Design and Analysis of Microcoils for NMR Microscopy

The signal-to-noise ratio achieved in a nuclear magnetic-resonance microscopy experiment is directly related to the performance of the radiofrequency coil. An accurate determination of coil performance requires that the resistance of the coil be well characterized, Traditional high-frequency electric-circuit models used to describe larger NMR coils are inadequate when the diameter of the conductor is reduced to the dimensions of the electrical skin depth (δ) at the frequency of operation. A more extensive model based on a scaling parameter that includes δ is presented. This model complements other existing circuit models that represent sample losses, ground-loop and parasitic losses, and the signal induced in the RF coil. Experimental verification is accomplished using a series of solenoidal microcoils in ¹H NMR microspectroscopy experiments at 4.7 T (200 MHz). This study demonstrates for the first time that a predictable performance enhancement is achieved using microcoils as small as 50 μm in diameter.