Motion: Ghosting and Smearing

Abstract

Ghosting typically occurs in the phase encoding direction, which is along the vertical axis in Fig. 103.1a and the horizontal axis in Fig. 103.1b. Note that wraparound artifact (see Chap. 101) is present in both images also along the respective phase encoding axes. Figures 103.1 and 103.2 display ghosts (arrows) caused by motion during acquisition of data that encodes edge information (high spatial frequency at the periphery of k-space). Thus, motion artifact appears as edge-related ghosts of structures of high signal intensity. Figure 103.2a presents a T1-weighted axial image of the brain without motion artifact. With a small amount of motion (Fig. 103.2b), a few ghosts appear, but with more substantial motion (Fig. 103.2c), increased number and better defined ghosts appear. Discrete ghosts of the entire spatial information (not illustrated) are caused by cyclic alteration in the MR signal from motion occurring throughout the period of data acquisition, often seen in pulsatile artifact (see Chap. 121). Blurring (not illustrated), another form of ghosting, is the loss of boundary definition due to slow modulation of the signal throughout data acquisition. Smearing (also not illustrated) occurs when signal is acutely disturbed during acquisition of the center lines of k-space. It appears as smudging across a major portion of an axis (usually the phase encoding axis) from any structure that produces signal, not just those at the edges of the image.