MRI: INSTRUMENTATION AND EQUIPMENT CHAP. 9
__________ uses additional solenoid electromagnets located around the outside of the main magnet coils at each end of the magnetic bore
Active shielding
________ is performed by an electromagnetic coil and can be used to shim the system for each patient or even each sequence within a protocol
Active shimming
______________ can significantly reduce the area affected by fringe field. there are two methods - passive and active.
Magnetic shielding
it stated that "the induced electromotive force in a closed loop equals the negative of time rate of change of magnetic flux through the loop"
Michael Faraday's law of electromagnetic induction
objects made of substances of this type can become dangerous projectiles when inadvertently brought near a strong magnetic field
ferromagnetic
once a _________ material is exposed to an external magnetic field it retains magnetization and therefore becomes magnetized
ferromagnetic
permanent magnet systems have almost no discernable _____________
fringe field
it a commonly known paramagnetic material used as MR contrast agents
gadolinium chelates
in MRI, the __________ is the unit used to measure fringe magnetic field that extends beyond the bore of the main magnet
gauss
the unit ________ is used to measure low field strengths
gauss
Gradient coils are powered by gradient amplifiers. faults in the gradient coils or gradient amplifiers can result into ____________________ in the MR image
geometric distortions
defines the time it takes for a given gradient to reach maximum amplitude. it is measured in microseconds
gradient speed/ gradient rise time
defines how steep or strong a particular gradient is. it is measured in milliTesla per meter (mT/m) or gauss per centimeter (G/cm)
gradient strength/ gradient amplitude
diamagnetic substances have _____________ magnetic susceptibility and show a slight decrease in magnetic field strength within the sample
low negative
Paramagnetic substances have _____________ susceptibility
low positive
the magnet aligns the nuclei into ____________ and _________ states
low-energy (parallel), high-energy (anti-parallel)
passive shimming is used to get the _________ to a particular level of homogeneity.
magnetic field
the way that materials behave in the presence of external magnetic field is determined by a property known as __________
magnetic susceptibility
like mass and electrical charge of a particular substance, __________is a fundamental property of matter
magnetism
another less popular configuration involved the use of three coils combined into one spherical arrangement known as
maxwell coil
the stronger the magnet, the ____________ in the low-energy state
more spins
generally, the range of the area that can be imaged is limited by the size of coil
multiple coils
the positive paramagnetic effect is greater than the _____________ effect and therefore the substance appears paramagnetic
negative diamagnetic
the ______________ of an atom is a combination of the magnetic moments of all electrons present
net magnetic moment
the more low-energy spins the greater the ________, the higher the signal.
spin excess
stray magnetic field outside the bore of the magnet is known as ___________
stray field/ fringe field
the temperature fluctuation should not fluctuate by more than _______ for optimal operation of the permanent magnet MR system
1 Kelvin
the weight of a permanent magnet can be on the order of ______ kg
15000
FDA increased a limit to __________ for infants up to one month and upto _______ for any age above this.
4T, 8T
the FDA limits the strength of stray field, located accessible by the public to ____________
5G
one liter of helium produces ________ liters of helium gas when allowed to boil off
748
the apparent magnetization of atom can be shown by this equation:
Bo= Ho (1+x)
_____________ appear in all substances
Diamagnetic effects
_____________ materials have half-filled electron shells
Ferromagnetic
MR signal is changed to an understandable format from a FID into a spectrum by series of mathematical equations known as __________
Fourier transform
___________ is an increasingly rare resource that is extracted from natural gas
Helium
this configuration is a primitive way of obtaining uniform signal across the volume of tissue
Helmholtz pair
these 1T systems give the advantage of a patient friendly, spacious scan environment, coupled with the benefits of high-field scanning - high SNR and ideal T1 contrast
High-field open systems
the strength, order and timing of the gradient pulses determines ___________
MR image resolution
the ___________ determine spatial location of RF signal
Magnetic Field gradients
the energy required to produce resonance of nuclear spins is expressed as a frequency and can be calculated by the ______________
Larmor equation
_______________ is accomplished by surrounding magnet with steel plates. can weigh up to 40 tonnes.
Passive Shielding
______________ is performed by scanning a phantom and adjusting the position of the shim plates until optimum field homogeneity is achieved. it is also performed at the time of installation.
