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