Dual optimization method of radiofrequency and quasistatic field simulations for reduction of eddy currents generated on 7T radiofrequency coil shielding

Zhao, Yang; Zhao, Tiejun; Raval, Shailesh B.; Krishnamurthy, Narayanan; Zheng, Haoran; Harris, Chad; Handler, William B.; Chronik, Blaine A.; Ibrahim, Tamer S.

doi:10.1002/mrm.25424

Cited by 8 publications

(11 citation statements)

References 42 publications

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“…In this study, we have developed an eight‐channel Tx‐only array based on a tic‐tac‐toe (TTT) design, and combined it with an eight‐channel Rx‐only array insert for breast MRI at 7 T for higher signal detectability. In order to predict the B 1 + field distribution and associated SAR in the breast tissues, RF numerical simulations were carried out using the finite‐difference time‐domain (FDTD) method .…”

Section: Introductionmentioning

confidence: 99%

Development of a 7 T RF coil system for breast imaging

et al. 2016

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In ultrahigh-field MRI, such as 7 T, the signal-to-noise ratio (SNR) increases while transmit (Tx) field (B1+) can be degraded due to inhomogeneity and elevated specific absorption rate (SAR). By applying new array coil concepts to both Tx and receive (Rx) coils, the B1+ homogeneity and SNR can be improved. In this study, we developed and tested in vivo a new RF coil system for 7 T breast MRI. An RF coil system composed of an eight-channel Tx-only array based on a tic-tac-toe design (can be combined to operate in single-Tx mode) in conjunction with an eight-channel Rx-only insert was developed. Characterizations of the B1+ field and associated SAR generated by the developed RF coil system were numerically calculated and empirically measured using an anatomically detailed breast model, phantom and human breasts. In vivo comparisons between 3 T (using standard commercial solutions) and 7 T (using the newly developed coil system) breast imaging were made. At 7 T, about 20% B1+ inhomogeneity (standard deviation over the mean) was measured within the breast tissue for both the RF simulations and 7 T experiments. The addition of the Rx-only array enhances the SNR by a factor of about three. High-quality MR images of human breast were acquired in vivo at 7 T. For the in vivo comparisons between 3 T and 7 T, an approximately fourfold increase of SNR was measured with 7 T imaging. The B1+ field distributions in the breast model, phantom and in vivo were in reasonable agreement. High-quality 7 T in vivo breast MRI was successfully acquired at 0.6 mm isotropic resolution using the newly developed RF coil system.

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Section: Introductionmentioning

confidence: 99%

Development of a 7 T RF coil system for breast imaging

et al. 2016

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“…The RF copper shielding is located at the back of the coil struts (with a gap of 15.8mm) and it functions as the ground of a cavity resonator, being responsible for both increasing the RF efficiency and preventing RF leaking. The RF copper shielding is composed of double layer 4μm thick copper sheets (Polyflon, Germany) and it was slotted with specific patterns to reduce eddy currents while the RF performance is maintained, as demonstrated in [ 25 ]. The non-metal parts of the array were 3D printed using polycarbonate (Stratasys, USA).…”

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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“…A Tx RF coil for foot and ankle imaging at 7T was designed based on the TTT family (a Tx RF coil design inspired by the arrangement of cross-pole antennas [27–31,33,34]). The Tx RF coil is composed of eight squared-shape transmission lines, electrically connected to each other in a 2× 2 Tic-Tac-Toe format (Fig.…”

Section: Methodsmentioning

confidence: 99%

“…1. Split cuts were made in the copper sheets to effectively open the low-frequency eddy currents (induced as a result of the gradient fields) pathways, thus avoiding the potential fast gradient switching artifacts while maintaining the RF characteristics associated with the Tx coil as described in [34]. The shields were extended to enclose the entire lower leg, except for a cavity on the top to accommodate the toes, thus allowing all sizes of feet and placement positions (Fig.…”

Section: Methodsmentioning

confidence: 99%

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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Dual optimization method of radiofrequency and quasistatic field simulations for reduction of eddy currents generated on 7T radiofrequency coil shielding

Cited by 8 publications

References 42 publications

Development of a 7 T RF coil system for breast imaging

Development of a 7 T RF coil system for breast imaging

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

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

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