RF-coil design

The compact MRI system has an advantage for rf coil design because solenoid coils can be used in most applications. The solenoid coil has about three times better SNR than that of the saddle-shaped coil (14). Even if the saddle-shaped or birdcage coil is used in the quadrature mode, the solenoid coil will still have better SNR because an SNR gain of only about 1.4 times is obtained in that mode. 

Microfabrication techniques with the features of integration, reproduction and precision are particularly suitable for the implementation of miniaturized designs and significant reduction in product costs. A miniaturized magnetic resonance probe for on-line/in-line flow studies can be microfabricated by combining an rf coil [45, 46] with microfabricated gradients [47] and electronics [48] around a small tube/capillary. 

A modification of the design of the flow cell can also lead to a further increase in sensitivity [7,8]. The addition of a proton RF coil for selective excitation of the protons before entering the 13C flow cell enables the full build-up of the nuclear Overhauser effect in a continuous-flow system (Figure 8.1.5). The combination of this NMR flow cell design, together with the use of immobilized paramagnetic species, could lead to the registration of continuous-flow 13C NMR spectra of separated species. 

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... 

To date, two arrangements of NMR coils, the solenoidal radio frequency (RF) coil (Figure 2A) and the saddle-type (Helmholtz) RF coil (Figure 2B) have been employed as on-line NMR detectors with CE and CEC. Theoretical studies have shown that reduction of the diameter of the RF coils increases the coil sensitivity [32], The miniaturized versions of saddle types are commonly used in commercial probes. As a major development, a saddle coil which houses 1.7-mm-diameter sample tubes has been introduced by Varian. Another significant contribution is the designing of an inverse coil to accommodate 3-mm-diameter sample tubes with a detection volume of 60 pL and a total volume of 140 pL [33], Fabrication procedures hinder further reduction of diameter of saddle-type coils that are optimized for sample volumes smaller than —1 pL. 

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