MR imaging with an RF pulse producing reduced magnetization transfer
View Patent ↗A system and method are provided herein for designing and transmitting RF pulses which cause a reduced off-resonance magnetization transfer saturation. An RF pulse shape may be optimized according to a set of Bloch solutions defining a desired magnetization profile. An RF pulse may be transmitted according to this optimized shape according to a k-space trajectory which traverses a high amplitude portion of the RF pulse more times than one or more low amplitude portions. In addition, a generally alternating slice select gradient may be applied during transmission of the RF pulse.
1. A method for MR imaging comprising:
selecting an RF pulse having a profile that includes at least one low amplitude portion and a high amplitude portion;
transmitting the RF pulse during an MR data acquisition sequence such that the high amplitude portion is played out more times than the at least one low amplitude portion;
acquiring MR data during the MR data acquisition; and
reconstructing an image using the acquired MR data.
2. The method of claim 1 further comprising optimizing the RF pulse to reduce off-resonance magnetization transfer saturation, based on a desired passband.
3. The method of claim 1 wherein transmitting the RF pulse includes playing out a first low amplitude portion once, then playing out the high amplitude portion five times, and then playing out a second low amplitude portion once.
4. The method of claim 3 further comprising controlling the transmission power for transmission of the RF pulse such that an aggregate flip angle thereof is distributed over the five times the high amplitude portion is played out as 1/9, 2/9, 3/9, 2/9, 1/9 of the aggregate flip angle, respectively.
5. The method of claim 1 further comprising applying an alternating slice select gradient while transmitting the RF pulse.
6. The method of claim 5 wherein applying the alternating slice select gradient includes applying the alternating slice select gradient at a reduced magnitude while playing out the high amplitude portion of the RF pulse.
7. The method of claim 6 wherein the alternating slice select gradient is reduced by an amount less than a corresponding slice select gradient of a VERSE sequence is reduced.
8. The method of claim 1 further comprising iteratively optimizing a spectral profile of the RF pulse to reduce magnetization transfer caused by transmission of the RF pulse.