Spin Echo Pulse Sequences Chapter 3
In conventional spin-echo if theres a phase matrix of 256, 256 phase encodings are performed and 256 TR must elapse to complete the scan. But if a turbo factor of 16 is applied in a TSE scan, what happens to the number of TR periods that must elapse to complete the scan?
16 (256/16 = 16) It decreases because more data is being collected in a shorter amount of time, therefor scan time is reduced.
After Tau, a RF rephasing pulse of what degree is applied?
180
Conventional spin-echo
90 deg pulse, followed by 1 or more 180 deg pulses to generate one or more spin-echos
During a spin-echo sequence using one echo, how are TE and TR applied to make a T1 weighted image and why?
A short TE ensures little T2 decay occurs. A short TR ensure fat and water do not fully recover
During T2* decay, why do magnetic moments of the same molecule dephase in a "fan" orientation, which contributes to spin-echo signal.
Because of inhomogeniety of the magnetic field. Some magnetic moments precess/ dephase slower, (the tail of the fan), and some precess/dephase faster (leading the fan).
How is proton density weighting highlighted during spin-echo with two echos?
During the first echo, A short TE, and a long TR are applied so that minimal T1 recovery and T2 decay are present so that PD weighting is dominant.
Increase in turbo factor means that it takes longer at each slice to collect data. Because information is taken at each spin-echo, the __________ is also important to consider because it affects the time between the acquisition at each slice.
Echo spacing
the number of lines of k-space selected per TR is equal to ______
Echo train length or turbo factor
How can we achieve a PD weighted image with a TSE when TSE is usually heavily T2 weighted?
Full echo train, Split echo train, shared eco train
________________ is controlled by phase reordering so that data collected from echos at or around the effective TE are placed in the signal and contrast areas of k-space
Image weighting
How is a signal built during a spin-echo sequence. Explain what happens to the magnetic moments.
Magnetic moments gradually come into phase after a 180 rf pulse, reaching a peak transverse magnetization at the TE, when all magnetic moments are in phase.
Explain how spin echo is produced when talking T2* decay and the fanning of magnetic moments.
Once T2* dephasing occurs (rf pulse is switched off), magnetic moments dephase at different times, some magnetic moments dephase faster and are the "leading" edge of the fan, while the tailing end of the fan dephases slower. Once an 180 rf pulse is applied, they flip on the axis and the tailing edge becomes the leading edge. Now dephasing again after the pulse, the previously leading (fast dephasing) magnetic moments, catch up with previously tailing magnetic moments and there is a momentary time (TE) where all of the magnetic moments are "inphase" (precess in same direction, at a moment in time) again without the application of the rf pulse. This causes "in-phase" transverse magnetization and maximum signal is induced in the coil. Called a spin-echo.
In Turbo spin echo (TSE), the scan time is decreased by modifying what parameter?
Phase matrix
is a spin-echo pulse sequence but with a much shorter scan time than conventional spin-echo.
RARE (Rapid acquisition with relaxation enhancement)
__________ phase-encoding slopes result in high amplitude. Explain their location in the k-space.
Shallow, they are placed in the central lines in the k-space
Technique that allows PD weighted imaging with TSE that permits shorter turbo factors and therefor more slices per TR
Shared echo train
During spin-echo using one echo, why is it important to have a short TE?
So that little T2 decay occurs so that the difference in T2 decay doesn't dominate over the spin-echo contrast.
What kind of weighted image involves spin-echo using one echo?
T1 weighted images
What 3 types of weighting are achievable using spin-echo?
T1, T2 and PD weighting
What kind of weighting is most pronounced with spin-echo sequences with two echos during the second echo?
T2 weighting
time between the rf pulse and the peak of the spin-echo
TE (echo time)
Explain the application of TR's when it comes to spin-echo using two echos and how it relates to T1 recovery.
TR is long with both echo's so that T1 recovery is minimized both times.
Scan time is a function of what three parameters? Scan time can be reduced by decreasing one or more parameters.
TR, number of signal averages, phase matrix
date from multiple spin-echos are placed into one image
TSE
A 90 deg rf pulse flips the NMV into the transverse plane, and once rf is switched off, and FID occurs and T2* dephasing brings the signal to 0. After a time called _______, another rf pulse is used to rephase magnetic moments.
Tau
Explain the application of TE's when it comes to spin-echo using two echos and how it relates to the T2 decay time.
The first TE is short and there isn't much T2 decay, so difference in T2 decay times are minimized. The second spin-echo has longer TE, and thus a longer T2 decay occurs, T2 decay is maximized.
turbo factor (echo train length)
The number of 180 degree rephasing pulses performed per TR corresponds to the number of echoes produced and the number of lines of K space filled.
