Sequence Parameters & Options Flashcards
What is the difference in SNR between 2 adjacent pixels (eye’s ability to detect difference)
CNR
Which parameter has the greatest influence on image quality
CNR
CNR is controlled by all the same factors as what
SNR
What is the ratio of signal amplitude to average noise amplitude called
SNR
Is signal predictable
yes
The induced voltage at the receiver coil is referred to as what
signal
Is noise predictible
no, it’s random
What is noise dependent on
body habitus & electrical noise of the system
What parameter has the greatest influence on SNR
size of FOV
What is the ability of the imaging system to detect 2 points as separate
spatial resolution
Which pixel types give better spatial resolution: square or rectangular
square
What is the amount of tissue within a 3 D volume called
voxel volume
What determines voxel volume
FOV, matrix & slice thickness
What determines spatial resolution
voxel volume only
Formula for voxel volume
pixel phase x pixel frequency x slice thickness
What is the voxel volume when using a 24cm FOV, a 256x128 matrix & 3mm slices?
(240mm/256) x (240mm/128) x 3 = 5.27mm³
Which provide better spatial resolution - square or rectangular pixels
square
Which pixel type is better for reformatting 2D/3D images - square or rectangular
square
What is the amount of time it takes to fill k-space called
acquisition time
What has the greatest impact on the amount of patient motion detected on an image
acquisition time
What parameters affect acquisition time
TR, NSA/NEX, phase matrix, ETL & # of slices (during 3D only)
What is the formula to calculate 2D acquisition time for conventional sequences
TR x NSA x Phase encodings
500ms x 1 x 256 = 128,000ms = 128sec = 2min 8 sec
What is the formula to calculate 2D acquisition time for fast sequences
(TR x NSA x Phase encodings)/ETL
(500ms x 1 x 256)/4 = 32,000ms = 32sec
What is the formula to calculate 3D acquisition time for conventional sequences
TR x NSA x Phase encodings x slice #
500ms x 1 x 256 x 24 = 3,072,000ms = 3,072sec = 51min 12sec
List the 7 intrinsic parameters
T1 recovery T2 decay proton density flow ADC perfusion diffusion
List the 8 extrinsic parameters
TR TE flip angle TI ETL b value FOV matrix
What are the 3 types of image contrast
T1
T2
PD
Which type of contrast appears as a difference in signal intensities between tissues with varying hydrogen proton concentrations
proton density contrast
What is the objective when trying to achieve T1 weighting
prevent recovery and decay
What is the objective when trying to achieve T2 weighting
allow recovery and decay
What are the TR & TE requirements for a T1 weighted image
short TR & TE
What are the TR & TE requirements for a T2 weighted image
long TR & TE
T1 images are dependent upon the differences between what
the T1 relaxation times of fat & water
T2 images are dependent upon the differences between what
the T2 decay times of fat & water
Proton density images are dependent upon the differences between what
the number of mobile hydrogen protons within adjacent tissues
What is the objective when trying to achieve PD weighting
allow recovery & prevent decay
What are the TR & TE requirements for a PD weighted image
long TR & short TE
What are the TR & TE requirements for a T2* weighted image
long TR & TE with use of a gradient echo sequence
T2* images are dependent upon what
T2 decay & magnetic field inhomogeneities
What controls T2* weighting
TE
What is TR
repetition time - the time between alpha pulses, measured in ms
What is TE
echo time - time between alpha pulse and peak of the echo, measured in ms
What is TI
time of inversion - time between 180° inversion pulse and 90° alpha pulse that nulls signal from specific tissues during IR sequence
What controls the amount of T1 contrast on T1 IR pulse sequences
TI & TR
What controls which tissues will be nulled on T2 IR pulse sequences
TI
Define NSA
number of signals averaged - number of times data is collected per TR period
How are NSA & scan time related
directly proportional: double NSA=double time
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)
What is the angle of the NMV to the direction of the main magnetic field called
flip angle
Which flip angles produce more signal
those closer to 90°
What is flip angle controlled by
the amplitude and duration of incoming RF pulses
Define FOV
area of anatomy covered in an image
What does FOV have the greatest impact on
SNR
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)
What is the total number of pixels in an image called
the matrix
Define phase matrix
of pixels in phase direction (has a direct affect on scan time)
Which matrix affects scan time (phase or frequency)
phase
The phase matrix affects what
scan time
Define frequency matrix
of pixels in frequency direction (has no affect on scan time)
What parameter limits the number of slices allowed
TR
What factors determine how many slices are allowed
TR & the systems SAR limitations
When does slice number affect scan time
only during 3D volumetric imaging
Does a higher TR produce more or less RF pulses
less
Does a higher TR produce more or less time between RF pulses
more
If you increase the TR, does more or less tissue heating occur
less
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
What determines slice thickness
the slope of the slice select gradient and the transmitted bandwidth
How are thin slices produced
with a steep slice select gradient slope and/or a narrow transmit bandwidth
How are thick slices produced
with a shallow slice select gradient slope and/or a broad transmit bandwidth
How is gap determined
by the thickness of the slice as well as the corresponding slice select gradient slope
What is gap important for reducing
image artifact (cross excitation)
The number of times the echo is sampled per TR period during SE pulse sequences is called
ETL
ETL corresponds to what
the number of rephasing 180° RF pulses applied
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)
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)
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)
