High field, instrumental requirements

Since each axis spans the entire chemical shift range, something on the order of a thousand individual FID patterns, each incremented in ti, must be recorded. With instruments operating at a high spectrometer frequency (high-field instruments), even more FID patterns must be collected. As a result, a typical COSY experiment may require about a half hour to be completed. Furthermore, since each FID pattern must be stored in a separate memory block in the computer, this type of experiment requires a computer with a large available memory. Nevertheless, most modem instraments are capable of performing COSY experiments routinely. 

NMR analysis allows characterization of proteins to an atomic level. The most frequently used nuclei on protein NMR are 41, 2H, 13C, 15N, and 170 with proton NMR (Jefson, 1988). The use of NMR methods for protein sequence and conformational studies was limited to the small proteins or peptides because high magnetic fields were required but not widely available to study larger molecules and it was very time consuming with the capability of instruments in the past. 

The present performance of high field NMR instruments allows the recording of a spectrum in quantities of micrograms. Under these conditions, it is possible to install a flow cell of only a few microlitres into the magnet of the instrument which allows the spectra of the analytes to be recorded. The experiment is conducted with a very small flow rate of the mobile phase (D20 or CD3CN) or in the stop-flow mode. In this mode, the mobile phase is momentarily stopped in order to record the spectrum. This technique, which requires very expensive materials, is of limited use. It is used mainly to isolate and identify very unstable compounds that cannot be isolated through classical means. 

Although the basic configurations of instruments tailored for structure-based design or for NMR drug screening are similar, there are some minor differences. For structure-based design applications a relatively high field spectrometer is required (>500 MHz), usually equipped with three or four radiofre-quency channels for the simultaneous irradiation of and in some cases... 

Quantitation of the CBF can be an important factor in studies of therapeutic intervention. The method of arterial spin-labeled (ASF) perfusion MRI allows quantitative determination of the perfusion rate in units of ml/100 g tissue/min. However, the technique requires exceptional instrumental stability, as the result depends on accurate determination of the difference between tw o images on the order of a few percent. With the appropriate acquisition parameters to achieve sufficient signal to noise ratio (>100) and current instrument stability, this technique is feasible, especially in high field clinical systems. Currently, arterial spin-tagged perfusion measures are not widely used in clinical settings. 

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