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MRI Physics > Unit 8 > Flashcards

Unit 8 Flashcards

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1
Q

What is signal?

A

RF received from relaxation of excited protons

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2
Q

What is noise?

A

RF received from background electrical interference and thermal motion in a patient

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3
Q

What is SNR?

A

signal to noise ratio; measure of image quality

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4
Q

What is contrast to noise ratio?

A

ratio between SNR of adjacent tissues

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5
Q

What is spatial resolution?

A

ability to distinguish points as separate and distinct

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6
Q

What is crosstalk?

A

mixing of signal from adjacent slices

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7
Q

What is cross excitation?

A

transmitted RF that leaks over from adjacent slices

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8
Q

What is respiratory compensation?

A

method whereby shallow phase encoding are acquired between breaths

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9
Q

What is phase/frequency oversampling?

A

method whereby extra encoding steps are acquired, but discarded prior to image reconstruction

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10
Q

Why is respiratory compensation used?

A

reduce respiratory motion artifact

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11
Q

Why is phase/frequency oversampling used?

A

reduce wrap-around

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12
Q

How is SNR affected by TR?

A

proportionally

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13
Q

How is SNR affected by TE

A

inversely proportional

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14
Q

How is SNR affected by slice thickness

A

proportionally

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15
Q

How is SNR affected by FOV?

A

proportionally

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16
Q

How is SNR affected by image matrix?

A

inversely proportional

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17
Q

How is SNR affected by Number of Signal Avgs (NSA)?

A

proportionally

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18
Q

How is SNR affected by slice gap?

A

proportionally

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19
Q

How is SNR affected by receiver bandwidth?

A

inversely proportionally

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20
Q

Describe phase mismapping/ghosting artifact

A

ghosting/blurring of image along phase econding axis

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21
Q

What causes phase mismapping/ghosting?

A

motion - resp, blood, swallowing

22
Q

Describe wrap around artifact

A

anatomy is placed in wrong posistion in image

23
Q

What causes wrap around?

A

FOV too narrow in phase direction

24
Q

Describe partial volume avg artifact

A

blurred edges of structures

25
Q

What causes partial volume avg?

A

large voxel size

26
Q

Describe Chemical shift artifact

A

dark edge on one side of structures, and bright on the opposite in frequency direction

27
Q

What causes chemical shift?

A

the different precessional frequencies of fat and water

28
Q

Describe chemical misregistration artifact

A

dark lining around tissue interfaces on GE

29
Q

What causes chemical misregistration?

A

fat and water precessions being out of phase

30
Q

Describe truncation/Gibbs artifact

A

low intensity bands that run parallel to tissue interface in phase direction

31
Q

What causes truncation/gibbs?

A

undersampled tissue interfaces

32
Q

Describe data clipping artifact

A

signal loss of image; exaggerated/inverted contrast

33
Q

What causes data clipping?

A

image data is beyond the range of the receiver

34
Q

What parameters affect voxel volume?

A

FOV, slice thickness, matrix size

35
Q

What parameters affect contrast and SNR?

A

TR, TE, flip angle

36
Q

What parameters affect scan timeand SNR?

A

TR, NSA, matrix size

37
Q

What parameters affect number of slices available during each TR?

A

TE, receive bandwidth

38
Q

What parameters affect resolution and SNR?

A

slice thickness, matrix size, FOV

39
Q

How much does SNR increase by doubling NSA?

A

44%

40
Q

How does slice gap affect SNR?

A

greater gap = less noise

41
Q

How is scan time calculated?

A

=TR x #phase encoding steps x NSA

42
Q

How is slice encoding done for 3D imaging?

A

phase encoding steps are performed in the slice direction

43
Q

What are advantages to 3D imaging?

A

thinner slices, no slice gap, higher SNR, manipulate into different orientations

44
Q

What is a disadvantage to 3D imaging?

A

longer scan time

45
Q

How do you compensate for motion/ghosting artifact?

A

swap phase and frequency direction, presatureation, resp comp, flow comp, gating

46
Q

How do you compensate for wrap-around?

A

oversample, presaturate

47
Q

How do you compensate for chemical shift?

A

increase bandwidth, decrease FOV, decrease voxel size

48
Q

How do you compensate for chemical misregistration?

A

SE squences, GE w/ TE when fat and water in phase

49
Q

Why is GE more susceptible to chemical misregistration?

A

there isn’t a refocusing pulse

50
Q

How do you compensate for truncation?

A

inc phase encoding steps

51
Q

How do you compensate for cross-excitation?

A

increase slice gap, interleaved slice excitation

MRI Physics (8 decks)

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