Lecture 2: Basic physical principles, MR equipment and safety issues Flashcards
at body temp when in a magnetic field what is the behaviour of the H atoms in our bodies
what does this produce
align with main magnetic field
gives net magnitization
if we add in radiation at special frequency when the person has been placed in the MRI machine what does it do to the net magnitization
tips the net mag to become perp to the main mag field
what is the energy we put into the system
radiofreq waves
what does the imaging gradient do
increase mag field slightly at one end and decrease the mag field at another end of the body
what is special resonant freq proportional to
the main mag field
what is the magnetic susceptibility
degree of magnetization of a material in response to an applied mag field
what are differences in mag susceptibility a source of in MRI
contrast
what are the 3 types of mag susceptibility
diamag
paramag
ferromag
what is diamagnetism’s mag susceptibility and what does it do in the applied field
negative mag susceptibility
opposed to applied field
what are examples of diamagnets
water, copper, tissue
what is paramagnetism’s mag susceptibility and what does it do in the applied field
small positive mag susceptibility
small attractive force
what is ferromagnetism’s mag susceptibility and what does it do in the applied field
high positive magnetic susceptibility
strong attractive forces
what are examples of paramagnets
aluminium
gadolinium
what are examples of ferromagnets
iron
nickle
cobalt
what is the precessionary motion aligned to
the main mag field
what freq do H atoms precess at
at the larmor freq
what are H atom spins
moving positive charge gives rise to magnetic moment
what happens when H protons with mag moments are outside and inside the mag field
outside = randomly distributed
inside = align with mag field, some align parallel and some align anti parallel (all precess in direction of mag field but point in opp directions)
which way do lower energy state H atoms point in the main mag field
in direction of main mag field
what produces the net energy mag
when the H atoms that point opposite directions cancel out and its the excess protons that produce the net mag
Radiofreq at special freq has to match what energy
energy difference between the 2 states
describe excitation/RF transmit
Put energy in resulting in the effect of net mag being tipped from longitudinal to trans plane
describe induction/RF recieve
As it’s a changing mag field, if we put the same RF coil next to the sample that changing magnetization will induce a current or voltage across the coil also oscillating at the lamor freq
what is T2* decay
When we tip net mag into transverse plane the individ spins that make up the net mag were all precessing at the same freq as we used the same freq to excite them
They can start to precess at diff freq so not all are in phase and making up net magnitization so effect of that is that when the Vectors added together they don’t make the same large net mag anymore and the net mag goes down and the sig in the coil goes down in the exponential way
what does T2* time characterize
time taken for signal to decrease when the protons dephase
what is T1 recovery what happens
longitudinal mag starts to recover even though the signal decays away
what plane does the T2 decay in
transverse plane
what happens to T1 recovery rate for different tissues
tissues with different T1 recover at a different rate
what doe the superconducting magnet provide
very strong and uniform magnetic field B0
what does the RF coils tuned to resonant freq provide
a way to transmit and detect the emitted RF waves
what type of magnetism does a permanent magnet produce
intrinsic
what is a tesla
unit of mag field strength - mag field density
what is a gauss
unit of magnetic induction - mag field density
what are the 2 positives of permanent magnets
easily shielded
less claustrophobic
what are resistive mag fields
reg wound copper coil carrying direct current gen mag field along axis
what are the 2 types of main mag fields
permanent and resistive
what is the drawback of resistive mag field
high operating costs due to large power to gen current
50% of power dissipated as heat
what is the positives of using resistive mag field
easily turned on/off
what is a superconducting magnet similar to and how is it different
similar to resistive magnets but uses liquid helium to cool coil windings
what happens to the electrical resistance in superconducting magnets and why
decreases and becomes super conducting (zero electrical resistance) when temperature falls below critical value ~263 degrees celsius
larger currents in windings of a superconducting magnet leads to what effect on the magnetic field
stronger mag field
what are the 2 positives of having a superconducting magnet
large homogenous magnetic field
little power input
what are the 2 cons of having a superconducting magnet
costly due to cooling system - liquid He
always on
what is the image quality of a 1.5T look like compared to a 3T
1.5T is a bit grainier than 3T
what is SNR used to measure
evaluates image quality
what is SNR proportional to
field strength
what do RF coils generate and what are they tuned to
generates and detects the RF pulses
tuned to the selected resonant freq
what are the 2 types of RF coils
body and surface coils
where does the body coil sit and what does it do
The body coil sits just on inside of magnet to do transmission that makes it nice and uniform - long cylinder that puts equal amount of radiofreq into the bore
where do we want the surface coil to be located
as close as possible to the object
what is done to correct the small signal of surface coils
amplifiers are used
do body RF coils transmit or receive
transmit
do surface RF coils transmit or receive
receive
what is resonance freq proportional to
main mag field
what happens to the spin precessions at either end of the body when you apply the gradient
spins precess faster at the higher and slower at the lower end where there is lower mag field
what are the 3 main safety concerns of the static field
cryogen
equipment malfunction
displacement effects
when does quench occur
cryogen is boiled off when need to shutdown mag field or if there is a mag fault
what is the safety measure to combat cryogen risks
quench pipe regularly checked
what are the 2 things that are considered in equipment malfunction
electrical equipment or active implants
what are the 3 safety measure to combat equipment malfunction
5Gauss line is marked
controlled areas for access
procedures in place for scanning conditional implants
what are the 2 things that are considered in displacement effects of static fields
projectile effect and rotation effect
what 2 things does projectile effect due to attractive force depend on
field strength and spatial mag gradient
what does the rotation due to torque depend on
B^2
what are the 2 safety measure to combat displacement effects of the static field
effective shielding
controlled access of people and objects to area
what are the 2 types of shielding for the fringe field
passive and active
what is passive shielding
added ferromag material that channels the field lines
what is active shielding
secondary gradient coils are used which oppose the field created outside the magnet bore
how do RF waves cause burns and heating
energy of RF wave is absorbed and converted to heat
what are some tissue factors that influence the degree of heating effects from RF waves
some tissues lack heating control mechanisms
depends on patient size/weight/age/health
depends on ambient conditions
what are the 2 safety measure to combat heating effects of RF waves
limit exposure of the patient with RF using SAR - specific absorption rate (W/kg)
keep room temp finely controlled
what are the 3 safety measure to combat burn effects of RF waves
good patient positioning (prevent skin-skin contact)
effective screening (remove conductive materials)
make sure any required materials/leads are well insulated
what are the 2 main safety concerns of the gradient field
nerve stimulation
acoustic noise
what causes the gradient field’s nerve stimulation
changing mag field with time will induce current in a conductor
in the body currents can be induced along nerves which can stimulate things
what are the 4 things that can be stimulated by the nerves by the gradient field and order them in order of seriousness
retina < peripheral nerve < peripheral muscle < cardiac
what 2 things does stimulation thresholds of the gradient field rely on
shape of gradient pulse
patient
conductivity/implants/size
what is a safety measure for gradient field nerve stimulation
limits on dB/dt - how fast the switching can go and limits set to never exceed peripheral nerve stimulation
