Use of metal or metal alloys with melting points lower than room temperature for electrical conductors provides potential flexibility in shape and self-healing capability. Mercury is typically avoided due to its toxicity and tendency to assume a spherical shape due to surface tension. In contrast, it is possible to pattern conductors composed of eutectic gallium indium (EGaIn)¹ into a variety of reconfigurable shapes². This material is attractive for MR applications, where a flexible coil is needed or for high field applications when coil reconfiguration is needed for each subject better control transmit efficiency or B₁ field distribution. However, since the conductivity of liquid metals is at least 17-times lower than of copper and gold^3, it is not clear whether it is practical to use transmitters made of such materials for MR applications. To answer this question, an electric dipole antenna made of EGaIn was compared to an electric dipole made of copper with similar size. MR images and B₁⁺ maps were acquired to compare the performance of both transmitters on a 7T scanner.

Gallium melts at 30 °C (i.e., it is a liquid at body temperature) and has metallic conductivity, a water-like viscosity, no vapor pressure (i.e. it does not evaporate), and low toxicity and has been used in multiple FDA approved activities including dental implants, MRI contrast agents, and pharmaceuticals. The addition of indium to gallium lowers the melting point below room temperature to ensure it is a liquid. Importantly, these liquid metal alloys containing gallium react rapidly with air to form a thin (~3 nm) oxide layer that imparts the metal with fascinating properties that allows the metal to be patterned despite its otherwise surface tension¹. The ability to pattern the metal into non-spherical shapes has led to new types of materials and devices that take advantage of the liquid properties including soft robotics, deformable sensors, stretchable electronics, conformal antennas, and microfluidic electrodes⁴.

To test the MR performance of this material, two 230 mm-long 1.3 mm-diameter conductor fibers made of EGaIn were built (Figure 1), from which an electric dipole antenna was constructed by injecting EGaIn into the hollow core of a fiber shell composed of Hytrel elastomer. A short piece of copper wire was inserted into each end for further electric connections. For comparison, a copper based electric dipole with similar dimensions was built. Both dipoles were matched to 50 Ohm and connected through a home-built interface to a Siemens Magnetom 7T-830-AS scanner. There were positioned on the side of a Siemens torso phantom by 3-cm thick foam (Figure 2). Gradient-echo images as well as Bloch-Siegert based B₁⁺ maps⁵ were acquired. Imaging parameters are: voxel size 1.6x1.6x5 mm³, image matrix size 256x112, TE 3.53 ms, TR 100 ms, flip angle 30 degrees, and 5 averages.

Gradient echo images (Figure 3) and B₁⁺ maps (Figure 4) showed very similar distribution between both dipoles. In fact, at 14 mm penetration depth, the transmit efficiency of the liquid-metal dipole was only about ~20% on average lower than the copper-based one, despite of a more than 17-fold difference in conductivity (σ_copper=59.6x10⁶ S/m v.s. σ_EGaIn=3.4x10⁶ S/m). This suggested that, at least at high field, when load is dominant, using EGaIn will not lead to a prohibitive loss in transmit efficiency. Transmit efficiency can be further improved by using thicker fibers or using multiple fibers in parallel.

With the liquid metal, the length of the dipole can be easily readjusted remotely² or by physical stretching^6, thus making it possible to personalize the transmitter for each patient to maximize transmit efficiency and thus ameliorating the SAR constraint. In addition, with the reconfigurability, truly flexible coils can be made from liquid metal fibers to accommodate various patient geometries to improve imaging performance. With low toxicity and self-healing ability, liquid metal-based coils may also be used as implanted coils for other MR applications.

MR experiments suggested that liquid metal based RF transmitters had a transmission efficiency nearly the same as solid copper despite a significantly lower conductivity. With other features of the liquid metal, this finding makes it attractive for some MR applications, such as personalized in-scanner reconfigurable coils.