MRI: INSTRUMENTATION AND EQUIPMENT CHAP. 9

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__________ 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


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