Parallel transmit and receive technology in high‐field magnetic resonance neuroimaging

The major radiofrequency engineering challenges of high‐field MR neuroimaging are as follows: (1) to produce a strong, homogeneous transmit B1 field, while remaining within regulatory guidelines for tissue power deposition and (2) to receive the signal with the maximum signal‐to‐noise and the greatest flexibility in terms of utilizing the benefits of parallel imaging. Borrowing from developments in electromagnetic hyperthermia, the first challenge has been met by the use of transmit arrays, in which the input power to each element of the array can be varied in terms of magnitude and phase. Optimization of these parameters, as well as the form of the applied RF pulse, leads to very homogeneous B1 fields throughout the brain. The design of large receive arrays, using impedance‐mismatched preamplifiers and geometrical overlap for interelement isolation, has resulted in significant sensitivity improvements as well as large acceleration factors in parallel imaging.