During FSE sequences, effective TE is a factor that determines what
image weighting
Values in the middle of K space affect what
image contrast
Values in the periphery of K space affect what
spatial resolution
The range of frequencies transmitted in an RF pulse is called
transmit bandwidth
Which bandwidth (transmit/receive) is automatically selected by the system upon slice thickness selection
transmit
The range of frequencies sampled during the time that the readout/frequency gradient is active is called
receive bandwidth
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)
Does receive bandwidth affect the minimum TE
yes
If you decrease the receive bandwidth, what happens to the minimum TE that is obtainable during a pulse sequence
the minimum obtainable TE increases
If you decrease the receive bandwidth, the minimum TE obtainable increases - what image weighting could this affect
T1 (need short TE)
The parameter that divides a sequence into multiple acquisitions is called
concatenations
If you increase concatenations, what affect does it have on TR
allows the use of a lower TR (limited by SAR)
Increasing concatenations has what affect on motion artifact
reduces it (bc it shortens acquisition time)
What is the time between alpha pulses, measured in ms called
TR
What is the time between alpha pulse and peak of the echo, measured in ms
TE
What is the time between 180° inversion pulse and 90° alpha pulse that nulls signal from specific tissues during IR sequence
TI
What is the number of times data is collected per TR period called
NSA
The slope of the slice select gradient and the transmitted bandwidth determine what
slice thickness
A steep slice select gradient slope and/or a narrow transmit bandwidth produce what kind of slices
thin slices
A shallow slice select gradient slope and/or a broad transmit bandwidth produce what kind of slices
thick slices
The thickness of the slice as well as the corresponding slice select gradient slope determines what
gap
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
What is the most common type of data acquisition
2D imaging
A type of image acquisition when all data is acquired as a volume with no gap space present is called
3D imaging
In what type of imaging does slice encoding occur post data acquisition for the determination of spatial localization
3D imaging
When does slice encoding occur in 3D imaging
post data acquisition
A gap space of at least how much is required to prevent cross excitation artifact in sequential slice ordering
30%
When data is acquired from alternate slices through two separate acquisitions it is called
interleaving slice order
What does interleaving slice order prevent
cross excitation artifact
How much slice gap is needed to prevent cross excitation with interleaving slice order
none
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)
Do spatial saturation pulses (sat bands) have a wide or narrow transmission bandwidth
wide (to saturate all tissues)
What flip angle do spatial saturation pulses (sat bands) have
90° flip angle
What effect do spatial saturation pulses (sat bands) have on SAR
SAR limits are reached sooner bc of increased tissue heating
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
Another word for flow comp or gradient moment rephasing
gradient moment nulling (GMN)
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
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
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
Which gradient is used to rephase blood flow with GMN
either the slice select or frequency encoding gradients
Which gradient is used to rephase blood flow in axial images with GMN
slice select gradient
Which gradient is used to rephase blood flow in coronal or sagittal images with GMN
frequency encoding gradient
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
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)
What does adding a shim over the anatomy of interest do to the magnetic field
improves field homogeneity
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
What is the process of tracking physiological motion so data acquisition can be properly timed for minimization of motion artifact
physiological gating/triggering
What are the 3 kinds of physiological gating/triggering
cardiac
nervous (CSF flow)
respiratory
How is nervous (CSF flow) system gating performed
with peripheral gating
How is cardiac system gating performed
with cardiac (ECG) or peripheral gating (pulse ox)
What is an ECG used for in MRI
to perform cardiac gating by determining the R to R interval within the cardiac cycle
What does the P wave on an ECG represent
atrial systole
What does the QRS complex on an ECG represent
ventricular systole
What does the T wave on an ECG represent
ventricular diastole
Define systole
contraction
Define diastole
relaxation
During cardiac gating, how is image weighting determined
by the patients R to R interval
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
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
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
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
Typically, when does the trigger window take place during cardiac gating
during the final 10% of the R to R interval
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
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
The trigger delay value is generally between what amounts of time (in ms)
5-10 ms
Altering the trigger delay in cardiac imaging allows you to do what
image at different phases of the cardiac cycle (during systole or diastole)
What is the R to R interval formula
60,000 ms/BPM = R to R interval
What is the R to R interval if the pt’s HR is 80 BPM
750 ms (60,000/80 = 750 ms)
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)
The process of tracking the physiological motion of the cardiovascular system & nervous system (CSF flow)
peripheral gating
How is peripheral gating usually achieved
with a pulse ox
An imaging option that uses phased array coils and an acceleration factor to fill K space in less time
