Sequence Parameters & Options Flashcards
(240 cards)
1
Q
What is the difference in SNR between 2 adjacent pixels (eye’s ability to detect difference)
A
CNR
2
Q
Which parameter has the greatest influence on image quality
A
CNR
3
Q
CNR is controlled by all the same factors as what
A
SNR
4
Q
What is the ratio of signal amplitude to average noise amplitude called
A
SNR
5
Q
Is signal predictable
A
yes
6
Q
The induced voltage at the receiver coil is referred to as what
A
signal
7
Q
Is noise predictible
A
no, it’s random
8
Q
What is noise dependent on
A
body habitus & electrical noise of the system
9
Q
What parameter has the greatest influence on SNR
A
size of FOV
10
Q
What is the ability of the imaging system to detect 2 points as separate
A
spatial resolution
11
Q
Which pixel types give better spatial resolution: square or rectangular
A
square
12
Q
What is the amount of tissue within a 3 D volume called
A
voxel volume
13
Q
What determines voxel volume
A
FOV, matrix & slice thickness
14
Q
What determines spatial resolution
A
voxel volume only
15
Q
Formula for voxel volume
A
pixel phase x pixel frequency x slice thickness
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
17
Q
Which provide better spatial resolution - square or rectangular pixels
A
square
18
Q
Which pixel type is better for reformatting 2D/3D images - square or rectangular
A
square
19
Q
What is the amount of time it takes to fill k-space called
A
acquisition time
20
Q
What has the greatest impact on the amount of patient motion detected on an image
A
acquisition time
21
Q
What parameters affect acquisition time
A
TR, NSA/NEX, phase matrix, ETL & # of slices (during 3D only)
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
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
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
25
List the 7 intrinsic parameters
```
T1 recovery
T2 decay
proton density
flow
ADC
perfusion
diffusion
```
26
List the 8 extrinsic parameters
```
TR
TE
flip angle
TI
ETL
b value
FOV
matrix
```
27
What are the 3 types of image contrast
T1
T2
PD
28
Which type of contrast appears as a difference in signal intensities between tissues with varying hydrogen proton concentrations
proton density contrast
29
What is the objective when trying to achieve T1 weighting
prevent recovery and decay
30
What is the objective when trying to achieve T2 weighting
allow recovery and decay
31
What are the TR & TE requirements for a T1 weighted image
short TR & TE
32
What are the TR & TE requirements for a T2 weighted image
long TR & TE
33
T1 images are dependent upon the differences between what
the T1 relaxation times of fat & water
34
T2 images are dependent upon the differences between what
the T2 decay times of fat & water
35
Proton density images are dependent upon the differences between what
the number of mobile hydrogen protons within adjacent tissues
36
What is the objective when trying to achieve PD weighting
allow recovery & prevent decay
37
What are the TR & TE requirements for a PD weighted image
long TR & short TE
38
What are the TR & TE requirements for a T2* weighted image
long TR & TE with use of a gradient echo sequence
39
T2* images are dependent upon what
T2 decay & magnetic field inhomogeneities
40
What controls T2* weighting
TE
41
What is TR
repetition time - the time between alpha pulses, measured in ms
42
What is TE
echo time - time between alpha pulse and peak of the echo, measured in ms
43
What is TI
time of inversion - time between 180° inversion pulse and 90° alpha pulse that nulls signal from specific tissues during IR sequence
44
What controls the amount of T1 contrast on T1 IR pulse sequences
TI & TR
45
What controls which tissues will be nulled on T2 IR pulse sequences
TI
46
Define NSA
number of signals averaged - number of times data is collected per TR period
47
How are NSA & scan time related
directly proportional: double NSA=double time
48
How are NSA & SNR related
square root relationship:
2 x NSA = 41% SNR increase (√2 = 1.41),
4 x NSA = 100% SNR increase (√4 = 2)
49
What is the angle of the NMV to the direction of the main magnetic field called
flip angle
50
Which flip angles produce more signal
those closer to 90°
51
What is flip angle controlled by
the amplitude and duration of incoming RF pulses
52
Define FOV
area of anatomy covered in an image
53
What does FOV have the greatest impact on
SNR
54
What relationship does FOV have with SNR
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)
55
What is the total number of pixels in an image called
the matrix
56
Define phase matrix
of pixels in phase direction (has a direct affect on scan time)
57
Which matrix affects scan time (phase or frequency)
phase
58
The phase matrix affects what
scan time
59
Define frequency matrix
of pixels in frequency direction (has no affect on scan time)
60
What parameter limits the number of slices allowed
TR
61
What factors determine how many slices are allowed
TR & the systems SAR limitations
62
When does slice number affect scan time
only during 3D volumetric imaging
63
Does a higher TR produce more or less RF pulses
less
64
Does a higher TR produce more or less time between RF pulses
more
65
If you increase the TR, does more or less tissue heating occur
less
66
Will a higher TR increase or decrease the # of allowable slices
increase, because less tissue heating will occur which keeps the system below its SAR limits
67
What determines slice thickness
