Refocusing pulse

These experiments yield T2 which, in the case of fast exchange, gives the ratio (Aoi) /k. However, since the experiments themselves have an implicit timescale, absolute rates can be obtained in favourable circumstances. For the CPMG experiment, the timescale is the repetition time of the refocusing pulse for the Tjp experiment, it is the rate of precession around the effective RF field. If this timescale is fast witli respect to the exchange rate, then the experiment effectively measures T2 in the absence of exchange. If the timescale is slow, the apparent T2 contains the effects of exchange. Therefore, the apparent T2 shows a dispersion as the... 

Figure 2.2 Effect of 180 pulse on phase imperfections resulting from magnetic field inhomogeneities. Spin-echo generated by 180 refocusing pulse removes the effects of magnetic field inhomogeneities.

Describe the effects of a 180° refocusing pulse in each of the following situations ... 

The shape of any rf pulse can be chosen in such a way that the excitation profile is a rectangular slice. In the light of experimental restrictions, which often require pulses as short as possible, the slice shape will never be perfect. For instance, the commonly used 900 pulse is still acceptable, while a 1800 pulse produces a good profile only if it is used as a refocusing pulse. Sometimes pulses of even smaller flip angles are used which provide a better slice selection (for a discussion of imaging with small flip angles, see Section 1.7). 

CPMG pulse sequence Carr-Purcell-Meiboom-Gill pulse sequence. A pulse sequence used for removing broad signals from a spectrum by multiple defocusing and refocusing pulses. 

Fig. 7 A chemical shift imaging pulse sequence. The MR signal is spatially encoded prior to acquiring the spectral signal in the absence of any applied magnetic field gradients. The shaded gradient pulses applied along z either side of the n refocusing pulse are homospoil gradients.

In contrast to the 13C shifts, the 13C resonance frequencies are not indicative of residue type, which makes the assignment more demanding in comparison to the 13C , 15N, Hn correlation map. Nevertheless, the HN(CA)CO-TROSY scheme equipped with the selective 13C refocusing pulses during the 13C -13C/ transfer steps is able to provide 13C (i), 15N(i),... 

Figure 11 shows the i 2 dependence on the particle size for systems containing a constant amount of magnetized material, without refocusing pulse (like in a gradient echo sequence), and with 7 different echo times used in CPMG sequences. 

A more phenomenological approach 25) allows to overcome this limitation. The space around each particle is divided between two regions the boundary is echo-time dependent. In the first one, near the particle, the gradients are too large for the refocusing pulses to be effective, so that the moments situated in this region will be rapidly dephased. These protons contribute a fast signal decay that is unobservable with MRI techniques. 

In the second region, relatively far from the particle, the refocusing pulses are effective, which corresponds to a weak magnetization condition. The relaxation rate can then be analyzed following Majumdar and Gore 26). The result of this calculation is in remarkable agreement with the slow rates, obtained from simulated multiexponential decays 27) ... 

The role of the refocusing pulses is generally understood in the following sense they lose their efficiency as soon as too many random events take place between two consecutive pulses, creating irreversibility. This idea is translated in a limitation of the allowed diffusion between pulses - the echo time must be shorter than the correlation time characterizing diffusion. However, one question remained and was long debated upon is this correlation time the diffusion time (xd = r jD), or the interdiffusion time (the time for diffus-... 

This model is close to the SDR. However, the SDR only deals with static spins, so that relaxation, more specifically the first-order effect we investigate, is automatically canceled by the refocusing pulses. On the contrary, our model accounts for irreversible dynamics the randomness of the jumps from... 

For large-weight Zn + complexes with broad (50-150 kHz) second-order quadrupolar powder patterns Zn QE NMR may be an experimental challenge. In such cases the sensitivity must be enhanced by isotope enrichment combined with, e.g., cross polarization (CP) from H ", low-temperature acquisition or sampling of the free-induction decay (FID) in the presence of a train of refocusing pulses. ... 

Fig. 18. (a) RARE pulse sequence, and (b) its associated k-space trajectory. The order of the phase encoding is shown by the numbers to the left of the raster. After each line in k-space, the spins are returned to the same point on the fcxqead) axis prior to the application of the refocusing pulse shown by the dashed line and arrow. 

Fig. 8.19. Vector representation of a H-13C HMQC experiment. The first 90° pulse along y rotates the equilibrium magnetization of the proton spin, /H, from the z axis to the x axis. After a time /d = 1/2/Hx, the antiphase coherence 2/J1/ t (see Appendix IX) is at its maximum. A 90° pulse on carbon along y then transforms the antiphase coherence into a MQ (multiple quantum) coherence (the 2/J1/ component is shown). During t the MQ evolves (with a 180 refocusing pulse on proton in the middle), until a further 90 pulse on carbon along x transforms the —2/ / component (shown at its maximum for clarity) into a 2/ /f antiphase coherence. After the time fd, in-phase coherence of the proton spin develops. The latter is detected during h. Its initial intensity is modulated by the carbon Larmor frequency during t (if proton refocusing has been used), thus originating a proton-carbon cross peak.

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