A Preliminary Experiment Based on One-step Measurement-trained Supervised Descent Method for Microwave Thorax Imaging

A preliminary experiment for microwave thorax imaging is conducted. A dual-band symmetrical dipole antenna is designed for microwave thorax imaging. It operates at 433 MHz and 915 MHz in Industrial, Scientific and Medical (ISM) Band. It is compact and low-profile with a size of 30 mm x 20 mm x 3mm. Short ended and triangular structures are applied to tune low and high frequency modes and enlarge bandwidth. The antenna radiates most power towards in-water direction. The antenna is then fabricated and measured. The measured reflection coefficients verify the antenna design. A rectangular water tank mimicking human thorax is built for experiments. Sixteen antennas are attached to the side wall with uniform spacing. Each antenna acts as transmitter alternately and other antennas act as receivers. Cylindrical objects are placed in water as imaging targets. Sixteen by sixteen S-parameter matrices are measured for reconstruction with the help of switch matrix and vector network analyzer (VNA). A One-Step Supervised Descent Method (SDM) is used for reconstruction. An average descent direction is trained before reconstruction. During the training stage a plastic graduated cylinder whose diameter is 52 mm is placed in water as training object. The object is placed on a 22 cm x 12 cm grid with 1 cm grid size. The S-parameter matrix is measured at each position and 299 training samples are acquired in total. The average descent direction is trained based on these training samples. In the testing stage, four cylindrical objects with circular, triangular, square and hexagonal section are fabricated by 3-D printing. Each target is separately put in water in single-object testing. With One-Step Measurement-Trained SDM, the position and size of the cylinder can be qualitatively reconstructed. This measurement-trained approach takes the characteristics of measurement environment into consideration through the training stage. The reconstruction results verify the feasibility of One-Step SDM in microwave thorax imaging.