MRI Physics and Instrumentation - Magnetism

Flashcards covering key concepts related to magnetism in MRI, including types of magnets, magnetic properties, electromagnetism principles, and instrumentation hardware.

What is the fundamental principle of electromagnetism?

When a charged particle (such as an electron) exhibits motion, a corresponding magnetic field is induced.

Name the three forms of manifestation of electromagnetism.

Light, Electricity, and Magnetism.

What is Magnetic susceptibility?

How easily a material is influenced by an external magnetic factor.

What is Magnetic retentivity?

Capacity of a magnetic field to stay in a substance after the external source has been removed.

What is Magnetic permeability?

A property that allows magnetic lines of force to pass through a material.

What is Angular Momentum?

A form of inertia, reflecting the object's size, shape, mass, and rotational velocity.

What determines the direction of the magnetic moment of any elementary particle?

Determined by the direction of its spin, with the negative value indicating that any electron's magnetic moment is antiparallel to its spin.

What are Maxwell's four equations in simple terms?

Wherever a charge exists, an electric field E diverges from it or into it; Magnetic field lines (B) can only exist in closed loops; An electric field (E) is produced by a changing magnetic field (dB/dt); Either a constant current or changing electric field creates a circulating magnetic field.

What is Faraday's Law of Induction?

The magnitude of the emf induced in a circuit is proportional to the rate of change with time t of the magnetic flux Φ that cuts across the circuit.

What does Lentz's Law state?

The direction of the induced current is always such that it will oppose the change in flux which produced it.

What are Lorentz Forces?

The force on a charged particle due to electric and magnetic fields.

List the categories of electron configuration changes between elements in order of increasing magnetic strength.

Diamagnetism, Paramagnetism, Ferromagnetism.

Describe Diamagnetism.

no magnetic moment without an external magnetic field, and in the presence of an external field, they are slightly repelled by the magnetic field, creating small, local decreases in magnetic field strength.

Describe Paramagnetism.

small magnetic moments due to unpaired electrons, and in the presence of an external magnetic field, their magnetic moments align with the direction of the field, creating local increases in magnetic field strength.

Describe Ferromagnetism.

strong attraction and alignment with an external magnetic field, remaining magnetic even without the presence of an external magnetic field.

What are the most Ferromagnetic elements?

Cobalt, Nickel, Iron, Gadolinium (both para and ferro), and some rare earth elements.

What are Gauss (G), Kilogauss (kG), and Tesla (T)?

Measurements of Magnetic Fields.

What hardware is required to complete MRI processes?

Magnet (Shim & Shielding coils), Radio frequency source, RF Amplifier, RF synthesizer, Spatial Encoding Gradients, RF amplifier, Image Processor & Host Computer system.

What is B0?

The main magnet field or static magnetic field or primary field

What is B1?

The RF field or radio frequency or secondary field

What are Permanent magnets?

They require no power supply or cryogens, are always on, and have vertical lines of flux.

What are Electromagnets?

They require a power supply, can be turned off immediately, and have horizontal flux lines.

What are Superconducting Magnets?

They are supercooled by cryogens, exhibit no resistance at certain temperatures, and have higher field strengths.

What is Fringe field?

Stray magnetic field outside the bore of the magnet also called spatial field gradient

What is Passive shielding?

sheets of metal line the walls of MRI suite to redirect the fringe field and prevent damage to equipment in nearby areas with diamagnetic substance

What is Active shielding?

Solenoid magnets or bucking coils oppose the flux and are located right outside the magnet rather than throughout the room. can reduce 5 gauss line to within a few feet of scanner.

Why is a Shim System used?

To create homogeneity within the magnet for imaging because without homogeneity, image distortion and processional frequency of atoms differences could occur.

What is Passive Shimming?

Sheets of metal line the walls of an MRI suite to redirect the fringe field and prevent damage to equipment in nearby areas.

What is the Shim system, and what is its function?

Improve the homogeneity of the magnetic field. It adjusts field gradients to enhance image quality by correcting inhomogeneities.

RF source

To generate and transmit pulses of electromagnetic radiation.

Magnetic field gradient system

To create linear slopes in field strength in any direction.

RF receiver coils

To detect signal from the patient.

Why are shim boxes used?

to improve the homogeneity of the magnetic field in an MRI by compensating for inhomogeneities and optimizing image quality .Also used on fat suppression if you’re looking at a body part that may not be isocenter in the magnet

How does inhomogeneity occur?

Inhomogeneity occurs due to variations in magnetic susceptibility of tissues, imperfections in coil design, and external magnetic interference, which result in uneven magnetic field distribution. Also if you move away from the center of the magnetic field or when something enters the magnetic field.