Types of Magnets and MRI Magnets

Magnet Types

• Magnets are objects surrounded by a magnetic field, attracting iron or steel. Natural or induced.
• MRI requires a large static magnetic field ("B₀ field") to magnetize tissue and obtain a signal.
• Types of magnets used in MRI include resistive, superconducting, and permanent.

Magnetic Field

• A magnetic field is a condition around a magnet or electric current, characterized by a detectable magnetic force and magnetic poles.
• It's a vector quantity with a north and south pole, inducing force on ferromagnetic and paramagnetic substances.
• Bipolar or dipolar magnets always have a north or south pole.

Classification of Magnets

• Magnets are classified by their origin: natural, permanent, or electromagnets.
• Natural magnets occur naturally.
• Permanent magnets retain their magnetism.
• Electromagnets create magnetism through electric current.

Magnets Used in MRI

• Types of MRI magnets are resistive, superconducting, and permanent.

MR Magnets

• Permanent Magnet: Made of ALNICO or rare-earth alloys, doesn't lose its magnetic field, and doesn't require electricity or cooling to low temperatures.
• Resistive Magnet: Large coils of copper or aluminum wire; produce heat during operation limiting the magnetic field strength, cooled by water.
• Superconducting Magnet: Niobium-titanium alloys in a copper matrix, cooled to 4K (-269°C), creating a super-conductor, with liquid helium as a coolant. The coils are often very long (several miles).

Resistive Magnets

• Are simple electromagnets used in MRI.
• Consist of coils of wire with electric current to create magnetic field.
• Electrical resistance generates heat.
• Cooling systems remove heat from the magnet.
• Produce a static magnetic field (direct current is required).

Resistive Magnet Characteristics

• Field strength: up to 0.5 Tesla (T).
• Magnetic field inhomogeneity: 10–50 parts per million (ppm).
• Power consumption: 50 to 100 kilowatts (kW).
• Weight: 4 tons.
• Field can be switched off immediately.
• Flux lines run horizontally.
• Modest fringe fields (~2 meters, 0.5 milliTesla (mT)).

Superconducting (Cryogenic) Magnets

• Are electromagnets cooled to very low temperatures (using liquid helium and liquid nitrogen).
• Wire loops are cooled to reduce electrical resistance.
• Used in MRI for high-field strengths.

Superconducting Magnet

• Built from multiple vacuum vessels that act as temperature shields.
• Superconducting wire coils are typically several miles long.
• Coils are kept at 4.2 Kelvin (K) by immersing them in liquid helium.
• A large dewar, surrounded by liquid nitrogen, insulates the coils.
• Typical liquid helium volume in an MRI magnet: about 1700 liters.

Superconducting Magnet Characteristics

• High field strength (0.37 to 4 T, up to 14 T for research).
• Low magnetic field inhomogeneity (0.1 to 5 ppm).
• Expensive to purchase and operate, complex technology to maintain.
• Large fringe fields (~10 meters, 0.5 mT).
• Weight: 10 tons
• Power consumption: ~20 kW.

Superconducting Magnet Advantages and Disadvantages

• Advantages: High field strength, homogeneity, low power consumption, high signal-to-noise ratio (SNR), fast scanning.
• Disadvantages: High capital cost, high cryogen cost, potential for acoustic noise, motion artifacts, technical complexity.

Magnet Quench

• Loss of superconductivity in an MRI magnet, resulting from sudden boiling off of liquid helium.
• Patients and staff must be evacuated from the scanner room immediately.
• Liquid helium can displace oxygen leading to unconsciousness if inhaled in large quantities.
• Planned quenches as well as accidental ones possible.
• Most superconductive magnets have a "quench button" to rapidly turn off the main field within seconds.

Permanent Magnets

• Made of naturally occurring ferromagnetic material.
• Constant magnetic field, requiring no electricity or cooling.
• Magnetic field doesn't extend as far away from the magnet as other types (smaller fringe field).

Fringe Field

• Portion of magnetic field extending outside the magnet.
• Cannot be used for imaging but can affect nearby equipment or personnel.

Permanent Magnets (Open Bore)

• Type of permanent magnet with an open center (bore), allowing for patient access.
• Field strengths range from 0.2 to 0.7 Tesla.

Permanent Magnets Advantages and Disadvantages

• Advantages: Low power consumption, low operating costs, no cryogens, smaller magnetic fringe field.
• Disadvantages: Limited field strength, magnets are very heavy, no quenches possible.

MR Field Strengths

• Low Field: below 0.3 Tesla
• Mid-field: 0.3 to 1.0 Tesla
• High field: 1.0 to 3.0 Tesla
• Very high field: 3.0 to 7.0 Tesla
• Ultra high field: above 7.0 Tesla

Advantages of Low Field Scanners

• Open design, low fringe field, Reduction of certain MR artifacts, lower energy deposition on tissues ( lower SAR), lower initial purchase price, and lower operational cost

Disadvantages of Low Field Scanners

• Lower signal-to-noise ratio, lower homogeneity, impaired detection of calcification & hemorrhage, and reduced detection of gadolinium enhancement.