Schematics show the mechanism that causes metal-related artifacts at MR imaging. (a) The spins in the main magnetic field predictably rotate at the same frequency.When the frequency encoding gradient is applied, the Larmor frequencies of the spins change along the gradient direction. The spins at locations of higher magnetic field strength (upper part of the triangle) rotate at higher frequencies. When a metallic object is placed in the region of interest (arrow inside the circle), the spins near the object rotate with higher frequency than those that are distant from it. (b) The region of interest near the metallic object is encoded as if it were at a higher gradient location (upper circle) than it actually is. The signal around the region of interest is summed at a higher gradient location (+) and subtracted at a lower one (-). The result is geometric distortion of the object (Lee et al., 2007).