1. Basic MRI I
Chapter 3 Notes

2. Terms to know Resonant Characteristic-- able to have resonance
Magnetized—makes a net magnetization vector in a magnetic field
Magnetization—process of forming a net magnetization vector
Resonance—capacity to absorb and re-radiate radiofrequency energy

3. Two ways to display MR signals
MR images
MR spectra

4. Magnetic nuclei Have magnetic “moment” Act like small magnets
Have an odd number of protons and neutrons
Have the quantum property of “spin”
Are sometimes called “spins”

5. Magnetic or non-Magnetic?
H-1
C-13
C-12
F-19
0-15
O-16
Na-23
N-15
P-31

6. RF signal strength
Most significant factor in determining image quality and the time it takes to acquire an image

7. Relative signal strength
Tissue concentration of the element
Isotopic abundance
Sensitivity of the specific nuclide

8. Tissue concentration
For hydrogen nuclei, or protons, this is the “proton density” or “PD”
For other nuclei, referred to as “spin density”

9. Isotopic abundance H-1 – 99.985% H-2 – 0.015% “deuterium”
H-3 -- <0.0011% “tritium”

10. Sensitivity Signal strength produced by an equal number of nuclei
H-1 is the strongest, considered 1.0 or 100%
See table 3 in your book.

11. Why proton imaging is so good
High tissue concentration
Most abundant magnetic isotope of hydrogen (H-1) is magnetic!

12. RF energy
RF pulses (transmitted)– usually 90 degrees or 180 degrees, but can be of any angle
RF signal (received)– received by coils that act as antennae
Stored in k-space
Fourier transform (FT) to make images
Received as echoes

13. Nuclear magnetic interactions
Nuclear alignment—only a few of every million nuclei align with the magnet, but this is enough to form a net magnetization vector
The rest are tumbling randomly, due to “thermal activity.”

14. Precession and resonance
The rate of precession is the same as the resonant frequency, called the Larmor Frequency

15. Excitation
Tip the net magnetization vector away from the z direction…any component in the x-y plane generates a signal that can be detected.

16. Relaxation
The net magnetization vector re-forms, lines up with the z direction, and the individual spins dephase.

17. Frequency is proportional to field strength
Freq = gyromagnetic ratio x field strength
Freq = “gamma” x B-naught
Freq = γHB0
Gyromagnetic ratio = γH = MHz/T
Field strength = B0