Physics: Sequences 3

Front 1

Is STIR mostly performed using a spin echo sequence

Back 1

yes

Front 2

How is fat nulled in a STIR sequence

Back 2

Fat has a short T1 (reaches 67% of original value rapidly). Hence by choosing a short TI of 140 milliseconds, the fat signal can be suppressed

Front 3

How is the time of acquisition reduced in STIR (given that there is added time to this sequence bc of the TI that is required)

Back 3

The combination of short TI inversion-recovery and fast spin echo sequences reduces acquisition time to acceptable limits for clinical practice.

Front 4

What is an advantage of STIR sequences

Back 4

The advantage of these sequences is that they offer a fat signal suppression technique with low sensitivity to magnetic field heterogeneities or to the effects of magnetic susceptibility in the presence of metal

Front 5

Can STIR have T1 or T2 weighting

Back 5

yes (remember that the T1 and T2 weighting has to do with the length of the TE and TR. These are controlled by when the RF pulses are programmed)
FYI: T1 is generally less than 500 and 30 and T2 is generally more than 1500 and 90.

Front 6

What happens to the nulled tissue in IR when it is in the transverse plan and is hit by the 90 pulse

Back 6

it goes antiparrelel and is 180 giving no signal. It will eventually start giving signal but the recievers will be turned off

see animations http://www.bigs.de/BLH/en/index.php?option=com_content&view=article&id=330:animation-aamp-explanation&catid=122:proton-spin-hf-impulse-focus&Itemid=298

Front 7

When a proton is antiparellel is it giving off signal

Back 7

no, it must have a component transverse to give off either T1 or T2 signal. This is the principal behind nulling a tissue.

Front 8

What is a potential shortfall of using STIR

Back 8

This technique must not be used to suppress a fat signal gadolinium injection because gadolinium–enhanced tissues have a shortened T1 and may be erased by short TI inversion-recovery (which is not specific to tissue but to its relaxation time T1).

Front 9

What is the aim of FLAIR

Back 9

The aim of a FLAIR sequence is to suppress liquid signals by inversion-recovery at an adapted TI.

Front 10

Does water have a long T1

Back 10

yes

Front 11

How long do you have to wait for TI for water

Back 11

Nulling of the water signal is seen at TI of 200 milliseconds

Front 12

Why is FSE used for inversion recovery sequences

Back 12

because of TI these sequences take a long time. A fast sequence is needed so it is done in a resonable amount of time.

Front 13

When is FLAIR most commonly used

Back 13

These sequences are routinely used in cerebral MRI for edema imaging

Front 14

What does a gradient echo sequence look like

Back 14

Front 15

What does a gradient echo sequence look like

Back 15

Front 16

What is the symbol of the pulse on gradient echo

Back 16

Front 17

What occurs as a result of a pulse less than 90 degrees

Back 17

there is less flip of the proton in the transverse plane which means less signal (for both T1 and T2)

Front 18

What is a result of a low flip angle in GE sequences

Back 18

The consequence of a low-flip angle excitation is a faster recovery of longitudinal magnetization that allows shorter TR/TE and decreases scan time.

Front 19

What are 2 benefits of low flip angles

Back 19

The advantages of low-flip angle excitations and gradient echo techniques are faster acquisitions, new contrasts between tissues and a stronger MR signal in case of short TR.

Front 20

What does the flip angle determine

Back 20

The flip angle determines the fraction of magnetization tipped in the transverse plane (which will produce the NMR signal) and the quantity of magnetization left on the longitudinal axis.

