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MRI in practice ch9

MRI day2

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
Created by: 1145508893