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### QUESTIONS» Image Creation

How is the MRI image created? This section explores slice selection, spatial encoding and the FFT.

# Flip Angle

## What is a flip angle?

The degree of rotation of a net magnetisation vector towards the xy-plane.

The initial angle of precession of a net magnetisation vector with B0.

The result of an RF pulse.

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.

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 B0 is only correct if we assume the net magnetisation is at thermal equilibrium to start with (aligned with B0). 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.)

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.