Probes Tuned to Multiple Frequencies for In-Vivo NMR

The growing interest in the application of NMR spectroscopy to intact biological systems has placed unique demands on instrumentation. Perhaps the most critical piece of instrumentation for the in-vivo spectroscopist is the NMR probe. The need to perform quick and reliable experiments, particularly in the clinical setting, makes probe sensitivity critical. Many studies involve the use of more than one nucleus. Typically, proton NMR is used to shim, obtain localizing images and/or spectra. Additional spectra may be acquired from 31P, 23Na, 13C or other nuclei. This multinuclear approach to in-vivo NMR spectroscopy has placed a premium on multiple tuned NMR probes that perform with high sensitivity. There are many designs for double tuned probes in the NMR literature. Some designs for probes tuned to three and four nuclei have also been presented. All of these designs must sacrifice some sensitivity at one or more of the resonant frequencies at which they operate when compared to a similar single tuned coil. One common strategy of all of the multiple tuned probe designs is to minimize the loss in sensitivity. In most designs it is possible to arbitrarily distribute the loss in sensitivity between the different operating frequencies of the probe by proper choice of component values. Thus the evaluation of any multiple tuned probe can only be performed if the probe is optimized for a particular application. A second, less frequently discussed aspect of multiple tuned NMR probes is orthogonal tuning adjustment. In the in vivo setting it is very difficult to control loading of the probe. Therefore, the tuning must be adjusted with each application. Independent tuning of the individual resonances is important to ensure reliable performance by the probe. In this paper, I will discuss in detail many of the multiple tuned designs that have appeared in the NMR literature, paying particular attention to the advantages and disadvantages of each. I will focus on multiple tuned surface coils, but will also treat double tuned volume coils, which are of importance in the context of volume selective MRS techniques.

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