Lecture 12 - MRI Flashcards
What magnet does MRI use
3-7 tesla
What does mri measure
the alignment of water molecules within a magnetic field, and how long it takes to get back
How does MRI work
no external field -> external magnetic field applied, water molecules align with the magnet (T1 IMAGE) –> external radio frequency pulse applied –> some molecules flip to antiparallel spins –> pulse stops –> protons align parallel again and give off radio frequency energy –> energy is detected by the reception coil and forms T2 image
What are the two features we adjust on MRI?
TE - ECHO TIME
TR - REPETITION TIME
What is T1
t1 relaxation, uses short TE (echo time) and short TR (repeition)
- longitudinal or spin lattice relaxation
- fat looks bright
- water looks dark
What are the properties of tissue that we are measuring on MRI
- proton density
- proton relaxation time (T1 and T2)
What is T2
T2 relaxation - uses long TR and TE
DIFFERENTIATES BETWEEN WHITE AND GREY MATTER BETTER THAN T1
- transverse, spin spin relaxation
- fat is dark and water is bright
What is FLAIR
it is a T2 image, but water is dark, providing better contrast
What is proton density
PD
uses short TE and long TR, reflection hydrogen concentration
What is VBM
Voxel based morphometry
- statstically compares voxels of the brain to examine its shape, volume and integrity
- you can look at one structure, compare it to other structures and other groups of people (within person or between groups)
What colour is water in a T1 and T2 scan?
T1 - dark
T2 - bright
What is fMRI?
BOLD - Blood oxygenation level dependent
- brain regions become oxygen rich after activity
- Hbr (deoxyhaemoglobin) acts as the contrast
What is DTI
Diffusion-tensor imaging
measures random motion of water molecules.
CSF in ventricles – molecules are unconstrained, so high velocity diffusion
BRAIN TISSUE - less diffusion as CSF constrained
