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530: Data Acquisition > Chapter 5 p. 1 > Flashcards

Chapter 5 p. 1 Flashcards

1
Q

in PD + T2 spin echo sequences, which echo happens first?

A

pd

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2
Q

conventional spin echo has a long __

A

TR

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3
Q

why is reducing the NEX and TR not a desirable way to reduce scan time in SE?

A

these affect SNR and T1-weighting

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4
Q

in FSE, scan time is reduced by __, such that we acquire __, thus reducing scan time

A

performing more than one phase encoding step (different amplitudes) within one TR, more than one line of k space at a time

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5
Q

FSE involves several __ pulses, called a __

A

180 degree rephasing; echo train

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6
Q

echo train length (ETL) =

A

number of 180 pulses in the echo train

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7
Q

the echo train length and the matrix size determine __

A

how scan time is reduced

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8
Q

because FSEs are generated at different values of the TE, collected data have __

A

variable weightings

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9
Q

FSE: the scanner operator selects an __, which determines the desired weighting of the image and which is a weighting of the __

A

effective TE; individual TE values associated with each spin echo

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10
Q

FSE: when each phase encoding is performed, __ is varied such that the signal is __

A

gradient slope; phase-shifted by a different amount and signal amplitude is different

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11
Q

when very steep phase gradients are turned on, __ nuclei are affected by the gradient, so signal amplitude is __

A

fewer; low

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12
Q

when very shallow phase gradients are turned on, __nuclei are affected and signal amplitude is __

A

more; high

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13
Q

FSE: scanner software arranges the recorded signals so that __

A

kspace waveforms with low amplitude (recorded when the phase gradient was steep) are on the upper or lower edges of kspace

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14
Q

FSE sequences result in much __ acquisition times than SE due to the __

A

shorter; echo train approach to data acquisition

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15
Q

because there are many, closely-spaced RF pulses in FSE imaging, the effects of __ are reduced, resulting in __ appearing bright in T2-weighted FSE images

A

spin-spin interactions in fat; fat

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16
Q

(FSE) because each line is filled during an echo train in which data with different TE values are acquired, __ may occur at __

A

image blurring; boundaries between tissues with different T2 values

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17
Q

why does image blurring occur in FSE?

A

because echos placed on the top and bottom edges of kspace have low signal amplitude – these top/bottom signals determine signal resolution, and so images with long ETLs have increased image blurring at boundaries because resolution is lost when signal amplitude determining resolution is low

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18
Q

image blurring in FSE sequences can be reduced by __ or by __

A

decreasing the ETL; decreasing the spacing between echoes (reducing the effective TE)

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19
Q

summary of differences between SE and FSE

A

FSE: shorter scan time, spin-spin interactions of fat are reduced and fat shows up bright in t2 images, but image blurring at boundaries between tissues of different t2 values

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20
Q

the __ application of FSE is SS-FSE, where __

A

most extreme; the entire kspace is filled in one TR (in a single shot) (but really its only half)

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21
Q

in SS-FSE, __ is acquired in one TR and then __

A

half of kspace; the rest is obtained by flipping the acquired data (making use of the conjugate symmetry property of kspace

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22
Q

SS-FSE makes use of what property of k space

A

the conjugate symmetry

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23
Q

advantage of SS-FSE

A

FAST

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24
Q

disadvantages of SS-FSE

A

decrease in SNR (due to partial k space acquisition), body temperature increases (due to many 180 degree pulses transferred a lot of energy to sample)

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25
Q

to prevent SAR increases in SS-FSE, __, though this results in __

A

the 180 pulse may be replaced with a 120 pulse; decrease in SNR

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26
Q

pi =

A

180 degree

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27
Q

all the 180 pulses in SS-FSE cause __

A

an increase in SAR of the body, and subject can overheat

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28
Q

in T1 and PD-weighted FSE imaging, __ ETLs introduce too much T2-weighting, so _ ETLs should be used

A

long; shorter

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29
Q

in T2-weighted FSE imaging, __ ETLs are more useful because __

A

long; T2 contrast is stronger than T1 contrast

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30
Q

using FSE, scan times are __ in T2 imaging than in T1 imaging

A

shorter

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31
Q

Driven-equilibrium FSE (DRIVE) applies a __ to avoid having to wait for __

A

reverse flip-angle excitation pulse at the end of the echo train; longitudinal magnetization to recover

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32
Q

(DRIVE) because water has the longest relaxation constants, most of the magnetization affected by the reverse pulse is __

A

due to water and thus water has high intensity in DRIVE images

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33
Q

DRIVE imaging produces increases in signal intensity in __ structures when using TRs which are __ than traditional FSE

A

CSF-rich; shorter

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34
Q

inversion recovery sequences:

A

begin with a 180 degree inversion pulse that saturates the sample

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35
Q

IR: some time is allowed to elapse after 180 saturating pulse and before 90 pulse as M0 begins to __

