Abstract-The adoption of microwave imaging as a tool for noninvasive monitoring of brain stroke has recently gained increasing attention. In this respect, the paper aims at providing a twofold contribution. First, we introduce a simple design tool to devise guidelines to properly set the working frequency as well as to choose the optimum matching medium needed to facilitate the penetration of the probing wave into the head. Second, we propose an imaging strategy based on a modified formulation of the linear sampling method, which allows a quasi real time monitoring of the disease's evolution. The accuracy of the design guidelines and performance of the imaging strategy are assessed through numerical examples dealing with 2D anthropomorphic phantoms.

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Abstract-Microwave tomography deserves attention in biomedical imaging, owing to its potential capability of providing a morphological and functional assessment of the inspected tissues. However, such a goal requires the not trivial task of solving a non linear inverse scattering problem. In this paper, the factors affecting the complexity of the inverse problem are exploited to trace guidelines aimed at setting the matching fluid, the frequency range and the number of probes in such a way that the dielectric parameters of female breast tissues can be reliably retrieved. Examples, concerning 2D realistic numerical phantoms obtained by NMR images, are given to asses a posteriori the effectiveness of the proposed guidelines.

We propose a simple method based on the Aperture Antennas Theory to understand in a simple fashion limitations of OAM antennas in far-field links. Additional insight is also given by analyzing the properties of the operators relating source and farfield distributions for a given order of the vortex. The outcomes fully agree with the results recently achieved by Edfors, Craeye, and co-authors, and emphasize some additional draw-back. The 'degrees of freedom' of the fields associated to the different orders of the vortices are also discussed. Index Terms-Antenna theory, Orbital Angular Momentum. I. A PREMISEAmongst the novel techniques developed for utilizing the radio spectrum with maximum efficiency, great attention has been recently devoted to Orbital Angular Momentum (OAM) antennas [1]- [20]. Such systems have been in some cases proposed as means to improve almost "indefinitely" the channel capacity in a link amongst two antennas [2].Roughly speaking, the idea is to take profit from the fact that an antenna could simultaneously generate different fields each one associated to a different amount of 'orbital momentum', i.e., to a different angular variation of the phase around the target direction (such as e jϕ , e 2jϕ , and so on, ϕ denoting the azimuth angle in the observation domain). Then, by associating a different information to each of these patterns, one could realize an "OAM multiplexing" [17] and eventually enlarge "at will" the channel capacity [2]. Saying it in other words, it is like using different modes of a channel, where the modes are associated to a free space link rather than to a guiding structure.A large interest has been devoted to such a topic, including spectacular public demonstrations for mass media (e.g., Piazza San Marco, Venice, Italy, June 24 th 2011 [2]) as well as contributions on top scientific journals, e.g., [3]-[6].Since then, doubts [7] and objections [8],[10] have begun to appear, emphasizing some expected limitations.In the attempt to contribute to such a debate, in the following we propose a very simple yet instructive point of view on the subject. In particular, we focus on the actual possibility to get such a multiplication of channels [2] in the far-field zone, which is the usual framework for antenna links. In this respect, we will focus on the case of aperture antennas. Notably, this assumption does not impair the general validity of the following discussion. In fact, any antenna can be regarded as an aperture antenna by a proper choice of the aperture plane [21]. Also, the arguments below could be rephrased in terms of multipole expansions as well as in terms of the appropriate radiation integrals. Then, additional insight is also given by an analysis of the operators relating source and field behaviors for a given order of the vortex. The outcomes confirm limitations indicated in [7],[8], and emphasize some further draw-backs of the proposed multiplexing scheme.The paper is organized as follows. In Sections II and III, respectively, we review some properties of the H...

The possibility of mimicking different types of breast tissues to realize experimental phantoms has been investigated by measuring the complex permittivity of polyethylene glycol mono phenyl ether (Triton X-100) and distilled water solutions. In this respect, broad band electromagnetic characterization of several mixtures, at different concentrations, has been carried out in the 0.5???12 GHz frequency range. The good fitting between the dielectric properties of mammary tissues and that of the proposed mixtures, and the stability against temperature in the range 18???30 °C, suggest the possibility of mimicking the dielectric characteristics of breast tissues using easily available and low cost materials