This paper presents the preparation and measurement of tissue-mimicking head phantom and its validation with the iteratively corrected coherence factor delay-multiply-and-sum (IC-CF-DMAS) algorithm for brain stroke detection. The phantom elements are fabricated by using different chemical mixtures that imitate the electrical properties of real head tissues (CSF, dura, gray matter, white matter, and blood/stroke) over the frequency band of 1–4 GHz. The electrical properties are measured using the open-ended dielectric coaxial probe connected to a vector network analyzer. Individual phantom elements are placed step by step in a three-dimensional skull. The IC-CF-DMAS image reconstruction algorithm is later applied to the phantom to evaluate the effectiveness of detecting stroke. The phantom elements are preserved and measured multiple times in a week to validate the overall performance over time. The electrical properties of the developed phantom emulate the similar properties of real head tissue. Moreover, the system can also effectively detect the stroke from the developed phantom. The experimental results demonstrate that the developed tissue-mimicking head phantom is time-stable, and it shows a good agreement with the theoretical results in detecting and reconstructing the stroke images that could be used in investigating as a supplement to the real head tissue.

Experimental tissue mimicking human head phantom for estimation of stroke using IC-CF-DMAS algorithm in microwave based imaging system

Medical microwave imaging (MMWI) is a multidisciplinary task that requires precise electromagnetic modeling of heterogeneous human tissues, solving non-linear and ill-posed inverse scattering problems with weak contrasts, and design of compact antenna arrays well-matched to adjacent body parts. If any of the activities listed above does not perform well, the resulting imaging will be poor. In this article, we devote a special effort to achieving high accuracy in electromagnetic simulations of MMWI scenarios. This complex task requires developing a suitable meshing procedure, which balances model fidelity and computational bottlenecks such as a large number of unknowns in the electromagnetic model. Thus, instead of a simple conversion from triangles to quads, we offer a technique that generates high-quality mesh (in terms of both simulation speed and geometrical accuracy) with controllable deviation from the initial model. In addition, we developed a strategy for self-convergence tests that prove the reliability of the simulation results in the MMWI framework. Moreover, an additional validity test has been proposed, based on the inverse scattering operator, commonly used in many microwave imaging algorithms. The effectiveness of these novel techniques is demonstrated with the example of a 21-antenna system placed around the 5-tissue head phantom with and without stroke. 

3-D EM Modeling of Medical Microwave Imaging Scenarios With Controllable Accuracy

Microwave imaging offers a variety of devices for patient diagnosis, monitoring, and treatment. To develop and evaluate these devices, it is necessary to design anthropomorphic reference phantoms with physical and dielectric characteristics similar to those of the human body. This chapter deals with the design and development of several anthropomorphic phantoms adapted to the geometry of the microwave imaging systems. They are composed of 3D printed anthropomorphic cavities filled with liquid mixtures of TritonX-100 and salt water. The concentrations of the latter, to mimic tissues over a wide band, are given by a Gauss–Newton algorithm combined with the use of a binary mixture law. The process is extended to the production of liquid mixtures mimicking any biological tissue, at least in the [0.5–6] GHz frequency range. This opens new avenues in the standardizing of phantoms for assessing microwave imaging systems. 

Standardized Phantoms

This paper describes the realization of a versatile anthropomorphic multi-tissue head phantom for testing microwave imaging devices, employing mixtures featuring readily available, not harmful, and stable components. Considering that the evaluation of devices, such as medical ones, might require the variation of the characteristics of the tissues for their assessment, the manufacturing approaches the issue using both unalterable and adjustable components. For the former, we reproduce skin, fat, bone, grey matter, white matter, cerebellum, and ventricles using flexible and solid compounds made with proper proportions of urethane rubber, graphite powder, and salt. While for the latter, water, triton X-100, and salt are used to generate a liquid mixture to mimic cerebrospinal fluid. The anatomical structures of the tissues are faithfully rendered using 3D-printed molds and assembled without additional intermediate plastic layers. The materials’ dielectric properties are measured via an open-ended coaxial probe method, investigating the frequency range of 0.5-2 GHz. The final prototype fits the nominal dielectric values in the literature, presenting stability and replicability, demonstrating its validity as a multi-purpose test object. 

Anthropomorphic Multi-tissue Head Phantom for Microwave Imaging Devices Testing

This work is devoted to the development and manufacturing of realistic benchmark phantoms to evaluate the performance of microwave imaging devices. The 3D (3 dimensional) printed phantoms contain several cavities, designed to be filled with liquid solutions that mimic biological tissues in terms of complex permittivity over a wide frequency range. Numerical versions (stereolithography (STL) format files) of these phantoms were used to perform simulations to investigate experimental parameters. The purpose of this paper is two-fold. First, a general methodology for the development of a biological phantom is presented. Second, this approach is applied to the particular case of the experimental device developed by the Department of Electronics and Telecommunications at Politecnico di Torino (POLITO) that currently uses a homogeneous version of the head phantom considered in this paper. Numerical versions of the introduced inhomogeneous head phantoms were used to evaluate the effect of various parameters related to their development, such as the permittivity of the equivalent biological tissue, coupling medium, thickness and nature of the phantom walls, and number of compartments. To shed light on the effects of blood circulation on the recognition of a randomly shaped stroke, a numerical brain model including blood vessels was considered.

Nicolas Phillips

Papers

A Simulation-Based Methodology of Developing 3D Printed Anthropomorphic Phantoms for Microwave Imaging Systems.