Microcoil
A microcoil is an tiny electrical conductor such as a wire in the shape of a spiral or helix which could be a solenoid or a planar structure. One field where these are found is nuclear magnetic resonance (NMR) spectroscopy, where it identifies radio frequency (RF) coils that are smaller than 1 mm.
The microcoils have also found usefulness in telemetry systems, where planar microcoils are used to supply energy to miniaturized implants.
The detection limits of Micro-MRI or MRM can be pushed further by taking advantage of microsystem fabrication techniques. In general, the RF receiver coil should closely conform to the sample to ensure good detection sensitivity. A properly designed NMR probe will maximize both the observe factor, which is the ratio of the sample volume being observed by the RF coil to the total sample volume required for analysis, and the filling factor, the ratio of the sample volume being observed by the RF coil to the coil volume.
The miniaturization of NMR probes thus involves two advantages:
Three microcoil types which are commonly used in NMR:
Is the classical geometry to create a magnetic field with an electric current. Even for a limited number of windings this geometry provides a reasonable homogeneous B1 field and a good filling factor is possible by winding the coil directly onto a holder containing the sample. Miniaturization to a scale of several hundred micrometers (µm) is not very difficult although the wire diameter (typically 20 to 50 µm) becomes very small and a freestanding coil is a very delicate object. A reduction to below 100 µm diameter is possible but the machining and handling of such coils will be rather tedious. For this reason other microsystem fabrication technology such as bulk micromachining, LIGA and micro-injection molding should be applied. For solenoid coils adding more turns to the coil will enhance the B1/i ratio and thus both the inductance and the signal response. At the same time the coil resistance will increase linearly, so the improvement in sensitivity will be proportional to the square root of the number of turns (n). At the same time we will have a larger ohmic heating at the center of the coil and an enhanced danger for arcing, so the optimum is generally found for only a limited number of turns. Besides RF performance, static field distortions due to susceptibility effects are an important factor in the design of microcoil probeheads.
Is the most common geometry used, based on a spiral design with the center winding contacted to the outside using a connection to another layer which is electrically isolated with a thin oxide layer. In this configuration the axis of the RF coil will be oriented perpendicular to the external static field B0.
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