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88 Cards in this Set

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How is a MR signal created from a magnetic field created by the proton after the RF pulse
How does a MR create a signal
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
How does a current in a wire produce a signal
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.
When is the T2 MR signal at its maximum
when the transverse magnetization is completely inphase
What is the definition of T2
The definition of T2 is the time that it takes for
the transverse magnetization to decay to 37% of
its original value
What does a T2 curve look like
If a tissue has a long T2 and dephases slowly will it be bright
yes
If a tissue has a short T2 and dephases rapidly what will it be
dark
What has a longer T2; CSF or white matter
CSF
Does CSF have a long T2 and dephase slowly
yes
What are the characteristics of grey matter on T2
Gray matter has an intermediate T2 and dephases
intermediately
What does it mean if the image is timed so the space between CSF and White matter is great
heavily T2 weighted
If a tissue has a long T2 and dephases slowly will it be bright or dark
bright
Do T1 and T2 relaxation process occur simultaneously
yes
What is the 'trick' that can be used to recover dephasing due to all effects (during T2) except spin spin interaction
spin echo
What happens to the protons after the 90 degree pulse is given
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)
How is this dephasing prevented
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.
What happens following the 180 degree pulse in spin echo
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
What does the T2 decay look like in spin echo
What is the immedicate signal after the 90 degree pulse
free induction decay
Note the difference between T2 star decay and T2 decay on the previous diagram
ok
What results from the 180 degree pulse
an echo
What is the time from the initial 90 degree pulse to the echo
Time to Echo (TE)
Can multiple 180 degree pulses be done
yes
What is the TE time
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
What is the time the curve formed by
connecting the peaks of the echoes
represents
decay by T2 effects (spin spin interaction)
What happens if a proton experiences a local increase magnetic field strength compared to a neighboring proton
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.
What happens to this proton prior to a 180 degree pulse
Prior to the 180° RF
pulse, the proton spins faster “away” from its neighbor
What happens to this proton after the 180 degree pulse
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
Why does spin echo eliminate magnetic field inhomgenties, Magnetic susceptibility and
Chemical shift effect
because these are constant and the 180 degree pulse can reverse these
Why does spin echo not work on spin spin interaction
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.
Can an echo be produced without application of a 180 degree pulse
yes
If the echo is not 180 degrees what can be said
the rate of rephasing will be due to all effects Spin-spin interactions
Magnetic field inho-
mogeneities
Magnetic susceptibility
Chemical shift effects
What is a MR pulse sequence
this is a way of depicting the timing of certain events during MR acquisition
What is responsible for localizing the signals of protons in the body
gradient pulses
What is typically shown in a pulse sequence diagram
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).
What is does a pulse sequence diagram look like
What is ADC
when the signal is digitized for storage in the ac-
quisition computer by the analog-to-digital con-
verter (ADC).
What is TE
the time be-tween the peak of the 90° RF pulse and the peak
of the echo that is forme
When does the 180 degree pulse occur
Note that the 180° RF
pulse occurs at half of the echo time TE
What is the repitition time
TR is the time that it takes to run through
the pulse sequence one time.
For basic pulse sequences one time through the pulse sequence provides how many rows of raw data
For basic pulse
sequences, one time through the pulse sequence
provides one row of raw data.
How many times must a pulse sequence be repeated in order to provide the necessary rows to reconstruct an image
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.
If you want to aquire an MR image with a matrix of 256 x256 pixels what does that mean
that is 256 rows of data and 256 columns of data
How many pulse sequences are required to aquire a matrix of 256 x 256 pixels
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.
What is TR time
TR is the time it takes
to go through the pulse sequence one time
If you wanted to aquire a pulse sequence for a 256 x 256 pixel matrix how long would it take
TR x 256
What can be used to control the T1 and T2 wieghting
TE and TR
nWhat does an example of a T1W chart look like
short TE long TR
What are the parameters of a T2W sequence
What are the parameters of a T2W sequence
What are the parameters of a proton density image
What are the TR and TE to create a proton density image
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.
What are the TR and TE for T2 weighting
Parameters for T2
weighting. Long TE (producing maximal T2 weighting) and long TR (producing minimal T1 weighting)
What are the parameters to produce T1W image
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.
What are typical TE and TR parameters for T1W
TE = 20msec
TR= 500msec
What are typical TE and TR parameters for T2W
TE  80 msec and
TR  2,000 msec
What is produced by the initial RF pulse
a free induction decay
Is the free induction decay used
no
When does the RF pulse occur
half the TE
What pulses are used in spin echo sequences
90 followed by a single 180
What is a multiecho spin echo
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.
What are typical parameters of a multiecho spin echo
TR 2000msec
TE1- 20 (proton density)
TE2 80 (T2W)
What does a sequence diagram look like for multiecho spin echo
What is turbo spin echo
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.
How is multiecho spin echo different from turbo spin echo pulse sequences
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
What does a sequence of turbo spin echo look like
What is the advantage of a turbo spin echo sequence
all the echoes are used to create a single image data set
at a faster rate.
What is the echo train length
this is the number of echoes that are formed in turbo spin echo
What happens to the echo once detected by the RF coils
it is digitized and the data is used to form one row of raw data
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
yes
If 256 rows of raw data are required and each pulse sequence gives 4 echoes how many are need
64 rather than 256 (4 times faster)
What happens if the echo train lenght is 8 or 16
then it will decrease the time by a factor of 8 or 16
Can turbo spin echos be used for both T1 and T2 W images
yes
What is inversion recovery sequences used to do
suppress unwanted signals in a MR image (fat, fluid)
What type of pulse is at the beginning of an inversion sequence
180 degree
What does an inversion recovery pulse sequence look like
What is the purpose of the initial 180 RF pulse with inversion recovery sequences
it causes initial inversion in the longitunidal magnetization ( negative Z direction)
What happens ones the protons are aligned antiparrellel
they begin dephase back in the direction of the main magnetic field (positive Z)
When is the second (90 degree pulse) given
When the signal to be suppressed crosses the zero axis (transverse)
Why doesnt the signal to be supressed give off a signal
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.
When do you apply the 90 degree RF pulse in inversion recovery sequences
when the tissue to be supressed is transverse
What is time to inversion
time between 180 pulse and the 90 degree pulse (inversion recovery sequences)
Does fat have a long or short TI
short TI of approxiametely 170msec
What is a gradient echo sequence
the initial pulse is less than 90 degrees (20 or 30 degrees) and lack of 180 degree pulse allows the sequence to be faster.
Can protons be dephased and rephased in gradient echo
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
Do gradient echoes have a short TR
yes and a resultant faster imaging sequence
What is produced in a GRE a T2 or T2star
In this
case, T2-weighted image contrast cannot be pro-
duced; rather, T1 and T2* image contrast can be
produced.