Loreto Di Donato

A FEASIBILITY STUDY ON MICROWAVE IMAGING FOR BRAIN STROKE MONITORING

The adoption of microwave imaging as a tool for non- invasive 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 modifled formulation of the linear sampling method, which allows a quasi real time monitoring of the diseases evolution. The accuracy of the design guidelines and performance of the imaging strategy are assessed through numerical examples dealing with 2D anthropomorphic phantoms.

On Quantitative Microwave Tomography of Female Breast

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 afiecting the complexity of the inverse problem are exploited to trace guidelines aimed at setting the matching ∞uid, 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 efiectiveness of the proposed guidelines.

Inverse Scattering Via Virtual Experiments and Contrast Source Regularization

In microwave imaging, the linearity of the relationship between the incident and the scattered field offers the possibility of a posteriori recombining the performed scattering experiments and then to cast the underlying inverse problem with respect to the resulting, virtual, ones. The interest of such a circumstance is that properly designed virtual experiments can enforce particular and convenient conditions. In this paper, we present an application of this paradigm to the popular contrast source inversion (CSI) method. In particular, we first design a set of virtual experiments capable to induce contrast sources exhibiting circular symmetries (with respect to some pivot points). Then, we devise an original and effective regularized CSI scheme, in which a penalty term is added to the usual cost functional, in order to account for the symmetry of the auxiliary unknowns. Notably, the approach does not require any apriori assumption on the unknown contrast, as it relies on the particular nature of the virtual contrast sources. Results with single frequency Fresnel experimental data are given to assess the capabilities of the proposed approach.

An Algebraic Solution Method for Nonlinear Inverse Scattering

By using properly designed synthetic (or “virtual”) experiments and an original approximation of the contrast sources, we are able to recast the inverse scattering problem in an algebraic form (in a subset of points of the imaged domain) and, hence, to solve it by means of closed form formulas. The new approximation relies on the assumption that the contrast sources induced by the different virtual experiments are focused in given points belonging to the scatterer. As such, the method involves a preprocessing step in which the outcome of the original scattering experiments is recombined into the new, virtual, ones capable of enforcing the expected contrast sources behavior. Examples with numerical and experimental data are provided to assess the actual possibility of setting such a virtual experiments framework, and show the effectiveness of the proposed method.

Assessing the Capabilities of a New Linear Inversion Method for Quantitative Microwave Imaging

We investigate the imaging capabilities of a new linear microwave imaging approach, which allows to quantitative retrieve the complex permittivity distribution of unknown nonweak targets. To this end, we carry out a parametric numerical analysis for a canonical scatterer (a homogeneous dielectric cylinder with circular cross section) and derive a quantitative criterion to foresee the method’s applicability. The reliability of the criterion is then tested against noncanonical scatterers to show the effectiveness of the method in imaging nonweak targets and in outperforming the linearized inversion method based on the standard Born approximation.

Permeability Changes of Cationic Liposomes Loaded with Carbonic Anhydrase Induced by Millimeter Waves Radiation

The interaction of millimeter wave radiation, in the 30–300 GHz range, with biological systems is a topic of great interest as many of the vibrational dynamics that occur in biochemical reactions of large macromolecules in living organisms fall in the 1–100 GHz range. Membranes and cellular organelles may have different ways of interacting with this radiation as well. In this article, we investigate the influence of 53.37 GHz of radiation on lipid membrane permeability by using cationic liposomes that contain dipalmitoylphosphatidylcholine (DPPC), cholesterol and stearylamine. Carbonic anhydrase (CA) is loaded inside the liposome and the substrate p-nitrophenyl acetate (p-NPA) is added in the bulk aqueous phase. Upon permeation across the lipid bilayer, the trapped CA catalyzes the conversion of the p-NPA molecules into products. Because the self-diffusion rate of p-NPA across intact liposomes is very low, the CA reaction rate expressed as ΔA/min is used to track membrane permeability changes. A highly significant (P < 0.0001) enhancement of the CA reaction rate, typically from ΔA/min = 0.0043 ± 0.0017 (n = 26) to ΔA/min = 0.0100 ± 0.0020 (n = 32) resulted at a low-level density power of 0.1 mW/cm2. The enhancement of the CA reaction rate was observed at a lesser extent on liposomes with a larger diameter and, in turn with leaflets less bent. The different packing of the phospholipid bilayer—due to the higher curvature—could be a critical factor in eliciting membrane permeability changes indicating a possible role for water molecules bound to functional groups in the glycerol region. Since numerical dosimetry indicates that the temperature rise during the exposure was negligible, the observed effects cannot be attributed to heating of the samples.

AN ADAPTIVE METHOD TO FOCUSING IN AN UNKNOWN SCENARIO

The problem of fleld focusing onto a target location in an unknown scenario is considered. In particular, we devise an adaptive procedure in which flrst an image of the unknown region where the target point is located is formed via the linear sampling method (LSM). Then, the LSM result is used also to deflne the excitations coe-cients for the array elements needed to focus the fleld. This novel approach to focusing is described and tested with numerical examples.

Imaging of 3D magnetic targets from multiview multistatic GPR data

Ground Penetrating Radar surveys aimed at imaging magnetic anomalies are gaining an increasing attention in several applications. In this respect, we introduce in this communication an inverse scattering strategy based on a suitable reformulation of the Linear Sampling Method (LSM). The LSM is a reliable and computationally effective imaging approach, which is exactly cast as a linear inverse problem (i.e., no approximation is required). This is made possible by restricting the processing task to the reconstruction of the shape of the anomaly and neglecting the retrieval of the electromagnetic features. Since the LSM has been never applied to the imaging of magnetic targets from electric field data, a preliminary assessment of its reconstruction capabilities towards purely magnetic anomalies is given by means of numerical examples.

Microwave Imaging via Distorted Iterated Virtual Experiments

The linearity of the scattering phenomenon with respect to primary sources allows to recombine a posteriori the available experiments and build, in a synthetic fashion, new “virtual” experiments. Starting from this circumstance, an iterative procedure is proposed as an effective approach to tackle nonlinear inverse scattering problems. In this procedure, the virtual experiments, the Green’s function, and the corresponding physical inspired field approximations are updated at each iteration. The structure and the complexity of the approach are comparable with those of the widely adopted distorted Born iterative method, but its performances are remarkably better, thanks to extended validity of the exploited field approximation. The overall approach also takes advantage of a compressive sensing inspired regularization scheme to promote sparsity in the search of piecewise constant dielectric profiles and further improve the accuracy of the imaging results. Examples with numerical and experimental data are given to assess the method.

A New Linear Distorted-Wave Inversion Method for Microwave Imaging via Virtual Experiments

A novel microwave imaging approach to reconstruct the dielectric properties of targets hosted in partially known, noncanonical, scenarios is proposed and assessed. The method takes joint advantage of the recently introduced virtual experiments paradigm and exploits a new linear approximation developed within such a framework. Such an approximation implicitly depends on the unknown targets and, therefore, has a broader applicability as compared with the traditional distorted Born approximation. Being noniterative, the resulting distorted-wave inversion method is capable of quasi-real-time imaging and successfully images nonweak perturbations. The performances of the novel imaging method have been assessed with simulated data and validated experimentally against some of Fresnel data sets.