Brainscape's Knowledge GenomeTM

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


See full index

530: Data Acquisition > Chapter 7 > Flashcards

Chapter 7 Flashcards

1
Q

phase mis-mapping (aka __) refers to the __ across an image in the __ direction

A

ghosting; replication of moving anatomy; phase-encoding

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

ghosting is typically due to __ (4)

A

periodic or quasi-periodic movements of the patient during scan; pulsatile movement of vessels and CSF; swallowing; eye movement

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

when ghosting is present in __ imaging, the __ direction can be determined by __

A

sequential; phase-encoding; visual inspection

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

ghosting occurs along the __ direction because that gradient has a __ whereas the __ gradient has __

A

phase-encoding; different amplitude during each TR; frequency encoding; same value during each TR

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

(ghosting) as anatomy moves, the scan is __ in the __ direction when each line of k space data are __

A

misplaced; phase-encoding; acquired within one TR

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

image from slide 3

A

— (he’s going to give an image and ask us to name the artifact)

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

when signal readout and frequency are done __ (which is often the case), there is a time delay between __ and __

A

simultaneously; phase encoding and signal readout

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

if anatomy has moved between __ and __, that movement will affect how the signal __

A

phase encoding and readout; is read into k space

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

no ghosting occurs in the __ direction because that type of encoding is done __

A

frequency encoding; simultaneously with the data readout and digitization processes

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

image from slide 4

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

one common remedy for ghosting involves swapping __ so that the direction of __ is not the same as the direction in which movement occurs

A

phase and frequency; phase encoding

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

in sagittal spine imaging, __ and __ along the phase encoding gradient produce ghosting

A

swallowing and pulsatile motion of the carotid arteries

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

in sagittal spine imaging, frequency encoding is usually performed by the __ gradient because this is the __

A

Z; longest axis of the patient in the sagittal plane

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

in sagittal spine imaging, freq encoding = Z, phase encoding is then along the __

A

anterior-posterior axis (Y)

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

in sagittal spine imaging, ghosting artifacts can be alleviated if the __ gradient is chosen to perform frequency encoding, and the __ gradient is chosen to perform phase encoding

A

Y; Z

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

images from slide 6

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

pre-saturation nulls the signal from __ so that __ are minimized

A

flowing nuclei; flow effects

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

pre-saturation: first a __ pulse is delivered to a volume of tissue __

A

90-degree; outside the FOV

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

nuclei which flow into the FOV following the pre-saturation pulse receive a 90-degree pulse and are thus __, producing __

A

saturated; no measurable signal

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

pre-saturation: if the second pulse is delivered within the FOV, __ will have their signal nulled

A

any flowing nuclei in it

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

__ produces ghosting along the __ axis in sagittal scans of the spinal cord

A

swallowing; phase

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

(sagittal scan of spinal cord): applying a pre-saturation pulse over the __ reduces __

A

throat; ghosting due to swallowing and blood flow

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

pre-saturation is most effective when __

A

placed between flow origin and FOV

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

image from slide 7

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

aliasing (aka __) occurs when __

A

wrap-around; anatomy outside the FOV is folded into the FOV

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

in sagittal imaging of the brain, the FOV in the __ direction is smaller than the __ dimension of the head, and signal __ the FOV is aliased

A

phase; anterior-posterior; outside

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

aliasing: anatomy outside the FOV still __. this must still be __ if it is sampled, i.e. __

A

produces signal if it is very close to the receiver coil; encoded; allocated a pixel position

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

aliasing: if data __ occurs, the signal is mis-mapped __ rather than __

A

under-sampling; into the FOV; outside

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

aliasing can occur along both __ axes

A

frequency and phase

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

image from slide 8

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

frequency wrap is __

A

aliasing along the frequency encoding direction

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

frequency wrap is caused by __

A

under-sampling frequencies present in the echo which are caused by signal recorded either inside or outside the FOV

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

if Nyquist’s theorem is not obeyed and __, signal from anatomy __ the FOV is mapped __ the FOV

