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Analytical modeling provides new insight into complex mutual coupling between surface loops at ultrahigh fields.

N I Avdievich1,2, A Pfrommer1, I A Giapitzakis1

  • 1High-Field MR Center, Max Planck Institute for Biological Cybernetics, Tübingen, Germany.

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|June 21, 2017
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Summary

Ultrahigh-field (UHF) MRI arrays require effective decoupling. A new analytical model accurately predicts element coupling, enabling optimized array designs for improved performance without extra circuits.

Keywords:
analytical modelarray optimizationdecouplingimpedance matrixtransceiver arraysultrahigh-field MRI

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Area of Science:

  • Magnetic Resonance Imaging (MRI)
  • Electromagnetics
  • Engineering

Background:

  • Ultrahigh-field (UHF) MRI systems (≥7 Tesla) utilize multi-channel transmit (Tx) arrays for enhanced efficiency and homogeneity compared to single-channel coils.
  • Decoupling multi-channel arrays is a significant design challenge, impacting overall performance.
  • Accurate prediction of element coupling is crucial for efficient array design but traditional electromagnetic (EM) simulations are computationally intensive.

Purpose of the Study:

  • To develop a fast and accurate analytical model for evaluating the impedance matrix (Z-matrix) of UHF Tx arrays.
  • To investigate the complex coupling (electric and magnetic) between array elements.
  • To optimize array geometry for improved decoupling and performance.

Main Methods:

  • Development of an analytical model based on dyadic Green's functions to predict the Z-matrix.
  • Validation of the analytical model using EM solvers and bench measurements.
  • Optimization of a two-loop transceiver array geometry using the developed model.

Main Results:

  • The analytical model accurately describes electric and magnetic coupling, performing well at lower fields and for low magnetic coupling at UHF.
  • Optimization by overlapping loops compensated for mutual inductance and resistance.
  • Achieved excellent decoupling (<-40 dB) without additional decoupling circuits.
  • An overlapped array prototype demonstrated favorable signal-to-noise ratio and Tx efficiency compared to a gapped array design.

Conclusions:

  • The dyadic Green's function-based analytical model provides a rapid and effective tool for assessing UHF MRI array coupling.
  • Simple geometric optimization, such as loop overlapping, can achieve superior decoupling in UHF Tx arrays.
  • This approach facilitates the design of more efficient and homogeneous UHF MRI systems.