MRI Magnets: Types and Principles

Magnets in MRI

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.

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.

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

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.

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.

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 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 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.

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.

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.

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.