Passive shimming
The instrumentation required to achieve resonance is the ______________ assembly, which includes RF transmitter coils and RF receiver coils
RF coil
a _________ perturbs or excites nuclei
RF source
the 90' RF pulse is created by oscillating secondary magnetic field formed as a result of alternating current flowing through a loop of wire called an
RF transmitter coil
Energy is transmitted at the resonant frequency of hydrogen in the form of a short intense burst of electromagnetic radiation known as radiofrequency pulse
RF transmitters
To produce MR images, energy must be added to the system
Radiofrequency
as the __________ enhanced when using the local coils, greater spatial resolution of small structures can often be achieved
SNR
ohm's law states:
V=IR
________________ can be configured in a solenoidal, saddle and/or birdcage configuration. it can both transmit RF and receive the MR signal and often called transceiver
Volume coils
______________ shielding implies current/activity
active
the most common material used to produce a permanent magnet is alloy of aluminum, nickel and cobalt known as _________
alnico
all substances will interact with an ___________, even those that one might consider non-magnetic
applied magnetic field
Fourier transformation occurs via _____
array processor
the degree of magnetism exhibited by a substance is related to a property known as _____________. these dipoles are generated in an atom by movement of electrons
atomic magnetic dipole (or moment)
each gradient pulse is balanced by an equal but opposite gradient pulse. this is known as _________ or balanced gradient system
bipolar
examples of diamagnetic substances include:
bismuth, carbon (diamond), carbon (graphite), copper, mercury, lead and water
a cylindrical array of electrically conductive elements positioned around the inner circumference of the magnet bore. main RF transmitter and transmits RF for most examinations that are acquired without a transmit/receive coil.
body coil
example of volume coils
body coil, birdcage coils, solenoid coils
there are further windings located at the length of the bore- known as ________________- to improve homogeneity. the entire structure is known as the _________
bucking coils, bobbin
if the current is passed through two parallel straight wires in opposing directions, the two magnetic fields tend to __________ out in the region between two wires
cancel each other
to maintain superconductivity the current- carrying loops of wire are super-cooled with substance known as ______ to eliminate resistance
cryogens
_________ is a stainless-steel tank configured in the shape of hollow cylinder. the inside of the cylinder contains layers known as heat shields, and the helium reservoir is isolated from the outer walls of cryostat by an evacuated chamber.
cryostat
__________ materials have paired electrons
diamagnetic
the prime safety concern related to helium gas is the fact that it ______________
displaces oxygen
defines the percentage of the time that the gradient is permintted to work. it is expressed in units of percentage (%)
duty cycle
the current passing along the wire is proportional to the magnetic field induced around it
electromagnets
elements can be classified as belonging to one of four main categories, depending on their _____________
electron configuration
at field strengths used in MRI, energy within the radiofrequency (RF) band of the electromagnetic spectrum is necessary to perturb or _____________
excite the spins
ferromagnetic materials retain their magnetic substances even when ____________ is removed
external magnetic field
__________ which are generally of a saddle configuration and are configured to accommodate the size of adult knee. these are generally used to image lower ex but can be also used to image the upper ex
extremity coil
a _____, which can be a saddle-shaped, or birdcage type configuration or a multichannel coil
head coil
coils do not provide ___________, but rather high signal.
high resolution
the more _________ the magnetic field, the better the image quality.
homogenous
the ________ computer oversees the process and allows a means for operator interference with the system
host
examples of superparamagnetic mateials
iron oxide contrast agents
examples of ferromagnetic materials
iron, steel, gadolinium (native state and below curie temperature)
examples of surface coils
linear coils, quadrature coils, helmholtz pair, maxwell pair, phased array, multi-coil elements
cryogens used in MRI include _____ and in some cases __________.
liquid helium, liquid nitrogen
Paramagnetic substances affect external magnetic fields in a positive way, by attraction to the field resulting in a _______________ in the magnetic field
local increase
system manufacturers have tried to invent variations in system designs for specialty imaging concerns. this system is known as ____________
niche magnets
examples of permanent magnet
nickel, iron, cobalt
with no external magnetic field present, diamagnetic substances such as lead and copper show ___________
no net magnetic moment
the main advantage of permanent magnets is that they require _____________ or ___________
no power supply, cryogenic cooling
the factor that governs the efficiency of the passage of current is inherent resistance of the coil. the degree of resistance along wire is determined by _____________
ohm's law
permanent magnet systems are typically designed so that the magnetic plates are located above and below the pt., providing good all- around access. these are known as _____________
open MRI system
active shimming is used to ___________ for each patient examination
optimize
electrons exhibit two principal kinds of movement-
orbittal motion around nucleus, spinning motion around their own axes
the magnetic behavior of an atom is therefore dictated by the configuration of __________
orbitting electron
with no external magnetic field, these magnetic moments occur in random pattern and thus cancel each other out
paramagnetic
inhomogeneity within particular imaging field is expressed in arbitrary unit known as _________
parts per million (ppm)
