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530: Data Acquisition > Chapter 5 p. 2 > Flashcards

Chapter 5 p. 2 Flashcards

1
Q

SSFP stands for

A

steady state free precession

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2
Q

the SSFP method overcomes the problem of __ (__) in GE sequences and allows one to obtain __

A

inhomogeneity artifacts (T2* weighting); true T2 images

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3
Q

in SSFP, signals are digitized only from the __, not from the __; this is down by repositioning the __ so that it does not occur at the same time as the __

A

stimulated acho; FID; following excitation pulse

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4
Q

repositioning is achieved by applying a __, which speeds up __ so that the __ occurs sooner

A

rewinding gradient; rephrasing; stimulated echo

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5
Q

SSFP: the resulting signal has more __ than regular GE

A

T2 weighting

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6
Q

there are 2 Te values

A

the actual TE (from echo to next RF); effective TE (from echo to RF pulse which created the FID)

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7
Q

TEeff = __

A

2TR - TE

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8
Q

image from slide 35

A
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9
Q

the longer the __ TE the lower the __ TE

A

actual; effective

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10
Q

advantages of SSFP (2)

A

signal rephasing is initiated by an RF pulse rather than by a gradient, so the images have more true T2 weighting than conventional GE sequences; scans can be 2D or 3D

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11
Q

disadvantages of SSFP (3)

A

susceptibility to artifacts; image quality may be poor; loud gradient noise due to very frequent gradient application

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12
Q

images from slide 37 (not coherent or spoiled images)

A
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13
Q

coherent GE sequences sample __ to produce T1 or T2 weighting depending on __

A

both the FID and the stimulated echo; the TE

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14
Q

incoherent (spoiled) GE sequences sample __ to produce mainly __ images

A

the FID only; T1-weighted

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15
Q

SSFP sequences sample __ which is done before the __ to produce images which are more __

A

only the stimulated echo; FID; T2-weighted

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16
Q

image from slide 38 (same as slide 37)

A
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17
Q

in balanced GE, a sequence of gradients with __ is used to correct for __ in __ (2)

A

opposite polarities; phase inhomogeneities; CSF or flowing blood

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18
Q

moving H nuclei accumulate a __ as they pass along the gradients, which means that nuclei in __ (2) are __ and thus achieve high __

A

zero phase change; CSF and blood; coherent; signal intensity

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19
Q

balanced GE: the gradients are applied in the __ directions

A

slice and frequency encoding

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20
Q

balanced GE: larger __ and shorter __ are used compared to in coherent GE, resulting in higher __ and shorter __

A

flip angles; TR values; SNR; scan times

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21
Q

image from slide 39

A
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22
Q

balanced GE: to avoid saturation occurring due to __ (which increases __ contrast), the excitation phase pulse is __

A

large flip angles; T1; changed every TR

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23
Q

avoiding saturation is achieved by using a flip angle of __ in the first TR and then a __ pulse in the succeeding TR

A

45 degrees; 90-degree pulse

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24
Q

balanced GE: alternating pulses allows the transverse magnetization to __ after each repetition, and thus __ is avoided

A

have opposite phases; saturation

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25
Q

balanced GE, avoiding saturation: tissues with __ will be bright, resulting in high __, good __, and short __

A

T1/T2 ratios approaching unity; SNR; CNR between fat, water, and surrounding tissues; scan times

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26
Q

image from slide 40

A
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27
Q

balance GE is used for __ (3)

A

spine imaging; imaging of the auditory canal, and imaging of structures where CSF flow is present

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28
Q

balanced GE is also used to image __

A

blood vessels in the brain, heart, etc.

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29
Q

balanced GE requires a short __ to reduce __ and __ as well as a long __ to enhance __

A

TR; scan time and flow artifact; TE; T2* weighting

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30
Q

fast GE sequences are either __ or __, where the __ is substantially shorter than in conventional ones

A

coherent or spoiled GE sequences; echo time

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31
Q

FGE: only __ is applied, because it takes less time to __, and after this is done only __ (as in __)

A

part of the RF excitation pulse; turn on and off an RF pulse; part of the echo is read (as in partial echo imaging)

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32
Q

FGE: because only part of the RF pulse is applied during each repetition, __ are applied __ of the sequence in order to __ (__0

A

some extra pulses; before the beginning; pre-magnetize the tissue (load it with RF energy)

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33
Q

FGE: there are several ways to pre-magnetize the sample (2):

A

an 180-degree pulse is applied to completely saturate the sample; driven equilibrium approach

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34
Q

driven equilibrium approach

A

a 90/180/90 degree combination is applied before the pulse sequence

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35
Q

FGE: driven equilibrium approach: the first pulse __, the second pulse __, the third pulse __, so that __ when the pulse sequence begins

A

generates transverse magnetization; rephases the transverse magnetization; drives the transverse magnetization into the longitudinal plane, so that it is available when the pulse sequence begins

36
Q

FGE: k space must be filled efficiently to __, and there are 6 types

A

minimize scan time; centric, keyhole, single-shot, spiral , elliptic, and propeller filling

37
Q

FGE: in centric filling, k space is filled __. the __ lines are filled first by applying the __ phase gradients, leaving the __ gradients for the __ of the sequence

A

linearly (line by line); central; shallowest; steepest; end

38
Q

FGE: centric filling allows both __ and __ to have reasonable quality and is important in FGE because __

