# Flip Angle

Partial flip angle. The rotation of net magnetisation into the xy-plane need not be 90°. Gradient echo pulse sequences use low flip angles (e.g. 5-20°.)

CORRECT. The "strength" of an RF pulse can be easily described by what it does to the net magnetisation vector. How far it is rotated from its position when the RF pulse is applied may be stated as an angle.

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Partial flip angle. The rotation of net magnetisation into the xy-plane need not be 90°. Gradient echo pulse sequences use low flip angles (e.g. 5-20°.)

CORRECT. But the initial angle of precession of a net magnetisation vector with B_{0} is only correct if we assume the net magnetisation is at thermal equilibrium to start with (aligned with B_{0}). But RF pulses can be appled at any time. For example, two 90° pulses cause net magnetisation to move 180°. The direction of an RF pulse can be varied too; magnetisation may be rotated from the z-axis down the x-axis, or from the z-axis down the y-axis, for example. (Any direction is theoretically possible.)

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Partial flip angle. The rotation of net magnetisation into the xy-plane need not be 90°. Gradient echo pulse sequences use low flip angles (e.g. 5-20°.)

CORRECT. An RF pulse (a magnetic field oscillating at the Larmor frequency) rotates net magnetisation. This rotation can be specified as the angle from the position of the net magnetisation before the pulse, to the position of the net magnetisation after the pulse.

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Further reading on this topic:

Books: Q&A in MRI p103, MRI From Picture to Proton p30, MRI The Basics p244

Online: MR-TIP