MRI Terms and Magnetic Principles

MRI Terms

• MRI is a medical imaging technique used to produce high-quality images of the inside of the human body
• MRI is based on the principles of nuclear magnetic resonance (NMR)
• NMR is a spectroscopic technique used by scientists to gather microscopic chemical and physical information about molecules

Outline of Presentation

• MRI
• Magnet
• Magnetic Field
• Classification of Magnets
• Magnetic States of Matter
• Magnets Used in MRI
• General style of magnets employed

Magnets

• A magnet is an object surrounded by a magnetic field, capable of attracting iron or steel.
• A high static magnetic field is required to obtain a magnetic resonance signal from tissues.
• The purpose of this static field(Bo field) is to magnetize the tissue.

Magnetic Field

• Magnetic field is a condition occurring in the space around magnets and electric currents.
• It's characterized by a detectable magnetic force at every point within the region, as well as by the existence of magnetic poles.
• A vector quantity with both a north and south pole, it induces force on ferromagnetic and paramagnetic substances.

Classification of Magnets

• Magnets are categorized according to their source of magnetic properties.
• Natural magnets
• Permanent magnets
• Electromagnets

Magnetic States of Matter

• All matter possesses magnetic properties.
• Four types of magnetic properties exist:
  • Nonmagnetic
  • Diamagnetic
  • Paramagnetic
  • Ferromagnetic.

Nonmagnetic

• Materials unaffected by magnetic fields
• Examples: Glass, Rubber, Wood

Diamagnetic

• Weakly repelled by both poles of a magnetic field
• Examples: Gold, Diamond, Lead, Silver

Paramagnetic

• Weakly attracted to both poles of a magnetic field
• Examples: Gadolinium, Tungsten, Aluminum

Ferromagnetic

• Can be strongly magnetized
• Examples: Iron, Nickel

Units of Magnetic Field Strength

• Gauss (smaller unit)
• Tesla (larger unit)
• 1 Tesla = 10,000 Gauss
• Earth's magnetic field ≈ 0.5 Gauss

Magnet Room

• The core component of an MRI system is the magnet.
• This large magnet surrounds the patient and any required antennas for radio wave transmission and reception.

Magnets Used in MRI

• Resistive
• Superconductive
• Permanent

Resistive Magnets

• Simple electromagnets, albeit large
• Earliest MRI magnet type
• Coils of wire used to create the magnetic field
• Heat and magnetic field strength limited by electrical resistance
• Heat is dissipated via a cooling system
• Direct current (DC) required for a static field

Resistive Magnets (cont.)

• Field strength typically up to 0.3 Tesla
• Can be turned off when not in use
• Temperature-sensitive

Superconductive Magnets

• Electromagnets
• Wire loops cooled to extremely low temperatures (using cryogens like liquid helium and liquid nitrogen) to reduce electrical resistance
• Enables higher magnetic field strengths than resistive magnets
• Clinical MRI strengths range from 0.5 to 1.5 Tesla

Superconductive Magnets (cont.)

• Advantages: High field strength which results in high signal-to-noise ratio (SNR)
• Disadvantages: High cost of acquisition, site installation, and maintenance.
• Quench: Sudden and violent loss of superconductivity occurs when the cooling environment is lost; the magnet coils face resistance, halting the static field

Permanent Magnets

• Composed of naturally occurring ferromagnetic material
• Produce a constant magnetic field without requiring additional electricity or cooling to low temperatures.
• Field strength (0.06–0.35 Tesla) increases with increased material amount
• Magnetic field does not extend as far away from the magnet (fringe field) as in other magnet types

Fringe Field

• Portion of the magnetic field outside the magnet's confines.
• Not usable for imaging.
• Can, however, affect nearby equipment or personnel.

General Styles of Magnets

• Vertical Field (Open MRI): Low capital cost, light-weight, can be shut off. High power consumption, limited field strength, water cooling required, large fringe field
• Conventional Horizontal Field: High field strength, high field homogeneity, low power consumption, high SNR, fast scanning. High capital costs, high cryogen costs, acoustic noise, motion artifacts, technical complexity