Magnetic Resonance Imaging (MRI) Quiz

Magnetic Resonance Imaging (MRI)

• MRI machines use a super-conducting magnet and coils to generate a strong magnetic field.
• A continuous supply of liquid helium at -269.1°C is needed to maintain the superconductivity.
• A typical MRI scanner uses 1,700 liters of liquid helium, which needs to be topped up periodically.
• Recent advancements include small refrigerators for recondensing evaporated helium, creating a closed refrigeration system.

Cooling of Magnets

• MRI machines work by generating a very large magnetic field using a super-conducting magnet with many coils.
• Superconductivity involves reducing wire resistance to almost zero by immersing the wires in liquid helium.
• The process requires careful control of the liquid helium.

MRI Machine Outline

• Cooling of Magnets
• LCS (Laser Cooling System)
• Shape of MRI Machine (Closed vs. Open)
• Comparison Between MRI Machines (Closed vs. Open)

Laser Cooling System (LCS)

• LCS is a recent technology used to cool MRI magnets.
• The temperature control in a laser cooling system determines its lifetime, performance, and safety.
• Laser cooling uses atomic and molecular samples cooled to nearly absolute zero through interaction with laser fields.
• The main principle is the Doppler effect, causing a change in wavelength and frequency due to movement of the observer relative to the source.
• Laser frequency is tuned just below an electronic transition in the atom, influencing photon absorption.
• Stationary atoms don't absorb photons. Moving atoms absorb more photons when they approach the photons source, which reduces the atom's speed and energy.

Laser Cooling System (LCS): Block Diagram

• A diagram shows a laser beam, superconducting magnet, temperature sensor, control signal for laser, temperature value, MRI, and controller.

MRI Machine Shape

• MRI machines come in closed and open designs.
• Closed MRI machines have a long tunnel-like bore, while open MRIs have an open structure.

Comparison of MRI Machines

• Closed MRI:
  • High field strength (1.5T–3T)
  • High image quality
  • Fast imaging
  • Advanced applications
  • Higher patient anxiety
  • Claustrophobic patient problems
  • High acoustic noise levels
• Open MRI:
  • Lower field strength (0.2T–0.4T)
  • Lower image quality
  • Slower imaging times
  • Limited applications
  • Less patient anxiety
  • Easier for claustrophobic patients
  • Lower acoustic noise levels

MRI Notes

• Magnet ramp-up is often done with captured boil-off helium to minimize helium loss and cryogenic margin.

• Normal operating conditions (NOC) may include extra heat loads from gradient heating.

• Ramp-down, shipping ('ride-through'), cooldown, and safe ramp-up are part of operational procedures.

• Cryocooler technology, like dual-stage cryocoolers, are constantly improving to re-liquefy escaping helium gas.

• A proposed system uses temperature sensors on the four sides of the superconducting magnet to predict and control the cooling of the magnet..