(FA) The flip angle a is used to define the angle of excitation for a field echo pulse sequence. It is the angle to which the net magnetization is rotated or tipped relative to the main magnetic field direction via the application of a RF excitation pulse at the Larmor frequency. It is also referred to as the tip angle, nutation angle or angle of nutation.
The radio frequency power (which is proportional to the square of the amplitude) of the pulse is proportional to a through which the spins are tilted under its influence. Flip angles between 0° and 90° are typically used in gradient echo sequences, 90° and a series of 180° pulses in spin echo sequences and an initial 180° pulse followed by a 90° and a 180° pulse in inversion recovery sequences.

Only the moveable (free) spins have a defined Larmor frequency and are visible in the image.

The mapping of the magnetic field by measuring or imaging the spatial distribution of magnetic field strength, can be performed by scanning with a probe and handles a large range of field strengths, but is slow and tedious. Accurate field maps can be made by measuring the Larmor frequency as a function of position.
The field must be homogeneous enough to allow MR imaging to be performed, than the magnetic field can be mapped by different methods.
1. The adaptation of chemical shift imaging.
2. The faster one measures the change in signal phase in an image obtained with a gradient echo pulse sequence resulting from a change in echo time TE, which is proportional to the local field strength.
Also useful is a spin echo pulse sequence with data collection from two time locations of the readout gradient and the data acquisition interval, where each having a known shift of the acquisition center away from the spin echo.

A pattern of multiple resonance's (spectral lines) observed when the initially single Larmor frequency of a given nucleus in a spin system is split by interactions with neighboring spins through the scalar or spin spin interaction. The magnitude of this interaction is independent of the applied magnetic field and is referred to as J, the spin spin coupling constant. The specific pattern produced depends on the number of coupled nuclei and their spin quantum numbers.

This pulse affects all of the tissue within the transmitting coil. It is only used for MTC at a frequency distant from the Larmor frequency or if another method is selected to define slices like 3D acquisition.