Low Energy Optimal Radiofrequency Pulses for MRI – LOOP

In Nuclear Magnetic Resonance (NMR), radiofrequency (RF) pulses are used to modify the magnetization state of the system. The amplitude and energy of the pulses must comply with both harware limitations (amplifier, coil), and clinical regulation (Specific Absorption Rate). When the desired pulse fails to comply with these constraints, several strategies can be applied with serious consequences on the acquired signal (lower SNR, contrast loss, increase of the scanning duration).
The objective of this proposal is to use optimal control theory to design RF pulses having the desired characteristics (flip angle, bandwidth, duration), while depositing significantly less energy than standard pulses. A major advance is the joint optimization of both the RF and the gradient fields, which should lead to important energy reduction. Various parametrization of the control fields will be investigated to improve convergence and computation time.
Three main applications are targeted:
1. B1-robust low energy pulses offering better performances than adiabatic pulses for a given energy value
2. Low SAR refocusing pulses allowing to reach higher acceleration factors than standard pulses for fast spin echo sequences
3. Self-refocusing excitation pulses for sodium imaging, which inherently requires more energy than hydrogen due to its lower gyromagnetic ratio. Such pulses would allow to implement ultra-short TE sequences with increased sensitivity, which could enhance the observation of metabolic processes related to salt absorption and digestion.