A

recover its direction along Bo

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36
Q

(IR) the time from inversion (Ti/tau)

A

the time between the 180 saturation/inversion pulse and the 90 excitation pulse

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37
Q

(IR) after the 90 excitation pulse, the resulting signal is then __ and then it is __ to produce a spin echo at the echo time

A

allowed to experience FID; rephrased by a 180 pulse

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38
Q

in IR sequences, contrast is primarily dependent on the value of __

A

the Ti/tau

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39
Q

in IR, the TE is the time between

A

the 90 excitation pulse and the echo

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40
Q

(IR) if the 90 pulse is applied after the magnetization vector has relaxed through the transverse plane, image contrast depends on __; resulting image is __-weighted because the 180 pulse achieves full saturation and ensures a large contrast difference between fat and water

A

the amount of longitudinal recovery in each vector; T1-weighted

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41
Q

(IR) if the 90 pulse is applied after the magnetization vector has achieved full recovery (in longitudinal plane), image contrast depends on __ because the magnetization vectors of both fat and water have fully relaxed; resulting image is _-weighted because signal amplitude is dependent on density of spins rather than on the difference in longitudinal magnetization between fat and water

A

proton density; PD

42
Q

advantages of IR sequences

A

can get large contrast between fat and water so excellent T1-weighting, can get pathology weighting, excellent SNR (because TR is long)

43
Q

IR is a sequence of choice for what type of weighting

A

T1

44
Q

we can get pathology-weighting if the echo time is __ in IR, because tissues with __ appear bright

A

long; long T2 decay constants

45
Q

the TR should always be long in IR to allow __

A

full recovery of longitudinal magnetization before next inversion pulse

46
Q

FIR involves the use of __

A

an echo train of 180 pulses after the initial 90 pulse

47
Q

FIR allows us to fill __

A

multiple rows of kspace at once, allowing shorter scan times

48
Q

FIR is typically used in conjunction with __, so that __ appear bright

A

T2-weighting; fluid and pathology

49
Q

the two main FIR sequences are __

A

short-tau inversion recovery (STIR) and fluid-attenuated inversion recovery (FLAIR)

50
Q

in STIR, the Ti is set to __

A

the amount of time needed for fat to recover into the transverse plane

51
Q

in FIR (STIR/FLAIR), what is the null point?

A

the amount of time needed for that tissue to recover into the transverse plane (null point of fat, CSF, gray matter, white matter)

52
Q

because of nulling _-rich tissues appear dark in STIR

A

fat

53
Q

main uses of STIR

A

(nulls fatty bone marrow) often used to identify bone bruises, fractures, and tumors

54
Q

FLAIR - Ti/tau is equal to __

A

the time needed for CSF to recover into the transverse plane (from 180 to 90 degrees)

55
Q

nulling: when the 90-degree excitation pulse is applied, whatever matter is nulled has no signal because

A

the transverse component becomes a completely longitudinal component and there is no signal

56
Q

FLAIR is used to suppress __ signal in __-weighted images so that __-adjacent pathology (__) can be seen as hyperintense

A

CSF; T2; CSF; edema

57
Q

uses of FLAIR imaging

A

useful for TBI, visualizing periventricular and spinal cord lesions, MS plaques, sub-arachnoid hemorrhages and brain swelling due to meningitis

58
Q

there are other types of FIR sequences, aimed at nulling signal due to __

A

white matter, muscle, etc.

59
Q

FIR sequences which null white matter are used to detect __

A

periventricular leukomalacia, a type of white matter necrosis which can occur in preterm infants

60
Q

GE: instead of using 180 pulse like spin echo to dephase/rephase signal, __

A

first dephase using a freq encoding gradient, then flip the gradient polarity to product the echo

61
Q

because GE sequences can involve small flip angles, __ images can be acquired using __ TR values

A

T2* and PD; short

62
Q

fundamental innovation of GE sequences

A

use variable flip angles and gradient to produce dephasing and rephasing faster than SE, reduce scan time

63
Q

GE: __ determines the degree of saturation and therefore T1 weighting

A

flip angle and TR

64
Q

GE: to prevent saturation, which is necessary in __ -weighting, the flip angle should be __ and the TR __

A

T2* and PD; short; long enough to permit full recovery (with a short flip angle, this “long” TR is still shorter than SE)

65
Q

GE: if saturation and therefore _-weighting is required, the flip angle should be __ and the TR __, so that __

A

T1; large; short; full recovery cannot occur

66
Q

GE: the TE controls the amount of __

A

T2* dephasing

67
Q

GE: to minimize T2*, the TE should be __

A

short (to maximize it should be long)

68
Q

GE T1 weighting: __ flip angle, __ TR, and __ TE

A

large (to maximize saturation); short (to maximize saturation); short (to minimize T2*)

