MRI Terms and Magnetic Concepts

MRI Terms

• MRI is an imaging technique used primarily in medical settings to produce high-quality images of the inside of the human body
• MRI is based on the principles of nuclear magnetic resonance
• Nuclear magnetic resonance is a spectroscopic technique used by scientists to obtain microscopic chemical and physical information about molecules
• Originally called nuclear magnetic resonance imaging (NMR).

Outline of Presentation

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

Magnets

• An object surrounded by a magnetic field (natural or induced) attracting iron or steel
• A large static magnetic field is needed to obtain an MRI signal from tissues
• The static magnetic field, called "B0" field, magnetizes the tissue.

Magnetic Field

• A condition found in the region around a magnet or electric current
• Characterized by a detectable magnetic force at every point in the region and the existence of magnetic poles
• A vector quantity with north and south poles
• Exerts an induction force on ferromagnetic and paramagnetic substances
• Bipolar/dipolar magnets, always have a north or south pole

Classification of Magnets

• Classified according to the origin of their magnetic property
• Natural magnets (naturally occurring)
• Permanent magnets (naturally occurring)
• Electromagnets (man-made)

Magnetic States of Matter

• All matter has magnetic properties
• Four types:
  • Nonmagnetic: Unaffected by magnetic fields (e.g., glass, rubber, wood)
  • Diamagnetic: Weakly repelled by both poles of a magnetic field (e.g., gold, diamond, lead, silver)
  • Paramagnetic: Weakly attracted to both poles of a magnetic field (e.g., gadolinium, tungsten, aluminum)
  • Ferromagnetic: Can be strongly magnetized(e.g., iron, nickel)

Units of Magnetic Field Strength

• Gauss and Tesla are units for measuring the strength of a magnetic field
• Gauss is a smaller unit compared to Tesla
• 1 Tesla equals 10,000 Gauss
• The earth's magnetic field is approximately 0.5 Gauss.

Magnet Room

• The primary component in the MRI magnet room is the magnet itself
• The magnet surrounds the patient and any necessary antennas for radio wave transmission and reception.

Magnets Used in MRI

• Three types:
  • Resistive
  • Superconductive
  • Permanent

Resistive Magnets

• Simple electromagnets
• Coils of wire
• Electric current produces a magnetic field; resistance produces heat
• Heat is conducted away by a cooling system
• Magnetic field strength is up to 0.3 Tesla
• Can be turned off when not in use
• Temperature sensitive

Superconductive (Cryogenic) Magnets

• Electromagnets
• Coils are cooled to very low temperatures with liquid helium or nitrogen to reduce electrical resistance and permit higher magnetic strengths
• Achieves high field strengths: 0.5 to 1.5 Tesla
• Major advantage: High signal-to-noise ratio (SNR)
• Major disadvantage: High cost and maintenance

Permanent Magnets

• Blocks or slabs of naturally occurring ferrous material
• Produces a constant magnetic field without additional electricity or cooling
• Field strength is 0.06 to 0.35 Tesla
• Magnetic field does not extend as far from the magnet as other magnetic field types.

Fringe Field

• Portion of the magnetic field extending away from the magnet's confines
• Cannot be used for imaging but can affect nearby equipment or personnel.

General Style of Magnets Employs

• Vertical Field (Open MRI): Low capital cost, light weight, can be shut off, high power consumption, limited field strength ( <0.2T), 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

Questions

• Q1: Key characteristics of permanent magnets
• Q2: Advantages of permanent magnets
• Q3: Disadvantages of permanent magnets
• Q4: Key characteristics of resistive electromagnets
• Q5: Advantages of resistive electromagnets
• Q6: Disadvantages of resistive electromagnets

This quiz explores the terminology and principles related to MRI technology, focusing on the magnetic fields and types of magnets used in medical imaging. Understand the relevance of nuclear magnetic resonance and how these concepts interact with the human body. Perfect for students of medical physics or radiology.