MRI Revision 1.2.24 Flashcards
What does M stand for?
Net magnetisation - generated in the patient
What is B created by?
Created by the scanner
What does TE stand for?
Time echo
What does the Larmor equation give?
Gives the processional frequency of something in a magnetic field
Why are Gradients are applied?
To temporarily vary the main magnetic field
What happens when the Gradients are off?
All spins have the same frequency
Rf pulse exited all spins
No localisation
What happens when theGradients are on?
Frequency depends on position.
RF pulse, only excite spins in a certain slice.
Signal only from that slice.
What is the Readout gradient ?
Gradient applied during signal detection
What Mega Hertz is a 1.5T scanner?
64 MHz
What megahertz is a 3T scanner?
128 MHz
What does FID stand for?
Free induction decay
What does PNS stand for?
Peripheral nerve stimulation
Which direction do gradients point ?
They point in the z direction otherwise the processional frequencies wouldn’t change
Perpendicular to the magnetic field (B0)
Which direction is Frequency encoding ?
Side to side
Which direction is Phase encoding ?
Up and down
What does TR stand for?
Repetition time
How does fat, fluid and muscle appear on T1 weighted images?
Fat - bright
Muscle - intermediate
Fluid - dark
What TR and TE do T1 weighted sequences have?
Short TR
Short TE
How does fat, fluid and muscle appear on T2 / T2* weighted images?
Muscle - dark
Fat - intermediate
Fluid - bright
How does Gradient echo work?
What does the signal strength depend on?
- Initial negative gradient dephases the signal.
- A positive gradient rephrases to form a complete echo
The signal strength depends of the T2* decay and the TE
How does spin echo (SE) work?
What does the signal strength depend on?
- Combination of the readout gradient and a 180’ RF pulse
- 180’ pulse reverses the effect of the main magnetic field variations which cause T2* decay
Signal depends on pure T2 decay.
T2 signal is larger than GE, T2* signal
SE VS GE
- SE - T2 rather than GE - T2*
- Signal and SNR higher from SE compared to GE
- The 180’ pulse in SE reverses dephasing
- SE less susceptible to artefacts
- GE uses shorter TR values so shorter scan times
What do RF pulses do to the Mz ?
The pulses flip Mz into Mxy
What is spatial resolution?
The ability of the image receptor to resolve small structures
