Topic 23: MRI principles Flashcards
(28 cards)
Describe magnetic predisposition
- Ability of external magnetic field = affect nuclei of atom
- Realted to e- configuration
- If nucleus surrounded by paired e- = protected + less affected
- If nucleus surronuded by unpaired e- = can be affected by external magnetic field
Give the 3 types of magnetic predisposition
1) Paramagnetism
2) Dimagnetism
4) Ferromagnetism
Describe paramagnetism
- Contains unpaired e-
- Magnetic moment = small magnetic field
- When no external MF = moments occur in random pattern = cancel each other out
- In presence of external MF = align in direction of field = moments add up
- Effect external MF = +ve
- E.g. O2
Describe diamagnetism
- Paired e-
- When no external MF = no net magnetic moment
- In presence of external MF = small magnetic moment = opposes applied field
- Repelled by MF
- E.g. H2O/fat/Ca/biological tissues
Describe ferromagnetism
- When in presence of MF = strong attraction + alignment
- Retains magnetisation when MF removed = permanently magnetized = become magnets
- E.g. iron
Describe a magnet
- Bipolar
- MF exerted = MF lines
- Run from magnetic north → south = magnetic lines of flux
Define magnetic flux density
- Number of lines per unit area
Give the units to measure MF strength
1) Gauss
2) Kilogauss
3) Tesla
How are magnetic fields are generated?
- Via moving charges = current
- CW/CCW
Define Ampere law/Fleming’s right hand rule
- Determines magnitude + direction of MF due to current
- Thumb = current direction
- Fingers = MF direction
How are electric currents generated?
- Changing magentic fields
- Magnet moving in/out of closed circuit = induced current
Describe electromagnetic induction in MRI
- Spinning charge of H protons = MF induced around it
- Movement of net magnetization vector across area of receiver coil = induces electrical charge in coil
Give the 3 types of magnets used in MRI
1) Permanent
2) Electromagnet
3) Superconducting
Describe permanent magnets
- Ferromagnetic substances
- E.g. Al/Ni/Co = Alnico
- Open design
- Need no power supply = low opeerating cost
- Vertical MF = keep MF confined within boundries of scan room
Disadvantages of permanent magnet
- Very heavy
- Low fixed strength = 0.2-0.7
- Low SNR + resolution
- Long scan time
Describe electromagnets
- Law of induction
- Current passes through series of wires = MF
- MF strength dependent on = current passing through coils
- Direction of MF = follows righ hand rule
- Produces line of flux = horizontal from head→foot of magnet
Give advantages of electromagnets
- Lighter than permanent
- Low capital cost
- Easy to install
- Easy to turn on/off inexpensively
Give disadvantages of electromagnets
- High operational cost = power requirements
- Max field strength < 0.3
- Longer scan times than permanent
- Low SNR + resolution
Describe superconducting electromagnets
- Use solenoid-shaped coil = niobium/titanium/tin + copper
- Alloys = 0 resistance to electrical current when cooled
- Coil kept below 10K
- Current = through loops of wires Ni/Ti = MF
- Wires cooled via liquid helium/nitrogen
Give the advantages of superconducting
- High MF with low power
- Low operating cost
- Accommodate advance applications
- Optimum image quality
Give disadvantages of superconducting
- High capital cost
- RF sheilding needed
- Tunnel design unsuiyable for large/claustrophobic patients
Explain shimming for inhomogeneities
- Design limitations = cannot create electromagnet with coil wires that are spaced evenly = inhomogeneities
- To correct = add another loop of wire in area
- Extra loop = shim coil
Give the 3 types of motion of particles in atom
1) Negative charged e- = spin on own axis
2) Negative charged e- = orbiting nucleus
3) Particles within nucleus spinning on own axis
- All produce MF
- In MRI mainly nucleus is of concern
Describe MR active nuclei
- Protons + neutrons = spins on own axis within nucleus
- Direction of spin = random
- Even mass number = spins cancel out = no net spin
- Odd mass number = don’t cancel out = nucleus spins = protons have charge = net charge + spin