How is T2 weighting highlighted during spin-echo with two echos?
The second echo allows for T2 decay to occur because the second spin echo has a long TE, and a long TR, highlighting T2 decay
T or F. Each 180 rf pulse, during conventional spin-echo, generates a separate spin-echo that is received by a coil and is used to create an image. A
True
After each echo, the phase-encoding is cancelled and a different phase-encoding is applied to the following echo. The number of echoes received in the same repetition is called ______
Turbo Factor or Echo Train Length.
During a 90 rf pulse, the time taken to "dephase" is called the Tau. When a 180 deg pulse is applied, it is flipped on its axis and the time it took to "dephase" after the first 90 deg withdrawl, equals the time to "rephase" after a 180 pulse is applied. Therefor TE is _________ the Tau. With spin-echo pulse sequences.
Twice
With dual echo sequence, how many images are produced per slice? What type of weighting are they?
Two, PD weighted and T2 weighted
Single Shot Fast Spin Echo (SS-FSE)
a fast spin echo sequence where all the lines of K space are acquired during a single TR period
If we select a long turbo factor and/or echo spacing, the TR is extended to acquire _________, this increasing scan time
a given number of slices
Driven Equilibrium
a modification of TSE, a reverse flip angle excitation rf pulse is applied at the end of the echo train
Why is T2 decay not affected by 180 rf rephasing pulse?
because T2 decay is caused by spin-spin interactions vs relaxation time
Explain why TSE the first echos are not "free" compared to dual sequence echos?
because data from every single echo fill a line of k -space and each image has its own k-space area. With the dual sequence, data for the PD weighted image fills one k-space area and the T2 weighted image fill another k-space area (scan time doubles).
Why does image blurring occur in TSE imaging at the edge of tissues with different T2 decay times?
because each line of k-space that is filled during an echo train contains data from echos with a different TE.
Why is artifact from metal implants significantly reduced in TSE imaging
because repeated 180 rf rephasing pulses compensate for magnetic field inhomogeneity
Even though spin-echo sequences are considered the gold standard in image contrast and weighting, FSE/TSA are used instead to create T2 and PD weighting. Why is that?
because spin-echo's have long scan times
with TSE, How is image blur reduced due to low signal amplitude?
by decreasing the space between echos and/or turbo factor
Do lines in the outer k-space or central k-space most contribute to the image?
central
raw image data from each from each spin echo are stored in k-space and each echo produce a separate image
conventional spin echo sequence
one phase-encoding step is applied per TR on each slice, and there for only one line of k-space is filled per TR
conventional spin-echo
Explain split echo train that helps achieve PD weighted imaging with TSE
data from the first half of the echo train is used for PD weighting and data from the last half is used for T2 weighting
Explain share echo train that helps achieve PD weighted imaging with TSE
data from the first spin echos are used to acquire PD weighted imaging and data from the last spin echos are used for T2 weighted imaging but data from other spin echos are shared for both images.
Name two advantages of TSE imaging
decreases scan times, high resolution imaging
Rephasing produces a signal called a _______
echo
Echos at the end or beginning of an echo train contribute PD weighting or very heavily T2 weighted data. Although data from these early or late spin-echos contribute to contrast, their impact is significantly less than spin-echos who is at or around the _________________
effective TE.
With TSE, the longer the turbo factor, the longer the the system spends at each slice collecting data. How does the affect the amount of slices per TR?
fewer slices are collected
Is the first or second spin-echo in charge of PD weighting when a spin-echo has two echos?
first
Driven equilibrium sequence produces increased signal intensity in __________ structures such as CSF when using a shorter TR than normal in TSE.
fluid-based
SS-TE (single short turbo spin echo) combines a partial _______ technique with TSE. Where half the lines of k-space are acquired in one TR, and the other half are transposed
fourier
In TSE, the number of phase encoding steps per TR is __________. As a result, k space is filled more efficiently, and the scan time __________
increases, decreases.
With TSE, increasing the TR to acquire more images is balanced by having a long echo train, which contributes to T2 weighting. With T1 weighting, the TR is kept short, why is that?
increasing TR will decrease T1 contrast. With a short TR several acquisitions are necessary to cover the anatomy
With TSE, when the number of phase encoding steps per repetition time (TR) increases, what happens to the scan time and why?
it decreases the scan time because k-space is filled more efficiently. More than one phase-encoding step is performed, filling more than one line of k-space per.
