MRI Magnetism and Principles

Magnetic susceptibility

The degree to which a material responds to an external magnetic field.

ex. Diamagnetic materials have low susceptibility while ferromagnetic materials have high susceptibility.

Helps determine how diff tissues or objects interact with MRI fields

Magnetic retentivity

The ability of a material to retain magnetization after the external field is removed

Ex. Permanent magnets have high entity while soft magnetic materials lose their magnetization quickly

Essential for designing MRI magnets that maintain stable fields

Magnetic permeability

The ease with which a material allows magnetic field lines to pass through it

Ex. Higher permeability means stronger interaction with magnetic fields

Key for shielding mri rooms and optimizing magnet design

Diamagnetic

Weakly repelled by a magnetic field

ex water copper

Paramagnetic

Aligns with a magnetic field but does not retain magnetization after. Repulsion, decrease in field strength

Ex. Gadolinium, oxygen

Superparamagnetic

Intermediate behavior seen in nanoparticles

Ferromagnetic

Strong permanent magnetization

Ex. Iron, cobalt, nickel

Curie point

The temperature at which ferromagnetic material transition to paramagnetic behavior

Above the curie point, magnetic properties are lost

Important for mri magnets to maintain stable performance under various conditions

B0

Main static magnetic field

main field for aligning protons

B1

Radiofrequency field

excites protons for signal generation

Gradient fields

Gx, Gy, Gz: used for spatial encoding of images

All three fields work together for mri imaging in spatial encoding within mri by allowing precise localization of signals within the body

X

Sagitta

Y

Coronal

Z

Axial

Hydrogen protons in MRI

Act as tiny magnets due to their spin and charge because they have 1 electron.

When placed in B0, they align and process at a specific frequency

Their behavior enables MRI signal detection

Magnetic dipole moment of hydrogen

Described the strength and orientation of a protons tint magnetic field because they can align to B0 parallel (low energy) or anti parallel (high energy). This alignment created a net magnetization which has weight, size, and direction at which the proton spins

Right hand grip rule

Determines the direction of a magnetic field around a current carrying wire. fingers represent the direction of the magnetic field lines and thumb represents the direction of the current

faradays law of induction

A changing magnetic field induces an electric current in a conductor

This principal is fundamental to MRI signal generation when protons returned to their equilibrium state they induce a measurable voltage in the receiver coil

Used in mri coils to detect signals from protons

Permanent magnets

Characteristics: naturally occurring ferrous materials, produces a vertical magnetic field

advantage: low power consumption, no need for cryogen’s

Disadvantage: low field strength, heavy, and difficult to install

Resistive magnets

Characteristics; generate a magnetic field, using electricity and wire loops and require constant power and cooling to maintain stability

Advantages:can be turned off easily and lower initial cost than superconducting magnets

disadvantages: requires continuous electrical power limited field strength typically below 0.3 T

Super conducting magnets

Principle of operation: when cooled to four Kelvin parentheses -269 Celsius parentheses exhibit zero electrical resistance(a material allows electric current to flow without losing energy as heat), allowing strong magnetic field.

Advantages:Highfield strength 1.5T 8T, stable, and efficient operation

Disadvantages: expensive to maintain and risk of quench events

Cryogens

Roll of liquid helium coolant and nitrogen (insulation) maintaining superconductivity

Quench

a sudden loss of superconductivity, causing rapid helium evaporation

can damage the magnet and pose safety risks.

only should happen for an emergency of someone’s life

Fringe Field

The magnetic field, extending outside the scanner. for must be controlled to prevent interference with nearby equipment and ensure patient safety

Main magnet coil

The core component that generates the B0 static main magnetic field and works with gradient and RF coils to produce MRI images

Passive shielding

Uses ferromagnetic materials shims inside the scanner and adjusts for large changes and reduces fringe field

Large metal plates like iron and steel structures are placed around the MRI scanner, which concentrate the magnetic field prevent preventing it from extending too far

active shielding

Uses electromagnets/bucking coils to counteract the main field and more effective at reducing Fringe field

ALWAYS ON

Shim system

Purpose is to achieve magnetic field homogeneity for clear images

Passive shimming

Uses ferromagnetic shims done once at installation

Improve field uniformity

Active shimming

Uses electromagnets done once at installation

Types resistive, and super conductive solenoids

Improve field uniform, buddy

gradient, offset (dynamic) Shimming

uses gradient, coils, done during image acquisition to correct minor in homogeneity, especially due to patient presence because the patient is a conductor

Shielding in mri

Ensure MRI scanner can be safely installed without affecting nearby equipment