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PulseAndFID< Bloch equations | Index | Spin echoes > One pulse and the Free Induction Decay We saw that the application of a radio frequency field B1 in the plane orthogonal to a stronger static field B0 determines a nutation of the nuclear spin, from its equilibrium direction, parallel to B0 . The motion which we call nutation was described as a precession around B1 in the rotating frame. In the laboratory frame this corresponds to precession around B0 in a cone with time dependent aperture. During this motion the tip of the spin arrow describes spirals on the surface of the sphere, which retain their axis along the direction of B0 . By switching off the radio frequency after an appropriate time we can turn the spin into any desired direction. We call pulse this train of radiofrequency. The simplest pulse to consider is the so-called π/2 pulse (where the name stands for the precession angle in the rotating frame), upon which the spin is turned into the x-y plane. The free spin evolution after a π/2 pulse is the precession around the static field B0 which gives rise to the Free Induction Decay (FID), i.e. the Faraday induction signal at frequency {$\omega_0=\gamma B_0$} in a receiver coil with axis lying in the x-y plane. This motion may be described as rest in the rotating frame. The term decay is justified by the fact that in practice the amplitude of the induction signal is reduced as time passes since the end of the pulse. This is due to a) relaxation of the nuclear magnetization towards equilibrium, and the characteristic time for this process is termed T1 ; b) dephasing of different components in the free induction signal, and its characteristic time is called T2 . The simplest possible cause of a T2 process is field inhomogeneity, causing different nuclear spins to precess at slightly different frequencies. In order to distinguish this, as well as other static, reversible causes of dephasing, from the dephasing originated by time dependent, irreversible processes, the time constant of the former is often referred to as T2* . If we consider an ensemble of nuclear spin subject to an inhomogeneous static field, their motion may be described in the rotating frame at the average Larmor frequency ω0 as a much slower precession of different isochromats, distributed evenly among positive and negative frequencies Δω = ω0 - ω, i.e. a fanning out of components around the initial roating frame direction, say y' . < Bloch equations | Index | Spin echoes > |