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32 Cards in this Set
- Front
- Back
•Using a very long echo trains
•Fill all lines of K space in one repetition (called single shot) •Gradients switched so many times – this is called blipping •This is currently fastest acquisition models in MRI •Real time imaging, Dynamic imaging, Functional imaging, Diffusion weighted imaging (DWI), Perfusion imaging are based on EPI THESE DESCRIBE WHICH TYPE OF IMAGING? |
Echo planar imaging (EPI)
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•Separating FID and MR signal
•TE is longer than TR •Rephase is initiated by an RF pulse, than with gradient •True T2 image |
Steady state free precession
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•This sequence should be used when T1 weighted images are required
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Incoherent gradient echo
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•SSFP is a steady state sequence that obtains images that have a sufficiently long TE and less T2* when using the steady state than other gradient echo pulse sequences.
TRUE OR FALSE? |
TRUE
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•Scan time is reduced
•Centric K space filling •Use for breath – hold, dynamic imaging after contrast •Gradient echo sequences do not demonstrate true T2 weighting because: - the TE is never long enough; -gradient rephasing is so inefficient, that any echo is dominated by T2* effects. - |
Incoherent gradient echo
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• Gradient spoiling is not as efficient as RF spoiling but T2* weighting can also be achieved. Gradient spoiled sequences are therefore more
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versatile than RF spoiled sequences.
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• Gradient spoiling involves
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applying a gradient that dephases the residual transverse magnetization
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. RF spoiling applies
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. RF spoiling applies RF excitation pulses at different phases every TR so that the residual transverse magnetization also has different phase values (and is therefore dephased).
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tissues with long T2 times are not allowed to dominate image contrast but T1/proton density contrast prevails; this is called
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spoiling
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•Variable flip angle (same as coherent)
•The steady state is maintained, so that residual transverse magnetization is left from previous repetitions. •That transverse magnetization from previous excitation is used This describes which type of pulse sequence? |
Incoherent gradient echo
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•These sequences keep this residual magnetization coherent by process known as rewinding.
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Coherent gradient echo
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•T2* used for blood pathologies
-Flow effect volume imaging, hemorrhages, angiographic Advantage: very fast scan time, breath hold possible, increase sensitivity to flow Disadvantage: poor SNR This describes what? |
Coherent gradient echo T2* (use in GE)
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These are a result of Coherent gradient echo,
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•These sequences keep this residual magnetization coherent by process known as rewinding.
•MR signal has FID and MR signal |
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•After the echo any residual transverse magnetization that still remains begins to dephase. The gradient used to phase encode at the beginning of the sequence is then
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switched on with opposite polarity and the same amplitude to the phase encode.
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•After the echo any residual transverse magnetization that still remains begins to dephase. The gradient used to phase encode at the beginning of the sequence is then switched on with opposite polarity and the same amplitude to the phase encode.
•This results in the residual transverse magnetization rephasing so that it is always in phase and therefore preserved when the next excitation pulse is applied. WHAT TYPE OF PS IS THIS? |
Coherent gradient echo
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The two main types of gradient echo sequence are called:
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• • coherent or rewind sequences;
• • incoherent or spoiled sequences. |
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•Gradient echo pulse sequences can generally be categorized according to whether they utilize the steady state
TRUE OR FALSE |
true
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•Most gradient echo sequences utilize the steady state because - ? -
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the TRs are so short that the fastest scan times are achievable.
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residual transverse magnetization affects ?
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image contrast.
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•Because the transverse magnetization does not have time to decay, its magnitude DOES WHAT?
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accumulates over successive TRs. This residual transverse magnetization therefore affects image contrast.
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•Steady state –
•The energy “in” equals |
stable condition
the energy “out” |
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•Steady state – stable condition
•The energy “in” equals the energy “out” •The NMV does not move between repetition times •This is a situation when the TR is shorter than both the T1 and T2 relaxation times of all the tissues. WHICH TYPE OF PS IS THIS/ |
The steady state and echo formation
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•Tissues with long T2 times (mainly water) appear bright. Most gradient echo sequences utilize the steady state because the 1. __ __ _ ___ ___ ___ that the fastest scan times are achievable
• |
TRs are so short
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•Parameters:
•T1 •PD •T2 |
>45
<20 >20, <45 |
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•Parameters:
•T1 |
>45
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Parameters:
•PD |
<20
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ParAmeters:
•T2 |
>20, <45
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•Gradient echo images are more sensitive to external magnetic field
TRUE OR FALSE |
TRUE
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•The longer the TE, the more ?
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•The longer the TE, the more T2 contrast in the image
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Echo planar imaging (EPI), • This is currently fastest acquisition models in MRI T or F
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TRUE
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Echo planar imaging (EPI)
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1 • Using a very long echo trains
2 • Fill all lines of K space in one repetition (called single shot) |
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• Gradients switched so many times – this is called 1. _____ and is used in whart type of imaging?
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1 blipping
2 Echo planar imaging (EPI) |