Being composed of individual nuclear spins, a transverse (in the x y plane) macroscopic magnetization Mxv(coherence)starts precessing about the axis with the Larmor frequency (in lab coordinate system)under the influence of the static Bo field, e.g., after a 90 pulse M (t=M cos(ot)+ M sin(ot) [2-12] thus inducing a voltage / current in the receiver coil (which is of course fixed in the probehead in a transverse orientation the Fid(free induction decay) typical H FID of a complex compound(cyclic hexapeptide) t(sec) According to eq. [2-12], the FID can be described as sine or cosine function, depending of its phase Relaxation The excited state of coherence is driven back to BOLTZMANN equilibrium by two mechanisms 1)spin-spin relaxation(transverse relaxation) dMx/dt=-Mxv/T2 [2-13] corresponds to a loss of phase coherence magnetixation is spread uniformly across the x y plane decay of net transverse magnetization/FID(entropic effect) due to the bo field being not perfectly uniform for all spins( disturbance by the presence of other spins), this inherent T2 relaxation is increased by experimental inhomogeneities(bad shim! ) T2* 2)spin-lattice relaxation(longitudinal relaxation11 Being composed of individual nuclear spins, a transverse (in the x,y plane) macroscopic magnetization Mx,y (coherence) starts precessing about the z axis with the Larmor frequency (in the lab coordinate system) under the influence of the static B0 field, e.g., after a 90ºx pulse: M -y (t) = M-y cos(wt) + Mx sin(wt) [2-12] thus inducing a voltage / current in the receiver coil (which is of course fixed in the probehead in a transverse orientation): the FID (free induction decay) typical 1H FID of a complex compound (cyclic hexapeptide) According to eq. [2-12], the FID can be described as sine or cosine function, depending of its phase. Relaxation The excited state of coherence is driven back to BOLTZMANN equilibrium by two mechanisms: 1) spin-spin relaxation (transverse relaxation) dMx,y / dt = -Mx,y / T2 [2-13] corresponds to a loss of phase coherence Þ magnetixation is spread uniformly across the x,y plane: decay of net transverse magnetization / FID (entropic effect) due to the B0 field being not perfectly uniform for all spins (disturbance by the presence of other spins); this inherent T2 relaxation is increased by experimental inhomogeneities (bad shim!): T2* 2) spin-lattice relaxation (longitudinal relaxation)