Week 5 Flashcards
image quality is controlled by __
resolution, scan time, SNR
what are constraints of image quality
tissue contrast, specific absorption rate, and stimulation
what is SNR
MR echo amplitude received by receiver coils
noises that can affect SNR are __
thermal motion, system electronics, patient
what parameters must increase signal to increase?
- higher B0
- larger voxels
- higher averages
- narrower receiver bandwidths
drawbacks of higher B0
more expensive
drawbacks of using larger voxels
lower resolution
drawbacks of using higher averages
longer scan time
drawbacks of narrower receiver bandwidth
artifacts present
slice resolution is increased by __
increasing slice thickness which raises SNR but causes more blurring & partial volume effects
what does FOV determine
total scan coverage
what is matrix
number of phases & frequency encoding steps
FOV formula
voxel size x matrix
what does smaller voxel size cause
higher spatial resolution but lower SNR
scan time is dependent on __
patient comfort, tolerance and co-operation
3 types of MRI artifacts
hardware, patient, radiographer
types of hardware MRI artifacts are __
- field in-homogeneity
- gradient non-linearity
- RF interference artifact
what causes field in-homogeneity
Large FOV with large frequency differences
how does field in-homogeneity affect image quality
degrades from iso-center to max FOV, leading to failed fat saturation & banding artifacts
solution to field in-homogeneity artifacts
- smaller FOV
- pulse sequences not sensitive to frequency differences
what causes Gradient non-linearity
large FOV
how does Gradient non-linearity affect image quality
Gradient pulses encode spatial info by applying linear field but at edge of field, linearity decreases causing shift in spatial info
solution to Gradient non-linearity artifacts
- smaller FOV
- filter to correct distortion within gradient specifications
what causes RF interference artifact
Randomly occurs along phase direction
how does RF interference affect image quality
RF emission comes from external or internal source
solution to RF interference artifacts
identify & remove source of interference
types of patient MRI artifacts are __
motion & metal artifacts
how does motion artifact affect image quality
Each TR repeated multiple times with incrementing phase encoding but with movement, data encoding in different spatial location = different K-space
solution to motion artifacts
stop motion by force if needed, immobilizations, sequences less sensitive to motion
what causes metal artifact
Severity of artifact dependent on metal type, size & orientation
how does metal artifact affect image quality
Susceptibility causes signal voids, distortions, failure to fat saturate
solution to metal artifacts
use Spin-Echo (instead of gradient echo), high bandwidth RF pulses, view angle tilted, multifrequency acquisitions, different fat saturation methods
types of radiographer MRI artifacts are __
- aliasing / phase-wrap
- cross talk
- chemical shift artifact
- signal intensity variations
what causes aliasing / phase-wrap
FOV in phase encoding direction smaller than object to be measured
how does aliasing / phase-wrap artifact affect image quality
Ambiguous phase info since outside range of encoding
solution to aliasing / phase-wrap
larger FOV in phase encoding direction, oversampling, radial encoding
what causes cross talk
Slices too closely spaced or non-ideal slice profiles used
how does cross talk artifact affect image quality
Excitation RF pulses do not excite perfect rectangle causing slices to interfere with each other & saturate spins in-between
solution to cross talk artifact
- Scan with gaps between slices
- scan in multiple concatenations
- change acquisition order
- 3D sequences
what causes chemical shift artifact
Black / white bands at structural edges containing fat & water
how does chemical shift artifact affect image quality
Since system frequency tuned to water, fat protons mis-mapped to different location in frequency direction
solution to chemical shift artifact
- change frequency encoding direction
- increase pixel bandwidth
- saturate fat
what causes signal intensity variation artifact
When using surface coils, anatomy nearest to coils have brighter signals than other parts
how does signal intensity artifact affect image quality
Receiver coil sensitivity decreases with distance and hence, tissues appear less bright
solution to signal intensity variation artifact
- different coil design used
- apply filter to normalize signal intensity
