[B₁] A conventional symbol for the radio frequency field strength (another symbol historically used, is H1). In MRI, B1 labels the field produced by the radio frequency coil.
The B1 field is often conceived of two vectors rotating in opposite directions, usually in a plane transverse to B0. At the Larmor frequency, the vector rotating in the same direction as the precessing spins will interact strongly with the spins.

[H₁] H1 is a conventional symbol historically used for the radio frequency magnetic field related to MRI; today B1 is in use.

See B1.

Quick Overview

Fat suppression (SPIR or FatSat) is very critical to the magnetic field homogeneity. Eddy currents in the patient results in B1 disturbance from left to right and from anterior to posterior. The artifact is seen as signal intensity variations with SPIR, like a signal intensity loss diagonal in the image. The short T1 inversion recovery (STIR) sequence is due to another type of fat suppression insensitive to this artifact.

Take short T1 inversion recovery (STIR) instead spectral presaturation inversion recovery (SPIR) for fat suppression.

A 'Rotating Frame of Reference' is a concept to simplify the complex motions of a magnetization vector. The frame of reference rotates about the axis of the external magnetic field B0 (z-axis) at the Larmor frequency of the applied RF magnetic field B1. In this simplified concept the rotating vector B1 appears stationary.

Adiabatic RF pulses are amplitude and frequency modulated pulses that are insensitive to the effects of B1-inhomogeneity and frequency offset (conventional RF pulses used in MRI are only amplitude modulated). Due to an extended application time adiabatic RF pulses are mostly used in NMR imaging applications.