MEDRADSC 3DE3 Lecture Notes - Lecture 8: Cryogenics, Magnetic Susceptibility, Ferromagnetism
18 Jun 2018
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What we need to have:
Magnet
RF source
Image processor- for image formation and user interface
Computer system
Magnetism
Property of matter
Magnetic susceptibility
Electron configurations
Paramagnetic- adds to main magnetic field
Diamagnetic- takes away from the magnetic field
Ferromagnetic- greatly positively adds to main magnetic field
Magnets
B0
B1
orefers to the small magnetic field that is produced in the receive coil as the FID is
produced
G
oGauss, measurement of field strength, usually used for low field strengths, stray field
oEarth’s magnetic field is measured as 0.6 G
kG
T
oused for high field strengths, an SI unit
1T= 10 kG=10 000 G
Homogeneity
oMeasured in parts per million (PPM)
Magnet Types
AIR CORE
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oResistive
oFirst generation, only seen now in science museums
o0.2T and under- very low field strengths, weak magnet
Permanent
oMade from Iron, cobalt, nickel
oSometimes an Alloy- combination of aluminum, nickel and cobalt (alnico)
oSometime made of ferromagnetic bricks/blocks
oFlux lines run vertically- the main field lines, the main fringe field lines
oNo power supply needed
oLow operating costs
oSmall fringe field
oHeavy
15,000kg
oLow field strength- don’t require cryogen
o“Open configuration” possible- useful for claustro, habitus, interventional (as long as it
doesn’t require high field strengths)
oVery temperature sensitive- needs to be maintained at a steady temperature, if it
fluctuates so does the homogeneity
Resistive
oMain advantage: can turn field off immediately
oFlux lines horizontal
oHigh operational costs- because power supply is constant
Initial buyout price is low but expensive to run
oLow fringe field
oLow field strength
oRight hand thumb rule
Decides where the flux lines are going to be
oLoops carrying current
oLightest magnet
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oLess than 0.3T
oHomogeneity measured at 5 ppm
Superconducting
oFlux lines horizontal
oLow operating costs- power supply not needed
oExpensive to purchase
oLarge fringe field
oHigh field strength
oResistance- temperature of wire
In order for it to be superconducting, we need the wires to be cooled to
absolute zero (0 Kelvin), so we use cryogens (either He or N)
o0.5 to 4T
oSome research magnets are 9T
oHomogeneity as 1 PPM (most homogenous, good for spectroscopy)
Quench
If windings cease to be superconductive- warm up
Heat spreads through the system
Quench results in collapse of field
Rapid boiling off of helium
Vent to outside
oIf not- asphyxiation
For a controlled quench, we call it a “ramping down”
oInvolves engineers, cools the system very slowly so the boiling off can happen so that it
vents to the outside
Fringe Field
Possible extension into contraindicated areas—front, side, back, on top, below
Patients
Mechanically operated devices
Monitoring equipment
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Document Summary
Image processor- for image formation and user interface. Diamagnetic- takes away from the magnetic field. Ferromagnetic- greatly positively adds to main magnetic field. Homogeneity: measured in parts per million (ppm) Air core: resistive, first generation, only seen now in science museums, 0. 2t and under- very low field strengths, weak magnet. Resistive: main advantage: can turn field off immediately, flux lines horizontal, high operational costs- because power supply is constant. Initial buyout price is low but expensive to run: low fringe field, low field strength, right hand thumb rule. Decides where the flux lines are going to be: loops carrying current, lightest magnet, less than 0. 3t, homogeneity measured at 5 ppm. Superconducting: flux lines horizontal, low operating costs- power supply not needed, expensive to purchase, large fringe field, high field strength, resistance- temperature of wire. If windings cease to be superconductive- warm up.
