180° pulse, poor performance with

Thalidomide is now a well-established agent for treating refractory or relapsed disease, and about 30% of patients will achieve a response to this therapy. More recently, thalidomide has been used in combination with dexamethasone, and response rates on the order of 65% have been observed. Studies are now under way to directly compare VAD with the combination of thalidomide and dexamethasone. In some patients, especially those with poor performance status, single-agent pulse dexamethasone administered on a weekly basis can be effective in palliating symptoms. 

Degenerate four-wave-mixing (DFWM, Section 10.2.3) in conjugated polymers has also been explored. Its use in optical image correlation has been demonstrated using Durham-route PAc and similar polymers. The very large optical non-linearity of PAc, Table 9.3, and the fast electronic response allow correlation to be performed with sub-picosecond laser pulses. The poor stability of PAc prevented this laboratory demonstration being turned into a practical device. 

Although in principle the simple scheme presented in Fig. 5.59 should provide TOCSY spectra, its suitability for practical use is limited by the effective bandwidth of the continuous-wave spin-lock. Spins which are off-resonance from the applied low-power pulse experience a reduced rf field causing the Hartmann-Hahn match to breakdown and transfer to cease. This is analogous to the poor performance of an off-resonance 180° pulse (Section 3.2.1). The solution to these problems is to replace the continuous-wave spin-lock with an extended sequence of composite 180 pulses which extend the effective bandwidth without excessive power requirements. Composite pulses themselves are described in Chapter 9 alongside the common mixing schemes employed in TOCSY, so shall not be discussed here. Suffice it to say at this point that these composite pulses act as more efficient broadband 180 pulses within the general scheme of Fig. 5.60. 

The current-time behaviour of membrane-covered microdisc clinical sensors was examined with the aim to explain their poor performance when pulsed (Sutton et al., 1996). It has been shown by Sutton and co-workers that the Cottrellian hypothesis is not ap>plicable to this type of sensor and it is not possible to predict this behaviour from an analytical expression, as might be the case for membrane-covered macrodisc sensors and imshielded microdisc electrodes. 

A version of this experiment was done by Mandel and his group at Rochester University. The experimental apparatus was slightly different from the one shown in Fig. 11, but the principle was essentially the same. The experimentalists concluded that the results do not confirm the existence of the 0 waves, because the visibility obtained was not 50%. The statistics of the experiment was very poor the mean coincidence rate was about six counts per second, but even so it was possible to fit the results with a visibility of 10%. This indicates that the experiment was not conclusive and therefore should have been performed again under better conditions, with more significant statistics, and with a larger coherence length for the overlapping pulses. 

In practice, one must also consider the properties of the available probe (additional channels in the commercial spectrometers are usually broad-band). Complete freedom for the experimentalist would be provided if the probe had a XH coil on the outside (as broad-band probes do) and the inner coil(s) for X and Y tunable and with equal sensitivity for X and Y. Such probes are not offered, and if they were, their performance would probably be poor (sensitivity, handling, pulse length etc.) as the price to pay for the flexibility. The second choice would be a XH—29Si—X probe such probes are available, but more common are XH—13C—X probes (with inverse configuration, i.e. with a XH... 

Such performance was beyond the capabilities of the NMR spectrometers used in the first few years of CP/MAS NMR studies of lignin. The problem of SSB strength was at first avoided by using low-field spectrometers, but this was unsatisfactory because of the poor sensitivity associated with low fields. The problem was later partly overcome by SSB-suppression pulse sequences (Dixon et al. 1981, Barron et al. 1985), or by applying correction factors to the centerband strength (Hemmingson and Newman 1985). 

Cardiac and respiratory stabilization are the first priorities following pentazocine poisoning. The patient s airway should be patent and adequate ventilation assured. If the patient has either inadequate ventilation or a poor gag reflex, then the patient may be at risk for subsequent CO2 narcosis with worsening acidosis or aspiration. If necessary, endotracheal tube intubation should be performed. Close monitoring of the patient s pulmonary exam should be performed to assure that pulmonary edema does not develop. The health care providers should place the patient on continuous cardiac monitoring with pulse oximetry and make frequent neurological checks. 

Figure 3.7. Excitation trajectories as a function of resonance offset for (a) a 90° pulse and (b) a 180° pulse. The offset moves from zero (on-resonance) to - -yB Hz in steps of 0.2yB] (as in Fig. 3.6). The 90° pulse has a degree of offset-compensation as judged by its ability to generate transverse magnetisation over a wide frequency bandwidth. In contrast the 180° pulse performs rather poorly away from resonance, leaving the vector far from the target South Pole and with a considerable transverse component.

Most of the sequences performed well even on this short and moderately shimmed sample. Despite not optimizing the shims, almost every sequence was able to suppress sufficiently to acquire useful spectra with minimal baseline distortions even close to the solvent. The exception occurred for the WET style sequences. The WET family did not suppress nearly as well and required the gain to be reduced by 6 dB (vertical scale in the figure was increased to compensate) to prevent a receiver overflow and ADC error. However, WET and SWET are very easy to include in pulse sequences (e.g. during the recycle delay or mixing periods) and allow simple suppression of multiple frequencies using composite shaped pulses. The poor results may come from several sources (e.g. coding error, optimization error, etc.) and therefore may not represent a limitation of the pulse sequences. Alternatively, as a real world example, some sequences simply perform better on some spectrometers and users must be prepared to adapt when necessary. We have t)q)ically found WET to be extremely reliable and robust, but certainly not on this particular sample. 

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