I've done alot of research on this. The most easy way is to use planar software (eg AXIEM or sonnet) and model your tissue as a very large lossy dielectric layer. Dielectric properties of body tissues are found in online calculators. Also, the smaller your coil the more easy your design will be. For coupling you should use inductive coupling ( a second small coil) . This is proven to be more better in terms of SNR and saves you an expensive variable non magnetic capacitor. Hope this helps