MRI Flashcards
What is precession?
The spin of the nuclei on its own axis due to the influence of the external magnetic field.
The frequency of spin of a nucleus on its own axis in the presence of an external magnetic field is called?
Larmor frequency
What is resonance?
An energy transition that occurs when an object is subjected to a frequency the same as its own.
What are two factors that make for a good image?
Signal & resolution
What does “signal” mean in MRI?
The amount of information on an image.
What can improve signal-to-noise ratio?
Anything that can increase number of protons in a voxel:
1) Inc slice thickness
2) Inc field of view
3) Inc voxel size (dec imaging matrix)
Also, signal acquisitions (or signal averages) – allowing for repeat signals (but inc. imaging time).
What are 2 causes that spins lose their phase coherence when a RF excitation pulse is removed?
1) Spin-spin energy transfer (= interactions of the intrinsic magnetic fields of adjacent nuclei)
2) Inhomogeneities of the external magnetic fields
Btw, another name for this process is T2*
What is another name for longitudinal relaxation?
Spin-lattice relaxation (called spin-lattice because the energy that the proton gives up from being excited by the RF is given off to the surrounding lattice).
What does T1 mean?
The time when 63% of the longitudinal magnetization is reached.
What is another name for transverse relaxation?
Spin-spin relaxation
What makes T2* different from T2?
T2* has a faster exponential decay
T2 has a 180 degree pulse that minimizes the inhomogeneities, and hence being longer than T2*
What are the sequence of events in a spin echo pulse sequence?
90 deg – TE/2 – 180 deg – TE/2 – record signal (echo) at TE
After the signal is given, a little while later, another round of 90 deg pulsation is sent.
What is unique about the pulse sequence in a STIR image?
It gives the 180 deg pulse first, then the 90 deg.
What is the most time consuming parameter of an imaging sequence?
TR
What do you do to TR in a fast spin echo sequence?
Use a short TR
What happens when a short TR is used (as in a fast spin echo sequence)?
1) An 180 deg pulse cannot be administered b/c it requires a long TR.
2) There’s a small longitudinal magnetization recovery before the next pulse is administered –> yields low signal
What is introduced into the magnetic field to get a gradient echo?
A magnetic field gradient –> causes further inhomogeneties to the external magnetic field
What does a stronger external magnetic field mean in respect to T1?
A longer T1.
Of the following, which is the fastest sequence?
a) T2W spin echo
b) T1W spin echo
c) Proton density spin echo
d) Gradient echo
e) STIR
Gradient echo
Why are GE sequences faster?
1) Don’t use 180 deg pulses
2) Shorter flip angles (5-30 deg)
3) Don’t have to wait for long TRs to decay to see longitudinal magnetization reappear (given the small flip angles)
What is cross talk in MRI?
When adjacent slices are acquired, some interference from one slice may spill over into the adjacent slice.
What is spatial resolution?
Ability to distinguish small objects.
What decreases spatial resolution on MRI?
The same things that increase signal-to-noise ratio.
What are the drawbacks of a fast spin echo sequence?
1) Blurring of edges
2) Fat intensity remains bright on T2 (obscures pathology b/c of similar fat & water intensities)
Which imaging is best to demonstrate fibrocartilage?
GE
