MRI Flashcards
What property does a material need to be considered MR active?
Odd mass number
What is it called when a proton is spinning in its own magnetic field?
Preccession
When B0 is active (RF is off) are there more parrallel or anti-parallel protons?
Parallel (spin-up)
Why does an increase in the magnetic field strength increase SNR?
More spin up protons
WHat is the NMV the product of?
spin up - spin down
The transverse axis is known as M__?
Mxy (mxy , maxymum signal)
What is the resonance frequency for hydrogen in a magnetic field of 5 Tesla
212.9 MHz
What affects the magnitude of the flip angle?
Amplitude (strength) and duration of RF pulse
What controls the flip angle?
The selected pulse sequence
When does saturation start to occur?
91+ degrees - 180
When is the signal considered fully saturated?
180 degree
WHat is phase coherence?
When the NMV is at 90 degrees and there is minimum inhomogenity
What are the two main relaxation processes?
T1 Recovery and T2 Decay
What is the fundamental difference between T1 and T2?
T1 looks at the amount of magnetisation in the longitudinal plane (as it increases) . It is the NMV moving back to B0. Time taken for 63% of NVM to recover.
T2 is the amount of magnetisation in the trans plane as it decreases. Time taken for protons to dephase
Which decay is faster: T2 or T2*?
T2*
If something is appearing black does it typically have a longer or shorter flip angle?
Shorter (not pushed as far into transverse plane).
Which Weighting relies on partial saturation?
T1
Why is a short TR desirable for T1?
Because it will push both water and fat into saturation
Will bone appear bright or dark on a Proton density weighting?
Dark
For T1 does the TR need to be long or short?
Short
for T2 does the TR need to be long or short?
Long and TE is Long too
Will a larger bore size improve SNR?
no. smaller increases SNR
What coil is used to increase homogeneity?
Shim coils
Is 15ppm good?
No we need less than 10 ppm for homogenity
Which provides better SNR: Surface coil or volume coil?
surface
What plane are receiver coils placed in?
transverse plane
What gives the spatial co-ordinates of anatomy?
gradient coils
Why does spin echo give better signal?
it diminishes the signal loss of T2*
What is TE?
Time between the 90° excitation pulse and the spin echo (i.e. signal)
What is TAU?
Time the NMV takes to dephase after the 90° RF pulse was withdrawn
Time taken to rephase after the application of the 180° RF pulse
Te=2TAU
What does TE control?
T2 weighting
Which coil is PEG?
Y
which coil is FEG
X
is PEG signal recorded when the PEG pulse is on?
no
Is the FEG signal recorded whilst the FEG is on?
yes
When is the SSG turned on?
during the RF pulse
In a chemical shift the higher frequencies are shifted which way?
down
What factors can affect chemical shift?
Higher B0 strength Thicker slices (lower gradients and wider bandwidths)
Will a short TE reduce chemical shift?
no we need a longer TE to allow the fat to dephase
We can have a thinner slice to decrease the chance of chemical shift and increase spatial resolution, but what is the trade off?
Lower SNR
How does partial volume averaging occur?
More than one tissue type in a voxel… use a thinner slice to remedy
How does zipper artefact occur?
Outside RF fields interfering with the scanner
3 ways to remedy excitation
1/3 slice thickness gap in contiguous slice mode
Interleaving
Optimised pulses with longer TE
