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