Physics: Mri Signals 1

Front 1

Are coils sensitive to transverse magnetization

Back 1

Coils are only sensitive to variations of transverse magnetization vector

Front 2

What signal occurs after a 90 degree pulse

Back 2

After a 90° RF pulse, the Free Induction Decay (FID) signal is oscillating at resonance frequency and signal enveloppe is a decay curve described as an exponential curve, depending on tissue-specific spin-spin relaxation and static field inhomogeneities

Front 3

What is another name for FID

Back 3

This decay is characterized by time constant T2*

Front 4

Is T2* longer than T2

Back 4

T2* is always shorter than T2.

Front 5

What is the function of the 180 pulse in spin echo sequences

Back 5

The 180° RF pulse reverses dephasing due to static field inhomogeneities (T2* effects) but not random spin-spin relaxation (T2 effects, tissue-specific)

Front 6

What are the 2 pulses involved in a spin echo sequence

Back 6

Spin Echo sequence requires an excitation pulse (90° RF pulse) and a 180° rephasing pulse

Front 7

What is the time between the the 90 pulse and the 180 pulse

Back 7

the TE/2

Front 8

What does a spine echo sequence look like

Back 8

Front 9

When is the MR signal acquired

Back 9

at TE when the signal is the strongest

Front 10

How many times must the pulse sequence be repeated

Back 10

The 90° - 180° RF pulses sequence must be repeated as many times as the number of lines in the data matri

Front 11

What is the time between each pulse sequence called

Back 11

The time between each 90° RF pulse (excitation pulse) is called Repetition Time (TR).

Front 12

Are transverse relaxation and longitudinal relaxation simultaneous

Back 12

tranverse relaxation (transverse magnetization decay, producing MR signal) and longitudinal relaxation (longitudinal magnetization recovery) are simultaneous

Front 13

What happens during a long TR

Back 13

The longer the TR is, the more longitudinal magnetization will recover.

Front 14

What happens if there is a long TR

Back 14

TR modifies T1-weighting : the longer is the TR, the more T1-weigthed the image is

Front 15

What happens if there is a long TE

Back 15

TE modifies T2-weighting : the shorter is the TE, the less T2-weigthed the image is

Front 16

What if there is a short TR and a short TE

Back 16

A short TR court and a short TE court give a T1-weighted image

Front 17

What if there is a long TR and a long TE

Back 17

A long TR long and a long TE long give a T2-weighted image

Front 18

What if there is a long TR and a short TE

Back 18

PD

Front 19

What happens when the 90 pulse occurs

Back 19

After a 90° RF pulse, net magnetization tips down so that longitudinal magnetization has disappeared and transverse magnetization has appeared.

Front 20

What 3 things occur after the 90 degree RF pulse

Back 20

transverse magnetization decays
longitudinal magnetization recovers
protons re-radiate the absorbed energy

Front 21

How do coils in the transverse plane receive a signal

Back 21

Coils can receive the signal in the transverse plane due to variations of transverse magnetization vector

Front 22

What is T2* in the abscence of a magnetic gradient

Back 22

In absence of any magnetic gradient, this signal is called Free Induction Decay (FID).

Front 23

What does T2* take into account when decaying

Back 23

tissue specific spin-spin relaxation (random interactions between spins) responsible for pure T2decay
static inhomogeneities in magnetic fields which accelerate spins dephasing

Front 24

FID and T2*

Back 24

Front 25

What is the function of the 180 degree pulse

Back 25

rephase spins and reverse static field inhomogeneities

Front 26

What happens after the 90 degree pulse

Back 26

After a 90° RF pulse, spins dephase and transverse magnetization decreases. If we apply a 180° RF pulse, spins rephase and transverse magnetization reappears

Front 27

What is the time after the 90 pulse called

Back 27

After the 90° RF pulse spins dephase (during a time defined as TE/2)

Front 28

What are the 2 parameters of spin echo sequences

Back 28

echo time
repitition time

Front 29

What is repetition time

Back 29

Repetition Time is the time between 2 excitations pulses (time between two 90° RF pulses)

Front 30

What is echo time

Back 30

Echo Time (TE) is the time between the 90° RF pulse and MR signal sampling, corresponding to maximum of echo. The 180° RF pulse is applied at time TE/2.

Front 31

What is the definition of T1

Back 31

After time T1, longitudinal magnetization has returned to 63 % of its final value. T1 defines the recovery rate of longitudinal magnetization

Front 32

What does a T1 curve look like

Back 32

Front 33

What is T1 time referring to

Back 33

the time at which the curve has recovered 67% look at the lines. If there is a short T1 (fat) that line will be at a short time

Front 34

What does a T2 curve look like

Back 34

Front 35

How does TR effect 2 different tissues

Back 35

Let's consider 2 tissues A and B with different T1s. If TR is very long, even if tissue A has a longuer T1 than tissue B, the longitudinal magnetization of both tissues will recover completely before the next excitation.

Front 36

What is the relative magnitude (amplitude)of the transverse magnetization of the hypotheical Tissue A and tissue B after a long TR

Back 36

they will have the same amplitude

Front 37

Long TR and notice both of these lines are the same height (magnitude)

Back 37

Long TR and notice both of these lines are the same height (magnitude)

Front 38

Shorter TR and tissue B (dark red) transverse amplitude is not as high

Back 38

Shorter TR and tissue B (dark red) transverse amplitude is not as high

Front 39

What happens if TR is set long

Back 39

Front 40

What is the result of a short TR and TE

Back 40

Front 41

Long TR and short TE

Back 41

Front 42

Long TR and TE

Back 42

Front 43

Do all images really have a combination of T1 T2 and PD

Back 43

yes

Front 44

What is the appearance of a tissue with a long T1 and T2

Back 44

A tissue with a long T1 and T2 (like water) is dark in the T1-weighted image and bright in the T2-weighted image.

Front 45

Are long T2 tissues bright

Back 45

yes

Front 46

Are long T1 tissues dark

Back 46

yes

Front 47

What is the appearance of fat in a T2W image

Back 47

A tissue with a short T1 and a long T2 (like fat) is bright in the T1-weighted image and gray in the T2-weighted image.

Front 48

What is the appearance of gadolinium

Back 48

Gadolinium contrast agents reduce T1 and T2 times, resulting in an enhanced signal in the T1-weighted image and a reduced signal in the T2-weighted imag

Front 49

In clinical practice what is considered a short TE

Back 49

A short TE is usually lower than 30 ms

Front 50

In clinical practice what is considered a long TR

Back 50

A long TR = 3 times the short TR, usually greater than 1500 ms

Front 51

In clinical practice what is considered a short TR

Back 51

A short TR = value approximately equal to the average T1 value, usually lower than 500 ms

Front 52

Is TE always shorter than TR

Back 52

yes, TE is always shorter than TR

Front 53

What is considered a long TE in clinical practice

Back 53

A long TE = 3 times the short TE, usually greater than 90 ms

Front 54

What is the key to a good MR sequence

Back 54

a good MRI sequence gives high tissue contrast but lasts the shortest time possible

Front 55

What is the incline portion of this graph

Back 55

before the 180 pulse

Front 56

What is the decline portion of this graph

Back 56

after the 180 pulse

Front 57

Can you see how the differenc that is detected is because of T1 effect (short TR and TE)

Back 57