________ shielding implies no current/passitivity
passive
___________ is achieved by using metal discs/plates. __________ is achieved by using additional solenoid magnets.
passive shimming, active shimming
as this magnetic field remains in the place when external magnetic field is no longer present, it is known as a _______________
permanent magnet
since ferromagnetic substances retain magnetism after being exposed to a magnetic field, these substances are used in the production of _________________
permanent magnet
ferromagnetic substances remain magnetic, are permanently magnetized and subsequently become
permanent magnets
____________ coils consists of multiple coils and multiple receivers whose individual signals combined to create one image with improved SNR and increased coverage
phase array
the signals from array processor are converted into shades of gray, represented as ________ in MR images
pixels
the application of enough power to create high gradient amplitudes shortens rise times but yields a __________
power overshoot
the ________ oversees or controls the order of the timing of RF and/or gradient pulses
pulse control unit
in the event of spontaneous boil-off over 1 000 000 liters of gas are liberated. this event is known as ________ and can be fairly explosive in nature
quench/quenching
time within the sequence is reduced if sampling is performed while the frequency encoding gradient is changing. this is accomplished with a technique known as
ramp sampling
the process of creating an electromagnetic field initially involves passing current through main superconducting coil of the scanner. this process is called ___________
ramping up
diamagnetic substances of this type are therefore not attracted to, but are slightly ___________ by the magnetic field.
repelled
an electromagnet at room temperature is subject to ohm's law and is said to be _______________
resistive magnet
the magnetic field strength in a __________ depends on the current that passes through its coils of wire
resistive magnet
How quickly a gradient can attain a particular gradient slope is called the ________________. this affects how fast a gradient can be switched on and off and this in turn affects the scan time.
rise time
MR signals are sampled during readout when the frequency encoding gradient is applied. Signals are sampled only after the gradient has reached the maximum amplitude
sampling
to maintain the magnetic evenness or homogeneity, a _______ system is necessary
shim
_______________ makes the field even.
shimming
________ often occurs when the imaging tissues deep within the patient
signal fall-off
Resonant gradient systems that oscillate at a particular frequency produce a sinusoidal readout gradient that permits ____________
sinusoidal sampling
defines the time it takes for a given gradient to reach maximum amplitude and what that amplitude is. it is the speed and strength of gradient and is measured in units of milliTesla per meter per second (mT/m/s)
slew rate
a gradient is simply a _____________, in this case a very linear slope in magnetic field strength across the imaging volume in a particular direction
slope
When an external magnetic field is applied, diamagnetic substance show a ________ magnetic moment that opposes the applied field
small
as a result of unpaired electrons within the atom, paramagnetic substances have ___________ magnetic moment
small
the loops of wire form a coil and act as though they are parallel straight wires. this 'spring-like' electromagnet is called __________
solenoid electromagnet
the strength of magnetic field is determined by the amount of current passed through wire, the number of loops in the spring and the distance of loops
solenoid electromagnet
gradient coils are used for _____________ and in certain imaging options such as GMN.
spatial encoding
to allow the high current necessary to obtain a very high magnetic field, the coils are constructed from an alloy of niobium and titanium. This material exhibits a property known as ______ when cooled below to a critical temperature
superconductivity
a ______ has virtually zero resistance and will continue to carry a powerful electrical current indefinitely without heating up
superconductor
______________ materials have an intermediate positive magnetic susceptibility that exhibited by paramagnetic and less than that of ferromagnetic materials
superparamagnetic
it has "medium" positive susceptibility
superparamagnetic materials
coils configured with a simple loop of wire and other components are known as linear coils. these coils typically offer high SNR because they only detect noise from a small area, and they are positioned close to the area of interest
surface coils
generally placed on or in the surface
surface coils
it refers to how something can be influenced by an external factor
susceptibility
As the resistance increases, the ___________ of the conductor also increases, and this in turn causes more resistance.
temperature
permanent scanners are ______________, and to maintain homogeneity and image quality a constant temperature must be maintained
temperature sensitive
___________ is the unit used to measure higher magnetic field strength.
tesla
in MRI, the strength of the magnetic field within the bore is expressed by the units of _________
tesla
in the presence of external magnetic field, paramagnetic substances align with _____________ and so the magnetic moments add together
the direction of the field
coils tend to be more symmetrical in design and rely on a ______________ arrangement to achieve the gradient field
three- terminal
the coils that both transmit and receive signal known as
transceivers
the RF system requires _____ and ______
transmitter and receiver
examples of paramagnetic materials`
tungsten, cesium, aluminum, lithium, magnesium and sodium
the resistive system is unique that the magnetic field associated with it can be _______________ at the flick of a switch- unlike permanent and superconducting magnets
turned off
permanent magnets commonly have ____________
two poles
Paramagnetic materials have _________________ electrons
unpaired
the magnetic field created by permanent magnet has lines of flux running ___________ from the south to north pole of the magnet, keeping the magnetic field virtually confined within boundaries of system
vertically
generally used to accomodate a 'volume' of tissue
volume coils
in diamagnetic, the electron currents caused by their motions add to ___________
zero