the extent to which a material becomes magnetized when placed within a magnetic field
Magnetic susceptibility <0. Slightly repelled by magnets.
Mercury, silver, copper, carbon, hydrogen have this type of magnetism
Magnetic susceptibility >0. Weakly influenced by external magnetic field. No measurable self-magnetism.
Platinum, oxygen, tungsten, manganese, aluminum, gadolinium contrast agents are this type of magnetic materials
Magnetic susceptibility >1. Easily magnetized
Iron, cobalt, nickel, dysprosium are this type of magnetic materials
type of magnet with no power needed and no cryo cooling
type of magnets that uses electrical current B0=KI Copper wire wrapped in loops to form a coil
type of magnets with horizontal flux lines Must have power supply = high operational cost .3T max strength
Hydrogen in the body
62% of all atoms found in the human body and 10% of total mass. Single charged spinning nucleon. No net charge.
type of magnets that must be cryo cooled with liquid helium High field strengths (1.5T, 3T, 7T, etc.) High capital cost, low ops cost
type of magnets that are a combination of superconducting and permanent
type of magnets that are low strength with a specific use (extremities, etc)
the magnetic field outside the bore. It is stronger the closer you get.
A moving electric charge, produces a magnetic field. Strength of the magnetic field is proportional to amplitude and velocity of the moving charge.
MRI Zones I-IV
I - general public II - screening are III - area around magnet, control room IV - magnet
Gradient coil strength depends on:
: number of loops in coil, current in loops, and diameter and spacing of loops
gradient strength, measured in mT/m or G/cm
Gradient rise time
:time to reach highest amplitude
:gradient strength over distance 70 mT/m/s average, 120 mT/m/s is high speed
:% of time during one TR that gradient can be at max amplitude
:can be a transmitter, receiver, or both Must be transmitted at resonant frequency of hydrogen for resonance to occur (wobble or jiggle)
Types of transmit/receive coils
head extremity some breast coils
Types of receiver coils
detect/encode signal volume or birdcage coils surface coils Hemholtz pair phased array
What are volume coils?
type of coil that transmit RF receives MR signal encompasses entire anatomy
What are surface coils?
type of RF coil that has better signal to noise small, placed close to anatomy can have signal drop off
What are array coils
type of RF coil that includes multiple coils and receivers, data from each coil is combined -spine, pelvic, breast, cardiac, TMJ
What is the minicomputer?
part of the operator interface -fast, high capacity -must be able to multi-task
What does the image processor do?
part of the operator interface that converts data to image
What is the hard disc drive?
part of the operator interface -image, parameter, and raw data storage
With the application of a magnetic field, magnetic moments align with OR against the external magnetic field
a rotational motion about an axis of a vector whose origin is fixed at the origin of the coordinate system
wobble associated with atom being exposed to the magnetic field
frequency of the precession is _______ _________ to the strength of the magnetic field.
The resonant frequency of a spin within a magnetic field of a given strength. It defines the frequency of electromagnetic radiation needed during excitation to make spins change to a high-energy state, as well as the frequency emitted by spins when they return to the low-energy state. (Rate of precession)
-W0 also known as larmor frequency or resonance frequency -λ is the gyromagnetic ratio --Unique to each atom
RF Frequency Pulse (RF pulse)
A short burst of electromagnetic energy delivered to the patient by the RF transmitter.
Absorption or emission of RF by a nucleus in a magnetic field following RF excitation at the Larmor frequency of that nucleus. Must apply an electromagnetic RF pulse at this type of frequency of hydrogen for the RF to be absorbed by hydrogen.
What happens when once the RF transmitter is turned off?
-Absorbed RF energy is retransmitted, producing a signal. -Excited spins return to original orientation. -Spins become "out of phase" as they return to their original orientation.
Will cause a change in the magnetic field strength within the magnet. -As spins are out of phase, this can be applied to rephase them.
process of losing energy.
when the amount of magnetization in the longitudinal plane increases
when the amount of magnetization in the transverse plane decreases