Velocity encoding and velocity compensation for multi-spoke RF excitation

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Biophysics
• Medical Physics

Background:

• Ultra-high field (UHF) MRI at 7 Tesla offers enhanced signal but faces challenges with radiofrequency (RF) pulse homogeneity and motion artifacts.
• Accurate quantification of blood flow is crucial for diagnosing cardiovascular diseases, requiring precise velocity measurements.
• Existing MRI techniques can suffer from displacement artifacts, particularly in the presence of flowing blood, complicating quantitative analysis.

Purpose of the Study:

• To investigate velocity-encoded and velocity-compensated multi-spoke RF pulses for flip-angle homogenization at ultra-high fields (UHF).
• To analyze displacement artifacts arising from flow in Fourier transform imaging with multi-spoke RF pulses.
• To evaluate the efficacy of novel gradient waveform designs for minimizing echo time (TE) and controlling velocity-induced artifacts.

Main Methods:

• Developed a gradient waveform design for multi-spoke excitation enabling minimal TE and two distinct encoding strategies.
• Compared proposed encoding schemes with established methods for multi-spoke excitations.
• Evaluated image quality and quantitative velocity maps in phantoms and in vivo at 7 Tesla using single- and two-spoke excitations.

Main Results:

• Maintaining a constant first gradient moment eliminated displacements in phase-encoding and slice-selection directions but caused readout artifacts for non-zero velocity.
• Introducing variable first gradient moments in the phase-encoding direction minimized velocity-induced geometrical distortions by creating displacements along the velocity vector.
• Phase-resolved mean volume flow measurements in the aorta using two-spoke excitation showed excellent agreement with single-spoke excitation (mean difference 0.8 ± 16.2 ml/s).

Conclusions:

• Successfully demonstrated the use of single- and multi-spoke RF pulses for flow quantification at 7 Tesla with controlled displacement artifacts.
• The presented techniques enable accurate velocity quantification and compensation for both conventional and multi-spoke RF pulses.
• These methods provide a foundation for in-plane B1+ homogenization using parallel transmission at UHF MRI.