Using Phase Difference Information to Detect Errors in the Flip Angle Measured with Actual Flip Angle Imaging at 7T

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Biophysics
• Medical Physics

Background:

• Accurate flip angle (FA) determination is crucial for quantitative MRI.
• Actual Flip Angle Imaging (AFI) is a common method for FA measurement.
• Ultrahigh field strengths (e.g., 7 tesla) can exacerbate errors in FA measurements using AFI.

Purpose of the Study:

• To develop and validate a method for detecting FA measurement errors in AFI at 7 tesla.
• To utilize phase information from AFI source images to identify inaccuracies.
• To ensure accurate FA measurements for reliable quantitative MRI at ultrahigh fields.

Main Methods:

• Computer simulations were conducted to model the relationship between FA calculation errors and phase differences between AFI source images.
• The proposed phase-based method was tested on phantoms to assess its error detection capabilities.
• The method was further validated in vivo using healthy human volunteers at 7 tesla.

Main Results:

• Simulations confirmed that FA calculation errors correlate with phase differences between AFI source images, particularly with inverted longitudinal magnetization.
• A phase difference cut-off of 90° effectively detected FA calculation errors in both phantom and human subject studies.
• The method demonstrated its ability to identify deviations from the prescribed nominal flip angle.

Conclusions:

• Phase information derived from AFI source images provides a robust mechanism for detecting FA measurement errors at 7 tesla.
• This phase-based error detection method enhances the reliability of quantitative MRI at ultrahigh fields.
• The validated technique contributes to more accurate FA measurements, improving diagnostic and research applications of MRI.