Physics: Mri Basics 3

Front 1

How is a MR signal created from a magnetic field created by the proton after the RF pulse

Back 1

Front 2

How does a MR create a signal

Back 2

Recall that an
electric current in a wire will produce a magnetic
field perpendicular to the loop of wire. Measure-ment of the transverse magnetization (which is
our “MR signal”) occurs through an opposite ef-fect. In this case, the transverse magnetization,
which is a magnetic field, can induce a current in
a loop of wire

Front 3

How does a current in a wire produce a signal

Back 3

This induced electric cur-rent is then digitized and recorded in the com-puter of the MR system for later reconstruction as
an MR image.

Front 4

When is the T2 MR signal at its maximum

Back 4

when the transverse magnetization is completely inphase

Front 5

What is the definition of T2

Back 5

The definition of T2 is the time that it takes for
the transverse magnetization to decay to 37% of
its original value

Front 6

What does a T2 curve look like

Back 6

Front 7

If a tissue has a long T2 and dephases slowly will it be bright

Back 7

yes

Front 8

If a tissue has a short T2 and dephases rapidly what will it be

Back 8

dark

Front 9

What has a longer T2; CSF or white matter

Back 9

CSF

Front 10

Does CSF have a long T2 and dephase slowly

Back 10

yes

Front 11

What are the characteristics of grey matter on T2

Back 11

Gray matter has an intermediate T2 and dephases
intermediately

Front 12

What does it mean if the image is timed so the space between CSF and White matter is great

Back 12

heavily T2 weighted

Front 13

If a tissue has a long T2 and dephases slowly will it be bright or dark

Back 13

bright

Front 14

Do T1 and T2 relaxation process occur simultaneously

Back 14

yes

Front 15

What is the 'trick' that can be used to recover dephasing due to all effects (during T2) except spin spin interaction

Back 15

spin echo

Front 16

What happens to the protons after the 90 degree pulse is given

Back 16

After a 90° RF pulse, protons
that were in phase begin to dephase in the trans-verse plane due to effects discussed earlier (repre-sented by some spins going faster than the aver-age and some spins going slower than the average)

Front 17

How is this dephasing prevented

Back 17

After a certain amount of time, if a
180° RF pulse is applied, the spins will rotate over
to the opposite axis. Now, rather than the spins
continuing to dephase, the spins will begin to
rephase.

Front 18

What happens following the 180 degree pulse in spin echo

Back 18

The spins will come back together and the sig-
nal measured with our receiver coil will increase,
form a maximum signal, and then decrease as the
spins once again dephase

Front 19

What does the T2 decay look like in spin echo

Back 19

Front 20

What is the immedicate signal after the 90 degree pulse

Back 20

free induction decay

Front 21

Note the difference between T2 star decay and T2 decay on the previous diagram

Back 21

ok

Front 22

What results from the 180 degree pulse

Back 22

an echo

Front 23

What is the time from the initial 90 degree pulse to the echo

Back 23

Time to Echo (TE)

Front 24

Can multiple 180 degree pulses be done

Back 24

yes

Front 25

What is the TE time

Back 25

The
time between the peak of the 90° RF pulse and
the peak of the echo is called the time to echo or
echo time (TE

Front 26

What is the time the curve formed by
connecting the peaks of the echoes

Back 26

represents
decay by T2 effects (spin spin interaction)

Front 27

What happens if a proton experiences a local increase magnetic field strength compared to a neighboring proton

Back 27

If a proton experiences a local increase in mag-netic field strength that is not experienced by a
neighboring proton, it will precess faster than its
neighbor.

Front 28

What happens to this proton prior to a 180 degree pulse

Back 28

Prior to the 180° RF
pulse, the proton spins faster “away” from its neighbor

Front 29

What happens to this proton after the 180 degree pulse

Back 29

After the 180° RF pulse, the spins are
“flipped” and their directions can be thought to
be reversed, so that now the faster proton is “be-
hind” its neighbor and can “catch up” to its
neighbor because it is still spinning faster. On the

Front 30

Why does spin echo eliminate magnetic field inhomgenties, Magnetic susceptibility and
Chemical shift effect

Back 30

because these are constant and the 180 degree pulse can reverse these

Front 31

Why does spin echo not work on spin spin interaction

Back 31

On the
other hand, spin-spin interactions are random
interactions between protons that cause random
local changes in the magnetic fields experienced
by the protons, and this causes dephasing. Be-cause this is a random process, dephasing due to this effect cannot be reversed.

