Chapter 1-2 Flashcards
The MR magnet produces the
external or “static magnetic field
The static field is also referred to as
B0 (B-zero)
The main purpose of B0 is to
magnetize the tissue
The term “static” means
that the field is constant
The units for magnetic field strength is expressed as
units of Tesla (T) or Gauss (G)
1 Tesla = ______ Gauss
10,000
There are 2 basic types of magnets:
permanent magnets and electromagnets
Permanent Magnet
made of materials which create their own persistent magnetic field; examples include simple bar magnet and horseshoe magnet
Characteristics of Permanent Magnet System
-magnet is produced by materials which are permanently magnetic
-magnetic field cannot be turned off
-no electric current is applied to create the magnetic field
-can only produce field strengths up to 0.35 T
-very heavy, 40,000lbs
A magnetic field is produced
any time current is flowing through a conductor
When a current flows through a conductor
a magnetic field is created perpendicular to the direction of current flow
Resistive MR System
-magnetic field is created by wrapping wire around an iron core
-when electrical current is applied, a magnetic field is generated
-field strength between 0.2-0.6 tesla
The main characteristic of resistive MR system is
the magnetic field is only present when electrical current is actively applied to the coils.
The only type of MR system that is routinely turned on and off
resistive MR system
The resistance of a conductor refers
to amount of opposition to current flow.
-higher the resistance, greater the opposition to current flow
-heat results because of this
Reducing the temperature of a conductor
will reduce resistance.
What does Superconductors refer to?
refers to materials whose resistance drops to zero when they are cooled below their critical temperature.
In a coil of superconductive wire,
electrical current can persist indefinitely with no external power source being applied.
How is a superconductive MR system cooled?
In a superconducting MR, the conductive wire is bathed in liquid helium to reduce it’s temperature.
In order to maintain helium in it’s liquid state,
it must be maintained at 4 Kelvin (4 K)
equal to 452.5 degree F
How does liquid helium maintain it’s temperature?
Liquid helium must be in vacuum to maintain a liquid at 4 Kelvin
What is cryostat?
the vacuum sealed chamber within a superconductive magnet which holds the conductive wire and liquid helium
What happens if the temp of liquid helium rises?
If the temp of liquid helium, rises ever so slightly for any reason it will expand to it’s gaseous state
What is the ratio at which liquid helium will expand?
760:1
one liter of liquid helium will expand to 760 liters of helium gas
Most superconducting magnets can hold how much helium?
upwards to 1000 liters which would result in 3/4 million liters of gas if it were to heat and expand
What is a quench?
sudden loss of magnetic field
helium gas must be vented out of the scan room
What is a major benefit of using superconductive magnets for MR systems?
higher field strengths
clinical MR systems range from 0.7 T to 3 T
research 7.0-11.0 T and higher
The direction of the magnetic field (depending on the system)
may be vertical or horizontal.
The head to foot direction of the body is annotated
z axis
transverse plane
The right to left direction of the body is annotated
x axis
sagittal plane
The anterior to posterior direction of the body is annotated
y axis
coronal plane
In cylindrical or horizontal bore, the direction of the B0 (magnetic field) would be along the
z axis
In a vertical field MR, the direction of the B0 (magnetic field) would be along the
y axis
Where should the fringe field line be?
As close to the MR system as possible.
it is desirable to confine the stray (fringe) field so the the 5 gauss line is contained as close to the MR system as possible.
What is magnetic shielding?
to reshape and/or reduce the fringe field
What is passive shielding?
uses metal (iron) in the scan room walls or around the magnet itself
What is active shielding?
using additional coils within the magnet itself with current actively applied
What are the FDA guidelines for exposure to a static magnetic field?
-Adults, children, and infants > 1 month - 8 T
-infants 1 month or less - 4 T
What is the magnetohydrodynamic effect?
this biological effect is observed when a patient is positioned in a system in which the direction of B0 is along the head to foot (z axis) of the body. Blood is an excellent conductor and when it flows around the aortic arch, you have a conductive medium moving perpendicular to the direction of B0. This results in an elevation of the T-wave segment on a ECG. It is reversible. When the patient is removed from the field, the ECG tracing returns to normal.
