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Hoorcolleges van vrijdag 4 juni 2021. Gaat over Neuro-MRI acquisitie en advanced MRI
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Hoorcolleges vrijdag 4 juniNeuro- MRI Acquisitie (principe van MRI)
Vergelijking met CT. CT werkt met röntgen, meet absorptie van de röntgen straling.
PET maak je afbeeldingen van een radioactieve traser bv FDG.
Echo meet je de reflexie van het ultrageluid van het weefsel.
Bij MRI kun je allemaal verschillende contrasten maken door de acquisitie aan te passen.
MRI basics:
Signal formation
Spins, Magnetic field
Image formative
Spatial encoding, spatial resolution
Contrast
white matter is white and grey matter is grey on a T1-weighted scan
MRI recipe:
Stap 1: Patiënt in een grote magneet
Protons align with magnetic field like compass needles
Veel waterstofkernen dus daarom wordt naar waterstofkernen gekeken. Er is netto een
heel groot veld, dus met heel veel waterstofkernen lukt het nog net om een beeld te
krijgen.
Step 2: apply radio waves
When you apply radio waves (RF pulse) at the appropriate frequency, you can change
the orientation of the spins as the protons absorb energy (Energie waterstofkernen
verhogen). Puls van 5 ms. Heel groot signaal
Resonance frequency: ω = ᵞB0
Step 3: Heel klein signaal terug
After you turn off the radio waves, as the protons return to their equilibrium state, they
emit energy in the form of radio waves with frequency: ω = ᵞB0
Step 4: use gradients to encode spatial location
Slice selection & excitation.
Field strength is ω = ᵞB0
Larmor frequency is proportional to strength of magnetic field ω = ᵞB0
Extra spoel in het apparaat zodat er een gradient wordt gevormd, statische elektrisch
magneet veld is anders aan het begin als aan het einde. Hierdoor is er ook een verschil
in resonantie frequentie.
Protons are excited only if frequency of RF wave equals Larmor frequency (alleen
spins die overeenkomen worden uitgezonden). Afhankelijk van de plek waar ze zitten
geven ze een signaal af waar ze zitten. Field strength van boven naar onder ook
anders.
Apply gradient during:
- RF excitation (slice selection)
- During reception (frequency encoding)
- Somewhere in between (phase encoding)
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