Development of a switchable DC coil based on a plasma conductor to minimize coupling with RF coils, gradient coils.

Coupling between metallic conducting structures is a major limitation for design of in-bore equipment. For example, the placement of copper B₀ shim arrays can be severely limited by the presence of RF coils [1]. Plasma conductors on the other hand can be turned on and off arbitrarily, completely removing the conducting structures and rendering them invisible to gradient and radio frequency (RF) activity.

As plasma based conductors have very low resistance and can be freely shaped they could be a promising alternative for designing in-bore equipment. This can be applied in B₀ shimming, signal de-phasing or ultimately even gradient coil designs.

Recently, RF coils based on a plasma conductor were introduced for B₁ field manipulation [2]. In this work we explore the principles of operation of a plasma conductor driven with a direct current (DC) for B₀ field manipulation such as field shimming or unwanted signal dephasing.

A commercial straight gas filled tube (45 cm) was used in an MRI (Figure 1). The tube was operated with a flash-circuitry where a 47µF capacitor is charged (up to 330V) and discharged over the tube to generate a DC current flow which generates a B₀ field variation. To trigger a discharge, a controlled high voltage starter circuit was used to ionize the first part of the tube; hereafter the voltage over the capacitor was high enough to initiate a discharge over the tube (discharge 7mC) (figure 2).

The straight tube was placed in a holder in the magnet with a small angulation to the field. A spherical phantom and a surface coil were placed next to the tube (figure 1). B­₀ field mapping was performed to measure the z-component of the induced magnetic field. The DC discharge was timed between the excitation and the acquisition of a single shot EPI sequence, B₀ field information was extracted from corresponding phase images.

The circuitry allowed for a controlled discharge of the capacitor over the tube. A clear B₀ field variation was observed due to the current discharge in the tube (figure 3). When the plasma was off, no field distortion was observed. At too large an angulation with the magnetic field, the current starter circuit did not result in a full ionization of the tube, most likely due to Lorentz forces on the plasma.

The proof of principle of a switchable DC plasma coil in MRI was shown. A controlled DC current was applied during an MRI field mapping sequence to observe the initiated B₀ field shift.

Several improvements are envisioned; different starting mechanism can be explored (e.g. high frequency RF ionization) to allow for angulated operation. More work will be needed to optimize gas mixtures, tube and cathode designs and investigate different coil shapes to create the best control in B₀ field shaping without distorting other electronics in the MRI bore.