PS267 Lecture Notes - Lecture 5: Voxel, Supercooling, Grey Matter
How does MRI work?
â—Ź The big magnet
â—‹ Very strong
â—‹ 1 tesla (T) = 10 000 gauss
○ 4 tesla = 4 x 10 000, 0.5 = 80 000x earth’s magnetic field
○ Continuously on… main field B₀
â—‹ Coils are supercooled, allows us to generate relatively little electrical input to get
large electrical field
â—Ź Necessary equipment
â—‹ Magnet, gradient coil, RF (radiofrequency) coil
â—Ź Step 1: put subject in big magnet
â—‹ Protons (hydrogen atoms) have spins (like tops). They have an orientation and a
frequency.
â—‹ When you put material (like your subject) in an MRI scanner, some of the protons
become oriented with the magnetic field → bulk of MRI is imaging of hydrogen…
MRI measures protons (ultimately imaging water)
â—Ź Step 2: apply radio waves
â—‹ When you apply radio waves (RF pulse) at the appropriate frequency, you can
change the orientation of the spins as the protons absorb energy
â—‹ After you turn off the radio waves, as the protons return to their original
orientations, they emit energy in the form of radio waves
â—Ź Step 3: measure radio waves
â—‹ T1 measures how quickly the protons realign with the main magnetic field
(myelin? goes back fastest, then grey matter, then CSF)
â—‹ T2 measures how quickly the protons give off energy as they recover to
equilibrium (CSF takes long to get back because has lots of water thus lots of
hydrogen)
â—Ź Step 4: use gradients to encode space
○ Lower magnetic field; lower frequencies… higher magnetic field; higher
frequencies
â—‹ Remember that radio waves have to be the right frequency to excite protons (at
4T = 170.3MHz) → no other part of person’s body will care
â—‹ The frequency is proportional to the strength of the magnetic field
â—‹ If we create gradients of magic fields, different frequencies will affect protons in
different parts of space
â—Ź Step 5: convert frequencies to brain space
â—‹ K-space contains info about frequencies in image, we want to see brains, not
frequencies
â—Ź What is a Voxel?
â—‹ Like a pixel
○ Google def… (in computer-based modeling or graphic simulation) each of an
array of elements of volume that constitute a notional three-dimensional space,
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
1 tesla (t) = 10 000 gauss. 4 tesla = 4 x 10 000, 0. 5 = 80 000x earth"s magnetic field. Continuously on main field b large electrical field. Coils are supercooled, allows us to generate relatively little electrical input to get. Step 1: put subject in big magnet. Protons (hydrogen atoms) have spins (like tops). When you put material (like your subject) in an mri scanner, some of the protons become oriented with the magnetic field bulk of mri is imaging of hydrogen . When you apply radio waves (rf pulse) at the appropriate frequency, you can change the orientation of the spins as the protons absorb energy. After you turn off the radio waves, as the protons return to their original orientations, they emit energy in the form of radio waves. T1 measures how quickly the protons realign with the main magnetic field (myelin? goes back fastest, then grey matter, then csf)
