Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

530: Data Acquisition > Chapter 3 > Flashcards

Chapter 3 Flashcards

1
Q

resonance occurs when __

A

an RF pulse is applied at the Larmor precession frequency

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

a resonating RF pulse gives H nuclei enough energy so that __ is created and so that nuclear moments __

A

transverse magnetization; precess in phase

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

a voltage is induced in the receiver coil of the scanner at the Larmor frequency, regardless of __

A

the spatial origin of the signal

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

to identify the spatial origin of the signal, we use

A

spatial encoding using gradients

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

first step in locating signal, then what

A

slice selection, then encoding along both axes of the image

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

gradients are __ and are generated by __

A

alterations to the static field; wire coils located within the magnet bore

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

the flow of electric current through gradient coils induces a magnetic fields which __

A

either increases of decreases the strength of Bo

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

in spatial encoding, the magnitude of Bo is __ by the gradient coils, so that __ can be predicted

A

altered linearly; magnetic field strength and precessional frequency

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

the three gradient coils

A

Z gradient = long, Bo
Y gradient = vertical
Z gradient = horizontal (short)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

the magnetic isocenter is located __

A

at the center of the magnet’s bore, and the center of the gradient coordinate system

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

magnitude of Bo at the isocenter is __ by gradients

A

unaltered

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

the slope of the gradient (steep vs shallow) determines __

A

how fast the magnitude of B changes along the gradient direction

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

when gradient coils are switched on, every point along each axis has a specific __

A

precessional frequency associated with it because the nuclei at that location have a specific Larmor frequency

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

slices are selectively excited when __

A

an RF pulse is transmitted at the Larmor frequency of spins in the slice defined by the slice gradient

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

the scan plane determines __

A

which gradient performs slice selection

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

if the slice selection gradient is Z, then you get __ slices

A

axial

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

if the slice selection gradient is X, you get __ slices

A

sagittal

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

if the slice selection gradient is Y, you get __ slices

A

coronal

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

oblique slices can also be selected by __

A

activating several gradients simultaneously, each at different strengths

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

(at the same transmit bandwidth)
steep gradient = __ slice
shallow gradient = __ slice

A

thin; thick

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

to give each slice a thickness, __

A

a band of nuclei is excited by the RF pulse

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

the steepness of a gradient’s slope determines __

A

the difference in Larmor frequency across space along that gradient’s direction

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

once a gradient is applied, the RF pulse transmitted to excited the slice must contain a certain __

A

transmit bandwidth (a range of frequencies to match the precessional frequency between two points)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

at same slope,
narrow bandwidth = __ slice
wide/broad bandwidth = __ slice

A

thin; thick

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Q

the slice gap is determined by __ (2)

A

gradient slope and slice thickness

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
26
Q

slice select gradients are turned on __

A

during all RF pulses

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
27
Q

once a slice has been selected, the signal must be located along both axes of the image. this is done using __

A

frequency and phase encoding

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
28
Q

for coronal slices, the freq gradient is __

A

Z

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
29
Q

for axial slices, the freq gradient is __

A

Y

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
30
Q

for sagittal slices, the freq gradient is __

A

X

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
31
Q

the frequency encoding gradient is applied/turned on when __, and thus is called the __

A

the signal is received; readout gradient

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
32
Q

the readout gradient/freq gradient is applied at the __

A

echo time

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
33
Q

the portion of space (size of anatomy) covered along the freq encoding axis is called

A

the field of view (FOV)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
34
Q

once slice selection and freq encoding have been performed, the image must be __, this is performed using __

A

localized along the remaining axis of the image; phase encoding

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
35
Q

when the phase encoding gradient is turned on, nuclei along the gradient __

A

precess at different frequencies along the gradient direction because their Larmor frequencies are different

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
36
Q

(phase encoding)

slower precessing nuclei __, faster precessing nuclei __

A

lose/ trail behind in the phase; gain/ move ahead in the phase

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
37
Q

phase encoding gradient is applied __

A

after initial dephasing after 90 pulse, before 180 pulse

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
38
Q

steep phase encoding gradient results in __

shallow gradient results in __

A

big phase shift from one end of the gradient to the other; small phase shift

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
39
Q

sampling interval I is the __

A

duration between successive samples

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
40
Q

the sampling interval is the inverse of __

A

the sampling frequency (I = 1/f)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
41
Q

as the sampling interval increases, the sampling frequency __

A

decreases

42
Q

to sample frequencies accurately, __

A

the highest frequency must be sampled at least twice as fast (twice per cycle)/ you must sample at a rate that is twice as fast as the highest frequency

43
Q

if sampling is performed slower than twice the highest frequency, __ results and the recorded frequency __

A

aliasing; is not the true frequency because sampling was implemented inappropriately

44
Q

the maximum frequency which can be sampled is called __

A

the Nyquist frequency

45
Q

sampling frequency = (equation)

A

2 x Nyquist frequency

46
Q

on an MR scanner, do we specify the Nyquist frequency or the receiver bandwidth?

