Constant-variable flip angles for hyperpolarized media MRI.

Area of Science:

• Medical Imaging
• Physics
• Biophysics

Background:

• Longitudinal magnetization in hyperpolarized media (e.g., (129)Xe, (3)He) is nonrenewable.
• Constant Flip Angle (CFA) MRI offers high SNR but poor spatial accuracy.
• Variable Flip Angle (VFA) MRI provides high accuracy but low SNR.

Purpose of the Study:

• To introduce and optimize a novel Constant-Variable Flip Angles (CVFA) scheme.
• To enhance both signal-to-noise ratio (SNR) and spatial accuracy in hyperpolarized MRI.
• To address the trade-offs between SNR and accuracy in existing MRI techniques.

Main Methods:

• Proposed a hybrid excitation scheme combining CFA and VFA pulse trains.
• Acquired hyperpolarized magnetic resonance signals using an initial train of n(∗) CFA pulses followed by N-n(∗) VFA pulses.
• Simulated and optimized flip angles for CFA and VFA sections, including pulse counts and initial/final VFA angles.

Main Results:

• The CVFA scheme was optimized through simulation and validated with phantom and in vivo experiments.
• Demonstrated the ability of CVFA designs to achieve high SNR and maintain high spatial resolution simultaneously.
• CVFA designs effectively balanced the SNR and accuracy limitations of CFA and VFA methods.

Conclusions:

• The proposed CVFA method offers a superior approach for hyperpolarized MRI.
• CVFA successfully overcomes the SNR-accuracy trade-off inherent in CFA and VFA techniques.
• This novel scheme enhances the performance of hyperpolarized MRI for both phantom and in vivo applications.