Lecture 1 Flashcards
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
What do wider and shorter bores help with?
patient tolerance but have lower homogenity + potential for increased geometric distortion
What does a faraday cage do?
Stops radio frequency energy getting in + out of (something) room
What is MR environment
MR environment is 3D volume of space surrounding MR magnet containing faraday shielded volume and 0.5 mT field contour
This volume is the region in which an item might pose a hazard from exposure to magnetic fields produced by MR
MR controlled access area contains MR environment
What is the missile effect?
When ferromagnetic objects act as projectiles towards magnet
Explain the Lenz effect
Movement of conducting object in magnetic field eddy currents which creates another magnetic field with opposite polarity to main magnetic field
effect of resistance to motion of conducer
can affect objects like artificial heart valves at high enough strengths
Fringe field < 0.5 mT
inhomogeneity (ppm) =
((variation (T)) / Nominal field strength (T) ) * 10^6
Gradient system
- coils generate gradient magnetic fields in the body
- current passes through gradient coil to augment static magnetic field
- spatial localisation
Safety issues (RF + gradient)
- Heating effect : normal mode = 2 W /kg
- conductive loops may cause burns
- Antenna effect: Resonant frequencies with lengths of conducting material can give extreme burns
- peripheral nerve stimulation possible for human = transient effect
- hearing damage - ear plugs must be worn