the slope of the slice select gradient and the transmitted bandwidth
68
How are thin slices produced
with a steep slice select gradient slope and/or a narrow transmit bandwidth
69
How are thick slices produced
with a shallow slice select gradient slope and/or a broad transmit bandwidth
70
How is gap determined
by the thickness of the slice as well as the corresponding slice select gradient slope
71
What is gap important for reducing
image artifact (cross excitation)
72
The number of times the echo is sampled per TR period during SE pulse sequences is called
ETL
73
ETL corresponds to what
the number of rephasing 180° RF pulses applied
74
When can effective TE be chosen
during fast spin echo techniques (since you can't choose the TE value for each echo in the train of 180° RF rephasing pulses)
75
In a fast spin echo technique, echos closest to the effective TE selected are place where in K space
the center of K space (where they affect image contrast)
76
In a fast spin echo technique, echos not close to the effective TE selected are place where in K space
the periphery of K space (where they affect spatial resolution)
77
During FSE sequences, effective TE is a factor that determines what
image weighting
78
Values in the middle of K space affect what
image contrast
79
Values in the periphery of K space affect what
spatial resolution
80
The range of frequencies transmitted in an RF pulse is called
transmit bandwidth
81
Which bandwidth (transmit/receive) is automatically selected by the system upon slice thickness selection
transmit
82
The range of frequencies sampled during the time that the readout/frequency gradient is active is called
receive bandwidth
83
Receive bandwidth has what kind of relationship with SNR
```
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)
```
84
Does receive bandwidth affect the minimum TE
yes
85
If you decrease the receive bandwidth, what happens to the minimum TE that is obtainable during a pulse sequence
the minimum obtainable TE increases
86
If you decrease the receive bandwidth, the minimum TE obtainable increases - what image weighting could this affect
T1 (need short TE)
87
The parameter that divides a sequence into multiple acquisitions is called
concatenations
88
If you increase concatenations, what affect does it have on TR
allows the use of a lower TR (limited by SAR)
89
Increasing concatenations has what affect on motion artifact
reduces it (bc it shortens acquisition time)
90
What is the time between alpha pulses, measured in ms called
TR
91
What is the time between alpha pulse and peak of the echo, measured in ms
TE
92
What is the time between 180° inversion pulse and 90° alpha pulse that nulls signal from specific tissues during IR sequence
TI
93
What is the number of times data is collected per TR period called
NSA
94
The slope of the slice select gradient and the transmitted bandwidth determine what
slice thickness
95
A steep slice select gradient slope and/or a narrow transmit bandwidth produce what kind of slices
thin slices
96
A shallow slice select gradient slope and/or a broad transmit bandwidth produce what kind of slices
thick slices
97
The thickness of the slice as well as the corresponding slice select gradient slope determines what
gap
98
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
2D imaging
99
What is the most common type of data acquisition
2D imaging
100
A type of image acquisition when all data is acquired as a volume with no gap space present is called
3D imaging
101
In what type of imaging does slice encoding occur post data acquisition for the determination of spatial localization
3D imaging
102
When does slice encoding occur in 3D imaging
post data acquisition
103
A gap space of at least how much is required to prevent cross excitation artifact in sequential slice ordering
30%
104
When data is acquired from alternate slices through two separate acquisitions it is called
interleaving slice order
105
What does interleaving slice order prevent
cross excitation artifact
106
How much slice gap is needed to prevent cross excitation with interleaving slice order
none
107
What is an extra RF pulse, with a 90° flip angle and a wide transmission bandwidth, strategically placed over areas of unwanted anatomy called
spatial saturation pulse (sat band)
108
Do spatial saturation pulses (sat bands) have a wide or narrow transmission bandwidth
wide (to saturate all tissues)
109
What flip angle do spatial saturation pulses (sat bands) have
90° flip angle
110
What effect do spatial saturation pulses (sat bands) have on SAR
SAR limits are reached sooner bc of increased tissue heating
111
What effect do spatial saturation pulses (sat bands) have on the number of available slices per acquisition
they decrease the number of slices available per acquisition
112
Another word for flow comp or gradient moment rephasing
gradient moment nulling (GMN)
113
What imaging option helps compensate for 1st order (laminar) flow withing the imaging volume
gradient moment nulling (GMN) aka flow comp or gradient moment rephasing
114
An imaging option that uses the bi-polar application of a gradient which acts to rephase flowing spins and enhance their signal
gradient moment nulling (GMN) aka flow comp or gradient moment rephasing
115
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
gradient moment nulling (GMN) aka flow comp or gradient moment rephasing
116
Which gradient is used to rephase blood flow with GMN