A

frequencies are undersampled; outside; inside

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

frequency wrap can be corrected by __ so that __

A

increasing the sampling frequency; Nyquist’s rule is obeyed

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

image from slide 9

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

phase wrap is __ and it is caused by __

A

aliasing along the phase encoding direction; under-sampling along this direction

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

the signal associated with each __ must be mapped into the FOV, and signal are allocated __ according to their __

A

precessional phase value (from 0 to pi radians); phase values; position along the phase curve

38
Q

image from slide 10

A
39
Q

phase wrap: because phase repeats, signal originating __ the FOV in the __ direction are allocated __ which have already been assigned to signal which originates __ the FOV (on the opposite side)

A

outside; phase direction; phase values; inside

40
Q

the rate of phase repeat is __

A

the pseudo-frequency

41
Q

in abdomen imaging, the FOV in the __ axis of the image (__) is smaller than the anatomy

A

phase; right to left

42
Q

abdomen imaging: signal originating in anatomy to the left of the FOV has the same phase as __. signal originating in anatomy to the right of the FOV has the same phase as __. this is why these regions are mapped __ rather than not mapped at all

A

the rightmost portion of the FOV; the leftmost portion of the FOV; within the FOV

43
Q

images from slide 11

A
44
Q

aliasing along the frequency direction can be reduced by __(4)

A

increasing the FOV; pre-saturating the areas outside FOV to null signal; increasing the sampling rate so that all frequencies are digitized properly; applying a digital filter so that only frequencies of interest are digitized, and frequencies outside the FOV are ignored

45
Q

one way to reduce frequency aliasing is increasing the __ but this causes a loss of __

A

FOV; spatial resolution

46
Q

one way to reduce frequency aliasing is to __ areas outside the FOV to __, but this is __

A

pre-saturating; null signal; time-consuming

47
Q

one way to reduce frequency aliasing is to increase the __ so that all frequencies are __, but this causes a decrease in __ due to increase in __

A

sampling rate; digitized properly; SNR; noise

48
Q

one way to reduce frequency aliasing is to __ so that only frequencies __ (__) are digitized, and frequencies __ are ignored. analogy: __. drawback: __

A

apply a digital filter; of interest (due to signals within the FOV); outside the FOV; filtering bass and treble on a music system using an equalizer; Gibbs ringing artifact

49
Q

apply a digital filter to reduce freq. aliasing can cause a __ artifact at __, resulting in artifactual increases in __ at the __ of the FOV

A

Gibbs ringing artifact; edges of the filter; image intensities; boundaries

50
Q

image from slide 12

A
51
Q

phase anti-aliasing is also called __ (3)

A

no-phase wrap, phase over-sampling, or anti-foldover

52
Q

phase anti-aliasing involves __ the original FOV to __ and prevent aliasing

A

extending; encompass all anatomy

53
Q

phase anti-aliasing: because enlarging the FOV while keeping all else constant results in loss of __, the __ is increased to keep this constant

A

spatial resolution; number of phase encodings

54
Q

obvious disadvantages of increasing the number of phase encodings in phase anti-aliasing (2)

A

increase in scan time; and if you want scan time to be constant you must decrease the NEX

55
Q

phase anti-aliasing: if NEX is reduced, only the __ is kept

A

anatomy image associated with the original FOV**

56
Q

phase anti-aliasing: reducing the NEX results in more __… there is no __

A

motion artifact; silver bullet

57
Q

images from slide 13

A
58
Q

truncation artifact produces a __ at the interface between __ (i.e. at the __)

A

banding effect; high- and low-intensity signal; at the edges of the brain in a T1-weighted scan

59
Q

the cause of truncation artifact is __, to the effect that __ (__) are poorly represented

A

under-sampling of data in k-space; contours (lines at the transition from high to low intensity)

