Magnetic Resonance Imaging Overview

Magnetic Resonance Imaging (MRI)

• MRI machines use super-conducting magnets and coils of wire to create a strong magnetic field.
• Superconductivity involves reducing the resistance in the wires to nearly zero.
• The wires are continuously bathed in liquid helium at -269.1°C.
• A typical MRI scanner uses 1,700 liters of liquid helium, needing periodic refills.
• Recent designs use small refrigerators for helium recondensation, creating a closed system.

Cooling of Magnets

• MRI magnets require a continuous supply of liquid helium for superconductivity.
• The cryostat (the machine holding the magnet) has an outer vacuum case with a thermal shield.
• Typical cryocooler setup is shown, with a vertical orientation to fit the cryocooler sleeve.
• The liquid helium fill level is kept between 1500-2000 liters.
• Temperature gradients and coil design influence the optimal fill volume.
• Any advanced cryogenic approach needs to optimize energy saving through pre-cooling using liquid nitrogen or a reusable helium facility.

Laser Cooling System (LCS)

• LCS is a modern technique to cool MRI magnets.
• The temperature of a laser system impacts its lifetime, performance, and safety.
• Atomic and molecular samples are cooled near absolute zero through laser interaction.
• The Doppler effect is fundamental. Doppler shift = change in wavelength/frequency due to relative movement between source and observer.
• By tuning the light frequency below the electronic transition in the atom, the atom absorbs more photons when moving towards the light source.
• As the atom moves and the laser shifts, the atom absorbs more light.
• This reduces atom speed and subsequently the kinetic energy, achieving cooling.

MRI Machine Shape

• MRI machines can be closed or open.
• Closed MRI machines have higher fields (1.5T or 3T), superior image quality, faster imaging, advanced applications, but higher patient anxiety for claustrophobic individuals and higher acoustic noise levels.
• Open MRI machines operate with lower fields (0.2T to 0.4T), lower image quality, slower imaging, fewer application options, less patient anxiety, and lower acoustic noise levels.

Comparison Between MRI Machines

• Closed MRI
  • High field strength (1.5T - 3T)
  • High-quality images
  • Fast imaging
  • Advanced applications
  • Increased patient anxiety, especially for claustrophobic patients
  • High acoustic noise levels
• Open MRI
  • Lower field strength (0.2T - 0.4T)
  • Lower image quality
  • Slower imaging
  • Limited applications
  • Lower patient anxiety
  • Lower acoustic noise levels

LCS Operation

• Four temperature sensors are placed on the four sides of the superconducting magnet to predict the temperature level.
• The control system transmits data to a controller.
• The controller uses the predicted model to provide a corresponding laser wavelength for optimal cooling.

Explore the fascinating world of Magnetic Resonance Imaging (MRI) technology. This quiz covers the principles of superconductivity, the role of liquid helium in MRI machines, and advanced cooling techniques for MRI magnets. Test your knowledge on the workings of MRI machines and their components.