either the slice select or frequency encoding gradients
117
Which gradient is used to rephase blood flow in axial images with GMN
slice select gradient
118
Which gradient is used to rephase blood flow in coronal or sagittal images with GMN
frequency encoding gradient
119
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
chemical suppression
120
How can you improve chemical suppression techniques
apply a shim over the anatomy of interest to improve field homogeneity (thus ensuring that fat is precessing at the same frequency)
121
What does adding a shim over the anatomy of interest do to the magnetic field
improves field homogeneity
122
What does adding a shim over the anatomy of interest do to the fat within the image
ensures that fat is precessing at the same frequency
123
What is the process of tracking physiological motion so data acquisition can be properly timed for minimization of motion artifact
physiological gating/triggering
124
What are the 3 kinds of physiological gating/triggering
cardiac
nervous (CSF flow)
respiratory
125
How is nervous (CSF flow) system gating performed
with peripheral gating
126
How is cardiac system gating performed
with cardiac (ECG) or peripheral gating (pulse ox)
127
What is an ECG used for in MRI
to perform cardiac gating by determining the R to R interval within the cardiac cycle
128
What does the P wave on an ECG represent
atrial systole
129
What does the QRS complex on an ECG represent
ventricular systole
130
What does the T wave on an ECG represent
ventricular diastole
131
Define systole
contraction
132
Define diastole
relaxation
133
During cardiac gating, how is image weighting determined
by the patients R to R interval
134
If pt has a low HR during cardiac gating, why might T1 images not be attainable
do to inability to calculate a low TR within a single R to R interval
135
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
do to inability to calculate a high TR within a single R to R interval
136
How can you get T2 images on a cardiac gated sequence with a pt that has an elevated HR
by using an R to R interval of greater than 1
137
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
trigger window
138
Typically, when does the trigger window take place during cardiac gating
during the final 10% of the R to R interval
139
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)
900 ms
140
What is the time period that the system delays (during cardiac gating) before beginning to scan again after sensing each R phase called
trigger delay
141
The trigger delay value is generally between what amounts of time (in ms)
5-10 ms
142
Altering the trigger delay in cardiac imaging allows you to do what
image at different phases of the cardiac cycle (during systole or diastole)
143
What is the R to R interval formula
60,000 ms/BPM = R to R interval
144
What is the R to R interval if the pt's HR is 80 BPM
```
750 ms
(60,000/80 = 750 ms)
```
145
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
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
The process of tracking the physiological motion of the cardiovascular system & nervous system (CSF flow)
peripheral gating
147
How is peripheral gating usually achieved
with a pulse ox
148
An imaging option that uses phased array coils and an acceleration factor to fill K space in less time
parallel imaging
149
What is the down side to using anti-aliasing/oversampling/no phase wrap/anti-fold over
increased scan times
150
What image option applies over sampling along the phase encoding axis by increasing the number of phase encodings to eliminate wrap around artifact
anti-aliasing/oversampling/no phase wrap/anti-foldover
151
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)
reduce the phase matrix
152
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.
out of phase imaging
153
What technique is also called the Dixon technique
out of phase imaging
154
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
In Phase imaging
155
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
In phase/Out of phase imaging
156
How is respiratory gating usually achieved
with a bellows
157
The process of tracking the physiological motion of the respiratory system during inspiration & expiration
respiratory gating/triggering
158
What can be done to the FOV to decrease scan time
decrease the phase FOV
159
Another name for a phased array coil
multichannel coil
160
What kind of coil uses multiple small coils and receivers to improve SNR and increase coverage area (combine benefits of large and small coils)
phased array/multi-channel coils
161
In which axis is anti-aliasing/oversampling/no phase wrap/anti-foldover done
phase encoding axis
162
How is anti-aliasing/oversampling/no phase wrap/anti-foldover achieved
by increasing the number of phase encodings to eliminate wrap around artifact
163
Accerleration factor is also called what
R factor
164
What does the acceleration factor/R factor indicate
the extent of scan time reduction
165
As the acceleration factor/R factor increases, what happens to scan time
decreases
166
As the acceleration factor/R factor increases, what happens to aliasing
increases
167
As the acceleration factor/R factor increases, what happens to noise within the image
decreases
168
A c is also called what
coil sensitivity map
169
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
calibration scan/coil sensitivity map
170
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.
