Purpose: Preamplifier decoupling is useful for minimizing interaction between MRI array elements. The purpose of this work is to propose a general approach to designing networks for preamplifier decoupling while keeping the number of elements to a minimum. The approach is applicable to arbitrary impedance preamplifiers and arbitrary coil impedances.
Methods: Closed form design equations for decoupling networks are derived based on maximum decoupling and minimum preamplifier noise conditions. The analytical solutions are verified using numerical simulations. Design examples at 32.1, 64, 128, and 298 MHz are shown. One of the examples is realized on a test bench. The fabricated circuit is tested for decoupling and minimum noise properties.
Results: The design equations are verified numerically and experimentally. The fabricated network demonstrates 30.7 dB of decoupling and minimum output noise at the design frequency.
Conclusion: The design equations lead to four alternative network solutions. Each network is realized as a T-shape or Π-shape three elements circuit topology. All four networks are identical in performance providing minimum amplifier noise and maximum decoupling for a given preamplifier and coil combination. An MRI array designer can choose any solution out of four. The considerations for choosing the most practical solution are given. The presented method enables the use of arbitrary impedance preamplifiers or transistors (not necessary 50 Ω) and provides the most compact design possible (with the least number of components), which is particularly useful in multi-element systems.
Keywords: MRI array; impedance matching; optimal noise matching; preamplifier decoupling.
© 2020 International Society for Magnetic Resonance in Medicine.