Fast three-dimensional inner volume excitations using parallel transmission and optimized k-space trajectories.

Area of Science:

• Magnetic Resonance Imaging
• Radiofrequency Engineering
• Biomedical Engineering

Background:

• Designing short parallel transmission (pTx) pulses for arbitrary 3D magnetization patterns is crucial for advanced MRI applications.
• Existing methods often face limitations in speed and precision for complex 3D excitations.

Purpose of the Study:

• To develop a joint optimization framework for pTx radiofrequency (RF) and gradient waveforms.
• To enable the excitation of arbitrary 3D magnetization patterns efficiently and rapidly.

Main Methods:

• Proposed a joint optimization of RF and gradient waveforms for 3D magnetization pattern excitation.
• Utilized parameterization of k-space trajectories (3D shells, stack-of-spirals, cross) for gradient waveform optimization.
• Integrated RF pulse design within each k-space trajectory optimization iteration using a small tip angle least-squares approach.
• Evaluated the framework using Bloch simulations and experimental data on a 7T scanner with eight transmit channels.

Main Results:

• Achieved excitation of a 3D cube (brain) shape with low normalized root-mean-square error (3.4% / 6.2%) in under 5 ms using an optimized 3D cross trajectory.
• Demonstrated superior performance compared to unoptimized trajectories (4.7% / 41.2% error).
• Observed significant alterations in k-space trajectories when incorporating B0 robustness into the pulse design.

Conclusions:

• The joint gradient and RF optimization approach enables excellent excitation of 3D shapes within 5 ms.
• This technique is suitable for applications like reduced field-of-view imaging and selective fat suppression in spectroscopy.
• The method offers a significant advancement in rapid and precise 3D magnetization pattern excitation in MRI.