Front 21

What happens as the flip angle decreases

Back 21

If the flip angle decreases, the residual longitudinal magnetization will be higher and the recovery of magnetization for a given T1 and TR will be more complete

Front 22

What happens as the flip angle is decreased

Back 22

less tipped magnetization

Front 23

Flip angle of 90 degrees

Back 23

Axial magnetization is greater

Front 24

small flip angle

Back 24

more longitudinal magnetization

Front 25

What is a gradient that is similar to the frequency encoding gradient that is required for GE sequences

Back 25

As there is no 180° RF pulse, a bipolar readout gradient (which is the same as the frequency-encoding gradient) is required to create an echo

Front 26

What is required prior to the frequency gradient (which makes it bipolar)

Back 26

The gradient echo formation results from applying a dephasing gradient before the frequency-encoding or readout gradient

Front 27

What is the goal of the dephasing gradient that is applied just prior to the frequency encoding gradient

Back 27

The goal of this dephasing gradient is to obtain an echo when the readout gradient is applied and the data are acquired

Front 28

What does the bipolar gradient look like

Back 28

Front 29

How long is the dephasing part of the bipolar gradient compared to the rephasing portion

Back 29

1/2

Front 30

What happens since the frequency encoding portion of the bipolar gradient is 2x longer than the dephasing lobe

Back 30

Consequently, during data acquisition, the readout gradient will rephase the spins in the first half of the readout (by reversing the dephasing effect of the dephasing lobe), and the spins will dephase in the second half (due to the dephasing effect of the readout gradient).

Front 31

What does the flip magnetization determine

Back 31

The flip angle determines the fraction of magnetization tipped in the transverse plane (which will produce the NMR signal) and the quantity of magnetization left on the longitudinal axis (because there is going to be residual magnetization during the next GRE pulse and the smaller the flip angle the less pulse there will be)

Front 32

What is the cause of decay of transverse magnetization
2

Back 32

-spin-spin tissue-specific relaxation (T2) which is random
-B0 field inhomogeneities and magnetic susceptibility, which are static

Front 33

Does longitudinal magnetization decay

Back 33

no, it goes back to alignment with the magnet of the MR machine

Front 34

Is a bipolar readout the same as a frequency encoding gradient (readout gradient)

Back 34

yes, a bipolar gradient is simply a frequency encoding gradient with a modified dephasing lobe.

Front 35

What is the purpose of the dephasing lobe of the bipolar readout gradient

Back 35

to create an echo

Front 36

How does the dephasing lobe create an echo

Back 36

The gradient echo formation results from applying a dephasing gradient part of the bipolar readout.

Front 37

How is the echo created in gradient echo different than spin echo

Back 37

the gradient causes the echo. Not a 180 degree pulse as in spin echo

Front 38

What does the dephasing lobe of the bipolar readout gradient do

Back 38

causes a reverse in transverse spin.

Front 39

What is the sequence of the gradient echo

Back 39

the dephasing lobe occurs and reverses the spin. Then the positive lobe occurs and this causes the spins to go back to positive. Then the second half of the positive readout occurs which will causes dephasing to occur

Front 40

When does rephasing occur in the gradient echo

Back 40

the first half of the positive lobe of the bipolar readout gradient

Front 41

What does the bipolar gradient look like

Back 41

Front 42

What is steadystate gradient echo

Back 42

Steady-state sequences are a class of rapid magnetic resonance (MR) imaging techniques based on fast gradient-echo acquisitions in which both longitudinal magnetization (LM) and transverse magnetization (TM) are kept constant. Both LM and TM reach a nonzero steady state through the use of a repetition time that is shorter than the T2 relaxation time of tissue.

Front 43

Is TR shorter than T2 in steady state gradient echo

Back 43

yes

Front 44

What happens to the residual transverse magnetization in gradient echo

Back 44

the transverse magnetization will not have completely disappeared at the onset of the following repetition and will also be submitted to the flip caused by the excitation pulse

Front 45

What are 2 techniques to manage residual transverse magnetization in gradient echo

Back 45

gradient echo sequences with spoiled residual transverse magnetization
steady state gradient echo sequences that conserve residual transverse magnetization and therefore participate in the signal.

Front 46

When a gradient echo has spoiled residual transverse magnetization what is the name of the sequence

Back 46

spoiled gradient echo sequence

Front 47

What are the different manufacture names of spoiled gradient echo sequences

Back 47

Front 48

What does a spoiled gradient echo sequence look like

Back 48

The gradient spoiler pulse is the rainbow pulse

Front 49

Does the longitudinal and transverse magnetization reach a t constant in a steady state GRE

Back 49

yes

Front 50

Why would someone decide to use spoiled gradient echo sequence instead of steady state

Back 50

In certain cases, the steady state can be detrimental, namely for obtaining T1 weighted sequences. To resolve this problem, gradients and/or RF pulses (spoilers) are used to eliminate residual transverse magnetization.

