chapter 9 Flashcards
how many guass’s are in a 1.5T scanner?
15,000
describe the right hand rule
right fingers wrapped around wire, thumb points to current, fingers point in the MF
material for RF shielding
copper
material to make permanent magnets
alnico
material for passive shielding
steel
material to make superconducting wire
niobium-titanium
maintain low temperatures
cryostat
how steep/strong the gradient is, measured mT/m or G/cm
gradient strength
time to reach maximum amplitude, measured in microseconds, longer for stronger gradients
gradient rise time
speed & strength, measured in mT/m/s
slew rate
% of time the gradient is working
duty cycle
converts signals to images
image processor
where the FFT occurs
array processor
the factors that change the strength of an electromagnet
- current passing through the windings
- number of windings
- diameter of the wire
- spacing between the windings
what are the two “bi-poles” of a magnet
north & south
Bo is vertical, heavy, low SNR
permanent magnets
requires power supply, switched on & off, Bo horizontal or vertical, lightweight 0.2-0.3T
resistive magnets
Bo is horizontal, higher SNR, Bo 1.5-14T
-269 C, -450 F, 4K
superconducting magnet
heavy steel plates around the magnet
passive shielding
uses solenoid electromagnets around the bore, equal but opposite effect on the magnet
active shielding
at the time of installation, iron plates counteract inhomogeneities due to surrounding structures
passive shimming
uses electromagnetic coils, between each patient, protocol or sequence
active shimming
purpose of shimming
magnetic inhomogeneities
purpose of shielding
reduce the fringe field
the degree of resistance along a wire is determined by
ohms law
V=IR
ohms law
V is equal to the applied voltage
I is the current
R is the resistance within the wire
the magnetic field strength in superconducting magnets is maintained with cryogens at a temperature of ___ K
4
positioning is not as critical, increased wrap with small FOV
large coil
high SNR, less likely for wrap
small coil
which cryogen is commonly used to super cool wires in MRI
liquid helium
apparently magnetization equation
Bo = H0(1+x)
slew rate equation
mT/m/sec
Tesla to Gauss conversion
1T = 10,000G
paired electrons, no magnetic moments
ex) diamond, graphite, wood, plastic
diamagnetism
unpaired electrons, small attraction to the magnetic field. ex) gadolinium contrast, platinum alloys, gold alloys
paramagnetism
half filled electron shells, strong attraction
ex) iron & nickel
ferromagnetism
units of MF homogeneity
parts per million (ppm)
max field strength for clinical imaging in USA
8T
given charge, motion, and magnetism, the interaction of 2 will induce the third
law of electromagnetic induction
stray magnetic field outside the bore of the magnet
fringe field
considered high field strength for clinical imaging
3T
