Free induction decay time constant

The procedure was tested on simulated time domain MRS data where the model data consisted of metabolite peaks at 3.2, 3.0 and 2.0 ppm representing choline, creatine and IV-acetylaspartate (NAA) respectively, with corresponding values of Ak of 1.0, 1.0 and 3.0 units.89 White noise of specified standard deviation, crt, was then added. The Levenberg-Marquardt method requires suitable initial values for each of the nine parameters being fitted. The initial values of the three frequencies were taken as their known values. An exponentially decaying curve with a constant offset parameter was fitted, using a nonlinear least-squares fit, to the envelope of the free induction decay, Mv(t), in order to obtain an initial value for T and for the amplitudes, each of which was taken to be one-third of the amplitude of the envelope. The constant offset was added to account for the presence of the noise. 

Figure 11 shows diagrammatically (Muller and Bloom, 1960) the arrangement of echoes and pulses which occur in a general type of three-pulse spin-echo experiment. In this diagram a pulse of duration 0 to t0 rotates the magnetization by the angle nj2. This pulse is followed by the free induction decay with time constant T determined by both extrinsic and intrinsic relaxation forces. At a time iq (iq > 11 t0) a pulse... 

The application of various multi-pulse methods in NQR requires a clear theoretical understanding of physical processes first of all behind the simplest "basic" multi-pulse sequences which include such a popular sequence as multi-pulse spin locking—MW-4, and a sequence of identically spaced coherent radio frequency (RF) pulses with intervals between them less than the time constant of free induction decay (FID) —strong off-resonance comb— SQRC. ... 

The fact that the duration of transient signals after SSFP type sequences can exceed T (Tj is a time constant of the free induction decay) can be used to eliminate spurious signals. The SSS can replace conventional pulses in the multiple-pulse sequences with intervals between pulses longer than T2 while detecting explosives at frequencies higher than 3 MHz (RDX, HMX, tetryl), where the duration of... 

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