What are the advantages of MRI
- Superior soft tissue contrast compared to CT
- No ionising radiation
- Various contrasts available (T1, T2, Diffusion)
- Both structural + functional info can be obtained (diffusion, perfusion, etc…)
- Chemical info can be obtained by MR spectroscopy
What are the disadvantages of MRI
- Scans take longer than CT
- Strong magnetic field can prevent patients with implants having scans
- more expensive than CT
What is the conversion of Tesla to Gauss
1 T = 10,000 Gauss
What does ferromagnetic mean?
- Strongly magnetised + large positive susceptibility
- Scissors
What does diamagnetic mean?
- Small negative susceptibility
- water
What does paramagnetic?
- Small positive susceptibility + enhance magnetic field
- Gadolinium
NMR (Nuclear Magnetic Resonance)
Is the principle that MRI relies on
- Magnetic field generated by a moving charge
- Nuclei of atoms possess magnetic moment
- 1/2 integer spin particles (helium)
The spin
What is spin
Spin = proton
w_o =
Angular frequency
Number of rotations per second about the axis
B_o =
Magnetic field
What happens when a proton is in an external magnetic field
- It precesses about the direction of the field
- has spin up (parallel)
- has spin down (anti-parallel)
m_o =
sum of all individual magnetic moments
= net magnetism
What does increasing B_o do
increases the energy gap, m_o + fraction of spin up states
Amount of protons (spin ratio)
spin up / spin down = exp( energy gap / Temp * k_B)
Temp in K
k_B = boltzmann constant in J/K
What is the equation for angular frequency, w_o?
w_o = y * B_o = 2 * pi * f_o
What is the equation for scalar frequency, f_o?
f_o = (y / (2* pi)) * B_o
What is the Gyromagnetic ratio?
y / (2 * pi) = 42.58 MHz T^-1
What happens when an RF pulse is applied?
- Some spins promoted to higher energy state
- Hydrogen atoms give rise to net magnetisation
- Larger pulse = smaller m_o
- Magnetisation grows in transverse (x, y) plane
Explain what RF pulse is
RF pulse = same frequency as precessional frequency of spins
- spins absorb energy
Individual spins
- Spins precess about B_o
- Magnetic moments out of phase w.r.t eachother
- apply 90 degree excitation pulse
- spins begin to precess in phase in (x, y) plane
- rotating frame of reference
- NMV precessing very fast
What is the flip angle?
- The angle through which an RF pulse rotates the NMV
- Any possible - Apply RF pulse as long as needed
- 90 degree flip angle gives maximum transverse magnetisation
- inversion pulses = flip angles of alpha = 180 degrees
Magnet properties
- Higher field strengths may yield a higher SNR but maybe a higher specific absorption rate (SAR) or increase in F-W shift
- want magnetic field to be very homogeneous
- want magnetic lines to be parallel to isocentre
When are permanent magnets useful?
In niche areas of MRI such as for claustrophobic patients
- homogeneity lower + magnets < 0.5 T
Superconducting magnets
Use materials at very low temp as have zero electrical resistance
- below temp = critical temp
- established current circulates
- cooled using liquid helium
- high fields reached (> 1.5 T)
- Cant be turned off without quenching
