Slice-selective excitation with B₁⁺-insensitive composite pulses

Area of Science:

• Magnetic Resonance Imaging (MRI)
• High-Field Imaging
• Radiofrequency (RF) Pulse Design

Background:

• 7 Tesla (T) MRI offers higher resolution but suffers from radiofrequency (RF) and static magnetic field inhomogeneities.
• These field variations lead to non-uniform flip angles, compromising image quality in human brain studies.
• Existing RF pulse designs struggle to maintain performance under such challenging conditions.

Purpose of the Study:

• To design and evaluate spatially selective RF pulses that produce uniform flip angles despite RF and static field inhomogeneities at 7 T.
• To assess the performance of these novel pulses in terms of slice profile, phase linearity, and signal loss.
• To establish the practical utility of these pulses for high-field human brain imaging.

Main Methods:

• Developed non-selective composite pulses optimized for field insensitivity.
• Transformed these into spatially selective pulses using modified spectral composition and oscillating gradient waveforms.
• Evaluated pulse performance through simulations and phantom/human brain measurements for 45° and 90° flip angles with a 2mm slice thickness.

Main Results:

• The designed field-insensitive RF pulses demonstrated improved flip-angle uniformity in 7 T human brain imaging.
• A subset of pulses with durations under 10 ms showed minimal magnetization phase non-linearities and acceptable signal loss.
• Performance was analyzed regarding pulse duration, slice profiles, phase linearity, field sensitivity, and T(2) decay effects.

Conclusions:

• The developed class of field-insensitive RF pulses offers a practical solution for achieving uniform flip angles in high-field MRI.
• These pulses effectively mitigate the impact of field inhomogeneities common in 7 T human brain imaging.
• The findings contribute to understanding the performance limits of single-channel transmission in high-field MRI systems.