Velocity selective spin labeling using parallel transmission.

Area of Science:

• Magnetic Resonance Imaging
• Biophysics
• Medical Physics

Background:

• Ultra-high field (UHF) MRI offers enhanced signal-to-noise ratio (SNR), beneficial for perfusion imaging.
• Transmit field (B1+) inhomogeneities at UHF degrade excitation uniformity, impacting imaging techniques.
• Velocity selective labeling is crucial for flow-based MRI but is sensitive to excitation uniformity.

Purpose of the Study:

• To investigate the use of parallel transmit (pTx) pulses to improve excitation uniformity in velocity selective labeling at UHF.
• To demonstrate how pTx pulses can enhance the efficiency and fidelity of velocity selective labeling compared to standard methods.

Main Methods:

• Standard excitation pulses in velocity selective preparation were replaced with tailored k-T points pTx pulses.
• Bloch simulations and experimental validation using a flow phantom and in vivo imaging were conducted.
• Pulse configurations were evaluated in circularly polarized (CP) mode and pTx mode.

Main Results:

• Tailored pTx pulses significantly improved labeling fidelity and signal uniformity, reducing RMS error from 0.489 to 0.047.
• Simulations indicated that time symmetry manipulation in pTx pulses is key to minimizing residual magnetization.
• In vivo experiments showed a 44% reduction in RF power and shorter pulse duration compared to adiabatic pulses in CP-mode.

Conclusions:

• Tailored pTx pulses effectively mitigate B1+ artifacts in velocity selective labeling at UHF, improving SNR and contrast fidelity.
• The enhanced labeling efficiency demonstrates the potential of pTx to improve the robustness and accessibility of flow-based sequences.
• pTx technology offers a promising approach for advancing UHF MRI techniques, particularly for applications like velocity selective spin labeling.