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530: Data Acquisition > Chapter 4 > Flashcards

Chapter 4 Flashcards

1
Q

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

A

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

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2
Q

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

A

spin up and spin down states grows and spin excess increases

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3
Q

a spin excess increase leads to __

A

larger Mo (magnetization) and higher SNR at high B

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4
Q

low proton densities yield __

A

low signal amplitudes and thus low SNR

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5
Q

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

A

the product of pixel area and slice thickness

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6
Q

the pixel area is equal to __

A

the FOV divided by the matrix dimension

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7
Q

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

A

higher; higher signal due to more protons/spins

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8
Q

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

A

SNR; resolution

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9
Q

7 ways to maximize SNR and their consequences

A

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)

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10
Q

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

A

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

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11
Q

4 ways to minimize scan time (and their consequences)

A

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)

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12
Q

benefits of increasing TR

A

higher SNR, more slices

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13
Q

limitations of increasing TR

A

increases scan time, decreases T1 weighting

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14
Q

benefits of increasing TE

A

increase T2 weighting

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15
Q

limitations of increasing TE

A

decrease SNR

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16
Q

benefits of increasing NEX

A

higher SNR and higher signal averaging

17
Q

limitations of increasing NEX

A

longer scan time

18
Q

benefits of increasing slice thickness

A

higher SNR, higher coverage

19
Q

limitations of increasing slice thickness

A

lower resolution and higher partial voluming

20
Q

benefits of increasing FOV

A

higher SNR, higher coverage, decreased aliasing

21
Q

limitations of increasing FOV

A

lower resolution

22
Q

benefits of increasing phase matrix

A

better resolution

23
Q

limitations of increasing phase matrix

A

longer scan time, lower SNR if pixel small

24
Q

benefits of increasing receive bandwidth

A

lower chemical shift, lower minimum TE

25
Q

limitations of increasing receive bandwidth

A

lower SNR

26
Q

benefits of using a large coil

A

more area of received signal

27
Q

limitations of using a large coil

A

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

28
Q

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

A

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

29
Q

spatial resolution is influenced by __ (3)

A

slice thickness, FOV, and matrix size

30
Q

square pixels provide better spatial resolution because __

A

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

31
Q

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

A

EMF which they can record; Faraday’s law

32
Q

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

A

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

33
Q

narrow receive bandwidth gets better SNR because __

A

same amount of signal, but less noise under the curve

34
Q

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

A

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

35
Q

if TE is too long, __

A

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

36
Q

if TR is too short

A

no signal

37
Q

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

A

greater signal (because greater transverse component

530: Data Acquisition (13 decks)

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