Microwave imaging is a relatively new modality to perform non- invasive diagnostic of biological tissues. The potential of these techniques results from the dependence of the dielectric properties of these tissues with respect to quantities of practical and clinical relevance such as water content, blood flow rate, temperature, etc. Initiated in the 80s, these techniques have suffered from the complexity of the interaction mechanisms between microwave beams and biological structures. The compensation of strong diffraction effects by high dielectric contrast structures requires efficient reconstruction algorithms. Recently, major advances have been achieved for improving the time resolution and the quantification of microwave images. This paper is focused on the first aspect. It reports some unique results obtained, for the first time, with a microwave camera providing qualitative images of biological targets at the rate of 15 images per second. Image reconstruction is performed by means of spectral diffraction tomography algorithms. The camera consists of a 2D array of 32 X 32 sensors, covering a square area of approximately 22 cm X 22 cm. The camera is operated at 2.45 GHz, according to the Modulated Scattering Technology (MST). The biological target is immersed in water (or can be inserted between bolus) and illuminated by a lens-compensated horn antenna. From the amplitude/phase measurement of the field scattered by the target, microwave images can be reconstructed, thanks to numerical focusing, in any plane located between the transmitting antenna and the camera. Typically, the investigation depth is 25 cm, and the spatial resolution is about 5 mm. The capabilities of this microwave camera will be illustrated by means of a short VHS video tape showing quick motions of living structures. Expected improvements of the camera performances are discussed and possible clinical applications are analyzed.
Nondestructive evaluation of concrete structures is an important practical issue in the construction industry. Evaluation encompasses many issues including concrete constituent property, compressive strength and chloride contamination determination to name a few. To this end, a combination of modulated scattering technique and near-field microwave nondestructive evaluation technique, is used to determine its potential for evaluating dielectric properties of a hardened mortar specimen. This technique utilizes a small resonant PIN diode-loaded dipole scatterer embedded inside the mortar while using an open-ended rectangular waveguide probe operating at 7 GHz to detect this dipole which is modulated at low frequencies. The results of this preliminary experiment, and its future ramifications for nondestructive concrete inspection structures are provided in this paper.
For portable, civil or military, communication systems, the characterization of the radiated field poses specific problems. First of all, due to its poor directivity, the radiated field has to be determined over a full sphere surrounding the test antenna. Secondly, the user has an evident but rather random effect on the radiating properties, and the knowledge of such effects is required to perform reliable predictions of the radiated field. Finally, due to their small volume, the measurement process can be seriously perturbed by the cable(s) used to feed the portable system. This paper presents a Near- Field (NF) approach which offers a convenient solution for the two first aspects. The third one has been solved by using an optical link to feed the portable equipment under test. The performances of this optical link have been assessed by comparison to standard RF coaxial feeding and to numerical results predicted from a computer code. The NF approach has allowed to investigate the effects of the user on the Far-Field (FF) radiation pattern of his telephone, both in co- and cross- polarization. Preliminary assessments, conducted in the Socrate spherical NF facility of the Centre d'Etudes de Gramat (CEG), have demonstrated the flexibility and the accuracy of the NF approach.
For a long time, microwaves have been considered as a possible sensing agent for nondestructive testing/evaluation purposes. This trend has still been reinforced these last years with the advent of new microwave penetrable materials, such as composites. Inspection of materials via a mechanically scanned probe has proven to offer a convenient, but time consuming, way to measure local reflexion or transmission coefficients and, hence, to evaluate defects, faults, etc... High speed measurements are now possible by using arrays of fixed probes, resulting in attractive imaging equipments. Indeed, the availability of amplitude/phase data allows us to consider different processing techniques, the complexity of which can be selected according to the required performances in terms of contrast, spatial and time resolutions. This paper reviews some of the most promising approaches, such as non-linear inverse scattering techniques and neural networks. Prospective considerations are devoted to the future of such sophisticated microwave sensing techniques.
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