MRI Magnet Types and Properties

Magnet Types

• Magnets are objects surrounded by a magnetic field, with the ability to attract iron or steel. They can be natural or induced.
• A strong static magnetic field, called the B₀ field, is essential for obtaining an MRI signal from tissues. Its primary function is to magnetize the tissue.
• MRI magnets come in different types: resistive, superconducting, and permanent.

Magnetic Field

• A magnetic field is a region surrounding a magnet or electric current. It's characterized by a detectable magnetic force at each point within the region and the existence of magnetic poles (north and south).
• It's a vector quantity, possessing both magnitude and direction, and exerts an induction force on ferromagnetic and paramagnetic substances.
• A magnetic field always has a north and south pole.

Classification of Magnets

• Magnets are categorized based on how their magnetic properties originate.
• Natural magnets are naturally occurring.
• Permanent magnets retain their magnetism.
• Electromagnets generate magnetism through electricity (i.e., wire coils).

Magnets Used in MRI

• MRI magnets utilize resistive, superconducting, or permanent magnet technologies.

MR Magnets

• Permanent magnets: Composed of magnetized material (e.g., ALNICO or rare earth materials) that doesn't lose magnetism. They offer a consistent magnetic field without the need for electricity or cooling at low temperatures. They have a limited field strength compared to other types.
• Resistive magnets: These are electromagnets made from coils of copper or aluminum wire. Passing an electrical current through these coils creates a magnetic field. They produce heat through electrical resistance, significantly affecting the maximum achievable field strength.
• Superconducting magnets: These are electromagnets that utilize superconducting wire loops. The wire's superconductivity arises by cooling it to extremely low temperatures (using liquid helium or liquid nitrogen) significantly reducing electrical resistance. This allows for high magnetic field strengths, however, maintaining them requires special cryogenic systems.

Resistive Magnets

• Resistive magnets are simple electromagnets.
• They usually consist of coils of wire.
• Passing direct current through the coils creates a magnetic field.
• The heat produced by electrical resistance needs to be dissipated through a cooling system.

Resistive Magnets Characteristics

• Field strength typically up to 0.5 Tesla (T).
• Significant magnetic field inhomogeneity (10-50 parts per million).
• Power consumption ranges from 50 to 100 kilowatts (kW).
• Weighs several tons (can reach 4 tons).
• The magnetic field can be quickly switched off.
• Magnetic flux lines run horizontally.
• A modest fringe field (~2 meters with 0.5mT).

Superconducting Magnets

• Superconducting magnets are a special type of electromagnet.
• Their wire coil loops are cooled to ultra low temperatures to achieve superconductivity (using liquid helium).
• This greatly minimizes resistance, leading to high magnetic fields.
• These magnets are typically solenoids in design.

Superconducting Magnet Characteristics

• Field strength ranges for clinical MRI from 0.37 to 4 Tesla (and up, up to 14T for research).
• Field homogeneity, which refers to how uniform the magnetic field is, are generally from 0.1 to 5 parts per million.
• They are generally more expensive to purchase and maintain, due to requirement of cryogenic cooling systems.
• Relatively large volumes of cryogen (liquid Helium) are needed and require careful maintaining.
• Flux lines run horizontally.
• Large fringe fields (~10 meters with 0.5mT).
• Weight can get very large (10 tons).
• Power consumption is about 20kW.
• A "quench" (loss of superconductivity) can be planned, or can occur accidentally. These have a special safety system.

Magnet Quench

• Quenching is the sudden loss of superconductivity in a superconducting magnet.
• Occurs when the cooling system fails (boil-off of liquid helium).
• It's extremely important to quickly evacuate personnel from the MRI room as the helium can displace oxygen, causing unconsciousness.

Permanent Magnets

• Permanent magnets are constructed from naturally occurring magnetic materials (e.g., ferrous materials).
• They consist of blocks or slabs of naturally occurring ferrous materials.
• They provide a consistent magnetic field without requiring electrical power or special cooling systems since the magnetization is inherent to these materials.
• The magnetic field does not extend very far away from the magnet itself.

Fringe Field

• The fringe field is the portion of the magnetic field that extends away from the confines of the magnet itself. It cannot be used for imaging, but can affect nearby equipment (such as pacemakers) and personnel.

Permanent Magnets Advantages / Disadvantages

• Often have low power consumption.
• Low operating costs (electricity).
• Small fringe field.
• No need for cryogenic cooling.
• On the downside, they have limited field strength.
• They are heavy and don't have a built–in "quench" protection.

MR Field Strengths

• Field strength measurement is reported in Tesla (T).
• Field strengths commonly used vary, categorizing them as low, mid, high, very high and ultra high.

Advantages of Low Field Scanners

• Open design, less confined.
• Lower fringe field, minimizes interactions with objects outside the bore.
• Reduced MR artifacts due to chemical shift, susceptibility, and flow movements (motion).
• Lower energy deposition on tissues (lower Specific Absorption Rate (SAR)).
• Lower initial purchase price.
• Lower operational costs.

Disadvantages of Low Field Scanners

• Lower signal-to-noise ratio.
• Reduced image homogeneity.
• Compromised ability to perform advanced pulse sequences.
• Difficult determining certain pathologies—like calcification, hemorrhage, or iron accumulation.
• Signal from gadolinium contrast is less apparent.

Explore the different types of magnets used in MRI technology while learning about their strengths, weaknesses, and operational principles. This quiz covers low field and high field scanners, as well as permanent, resistive, and superconducting magnets in the medical imaging field.