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MRI in practice ch9
MRI day2
| Question | Answer |
|---|---|
| fundamental property of matter | magnetism |
| the degree of magnetism exhibited by a substance is due to a property known as | atomic magnetic dipole (or moment) |
| dipoles are generated in an atom by the movement of | electrons |
| no external magnetic field present, no net magnetic moment | Diamagnetism |
| silver and copper; inert gases, sodium chloride and sulfur | diamagnetism |
| Negative magnetic susceptibilities slight decrease in magnetic field strength | Diamagnetic substances |
| as result of unpaired electrons within the atom....have small magnetic moments | paramagnetic substances |
| No external magnetic field | paramagnetic |
| effect external magnetic fields positive way, resulting in local increase in magnetic field | paramagnetism |
| Oxygen, gadolinium chelates | paramagnetism substance |
| strong attraction and alignment, dangerous prjectiles | ferromagnetism |
| remain magnetic after external magnetic field has been removed and permanently magnetized | ferromagnetism |
| permanent magnet produces magnetic field lines or lines of force running from the | magnetic south to north |
| primary field or BO (B sub O) | main magnetic or static field |
| Used to excite spins and produce resonance | RF field or secondary magnetic field B1 (B sub 1) |
| Mesured in one of three units | strength of magnetic field |
| strength of low magnetic fields such as earth's magneic field or fringe field produced by magnet | Gauss (G) |
| This is equal to 1000G | Kilogauss (KG) |
| this is the strength of the main magnetic field or the stronges magnetic field | Tesla (T) |
| 1T= | 10KG=1,000G |
| FDA increased the limit from 2T to ___ for infants up to one month and up to ___ for any age above this | 4T, 8T |
| Magnets that are used in the production on permanent magnets | ferromagnets |
| most common material used in permanent magnets | alloy of aluminum, nickel and cobalt known as alnico |
| require no power supply or cryogenic cooling, and low operating costs | advantage of permanent magnet |
| has a vertical field | permanent magnet |
| has lines of flux running from south to north keeping magnetic field virtually confined within the boudaries of the system and hence the scan room | main magnetic field of permanent magnet |
| almost has no fringe, low safety considerations, | Permanent Magnet |
| Has Very low SNT and low field strength, but very heavy and require reinforced floors | Permanent magnet |
| state that moving electrical charges induce magnetic fields around themselves | laws of electromagnetism |
| a current is passed through a long straight, wire a magnetic field is created around that wire | Farraday's Law |
| degree of resistance along a wire is determinded by | ohm's law v=ir |
| often said to be a resistive magnet | solenoid electromagnet |
| one wire can be wrapped around to form many loops like a spring and the loops form a coil and act as though they are parallel straight wires | solenoid electromagnets |
| Kind of magnets that are used today | Resistive Magnets |
| Have a horizontal field | Resistive Magnets |
| primarily consist of loops carrying current, lighter in weight than permanent magnet and capital cost are low, operational cost are high | Resistive magnet |
| Require large quantities of power to maintain the magnetic field | resistive magnets |
| Max strenght of resistive system is less than | .3T |
| Can be turned of in a flick of a switch | resistive system |
| As resistance decreases the current dissipation also decreases | superconduction magnets |
| resistance depends on the | material of loops of wire are made length of wire in loop and cross sectional area of wire itself also temp |
| superconductors exhibit zero resistance below a certain very low temp called | critical temp |
| niobium and titanium becomes superconductive below | 4K |
| Magnet must stay at | -181 F |
| Wires are supercooled with | cryogens |
| Cryogens are used to | |
| it surrounds the coils of wire and is housed in the system between insulated vacuums | Dewar |
| Greater safety Hazard | superconductive magnets |
| remain magnetized permanetly | Permanent Magnets |
| flux lines run vertically | Permanent Magnets |
| require no power supply | Permanent Magnets |
| low operational costs | permanent Magnets |
| small fringe fields | permanent magnets |
| Very Heavy | permanent magnets |
| low field strength (SNR lower.usually longer scan times) | permanent magnets |
| field can be switched off immediately | Resistive magnets |
| flux lines are horizontal | resistive magnets and super conducting magnets |
| high operational cost as power supply required | resistive magents |
| large fringe fields | resistive magnet and superconducting magnets |
| poor homogeneity at higher field strengths | resistive magnets |
| lower power requirements (cheap to run) | superconducting magnets |
| expensive to buy | superconducting magnets |
| high field strength (high SNR/usually shorter scan times) | superconducting Magnets |
| when magnet is shimmed with metal it is termed | passive shimming |
| when simming is performed with loops of current carrying wire it is known as | active shimmin |
| produce magnetic field evenness or homogeneity | passive and active shimming |
| for imaging purposes homogeneity or the order of ____ is required | 10 ppm |
| Spectroscopic procedures require a homogeneous environment of ___ | 1ppm |
| Made of niobium titanium | gradient coils |
| Lining the walls of the MR scan room with steel | passive shielding |
| uses additional solenoid magnets ouside cryogen bath that restrict the magnetic fields lines to an acceptable location | active shielding |
| has no respect for the confines of conventional walls, floors or ceilings | stray magnetic field |
| the stray magnetic field outside the bore of the magnet is known as | fringe field |
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