A breast phantom developed at the Supelec Institute was interrogated to study its suitability for microwave tomography measurements. A microwave measurement system based on 16 monopole antennas and a vector network analyzer was used to study how the S-parameters are influenced by insertion of the phantom. The phantom is a 3D-printed structure consisting of plastic shells that can be filled with tissue mimicking liquids. The phantom was filled with different liquids and tested with the measurement system to determine whether the plastic has any effects on the recovered images or not. Measurements of the phantom when it is filled with the same liquid as the surrounding coupling medium are of particular interest. In this case, the phantom plastic has a substantial effects on the measurements which ultimately detracts from the desired images.

We have developed a transmission-based, open-ended coaxial dielectric probe that can be used in clinical situations and overcomes many of the limitations related to the typical reflection-based dielectric probes. The approach utilizes the low profile, open-ended coaxial cables enabling clinicians to still probe relatively compact spaces. The sensing depth can be extended to as large as 1.5 to 2 cm compared with the more typical range of 0.3 mm for conventional probes and is dramatically less affected by measurement technique variability including poor sample contact and cable bending. As a precursor to an actual clinical implementation, we study the technique in a range of homogeneous liquids with substantially varying dielectric properties. The initial results demonstrate good agreement between the transmission-based probe and commercial, reflection-based probes and pave the way for more substantial clinical implementation.

A microwave tomographic system is used to evaluate the realistic breast phantom developed at the Supelec institute. The system utilizes 16 monopole antennas and a modern vector network analyzer (VNA) to measure the phantoms influence on the S-parameters. An iterative algorithm is then used to solve the inverse problem and reconstruct a 2-D plane transecting the phantom. The reconstructed images are compared to the ones recovered from a cylindrical phantom of equivalent phantom media. The results show that both phantoms are possible to reconstruct, although the interior of the Supelec phantom is more challenging.

The realistic breast phantom developed by the French team at Supelec provides an excellent opportunity to explore various measurement systems and associated algorithms. In this case there is a known ground truth for the geometry of the segmented regions for direct comparison of images with actual sizes, shapes and locations of the different features. It also allows for testing utilizing a range of dielectric materials for both the adipose and fibroglandular region which can be useful given the disparity of published property values. For this experiment, we are exploring our tomographic imaging algorithm with a log transformation in the context of restricting the imaging zone strictly to that space occupied by the phantom. In this case, the 2-dimensional perimeter of the breast was approximated by a simple ellipse which was extracted from the original image. All images were recovered without the assistance of a priori information and also included our 2-step imaging scheme. Simple parameter tests were performed to assess the limitations of the technique, especially when the imaging zone was not accurately determined. The final results were also analyzed by examination of histograms of the field residuals to assess whether the algorithm is robust and unbiased.