times, B1 inhomogeneity, and increased artifacts at 7 T; specific absorption rate limitation issues quickly arise for turbo spin echo pulse sequences. The development of dedicated pulse sequence techniques in the last 2 decades and the increasing availability of specialized coils now facilitate several clinical musculoskeletal applications. 7 T MRI is performed in vivo in a wide range of applications for the knee joint and other anatomical areas, such as ultra-high-resolution nerve imaging or bone trabecular microarchitecture imaging. So far, however, it has not been shown systematically whether the higher field strength compared with the established 3 T MRI systems translates into clinical advantages, such as an early-stage identification of tissue damage allowing for preventive therapy or an influence on treatment decisions and patient outcome. At the moment, results tend to suggest that 7 T MRI will be reserved for answering specific, targeted musculoskeletal questions rather than for a broad application, as is the case for 3 T MRI. Future data regarding the implementation of clinical use cases are expected to clarify if 7 T musculoskeletal MRI applications with higher diagnostic accuracy result in patient benefits compared with MRI at lower field strengths.

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Section: Technological Challenges and Image Optimizationmentioning

confidence: 99%

“…Promising steps toward a 7 T parallel-transmit body coil were recently reported. 68 Post–data acquisition attempts to homogenize signal intensity seem moderately successful. 69 …”

Section: Technological Challenges and Image Optimizationmentioning

confidence: 99%

7 T Musculoskeletal MRI

2022

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“…The capacitors include a corresponding equivalent series resistance (ESR) according to Rietsch et al 21 The 3D simulation model contains the MR environment consisting of a bore liner, patient table, gradient coil with RF shield (inner diameter 68 cm, length 122 cm), and magnet. The conductive antenna parts, gradient coil with RF shield, and magnet were modeled as perfect electric conductors, the PCB as FR‐4, 8,13 and the bore liner and patient table as glass‐fiber reinforced plastic (

ε_{r}^{'} = 4.805

\tan ((), δ) = 0.0045

) 22 . Open boundaries were applied around the model to absorb RF power radiated from the MR system.…”

Section: Methodsmentioning

confidence: 99%

“…The conductive antenna parts, gradient coil with RF shield, and magnet were modeled as perfect electric conductors, the PCB as FR-4, 8,13 and the bore liner and patient table as glass-fiber reinforced plastic (ε 0 r ¼ 4:805, tan δ ð Þ ¼ 0:0045). 22 Open boundaries were applied around the model to absorb RF power radiated from the MR system. The input connection of the impedance matching network was set as the reference plane.…”

Section: Antenna Design and Simulation Setupmentioning

confidence: 99%

Numerical comparison of local transceiver arrays of fractionated dipoles and microstrip antennas for body imaging at 7 T

2022

Self Cite

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Longitudinally orientated dipoles and microstrip antennas have both demonstrated superior results as RF transmit elements for body imaging at 7 T MRI, and are as of today the most commonly used transmit elements. In this study, the performances of the two antenna concepts were compared for use in local RF antenna arrays by numerical simulations. Antenna elements investigated are the fractionated dipole and the microstrip line with meander structures. Phantom simulations with a single antenna element were performed and evaluated with regard to specific absorption rate (SAR) efficiency in the center of the subject. Simulations of array configurations with 8 and 16 elements were performed with anatomical body models. Both antenna elements were combined with a loop coil to compare hybrid configurations. Singular value decomposition of the B1+ fields, RF shimming, and calculation of the voxel‐wise power and SAR efficiencies were performed in regions of interest with varying sizes to evaluate the transmit performance. The signal‐to‐noise ratio (SNR) was evaluated to estimate the receive performance. Simulated data show similar transmit profiles for the two antenna types in the center of the phantom (penetration depth > 20 mm). For body imaging, no considerable differences were determined for the different antenna configurations with regard to the transmit performance. Results show the advantage of 16 transmit channels compared with today's commonly used 8‐channel systems (minimum RF shimming excitation error of 4.7% (4.3%) versus 2.7% (2.8%) for the 8‐channel and 16‐channel configurations with the microstrip antennas in a (5 cm)3 cube in the center of a male (female) body model). Highest SNR is achieved for the 16‐channel configuration with fractionated dipoles. The combination of either fractionated dipoles or microstrip antennas with loop coils is more favorable with regard to the transmit performance compared with only increasing the number of elements.

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“…In particular, an EM-field simulation is essential to more accurately predict the safety of RF coils as EM-fields play an important role in the generation of B 1 -fields and SAR distributions . Owing to these problems, various studies have been conducted with an aim to increase the uniformity of the B 1 -field [26][27][28][29][30][31][32][33][34][35][36][37] and to improve the sensitivity of the B 1 -field [38][39][40][41][42][43][44][45] along with RF safety [46][47][48][49][50][51][52][53][54][55][56][57].…”

Section: Introductionmentioning

confidence: 99%

A Preliminary Study for Reference RF Coil at 11.7 T MRI: Based on Electromagnetic Field Simulation of Hybrid-BC RF Coil According to Diameter and Length at 3.0, 7.0 and 11.7 T

Seo

Chung

2022

Sensors

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Magnetic resonance imaging (MRI) systems must undergo quantitative evaluation through daily and periodic performance assessments. In general, the reference or standard radiofrequency (RF) coils for these performance assessments of 1.5 to 7.0 T MRI systems have been low-pass-type birdcage (LP-BC) RF coils. However, LP-BC RF coils are inappropriate for use as reference RF coils because of their relatively lower magnetic field (B1-field) sensitivity than other types of BC RF coils, especially in ultrahigh-field (UHF) MRI systems above 3.0 T. Herein, we propose a hybrid-type BC (Hybrid-BC) RF coil as a reference RF coil with improved B1-field sensitivity in UHF MRI system and applied it to an 11.7 T MRI system. An electromagnetic field (EM-field) analysis on the Hybrid-BC RF coil was performed to provide the proper dimensions for its use as a reference RF coil. Commercial finite difference time-domain program was used in EM-field simulation, and home-made analysis programs were used in analysis. The optimal specifications of the proposed Hybrid-BC RF coils for them to qualify as reference RF coils are proposed based on their B1+-field sensitivity under unnormalized conditions, as well as by considering their B1+-field uniformity and RF safety under normalized conditions.

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Performance and safety assessment of an integrated transmit array for body imaging at 7 T under consideration of specific absorption rate, tissue temperature, and thermal dose

Cited by 11 publications

References 30 publications

7 T Musculoskeletal MRI

7 T Musculoskeletal MRI

Numerical comparison of local transceiver arrays of fractionated dipoles and microstrip antennas for body imaging at 7 T

A Preliminary Study for Reference RF Coil at 11.7 T MRI: Based on Electromagnetic Field Simulation of Hybrid-BC RF Coil According to Diameter and Length at 3.0, 7.0 and 11.7 T

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