1. MRI: From Protons to Pixels
By Wesley Eastridge, adapted from and with illustrations from The Basics of MRI by Joseph P. Hornak, Ph.D.

2. What MRI Is
Magnetic Resonance Imaging uses the magnetic properties of protons to make an image of the body.
No ionizing radiation
Demonstrates physiologic properties
Relatively expensive and slow

3. Q: How do we get an image of the body from inside a big magnet
Q: How do we get an image of the body from inside a big magnet? A: With radio waves!

4. How MRI Works... Protons align in a magnetic field like tiny magnets.
They precess with a frequency proportional to the magnetic field strength.
They resonate when radio waves of their particular frequency are applied.
Then they give off their own radio waves at their particular frequency

5. ...How MRI Works
Computer analysis of their radio waves uses the Fourier Transform to determine their location and generate the image.
Special effects include T1-weighted images, T2- weighted images and gadolinium contrast.

6. Q: Did you hear about the two hydrogen atoms that walked into a bar?

7. Protons have “spin” and therefore magnetic moments

8. Precession

9. Protons align and precess in a magnetic field

10. Radio waves are alternating magnetic fields perpendicular to alternating electric fields

11. Resonance
Repeated pushing has amplified effect when timed with the object's intrinsic frequency.

12. Protons resonate at their characteristic frequencies
Dependent on the strength of magnetic field

13. Once synchronized they emit their own radio waves
Which we can detect with an antenna

14. We localize them by making their radio wave characteristics dependent on location
If all points in the body experience the same magnetic field they would emit only one frequency

15. Our goal: Protons Precessing Proportionally to Position

16. Localization 1st dimension
The main magnetic field strength varies along the z-axis so only one plane of the body experiences the strength that creates a given precession frequency

17. Phase Shift

18. Localization 2nd Dimension
A temporary orthogonal magnetic gradient in the x-axis then makes protons precess a little faster or slower than the others while the gradient is on, getting out of phase with each other.

19. Localization 2nd Dimension

20. Localization 2nd Dimension

21. Localization 2nd Dimension

22. Localization 3rd Dimension
An orthogonal magnetic field gradient in the y- direction is then left on while we detect the protons' signals, making their frequency dependent on location along the y-axis

23. Localization
Now the radio waves emitted by the protons are dependent on their position. Each pulse sequence detects protons in a plane along the z-axis, whose frequency represents their location along the y-axis and whose phase shift represents their location along the x-axis!

24. Fourier Transform

25. Fourier Transform
For example in in real time analysis of sound waves:

26. Fourier Transform
Bottom line: FT takes a wave function and creates a function of how much of each frequency made up that wave.
Since proton frequency varies along the magnetic gradient, the FT is a function of distance along that axis.

27. Decoding the waveform Requires two applications of Fourier transform
See for more

28. Recap of How MRI Works
Protons have charge and spin and magnetic moments
They align with a magnetic field and precess
They resonate when an orthogonal magnetic field oscillates at the same frequency of their precession
Once synchronized with the oscillating field they emit their own oscillating fields (radio waves)
We localize them by making their frequencies and phases dependent on their position

29. Beyond the Basics: Physiology
More than just displaying proton density, MRI can detect physiological properties of the body such as blood flow or metabolic activity of tissues.

30. Physiology
T1-weighted vs T2-weighted images

31. Spin-echo sequence
Produces a T1 or T2-weighted image, depending on timing

32. T1 vs T2 weighted images
T1 on the left, T2 on the right

33. Physiology Metabolic activity Gadolinium
FLAIR (fluid attenuated inversion recovery)
Etc...
Gadolinium, gadolinium, t2
FLAIR, axial gradient