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5 B0 Mapping/1 B0 Inhomogeneities/1-Introduction.md

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-The main magnetic field, also called the _B_{sub}`0` field, plays a crucial role in MRI. It dictates the precessional frequency of the spins and sets-up the bulk magnetization, which plays an important role in the image signal-to-noise ratio. Moreover, the radio frequency coils, tuned to the _B_{sub}`0` field, are responsible for flipping the spins in the transverse plane and for acquiring the signal. However, imaging reconstruction techniques assume a perfectly homogeneous _B_{sub}`0` field to reconstruct the signal from k-space data. An inhomogeneous _B_{sub}`0` field can lead to image artifacts such as signal loss, distortions [@Jezzard1995-qd], poor fat saturation [@Anzai1992-j] and many other image artifacts. In extreme cases, it can completely hinder the ability to create an image. _B_{sub}`0` inhomogeneities are also problematic for MR spectroscopy (MRS), because they widen the spectral linewidth.
+The main magnetic field, also called the _B_{sub}`0` field, plays a crucial role in MRI. It dictates the precessional frequency of the spins and sets-up the bulk magnetization, which plays an important role in the image signal-to-noise ratio. Moreover, the radio frequency coils, tuned to the _B_{sub}`0` field, are responsible for flipping the spins in the transverse plane and for acquiring the signal. However, imaging reconstruction techniques assume a perfectly homogeneous _B_{sub}`0` field to reconstruct the signal from k-space data. An inhomogeneous _B_{sub}`0` field can lead to image artifacts such as signal loss, distortions [@Jezzard1995-qd], poor fat saturation [@Anzai1992-jl] and many other image artifacts. In extreme cases, it can completely hinder the ability to create an image. _B_{sub}`0` inhomogeneities are also problematic for MR spectroscopy (MRS), because they widen the spectral linewidth.
 
 When a subject is introduced in the scanner, the static _B_{sub}`0` field can be rendered more homogeneous through a technique called active shimming. Active shimming sends the appropriate amount of current through specific gradient and shim coils, in order to generate a magnetic field that will compensate for the existing (inhomogeneous) magnetic field. This procedure requires precise and accurate mapping of the _B_{sub}`0` field. _B_{sub}`0` maps show the difference between the current field and the expected field, and are typically displayed in units of magnetic field strength (Tesla [T]), precessional frequency (Hertz [Hz]) or in parts per million (ppm). [](#b0Eq1) can be used to convert from the different units.