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MRI AllinOne > Sequence Parameters & Options > Flashcards

Sequence Parameters & Options Flashcards

1
Q

What is the difference in SNR between 2 adjacent pixels (eye’s ability to detect difference)

A

CNR

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2
Q

Which parameter has the greatest influence on image quality

A

CNR

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3
Q

CNR is controlled by all the same factors as what

A

SNR

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4
Q

What is the ratio of signal amplitude to average noise amplitude called

A

SNR

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5
Q

Is signal predictable

A

yes

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6
Q

The induced voltage at the receiver coil is referred to as what

A

signal

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7
Q

Is noise predictible

A

no, it’s random

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8
Q

What is noise dependent on

A

body habitus & electrical noise of the system

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9
Q

What parameter has the greatest influence on SNR

A

size of FOV

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10
Q

What is the ability of the imaging system to detect 2 points as separate

A

spatial resolution

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11
Q

Which pixel types give better spatial resolution: square or rectangular

A

square

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12
Q

What is the amount of tissue within a 3 D volume called

A

voxel volume

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13
Q

What determines voxel volume

A

FOV, matrix & slice thickness

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14
Q

What determines spatial resolution

A

voxel volume only

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15
Q

Formula for voxel volume

A

pixel phase x pixel frequency x slice thickness

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16
Q

What is the voxel volume when using a 24cm FOV, a 256x128 matrix & 3mm slices?

A

(240mm/256) x (240mm/128) x 3 = 5.27mm³

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17
Q

Which provide better spatial resolution - square or rectangular pixels

A

square

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18
Q

Which pixel type is better for reformatting 2D/3D images - square or rectangular

A

square

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19
Q

What is the amount of time it takes to fill k-space called

A

acquisition time

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20
Q

What has the greatest impact on the amount of patient motion detected on an image

A

acquisition time

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21
Q

What parameters affect acquisition time

A

TR, NSA/NEX, phase matrix, ETL & # of slices (during 3D only)

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22
Q

What is the formula to calculate 2D acquisition time for conventional sequences

A

TR x NSA x Phase encodings

500ms x 1 x 256 = 128,000ms = 128sec = 2min 8 sec

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23
Q

What is the formula to calculate 2D acquisition time for fast sequences

A

(TR x NSA x Phase encodings)/ETL

(500ms x 1 x 256)/4 = 32,000ms = 32sec

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24
Q

What is the formula to calculate 3D acquisition time for conventional sequences

A

TR x NSA x Phase encodings x slice #

500ms x 1 x 256 x 24 = 3,072,000ms = 3,072sec = 51min 12sec

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25
Q

List the 7 intrinsic parameters

A 
T1 recovery
T2 decay
proton density
flow
ADC
perfusion
diffusion
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26
Q

List the 8 extrinsic parameters

A 
TR
TE
flip angle
TI
ETL
b value
FOV
matrix
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27
Q

What are the 3 types of image contrast

A

T1
T2
PD

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28
Q

Which type of contrast appears as a difference in signal intensities between tissues with varying hydrogen proton concentrations

A

proton density contrast

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29
Q

What is the objective when trying to achieve T1 weighting

A

prevent recovery and decay

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30
Q

What is the objective when trying to achieve T2 weighting

A

allow recovery and decay

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31
Q

What are the TR & TE requirements for a T1 weighted image

A

short TR & TE

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32
Q

What are the TR & TE requirements for a T2 weighted image

A

long TR & TE

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33
Q

T1 images are dependent upon the differences between what

A

the T1 relaxation times of fat & water

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34
Q

T2 images are dependent upon the differences between what

A

the T2 decay times of fat & water

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35
Q

Proton density images are dependent upon the differences between what

A

the number of mobile hydrogen protons within adjacent tissues

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36
Q

What is the objective when trying to achieve PD weighting

A

allow recovery & prevent decay

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37
Q

What are the TR & TE requirements for a PD weighted image

A

long TR & short TE

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38
Q

What are the TR & TE requirements for a T2* weighted image

A

long TR & TE with use of a gradient echo sequence

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39
Q

T2* images are dependent upon what

A

T2 decay & magnetic field inhomogeneities

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40
Q

What controls T2* weighting

A

TE

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41
Q

What is TR

A

repetition time - the time between alpha pulses, measured in ms

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42
Q

What is TE

A

echo time - time between alpha pulse and peak of the echo, measured in ms

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43
Q

What is TI

A

time of inversion - time between 180° inversion pulse and 90° alpha pulse that nulls signal from specific tissues during IR sequence

