Physics: Mri Gre (In Practice)

Front 1

What is the flip angle of a gradient echo

Back 1

the gradient echo uses a flip angle

Front 2

Why does gre use flip angles.

Back 2

So TR and therefore scan time is reduced

Front 3

What is used to rephase FID in GRE sequences

Back 3

gradient

Front 4

What is an advantage of a gradient echo over a pulse

Back 4

it is faster.

Front 5

What has a greater transverse magnetization after excitation; spin echo or gradient echo

Back 5

gradient echo

Front 6

What happens after the RF pulse is withdrawn

Back 6

the FID signal is produced due to inhomogeneties

Front 7

What is the cause of the echo in gradient echo

Back 7

the FID is dephased then rephased (and subsequently dephased) by the bipolar readout gradient

Front 8

What is the name of the center of the gradient

Back 8

the isocenter

Front 9

What does the isocenter look like

Back 9

Front 10

What happens to the protons above and below the isocenter

Back 10

they will precess and a slower or faster frequency then that of the main magnetic field

Front 11

What is the polarity of a gradient

Back 11

The
direction of the gradient that determines which end is high and which end
is low is called the polarity.

Front 12

What determines the polarity of the gradient

Back 12

the direction of the current in the gradient coil

Front 13

What happens to the precession frequency above and below the isocenter

Back 13

Magnetic moments experiencing an increased field strength due to the
gradient speed up, i.e. their precessional frequency increases.
. Magnetic moments experiencing a decreased magnetic field strength
slow down, i.e. their precessional frequency decreases

Front 14

Since gradients can change the frequency of protons what does this mean

Back 14

As gradients can cause nuclei to speed up or slow down, they can be
used to either dephase or rephase their magnetic moments

Front 15

How does the gradient cause dephasing

Back 15

It all depends what condition the nuclei are in when the gradient is turned on. If they are inphase it will cause them to get out of phase. On the left the protons are in phase and therefore they will become out of phase (kinda how the positive lobe of the readout gradient gets every thing in phase and then out of phase)

Front 16

Now the gradient is causing rephasing

Back 16

Front 17

What are 2 ways that a gradient is faster

Back 17

gradients rephase faster
smaller flip angle

Front 18

What is meant by the term saturation

Back 18

this is when complete longitudinal relaxation does not occur in a single TR interval such that the next excitation will push the transverse magnetization beyond 90 degrees (thereby lessening its signal) to eventually it will go to 180 degrees where there is no signal .

Front 19

IMPORTANT:Why are we able to shorten the TR interval in GRE

Back 19

since the flip angle is smaller less time is required to get to longitudinal relaxation. This is important because it prevents saturation (which results in loss of signal) Therefore the TR interval can be small

Front 20

What is the main disadvantage of GRE sequences

Back 20

there is no compensation for magnetic field inhomogenities

Front 21

What does saturation look like

Back 21

Front 22

What does the TR control in gre

Back 22

the amount of relaxation that occurs before the next pulse is applied

Front 23

How is the T1 contrast of the image maximized in GRE

Back 23

In order to maximise Tl differences, neither the fat nor
water vectors must have had time to recover full longitudinal magnetisa-tion before the next RF pulse is applied

Front 24

How is longitudinal relaxation prevented

Back 24

To avoid full recovery, the flip
angle is large and the TR short

Front 25

How is the T2 effect minimized

Back 25

To minimise T2*
differences, the TE is short so that neither fat nor water has had time to
decay

Front 26

What is the objective behind maximized in GRE

Back 26

To maximise T2* decay, the TE is long so that the fat and water vectors
have had time to decay sufficiently to show their decay differences.

Front 27

How is the T2 star effect increased

Back 27

To
minimise Tl recovery, the flip angle is small and the TR long enough to
permit full recovery of the fat and water vectors.

Front 28

How is the TE controlled

Back 28

by adjusting the rephasing gradient timing

Front 29

Does saturation occur with T2 weighting images

Back 29

no

Also note how the TE time is longer (by slightly delaying the readout gradient)

Front 30

How is the T1 time decreased

Back 30

To minimise Tl
recovery, the flip angle is small and the TR long enough to permit full
recovery of longitudinal magnetisation.

Front 31

Can the TE be shorter than in regular spin echo sequences

Back 31

yes, because the gradient will allow rephasing faster

Front 32

How is T1 increased

Back 32

by increasing the flip angle (saturation) and decreasing the TR. Also be shortening the TE time

Front 33

How is T2 star increased

Back 33

decreasing the flip angle
increasing the TR
lengthen TE time where the contrast differences are greatest

Front 34

What are the typical values in GRE sequences

Back 34

Long TR 100 ms+
Short TR less than 50 ms
Short TE 5-10 ms
Long TE 15-25 ms
Low flip angles 5o-20o
Large flip angles 70o-110o

