Flip Angle Errors in Actual Flip Angle Imaging Using Polyvinylpyrrolidone/Water-Based Phantoms

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Biomedical Engineering
• Materials Science

Background:

• Actual Flip Angle Imaging (AFI) is a crucial technique for accurate B1+ mapping in MRI.
• Polyvinylpyrrolidone (PVP) is used in phantoms for MRI research but can introduce imaging artifacts.
• Understanding these artifacts is essential for reliable MRI quantitative measurements.

Purpose of the Study:

• To identify and analyze unexpected errors in AFI when applied to polyvinylpyrrolidone (PVP) phantoms.
• To investigate the influence of RF-spoiling parameters and echo time (TE) on AFI accuracy in PVP/water mixtures.
• To compare experimental AFI results with reference methods and EPG simulations.

Main Methods:

• Acquired 3D Cartesian AFI at 3T and 7T for PVP/water mixtures with varying PVP ratios.
• Investigated RF-spoiling effects by altering phase increment (Φ0) and gradient moments.
• Performed AFI measurements with varied TE and 1H-NMR spectroscopy on PVP samples.
• Compared results with a reference method and EPG simulations.

Main Results:

• Observed asymmetric spoiling curves in PVP/water mixtures, deviating from expected symmetry around Φ0 = 90°.
• Detected significant deviations from reference values in PVP/water mixtures, unlike pure water samples.
• Identified oscillatory AFI signal behavior in PVP/water mixtures correlating with PVP NMR spectral bands.
• Simulations incorporating PVP signal components reproduced the observed asymmetric spoiling curves.

Conclusions:

• AFI accuracy is compromised in PVP/water mixtures due to interference between water and PVP signal components.
• These errors can significantly impact B1+ mapping accuracy using AFI.
• Increasing echo time (TE) and PVP concentration can mitigate these AFI-related errors.