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44
Q

What controls the amount of T1 contrast on T1 IR pulse sequences

A

TI & TR

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45
Q

What controls which tissues will be nulled on T2 IR pulse sequences

A

TI

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46
Q

Define NSA

A

number of signals averaged - number of times data is collected per TR period

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47
Q

How are NSA & scan time related

A

directly proportional: double NSA=double time

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48
Q

How are NSA & SNR related

A

square root relationship:
2 x NSA = 41% SNR increase (√2 = 1.41),
4 x NSA = 100% SNR increase (√4 = 2)

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49
Q

What is the angle of the NMV to the direction of the main magnetic field called

A

flip angle

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50
Q

Which flip angles produce more signal

A

those closer to 90°

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51
Q

What is flip angle controlled by

A

the amplitude and duration of incoming RF pulses

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52
Q

Define FOV

A

area of anatomy covered in an image

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53
Q

What does FOV have the greatest impact on

A

SNR

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54
Q

What relationship does FOV have with SNR

A

FOV has a directly squared relationship with SNR:
2 x FOV = 4 times the signal (2² = 4)
1/2 the FOV = 1/4th the signal (0.5² =.25)

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55
Q

What is the total number of pixels in an image called

A

the matrix

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56
Q

Define phase matrix

A

of pixels in phase direction (has a direct affect on scan time)

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57
Q

Which matrix affects scan time (phase or frequency)

A

phase

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58
Q

The phase matrix affects what

A

scan time

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59
Q

Define frequency matrix

A

of pixels in frequency direction (has no affect on scan time)

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60
Q

What parameter limits the number of slices allowed

A

TR

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61
Q

What factors determine how many slices are allowed

A

TR & the systems SAR limitations

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62
Q

When does slice number affect scan time

A

only during 3D volumetric imaging

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63
Q

Does a higher TR produce more or less RF pulses

A

less

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64
Q

Does a higher TR produce more or less time between RF pulses

A

more

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65
Q

If you increase the TR, does more or less tissue heating occur

A

less

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66
Q

Will a higher TR increase or decrease the # of allowable slices

A

increase, because less tissue heating will occur which keeps the system below its SAR limits

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67
Q

What determines slice thickness

A

the slope of the slice select gradient and the transmitted bandwidth

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68
Q

How are thin slices produced

A

with a steep slice select gradient slope and/or a narrow transmit bandwidth

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69
Q

How are thick slices produced

A

with a shallow slice select gradient slope and/or a broad transmit bandwidth

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70
Q

How is gap determined

A

by the thickness of the slice as well as the corresponding slice select gradient slope

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71
Q

What is gap important for reducing

A

image artifact (cross excitation)

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72
Q

The number of times the echo is sampled per TR period during SE pulse sequences is called

A

ETL

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73
Q

ETL corresponds to what

A

the number of rephasing 180° RF pulses applied

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74
Q

When can effective TE be chosen

A

during fast spin echo techniques (since you can’t choose the TE value for each echo in the train of 180° RF rephasing pulses)

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75
Q

In a fast spin echo technique, echos closest to the effective TE selected are place where in K space

A

the center of K space (where they affect image contrast)

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76
Q

In a fast spin echo technique, echos not close to the effective TE selected are place where in K space

A

the periphery of K space (where they affect spatial resolution)

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77
Q

During FSE sequences, effective TE is a factor that determines what

A

image weighting

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78
Q

Values in the middle of K space affect what

A

image contrast

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79
Q

Values in the periphery of K space affect what

A

spatial resolution

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80
Q

The range of frequencies transmitted in an RF pulse is called

A

transmit bandwidth

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81
Q

Which bandwidth (transmit/receive) is automatically selected by the system upon slice thickness selection

