Motion and Ghosting artifacts : Remedies
Physical restraint , breath-hold
Reducing the patient's movements sometimes requires using physical restraint (compression devices) or sedation (or general anesthetic).
Breath-hold sequences with scan time less than 30 seconds can be used to eliminate respiratory artifacts.
There are several methods for reducing respiratory artifacts when breath-hold is not possible. For instance, respiratory gating consists in synchronizing the TR (phase-sampling interval) and the periodic motion, so that the measurements are always performed in the same position. The penalty of this technique is a much longer acquisition time.
Some other techniques have a lower acquisition time penalty. Instead of acquiring each phase-encoding step randomly in function of the patient's respiration, phase-reordering methods synchronize the phase-encoding steps with the respiratory cycle: the central lines of k-space are acquired when movement amplitude is the lowest, and the periphery of k-space is acquired when movement amplitude is the highest. As the central lines correspond to image contrast and shape, and the periphery to the edges, the penalty of this technique is a blur of edges. However, the time penalty is much lower than with respiratory gating.
Navigator echoes allow for correction of respiratory motion. Additional echoes that are not phase encoded are acquired at each repetition to identify the location of the diaphragm: A vertical line centered on the diaphragm boundary is obtained thanks to two slice-selective pulses in orthogonal planes and frequency-encoding in the direction of the motion. Navigator echoes give some information about the position of the diaphragm boundary that can be used to trigger the acquisition or correct phase shifts in the raw data before reconstruction of the image.
In the same way as with respiratory gating, cardiac gating consists in synchronizing the TR with the cardiac cycle. This reduces the cardiac motion artifacts and also allows for cine imaging of the cardiac movement.
(See Cardiac MRI for more details about cardiac triggering)
Phase-encode and frequency-encode directions swap
Swapping the phase-encode and frequency-encode directions can be useful to move the motion artifacts away from the region of interest.
Another way to reduce motion artifacts is to suppress the signal from moving tissues with saturation bands or fat-signal suppression techniques.