RF phase modulation improves quantitative transient state sequences under constrained conditions

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Quantitative Imaging
• Biomedical Engineering

Background:

• Gradient-spoiled transient-state MRI sequences, such as Magnetic Resonance Fingerprinting (MRF) and Magnetic Resonance Spin Tomography in Time-domain (MR-STAT), are crucial for quantitative relaxometry.
• Optimizing radiofrequency (RF) pulse sequences is key to improving the precision of T1 and T2 maps.

Purpose of the Study:

• To investigate the impact of optimized RF phase modulation on the precision of relaxometry maps in transient-state MRI.
• To compare the effectiveness of amplitude-only modulation versus amplitude + phase modulation for RF pulse sequences.

Main Methods:

• Utilized the Cramer-Rao based method BLAKJac to generate optimized RF pulse sequences.
• Tested sequences with amplitude-only and amplitude + phase modulation on phantoms, human brain, and leg.
• Reconstructed T1 and T2 parametric maps and their standard deviations.

Main Results:

• Amplitude + phase modulation consistently yielded lower noise levels (average 34% reduction) compared to amplitude-only modulation.
• The benefits of amplitude + phase modulation were more pronounced under Specific Absorption Rate (SAR) restrictions.
• Reduced the necessity for inversion pulses and pauses in sequence design.

Conclusions:

• Optimized RF phase modulation is beneficial for quantitative transient-state MRI, particularly under SAR restrictions.
• RF phase modulation enhances the precision of T1 and T2 mapping.
• This optimization simplifies sequence design by reducing the need for inversion pulses and pauses.