Magnetic Resonance Imaging (MRI) Practice Test

All of the following artifacts occur along the phase encoding axis except what?

1. Motion artifact

2. Chemical shift

3. Magnetic susceptibility artifact

4. Phantom phenomenon

The correct answer is: Chemical shift

The correct answer highlights that chemical shift artifacts do not occur along the phase encoding axis. This is because chemical shift artifacts arise due to the difference in resonance frequencies of protons in different chemical environments, notably fat and water. In MRI, these different frequencies cause signal displacement along the frequency encoding axis rather than the phase encoding axis. In contrast, motion artifacts, magnetic susceptibility artifacts, and phantom phenomenon do manifest along the phase encoding axis. Motion artifacts occur when there is patient movement during the acquisition, leading to misregistration of tissue locations along that axis. Magnetic susceptibility artifacts occur due to differences in the magnetic field caused by variations in tissue composition, again affecting the phase encoding direction. Phantom phenomenon, which can happen during imaging, refers to artifacts that might create fictitious areas or shapes, typically influenced by inconsistencies in the phase encoding process. Thus, focusing on where each of these artifacts occurs in relation to the phase encoding axis clarifies why chemical shift artifacts are the exception.