69
Q

GE T2* weighting: __ flip angle, __ TR, and __ TE

A

small (to minimize saturation); long (to minimize saturation); long (to maximize T2*)

70
Q

GE PD weighting: __ flip angle, __ TR, and __ TE

A

small (to minimize saturation); long (to minimize saturation); short (to minimize T2*)

71
Q

the the steady state, the amount of energy imparted to a system is __

A

equal to the amount of energy flowing out of the system

72
Q

in GE imaging, the __ (2) determine the amount of energy imparted to the protons as well as the amount of energy allowed to dissipate into the lattice, so one can __

A

flip angle and TR; choose the right combination of flip angle and TR so that the spin-lattice system remains in a steady state as a function of time

73
Q

RF pulses have __ energy, ad thus __ values of the TR are needed to achieve a steady state

A

low; short

74
Q

because TR is short in steady-state sequences, there is always __ at the end of each repetition

A

residual transverse magnetization

75
Q

in steady-state sequences, the TR is so short that __

A

the magnetization does not have enough time to reach its T1 and T2 values before the end of each repetition

76
Q

steady-state image contrast is not due to difference in __ between tissues, but rather __

A

T1 or T2; the ratio of T1/T2

77
Q

in steady-state images, tissues where T1 and T2 are very similar (like water) will appear __

A

bright

78
Q

in the human body, the T1/T2 ratio is high for __ (2) compared to other tissues, so they appear brighter

A

water and fat

79
Q

because the Tr is very short in steady-state sequences, __

A

most GE sequences use a steady-state design

80
Q

GE sequences are classified depending on whether __

A

the residual transverse magnetization of protons at the beginning of each repetition is in phase (coherent GE) or out of phase (incoherent GE)

81
Q

in steady state, residual transverse magnetization is __

A

rephrased by an RF pulse to generate a SE, which occurs at precisely the time of the next RF pulse because it takes as much time to rephrase spins as it does to dephase them

82
Q

the TR in steady state is equal to

A

the tau of the spin echo

83
Q

in steady state, the __ occurs as a result of the initial pulse, and the __ occurs as a results of the second pulse

A

FID; Hahn or stimulated echo

84
Q

two consecutive RF pulses produce a __ in the steady state

A

stimulated echo

85
Q

in steady state, because the TR between RF pulses is shorter than __, there is residual magnetization after each RF pulse, and this magnetization is __

A

the T1 and T2 of the tissues; excited subsequent to each TR

86
Q

(steady state) the first RF pulse excited nuclei __

A

regardless of its net amplitude

87
Q

(steady state) the second RF pulse __

A

rephases the FID nuclei and the residual magnetization to generate a SE

88
Q

(steady state) if both RF pulses have a flip angle of 90 degrees, they generate __ (otherwise they produce __)

A

Hahn echoes; stimulated echoes

89
Q

coherent GE sequences use an RF pulse with a __ followed by __ to produce a GE

A

variable flip angle; gradient rephasing

90
Q

coherent GE sequences involve a steady state because __ and there is __

A

the TR is short; residual transverse magnetization when the next excitation pulse is delivered

91
Q

(coherent GE) the residual magnetization is kept in phase (coherent) via __ , where __

A

rewinding; the slope of the phase encoding gradient is reversed after readout

92
Q

rewinding results in the __ so that __

A

rephasing of residual magnetization; in phase (coherent) at the beginning of each repetition

93
Q

4 advantages of coherent GE

A

very short TR and thus scan time; water is bright so good T2*; useful for angiographic, myelographic, or arthorgraphic imaging; excellent for determining whether brain vessels are occluded or whether there is fluid accumulation in certain brain areas

94
Q

3 drawbacks of coherent GE

A

SNR is low when images are acquired 2Dlly; increased magnetic susceptibility artifacts; loud (due to gradient noise)

95
Q

spoiled/incoherent GE sequences involve a __ and __

A

variable flip angle excitation pulse and rephrasing

96
Q

residual magnetization is spoiled so that __

A

its effect on image contrast is minimal

97
Q

RF spoiling: when the signal produced by the second pulse is rephrased, __

A

there is some residual signal from the first pulse

98
Q

the two overlapping signals in RF spoiling are kept separate because __, and the signal due to the dephasing residual magnetization is not recorded

A

the two signals are at different frequencies and phases

99
Q

RF spoiling allows only information from the __

A

most recent signal to affect image contrast

100
Q

4 advantages of spoiled GE

A

accommodates 2D and 3D imaging; short TR so short scans; good SNR and anatomical detail; amenable to the use of contrast agents

101
Q

2 drawbacks of spoiled GE

A

low SNR when images are acquire 2Dlly (same as coherent GE); loud (due to gradient noise) (also same as coherent GE)

102
Q

what disadvantage does coherent GE have that spoiled GE does not?

A

greater magnetic susceptibility artifacts

530: Data Acquisition (13 decks)

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