  The radiofrequency (RF) transmit field is severely inhomogeneous at ultrahigh
field due to both RF penetration and RF coil design issues. This particularly
impairs image quality for sequences that use inversion pulses such as
magnetization prepared rapid acquisition gradient echo and limits the use of
quantitative arterial spin labeling sequences such as flow-attenuated inversion
recovery. Here we have used a search algorithm to produce inversion pulses
tailored to take into account the heterogeneity of the RF transmit field at 7
T. This created a slice selective inversion pulse that worked well (good slice
profile and uniform inversion) over the range of RF amplitudes typically
obtained in the head at 7 T while still maintaining an experimentally
achievable pulse length and pulse amplitude in the brain at 7 T. The pulses
used were based on the frequency offset correction inversion technique, as well
as time dilation of functions, but the RF amplitude, frequency sweep, and
gradient functions were all generated using a genetic algorithm with an
evaluation function that took into account both the desired inversion profile
and the transmit field inhomogeneity.
