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Magn Reson Med. 2015 Oct;74(4):1165-76. doi: 10.1002/mrm.25493. Epub 2014 Oct 16.

Power balance and loss mechanism analysis in RF transmit coil arrays.

Author information

1
Center for Medical Physics and Biomedical Engineering, Medical University of Vienna, Vienna, Austria.
2
MR Centre of Excellence, Medical University of Vienna, Vienna, Austria.
3
Physikalisch-Technische Bundesanstalt (PTB), Braunschweig und Berlin, Germany.

Abstract

PURPOSE:

To establish a framework for transmit array power balance calculations based on power correlation matrices to accurately quantify the loss contributions from different mechanisms such as coupling, lumped components, and radiation.

THEORY AND METHODS:

Starting from Poynting's theorem, power correlation matrices are derived for all terms in the power balance, which is formulated as a matrix equation. Finite-difference time-domain simulations of two 7 T eight-channel head array coils at 297.2 MHz are used to verify the theoretical considerations and demonstrate their application. Care is taken to accurately incorporate all loss mechanisms. The power balance for static B1 phase shims as well as two-dimensional spatially selective transmit SENSE pulses is shown.

RESULTS:

The simulated power balance shows an excellent agreement with theory, with a maximum power imbalance of less than 0.11%. Power loss contributions from the different loss mechanisms vary significantly between the investigated setups, and depending on the excitation mode imposed on the coil.

CONCLUSION:

The presented approach enables a straightforward loss evaluation for an arbitrary excitation of transmit coil arrays. Worst-case power imbalance and losses are calculated in a straightforward manner. This allows for deeper insight into transmit array loss mechanisms, incorporation of radiated power components in specific absorption rate calculations and verification of electromagnetic simulations.

KEYWORDS:

Poynting theorem; coil array; electromagnetic simulation; loss analysis; parallel transmission; power balance

PMID:
25324179
DOI:
10.1002/mrm.25493
[Indexed for MEDLINE]

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