A

transmit

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82
Q

The range of frequencies sampled during the time that the readout/frequency gradient is active is called

A

receive bandwidth

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83
Q

Receive bandwidth has what kind of relationship with SNR

A 
square root (+/- √ of the increase or decrease factor)
2 x receive bandwidth = 41% signal loss (-√ 2)
1/2 x receive bandwidth = 41% signal gain (+√ 2)
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84
Q

Does receive bandwidth affect the minimum TE

A

yes

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85
Q

If you decrease the receive bandwidth, what happens to the minimum TE that is obtainable during a pulse sequence

A

the minimum obtainable TE increases

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86
Q

If you decrease the receive bandwidth, the minimum TE obtainable increases - what image weighting could this affect

A

T1 (need short TE)

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87
Q

The parameter that divides a sequence into multiple acquisitions is called

A

concatenations

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88
Q

If you increase concatenations, what affect does it have on TR

A

allows the use of a lower TR (limited by SAR)

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89
Q

Increasing concatenations has what affect on motion artifact

A

reduces it (bc it shortens acquisition time)

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90
Q

What is the time between alpha pulses, measured in ms called

A

TR

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91
Q

What is the time between alpha pulse and peak of the echo, measured in ms

A

TE

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92
Q

What is the time between 180° inversion pulse and 90° alpha pulse that nulls signal from specific tissues during IR sequence

A

TI

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93
Q

What is the number of times data is collected per TR period called

A

NSA

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94
Q

The slope of the slice select gradient and the transmitted bandwidth determine what

A

slice thickness

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95
Q

A steep slice select gradient slope and/or a narrow transmit bandwidth produce what kind of slices

A

thin slices

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96
Q

A shallow slice select gradient slope and/or a broad transmit bandwidth produce what kind of slices

A

thick slices

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97
Q

The thickness of the slice as well as the corresponding slice select gradient slope determines what

A

gap

98
Q

A type of image acquisition where a single line of K space is filled by data acquired from each slice before repeating the TR is called

A

2D imaging

99
Q

What is the most common type of data acquisition

A

2D imaging

100
Q

A type of image acquisition when all data is acquired as a volume with no gap space present is called

A

3D imaging

101
Q

In what type of imaging does slice encoding occur post data acquisition for the determination of spatial localization

A

3D imaging

102
Q

When does slice encoding occur in 3D imaging

A

post data acquisition

103
Q

A gap space of at least how much is required to prevent cross excitation artifact in sequential slice ordering

A

30%

104
Q

When data is acquired from alternate slices through two separate acquisitions it is called

A

interleaving slice order

105
Q

What does interleaving slice order prevent

A

cross excitation artifact

106
Q

How much slice gap is needed to prevent cross excitation with interleaving slice order

A

none

107
Q

What is an extra RF pulse, with a 90° flip angle and a wide transmission bandwidth, strategically placed over areas of unwanted anatomy called

A

spatial saturation pulse (sat band)

108
Q

Do spatial saturation pulses (sat bands) have a wide or narrow transmission bandwidth

A

wide (to saturate all tissues)

109
Q

What flip angle do spatial saturation pulses (sat bands) have

A

90° flip angle

110
Q

What effect do spatial saturation pulses (sat bands) have on SAR

A

SAR limits are reached sooner bc of increased tissue heating

111
Q

What effect do spatial saturation pulses (sat bands) have on the number of available slices per acquisition

A

they decrease the number of slices available per acquisition

112
Q

Another word for flow comp or gradient moment rephasing

A

gradient moment nulling (GMN)

113
Q

What imaging option helps compensate for 1st order (laminar) flow withing the imaging volume

A

gradient moment nulling (GMN) aka flow comp or gradient moment rephasing

114
Q

An imaging option that uses the bi-polar application of a gradient which acts to rephase flowing spins and enhance their signal

A

gradient moment nulling (GMN) aka flow comp or gradient moment rephasing

115
Q

An imaging option that requires the use of either the slice select or frequency encoding gradients in order to properly rephase blood flow when acquiring images in different planes