Front 32

Can an echo be produced without application of a 180 degree pulse

Back 32

yes

Front 33

If the echo is not 180 degrees what can be said

Back 33

the rate of rephasing will be due to all effects Spin-spin interactions
Magnetic field inho-
mogeneities
Magnetic susceptibility
Chemical shift effects

Front 34

What is a MR pulse sequence

Back 34

this is a way of depicting the timing of certain events during MR acquisition

Front 35

What is responsible for localizing the signals of protons in the body

Back 35

gradient pulses

Front 36

What is typically shown in a pulse sequence diagram

Back 36

Lines are shown for the timing of RF
pulses, the signal formed from these pulses, and
when the signal is digitized for storage in the ac-quisition computer by the analog-to-digital con-verter (ADC).

Front 37

What is does a pulse sequence diagram look like

Back 37

Front 38

What is ADC

Back 38

when the signal is digitized for storage in the ac-
quisition computer by the analog-to-digital con-
verter (ADC).

Front 39

What is TE

Back 39

the time be-tween the peak of the 90° RF pulse and the peak
of the echo that is forme

Front 40

When does the 180 degree pulse occur

Back 40

Note that the 180° RF
pulse occurs at half of the echo time TE

Front 41

What is the repitition time

Back 41

TR is the time that it takes to run through
the pulse sequence one time.

Front 42

For basic pulse sequences one time through the pulse sequence provides how many rows of raw data

Back 42

For basic pulse
sequences, one time through the pulse sequence
provides one row of raw data.

Front 43

How many times must a pulse sequence be repeated in order to provide the necessary rows to reconstruct an image

Back 43

must repeat the
pulse sequence as many times as necessary to pro-vide as many rows of data as are needed to recon-struct the image.

Front 44

If you want to aquire an MR image with a matrix of 256 x256 pixels what does that mean

Back 44

that is 256 rows of data and 256 columns of data

Front 45

How many pulse sequences are required to aquire a matrix of 256 x 256 pixels

Back 45

Because one time through the pulse sequence
provides one row of data (in 256 columns), we
must repeat the pulse sequence 256 times to ac quire all the rows needed.

Front 46

What is TR time

Back 46

TR is the time it takes
to go through the pulse sequence one time

Front 47

If you wanted to aquire a pulse sequence for a 256 x 256 pixel matrix how long would it take

Back 47

TR x 256

Front 48

What can be used to control the T1 and T2 wieghting

Back 48

TE and TR

Front 49

nWhat does an example of a T1W chart look like

Back 49

short TE long TR

Front 50

What are the parameters of a T2W sequence

Back 50

Front 51

What are the parameters of a T2W sequence

Back 51

Front 52

What are the parameters of a proton density image

Back 52

Front 53

What are the TR and TE to create a proton density image

Back 53

proton density weighting. Short TE (producing mini-
mal T2 weighting) and long TR (producing minimal
T1 weighting) will result in a proton density–weighted
image.

Front 54

What are the TR and TE for T2 weighting

Back 54

Parameters for T2
weighting. Long TE (producing maximal T2 weighting) and long TR (producing minimal T1 weighting)

Front 55

What are the parameters to produce T1W image

Back 55

Parameters for T1 weighting. Short TE (producing minimal T2 weighting) and inter-
mediate TR (producing maximal T1 weighting) will
result in a T1-weighted image.

Front 56

What are typical TE and TR parameters for T1W

Back 56

TE = 20msec
TR= 500msec

Front 57

What are typical TE and TR parameters for T2W

Back 57

TE
80 msec and
TR
2,000 msec

Front 58

What is produced by the initial RF pulse

Back 58

a free induction decay

Front 59

Is the free induction decay used

Back 59

no

Front 60

When does the RF pulse occur

Back 60

half the TE

Front 61

What pulses are used in spin echo sequences

Back 61

90 followed by a single 180

Front 62

What is a multiecho spin echo

Back 62

The multiecho spin-echo pulse sequence uses
multiple 180° RF pulses to generate multiple ech-
oes Each echo occurs at a different TE and is used to generate a seperate image data set.

