Inversion pulse

Figure Bl.14.9. Imaging pulse sequence including flow and/or diflfiision encoding. Gradient pulses before and after the inversion pulse are supplemented in any of the spatial dimensions of the standard spin-echo imaging sequence. Motion weighting is achieved by switching a strong gradient pulse pair G, (see solid black line). The steady-state distribution of flow (coherent motion) as well as diffusion (spatially...

Figure B2.4.8. Relaxation of two of tlie exchanging methyl groups in the TEMPO derivative in figure B2.4.7. The dotted lines show the relaxation of the two methyl signals after a non-selective inversion pulse (a typical experunent). The heavy solid line shows the recovery after the selective inversion of one of the methyl signals. The inverted signal (circles) recovers more quickly, under the combined influence of relaxation and exchange with the non-inverted peak. The signal that was not inverted (squares) shows a characteristic transient. The lines represent a non-linear least-squares fit to the data.

Fig. 9.2 Schematic representation of the three basic experiments useful for the determination of (A) transient NOE experiment, (B) 2D NOESY and (C) 2D ROESY. The gray-filled half-circle represents a frequency-selective inversion pulse which inverts the spin to which the cross-relaxation...

The best and easiest way to implement such an experiment is to use adiabatic inversion pulses, in order to introduce heterogeneity for evolution under 13C-1H scalar or residual dipolar couplings by means of a frequency-swept 180° pulse on 13C that inverts 13C nuclei at different positions in the NMR sample at different times (Figure 13) 40,45 This filter is robust with respect to pulse miscalibration and operates efficiently without the need to cycle the phases of pulses that otherwise is a common feature of non-destructive LPJFs. 

Hyperbolic Secant Inversion Pulses and Wideband Uniform Rate Smooth Truncation... 

In STARTMAS, SQ STs of order q (with q = 2 and p = —1) are initially excited as in STMAS (Fig. 14). The ST coherences then evolve during the first time period r/[2(l + k)], before being converted to a DQ transition by a selective n inversion pulse on the CT (as in DQF-STMAS [161]) and allowed to evolve during kx/(l + k). A second selective n pulse brings the system back to an SQ transition for the last part of the ST evolution. At time t = x, the phase of the detected SQ ST coherence is thus... 

The idea of using the linear phase increments to achieve frequency-shifted excitation can be adopted almost to any pulses, such as hard (amplitude fixed) pulses, shaped pulses, and even adiabatic inversion pulses. Unlike any other pulses, the adiabatic pulses have already used non-linear phase increments for tilting the effective RF field slowly compared with the Larmor frequency of the spins in the rotating frame in order to fulfill the adiabatic condition. 

Bloch-Siegert phase shift is usually introduced in homonuclear nuclear decoupling with an inversion pulse that is far off-resonance to the spins in precession. To minimize the disturbance, the strength of the inversion pulse... 

The 13CO inversion pulse can be achieved by a PIP with the carrier placed at the centre of the 13C . This inversion PIP causes an additional procession of the 13C spins. The amount of the phase shift in the rotating frame of the 13CO, where the centre of the 13CO spins is on-resonance to the inversion PIP, can be calculated... 

To overcome these problems, a compensating PIP can be applied on the other side of the 13C region immediately after the first inversion pulse. The first pulse is a 180° pulse with a x phase and is on-resonance to the centre of the 13CO. As mentioned above, it can still be denoted as a PIP O0, 0°, 0.6 ps,... 

The idea of using phase increment to achieve frequency-shifted excitation can be extended virtually to any sort of RF pulses, including the most complicated adiabatic inversion pulses where a non-linear phase increment has already been applied. Using the phase increment, double or multiple pulses can be constructed with only a single waveform generator in order to excite different regions of a NMR spectrum or to compensate the BSFS, BSPS, as well as BSOS. 

Fig. 17.5 Use of an adiabatic inversion pulse in a 13C filter. An almost linear relationship exists between a 13C chemical shift and the corresponding Yh,13C coupling constant (left panel). With an adiabatic 180° pulse on 13C, sweeping from high...

Fig. 28. FFC Inversion Recovery sequence. In the upper case the sample is first prepolarized in a filed Bp, then switched to the acquisition field Ba where the first RF pulse of 180° is applied and the sample magnetization is inverted. The field is then switched to B,. and the sample is allowed to relax for the variable time t. Finally, the field is switched again to the acquisition value and the magnetization is sampled by any of the sample-detection methods (here, a simple FID following a 90° RF pulse). Notice that, as shown in the lower diagram, in the special case when Bp = Ba it is possible to neatly avoid the extra switching interval prior to the inversion pulse.

For selective irradiations with a flip angle of 180°, one can distinguish two groups of shaped pulses inversion pulses, which change the sign of Zeeman... 

Fig. 3. Numerical simulations of four different selective inversion pulses. (Top) Pulse shapes. From left to right 180° rectangular, 180° Gaussian truncated at 2.5%, Quaternion cascade Q, and l-BURP-2. (Middle) Trajectories of Cartesian operators in the rotating frame during...

A universal inversion pulse can also be used for selective refocussing. 

The peak rf amplitude required to achieve optimum inversion with a selective inversion pulse is given in comparison to the rf amplitude required to achieve an on-resonance 90° flip-angle with a selective rectangular pulse, the simplest conceivable shape. 

Selectivity of the selective inversion pulses of table 3, all numbers given for properly... 

Inversion pulses can be calibrated either by the crude method which consists of applying the pulse to an isolated multiplet and searching the... 

Fig. 6. Simulation of the effect of longitudinal and transverse relaxation during a 30 ms selective inversion pulse. The trajectories are shown on a Bloch sphere and in the...

For selective inversion or refocussing, a universal pulse is a good choice. In cases where singlets are to be inverted and where relaxation or exchange during pulses is critical, one may need to use a 180° Gaussian pulse which is the shortest selective inversion pulse available [24]. 

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