MRI
Advantages and Disadvantages of Superconductive Magnets
Adv: Disadv: - high field strength - high capital cost -high field homogeneity - high cryogen cost -low power consumption - intense fringe field
Advantages and Disadvantages of Resistive Magnets
Adv: Disadv: - low capital cost - High power consumption -easy coil maintenance - Water cooling required -negligible fringe field - Significant fringe field
What was MRI initially called
NMR (Nuclear Magnetic Resonance)
Gyromagnetic Ratio is ____ for each type of element, which allows it to be ______ during MRI
fixed differentiated
contrast agent is common with strong relaxation rate properties, but highly toxic and ust be administered with DTPA
gadolinium
What does an MRI consist of?
gantry, table, and computer
uses electrocardiographic waves to trigger scanning pulses and reduces motion in t, c-spine, and cardiac imaging
gating
act as spatial localizers within the anatomic section
gradient coils
What does MRI utilize to create diagnostic cross-sectional images of the body
magnetism and radio frequencies
this can be a problem due to long scan times
motion
What is precession
spinning nuclei have a stationary axis and as it starts to lose speed the axis begins to lean and rotated causing it to wobble
Proton Spin Density
-Quantity of resonating spins within a tissue -The more concentrated the protons, the denser the tissue and stronger the signal
What does the MRI computer do
-controls gradient and RF coils and their pulsing -collects received RF data and creates image -stores data -allows for manipulation of the image
Why does MRI focus on the hydrogen nuclei
-hydrogen nuclei are the strongest nuclear magnets -give strongest MR signals -most common element in the body
Safety
-magnetic field attraction increases by the inverse cube law, so metal can become lethal -surgically implanted devices or FBs may be torqued within the tissue -magnetic media may be erased -electronic devices pacemakers may be rendered non-functional
MRI Parameters: relies upon 3 properties of matter:
-nuclear density -relaxation rates -flow phenomena
What is fringe field
-part of the magnetic field which extends outside of the patient aperature. -It is 3D. -can interfere with proper operation of nearby equpiment
What could quenching cause
-severe damage due to heating of machine -asphyxiation -explosion
What does resonance refer to
-the state of being resonant(echoing) -intensification and enriching of a musical tone by supplementary vibration -vibration of lrg amplitude in a mechanical or electrical system caused by a relatively small periodic stimulus of the same or near the same pd as the natural vibration pd of the system
What is the typical range for stationary magnets in Teslas
.06 - 4.0
Nuclear Magnetic Resonance was not reported until
1946
First images were produced in what year
1972
FDA requires prominent posting of triangular warning signs at the __ __ line
5 G
Advantages and Disadvantages of Permanent Magnets
Adv: Disadv: -low capital cost - limited field strength -low operating cost - Fixed field strength -negligible fringe field - very heavy
T1 Relaxation Time
After the RF signal is removed, the time required for the spins to realign with the external magnet also called: longitudinal, thermal, spin-lattice
A rotating, charged nucleus behaves as a spinning bar magnet, emitting a moving magnetic field with magnetic lines of flux. This is referred to as
Angular momentum Nuclear Magnetic Moment Magnetic Field
Gyromagnetic Ratio is measured in units of
MHz/T
electromagnetic shielding is known as
Faraday Shielding or RF Shielding
stronger magnetic fiels produce stronger RF signals
Field Strength
eliminates need for contrast in the traditional sense for visualization of vascular structures moving substances usually have weak signals (black)
Flow Phenomena
Less than 90 degrees pulse - basis for fast imaging
Gradient Echo
the resistance to lining up with the external magnetic field is known as the ___ ____
Gyromagnetic Ratio
Rate of Precession: definition & what is it also referred to as
Gyromagnetic Ratio X Magnetic Field Strength Angular Velocity
Gyromagnetic Ratio is:
Magnetic Moment/Inertia or Angular Momentum/Inertia
2 pulse sequence - 180 degrees pulse followed by a partial saturation sequence
Inversion Recovery
Larmor Frequency
Is when all protons in a nucleus of a given element will precess at exactly the same frequency
MR signals are a combo of what 3 pieces of information
PD, T1, T2
Series of 90 degrees RF pulses with signal measured after each
Partial Saturation
made of permanently magnetized materials. may be very heavy (up to 100 tons) but low maintenance
Permanent Magnets
primary variable in the data collection procedure during MR
Pulse Sequence
____ ____ is used to cause protons to precess and intervals for each
Pulse Sequences
-serve as both the transmitter and receiver of the ____ signal -should be as close as possible to the part being visusalized, so are often sized and shaped to fit specific body parts(surface coil)
Radiofrequency Coils
electromagnet with loops of wire uses lots of power and generates much heat
Resistive Magnets
2 types of RF coils
Saddle or Birdcage
Nuclear Spin Density: many ____ & ____ have similar #s of protons, and thus reading _____ seperate those tissues
Soft tissues & fat Poorly
90 degrees initiating partial saturation and followed by a 180 degree pulse @ 50 % of echo times
Spin-Echo
Signal Strength varies according to each property. The computer converts signal strengths to shades of gray:
Strong Signals = white Weak Signals = black
-Uses many loops of wire made of superconductors who lose nearly all resistance when their temp is dropped near absolute 0 ( 0 kelvin) -low temp is achieved using liquid cryogens -requires an insulating chamber (dewar) filled with liquid helium and liquid nitrogen
Superconductive Magnets
T2 Relaxation Time
Time required for spins to realign with the external magnet due to the interactions with each other also called: spin-spin or transverse
Who suggested that nuclei spin and in what year
Wolfgang Pauli 1924
3 sets of gradient coils:
X - coronal axis Y - sagittal axis Z - transverse axis
what happens to spinning nuclei with magnetic moments when they are placed in an external magnetic field _________ BUT.... Nuclei also have ______, and bc of this the spinning nucleus is also subject to ______ Inertia tries to keep things moving in the same direction, thus it _____ ____ ___ with the external magnetic field
align with the field as does any magnet Mass Inertia resists lining up
What is the primary visual factor called
brightness
Nuclear Magnetism: A nucleus is a small, but highly ______ piece of matter
charged
Faraday Shielding is used to
improve image quality by restricting interference from outside sources
What does the gantry contain
large stationary magnet shim coils gradient coils RF coils
FDA Recommendation
no published reports of harmful effects due to the use of MRI. FDA suggests limiting whole or partial body exposure to: 2.0 T for static magnetic fields 3.0 T/sec for gradient fields specific absorption rate (SAR) of .4 W/kg for RD fields
Pulse Sequence is set by
operator
contrast agents for MRI agents are primarily?
paramagnetic agents
Not only do protons, neutrons, and electrons have charge, they also possess a property called
spin
determined by the size or depth of voxel
section thickness
Production of the Nuclear Magnetic Resonance Signal
see yellow paper
-a series of corrective magnets added to even out the magnetic field of the stationary magnet -irregularities occur in the field strength as the magnetic field strength varies with the inverse cube law so that distance has a much faster effect
shim coils
what are paramagnetic agents designed for
to enhance T1 and T2 relaxation times of adjacent hydrogen nuclei
What does Faraday Shielding consist of
wire mesh in walls