Overcoming motion in abdominal MR imaging.

Anatomic structures that move periodically during the acquisition of data for an MR image become multiple ghosts in the phase-encoding direction. There is a constant spacing in pixels between consecutive ghosts, which is equal to the number of cycles of motion that occurred during the acquisition of data. The intensity of ghosts depends on the intensity of the moving structure and the number of pixels over which the motion occurred. No single method is completely satisfactory at suppressing motion artifacts. The major attributes and limitations of each method are summarized in Table 2, with plus (+) signs denoting merit. Theoretically, some methods perform better in reducing the intensity of ghosts and restoring the image intensity to its proper place. This certainly is not the final criterion, however. Some methods reduce the blurring in addition to suppressing the ghosts, or they suppress ghosts without prolonging the time for imaging. Certain methods also reduce ghosts from other kinds of motion. It is very appealing for a method to function without monitoring. The success of monitoring often depends too much on the cooperation of both the patient and technologist. The theoretical performance, attributes, and deficiencies of the various methods have been combined into a subjective overall rating in the last column of Table 2. All of the methods can be effective under the appropriate circumstances. Moreover, the methods are not mutually exclusive. It is advantageous, therefore, to combine methods to achieve even greater suppression. For example, physical restraint can be used for all but the most uncooperative patients. Most imaging techniques can be designed with gradients that rephase the signals from moving structures. Then other methods, such as averaging or reordering, can be applied as necessary. Fortunately, there are effective motion artifact suppression methods, even though not all are widely available yet on commercial equipment. Consistent suppression of motion artifacts will enhance the quality of MR images. Elimination of motion artifacts will improve the capability of MR to detect lesions and will provide a higher standard of performance for MR in the body.