A

gradient moment nulling (GMN) aka flow comp or gradient moment rephasing

116
Q

Which gradient is used to rephase blood flow with GMN

A

either the slice select or frequency encoding gradients

117
Q

Which gradient is used to rephase blood flow in axial images with GMN

A

slice select gradient

118
Q

Which gradient is used to rephase blood flow in coronal or sagittal images with GMN

A

frequency encoding gradient

119
Q

An imaging technique that applies an extra 90° RF pulse with a narrow transmission bandwidth (at the precessional frequency of fat, water or sometimes silicone) before application of the alpha pulse

A

chemical suppression

120
Q

How can you improve chemical suppression techniques

A

apply a shim over the anatomy of interest to improve field homogeneity (thus ensuring that fat is precessing at the same frequency)

121
Q

What does adding a shim over the anatomy of interest do to the magnetic field

A

improves field homogeneity

122
Q

What does adding a shim over the anatomy of interest do to the fat within the image

A

ensures that fat is precessing at the same frequency

123
Q

What is the process of tracking physiological motion so data acquisition can be properly timed for minimization of motion artifact

A

physiological gating/triggering

124
Q

What are the 3 kinds of physiological gating/triggering

A

cardiac
nervous (CSF flow)
respiratory

125
Q

How is nervous (CSF flow) system gating performed

A

with peripheral gating

126
Q

How is cardiac system gating performed

A

with cardiac (ECG) or peripheral gating (pulse ox)

127
Q

What is an ECG used for in MRI

A

to perform cardiac gating by determining the R to R interval within the cardiac cycle

128
Q

What does the P wave on an ECG represent

A

atrial systole

129
Q

What does the QRS complex on an ECG represent

A

ventricular systole

130
Q

What does the T wave on an ECG represent

A

ventricular diastole

131
Q

Define systole

A

contraction

132
Q

Define diastole

A

relaxation

133
Q

During cardiac gating, how is image weighting determined

A

by the patients R to R interval

134
Q

If pt has a low HR during cardiac gating, why might T1 images not be attainable

A

do to inability to calculate a low TR within a single R to R interval

135
Q

If pt has an elevated HR during cardiac gating, why might T2 images only be attainable using an R to R interval of greater than 1

A

do to inability to calculate a high TR within a single R to R interval

136
Q

How can you get T2 images on a cardiac gated sequence with a pt that has an elevated HR

A

by using an R to R interval of greater than 1

137
Q

What is the time period (during cardiac gating) towards the end of each R to R interval when the system stops scanning so that it can sense the next heart beat and prepare for the next excitation pulse called

A

trigger window

138
Q

Typically, when does the trigger window take place during cardiac gating

A

during the final 10% of the R to R interval

139
Q

If the R to R interval in cardiac gating is 1000 ms, then the system would stop scanning and the trigger window would take place when (in ms)

A

900 ms

140
Q

What is the time period that the system delays (during cardiac gating) before beginning to scan again after sensing each R phase called

A

trigger delay

141
Q

The trigger delay value is generally between what amounts of time (in ms)

A

5-10 ms

142
Q

Altering the trigger delay in cardiac imaging allows you to do what

A

image at different phases of the cardiac cycle (during systole or diastole)

143
Q

What is the R to R interval formula

A

60,000 ms/BPM = R to R interval

144
Q

What is the R to R interval if the pt’s HR is 80 BPM

A 
750 ms
(60,000/80 = 750 ms)
145
Q

During cardiac gating, if the pt’s R to R interval is 750 ms, how high will the R to R interval need to be set in order to obtain T2 images

A

to an R to R interval of 2 or greater (750 ms x 2 = 1500ms, which is a high enough TR for a T2 image)

146
Q

The process of tracking the physiological motion of the cardiovascular system & nervous system (CSF flow)

A

peripheral gating

147
Q

How is peripheral gating usually achieved

A

with a pulse ox

148
Q

An imaging option that uses phased array coils and an acceleration factor to fill K space in less time

A

parallel imaging

149
Q

What is the down side to using anti-aliasing/oversampling/no phase wrap/anti-fold over

