Tuning ultra-low output impedance amplifiers for optimal power and decoupling in parallel transmit MRI

Higher strength main magnets and more demanding pulse sequences have driven interest in parallel excitation systems. Parallel transmit array coils need to be decoupled to be used effectively. Ultra-low output impedance power amplifiers have been proposed as a possible solution for this. They provide decoupling in a manner analogous to low input impedance preamplifiers. Chu et al suggest a tuning method that does not rely on direct measurements of the matching network. This does not result in an optimal tuning, sacrificing peak output power in favor of decoupling. We have developed an alternate tuning strategy that allows optimize for both power and decoupling. It also reveals a second tuning mode for the matching network that causes the amplifier to fail, but is difficult to detect. When tuned in the manner reported the amplifiers provide high output power and good decoupling when used with an eight channel head array.