Front 35

What is considered a long TR time in gradient echo

Back 35

100ms or longer

Front 36

What is considered a short TR time in gradient echo

Back 36

less than 50ms

Front 37

What is consideered a long TE in gradient echo

Back 37

15-25ms

Front 38

What is consdered a short TE in GRE

Back 38

5-10

Front 39

remember as saturation occurs then there will be decreased t2

Back 39

Front 40

Why is the frequency encoding gradient initialy applied negatively

Back 40

The frequency encoding gradient is initially applied negatively to
speed up the dephasing of the FID, and then its polarity is reversed
producing rephasing of the gradient echo

Front 41

What does a gradient echo sequence look like

Back 41

Front 42

What determines saturation of the transverse magnetization

Back 42

flip angle and TR time. The more saturation the greater the T1

Front 43

What does TE controld

Back 43

The TE controls the amount of T2*
dephasing. To minimise T2* the TE should be short. To maximise it,
the
TE should be long.

Front 44

What causes steady state to occur

Back 44

The steady state is a condition where the TR is shorter than the Tl and T2
times of the tissues.

Front 45

What is the result of steady state

Back 45

the longitudinal AND the transverse magnetization are held in a steady state

Front 46

Is there time for the transverse magnetization to decay before the repeated flip angle

Back 46

no

Front 47

What are the standard parameters to achieve steady state

Back 47

angles of 30°
to 45° in conjunction with a TR of 20 to 50 ms achieves the
steady state.

Front 48

What is the transverse magnetization that is produced as a result of previous transverse magnetization called

Back 48

This transverse magnetisation, produced as a result of
previous excitations, is called the residual transverse magnetisation.

Front 49

What is the appearance of fluid in SSGRE

Back 49

It
affects image contrast as it results in tissues with long T2 times (such as
fluid), appearing bright on the image.

Front 50

Why is SSGRE commonly used

Back 50

Most gradient echo sequences use the steady state as the shortest TR
and scan time is achieved

Front 51

What is the fluid signal for coherent (inphase) gre sequences

Back 51

Generally, in phase residual
transverse magnetisation gives tissues with a long T2 time a bright signal.

Front 52

What essential for the timing of the RF pulse in SSGRE

Back 52

The steady state involves repeatedly applying RF pulses at time intervals
less than the T2 and T1 times of all the tissues

Front 53

What 2 signals are generated from SSGRE sequences

Back 53

(1) a FID signal which occurs as a result of the withdrawal of the RF pulse
and contains T2* information,
(2) a spin echo whose peak occurs at the same time as an RF pulse

Front 54

When does the spin echo signal of SSGRE occur

Back 54

during the next RF pulse

Front 55

How can a rephasing occur at the RF pulse if it is causing its own FID

Back 55

This happens because every RF pulse (regardless of its net amplitude)
contains individual radio waves that have sufficient energy to rephase a previous FID

Front 56

Note that there is an echo of the first FID that occurs during the next sequence

Back 56

Note that the echo occurs during the 3rd RF pulse

Front 57

What is the name of the echo that is produced in SSGRE

Back 57

. The spin echoes produced are
sometimes called Hahn or stimulated echoes

Front 58

What is the T1 time of water, fat and CSF

Back 58

2500 water
200 fat
2000 CSF

Front 59

What is the T2 time of water, fat and CSF

Back 59

2500ms water
100ms fat
300ms CSF

Front 60

What is the TR equivalent to in SSGRE

Back 60

the Tau of the spin echo

Front 61

What is coherent gradient echo

Back 61

Pulse sequences that use coherent magnetisation use a variable flip angle
excitation pulse followed by gradient rephasing, to produce a gradient
echo

Front 62

Are coherent gradient echos steady state

Back 62

yes

Front 63

How is the residual magnetization in gradient echo kept coherent

Back 63

These sequences keep this residual magnetisation coherent by a
process known as rewinding

Front 64

How does phase rewinding work

Back 64

Rewinding is achieved by reversing the
slope of the phase encoding gradient after readout. This results in the
residual magnetisation rephasing, so that it is in phase at the beginning of
the next repetition.

Front 65

What does phase encoding look like

Back 65

Rewinding is achieved by reversing the
slope of the phase encoding gradient after readout. This results in the residual magnetisation rephasing, so that it is in phase at the beginning of
the next repetition.

Front 66

What is the appearance of a coherent GRE

Back 66

Coherent gradient echo pulse sequences produce images that are T2* weighted . As fluid is bright they are often said to give an angiographic, myelographic or arthrographic effect.

Front 67

What are the parameters of a coherent SSGRE

Back 67

To maintain the steady state
Flip angles 30o-45o
TR 20-50 ms
To maximise T2*
Long TE 15-25 ms

Front 68

What are the advantages of coherent SSGRE

Back 68

Advantages
Very fast scans, breath holding possible
Very sensitive to flow so good for angiography
Can be acquired in a volume acquisition

Front 69

What does a coherent SSGRE study look like

Back 69

Front 70

How do incoherent GRE sequences work

Back 70

Pulse sequences that use incoherent residual transverse magnetisation,
begin with a variable flip angle excitation pulse, and use gradient rephasing to produce a gradient echo.

