Dataset Information

SAR reduction in 7T C-spine imaging using a "dark modes" transmit array strategy.

ABSTRACT: Local specific absorption rate (SAR) limits many applications of parallel transmit (pTx) in ultra high-field imaging. In this Note, we introduce the use of an array element, which is intentionally inefficient at generating spin excitation (a "dark mode") to attempt a partial cancellation of the electric field from those elements that do generate excitation. We show that adding dipole elements oriented orthogonal to their conventional orientation to a linear array of conventional loop elements can lower the local SAR hotspot in a C-spine array at 7 T.We model electromagnetic fields in a head/torso model to calculate SAR and excitation B1 (+) patterns generated by conventional loop arrays and loop arrays with added electric dipole elements. We utilize the dark modes that are generated by the intentional and inefficient orientation of dipole elements in order to reduce peak 10g local SAR while maintaining excitation fidelity.For B1 (+) shimming in the spine, the addition of dipole elements did not significantly alter the B1 (+) spatial pattern but reduced local SAR by 36%.The dipole elements provide a sufficiently complimentary B1 (+) and electric field pattern to the loop array that can be exploited by the radiofrequency shimming algorithm to reduce local SAR.

SUBMITTER: Eryaman Y

PROVIDER: S-EPMC4761435 | biostudies-literature | 2015 Apr

REPOSITORIES: biostudies-literature

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SAR reduction in 7T C-spine imaging using a "dark modes" transmit array strategy.

Eryaman Yigitcan Y Guerin Bastien B Keil Boris B Mareyam Azma A Herraiz Joaquin L JL Kosior Robert K RK Martin Adrian A Torrado-Carvajal Angel A Malpica Norberto N Hernandez-Tamames Juan A JA Schiavi Emanuele E Adalsteinsson Elfar E Wald Lawrence L LL

Magnetic resonance in medicine 20140418 4

<h4>Purpose</h4>Local specific absorption rate (SAR) limits many applications of parallel transmit (pTx) in ultra high-field imaging. In this Note, we introduce the use of an array element, which is intentionally inefficient at generating spin excitation (a "dark mode") to attempt a partial cancellation of the electric field from those elements that do generate excitation. We show that adding dipole elements oriented orthogonal to their conventional orientation to a linear array of conventional ...[more]

PMID: 24753012

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Project description:PurposeTo establish a cardiac signal from scattering matrix or scattering coefficient measurements made on a 7T 8-channel parallel transmit (pTx) system, and to evaluate its use for cardiac gating.MethodsMeasurements of the scattering matrix and scattering coefficients were acquired using a monitoring pulse sequence and during a standard cine acquisition, respectively. Postprocessing used an independent component analysis and gating feature identification. The effect of the phase of the excitation radiofrequency (RF) field ( B1+ shim) on the cardiac signal was simulated for multiple B1+ shim configurations, and cine images were reconstructed from both the scattering coefficients and electrocardiogram (ECG).ResultsThe cardiac motion signal was successfully identified in all subjects with a mean signal-to-noise ratio of 33.1 and 5.7 using the scattering matrix and scattering coefficient measurements, respectively. The dominant gating feature in the cardiac signal was a peak aligned with end-systole that occurred on average at 311 and 391 ms after the ECG trigger, with a mean standard deviation of 13.4 and 18.1 ms relative to ECG when using the scattering matrix and scattering coefficients measurements, respectively. The scattering coefficients showed a dependence on B1+ shim with some shim configurations not showing any cardiac signal. Cine images were successfully reconstructed using the scattering coefficients with minimal differences compared to those using ECG.ConclusionWe have shown that the scattering of a pTx RF coil can be used to estimate a cardiac signal, and that scattering matrix and coefficients can be used to cardiac gate MRI acquisitions with the scattering matrix providing a superior cardiac signal. Magn Reson Med 80:633-640, 2018. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Dataset Information

SAR reduction in 7T C-spine imaging using a "dark modes" transmit array strategy.

Publications

SAR reduction in 7T C-spine imaging using a "dark modes" transmit array strategy.

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