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22 Cards in this Set
- Front
- Back
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Describe Diamagnetism |
Paired electrons No magnetic moment with no external field present Small magnetic moment that opposes the applied field when external field present. |
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Examples of diamagnetic substances |
lead and copper bismuth, carbon, graphite, mercury, lead, water |
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Describe Paramagnetism |
Unpaired electrons Small magnetic moment Effect magnetic field in a positive way Ex/ Tungstun, cesium, aluminum, lithium, magnesium, and sodium |
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Ferromagnetism |
half filled electron shells remain magnetic very high positive susceptibility Ex/ iron, steel, gadolinium |
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Superparamagnetism |
intermediate positive magnetic susceptibility greater than paramagnetic matererials ex/ iron oxide |
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1 Tesla equals how many gausian? |
10 000 |
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What are permanent magnets? |
remain magnetized permantently usually open design require no power supply low operational cost small fringe field heavy low field strength |
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What are restrictive magnets? |
field can be switched on and off on-going cost for power supply larger fringe field |
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What is a superconducting magnet? |
flux lines horizontal lower power requirements expensive to purchase high field strengths |
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What is the fringe field? |
5G around scanner (walls, floors, and ceillings) |
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Describe passive shielding? |
Surrounding magnet or lining magnetic room with steel plates. This method is both expensive and inconvenient due to the weight. |
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Describe active shielding? |
Uses additional solenoid electromagnets located around the outside of the main magnet coils at each end of magnet bore. They are located inside of cryostat and are superconducting coils. They exhibit an equal but opposite effect to the main magnet. |
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What are the imaging requirements for homegeneity? |
4 ppm |
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What is shimming? |
Makes field even |
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What is passive shimming? |
Small ferromagnetic plates n specially constructed non ferrous metal trays located around circumfrence of warm bore of magnet. 16 trays which can hold 15 shims |
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What is active shimming? |
Performed by an electromagnetic coil and can be used to shim the sytem for each patient or even each sequence within protocol. |
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What is the main use for gradient coils? |
spatial encoding and GMN Can be used to rephase spins and produce echoes in GRE |
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What is gradient strength or amplitude? |
defines how steep or strong a particular gradient is.( mT/m or G/cm) |
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What is gradient rise time? |
defines the time it takes for a given gradient to reach max amplitude (microseconds) |
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What is slew rate? |
Times it takes for a given gradient to reach max (mT/m/S) |
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What is duty cycle? |
defined as percentage of time that gradient is permitted to work |
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Typical gradient amplitudes |
10-40mT/m |
