Experimental and numerical analysis of B 1 + field and SAR with a new transmit array design for 7 T breast MRI

Kim, Junghwan; Krishnamurthy, Narayan; Santini, Tales; Zhao, Yang; Zhao, Tiejun; Bae, Kyongtae T.; Ibrahim, Tamer S.

doi:10.1016/j.jmr.2016.04.012

Cited by 20 publications

(18 citation statements)

References 35 publications

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“…Slight differences in the offset of transmission for opposite ports (S13) can be explained by losses in the system that were not included in the simulations, such as losses in the cables, coil copper, and connectors. The high coupling between opposite ports in the array (S13) may decrease the coil efficiency, yet other studies have demonstrated its potential to increase the B 1 + fields homogeneity, decrease SAR per mean B 1 + intensity and increase load insensitivity [27,28,33]. …”

Section: Discussionmentioning

confidence: 99%

“…The coupling of the RF coil is inherently high by its design [28]; therefore, changing one rod can affect all the channels. The s-matrix was measured using a network analyzer (Model E5062A, manufactured by Agilent Technologies).…”

Section: Methodsmentioning

confidence: 99%

“…1d). The FDTD package incorporates true transmission line modeling for capturing the mechanism of excitation and reception, and, therefore, accurately predicting coil input impedance and coupling between coil elements [27,28,36,37]. …”

Section: Methodsmentioning

confidence: 99%

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A new RF transmit coil for foot and ankle imaging at 7T MRI

Santini

Kim

Wood

et al. 2018

Magnetic Resonance Imaging

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A four-channel Tic-Tac-Toe (TTT) transmit RF coil was designed and constructed for foot and ankle imaging at 7T MRI. Numerical simulations using an in-house developed FDTD package and experimental analyses using a homogenous phantom show an excellent agreement in terms of B1 + field distribution and s-parameters. Simulations performed on an anatomically detailed human lower leg model demonstrated an B1+ field distribution with a coefficient of variation (CV) of 23.9%/15.6%/28.8% and average B1 + of 0.33 μT/0.56 μT/0.43 μT for 1 W input power (i.e., 0.25 W per channel) in the ankle/calcaneus/mid foot respectively. In-vivo B1 + mapping shows an average B1 + of 0.29 μT over the entire foot/ankle. This newly developed RF coil also presents acceptable levels of average SAR (0.07 W/kg for 10 g per 1 W of input power) and peak SAR (0.34 W/kg for 10 g per 1 W of input power) over the whole lower leg. Preliminary in-vivo images in the foot/ankle were acquired using the T2-DESS MRI sequence without the use of a dedicated receive-only array.

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Section: Discussionmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

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A new RF transmit coil for foot and ankle imaging at 7T MRI

Santini

Kim

Wood

et al. 2018

Magnetic Resonance Imaging

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“…The TTT coil design has been applied to several UHF human MRI applications, including head [ 9 , 17 – 19 ], breast [ 20 , 21 ], torso [ 22 ], and foot [ 23 , 24 ]. Fig 1(A) shows the schematic diagram of a four-element 2x2 TTT transmit array design.…”

Section: Methodsmentioning

confidence: 99%

In-vivo and numerical analysis of the eigenmodes produced by a multi-level Tic-Tac-Toe head transmit array for 7 Tesla MRI

et al. 2018

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Radio-frequency (RF) field inhomogeneities and higher levels of specific absorption rate (SAR) still present great challenges in ultrahigh-field (UHF) MRI. In this study, an in-depth analysis of the eigenmodes of a 20-channel transmit Tic-Tac-Toe (TTT) RF array for 7T neuro MRI is presented. The eigenmodes were calculated for five different Z levels (along the static magnetic field direction) of the coil. Four eigenmodes were obtained for each Z level (composed of 4 excitation ports), and they were named based on the characteristics of their field distributions: quadrature, opposite-phase, anti-quadrature, and zero-phase. Corresponding finite-difference time-domain (FDTD) simulations were performed and experimental B1+ field maps were acquired using a homogeneous spherical phantom and human head (in-vivo). The quadrature mode is the most efficient and it excites the central brain regions; the opposite-phase mode excites the brain peripheral regions; anti-quadrature mode excites the head periphery; and the zero-phase mode excites cerebellum and temporal lobes. Using this RF array, up to five eigenmodes (from five different Z levels) can be simultaneously excited. The superposition of these modes has the potential to produce homogeneous excitation with full brain coverage and low levels of SAR at 7T MRI.

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“…However, magnetic field distortions in both breasts cannot be completely removed, even with the availability of third-order B 0 shimming, available on most 7T systems (16). Fortunately, the development of hardware for breast MRI at 7 T is an active field of research (17)(18)(19)(20)(21)(22). Although not widely available, recently developed 7T breast coils can make use of the possibilities that parallel transmit systems provide (ie, the transmitter for each breast is connected to a separate RF amplifier) (18).…”

Section: Introductionmentioning

confidence: 99%

Improved fat suppression of the breast using discretized frequency shimming

Velden

Luijten

Klomp

2017

Magnetic Resonance in Med

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Purpose: Robust fat suppression is essential in bilateral breast MRI at 7 Tesla. The lack of good fat suppression can result in errors when calculating the enhancement curve from dynamic contrast-enhanced acquisitions. In this work we propose discretized frequency shimming to improve the quality of fat suppression by exploiting the capabilities of a parallel transmit setup. Methods: With discretized frequency shimming, different resonance frequencies are used for the different transmitters of the radiofrequency coil. This can be particularly useful for frequency-selective radiofrequency pulses, such as water excitation. The potential of this method has been investigated by simulations on seven acquired B 0 maps. Prospective experiments on phantoms and four healthy volunteers were performed to illustrate the performance of discretized frequency shimming.Results: Simulations show a decrease in off-resonance excitation for all subjects when discretized frequency shimming is used. This decrease is highly subject-dependent, but can result in up to 30% better fat suppression, as observed in phantom and volunteer experiments. Conclusions: Discretized frequency shimming improves fat suppression in bilateral breast MRI at 7 Tesla. Magn Reson Med 79:593-599,

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Experimental and numerical analysis of B 1 + field and SAR with a new transmit array design for 7 T breast MRI

Abstract: Graphical Abstract

Cited by 20 publications

References 35 publications

A new RF transmit coil for foot and ankle imaging at 7T MRI

A new RF transmit coil for foot and ankle imaging at 7T MRI

In-vivo and numerical analysis of the eigenmodes produced by a multi-level Tic-Tac-Toe head transmit array for 7 Tesla MRI

Improved fat suppression of the breast using discretized frequency shimming

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