MRI Magnets: Types and Principles

Magnets in MRI

Permanent Magnets

• Ferromagnetic material exposed to an external magnetic field.
• Have a north and south pole.
• Magnetic field lines run from south to north.

Magnetic Field Strength

• Strength around the scanner in public areas: Generally 5 Gauss, possibly updated to 9 Gauss in many facilities.
• Medium to large hospitals with multiple machines typically have 1.5 Tesla and 3 Tesla scanners; facilities with one MRI usually have a 1.5 Tesla scanner.
• FDA criteria for significant risk investigations:
  • Infants (newborn to 1 month): Limited to 4 Tesla.
  • One month and older: Up to 8 Tesla.
• Field homogeneity is measured in parts per million (ppm).
  • Inhomogeneity of 1 ppm in a 1 Tesla magnet: 10,000 \times 0.000001 Gauss.
  • Inhomogeneity of 1 ppm results in a frequency difference of 42.57 Hz.

Types of Magnets for Clinical Imaging

• Permanent magnets
• Electromagnets
  • Solenoid magnets
  • Resistive magnets
  • Superconducting magnets
• Hybrid magnets

Permanent Magnets in Detail

• Made of permanently magnetized ferrous materials (ferromagnetic substances).
• Common materials: iron, cobalt, nickel.
• Most common alloy: aluminum, nickel, cobalt (alnico).
• Ceramic bricks with ferromagnetic properties can also be magnetized.
• Smaller fringe field, so they can be placed in public areas.
• Significantly heavy: up to 15,000 kg.
• Temperature-sensitive: Temperature fluctuations must be less than 1 Kelvin for optimal operation to maintain homogeneity.
• Open MRI system design: Plates above and below the patient, providing good all-around access also known as open bore systems.

Negatives

• Lower field strength (e.g., 1.2 Tesla) compared to typical hospital scanners (1.5T, 3T), which means lower Signal-to-Noise Ratio (SNR).

Positives

• Lower initial and operational costs.
• Accommodate larger patients and reduce claustrophobia.
• Open side to side but vertical space may still be close.
• Can be either permanent or superconducting solenoids.

Electromagnets

• Based on Michael Faraday's law of electromagnetic induction.
• States that an induced electromotive force in a closed loop equals the negative of the time rate of change of magnetic flux through the loop.
• Interaction between charge, motion, and magnetism: if two of these variables exist, the third will be created.

Current Through a Wire

• Passing current through a long straight wire creates a magnetic field around it.
• Direction of the magnetic field is determined by the right-hand thumb rule.
  • Fingers of the right hand curled around the wire indicate the direction of the magnetic field, and the thumb points in the direction of the current.
• If two wires have current in opposing directions, their magnetic fields cancel; if in the same direction, the fields are additive.
• Exploited by using many current-carrying wires to create larger magnetic fields.

Solenoid Electromagnets

• Spring-like electromagnets formed by wrapping a wire into loops.
• Loops of wire form a coil that strengthens the field.
• Efficiency is governed by the inherent resistance of the wire.
• Follows the right-hand thumb rule; direction can be horizontal or vertical.
• Horizontal field: Magnetic field lines run from head to foot.
• Vertical Field: magnetic field lines run from above and below.
• Lighter in weight than permanent magnets.

Superconducting Electromagnets

• Used to achieve and maintain a strong magnetic field without overheating.
• Superconducting electromagnets use coolants.
• More expensive to buy, but have lower operating costs.
• Whole-body superconducting systems have various field strengths.
• Two main solenoids: one at the head and one known as a bobbin.
• Process: First, current is passed through the main superconducting coil
  *Process is called ramping up.

Ramping Up

• Ramping up done after scanner delivered and fixed in place by service engineers.
• Temperature: Niobium-titanium alloy wire requires supercooling by cryogens to eliminate resistance, must maintain superconductivity and supercooled by cryogens to help eliminate resistance.

Cryogens

• Cryogens used: liquid helium or liquid nitrogen.
• Loops of wire are submerged in the cryogen.
• Liquid nitrogen is used to keep helium cold.
• Liquid helium is used to create superconductivity.
• Helium boils away to gas quickly at room temperature.
• Cryostat: stainless steel tank in the shape of a hollow cylinder with heat shield layers. The helium reservoir is isolated from the outer walls of the cryostat by an evacuated chamber.
• Refrigeration unit cools the entire structure.
• Features reduce heat transfer by radiation, convection, and conduction.
• Modern cryostats may have helium condensers that recycle boil off which save cryogen refills, reducing the need for refills.

Helium Safety Concerns

• Helium displaces oxygen, posing an anoxia risk.
• One liter of helium produces 748 liters of helium gas when it boils off.

Quenching

• Occurs when your magnet is quenched.
• Quenching is when the magnetic field rapidly stifles.
• Venting: MRI is designed to vent helium gas outside via a piping tube; however, malfunctions can cause helium to re-enter the room.
• Cryostat capacity varies by machine design.
• An average is about 15.
• Quenching is only done when life is in danger.
• Spontaneous quenches can occur.
• Quenching can be explosive.

High-Field Open MRI Systems

• Ideal for T1 contrast.
• Use superconducting solenoid magnets above and below the patient to create a vertical magnetic field.
• Vertical field is always associated with open MRIs.
• Open MRIs can be permanent magnets or superconducting solenoids.

Niche Magnets

• Specialty imaging concerns (e.g., extremity imaging in orthopedist offices).
• Lower quality due to lower SNR; trade-offs in imaging parameters are necessary.
• Improving SNR increases scan time and the likelihood of patient movement, reducing image quality.
• Superconducting magnets have a larger fringe field and horizontal static field, lower power.