Front 63

What are typical parameters of a multiecho spin echo

Back 63

TR 2000msec
TE1- 20 (proton density)
TE2 80 (T2W)

Front 64

What does a sequence diagram look like for multiecho spin echo

Back 64

Front 65

What is turbo spin echo

Back 65

This sequence uses a 90° RF pulse with multiple 180° RF pulses.Multiple echoes are formed, and the data are
used to create a single data set.Multiple rows of raw
data are filled during one TR period; this feature allows
the pulse sequence to be run fewer times, thus saving imaging time.

Front 66

How is multiecho spin echo different from turbo spin echo pulse sequences

Back 66

turbo spin echo will look at a single data set in a single TR interval where multiecho spin echo will create mulitple data sets in a single TR interval

Front 67

What does a sequence of turbo spin echo look like

Back 67

Front 68

What is the advantage of a turbo spin echo sequence

Back 68

all the echoes are used to create a single image data set
at a faster rate.

Front 69

What is the echo train length

Back 69

this is the number of echoes that are formed in turbo spin echo

Front 70

What happens to the echo once detected by the RF coils

Back 70

it is digitized and the data is used to form one row of raw data

Front 71

In turbo spin echo if four echos are produced each time through the pulse sequence the digitized data from these 4 echo will fill 4 rows of raw data

Back 71

yes

Front 72

If 256 rows of raw data are required and each pulse sequence gives 4 echoes how many are need

Back 72

64 rather than 256 (4 times faster)

Front 73

What happens if the echo train lenght is 8 or 16

Back 73

then it will decrease the time by a factor of 8 or 16

Front 74

Can turbo spin echos be used for both T1 and T2 W images

Back 74

yes

Front 75

What is inversion recovery sequences used to do

Back 75

suppress unwanted signals in a MR image (fat, fluid)

Front 76

What type of pulse is at the beginning of an inversion sequence

Back 76

180 degree

Front 77

What does an inversion recovery pulse sequence look like

Back 77

Front 78

What is the purpose of the initial 180 RF pulse with inversion recovery sequences

Back 78

it causes initial inversion in the longitunidal magnetization ( negative Z direction)

Front 79

What happens ones the protons are aligned antiparrellel

Back 79

they begin dephase back in the direction of the main magnetic field (positive Z)

Front 80

When is the second (90 degree pulse) given

Back 80

When the signal to be suppressed crosses the zero axis (transverse)

Front 81

Why doesnt the signal to be supressed give off a signal

Back 81

since the signal to be suppressed crosses the zero axis application of a 90 degree RF pulse will not rotate that tissue (already transverse) thus this tissue will not contribute to any brightness to the resultant image.

Front 82

When do you apply the 90 degree RF pulse in inversion recovery sequences

Back 82

when the tissue to be supressed is transverse

Front 83

What is time to inversion

Back 83

time between 180 pulse and the 90 degree pulse (inversion recovery sequences)

Front 84

Does fat have a long or short TI

Back 84

short TI of approxiametely 170msec

Front 85

What is a gradient echo sequence

Back 85

the initial pulse is less than 90 degrees (20 or 30 degrees) and lack of 180 degree pulse allows the sequence to be faster.

Front 86

Can protons be dephased and rephased in gradient echo

Back 86

yes, Even though there
is no 180° RF pulse to produce a spin echo, gradi-
ent pulses (which we have not discussed) can be
used to dephase and rephase the signal in the
transverse plane to form gradient echoes

Front 87

Do gradient echoes have a short TR

Back 87

yes and a resultant faster imaging sequence

Front 88

What is produced in a GRE a T2 or T2star

Back 88

In this
case, T2-weighted image contrast cannot be pro-
duced; rather, T1 and T2* image contrast can be
produced.