propeller/blade
171
When is the central portion of K space acquired in propeller/blade imaging
every TR
172
Use of propeller/blade sequences does what to SNR
increases it
173
Use of propeller/blade sequences does what to CNR
increases it
174
Use of propeller/blade sequences does what to scan time
decreases it
175
Propeller/blade sequences are similar to using what
multiple NEX
176
Use of propeller/blade sequences does what to motion artifact
reduces it
177
The MR system automatically applies what during image acquisition to prevent aliasing from occurring in the frequency direction
frequency filter
178
Increasing TR does what to CNR
increases it
179
Increasing TR does what to SNR
increases it
180
Increasing TR does what to scan time
increases it
181
Increasing TR does what to T1 contrast
reduces it
182
Increasing TE does what to CNR
reduces it
183
Increasing TE does what to SNR
reduces it
184
Increasing TE does what to T2 contrast
increases it
185
Increasing TE does what to T2* contrast
increases it
186
Increasing TE does what to scan time
nothing
187
Increasing TI does what to CNR
increases it
188
Increasing TI does what to SNR
increases it
189
Increasing TI does what to scan time
nothing
190
Increasing TI does what to T1 contrast
reduces it
191
Increasing NSA/NEX does what to SNR
increases it
192
Increasing NSA/NEX does what to CNR
increases it
193
Increasing NSA/NEX does what to scan time
increases it
194
Increasing NSA/NEX does what to spatial resolution
nothing
195
Increasing the flip angle does what to CNR
approaching 90° = increases it
| past 90° = reduces it
196
Increasing the flip angle does what to SNR
approaching 90° = increases it
| past 90° = reduces it
197
Increasing the flip angle does what to T1 contrast
increases it
198
Increasing the flip angle does what to scan time
nothing
199
Increasing the FOV does what to CNR
increase it
200
Increasing the FOV does what to SNR
increase it
201
Increasing the FOV does what to spatial resolution
reduces it
202
Increasing the FOV does what to scan time
nothing
203
Increasing the FOV does what to proton (spin) density
increases it
204
Increasing the matrix does what to CNR
reduces it
205
Increasing the matrix does what to SNR
reduces it
206
Increasing the matrix does what to spatial resolution
increases it
207
Increasing the matrix does what to scan time
increase of phase = increase scan time
| increase of frequency = no effect
208
Increasing the matrix does what to proton (spin) density
reduces it (small voxels have less protons)
209
Increasing the # of slices does what to CNR
nothing
210
Increasing the # of slices does what to SNR
nothing
211
Increasing the # of slices does what to spatial resolution
nothing
212
Increasing the # of slices does what to scan time
```
2D = no effect
3D = increased scan time
```
213
Increasing the slice thickness does what to CNR
increases it
214
Increasing the slice thickness does what to SNR
increases it
215
Increasing the slice thickness does what to spatial resolution
reduces it
216
Increasing the slice thickness does what to scan time
nothing
217
Increasing the slice thickness does what to proton (spin) density
increases it
218
Increasing the ETL does what to SNR
reduce it
219
Increasing the ETL does what to CNR
reduce it
220
Increasing the ETL does what to spatial resolution
nothing
221
Increasing the ETL does what to scan time
reduces it
222
Increasing the ETL does what to T2 contrast
increases it
223
Increasing the ETL does what to T2* contrast
increases it
224
Increasing the effective TE does what to CNR
reduces it
225
Increasing the effective TE does what to SNR
reduces it
226
Increasing the effective TE does what to spatial resolution
nothing
227
Increasing the effective TE does what to scan time
nothing
228
Increasing the effective TE does what to T2 contrast
increases it
229
Increasing the effective TE does what to T2* contrast
increases it
230
Increasing the receive bandwidth does what to CNR
reduces it
231
Increasing the receive bandwidth does what to SNR
reduces it
232
Increasing the receive bandwidth does what to spatial resolution
nothing
233
Increasing the receive bandwidth does what to scan time
nothing
234
What 3 factors affect spatial resolution
FOV, matrix and slice thickness
235
What 3 factors affect proton (spin) density
FOV, matrix and slice thickness
236
What kind of a relationship do spatial resolution and proton (spin) density have
inverse
237
What 3 factors affect T2 and T2* contrast
TE, effective TE, and ETL
238
What 3 factors affect T1 contrast
TR, TI, and flip angle
239
What factors affect scan time
TR, NSA/NEX, ETL, phase matrix, and # of slices (3D only)
240
What is the only factor that does not affect CNR or SNR
of slices
