Multi-frequency detection of a dielectric object using flexible contactless RF sensors for tissue diagnosis

The aim of this paper is to experimentally assess the capacity of radio-frequency flexible sensors to localize a dielectric object inside a fluid, which could be used as a non contact sensor detected to the detection or the monitoring a local modification of a tissue, such as a tumor or a lesion. The used sensor is an MRI-like antenna, which consists in a flat, flexible and low cost multi-turn split resonator (MTLR), which features a geometrically pre-determined resonance frequency. The MTLR is used here as a transmit and receive sensor monitored by a distant loop coil. The complex impedance changes observed at the ends of the monitoring coil is known to be linked to the dielectric properties of the resonator environment. If a dielectric object is placed close to the resonator, the complex impedance is altered. In this work, an experimental set up is used to assess the relevance of such a measurement scheme to detect the presence of a dielectric inclusion embedded in another dielectric medium. The setup includes a spherical object of 1.5 cm diameter filled with various NaCl solutions moved vertically inside a tank filled with deionized water by means of a three axis robotic arm, to create an electrical conductivity contrast between the inclusion and the media, and three 2 cm diameter MTLR sensors featuring 30, 47 and 70 MHz resonance frequencies, respectively. The sensors are operated through the use of monitoring coils connected to a network analyzer, and measurements are carried out at each position of the spherical object. The resulting sensor responses are plotted and used for the assessment of sensor performances. In addition, a method to combine the multi-frequency data provided by the three different sensors is proposed. Two different metrics regarding the spatial resolution (SR) and the peak signal to noise ratio (PSNR) are computed to characterize the single sensor performances, as well as the enhancement provided by the proposed multi-frequency approach.