Pulse Sequences and Data Processing

To improve reproducibility in MRSI in human brain, simultaneous acquisition of the internal water reference and metabolite signals was evaluated. Use of singular value decomposition techniques and finite impulse response filters proved effective in separating water and metabolite signals and providing estimations of the metabolite concentrations. 

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Pulse sequences and data processing 4.1 Pulse techniques strategy... 

Introducing parallelism into NMR data acquisition requires simultaneous acquisition of signals from more than one sample. The simplest way to achieve this is to use multiple samples within a single radiofrequency (RF) coil, and to design either pulse sequences or post-processing routines to separate the signals... 

NMR spectroscopy represents a valuable and versatile tool for the characterization of dispersed nanoparticles. In contrast to alternative analytical techniques, it combines a distinctly non-invasive character with the ability to analyse for chemical composition as well as for local mobility of individual system components. Its main disadvantages - poor sensitivity and time consuming acquisition of experimental data - can be overcome by a suitable choice of the pulse sequence and the experimental conditions. The advantages of the NMR approach are especially promising for the study of nanoparticle dispersions used as drug carriers, where many important system characteristics such as release properties, surface exchange processes or decomposition pathways are readily available by relatively simple pulse experiments. 

In the context of the NMR-SIM simulation, the recording of a one-dimensional experiment consists of the definition of the spin system, the pulse sequence and the experiment parameters. The result of the simulation is a FID that is loaded into the ID WIN-NMR processing program. Both the time domain and the frequency domain data may be processed using a variety of domain specific mathematical functions. 

There have been major advances in the last few years both in the number and variety of the pulse sequences used to perturb the spins, and in the computing power and data processing techniques employed. 

Data acquisition and processing. Collect NMR data using high-resolution (500 MHz or more) spectrometers, using different pulse sequences and multiple nuclei. 

During the detection period denoted t2 (not the relaxation time T2 ) the NMR signal is captured electronically and stored in a computer for subsequent workup. Although detection occurs after evolution, the first Fourier transformation is applied to the time domain data detected during the t2 detection period to generate the f2 frequency axis. That is, the t2 time domain is converted using the Fourier transformation into the f2 frequency domain before the tj time domain is converted to the fj frequency domain. This ordering may seem counterintuitive, but recall that q and t2 get their names from the order in which they occur in the pulse sequence, and not from the order in which the data set is processed. 

For applied NQR spectroscopy, pulsed techniques are used in conjuction with a variety of pulse sequences and FT data processing. This approach provides for maximum sensitivity and versatility. 

Radiofrequency spectroscopy (NMR) was introduced in 1946 [158,159]. The development of the NMR method over the last 30 years has been characterised by evolution in magnet design and cryotechnology, the introduction of computer-based operating systems and pulsed Fourier transform methods, which permit the performance of new types of experiment that control production, acquisition and processing of the experimental data. New pulse sequences, double-resonance techniques and gradient spectroscopy allow different experiments and have opened up the area of multidimensional NMR and NMRI. 

B) up-down HMBC pulse sequence inverting BCH and 13CH3 peaks relative to the standard sequence. (C) up-down + HMBC pulse sequence inverting 13CH and 13CH3 peaks relative to the standard sequence and in the opposite sense to the up-down sequence. The data of the different pulse sequences are recorded in an interleaved manner. After formation of the required two linear combinations in the time domain, the data are processed in the same way as other HMBC-type data. 

Throughout the cross-polarization pulse sequence, a number of competing relaxation processes are occurring simultaneously. The recognition and understanding of these relaxation processes are critical in order to apply CP pulse sequences for quantitative solid state NMR data acquisition or ascertaining molecular motions occurring in the solid state. 

The use of magnetic resonance imaging (MRI) to study flow patterns in reactors as well as to perform spatially resolved spectroscopy is reviewed by Lynn Gladden, Michael Mantle, and Andrew Sederman (University of Cambridge). This method allows even unsteady-state processes to be studied because of the rapid data acquisition pulse sequence methods that can now be used. In addition, MRI can be used to study systems with short nuclear spin relaxation times—e.g., to study coke distribution in catalytic reactors. 

Fig. 10.12. Pulse sequence for amplitude modulated 2D J-resolved spectroscopy. The experiment is effectively a spin echo, with the 13C signal amplitude modulated by the heteronuclear coupling constant(s) during the second half of the evolution period when the decoupler is gated off. Fourier transformation of the 2D-data matrix displays 13C chemical shift information along the F2 axis of the processed data and heteronuclear coupling constant information, scaled by J/2, in the F1 dimension.

The NMR scan is basically a series of pulses and delays. A short pulse of radiation is applied to the sample as the magnetic field is scanned. If the signal due to water is to be suppressed there is an initial saturation signal at the water frequency as described earlier. Before another test scan can be taken, the sample must be allowed to relax and during this period a further saturation of the water signal can be done and the process repeated. The sequence of events and collection of data are automated. 

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