Types of Magnets and Magnetic Fields

Magnet Types

• Magnets are objects surrounded by a magnetic field, capable of attracting iron or steel.
• Obtaining an MRI signal from tissues requires a large static magnetic field (Bo field).
• The Bo field's primary purpose is to magnetize the tissue.

Magnetic Field

• A magnetic field exists around magnets or electric currents.
• It's characterized by detectable magnetic forces and magnetic poles.
• A magnetic field is a vector quantity with both a north and south pole.
• It exerts induction forces on ferromagnetic and paramagnetic substances.
• Dipolar magnets always have a north and south pole.

Classification of Magnets

• Magnets are categorized based on their magnetic properties' origin.
• Natural occurring magnets exist in nature.
• Permanent magnets retain their magnetism without external energy.
• Electromagnets produce magnetism through electrical currents.

Magnets Used in MRI

• MRI uses different types of magnets.
• Resistive magnets are electromagnets, the earliest MRI magnets.
• Superconducting magnets are more powerful electromagnets, requiring very low temperatures to maintain their superconductivity.
• Permanent magnets have a constant magnetic field, require no additional electricity or cooling, but have limitations in field strength.

MR Magnets

• Permanent Magnet: A material that has been magnetized and won't lose its magnetic field. Commonly made from an ALNICO alloy or rare-earth materials.
• Resistive Magnet: Consists of large copper or aluminum coils. Produces heat that requires water cooling and limits the maximum magnetic field strength.
• Superconducting Magnet: Special alloys (like Niobium-Titanium) in a copper matrix cooled to extremely low temperatures (4K) become superconductors. The coolant used is liquid helium, and Niobium-Tin and Magnesium diboride are emerging new alloys.

Resistive Magnets

• Resistive magnets are simple electromagnets used in MRI.
• They consist of coils of wire, and passing an electrical current through these coils creates a magnetic field.
• The electrical resistance in the wires generates heat, limiting the maximum magnetic field strength.
• These magnets' heat is typically conducted away by a cooling system.
• Direct current must be used to achieve a stable field.

Resistive Magnets Characteristics

• Field strength is up to 0.5 Tesla (T).
• Magnetic field inhomogeneity is between 10 and 50 parts per million (ppm).
• Power consumption ranges from 50 to 100 kilowatts (kW).
• Weight is about 4 tons.
• The field can be turned off quickly.
• Flux lines run horizontally.
• Modest fringe fields (~2 meters, 0.5 militesla).

Superconducting (Cryogenic) Magnets

• These are also electromagnets.
• Their coils are cooled to extremely low temperatures using liquid helium and liquid nitrogen (cryogens) to minimize electrical resistance.

Superconducting Magnet

• A superconducting magnet comprises many vacuum vessels acting as temperature shields.
• Superconducting wires can be several miles long.
• Coils are kept at 4.2 Kelvin by immersion in liquid helium.
• Liquid helium in MRI magnets averages 1700 liters.
• The Dewar is surrounded by liquid nitrogen (77.4K) to act as a buffer between room temperature and liquid helium.

Superconducting Magnet Characteristics

• Field strength range from 0.37 to 4 Tesla (up to 14T in research settings).
• Magnetic field inhomogeneity is between 0.1 and 5 ppm.
• They are expensive to purchase and operate, and are difficult to maintain.
• Flux lines are horizontal.
• Large fringe fields extend ~10 meters (0.5 milliTesla).
• Weight is around 10 tons.
• Power consumption is around 20 kilowatts (kW).

Superconducting Magnet Advantages and Disadvantages

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

Magnet Quench

• Loss of superconductivity in a magnet, often caused by a rapid boil-off of liquid helium.
• Patients and staff must be quickly evacuated when this occurs.
• Large quantities of liquid helium can cause unconsciousness due to oxygen displacement.
• Quenches can be planned or accidental.
• All superconducting magnets have a quench button to turn off the field quickly.

Permanent Magnets

• Made of naturally occurring ferrous materials (blocks or slabs).
• Maintain a constant magnetic field without additional electricity or cooling to low temperatures.
• Their magnetic field doesn't extend as far as resistive or superconducting magnets (smaller fringe field).

Fringe Field

• The portion of the magnetic field that extends beyond the magnet's boundaries.
• It's not used for imaging but can affect nearby equipment or personnel.

Permanent Magnets (Open Bore)

• A specific type of permanent magnet for MRI, featuring a hollow bore.
• Field strength typically ranges from 0.2 to 0.7 Tesla.

Permanent Magnets Advantages and Disadvantages

• Advantages: Low power consumption, low operating costs, small fringe field, no cryogen.
• Disadvantages: Limited field strength, very heavy weight, no quench possibility.

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
• Lower fringe field
• Reduced susceptibility and flow artifacts
• Lower energy deposition on tissues (SAR)
• Lower initial purchase price
• Lower operational cost

Disadvantages of Low-Field Scanners

• Lower signal-to-noise ratio
• Lower homogeneity
• Impaired detection of calcification, hemorrhage, or gadolinium enhancement.