How MRI Really Works

Richard Watts, Yale University

One Question, Many Answers

I’m a physicist, which means that I don’t get invited to cocktail parties very much. On the rare occasions when I have, explaining that I am a medical physicist who works with MRI scanners usually elicits a sudden interest in anything else, especially something on the other side of the room. However, very, very occasionally my fellow partygoer cannot think quickly enough how to kill the conversation and instead feels obliged to show an interest and asks a question such as “How does MRI work?” The unfortunate problem with this question is that I have yet to come across a simple and yet even moderately satisfying explanation, so I resort to mumbling things about magnetic fields, radio waves and hydrogen nuclei.

The answer to the question “How does MRI work?” depends very much on what the person asking the question wants to know. Simplistic explanations may not be satisfying to someone who wants to really understand both the physics and mathematics of how images are acquired. Fully rigorous explanations are likely to be accessible only to those starting with a strong background in physics, chemistry and mathematics.

The concept behind this document is to provide several explanations at increasingly sophisticated levels. At the early levels, we use analogies and simplifications. Later levels increase the level of detail.

Contents:

• The Cocktail Party Answer
• The Coffee Room Answer
  • Precession
  • Excitation
  • Imaging
  • A Nobel-Prize Winning Idea
• Contrast in MRI
  • Spin Physics
  • T2 decay
  • T2-weighted Images
  • T1 Recovery
  • T1-weighted Images
  • Basic MRI Contrasts: Proton Density, T1 and T2-weighted Images
  • TE and TR: What is Short and What is Long?
  • What’s Bright and What’s Dark?
• Imaging in MRI
  • Slice Selective Excitation
  • Frequency Encoding
  • Pulse Sequences
  • Components of an MRI Scanner
• Echo, echo, echo…
  • Gradient Echoes and T2* Decay
  • The Discovery of the Spin Echo
  • Flip Angles
  • Why do we get a Spin Echo?
  • Spin Echoes vs. Gradient Echoes, T2 vs. T2*
• Entering the Third Dimension
  • Phase Encoding
• k-Space without Math
  • Another View on What we Measure with MRI
  • MRI Pulse Sequences
  • A Realistic Pulse Sequence – Gradient Echo
  • Slice Selective Excitation and Slice Rewinder
  • Phase Encoding
  • Frequency Rewinder and Readout
  • Spin Echoes
• k-Space with Math
• The Hitchhiker’s Guide to k-Space
• Animated Pulse Sequences
• FAQ
  • Why all the noise in MRI?
  • Why do we need such a strong magnetic field?
  • Why are MRI scanners so expensive?
• Acknowledgements