Image Quality Flashcards
How does TE affect SNR?
Low TE = high SNR
High TE = better T2
How does TR affect SNR?
High TR = high SNR
Low TR = better T1
How does voxel size affect SNR and spatial resolution?
Large = high SNR
Small = high res
How NEX affect SNR?
High = high SNR
How does bandwidth affect SNR?
Low = high SNR
High = less chemical shift
How does the coil affect SNR?
Small = high SNR
How does magnetic field strength affect SNR and spatial res?
High = high SNR, high res
How does slice thickness affect SNR or spatial res?
Thick = high SNR
Thin = high res
How does slice gap affect SNR?
No overlap = high SNR
How does field of view affect SNR and spatial res?
Large = high SNR
Small = high res
How does matrix size affect SNR and spatial res?
Small = high SNR
Large = high res
How does the reconstruction process affect SNR?
Low-pass filtration = high SNR, less edge definion
How can you correct a motion artifact?
Stop the motion (ex. holding breath during imaging)
What is a magnetic susceptibility artifact? What does it look like? How to correct?
Metal deflecting the magnetic field and changing the resonance frequency
Appears like a chunk has been taken out of the body
Corrected by removing the source (if possible)
What is Gibb’s artifact? What does it look like? How to correct?
Occurs when two objects with large contrast are imaged; inability to sample high frequencies
Creates lines of high and low intensity along sharp edges
Corrected by increasing the number of phase-encoding steps or reducing FOV, increase Nyquist
What is a chemical shift (of the first kind) artifact? What does it look like? How to correct?
Mismapping of fat due to overlapping fat and water signals
Appears with high contrast (bright) on one side of an organ and low contrast (dark) on the other (ex. kidney)
Corrected with fat-suppressing pulse sequences (like STIR), lower gradient strengths, larger voxel size (greater slice thickness)
What is a chemical shift (of the second kind) artifact? What does it look like? How to correct?
Occurs when water and fat protons are out of phase (cancelling one another out), causing a complete loss of signal
** Exclusive to GRE
Appears like structures have been outlined in black (“sharpie”)
Corrected with fat-suppressing pulse sequences (like STIR). A larger bandwidth leads to a smaller relative shift
What is a Wrap-Around / Phase-Wrap artifact? What does it look like? How to correct?
Occurs when the FOV is smaller than the object being imaged; mismapped phase (aliasing)
Appears like some of the image has been warped from one side to the other
Corrected by increasing FOV, increasing number of phase-encoding steps
