SummaryMotion and Ghosting artifacts
Origin
The patient's movements are the most common cause of image artifacts.
There are 2 types of artifacts due to motion:
- Random movements which produce a blurry and noisy image, mainly in the phase-encode direction.
- Periodic motion which creates ghost images in the phase-encode direction.
Motion artifacts mainly propagate in the phase-encode direction. This is due to movement of the spins between 2 excitations or between phase-encoding and signal reading: in the first case, the spins will not be recorded at the same position between excitations, in the second case, their phase-encoding will not be correct. As a result, the phase-encoding of these voxels is corrupted and this will be responsible for artifacts in the phase-encode direction.
On the other hand, signal sampling and spatial-encoding in the frequency-encode direction are done so fast that physiological motion will only produce a small amount of spatial blurring in that direction.
Ghost images
When the movements are periodical (cardiac beats, arterial or CSF pulsations, respiration), they can produce ghost images, propagated in the phase-encode direction, even outside the anatomic limits. The intensity of these ghost images becomes more extreme with the intensity of the moving structure and with the amplitude of movement. These ghost images can show up as an increase or decrease of the true image signal.
The spacing between ghost images varies with the direction of the movement, its amplitude and its periodicity relative to the phase-sampling interval (TR).
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.
Respiratory triggering
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.
Respiratory compensation
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
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.
Cardiac gating
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.
Saturation bands
Another way to reduce motion artifacts is to suppress the signal from moving tissues with saturation bands or fat-signal suppression techniques.