What are rotation/torque forces?
forces exerted on ferromagnetic objects in the body in magnetic field (implants and devices)
-Rotation or torque forces are forces which when exerted on an elongated object, tend to cause it to turn and align with the direction of the external magnetic field.
-will also scale with field strength (the higher the field strength, the greater the torque or rotational force which will be exerted on an elongated object or device)
Where are rotation/torque forces the greatest?
at the isocenter of the magnet
What is translational force?
-missile effect
-when a ferromagnetic object encounters a magnetic field that changes from point a to point b, it can encounter a force that causes it to move toward the higher field
What does translational force exertion depend on?
-the mass and ferromagnetic properties of the object
-the amount of change in the magnetic field over distance (dB/dx)
What is spatial field gradient?
the change in field over distance (dB/dx)
-the change is greatest at the point where the patient enters the magnet
What is required to produce an MR signal from the tissue?
A secondary magnetic field. This is the radiofrequency (RF) field - also known as the B1 field
What is the frequency of the RF field at 1.5 T?
approximately 64 MHz
Is it necessary to shield the MR environment from interference by external or stray RF signals?
Yes, they must use RF shielding
What is RF shielding?
-shields the MR environment from interference by external or stray RF signals
-encompasses the MR room in copper
What is Faraday Cage?
Metal enclosure that prevents entry or escape of electromagnetic fields
-line the walls with copper before sheetrock is installed
what is the main purpose of RF shielding?
to eliminate RF noise (extraneous signals) from the MR environment
What are the main components of the RF system?
the RF coil and the associated electronics which produce RF waveforms
what are the RF coils responsible for?
the RF coils are responsible for transmitting RF and/or detecting the MR signals from the tissues in the body
RF coils can be designed to be which types?
-can be designed to transmit RF energy to the tissue (Transmit Only)
-detect (receive) the signal from the tissue (Receive Only)
-both transmit and receive
In a cylindrical whole body MR system, the transmit coil is located
within the magnet enclosure.
-referred to as integrated body coil or body coil which can be transmit only or transmit/receive
Coils are typically classified as
-linear
-quadrature
-multichannel
What is a linear coil?
-simplest design
-consists of single element and a single receiving channel (receiver)
-as the net magnetization precesses through the coil, waveform is induced in the coil (Faraday’s law of induction)
-linear refers to the coil being linearly polarized
What is a quadrature coil?
-circularly polarized (aka quadrature)
-consists of 2 elements
-electronically arranged so the waveforms induced the elements are 90 degrees out of phase
-the elements and their associated circuitry are designated as either an I or Q channel which are connected to a single receiver
-the signals are combined in the receiver
-approx 41% more efficient than a linear coil of the same size
What is a multichannel coil?
-aka Phased Array
-consists of multiple elements and multiple independent receivers
-can achieve greater coverage and high SNR
-each element in the array is electronically mapped to a receiving channel & receiver
-boosts SNR while maintaining the required coverage
RF energy can result in
heating.
Heating can occur in both conductive materials (implants/devices) and human tissue
What is SAR?
Specific Absorption Rate
-the measure of RF energy absorbed in tissue
What is SAR expressed in?
units of Watts/kg of body weight
What are the FDA guidelines for SAR limits?
4.0 W/kg or less of whole body average
8.0 W/kg or less spatial peak in any 1 g of tissue
3.2 W/kg or less averaged over the head
or
core temp increase of 1 degree C
What are the 2 scanning modes on clinical MR systems?
Normal Operating Mode and First Level Controlled Mode
Normal Operating Mode
-will limit the SAR to no more than 2.0 W/kg whole body average and 3.2 W/kg for the head
-in normal mode, no physiologic stress is expected
First Level Controlled Mode
-will allow SAR up to the maximum of 4.0 W/kg whole body average and 3.2 W/kg for the head
-physiologic stress may be seen in certain patients (some patients may not be able to adequately dissipate the energy/heat)
SAR is proportional to the power of __ for a systems resonant frequency
2
this means when you double the field strength (going from 1.5T to 3T), it takes 4 times the RF power for a given RF pulse
SAR is proportional to the power of __ for the B1 amplitude of the RF field
2
