Topic 24: MRI intrumentation Flashcards
(27 cards)
What are the 2 theories of alignment?
1) Classical
2) Quantum
Define anti/parallel alignment
- Parallel = moments same direction as main field
- Anti = moments opposite direction to field
How do nuclei moments align at room temp?
- More = parallel than antiparallel
Define net magnetization vector
- Net magnetism of patient
- Reflects balance between anti/parallel moments
Describe low/high energy states of nuclei
- Low = align moments parallel to external MF
- High = align moments anti-parallel to external MF
Explain precession
- Nuclei produce secondary spin/wobble
- Due to influence of external MF on nuclei spinning
- Causes moments of MR nuclei = circular path around strength of MF
- Speed of moment wobble = LARMOR/precessional frequency
How to calculate speed of precession using LARMOR equation?
𝝎𝟎 = 𝑩𝟎 × ɣ
- ω0 = precessional frequency in Hz
- B0 = strength of external MF in tesla
- γ = gyromagnetic ratio in MHz/T
Define gyromagnetic ration
- If precession = CW/CCW
- H1 + C13 = +ve
How is moments during equilibrium?
- Nuclei out of phase with each other
- Phase = position of moments on path
Define resonance
- Energy transition = when object subjected to frequency same as own
- Induced = applying RF pulse
How is resonance induced?
1) Apply RF pulse at same frequency as precessing H nuclei
2) At 90° to B0
- Causes H molecule to resonate + MR nuclei to not
What 2 things happen at resonance?
1) Energy absorption
2) Phase coherence
Describe MR signal
- NMV rotates around transverse plane = result of resonance = pass across reciver coil = induce voltage = MR signal
- Receiver coil = in transverse plane
Describe free induction decay
- RF pulse removed
- Induced signal = decreases
- In-phase NMV in transverse plane = decrease
- Amplitude of voltage in receiver coil = decreases
When will image have contrast?
- If areas of high + low signal together
Give the individual vectors NMV can be seperated into of tissue present
1) High signal = white = large transverse component
2) Intermediate signal = grey = medium transverse component
3) Low signal = black = small transverse
Give contrast parameters to control image contrast via operator
1) Repetition time = from application of FR pulse to next
2) Echo time = between RF excitation pulse + signal collection
3) Flip angle = angle NMV moved due to RF excitation
4) Turbo factor/echo train length = number of 180° RF pulses + resultant echoes
5) Time from inversion = inversion recovery sequences
6) b value= diffusion weighted imaging
Give contrast parameters to control image contrast not controlled by operator
1) T1 recovery
2) T2 decay
3) Proton density
4) Flow
5) Apparent diffusion coefficient
Define T2 decay
- Cumulative dephasing of spin-spin interactions + inhomogeneities
- Nuclei diphase
- NMV decays in transverse plane
Describe relaxation
- Energy loss
- RF excitation pulse = resonance + flip angle achieved = RF removed = voltage induced decreases
Why does NMV in trasnverse plane decrease ?
1) Relaxation
2) Field inhomogeneities
3) Susceptibility effects
How does rate affect image contrast?
- Magnetization in each tissue relax = different rates = creates image contrast
Describe the effect of RF withdrawal
- AKA T1 recovery
- Nuclei emit energy absorbed from RF pulse via spin lattice energy transfer
- Shift moments from high energy →low energy
- NMV recovers + realigns to B0
How do nuclei lose their consistency?
- Interactions of MF of adjacent nuceli = spin-spin energy transfer
- Inhomogeneities of external MF
