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Understanding MRI Machine Components and Physics

May 5, 2025

MRI Physics Lecture Notes

Introduction

  • Recap of the introductory talk on MRI principles.
  • Focus on understanding components in MRI physics.

MRI Machine Components

  • MRI machine consists of multiple layers, each representing a different magnet.

Main Coil

  • Function: Create the main magnetic field (Bâ‚€) along the longitudinal (Z) axis.
  • Principle: Ampere's Law - Current through a wire generates a magnetic field.
  • Factors:
    • Number of coils.
    • Amount of current.
  • Superconductivity
    • Used to generate sufficient current for a strong magnetic field.
    • Material: Niobium-titanium alloys.
    • Requires cooling to below 4 degrees Kelvin with liquid helium.
    • Quenching: Release of gaseous helium if temperature rises above 4K, leading to loss of superconductivity.

Shims

  • Purpose: Make the main magnetic field homogeneous.
  • Types:
    • Passive Shims: Ferromagnetic materials that passively adjust the field.
    • Active Shims: Coils with their own power supply, actively adjust the field.

Gradient Coils

  • Function: Apply a gradient along the magnetic field.
  • Orientation: Perpendicular in X, Y, and Z planes.
  • Purpose: Spatial localization of MRI signals.
  • Mechanism:
    • Superimpose magnetic fields to create gradients.
    • Change magnetic field strength, not direction.

Radio Frequency Coils

  • Function: Generate a magnetic field perpendicular to the main field.
  • Purpose: Isolate specific hydrogen protons by matching their precessional frequency.
  • Mechanism:
    • Push hydrogen protons into phase (like children on swings).
    • Flip net magnetization vector to the transverse plane.

Summary

  • Main Coil: Generates primary magnetic field.
  • Shims: Ensure homogeneity of the magnetic field.
  • Gradient Coils: Manipulate field strength for spatial localization.
  • Radio Frequency Coils: Select specific hydrogen protons and measure signals.

Next Steps

  • Explore nuclear magnetic resonance.
  • Understand its role in generating MRI signals.
  • For further study, refer to curated question banks linked in the provided resources.

Full transcript