Magnetic Resonance Imaging Overview

Magnetic Resonance Imaging (MRI)

• MRI machines use superconducting magnets and coils of wire to generate a strong magnetic field.
• Maintaining this field requires a continuous supply of liquid helium at -269.1°C.
• A typical MRI scanner uses 1,700 liters of liquid helium.
• Periodic topping-up is necessary.
• Recently, small refrigerators are proposed for helium recondensation, achieving a closed refrigeration system.

Cooling of Magnets

• MRI relies on superconductivity to minimize wire resistance and maintain a consistent magnetic field.
• A cryostat houses the magnet assembly with a vacuum case, thermal shield, and helium vessel.
• Cryostats commonly have a temperature range of 40-50 K.
• The liquid helium fill level for superconductivity is typically 1500-2000 liters.
• Lower volumes are sometimes tolerable based on specific coil designs.
• Ramping processes may have different minimum volumes.

Laser Cooling System (LCS)

• LCS is a recent technology used to cool MRI magnets.
• Laser cooling reduces atomic and molecular sample temperatures close to absolute zero.
• The Doppler effect is a core principle in laser cooling.
• Doppler effect results in a change in wavelength/frequency due to an observer/source relative motion.
• This effect is utilized in decreasing atom speeds in the cooling process.

LCS for MRI Scanners

• The system uses temperature sensors on the superconducting magnet.
• Sensors record and transmit data to a controller (system level).
• The controller adjusts laser wavelengths to maintain optimal operating temperature on the magnet.

Shape of MRI Machines

• Two main types:
  • Closed MRI: Cylindrical designs, creating a confined environment. High field strength (1.5T to 3T), high-quality images, faster imaging, advanced applications, but may increase claustrophobia and acoustic noise.
  • Open MRI: Non-tubular designs, offer more open space. Lower field strength (0.2T to 0.4T), slower image quality, less claustrophobia and acoustic noise. This type of design may be suitable for patients with claustrophobia.

Comparison of Closed and Open MRI