MRI Basic principles
Atom
What the inherent spin of the atom?
protons and neutrons spin in opposite directions
Unequal number of protons and neutrons = MRI active nucleui
What is the practical spin of the atom?
MRI active nuclei
net spin
aligns axis of rotation —→ applied magnetic moment 🔺
Why is hydrogen measured in MRI?
Hydrogen: contains one proton in its nucleus
large magnetic moment (momentum) (direction of force)
Abundant in human body
What is alignment of hydrogen atoms with the MRI magnetic field?
🔺ALIGNMENT
magnetic moments are randomly orientated in MRI active nuclei
—→ Strong magnetic field applied
magnetic moments align with magnetic field
What is the difference in alignement between different nuclei?
What is the net magnetic vector?
Low energy nuclei: align parallel to the magnetic field
High energy nuclei: align anti parallel to the magnetic field
Net magnetic vector = all parallel nuclei
What is precession?
Hydrogen spins on its own axis.
Precession: magnetic field strength creates an additional spin, so hydrogen nucleus spins around an additional axis
Precession frequency: speed of spinning
What is excitation?
What is resonance?
Excitation= rf pulse is applied causing resonance
RESONANCE
nucleus exposed to external force with similar frequency
gains ENERGY, RESONATES
Hydrogen = 1.5 T and 63.86 MHz
What happens to the net magnetic vector when resonance occurs?
Radiofrequency pulse is applied
Hydrogen resonates
Net magentic vector move out of alignment with the magnetic field
Flip angle: angle between magnetic field and net magnetic vector
(CHANGE)
What happens to the magnetic moments of the hydrogen nuclei when resonance occurs?
IN PHASE: with each other
How is MRI signal produced?
IN PHASE MAGNETISM: across the receiver coil
- moving transverse magnetisation of spinning nuclei
- causes magnetic field fluctuations in coil
- induce electric voltage in coil
- read by detectors and produce signal
Relaxation: signal switched off and nuclei lose energy
Realignment of NMV with Bo
T1 Relaxation: amount of magnetisation in the longitudinal plane increases (67% recovery)
T2 decay: amount of magnetisation in the transverse plane decreases (33% decrease)
Why does hydrogen show up differently in different environments?
OR
If only hydrogen signal is being received, why does the signal look different in images?
Image Weighting
Influence of different electric charges of surrounding atoms
Free Induction Decay: loss of signal in the receiver coil due to relaxatio
Relaxation —→ magnitude of transverse magnetisation decreases
Relaxation —→ nuclei dephase
What two factors determine the relaxation time of tissues?
Relaxation times
inherent energy of the tissue
low inherent energy = easily absorbent of energy from hydrogen
How closely packed the molecules are
closely packed = more efficient interaction between magnetic fields
FAT
- large molecules
- closely packed together
- low inherent energy
- gain longitudinal magnetisation quickly
- transverse magnetisation is lost rapidly
WATER
- spaced apart
- high inherent energy
- regain longitudinal magnetisation slowly
- loss of transverse magnetisation is slow
IMAGE WEIGHTING
What is repetition time and echo time?
Repetition time: time from application of one radiofrequency pulse to another
Echo time: time from application of radiofrequency pulse to the peak of the signal
High signal of transverse magnetisation at time of echo time = white
Low signal of transverse magnetisation at the time of echo time = black
T1 WEIGHTED
differences in t1 relaxation times
Repetition time controls how far each vector recovers before the slice is excited again
Short echo time so neither fat or water have enough time to recover to the magnetic field
T2 WEIGHTED
differences in t2 delay
Echo time controls how much t2 decay occurs before signal is recieved
Long echo time is used for signal differences betwen tissues to be large
PROTON DENSITY
amount of signal generated based on concentration of hydrogen protons
Minimise t1 and t2 effects
Long repetition time to reduce t1 weighting
Short echo time to reduce T2 weighting