Chapter 9 Flashcards
magnetism is a class of __ mediated by __
physical phenomena; B fields
magnetic susceptibility (__) is a physical property of materials which indicates whether they are __
chi; attracted into or repelled out of a magnetic field
by the Law of Biot & Savart, __ give rise to a magnetic field which acts upon __
electric currents; other current and magnetic moments in materials
by Faraday’s law, a __ induces a magnetic field, and vice versa
changing electric current
images from slide 3
the magnetic susceptibility of a material is determined by __
the magnetic moment of the atoms which constitute the material
the motion of an electrically charged particle induces a __ which is proportional to the __
magnetic moment (miu); spin angular momentum (S)
in the classical view, an atom’s magnetic moment is determined by the __ and by the __
orbital angular momentum (L); spins (S) of its electrons
electrons orbiting the nucleus are __
organized in shells according to the laws of quantum physics
images from slide 4
image from slide 5
in each shell, electrons __ so that their net magnetic moment __
pair up; has the lowest possible value
depending on the extent to which its electrons are __, and atom can be __(3)
paired up (i.e. the extent to which atomic shells are filled); diamagnetic, paramagnetic, or ferromagnetic
diamagnetic = __
all electrons are paired up
paramagnetic = __
a few electrons are unpaired
ferromagnetic = __
many electrons are unpaired because the outermost shell is only half filled
image from slide 6
diamagnetic materials have __ electrons
paired
if no external B field is present, diamagnetic substances __
have no net magnetic moment
5 examples of diamagnetic materials
bismuth (Bi), carbon (C), lead (Pb), copper (Cu), and water (H2O)
if an external B field is present, diamagnetic substances __. this causes diamagnetic substances to be __
exhibit a small magnetic moment which opposes the applied B field; slightly repelled by applied magnetic fields
(diamagnetism) the presence of a small magnetic moment implied the presence of __
a small non-zero magnetic susceptibility
(diamagnetism) the fact that the material is repelled by the external field implies that __
the susceptibility is negative
image from slide 7
paramagnetic materials have __ electrons
unpaired
even if no external B field is present, paramagnetic substances __
have a small net magnetic moment
4 examples of paramagnetic materials
aluminum (Al), lithium (Li), magnesium (Mg), sodium (Na)
if an external B field is present, paramagnetic substances exhibit __. this causes the substances to be __, implying that the susceptibility is __
a small magnetic moment which is aligned with the applied B field; slightly attracted by the applied magnetic fields; positive
many materials consist of both __ atoms and __ atoms - such materials are __ overall because it is a stronger effect
diamagnetic; paramagnetic; paramagnetic (because paramagnetism is a stronger effect)
images from slide 8
ferromagnetic materials also have __ electrons, however the __ of their atoms align themselves __, such that they generate a __ even in the absence of an external magnetic field
unpaired; magnetic spins; in the same direction spontaneously; intrinsic magnetic field
2 examples of ferromagnetic materials
iron (Fe) and gadolinum (Gd)
ferromagnetic materials have __ susceptibilities
very large
when a ferromagnetic material comes into contact with an external magnetic field, the result is __
a strong attraction of the material to the source of the field
in the absence of an external magnetic field, ferromagnetic materials __
retain their permanent magnet property
images from slide 9
permanent MR magnets are typically made of __
an alloy of Al, Ni, and Co (AlNiCo)
2 advantages of permanent MR magnets
no need for a power supply or cryogenic cooling; B field is confined to the bore (no fringe field)
4 disadvantages of permanent MR magnets
extremely heavy; very sensitive to ambient temperature; field strengths are low; difficult to deploy in the field due to weight and permanent magnetic properties
the main reason that permanent MR magnets are still used is __
cost
image from slide 10
Faraday’s law of induction: the __ in any closed circuit is equal to the negative of the __
induced electromotive force (voltage); time rate of change of the magnetic flux enclosed by the circuit
equation from slide 11
a solenoid is a __ which allows a __ magnetic field to be generated in the space __
long wire wound in the form a helix; fairly uniform; surrounded by the turns of the wire
images from slide 11
the efficiency of __ is determined by Ohm’s law (__)
a electric current passage through a solenoid’s coil; V=IR
Ohm’s law: V=IR. define these variables. how does it relate to meltdown of permanent magnets?
