Adiabatic null passage for on-resonance magnetization transfer preparation
Shahrokh Abbasi-Rad, David G. Norris
Abstract
Purpose
We propose a novel RF pulse providing an adiabatic null passage (ANP) for magnetization transfer preparation with improved insensitivity to B1+ and B0 inhomogeneities and mitigated direct saturation and T2 effects.
Method
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 B1+ was investigated and compared with a phase-inverted (12¯1-1¯21¯) 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 TRslice = 59 ms, three experiments with 6-ms ANP and constant MT used overdrive factor (OF)/PRP values of 1/TRslice, √2/2TRslice, and √3/3TRslice.
Results
At gray matter (white matter) in vivo, the MTR decreased from 61% (64%) at OF = 1 to 38% (42%) applying ANP with an OF = √3 and PRP = 3 TRslice, 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.
Conclusion
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 B1+ and B0 inhomogeneity.