Rf pulse sequences

Fig. 2.9.7 Hahn spin-echo rf pulse sequence combined with bipolar magnetic field gradient pulses for hydrodynamic-dispersion mapping experiments. The lower left box indicates field-gradient pulses for the attenuation of spin coherences by incoherent displacements while phase shifts due to coherent displacements on the time scale of the experiment are compensated. The box on the right-hand side represents the usual gradient pulses for ordinary two-dimensional imaging. The latter is equivalent to the sequence shown in Figure 2.9.2(a).

Fig. 5.5.8 The volume selective spectroscopy rf pulse sequence used to acquire the data shown in Figures 5.5.9-5.5.11. Magnetic field gradients applied alongthex, y and zdirections enable localized spectra to be recorded from...

One thus arrives at the concept of pulser sequences (rather than RF pulse sequence) in which the programmable pulser generates a sequence of synchronized events and controls a number of distinct devices. Analogously, the term RF phase-cycling is no longer appropriate (in Stelar terminology, it is replaced by X-device cycling). 

Figure 5. The rf pulse sequences for determining SL cross polarization time constant Ten, C-13 T,p and C-13 Tj under proton decoupling. Each experiment starts with a SL cross polarization.

The parameter to be measured in spin-echo diffusion experiments is the amplitude A of the echo. Its magnitude depends 3) on the spin-spin relaxation time T2 and on the diffusion coefficient D. For an SGSE experiment employing a 90°-t-180° rf pulse sequence in the presence of a steady field gradient of magnitude G0 4),... 

Cosgrove and Warren 33) used the PGSE method of Packer, et al. 9). with the Meiboom-Gill rf pulse sequence 7> to investigate concentrated polystyrene solutions. 

The value of n, set by the experimenter, determines how long the system evolves under Hd. The terms in Hd in Equation (6.1.18) are simply those involving a sum over two-body products, (Equation (6.1.7)). These contain internuclear distances r,y, an average over the orientational factors (3 cos 0,y - 1), and the two-body spin operators (I, Ij - 31., ), as manipulated by rf-pulse sequences. Now and similarly, the two body terms... 

The main application at low frequencies is, of course, NMR. In FT-NMR, a magnetized sample is irradiated by radio frequency (RF) radiation to manipulate the bulk magnetization. The basic signal is the free induction decay, but hundreds of sophisticated RF pulse sequences have been invented for specific purposes, including medical imaging. 

Figure 10 The spin-echo RF pulse sequence (A) and its result on the amplitude of the NMR signal (B) when applied to com samples.

Figure 3 Three RF pulse sequences used in combination with MAS. The 90° pulses are black while the 180° pulses are gray (A) SP-MAS (B) PASS (C) the prototype PHORMAT. The various timing parameters are explained in the text. (Reprinted with permission from Hu JZ and Wind RA (2002) The evaluation of different MAS techniques at low spinning rates in aqueous samples and in the presence of magnetic susceptibility gradients. Journal of Magnetic Resonance 159 92-100 Elsevier)...

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