Why is MRI so popular? Flashcards
Why MRI?
- Based on magnetic resonance of water molecules
- Non-ionising
- Full 3D imaging technique
- Has excellent contrast in soft tissue
- Very versatile
What are the MRI Field strength?
1T = 1 Tesla 0.00005T = Average Earth's field 1.5T = Typical clinical MR scanner 3T = becoming standard MR in UK 7T = many in US, Europe, Asia, Australia 10.5T = Highest human MR scanner
What does MRI look at?
Molecular level =10-9 m
Atomic level =10-10 m
Nucleus of hydrogen atom=10-13 m
Where does spin rotate (or precess)?
About applied magnetic field B0 i.e. along z axis
What is the frequency of this precession proportional to?
Applied field
What does each water molecule within body have?
Small magnet associated with it
What is Larmor equation?
v= γB0
What does MRI rely on?
Magnetic properties of hydrogen atom to produce images
What is H nucleus composed of?
- Single proton
- Spinning charged particle
- Produces a magnetic field = magnetic moment
How are the protons orientated?
Randomly orientated with no applied field
What are the MRI components?
- Primary magnet
- Gradient magnet
- Radiofrequency (RF) coils
- Computer system
What does the primary magnetic field refer to?
The strength of the static permanent field
e.g. 1.5 or 3T
How does hydrogen atoms align?
parallel or antiparallel to primary field (B0)
known as longitudinal magnetisation
How does a greater proportion of the hydrogen protons align?
Direction parallel to the primary magnetic field or low energy state than align anti-parallel to the primary magnetic field (high energy state)
net result = net magnetic vector (M)
What is precession?
The protons spin on their axis
protons spin around long axis of primary magnetic field
What is the precession rate termed?
Larmor frequency
What is in phase?
When protons precess together?
What is out of phase?
When protons precess separately
What does frequency change with?
In proportion to the magnetic field strength
At 1.5T = 63.9 MHz