·
Also called spin-spin
relaxation
·
A 90O RF pulse
causes phase coherence of the individual spinning protons and generates the
maximum transverse magnetization (MXY) for a given sample
·
As MXY rotates at a
given frequency (Larmor frequency) – the
receiver antenna is induced (by magnetic induction – changing magnetic field
creating electronic current) to produce a sinusoidal electronic signal - The FREE INDUCTION DECAY (FID) signal
·
FID decay is as a result of the
loss of phase coherence of the individual spins
·
Inhomogeneities intrinsic to
and extrinsic (from the magnet or ferromagnetic and paramagnetic materials –
casue faster dephasing) to the sample structure cause a spin-spin interaction
and the individual spins precess at different frequencies due to slight changes
in the local magnetic field strength
·
The elapsed time between the
peak MXY and 37% of the peak is the T2* constant (when both
intrinsic and extrinsic inhomogeneities are taken into account)
·
T2* mechanisms are determined
by the molecular structure of the sample
|
|
T2
|
|
General time
|
Short
(5-10x)
|
|
Large; slowly vibrating;
bound molecules
|
Short
(++
intrinsic inhomogeneities)
|
|
Intermediatly sized,
vibrating and bound molecules (Viscous materials – lipids/proteins/fats)
|
Short
(also
extrinxic inhomogeneities speed up dephasing)
|
|
Small; fast vibrating;
free molecules
|
Long
(decreased
intrinsic inhomogeneities)
|
|
Influence of magnetic
field strength (increase)
|
Insignificant
|
|
Paramagnetic blood
degredation products; gadolinium; ferromagnetic materials
|
Shortened
T*2
(Disruption
of local magnetic field environment)
|