V = applied electric voltage I = applied electric current R = resistance of the solenoid wire
to generate large magnetic field through an electromagnet, a __ is required and this can increase __, ultimately leading to meltdown
huge current; the temperature of the coil
though __ than permanent magnets, room-temperature electromagnets have __ and their B fields are __
lighter; large electric power requirements; weak
image from slide 12
to circumvent the limitations of permanent magnets and electromagnets, MR engineers have designed __
superconducting electromagnets
super conducting electromagnets have coils made of __ which exhibits __ when cooled below __
titanium-niobium alloy; superconductivity; critical temperature Tc
superconductivity is the ability of materials to __
conduct electric currents with extremely low resistance
the critical temperature is the absolute temperature below which a metal __
superconductive
2 advantages of superconducting magnets
can generate very strong magnetic fields; have low electric power requirements
3 disadvantages of superconducting magnets
can be very expensive to manufacture; require periodic refills of liquid He (rare on Earth, difficult to extract and process, challenging to cool down and transport, thus very expensive); because He stays in liquid form at such a low temp, requires additional thermal shielding using liquid N (also expensive)
superconducting magnets: thermal shielding must be implemented using __ which contain __ (__)
cryostats; cryogens (He and N)
3 reasons liquid He is expensive
rare on Earth, difficult to extract and process, challenging to cool down and transport
image from slide 14
the stray magnetic field outside the bore of an MR magnet is called a __ and __
fringe field; its magnitude can be significant
due to safety concerns regarding the presence of fringe fields, __ must be implemented
magnetic shielding
two types of magnetic shielding
passive and active
passive shielding involves __
surrounding the magnet and lining the magnet room with steel plates to attenuate the fringe field
3 disadvantages to passive shielding
very heavy (steel plates), expensive, and inconvenient
active shielding involves installing additional __ and __
superconducting electromagnet coils just outside the main magnet coils; running a current through them whose direction is opposite the direction of the current through the main coil
superconducting MR magnets produce static fields with inhomogeneities due to __
tiny imperfections in the geometry of the magnet
because MR imaging is very sensitive to inhomogeneities, __ must be used to obtain high-quality images
shim coils
passive shimming involves placing __ around the circumference of the inner wall of the __. once these have been placed, a __ is scanned and the __ is measured. the positions of the __ are then __.
about 15 small ferromagnetic shim plates in sophisticated non-ferrous metal trays; cryostat; phantom; amount of field inhomogeneities; shim plates; adjusted to minimize inhomogeneities
active shimming involves using __ to shim the system for __
an electromagnetic coil; each patient and each sequence used
typical MRI scanners use __ shimming to attenuate the largest inhomogeneities, and __ shimming to further reduce remaining inhomogeneities and optimize image quality
passive; active
the gradient set is a __ structure which contains __
cylindrical; 3 individual electromagnets, one for each gradient direction (X, Y, Z)
gradient coils are used for __ (3)
spatial encoding, dephasing, and rephasing spins
by changing the direction of the current which passes through a gradient coil, one can __
alter the polarity of the corresponding gradient
image from slide 18
image from slide 19
gradient strength (__) defines gradient __
amplitude; steepness
gradient rise time is the time needed for the gradient to __
reach maximum magnitude
reducing the rise time results in a __ and subsequent __
power overshoot; undershoot
slew rate is the time it take the gradient to __
some given magnitude
duty cycle is the __
percentage of time the gradient is allowed to work
image from slide 20
balanced gradient systems involve each gradient pulse __
being balanced by an equal but opposite gradient pulse
gradient lobe amplitude is limited by __ (2) of the sequence
bandwidth and acquisition window
gradient duty cycle is determined by __
the receiver bandwidth
the application of symmetric/balanced gradients is not very __, so asymmetric gradient systems have been developed which decrease the __, thus shortening __ and increasing __
time-efficient; amount of time that the gradient is on; scan time; turbo factor
images from slide 21
in conventional sampling, data are read out after the gradient __
has achieved maximum magnitude
a disadvantage of conventional sampling is __
wasting time while gradients are turned on
to alleviate wasting time while gradients are turned on in conventional sampling, one can implement __, where data are acquired __, although data must be __
ramp sampling; while the gradient is still changing; weighted using complicated mathematical techniques
to alleviate wasting time while gradients are turned on in conventional sampling, one can also implement __ where the gradient amplitude __, resulting in shorter __ though potentially more __
sinusoidal sampling; varies nonlinearly; scan times; artifacts
images from slide 22
RF coils allow scanner operators to generate RF pulses via __, which instructs the coil to __
pulse control unit; transfer energy to the body, tip the magnetization at an angle with respect to the static magnetic field, etc.
in neuroimaging, the __ coil is the most important, though other coils may also be used for imaging the __ (__ coil) or __ (__ coils)
head; spine (body); peripheral nerves (extremity coils)
images from slide 23
receiver coils are the coils which __
exhibit an induced EMF due to the MR signal
there are two types of receiver coils: __
volume coils (go all the way around the body/ body part) and surface coils (sit atop the body part)
a transceiver is a __
receiver coil combined with an RF coil (very common)
the smaller the RF coil, the __ the SNR
better
using more coils __ the SNR
increases
disadvantage of small coil
small area of coverage
advantage of small coil
better SNR
to alleviate the drawbacks of small coils, MRI engineers have designed phased array coils, where __
multiple small coils are paired with multiple receivers
with phased array coils, each coil __
receives signal from its own FOV
phased array coils: MRI software is used to automatically __ in order to produce an MR volume which __
combine the signals from all the coils; covers all the anatomy of interest
images from slide 24