Front 71

How does the RF pulse in GRE spoiled sequences work

Back 71

It does it by encoding the signal that is going to be received to a certain phase of the frequency. This means there is residual transverse magnetization but it is not detected by the reciever coil.

Front 72

The top image shows that the RF pulse was in a certain phase and that phase will be recieved during the readout gradient like normal

Back 72

The second RF pulse gives a different phase excitation. It does rephase the transverse magnetization of the previous pulse but since it is in a different phase it will not be detected at the end of the sequence

Front 73

See ppt for gradient spoiling

Back 73

yes

Front 74

Why is gradient echo not good at measuring the T2 of a tissue

Back 74

this is because the TE is very short and not longer than the approximately 70ms required for T2W imaging

Front 75

What other factor contributes to the crappyness of T2 weighting in gradietn echo sequences

Back 75

the gradient echo is very inefficient in eliminating magnetic inhomogenities and therefore the T2 star (magnetic inhomogenities dominate)

Front 76

What is the function of the SSFP images (steady state free precession)

Back 76

this sequence overcomes the problems with acquiring T2 during GRE

Front 77

How does SSFP work

Back 77

this works by eliminating the signal from the FID and only transmitting the frequency from the hahn (stimulated echo). This makes it sort of like a spin echo sequence except the echo is caused by the gradient.

Front 78

What is the function of the rewinder gradient in the SSFP sequence

Back 78

in order for this sequence to work the echo signal can not occur at the same time as the 3rd RF pulse. In order to have the echo signal occur earlier the 3rd RF pulse a rewinder gradient is used to speed up the rephasing of the protons.

Front 79

How many TE's are there in SSFP

Back 79

2

Front 80

What are the 2 types of TE in SSFP

Back 80

actual TE
effective TE

Front 81

What is the actual TE

Back 81

this is the time from the echo (signal) to the next excitation pulse

Front 82

What is the effective TE

Back 82

this is from the initial RF pulse to the echo being measured (greater than TR)

Front 83

What equation will give you effective TE

Back 83

2 x TR - actual TE

actual TE is the time from the signal to the next RF pulse

Front 84

What is the function of the rewinder gradient in the SSFP sequence

Back 84

to make it so the signal does not occur during the the 3rd RF pulse

Front 85

By using the rewinder gradient to move the echo (signal) earlier what else is achieved besides moving the signal before the RF pulse

Back 85

it gives the protons more time to dephase and therefore they can be T2 weighted.

Front 86

Prior incorrect slide. The echo in steady state free precession is caused by an RF pulse not the gradient

Back 86

ok

Front 87

When is SSFP used

Back 87

this is used in brains and knees with both 2D and 3D volumetric acquisition.

Front 88

What has replaced SSFP imaging

Back 88

FSE because better quality images with equal scan time

Front 89

What is another type of imaging besides SSFP that use shifting of the echo

Back 89

MR perfusion

Front 90

So RF pulses have many different types of frequencies with the net pulse being the sum of these

Back 90

yes

Front 91

How long does it take the FID to have a echo signal

Back 91

at the same time as the 3rd RF pulse

Front 92

What is done to overcome this problem

Back 92

rewinder gradient

Front 93

Is the FID signal used in SSFP

Back 93

no, not a direct signal. The only signal detected is a result of the echo of the FID

Front 94

What occurs at the RF pulse during SSFP

Back 94

2 different many RF pulses are created but the important ones are the 180 for the spin echo and the small flip angle to create the FID

Front 95

Why is the T2 signal in SSFP of better quality than regular GE

Back 95

The rephasing has been initiated by an RF pulse rather than a gradient
so that more T2 and less T2*
information is present.

Front 96

What 2 signals are produced in a steady state sequence

Back 96

(1) a FID,
(2) a spin echo made up of the residual transverse magnetisation
component.

Front 97

What signal is actually used in SSFP

Back 97

the spin echo (FID is not directly used)

Front 98

IMPORTANT: Gradient echo sequences are classified
according to whether the residual transverse magnetisation is in phase,
(coherent), or out of phase, (incoherent).

Back 98

Gradient echo sequences are classified
according to whether the residual transverse magnetisation is in phase,
(coherent), or out of phase, (incoherent).

Front 99

How is the residual transverse magnetization put in phase

Back 99

rewinder gradient

Front 100

What do in phase residual transverse magnetization look like when it is in phase

Back 100

Generally, in phase residual
transverse magnetisation gives tissues with a long T2 time a bright signal.

Front 101

Do coherent steady state sequences have a high T2

Back 101

yes

Front 102

What are incoherent GRE sequences also known as

Back 102

spoiled

Front 103

Do incoherent pulses use FID only

Back 103

yes

Front 104

How do you differentiate the coherent steady state, spoiled steady state and SSFP

Back 104

coherent- FID and spin echo
spoiled- FID only
SSFP- Spin Echo only