- Spin Relaxation
- The return of a population distribution to its equilibrium value.
Photons are the carriers of magnetic and electric fields.
Motions resulting in time varying magnetic fields cause transitions and hence spin relaxation.
Time varying fields at Larmor frequency cause transitions between the spin states and hence a change in MZ.
1/T1 is proportional to the density of molecular motions at the Larmor frequency.
T1 will vary as a function of temperature.
For example, at a constant no, T1 (280 K ) < T1 (340 K).
The temperature of the human body does not vary by enough to cause a significant influence on T1.
The viscosity does however vary significantly from tissue to tissue and influences T1.
Fluctuating fields perturb the energy levels and cause the transverse magnetization to dephase.
- Chemical Shift (d)
- A shift in te resonance frequency of a nucleus due to its surroundings.
d = (n - nREF) x106 / nREF A portion of the applied magnetic field does not make it into the nucleus.
This portion is said to be screeded out.
B = Bo (1-s)
s = Screening Constant
Consider methanol - each hydrogen type experiences a different field.