A

increased scan times

150
Q

What image option applies over sampling along the phase encoding axis by increasing the number of phase encodings to eliminate wrap around artifact

A

anti-aliasing/oversampling/no phase wrap/anti-foldover

151
Q

If you use a rectangular field of view (smaller in phase direction) to shorten scan time what must you do in order to maintain square pixels (to maintain spatial resolution)

A

reduce the phase matrix

152
Q

An imaging option used in gradient echo sequences to null the signal from voxels in which fat and water interface by selecting TE values in multiples of 2.1 ms (when fat and water precess out of phase/incoherently with each other), resulting in a black outline around structures where fat and water interface.

A

out of phase imaging

153
Q

What technique is also called the Dixon technique

A

out of phase imaging

154
Q

An imaging option used in gradient echo sequences to increase the signal from voxels in which fat and water interface by selecting TE values in multiples of 4.2 ms (when fat and water precess in phase/coherently with each other), resulting in a bright outline around structures where fat and water interface

A

In Phase imaging

155
Q

An imaging option that uses a gradient echo pulse sequence with specific TE values in order to better demonstrate areas where fat and water interface

A

In phase/Out of phase imaging

156
Q

How is respiratory gating usually achieved

A

with a bellows

157
Q

The process of tracking the physiological motion of the respiratory system during inspiration & expiration

A

respiratory gating/triggering

158
Q

What can be done to the FOV to decrease scan time

A

decrease the phase FOV

159
Q

Another name for a phased array coil

A

multichannel coil

160
Q

What kind of coil uses multiple small coils and receivers to improve SNR and increase coverage area (combine benefits of large and small coils)

A

phased array/multi-channel coils

161
Q

In which axis is anti-aliasing/oversampling/no phase wrap/anti-foldover done

A

phase encoding axis

162
Q

How is anti-aliasing/oversampling/no phase wrap/anti-foldover achieved

A

by increasing the number of phase encodings to eliminate wrap around artifact

163
Q

Accerleration factor is also called what

A

R factor

164
Q

What does the acceleration factor/R factor indicate

A

the extent of scan time reduction

165
Q

As the acceleration factor/R factor increases, what happens to scan time

A

decreases

166
Q

As the acceleration factor/R factor increases, what happens to aliasing

A

increases

167
Q

As the acceleration factor/R factor increases, what happens to noise within the image

A

decreases

168
Q

A c is also called what

A

coil sensitivity map

169
Q

What acquires signal seen by individual channels of the phased array coil and functions to locate anatomy withing the imaging volume to prevent aliasing artifact during parallel imaging

A

calibration scan/coil sensitivity map

170
Q

An imaging option that fills K space radially with multiple lines of K space being acquired as a block, and the central portion of K space being acquired every TR.

A

propeller/blade

171
Q

When is the central portion of K space acquired in propeller/blade imaging

A

every TR

172
Q

Use of propeller/blade sequences does what to SNR

A

increases it

173
Q

Use of propeller/blade sequences does what to CNR

A

increases it

174
Q

Use of propeller/blade sequences does what to scan time

A

decreases it

175
Q

Propeller/blade sequences are similar to using what

A

multiple NEX

176
Q

Use of propeller/blade sequences does what to motion artifact

A

reduces it

177
Q

The MR system automatically applies what during image acquisition to prevent aliasing from occurring in the frequency direction

