Magnetic Resonance Flashcards
Nuclear Spin
absence of external magnetic field - spins randomly oriented
apply a magnetic field - spins align along direction of field, generate bulk nuclear magnetisation Mz
Longitudinal Relaxation
thermal equilibrium with external magnetic field
NMR Precession
net magnetisation oriented along direction of magnetic field
if not parallel then feel torque
magnetisation precesses
Rotating Frame: On Resonance
reference frame rotating at same frequency as magnetisation vector
does not rotate
Rotating Frame: Off Resonance
all spins do not have same Larmor frequency
cannot generate a rotating frame
precesses at difference between rotating frame frequency and Larmor frequency
offset frequency
Signal Excitation: On Resonance
rotate magnetisation into transverse plane
Signal Excitation: Off Resonance
offset field and rf field - effective field
Phase and Coherence
precession arise from phase and coherence
(Mxycos¥) x component
(Mxysin¥) y component
when added, signals with different phases will cancel - dephasing
Transverse NMR Relaxation (Tz)
irreversible dephasing
liquids - weak spin interaction T2 = T1
solids - strong spin interaction T2
Effective Transverse NMR Relaxation (Tz*)
Mxy decay due to reversible dephasing
experience different local magnetic fields
NMR Signal Detection
oscillating voltage/current = signal
detect with a coil - magnetic induction
free induction decay
Signal Amplitude
proportional to:
- amplitude of precessing magnetisation
- precession frequency
Receptivity
how easy it is to detect the signal
Zeeman Interaction
Ha external field and nuclear spin
Chemical Shift
Hcs external field and spin
electrons give rise to small induced field
