Relaxation Mechanisms Flashcards
What is spin-lattice (T1) relaxation?
The process that is continually trying to establish the thermal equilibrium population distirbution, so that the net magnetisation lies along the longitudinal axis.
What is the cause of spin-spin (T2) relaxation?
The loss of phase coherence between individual spins due to energy loss through the interactions between the spins.
What is the equation for T1 relaxation?
M_z=M_0.[1-exp(-t/T1)]
What is the cause of T1 relaxation
Transfer of energy between the nucleus and its environment. This transfer occurs through energy exchange which is stimulated when a proton encounters a magnetic field which is fluctuating at the Larmor frequency (e.g. a proton or electron attached to a molecule that is rotating/tumbling at that frequency).
What is the equation for T2 relaxation?
M_xy=M_0.[exp(-t/T2)]
How does correlation time affect T1 relaxation?
T1 is quickest when tau_c=1/omega_0.
What type of molecules have short T1?
Water molecules transiently bound to proteins and free lipid molecules. Water molecules on their own are too quick at tumbling, and large macromolecules are too slow.
How does the correlation time affect T2 relaxation?
Longer correlation time, more time to interact with neighbours, faster T2 relaxation.
What types of molecules have long T2?
Free water - tumble fast, short tau_c, long T2
Lipids - Intermediate tumble, intermediate tau_c, intermediate T2.
How does T2 relate to T1?
Causes of T1 also cause T2, but T2 can occur without T1.
T1>T2.
What is the difference between T2 and T2*?
T2 is caused by a random process and so is reversible.
T2* occurs primarily due to inhomogeneities in the B_0 field. AS these are fixed in time and space T2* is reversible.
What can cause the B_0 inhomogeneities that affect T2*?
- Intrinsic defects in the B_0 field.
- Magnetic susceptibility effects from tissue or other materials in the field.
What is the equation that links T2 and T2*?
1/T2*=(1/T2)+gamma. (dB_0/2)
How is T1 measured experimentally?
S(T_D) = S(0)[1-exp(-T_D/T1)}
TD is either the delay time between a 90deg saturation pulse and the second 90deg read-out pulse or the inversion time between the 180deg inversion pulse and the read-out pulse.
Repeating the scan for multiple T_Ds allows the curve to be characterised, and by plotting ln[S(0)-S(T-D)] against T_D the gradient of the line will be 1/T1.
This is assuming heterogeneous tissue with a single compartment, otherwise, there could be multiexponential behaviour.
What is the minimum TR for an inversion recovery sequence?
5xT1 to allow 99.9% of the magnetisation to recover.