A

signal and contrast; the SNR is comparatively low in FGE

39
Q

image from slide 43

A
40
Q

FGE: in keyhole filling, k space is filled __ but the __ lines are filled __

A

linearly; central; certain latencies after the beginning of the sequence

41
Q

FGE: keyhole filling is employed primarily in conjunction with __, because __

A

contrast agents; one must wait for the agent to reach the tissues whose signal intensity needs to be enhanced

42
Q

image from slide 44

A
43
Q

FGE: when SS imaging (EPI) is done, __

A

all k-space is filled within one repetition

44
Q

FGE: in SS (EPI), echo trains may consist of __ (as in __) or a train of __, in a sequence called __

A

spin echoes (as in SS-FSE); gradient echoes; echo-planar imaging (EPI)

45
Q

in EPI, __ are generated and each phase is encoded __ to fill all the lines of k space in one repetition

A

multiple echoes; by a different slope

46
Q

image from slide 45

A
47
Q

FGE: spiral filling is used when __

A

signal amplitude is paramount

48
Q

FGE: elliptic filling is used in certain sequences aiming to __

A

emphasize blood flow

49
Q

FGE: propeller filling increases __ because __

A

SNR and CNR; the origin is traversed repeatedly in each repetition

50
Q

images from slide 46

A
51
Q

in EPI, gradient echoes are generated by __

A

oscillation (ramping up) of the readout gradient

52
Q

EPI: different contrasts are achieved by either beginning the sequence using __ or with a set of __

A

a variable RF excitation pulse called a GE-EPI; 90- and 180-degree RF pulses called SE-EPI

53
Q

GE-EPI begins with a __ followed by __

A

variable flip angle; phase/frequency encoding

54
Q

SE-EPI begins with a __, followed by __, and then a __, followed by __

A

90-degree RF pulse; phase and frequency encoding; 180-degree pulse; echo trains

55
Q

image from slide 47

A
56
Q

because all k space is filled at once during EPI imaging, the repetition time is __. this is also the case for other sequences where all k space is acquired at once

A

said to be “infinitely long”

57
Q

EPI: because the TR is infinitely long, __ is not critical and thus the __ ceases to be a parameter which determines __. for this reason, __ weighting are achieved by selecting either a short or a long TE, respectively

A

the recovery rate of tissues; TR; weighting; PD and T2

58
Q

EPI: PD requires a __ TE

A

short

59
Q

EPI: TE requires a __ TE

A

long

60
Q

in EPI, the TE is the time interval between __

A

the excitation pulse and the moment when the center of k space is filled

61
Q

in EPI, T1 weighting can only be achieved by first __ before __

A

saturating the sample; the excitation pulse is applied

62
Q

why did people try to combine SE and GE?

A

because they have complementary advantages

63
Q

GRASE sequences

A

hybrid sequences combined gradient echoes and spin echoes

64
Q

GRASE sequences use a __ followed by a __

A

RF pulse; series of gradient rephrasing steps

65
Q

benefits of GRASE (2)

A

ability to use gradient dephasing/rephrasing (as in GE) and the ability to use RF pulses to compensate for T2* effects (as in SE)

66
Q

GRASE sequences can have scan times __ but __

A

which are a bit longer than steady state sequences; image quality is often higher

67
Q

image from slide 49

A
68
Q

EPI and GRASE sequences are often used for fMRI because

A

they are relatively fast and they can be designed to be sensitive to rapidly changing concentration in hemoglobin

69
Q

the BOLD signal (__) is usually measured using __ sequences, increasing the popularity of these sequences

A

blood oxygen level dependent; EPI

70
Q

in EPI, __ can vary so much that __ can occur

A

gradient polarities and amplitudes; peripheral nerve stimulation

71
Q

why/how does EPI cause peripheral nerve stimulation?

A

— changing the gradient polarities and amplitudes could trigger action potentials

72
Q

in EPI, patients should wear __ because of __

A

ear plugs and noise-cancelling headphones; very severe gradient noise

73
Q

other disadvantages of EPI (2)

A

blurring and ghosting

74
Q

images from slide 51

A
75
Q

parallel imaging (aka __) involves filling kspace __ (as in __)

A

sensitivity encoding; several lines at a time (as in FSE) — also involves aliasing

76
Q

the difference between parallel imaging and FSE is that __

A

different lines of kspace are acquired by different coils in the magnet

77
Q

parallel imaging: because each coil records __, the scan time is reduced by an amount equal to __, called the __

A

a different line; the number of coils; acceleration factor

78
Q

image from slide 52

A
79
Q

when each coil acquires a line of k space, the gap (delta k) between lines is __

A

x (= # of coils) times larger than it would be if the lines were acquired using one coil

80
Q

the data spacing (gap) parameter delta k is similar to the __ in our chest analogy, which means that the __ has increased by a factor of __

A

drawer depth; drawer depth; x (= # of coils)

81
Q

(gap) delta k = __. this means that the __ has narrowed by a factor of _

A

1/FOV; FOV; x (= # of coils)

82
Q

because FOV is narrowed, tissue __ the FOV is __ and the image is __

A

outside; aliased; ‘wrapped around’

83
Q

aliasing can be corrected via a sophisticated calculation which relates the acquired signal location to the __ and __ signals by a different factor in order to ‘unwrap’ the image

A

coil position; weighs

84
Q

parallel imaging techniques: there is a lot of them, they are complex both conceptually and and programmatically, therefore __, but they are __

A

it can take years for MR physicists and researchers to become familiar with fine details and subtle differences between them; commonly used

85
Q

image from slide 54

A

— at least know the first column (the ones we’ve talked about)

530: Data Acquisition (13 decks)

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