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

The aim of this work is to report and investigate unexpected errors in actual flip angle imaging (AFI) applied to phantom fillings based on polyvinylpyrrolidone (PVP).

Flip angle maps of PVP/water mixtures with different PVP ratios were acquired at 3 T and 7 T using a 3D cartesian AFI. The influence of spoiling was investigated by varying the RF-spoiling phase increment Φ₀ for different settings of the spoiling gradient moments. The results were compared with a reference method and EPG simulations. AFI measurements with varying TE were carried out to investigate non-spoiling-related errors. Furthermore, ¹H-NMR spectroscopy was performed on PVP samples.

Measurements reveal spoiling curves of PVP/water mixtures that lack the symmetry around Φ₀ = 90° that has been observed in prior work. Depending on the TE and PVP ratio, substantial deviations from the reference value are found, which are not observed in water samples. Varying the TE shows oscillatory behavior of the AFI signals in PVP/water mixtures that match with the existence of PVP bands found in ¹H-NMR spectra. Including a signal component with a precession frequency that matches a PVP band into the simulations allows the reproduction of the asymmetric spoiling curves.

Under certain conditions, AFI of PVP/water mixtures suffers from errors originating from interferences of the water and PVP signal components. This can severely reduce the accuracy of the AFI and must be considered for $B_1^{+}$ mapping with AFI. The impact of this effect can be reduced by increased TE as well as high PVP ratios.