parallel imaging
What is the down side to using anti-aliasing/oversampling/no phase wrap/anti-fold over
increased scan times
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
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
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
What technique is also called the Dixon technique
out of phase imaging
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
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
How is respiratory gating usually achieved
with a bellows
The process of tracking the physiological motion of the respiratory system during inspiration & expiration
respiratory gating/triggering
What can be done to the FOV to decrease scan time
decrease the phase FOV
Another name for a phased array coil
multichannel coil
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
In which axis is anti-aliasing/oversampling/no phase wrap/anti-foldover done
phase encoding axis
How is anti-aliasing/oversampling/no phase wrap/anti-foldover achieved
by increasing the number of phase encodings to eliminate wrap around artifact
Accerleration factor is also called what
R factor
What does the acceleration factor/R factor indicate
the extent of scan time reduction
As the acceleration factor/R factor increases, what happens to scan time
decreases
As the acceleration factor/R factor increases, what happens to aliasing
increases
As the acceleration factor/R factor increases, what happens to noise within the image
decreases
A c is also called what
coil sensitivity map
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
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
When is the central portion of K space acquired in propeller/blade imaging
every TR
Use of propeller/blade sequences does what to SNR
increases it
Use of propeller/blade sequences does what to CNR
increases it
Use of propeller/blade sequences does what to scan time
decreases it
Propeller/blade sequences are similar to using what
multiple NEX
Use of propeller/blade sequences does what to motion artifact
reduces it
The MR system automatically applies what during image acquisition to prevent aliasing from occurring in the frequency direction
frequency filter
Increasing TR does what to CNR
increases it
Increasing TR does what to SNR
increases it
Increasing TR does what to scan time
increases it
Increasing TR does what to T1 contrast
reduces it
Increasing TE does what to CNR
reduces it
Increasing TE does what to SNR
reduces it
Increasing TE does what to T2 contrast
increases it
Increasing TE does what to T2* contrast
increases it
Increasing TE does what to scan time
nothing
Increasing TI does what to CNR
increases it
Increasing TI does what to SNR
increases it
Increasing TI does what to scan time
nothing
Increasing TI does what to T1 contrast
reduces it
Increasing NSA/NEX does what to SNR
increases it
Increasing NSA/NEX does what to CNR
increases it
Increasing NSA/NEX does what to scan time
increases it
Increasing NSA/NEX does what to spatial resolution
nothing
Increasing the flip angle does what to CNR
approaching 90° = increases it
past 90° = reduces it
Increasing the flip angle does what to SNR
approaching 90° = increases it
past 90° = reduces it
Increasing the flip angle does what to T1 contrast
increases it
Increasing the flip angle does what to scan time
nothing
Increasing the FOV does what to CNR
increase it
Increasing the FOV does what to SNR
increase it
Increasing the FOV does what to spatial resolution
reduces it
Increasing the FOV does what to scan time
nothing
Increasing the FOV does what to proton (spin) density
increases it
Increasing the matrix does what to CNR
reduces it
Increasing the matrix does what to SNR
reduces it
Increasing the matrix does what to spatial resolution
increases it
Increasing the matrix does what to scan time
increase of phase = increase scan time
increase of frequency = no effect
Increasing the matrix does what to proton (spin) density
reduces it (small voxels have less protons)
Increasing the # of slices does what to CNR
nothing
Increasing the # of slices does what to SNR
nothing
Increasing the # of slices does what to spatial resolution
nothing
Increasing the # of slices does what to scan time
2D = no effect 3D = increased scan time
Increasing the slice thickness does what to CNR
increases it
Increasing the slice thickness does what to SNR
increases it
Increasing the slice thickness does what to spatial resolution
reduces it
Increasing the slice thickness does what to scan time
nothing
Increasing the slice thickness does what to proton (spin) density
increases it
Increasing the ETL does what to SNR
reduce it
Increasing the ETL does what to CNR
reduce it
Increasing the ETL does what to spatial resolution
nothing
Increasing the ETL does what to scan time
reduces it
Increasing the ETL does what to T2 contrast
increases it
Increasing the ETL does what to T2* contrast
increases it
Increasing the effective TE does what to CNR
reduces it
Increasing the effective TE does what to SNR
reduces it
Increasing the effective TE does what to spatial resolution
nothing
Increasing the effective TE does what to scan time
nothing
Increasing the effective TE does what to T2 contrast
increases it
Increasing the effective TE does what to T2* contrast
increases it
Increasing the receive bandwidth does what to CNR
reduces it
Increasing the receive bandwidth does what to SNR
reduces it
Increasing the receive bandwidth does what to spatial resolution
nothing
Increasing the receive bandwidth does what to scan time
nothing
What 3 factors affect spatial resolution
FOV, matrix and slice thickness
What 3 factors affect proton (spin) density
FOV, matrix and slice thickness
What kind of a relationship do spatial resolution and proton (spin) density have
inverse
What 3 factors affect T2 and T2* contrast
TE, effective TE, and ETL
What 3 factors affect T1 contrast
TR, TI, and flip angle
What factors affect scan time
TR, NSA/NEX, ETL, phase matrix, and # of slices (3D only)
What is the only factor that does not affect CNR or SNR
of slices