Front 51

Does steady state gradient echo sometimes produce low quality T1W images

Back 51

yes and this is why spoiled gradient echo sequences may be used

Front 52

What causes increased T1W images in a gradient echo sequence

Back 52

A large flip angle. The larger the angle the more T1 W. This occurs because the longitudinal magnetization has not recovered completely.

Front 53

What are 2 techniques to increase the T1W in spoiled gradient echo

Back 53

the flip angle for T1 weighting (the greater the angle, the more T1 weighting)
the TE for T2* weighting (the shorter the TE, the more T2* deweighting)

Front 54

In Gradient echo what are the 3 ways to weight images as T1 or T2

Back 54

As in spin echo the TE and TR can be adjusted but also the flip angle. A larger flip angle will increase T1 weighting

Front 55

What are spoiled gradient echo sequences used for

Back 55

rapid 3D imaging during short periods of apnea (10-20 sec)

Front 56

They are used in angiography after gadolinium injection, possibly completed by a prior acquisition to allow subtraction

Back 56

yes

Front 57

What are the main 2 types of GRE sequences

Back 57

Ignore this slied

Front 58

Is spoiled GRE sometimes called steady state

Back 58

yes, and this is confusing but these sequences have been called steady-state INCOHERENT sequences, while the classic 'steady state' are called coherent sequences.

Front 59

Is spoiled gradient echo considered a classic steady state sequence

Back 59

no it is not a classic steady sate but a point of confusion is when it is called steady state incoherent sequence

Front 60

How does steady state work

Back 60

Graphs show steady-state formation in GRE pulse sequences. In A, magnetization before application of an RF excitation pulse is shown.

Front 61

B, the RF pulse tips the magnetization by θ°

Back 61

In C, the resulting tipped magnetization has an LM that recovers and a TM that decays during the TR period.

Front 62

seen in D, during the TR period, the TM can precess through a 180° phase shift in the transverse plane

Back 62

n E, the succeeding RF pulse simultaneously tips a component of residual TM back along the +Z axis and a portion of LM into the transverse plane. In F, after several TR periods, this feeding of LM into TM and vice versa establishes a steady state of both LM and TM.

Front 63

How many lines of K space are filled with each bipolar readout gradient

Back 63

only one but this is still really fast

Front 64

How is the fact that GRE has a sensitivity to magnetic field inhomogenites good and bad

Back 64

more sensitive to magnetic field inhomogeneity and magnetic susceptibility then SE
can be an advantage (imaging of hemorrhage)
can be a disadvantage (loss of signal and geometric distortion)

Front 65

Does GRE typically have decreased signal to noise

Back 65

yes

Front 66

What are some differences of GRE to spin echo

Back 66

Front 67

What determines the contrast of a GRE sequence

Back 67

Gradient Recalled Echo (GRE) produces T2*, T1, and mixed-weighted images (T1/T2*, T2/T1) depending mainly on flip angle, TR and TE and specific GRE technique used.

Front 68

What does the term spoiled mean in spoiled GRE

Back 68

Residual transverse magnetization is destroyed (spoiled) after each echo

Front 69

What are the 2 ways to spoil a gradient

Back 69

Spoiling occurs by either:
gradient spoiling: strong spoiler gradient (shown above)
RF spoiling: phase changes in RF excitation (most common)

Front 70

What does destroying the transverse magnetization by use of a strong spoiler gradient look like

Back 70

Front 71

Why is spoiled GRE sequences used

Back 71

Spoiled GRE imaging allows for very short TR values and high T1 contrast by using a spoiler gradient to destroy any transverse magnetization remaining after each echo

Front 72

What is an example of an RF pulse that is used to spoil transverse magnetization and therefore increase T1 weighting in spoil GRE

Back 72

Front 73

How many rows of K-space are filled in this example

Back 73

4