What is the point of the reverse flip angle at the end of an echo train of driven equilibrium TSE?
it drives the transverse magnetization into the longitudinal plane so that it is available for excitation at the beginning of the next TR period. Reducing the time for T1 relaxation to occur
Explain how in conventional dual echo sequence the first echo is "free"
it is considered free because the acquisition for the first pulse is collected while waiting for the second pulse acquisition, therefor there is no "time expense" because data is collected during the waiting period
At each 180 rf, a different amplitude of phase encoding gradient slope is applied to fill a different line of k-space. With TSE, several lines corresponding to turbo factor are filled every TR, therfor, _________ is filled more rapidly, and scan time decreases
k-space
In TSE, how does the system orders phase-encoding steps in the k-space
lines are filled depending on how close the TE of each spin-echo matches the effective TE. Data from spin-echos collected close to the effective TE (central lines of the k-space) contribute to the signal and contrast of the images.
The longer TRs associated with TSE are relevant in T2 weighting but are far less significant than the huge scan time savings produces by a ________.
long echo train
The TR is often much _______ than that used in conventional spin-echo
longer
Why are shorter turbo factors used in T1 and PD weighted imaging?
longer turbo factors place too much T2 weighting in the image
Very steep phase encoding slopes result in spin-echo's with a __________ amplitude. Explain their location in the k-space.
low, they are placed in the outer lines of the k-space
Long turbo factors leads to _________ T2 contrast
more
Why does a higher the turbo factor, or echo train length, shorter the scan time?
more phase-encoding steps are performed per TR.
Phase reordering is carried out in some techniques to reduce __________ artifacts
motion
the amplitude and polarity of the phase-encoding gradients are varied by a process called ___________
phase reordering
A _________ is a carefully coordinated and times sequence of events that generates certain type of image weighting
pulse sequence
series of rf pulses, gradient application, and intervening time periods
pulse sequence
used in MR to rephase magnetic moments at a later point in time
pulse sequences
Name a major disadvantage of TSE imaging
reduces magnetic susceptibility which can blur images and hide a hemorrhage
Contrast is mainly determined by spin-echo in conventional spin echo but there is contribution from what two factors?
rephasing by negative polarity applications of slice select, and frequency encoding gradients
In TSE, if T2 weighting is required, an effective TE of 100ms, a TR of 4000ms and turbo factor of 16 might be used. During the sequence, the ________ phase encodings are performed on echos occuring at 100ms. Data acquired at these echos have a TE at or close to 100ms
shallow
Phase encoding
signal along the short axis of the slice
used to produce a proton density and T2 weighted image in the TR period
spin-echo using two echos
In TSE, if T2 weighting is required, an effective TE of 100ms, a TR of 4000ms and turbo factor of 16 might be used. Phase encodings performed at the very beginning or end of an echo train, are __________. What is the amplitude of the signal in this case?
steep, low signal
An effective TE is chosen to weight images with TSE, explain how this is achieved in relation to k-space
the system organizes where to put data from each spin-echo in the k-space. Each spin-echo is phase-encoded based on the gradient that is applied and then it is organized in different lines in the k-space depending on how close the TE of each spin echo matches the effective TE.
Intervening Time Period
the time intervals between various functions, some are extrinsic that we select in scan protocol
The higher the turbo factor the shorter the scan time, but the resultant image has a mixture of weighting because why?
there is more data collected at a TE that isn't as close to the effective TE
Echo Spacing
time between each echo in the echo train in TSE or FSE.
What two factors determine image weighting of pulse sequences
timing and coordination of rf pulses and gradients
Scan times are reduced by a factor equal to the __________
turbo factor or echo train length
this technique yields a reduction in imaging time as all the image data are acquired at once.
turbo spin echo
Spin echo pulse sequence
uses a 180 degree rephasing pulse to generate an echo.
Gradient echo pulse sequence
uses a gradient to regenerate an echo
Two ways of rephasing magnetic moments to produce an echo
using 180 rf pulse, gradients
Echo Train
using several 180 RF pulses to produce several spin-echos
In TSE with the driven equilibrium, __________ has the longest T1 and T2 relaxation times. So when the T1 relaxation time is shortened due to the reverse flip angle, the magnetization of this structure is affected and it appears white in the image due to higher signal intensity.
water
When we determine turbo factor we there determine what when it comes to k-space?
we determine how many lines of k-space are filled every TR
Explain how image blurring occurs in TSE imaging
with long echo trains, late echos have low signal amplitude and resolution in the k-space is lost, and blurring may occur
