MRI Notes

1. Apply B0 field (1-3T)
2. Spins line up in the direction of B0 (lab frame Z-axis)
3. Apply (90 degree) RF pulse at resonance frequency
4. RF energy is absorbed by the proton and it precessess at 90 degrees from the B0 field. The precessing frequency (time to go around (360 degrees)) is the same in any orientation.
This precession frequency depends on the particular atomic nuclie and the applied field B0.
This is also known as resonance frequency.
The Mag field component of the RF field is perpendicular to the B0 field.
How do you know when to stop the RF signal?
Apply the RF signal for the right amount of time (until the precession is 90 deg).
5. When the RF is turned OFF, the proton goes back to the Z axis and re-aligns with the B0 field.
6. How long does it take to go back to Z axis?
depends on 2 factors - T1 and T2. These are called relaxation time constants.
T1: is the time taken to re-align with B0 field.
T2: is the time until which the spins of neighboring atoms are in sync. After this spin they go out of phase (dephase time).