A

the receiver bandwidth

47
Q

receiver bandwidth =

A

the range of frequencies we wish to sample

48
Q

to specify a receive bandwidth, we select __

A

a center frequency

49
Q

the acquisition window is __

A

the amount of time that the readout gradient is on

50
Q

the duration of the acquisition window affects __

A

the Te (ex: echo must occur halfway through the freq encoding gradient/acquisition window, so if the window is increase, Te increases because the peak of the echo occurs later)

51
Q

the number of times that the magnetic moments of nuclei cross the receiver coil indicates __

A

the frequencies encoded in the slice

52
Q

the positions of the magnetic moments along their precessional paths indicates __

A

their phases

53
Q

these data (signal frequency, phase) are stored in __, which constitutes a __

A

k space; spatial frequency domain

54
Q

vertical axis of kspace = __

horizontal axis of kspace = __

A

phase; frequency

55
Q

frequency encoding gradient (Gr) is turned on while the system __

A

records the frequencies present in the signal and digitizes them

56
Q

the duration of the readout gradient Gr is called __

A

the sampling time or acquisition window

57
Q

during the acquisition window, the scanner samples frequencies a large number of times, and the value of each frequency __

A

is stored as a data point

58
Q

the sampling frequency is _-

A

the rate at which frequencies are sampled and digitized every second

59
Q

the number of data points (__) is determined by the __

A

frequencies recorded; frequency matrix

60
Q

if the freq matrix has 256 columns, __

A

256 frequencies are recorded during the acquisition window

61
Q

the spatial interval over which the reconstructed image repeats itself is the __ (__ equation)

A

field of view/FOV; 1/deltak = FOV

62
Q

phase gradients are turned on __ in order to fill different k space __ with data

A

every Tr; lines

63
Q

phase gradients with positive (negative) polarity aid in recording the __

A

top (bottom) half of k space

64
Q

steep (shallow) phase gradients select the __

A

outer (inner) lines of k space

65
Q

usually, k space is filled __, but there are many ways to fill it

A

in a linear fashion

66
Q

chest of drawers analogy: slice select gradient __, phase encoding gradient __, freq encoding gradient __

A

chooses which chest; chooses with drawer; choose which point in drawer to put sock

67
Q

an MR image consists of a series of __

A

pixels

68
Q

the number of pixels is determined by __

A

the size of k space

69
Q

size of k space = number of lines filled (__) and number of data points in each line (__)

A

phase matrix; frequency matrix

70
Q

FFT allows one to __

A

assign an image intensity corresponding to the amplitude of specific frequencies coming from the spatial location corresponding to the pixel whose intensity is being determined

71
Q

FFT converts time-domain information to __

A

frequency-domain information

72
Q

in kspace, within a row = __ pseudofreq, __ freq; along a column = opposite

A

same; different

73
Q

in each row of kspace, the pseudofreq data in each data point are unchanged because __

A

they result from a particular slope of phase encoding gradient

74
Q

in each row of kspace, the freq data are different because _-

A

each data point was acquired at a different time during readout while freq encoding gradient was on

75
Q

in each column of kspace, the pseudofreq data in each data point are different because __

A

they were acquired with a different slope of the phase encoding gradient

76
Q

in each column of kspace, the freq data in each data point are the same because __

A

they were acquired at the same time during readout while freq encoding gradient was on

77
Q

conjugate symmetry

A

kspace is symmetric with respect to the horizontal and vertical center lines

78
Q

data acquired in central lines contribute __, data acquired in outer lines contribute __

A

signal and contrast; resolution

79
Q

phases of same amplitude but different polarity yield __ pseudofreq

A

the same

80
Q

shallow gradient slopes __ phase differences, and the resulting signal has __, thereby contributing more to __

A

minimize; high amplitude; image intensity and contrast

81
Q

steep gradient slopes __ phase differences, resulting signal has __ and thus does not contribute much to __

A

maximize; low amplitude; signal intensity and contrast

82
Q

the presence of large phase differences indicates that two points close together in space will __. this is why __

A

be well-differentiated in the image; outer lines of kspace provide resolution information

83
Q

TR is actually the time __

A

required to get a line of kspace for each slice, before going back to fill the next line of each slice in the next TR

84
Q

the amplitude of the freq encoding gradient determines how __ k space is filled (i.e. __)

A

far to the left and right (i.e. what the FOV is in the freq direction)

85
Q

the amplitude of the phase encoding gradient determines how __

A

far up and down k space is filled

86
Q

gradient polarity determines __ k space is filled

A

the direction in which

87
Q

positive freq gradient = __ traversal

A

left to right

88
Q

negative freq gradient = __ traversal

A

right to left

89
Q

positive phase gradient = filling the __

A

top

90
Q

negative phase gradient = filling the __

A

bottom

91
Q

partial echo allows for __

A

a very short TE

92
Q

partial echo allows the freq gradient to be switched on at a normal time but have the peak signal occur __

A

earlier rather than at the middle of the acquisition window

93
Q

partial echo: because k space has conjugate symmetry, __

A

we only need to fill half of it with data and the remainder can be extrapolated

94
Q

NEX (number of excitations) indicates the __

A

fraction of kspace to fill

95
Q

the lower the NEX, the __ the scan time

A

shorter

96
Q

though scan time can be reduced by reducing NEX, __

A

fewer data are acquired and signal is weaker

97
Q

(partial averaging) one cannot extrapolate data as with partial echo, because __

A

each k space line takes a Tr to acquire and so subject movement is very likely to have occurred between the beginning and the end of scan

98
Q

in partial averaging, instead of extrapolating the data to the missing lines, __

A

they are filled with zeroes

99
Q

3 types of acquisition

A

sequential, 2D volumetric, and 3D volumetric

100
Q

sequential acquisition

A

data are acquired in order for each slice and line of k space

101
Q

2D volumetric acquisitions

A

one line of kspace is filled for all slices first, then the procedure is repeated for subsequent lines of kspace

102
Q

3D volumetric acquisitions

A

excitation pulse does not perform slice selection and the entire volume is excited instead. after all data have been acquired, the slice select gradient is turned on and slices are identified based on their phase. this is called slice encoding.

530: Data Acquisition (13 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions