flip angle
The angle through which the net magnetization vector is rotated away from equilibrium by a radiofrequency pulse in MRI, measured in degrees.
Full Definition
Flip angle is a fundamental MRI parameter that describes the degree of rotation imparted to tissue magnetization by radiofrequency pulses. At equilibrium, tissue magnetization aligns with the static magnetic field (longitudinal direction). RF pulses rotate this magnetization toward the transverse plane, with the flip angle determining the extent of this rotation. Common flip angles include 90° for spin-echo sequences and variable angles (5-30°) for gradient-echo sequences. The choice of flip angle affects image contrast, signal-to-noise ratio, and acquisition speed. Smaller flip angles allow faster repetition times and are essential for rapid imaging techniques, while larger angles maximize signal from specific tissue types.
Usage
Usage note: Always include degree symbol (°) when specifying numerical values.
In Context
- "The T1-weighted gradient echo sequence used a flip angle of 15° to optimize gray-white matter contrast." — MRI protocol documentation
- "Reducing the flip angle to 5° enabled sub-second acquisition times for the dynamic series." — Technical parameter optimization