Spatially selective excitation

The inhomogeneity in Bi, especially when using surface coils, can lead to a spatially dependent population of desired and undesired coherence pathways. Field gradient pulses in combination with shaped RF pulses lead to spatially selective excitation or refocusing and, in such cases, localization can be viewed as a type of coherence pathway selection. The inherent Bi gradients resulting from the inhomogeneous RF fields of surface coils have also been used for water suppression in in vivo experiments. ... 

Figure 10.44. Spatially selective excitation for single-scan 2D spectra showing a hypothetical experiment of six ti increments, each initiated by selective excitation applied at frequencies fi-fe corresponding to slices si- 6-...

Another approach to obtain spatially selective chemical shift information is, instead of obtaining the entire image, to select only the voxel of interest of the sample and record a spectrum. This method called Volume Selective spectroscopY (VOSY) is a ID NMR method and is accordingly fast compared with a 3D sequence such as the CSI method displayed in Figure 1.25(a). In Figure 1.25(b), a VOSY sequence based on a stimulated echo sequence is displayed, where three slice selective pulses excite coherences only inside the voxel of interest. The offset frequency of the slice selective pulse defines the location of the voxel. Along the receiver axis (rx) all echoes created by a stimulated echo sequence are displayed. The echoes V2, VI, L2 and L3 can be utilized, where such multiple echoes can be employed for signal accumulation. 

Fig.l Demonstration of the spatial selectivity of one-photon excitation (left) vs. two-photon excitation (right) in a fluorescein solution... 

A way to reduce the data acquisition time is to use chemically selective excitation. According to this approach, the experimenter may decide that it is not necessary to acquire the full spectrum at each position instead, one simply acquires data from a particular spectral resonance (i.e., chemical species) within the full spectrum. In this way, the spatial location of just one particular species is mapped in 1-, 2-, or 3-D. 

Femtosecond photoionization mass spectrometry might be useful in the study of the three-dimensional structure of large biomolecules. When a selectively excitable and ionizable chromophore is located on the outer (surface) part of large molecule, one can be detached in the picosecond time scale. However, when the excitable chromophore is located in the inner part of the big molecule, its detachment will require a much longer time, which is needed for spatial rearrangement of the molecule. So, even the simple mass spectrometry of bioorganic molecules with femtosecond laser ionization can reveal some details of their spatial structure. 

Applications have been reported for photoelectrochemical experiments, for example, splitting of water [11], local generation of photoelectrodes by spatially selective laser excitation [12], and steady-state electrochemiluminescence at a band electrode array [13,14]. Band electrodes prepared from very thin films approaching molecular dimensions have been used to assess the limits of theory describing electrode kinetics at ultramicroelectrodes [9]. Spectroelectrochemical applications have been extensively reviewed [1], In an intriguing approach, thin, discontinuous metal films have been prepared on a transparent semiconductor substrate they are essentially transparent under conditions in which a continuous metal film containing the same quantity of metal would be expected to substantially absorb [15]. 

As illustrated by the model compound work summarized in Fig. 3, Na spin echo decay spectroscopy is suitable in principle for elucidating the spatial distribution of sodium ions in sohds [11,44-47]. Meaningful results can be obtained provided that the measurement is carried out by selective excitation of the central j 1/2> -1/2> transition, which needs to be ascertained by nutation NMR. Furthermore, sub-ambient temperatures may be required to suppress any potential influence of sodium dynamics. To date, promising applications have been pubhshed using Na spin echo decay spectroscopy (1=3/2) on sodium-containing glasses, to address the vital issue of cation clustering. Fig-... 

The inhomogeneity of the excitation RF fields from transmit-receive surface coils can be used to advantage for selecting a sensitive volume slab based on approximate distance from the coil. Limited spatial selectivity can be achieved by adjusting the flip angle to best select the depth of interest. 

With this technique, the spatial resolution is not limited by the requirement that B must be larger than the largest frequency offset. In fact, selective excitation can improve... 

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