MRI: From Protons to Pixels By Wesley Eastridge, adapted from and with illustrations from The Basics of MRI by Joseph P. Hornak, Ph.D. http://www.cis.rit.edu/htbooks/mri/

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

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!

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

...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.

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

Protons have “spin” and therefore magnetic moments

Precession

Protons align and precess in a magnetic field

Radio waves are alternating magnetic fields perpendicular to alternating electric fields

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

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

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

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

Our goal: Protons Precessing Proportionally to Position

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

Phase Shift

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.

Localization 2nd Dimension

Localization 2nd Dimension

Localization 2nd Dimension

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

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!

Fourier Transform

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

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.

Decoding the waveform Requires two applications of Fourier transform See http://www.revisemri.com/tutorials/what_is_k_space/ for more

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

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.

Physiology T1-weighted vs T2-weighted images

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

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

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