Week 5

Question 1

image quality is controlled by __

Answer 1

resolution, scan time, SNR

Question 2

what are constraints of image quality

Answer 2

tissue contrast, specific absorption rate, and stimulation

Question 3

what is SNR

Answer 3

MR echo amplitude received by receiver coils

Question 4

noises that can affect SNR are __

Answer 4

thermal motion, system electronics, patient

Question 5

what parameters must increase signal to increase?

Answer 5

• higher B0
• larger voxels
• higher averages
• narrower receiver bandwidths

Question 6

drawbacks of higher B0

Answer 6

more expensive

Question 7

drawbacks of using larger voxels

Answer 7

lower resolution

Question 8

drawbacks of using higher averages

Answer 8

longer scan time

Question 9

drawbacks of narrower receiver bandwidth

Answer 9

artifacts present

Question 10

slice resolution is increased by __

Answer 10

increasing slice thickness which raises SNR but causes more blurring & partial volume effects

Question 11

what does FOV determine

Answer 11

total scan coverage

Question 12

what is matrix

Answer 12

number of phases & frequency encoding steps

Question 13

FOV formula

Answer 13

voxel size x matrix

Question 14

what does smaller voxel size cause

Answer 14

higher spatial resolution but lower SNR

Question 15

scan time is dependent on __

Answer 15

patient comfort, tolerance and co-operation

Question 16

3 types of MRI artifacts

Answer 16

hardware, patient, radiographer

Question 17

types of hardware MRI artifacts are __

Answer 17

• field in-homogeneity
• gradient non-linearity
• RF interference artifact

Question 18

what causes field in-homogeneity

Answer 18

Large FOV with large frequency differences

Question 19

how does field in-homogeneity affect image quality

Answer 19

degrades from iso-center to max FOV, leading to failed fat saturation & banding artifacts

Question 20

solution to field in-homogeneity artifacts

Answer 20

• smaller FOV
• pulse sequences not sensitive to frequency differences

Question 21

what causes Gradient non-linearity

Answer 21

large FOV

Question 22

how does Gradient non-linearity affect image quality

Answer 22

Gradient pulses encode spatial info by applying linear field but at edge of field, linearity decreases causing shift in spatial info

Question 23

solution to Gradient non-linearity artifacts

Answer 23

• smaller FOV
• filter to correct distortion within gradient specifications

Question 24

what causes RF interference artifact

Answer 24

Randomly occurs along phase direction

Question 25

how does RF interference affect image quality

Answer 25

RF emission comes from external or internal source

Question 26

solution to RF interference artifacts

Answer 26

identify & remove source of interference

Question 27

types of patient MRI artifacts are __

Answer 27

motion & metal artifacts

Question 28

how does motion artifact affect image quality

Answer 28

Each TR repeated multiple times with incrementing phase encoding but with movement, data encoding in different spatial location = different K-space

Question 29

solution to motion artifacts

Answer 29

stop motion by force if needed, immobilizations, sequences less sensitive to motion

Question 30

what causes metal artifact

Answer 30

Severity of artifact dependent on metal type, size & orientation

Question 31

how does metal artifact affect image quality

Answer 31

Susceptibility causes signal voids, distortions, failure to fat saturate

Question 32

solution to metal artifacts

Answer 32

use Spin-Echo (instead of gradient echo), high bandwidth RF pulses, view angle tilted, multifrequency acquisitions, different fat saturation methods

Question 33

types of radiographer MRI artifacts are __

Answer 33

- aliasing / phase-wrap - cross talk - chemical shift artifact - signal intensity variations

Question 34

what causes aliasing / phase-wrap

Answer 34

FOV in phase encoding direction smaller than object to be measured

Question 35

how does aliasing / phase-wrap artifact affect image quality

Answer 35

Ambiguous phase info since outside range of encoding

Question 36

solution to aliasing / phase-wrap

Answer 36

larger FOV in phase encoding direction, oversampling, radial encoding

Question 37

what causes cross talk

Answer 37

Slices too closely spaced or non-ideal slice profiles used

Question 38

how does cross talk artifact affect image quality

Answer 38

Excitation RF pulses do not excite perfect rectangle causing slices to interfere with each other & saturate spins in-between

Question 39

solution to cross talk artifact

Answer 39

- Scan with gaps between slices - scan in multiple concatenations - change acquisition order - 3D sequences

Question 40

what causes chemical shift artifact

Answer 40

Black / white bands at structural edges containing fat & water

Question 41

how does chemical shift artifact affect image quality

Answer 41

Since system frequency tuned to water, fat protons mis-mapped to different location in frequency direction

Question 42

solution to chemical shift artifact

Answer 42

- change frequency encoding direction - increase pixel bandwidth - saturate fat

Question 43

what causes signal intensity variation artifact

Answer 43

When using surface coils, anatomy nearest to coils have brighter signals than other parts

Question 44

how does signal intensity artifact affect image quality

Answer 44

Receiver coil sensitivity decreases with distance and hence, tissues appear less bright

Question 45

solution to signal intensity variation artifact

Answer 45

- different coil design used - apply filter to normalize signal intensity