A

frequency filter

178
Q

Increasing TR does what to CNR

A

increases it

179
Q

Increasing TR does what to SNR

A

increases it

180
Q

Increasing TR does what to scan time

A

increases it

181
Q

Increasing TR does what to T1 contrast

A

reduces it

182
Q

Increasing TE does what to CNR

A

reduces it

183
Q

Increasing TE does what to SNR

A

reduces it

184
Q

Increasing TE does what to T2 contrast

A

increases it

185
Q

Increasing TE does what to T2* contrast

A

increases it

186
Q

Increasing TE does what to scan time

A

nothing

187
Q

Increasing TI does what to CNR

A

increases it

188
Q

Increasing TI does what to SNR

A

increases it

189
Q

Increasing TI does what to scan time

A

nothing

190
Q

Increasing TI does what to T1 contrast

A

reduces it

191
Q

Increasing NSA/NEX does what to SNR

A

increases it

192
Q

Increasing NSA/NEX does what to CNR

A

increases it

193
Q

Increasing NSA/NEX does what to scan time

A

increases it

194
Q

Increasing NSA/NEX does what to spatial resolution

A

nothing

195
Q

Increasing the flip angle does what to CNR

A

approaching 90° = increases it

past 90° = reduces it

196
Q

Increasing the flip angle does what to SNR

A

approaching 90° = increases it

past 90° = reduces it

197
Q

Increasing the flip angle does what to T1 contrast

A

increases it

198
Q

Increasing the flip angle does what to scan time

A

nothing

199
Q

Increasing the FOV does what to CNR

A

increase it

200
Q

Increasing the FOV does what to SNR

A

increase it

201
Q

Increasing the FOV does what to spatial resolution

A

reduces it

202
Q

Increasing the FOV does what to scan time

A

nothing

203
Q

Increasing the FOV does what to proton (spin) density

A

increases it

204
Q

Increasing the matrix does what to CNR

A

reduces it

205
Q

Increasing the matrix does what to SNR

A

reduces it

206
Q

Increasing the matrix does what to spatial resolution

A

increases it

207
Q

Increasing the matrix does what to scan time

A

increase of phase = increase scan time

increase of frequency = no effect

208
Q

Increasing the matrix does what to proton (spin) density

A

reduces it (small voxels have less protons)

209
Q

Increasing the # of slices does what to CNR

A

nothing

210
Q

Increasing the # of slices does what to SNR

A

nothing

211
Q

Increasing the # of slices does what to spatial resolution

A

nothing

212
Q

Increasing the # of slices does what to scan time

A 
2D = no effect
3D = increased scan time
213
Q

Increasing the slice thickness does what to CNR

A

increases it

214
Q

Increasing the slice thickness does what to SNR

A

increases it

215
Q

Increasing the slice thickness does what to spatial resolution

A

reduces it

216
Q

Increasing the slice thickness does what to scan time

A

nothing

217
Q

Increasing the slice thickness does what to proton (spin) density

A

increases it

218
Q

Increasing the ETL does what to SNR

A

reduce it

219
Q

Increasing the ETL does what to CNR

A

reduce it

220
Q

Increasing the ETL does what to spatial resolution

A

nothing

221
Q

Increasing the ETL does what to scan time

A

reduces it

222
Q

Increasing the ETL does what to T2 contrast

A

increases it

223
Q

Increasing the ETL does what to T2* contrast

A

increases it

224
Q

Increasing the effective TE does what to CNR

A

reduces it

225
Q

Increasing the effective TE does what to SNR

A

reduces it

226
Q

Increasing the effective TE does what to spatial resolution

A

nothing

227
Q

Increasing the effective TE does what to scan time

A

nothing

228
Q

Increasing the effective TE does what to T2 contrast

A

increases it

229
Q

Increasing the effective TE does what to T2* contrast

A

increases it

230
Q

Increasing the receive bandwidth does what to CNR

A

reduces it

231
Q

Increasing the receive bandwidth does what to SNR

A

reduces it

232
Q

Increasing the receive bandwidth does what to spatial resolution

A

nothing

233
Q

Increasing the receive bandwidth does what to scan time

A

nothing

234
Q

What 3 factors affect spatial resolution

A

FOV, matrix and slice thickness

235
Q

What 3 factors affect proton (spin) density

A

FOV, matrix and slice thickness

236
Q

What kind of a relationship do spatial resolution and proton (spin) density have

A

inverse

237
Q

What 3 factors affect T2 and T2* contrast

A

TE, effective TE, and ETL

238
Q

What 3 factors affect T1 contrast

A

TR, TI, and flip angle

239
Q

What factors affect scan time

A

TR, NSA/NEX, ETL, phase matrix, and # of slices (3D only)

240
Q

What is the only factor that does not affect CNR or SNR

A

of slices

MRI AllinOne (2 decks)

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