Imaging experiments

Once a slice has been selected and excited, it is necessary to encode the ensuing NMR signal with the coordinates of nuclei within the slice. For each coordinate (x andy) this is achieved by one of two very closely related means, frequency encoding or phase encoding [1]. In this section we consider the fonner and in the next, the latter. In tlie section after that we show how the two are combined in the most coimnon imaging experiment. 

What needs to be known to discuss the feasibility of a particular imaging experiment is a good estimate for the relaxation times of the system under study, which then need to be compared with the times required for executing the sequence, frequently determined by hardware limitations. In a simple liquid, one usually has T = T2 and both are in the range of several seconds. This is... 

A single cell layer of an excised piece of onion epidermis was selected as a test object for this micro-coil, (Figure 2.1.14), and an imaging experiment was carried out, similar to the one by Mansfield et al., who used a laboratory-made micro-coil probe and gradient system [31]. Other micro-coil types, e.g., volume coils or coils that are immersed into the objects, can be adapted to specific applications and mounted on commercial imaging probes. 

Our method is demonstrated with experiments on a Bentheimer sandstone sample. The sample was prepared to be cylindrically shaped with a diameter of 2.5 cm and a length of 2.0 cm. The sample was fully saturated with de-ionized water under vacuum. We performed the CPMG imaging experiment described in the previous section to measure the magnetization intensity at 50 echoes spaced by 4.6 ms for each of 32 x 16 x 16 voxels within the field of view of 3.0 cm x 3.0 cm x 3.0 cm. The corresponding voxel size is 0.938 mm x 1.88 mm x 1.88 mm. We used 1 s of repetition time (TR) and the total imaging time was 4.3 min. 

An example of the spin-velocity density function is demonstrated in Figure 4.1.6. A velocity imaging experiment was performed on water flowing through a 6-mm diameter tube. The velocity density function was spatially resolved along the axial direction of the tube, denoted by z in the figure. It is observed that the velocity density function has a steep peak at zero velocity when the fluid is not flowing, but is shifted to a positive velocity when the flow rate was increased to 2.5 mL min-1. 

Now let us take a look at a recent NMR imaging experiment of Fano flow, in which the local velocities in the tubeless column were mapped out quantitatively and nondestructively [20], For such a set-up, the weight force of the column is balanced by the extensional stress difference azz - axx associated with the vertical velocity gradient (dvz/dz), as... 

The previous two sections described the development of hp-xenon techniques for the investigation of gas dynamics in porous or opaque media. The usefulness of continuous flow hp-xenon as a tool for dynamic NMR imaging experiments was... 

NaX, the same type that was used for the imaging experiments in Figure 5.3.3) and ignited above the pellets. The probe was subsequently inserted into the magnet. 

All imaging experiments used the two-pulse spin-echo sequence a-T-2a-t-echo. In 3H experiments, a = 90 ° was used. For 2D 1H M RI of liquids, a 2-mm thick axial... 

The rf pulses for 27A1 NMR experiments were calibrated using an aqueous solution of A1C13. For the rf power level attenuated by 10 dB, the duration of the 180 "-pulse of the broadband probe was 60 ps. All solids imaging experiments were performed with t => 300 ps and the nominal flip angle a = 90°/(J + 1/2). The two pulses had the same amplitude and for 27A1 MRI were 10- and 20-ps long, respectively. 

Coupled on-line techniques (GC-MS, LC-MS, MS/ MS, etc.) provide for indirect mixture analysis, while many of the newer desorption/ionisation methods are well suited for direct analysis of mixtures. DI techniques, applied either directly or with prior liquid chromatographic separations, provide molecular weight information up to 5000 Da, but little or no additional structural information. Higher molecular weight (or more labile) additives can be detected more readily in the isolated extract, since desorption/ionisation techniques (e.g. FD and FAB) can be used with the extract but not with the compounded polymer. Major increases in sensitivity will be needed to support imaging experiments with DI in which the spatial distribution of ions in the x — y plane are followed with resolutions of a few tens of microns, and the total ion current obtained is a few hundreds of ions. 

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