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57 Cards in this Set

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Are coils sensitive to transverse magnetization
Coils are only sensitive to variations of transverse magnetization vector
What signal occurs after a 90 degree pulse
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
What is another name for FID
This decay is characterized by time constant T2*
Is T2* longer than T2
T2* is always shorter than T2.
What is the function of the 180 pulse in spin echo sequences
The 180° RF pulse reverses dephasing due to static field inhomogeneities (T2* effects) but not random spin-spin relaxation (T2 effects, tissue-specific)
What are the 2 pulses involved in a spin echo sequence
Spin Echo sequence requires an excitation pulse (90° RF pulse) and a 180° rephasing pulse
What is the time between the the 90 pulse and the 180 pulse
the TE/2
What does a spine echo sequence look like
When is the MR signal acquired
at TE when the signal is the strongest
How many times must the pulse sequence be repeated
The 90° - 180° RF pulses sequence must be repeated as many times as the number of lines in the data matri
What is the time between each pulse sequence called
The time between each 90° RF pulse (excitation pulse) is called Repetition Time (TR).
Are transverse relaxation and longitudinal relaxation simultaneous
tranverse relaxation (transverse magnetization decay, producing MR signal) and longitudinal relaxation (longitudinal magnetization recovery) are simultaneous
What happens during a long TR
The longer the TR is, the more longitudinal magnetization will recover.
What happens if there is a long TR
TR modifies T1-weighting : the longer is the TR, the more T1-weigthed the image is
What happens if there is a long TE
TE modifies T2-weighting : the shorter is the TE, the less T2-weigthed the image is
What if there is a short TR and a short TE
A short TR court and a short TE court give a T1-weighted image
What if there is a long TR and a long TE
A long TR long and a long TE long give a T2-weighted image
What if there is a long TR and a short TE
PD
What happens when the 90 pulse occurs
After a 90° RF pulse, net magnetization tips down so that longitudinal magnetization has disappeared and transverse magnetization has appeared.
What 3 things occur after the 90 degree RF pulse
transverse magnetization decays
longitudinal magnetization recovers
protons re-radiate the absorbed energy
How do coils in the transverse plane receive a signal
Coils can receive the signal in the transverse plane due to variations of transverse magnetization vector
What is T2* in the abscence of a magnetic gradient
In absence of any magnetic gradient, this signal is called Free Induction Decay (FID).
What does T2* take into account when decaying
tissue specific spin-spin relaxation (random interactions between spins) responsible for pure T2decay
static inhomogeneities in magnetic fields which accelerate spins dephasing
FID and T2*
What is the function of the 180 degree pulse
rephase spins and reverse static field inhomogeneities
What happens after the 90 degree pulse
After a 90° RF pulse, spins dephase and transverse magnetization decreases. If we apply a 180° RF pulse, spins rephase and transverse magnetization reappears
What is the time after the 90 pulse called
After the 90° RF pulse spins dephase (during a time defined as TE/2)
What are the 2 parameters of spin echo sequences
echo time
repitition time
What is repetition time
Repetition Time is the time between 2 excitations pulses (time between two 90° RF pulses)
What is echo time
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.
What is the definition of T1
After time T1, longitudinal magnetization has returned to 63 % of its final value. T1 defines the recovery rate of longitudinal magnetization
What does a T1 curve look like
What is T1 time referring to
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
What does a T2 curve look like
How does TR effect 2 different tissues
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.
What is the relative magnitude (amplitude)of the transverse magnetization of the hypotheical Tissue A and tissue B after a long TR
they will have the same amplitude
Long TR and notice both of these lines are the same height (magnitude)
Long TR and notice both of these lines are the same height (magnitude)
Shorter TR and tissue B (dark red) transverse amplitude is not as high
Shorter TR and tissue B (dark red) transverse amplitude is not as high
What happens if TR is set long
What is the result of a short TR and TE
Long TR and short TE
Long TR and TE
Do all images really have a combination of T1 T2 and PD
yes
What is the appearance of a tissue with a long T1 and T2
A tissue with a long T1 and T2 (like water) is dark in the T1-weighted image and bright in the T2-weighted image.
Are long T2 tissues bright
yes
Are long T1 tissues dark
yes
What is the appearance of fat in a T2W image
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.
What is the appearance of gadolinium
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
In clinical practice what is considered a short TE
A short TE is usually lower than 30 ms
In clinical practice what is considered a long TR
A long TR = 3 times the short TR, usually greater than 1500 ms
In clinical practice what is considered a short TR
A short TR = value approximately equal to the average T1 value, usually lower than 500 ms
Is TE always shorter than TR
yes, TE is always shorter than TR
What is considered a long TE in clinical practice
A long TE = 3 times the short TE, usually greater than 90 ms
What is the key to a good MR sequence
a good MRI sequence gives high tissue contrast but lasts the shortest time possible
What is the incline portion of this graph
before the 180 pulse
What is the decline portion of this graph
after the 180 pulse
Can you see how the differenc that is detected is because of T1 effect (short TR and TE)