Longitudinal relaxation time

·         Also called spin lattice relaxation

·         Release of energy by the excited spin/proton to the local tissue

·         Basically the longitudinal re-growth of M_Z (longitudinal magnetization – the component of the proton’s magnetic moment parallel to the applied external magnetic field – at maximum equilibrium = M₀, with amplitude determined by excess number of protons in the lower energy state)

·         The T₁ constant of a tissue is the time needed for protons/spins of a specific tissue to return to 63% of the M₀ following a 90^O RF pulse

·         After a time of 5x T₁the sample is considered to be back to full M₀ (back to equilibrium)

·         Because only M_XY (transverse magnetization) can be measured, repeated 90^O pulses with different delay times data points that lie only the T1 recovery curve can be determined

·         T₁ Depends on the characteristics of the spin/proton interaction with the lattice (molecular arrangement or structure) and the number of protons available for the transition

	T1
General time	Long
Large; slowly vibrating; bound molecules	Long
Intermediatly sized, vibrating and bound molecules (Viscous materials – lipids/proteins/fats)	Short
Small; fast vibrating; free molecules	Long
Influence of magnetic field strength (increase)	Increase (Related to dependence of Larmor frequency on mag. Field strength and degree of overlap with molecular vibration spectrum (decreases with increased mag. field)
Paramagnetic blood degredation products; gadolinium; ferromagnetic materials	In situations when macromolecule binds free water in a hydration layer –  ++ Shorter