MRI Artifacts Flashcards
What are other names for phase mismapping?
Ghosting or blurring
What is magnetic susceptibility and how do you reduce it?
“It is the ability of a substance to become magnetized.” This artefact distorts the image and has large signal voids.
It is usually caused by metal objects within the imaging volume e.g. metal screws, braces surgical clips etc.
Remove all metal objects
Use SE instead of GRE sequences
Decrease the TE. A longer TE will allow for more dephasing to between tissues with susceptibility differences so in using a short TE it will reduce this artefact.
When imaging a patient with a prosthesis in situ, what artefact can you except and how would you try to reduce it?
Magnetic susceptibility artefact
Use SE instead of GRE sequences
Decrease the TE. A longer TE will allow for more dephasing to between tissues with susceptibility differences so in using a short TE it will reduce this artefact.
What is the difference between chemical shift and chemical misregistration?
Chemical shift:
A dark edge is produced at the interface between fat and water and occurs in the frequency direction only.
This artefact is caused by the different chemical environments of fat and water.
Chemical misregistration:
When the signals of fat and water are in phase they add constructively but when they are out of phase they cancel each other out.
The cancellation effect is known as chemical misregistration where a dark ring is produced around a certain organ where fat and water interfaces occur within the same voxel e.g. in the kidneys.
It mainly occurs in the phase direction because of the phase difference between fat and water
When would you get wrap?
When anatomy that exists outside the FOV are folded or wrapped over the anatomy inside the FOV.
What is the difference between cross-talk and cross- excitation?
Cross excitation: Energy is given to nuclei in adjacent slices by the RF pulse.
Cross-talk: Energy is given to adjacent slices due to spin lattice relaxation.
Which one of Cross-talk and Cross-excitation can be reduced and how?
There is no remedy for cross-talk as it is caused by the natural dissipation of energy by the nuclei.
A slice gap of at least 30% (of the slice thickness itself) can help reduce cross-excitation.
How can you reduce phase mismapping?
Swap frequency and phase to avoid artefact over anatomy of interest.
Use pre-saturation bands (pulses). These pulses nullifies the signal in the area and can reduce the artefacts e.g. in a cervical spine an AP saturation band can be placed for swallowing or flow (carotids) when the phase is AP.
Respiratory compensation techniques (ROPE) respiratory ordered phase encoding is used when the sequences are too long for the patients to hold their breath.
Cardiac gating. A general term used to reduce phase artefacts coming from cardiac, respiratory and pulsatile flow motion. Cardiac motion is monitored by co-ordinating the excitation pulse with the R-wave of systole. The two ways of doing this are:
- Electrocardiogram (ECG).
- Peripheral gating uses a light switch clipped onto the patients’ finger to detect the pulsation of blood through the capillaries.
Gradient moment nulling.
More ways to reduce phase mismapping….
Other methods -
For voluntary movements patients can be asked not to swallow or look around for a c-spine or orbits MRI.
In the case of bowel movements an antispasmodic agent e.g. buscopan can be administered.
NEX (number of averages) can be increased.
What is the cause of truncation or ringing artefact?
This is a banding artefact at the interfaces between high and low signal e.g. at the edges of the brain where high signal from fat in the scalp lies next to the low signal from the skull.
It is called Gibbs artefact when seen on a sagittal cervical spine image.
What do you do when you see the zipper artefact?
Call the service engineer