60
Q

truncation artifact involves bands of __ running through __

A

low-intensity artifact; high-intensity regions

61
Q

to avoid truncation artifact, the __ must be increased

A

number of phase encoding steps

62
Q

image from slide 14

A
63
Q

magnetic susceptibility is a __ which indicates how susceptible they are to __

A

physical property of materials; becoming magnetized

64
Q

magnetic susceptibility artifacts result in partial or complete __ in the region where __ is located

A

loss of signal; an object made of magnetically-susceptible material

65
Q

__ materials have very high susceptibilities and cause __

A

ferromagnetic; distortion

66
Q

susceptibility artifacts are more common in __ sequences because __ followed by __ does not compensate for __

A

GE; gradient dephasing; rephasing; FID-related T2* effects

67
Q

to reduce susceptibility artifacts, one should __, use __ sequences, and/or decrease __ to minimize __

A

remove metal objects; SE; TE; dephasing between tissues with susceptibility differences

68
Q

3 ways to reduce susceptibility artifacts

A

remove metal objects; use SE sequences; decrease TE

69
Q

image from slide 15

A
70
Q

cross-excitation occurs when __

A

adjacent slices have different contrasts

71
Q

adjacent slices have different contrasts because RF pulse is __ and if there is no __ then adjacent RF pulses can __, resulting in __ across adjacent slices (hence the name ‘cross-excitation’)

A

fully rectangular; slice gap; overlap; excessive excitation

72
Q

when cross-excitation occurs, the __ of protons at the __ of slices are tipped farther away from the __, which may cause __

A

magnetization vector; edges; direction of the static field; partial saturation

73
Q

cross-talk involves nuclei __ during recovery of the __ (due to __ interactions)

A

losing energy to protons in adjacent slices; longitudinal magnetization; spin-lattice

74
Q

images from slide 17

A
75
Q

cross-talk __ (can/cannot) be eliminated because it is __

A

cannot; an uncontrollable natural phenomenon

76
Q

cross-talk — cross-excitation (which can be reduced/prevented?)

A

cross-excitation

77
Q

one way of reducing cross-excitation: implement two scans, one where you __ and another when you __ (called __)

A

excite all odd slices first; excite even slices; interleaving/concatenation

78
Q

interleaving/concatenation ensures that __

A

slices do not overlap at all while they are excited

79
Q

interleaving has the benefit of __ however it results in __

A

not having to use a slice gap; longer scans

80
Q

one way of reducing cross-excitation: __ first, then __, the again __, and so on until all volume has been imaged

A

excite a few odd slices; excite a few even slices; a few odd slices

81
Q

alternating odd/even slices being excited results in __ because it takes about __ for __

A

a faster scan; TR/2; cross-excitation of slices to decay

82
Q

alternating odd/even slices being excited: disadvantage is that __ is needed to reduce cross-excitation effects

A

a slice gap of at least about 30% the slice thickness

83
Q

two ways to prevent cross-excitation

A

interleaving; alternating exciting a few odd slices, then a few even, then a few odd, etc.

84
Q

zipper artifacts occur when __

A

a dense line appear in the image at a specific point

85
Q

the cause of zipper artifact is the presence of __, which allows __ to interfere with the RF pulse emitted by the scanner

A

a leak in the RF shielding of the MR room; RF energy from electrical sources outside the room

86
Q

remedy of zipper artifact: __ (may be __)

A

locate the RF shielding leak and repair it (expensive)

87
Q

shading refers to __

A

differences in intensity across the image volume

88
Q

3 causes of shading

A

uneven excitation of nuclei across the sample; undesirably close proximity of the anatomy to the receiver coil, causing ‘coupling’ or even contact of the receiver coil and anatomy; inhomogeneities in the magnetic field (especially at high field)

89
Q

4 remedies of shading

A

reposition the patient/use padding around the body; apply shimming; ensure that the coil is of appropriate size; apply from pre-scan pulses to determine the correct RF frequency and amplitude

90
Q

image from slide 20

A
91
Q

what is coupling of the coil and anatomy and how does it cause shading?

A

530: Data Acquisition (13 decks)

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