However, these techniques are approximate due to phase errors in the MRI data which can rarely be completely controlled (due to imperfect static field shim, effects of spatially selective excitation, signal detection coil properties, motion etc.)
However, these approaches are approximate because of inaccuracies in the MRI data caused by factors such as phase errors, static field shimming issues, spatially selective excitation effects, signal detection coil properties, and motion artifacts.