Chapter 4

Question 1

signal-to-noise ratio (SNR) is the ratio of __ and is dependent on __ (2)

Answer 1

signal amplitude to the average noise amplitude; number of acquisitions N and the noise variance

Question 2

as the magnetic field strength increases, the energy difference between __

Answer 2

spin up and spin down states grows and spin excess increases

Question 3

a spin excess increase leads to __

Answer 3

larger Mo (magnetization) and higher SNR at high B

Question 4

low proton densities yield __

Answer 4

low signal amplitudes and thus low SNR

Question 5

the voxel is the MRI unit of volume, defined as __

Answer 5

the product of pixel area and slice thickness

Question 6

the pixel area is equal to __

Answer 6

the FOV divided by the matrix dimension

Question 7

the larger the voxel, the __ the SNR because _-

Answer 7

higher; higher signal due to more protons/spins

Question 8

greater slice thickness = better __ but poorer __ (and vice versa)

Answer 8

SNR; resolution

Question 9

7 ways to maximize SNR and their consequences

Answer 9

increase NEX (but increase scan time); decrease matrix size (decrease scan time, but decrease resolution); increase slice thickness (but decrease resolution); decrease receive bandwidth (but increase TE and chemical shift); increase FOV (but decrease resolution); increase TR (but decrease T1 weighting and increase # of slices); decrease TE (but decrease T2 weighting)

Question 10

3 ways to maximize resolution (assuming a square FOV) and their consequences

Answer 10

decrease slice thickness (but decrease SNR); increase matrix size (but decrease SNR and increase scan time); decrease FOV (but decrease SNR)

Question 11

4 ways to minimize scan time (and their consequences)

Answer 11

decrease TR (but increase T1 weighting, decrease SNR, and decrease number of slices); decrease phase matrix (but decrease resolution and increase SNR); decrease NEX (but decrease SNR and increase movement artifacts); decrease slice number in volume imaging (but decrease SNR)

Question 12

benefits of increasing TR

Answer 12

higher SNR, more slices

Question 13

limitations of increasing TR

Answer 13

increases scan time, decreases T1 weighting

Question 14

benefits of increasing TE

Answer 14

increase T2 weighting

Question 15

limitations of increasing TE

Answer 15

decrease SNR

Question 16

benefits of increasing NEX

Answer 16

higher SNR and higher signal averaging

Question 17

limitations of increasing NEX

Answer 17

longer scan time

Question 18

benefits of increasing slice thickness

Answer 18

higher SNR, higher coverage

Question 19

limitations of increasing slice thickness

Answer 19

lower resolution and higher partial voluming

Question 20

benefits of increasing FOV

Answer 20

higher SNR, higher coverage, decreased aliasing

Question 21

limitations of increasing FOV

Answer 21

lower resolution

Question 22

benefits of increasing phase matrix

Answer 22

better resolution

Question 23

limitations of increasing phase matrix

Answer 23

longer scan time, lower SNR if pixel small

Question 24

benefits of increasing receive bandwidth

Answer 24

lower chemical shift, lower minimum TE

Question 25

limitations of increasing receive bandwidth

Answer 25

lower SNR

Question 26

benefits of using a large coil

Answer 26

more area of received signal

Question 27

limitations of using a large coil

Answer 27

lower SNR, more sensitive to artifacts, more prone to aliasing with small FOV

Question 28

when the anatomy has a smaller dimension in one axis vs another, a __ may be desired. this can __

Answer 28

a rectangular FOV; reduce scan time a lot (imaging spinal cord)

Question 29

spatial resolution is influenced by __ (3)

Answer 29

slice thickness, FOV, and matrix size

Question 30

square pixels provide better spatial resolution because __

Answer 30

the image is equally resolved along both the freq and phase axes

Question 31

receiver coils should be positioned to maximize the __, according to __

Answer 31

EMF which they can record; Faraday's law

Question 32

the contrast-to-noise ratio (CNR) can be increased by __ (3)

Answer 32

using T2 weighting (since tumor contrast is better), using contrast agents, using more sophisticated techniques such as magnetization transfer contrast (MTC)

Question 33

narrow receive bandwidth gets better SNR because __

Answer 33

same amount of signal, but less noise under the curve

Question 34

doubling the NEX increases the SNR only by __, so __

Answer 34

square root of 2; increasing the NEX is not an efficient way to increase SNR

Question 35

if TE is too long, __

Answer 35

signal intensity is too low and therefore SNR is too low too

Question 36

if TR is too short

Answer 36

no signal

Question 37

the closer to 90 degrees the flip angle is, the __. the closer to 0/180 the flip angle is, vice versa

Answer 37

greater signal (because greater transverse component