MRI Flashcards
1
Q
What happens to hydrogen protons to cause MRI?
A
- They align with the external magnetic field
- they precess around this field (spin around it at a specific frequency) and the precession is determined by the strength of the magnetic field
- If enough protons allign in the same direction at the same frequency a signal can be detected
- when protons are put in a field they align parallel which is lower energy
- when a force is applied to a spinning object the resulting force is perpendicular
2
Q
What does the magnet in MRI do?
A
Causes protons to align longitudinally and precess at a specific frequency
3
Q
What do RF coils do and detect
A
- They give a radio frequency signal to patients what cause protons to flip to a new orientation. This flip angle is determined by the strength of the RF pulse applied
- They then detect the signal produced by the patient as the signal is a magnetic field that rotates around the patient and induces an EMF in the RF coils
- only voxels with a lot of hydrogen protons precessing at the right frequency will produce a signal to be detected
4
Q
What is T2
A
- Transverse decay- the measure of time taken for protons to lose transverse alignment
- occurs faster in fatty tissues where the water (hydrogen is tightly bound)
5
Q
What is T1
A
- longitudinal recovery- measure of time taken for protons to realign with magnetic field
- T1 recovery is faster in tissues where the water is tightly bound so fatty tissues
6
Q
How to get T2 weighting
A
- Long TE to get T2
- Long TR to suppress T1
- Free water images Will appear brighter as they remain aligned in transverse direction for longer
7
Q
How to get T1 weighting
A
- Short TR to get T1
- Short TE to suppress T2
- Fatty images appear bright because they are strongly aligned because they recover longitudinal alignment faster
8
Q
What impacts pixel brightness
A
- Pixels with all the protons aligned appear bright
- Pixels where there is little proton alignment appear dark
- Tissues which have a lot of protons can also appear fairly brights even if the degree of alignment is low
9
Q
What do you do to get proton density weighting
A
- Long TR to suppress T1
* Short TE to suppress T2
10
Q
What is a stir image
A
Fat suppression
11
Q
What is a flair image
A
Fluid suppression
12
Q
What is TE
A
Echo time so time at which signal is sampled
13
Q
What is TR
A
Repetition time so time at which pulse sequence starts again
14
Q
What is TI
A
Time from inversion
