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MRI Physics > Unit 7 > Flashcards

Unit 7 Flashcards

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1
Q

What is spacial localization?

A

process of locating each voxel of tissue for positioning

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2
Q

What is gradient slope?

A

amount of change in strength of a gradient magnetic field

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3
Q

What is magnetic isocenter?

A

center of B0

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4
Q

What is slice selection gradient

A

logical gradient that localizes tissue in imaging plane; on during RF transmission

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5
Q

What is frequency encoding/readout gradient?

A

logical gradient that localizes perpendicular to imaging plane; on during signal reception

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6
Q

What is phase encoding gradient

A

logical gradient that localizes perpendicular to imaging plane; on before refocusing pulse

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7
Q

What is sampling rate?

A

frequency at which a waveform/signal is sampled

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8
Q

What is k-space?

A

frequency and phase encoded raw data map

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9
Q

What is image matrix?

A

the interpreted k-space

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10
Q

What is partial/fractional echo imaging?

A

method of imaging whereby only half of k-space in frequency encoding axis is filled, the rest is calculated by MR system

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11
Q

What is partial/fractional averaging?

A

method of imaging whereby only half of k-space in phase encoding axis is filled

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12
Q

What is sequential k-space filling?

A

method of imaging whereby all of k-space is filled one slice at a time

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13
Q

What is 2D k-space filling?

A

method of imaging whereby k-space is filled one line at a time for every slice

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14
Q

What is 3D k-space filling?

A

method of imaging whereby an entire volume of tissue is excited by the RF, and images are divided up after acquisition

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15
Q

What is Fourier Transformation?

A

math calculation that converts waveforms into discrete values for signal strength at each frequency

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16
Q

What is slice profile?

A

shape of transmitted RF pulse

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17
Q

What is the relation between gradient M and local M experienced by tissue?

A

when gradient > B0, tissue exp higher local M; vice versa

18
Q

What is the relationship between gradient M and precession?

A

when gradient > B0, precession higher frequency; vice versa

19
Q

What is a steep vs shallow gradient?

A

steep gradient has a greater change in M

20
Q

What are the physical gradient coil for each slice orientation?

A

axial - Z
Sagittal - X
Coronal/Dorsal - Y

21
Q

How does gradient slope affect slice thickness?

A

stepper gradient = thinner slice; vice versa

22
Q

How does RF pulse transmit bandwidth affect slice thickness?

A

narrower bandwidth = thinner slice; vice versa

23
Q

Which logical gradient is on during RF transmission?

A

slice select

24
Q

Which logical gradient is on during signal reception?

A

frequency encoding

25
Q

Which logical gradient is on before the refocusing pulse?

A

phase encoding

26
Q

What is the Nyquist Theorem?

A

waveform must be sampled at least twice per cycle to be accurate

27
Q

What is the relationship between RF receive bandwidth and sampling rate to satisfy the Nyquist theorem?

A

proportionally related

28
Q

What is the relationship between sampling time and sampling rate to satisfy the Nyquist theorem?

A

inversely proportional

29
Q

What is the relationship between RF receive bandwidth and sampling time to satisfy the Nyquist theorem?

A

inversely proportional

30
Q

What is a fine vs coarse image matrix?

A

fine has smaller pixels

31
Q

How does the number of phase and frequency encodings affect image matrix?

A

fine matrix has many frequency and/or phase encoding steps; vice versa

32
Q

How many phase encoding lines of k-space are acquired during each TR period?

A

one

33
Q

How does number of phase encoding affect image time?

A

proportionally

34
Q

Where is the highest amplitude signals placed in k-space?

A

center

35
Q

What part of k-space determines resolution?

A

outer

36
Q

What part of k-space determined contrast?

A

center

37
Q

How does RF transmit bandwidth and sampling time affect slice profile?

A

narrow bandwidth => squarer slice profile;

longer sampling time => squarer slice profile

38
Q

Howe does RF transmit bandwidth affect duration of RF pulse?

A

narrow bandwidth requires longer RF pulse duration; vice versa

39
Q

How does RF receive bandwidth affect min TE and TR?

A

wider bandwidth makes shorter TE and TR; vice versa

40
Q

Hose does sampling rate affect minimum TE and TR?

A

faster sampling rate makes a shorter TE and TR; vice versa

41
Q

How does frequency encoding gradient and RF receive bandwidth affect FOV?

A

FOV can be decreased by steeper frequency encoding gradient, narrow receive bandwidth, or both

42
Q

What part of k-space is filled w/ steep phase encoding gradient?

A

outer edges are filled w/ steeper phase encoding gradient

MRI Physics (8 decks)

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