Summary of Last Week's Class and Today's Focus

• Have an idea of how the proton would behave in a static magnetic field if it were not aligned with the magnetic field. Always looking for lowest energy, the spins will line up with the field, in practice. Need to look at ways of disturbing the protons to make measurements.

• Introduce the rf magnetic field to ``tip'' the spins away from the static magnetic field.

• Will work on understanding resonance. Turns out that the precession frequency will be a key in all experiments.

• Like all physical systems the spins are always looking for ways to get into the lowest energy state. Study the ``relaxation'' processes that take the spins back to the lowest energy state such that they are all aligned with the static magnetic field.

• Will find that there are reasons to consider more than one relaxation rate. Will consider spins re-aligning with the main magnetic field. Will also consider spins tipped away from the static field de-phasing with respect to one another. Dephasing of the spins reduces the measured MR signal.