Adiabatic null passage for on-resonance magnetization transfer preparation

We propose a novel RF pulse providing an adiabatic null passage (ANP) for magnetization transfer preparation with improved insensitivity to $$ and B₀ inhomogeneities and mitigated direct saturation and T₂ effects.

The phase modulation function of a 6-ms time-resampled frequency offset-corrected pulse was modified to achieve zero flip angle at the end of the pulse. The spectral response was simulated, and its insensitivity to B₀ and $$ was investigated and compared with a phase-inverted (1 $$ 1- $$ 2 $$ ) binomial pulse. The proposed pulse was implemented in a 2D-EPI pulse sequence to generate magnetization transfer (MT) contrast and MT ratio (MTR) maps. In vivo experiments were performed on 3 healthy participants with power-matched settings for ANP and the binomial pulse with the following parameters: 6-ms binomial pulse with a flip angle of 107° (shortest element) and pulse repetition period (PRP) of TR_slice  = 59 ms, three experiments with 6-ms ANP and constant MT used overdrive factor (OF)/PRP values of 1/TR_slice , $$ /2TR_slice , and $$ /3TR_slice .

At gray matter (white matter) in vivo, the MTR decreased from 61% (64%) at OF = 1 to 38% (42%) applying ANP with an OF = $$ and PRP = 3 TR_slice , demonstrating the mitigation of T₂ /direct effect by 22% (22%). Bloch-McConnell simulations gave similar values. In vivo experiments showed significant improvement in the MTR values for areas with high B₀ inhomogeneity.

ANP pulse was shown to be advantageous over its binomial counterpart in providing MT contrast by mitigating the T₂ effect and direct saturation of the liquid pool as well as reduced sensitivity to $$ and B₀ inhomogeneity.