Unconventional trajectories on the Bloch Sphere:  A closer look at the effects and consequences of the breakdown of the rotating wave approximation

TRASE MRI uses rapid π-pulses of phase gradient fields, and in general requires as many as two distinct phase-gradient coils per encoding direction. This tends to restrict one to large amplitude, linear B₁ fields, which in low B₀ field leads to a breakdown of the rotating wave approximation. We have studied this regime both numerically and experimentally. Our results show a rich behavior involving a complex interplay of the Bloch-Siegert shift, the B₁ start and stop phase, and B₁ amplitude transients.