Flashcards in Physics of Image Artefacts Deck (17)
What is the definition of an artefact?
A signal in an image which obscures the reality of the object being imaged
What types of patient motion can cause artefacts?
Gross patient movement
What direction of an image does patient motion affect?
The phase encoding direction
How can patient movement effects be minimised?
Improve patient compliance/sedation
Triggering or monitoring of the scan by respiratory bellows, navigators or ECG leads
Select a sequence type that is less sensitive to motion
What is the cause of the chemical shift effect in MRI?
The motion of electrons produces a weak magnetic field due to circulating charge to oppose the applied magnetic field. The magnetic field of the electrons is proportional to the applied magnetic field
What effect does chemical shifts have on the MRI signal?
The nucleus experiences the vector sum of the 2 fields
The larmor frequency is now w0 = gamma(B0-Be)
This can cause artefacts for fat and water
How is the bandwidth per pixel for frequency encoding expressed on the MRI scanner?
Either water-fat shift (in pixels) or as total bandwidth for all pixels
What is the compromise needed when choosing a bandwidth?
A high bandwidth minimises the water-fat shift but reduces the SNR
How does phase wrap occur?
If the FoV doesn't cover the anatomy then the signal excited from outside will alias on the other side of the image
How can phase wrap be minimised?
Prescribe an adequate phase FoV (phase oversampling)
When is phase wrap most commonly a problem?
For double oblique scans of the heart where phase relationships may change during prescription
What causes parallel imaging artefacts?
When the whole FoV of the subject is not covered by the reference scan and the data is undersampled in the phase direction you get 'hot lips' where the signal wraps to the middle of the FoV
What is fast imaging with steady state precession and how is it used?
Can either spoil magnetisation between each TR or rewind gradients so that you can produce high SNR images where both water and fat are bright
What is the effect of the off-resonance artefact?
Fat spins can null the water spins
How is the off-resonance artefact effect minimised?
Use the shortest TR and TE possible - gives the widest passband
Ensure shimming and F0 routines locate the resonant frequency at the centre of the anatomical ROI
Manually offset F0 to move the artefact
Not appropriate near implanted objects
What is the zipper artefact?
External RF is detected by the scanner creating bands of speckled interference
The interference isn't coherent with phase encode gradient so it extends across the image in phase encoding direction