Shading artifact is an MRI artifacts and refers to loss of signal intensity in one part of the image, leading to dark shading in this portion of the image.
Causes
uneven excitation of nuclei within the field; due to RF pulses applied at flip angles other than 90 and 180 degrees
abnormal loading of coil or coupling of the coil at a point (as with a large patient who touches one side of the coil)
inhomogeneity in the magnetic field
overflow of analog to digital converter
Axis
frequency encoding
phase encoding
Remedy
load the coil correctly
use the proper size coil for patient size and the examined part
prevent the patient from touching the coil (you can use foam pads between the patient and the coil)
shimming to reduce the inhomogeneity of the magnetic field
use the proper scanning parameters to set proper amplitude of applied RF pulses (less amplification to avoid analog to digital converter over flow)